JP2018129993A - Voltage converter, voltage step-down control method of voltage conversion circuit, and computer program - Google Patents

Abstract

When performing so-called soft switching using LC resonance, a voltage conversion device, a voltage conversion circuit step-down control method, and a computer capable of preventing and reducing deterioration in conversion efficiency even when the resonance period fluctuates Provide a program. A control unit included in the voltage conversion device includes: an efficiency calculation unit that calculates a conversion efficiency of the voltage conversion circuit based on detection results of the first and second detectors; and resonance of resonance current by the first inductor and the capacitor. The efficiency when the specifying unit for specifying a second time different from the first time according to the length of the cycle, and the high-voltage side switching element for the second time specified by the first time and the specifying unit are turned on. An efficiency comparison unit that compares the conversion efficiencies calculated by the calculation unit; and an on-time determination unit that determines an on-time to turn on the high-voltage side switching element based on a comparison result of the efficiency comparison unit. [Selection] Figure 1

Description

  The present invention relates to a voltage conversion device that converts a DC voltage into a voltage, a step-down control method for a voltage conversion circuit, and a computer program.

  A DC-DC converter (hereinafter simply referred to as a converter) that steps up and down a DC voltage is widely used as a power source for in-vehicle devices and industrial devices. In response to the demand for miniaturization of power supplies, the operating frequency of converters tends to be increased in order to make it possible to use passive components such as inductors and capacitors having a small volume. On the other hand, another problem that the switching loss of the switching element that switches the current flowing through the inductor increases as the operating frequency increases.

  On the other hand, Patent Document 1 discloses a resonance current flowing in a resonance circuit composed of a resonance reactor (inductor) and a resonance capacitor (capacitor) connected in series to a transistor (switching element) that switches an input voltage. A step-down converter is disclosed in which a transistor is switched from on to off when it becomes 0 or less. By performing such zero current switching, the switching loss of the transistor that switches the current flowing through the inductor is reduced.

  The converter disclosed in Patent Document 1 is a transistor at the time when a time To = Tn · (1 + Zn · Io / Vi) / 2 during which a resonance current i of zero or more flows through a resonance reactor from the time when the transistor is turned on. The basic configuration is to switch from on to off. Also disclosed is a configuration in which the transistor is switched from on to off after a predetermined time when the resonance current i becomes equal to or less than zero after the resonance current i becomes equal to or less than the predetermined current.

JP 2002-58240 A

  However, the inductance of the resonance reactor and the capacitance of the capacitor have fluctuation factors such as manufacturing variations, temperature changes, and time changes. In the technique described in Patent Document 1, the resonance period is designed by these factors. May deviate from the value. When such a divergence occurs, there is a problem that the conversion efficiency of the voltage conversion circuit deteriorates due to a shift in the time T0 or a shift in the predetermined time.

  The present invention has been made in view of such circumstances. The purpose of the present invention is to perform so-called soft switching using LC resonance and to reduce the conversion efficiency even when the resonance period fluctuates. An object of the present invention is to provide a voltage conversion device, a step-down control method for a voltage conversion circuit, and a computer program that can be prevented.

  A voltage conversion device according to an aspect of the present invention includes a first inductor, a high-voltage side switching element and a low-voltage side switching element connected in series via the first inductor, the first inductor and the low-voltage side switching element. A voltage conversion circuit having a second inductor having one end connected to the connection point of the capacitor and a capacitor connected in parallel to the low-voltage side switching element, and turning on / off the high-voltage side switching element to the voltage conversion circuit A voltage conversion device including a control unit that performs step-down conversion, a first detector that detects a voltage and a current input to the voltage conversion circuit, and a second that detects a voltage and a current output from the voltage conversion circuit. An efficiency of calculating a conversion efficiency of the voltage conversion circuit based on detection results of the first and second detectors. The specifying unit for specifying the second time different from the first time according to the length of the resonance period of the resonance current by the first inductor and the capacitor, and the first time and the specifying unit When the high-voltage side switching element is turned on only for the second time, the efficiency comparison unit that compares the conversion efficiencies calculated by the efficiency calculation unit and the high-voltage side switching element are turned on based on the comparison result of the efficiency comparison unit. An on-time determining unit for determining an on-time to be performed.

  A step-down control method for a voltage conversion circuit according to an aspect of the present invention includes a first inductor, a high-voltage side switching element and a low-voltage side switching element connected in series via the first inductor, the first inductor, A voltage conversion circuit having a second inductor having one end connected to a connection point of the low-voltage side switching element and a capacitor connected in parallel to the low-voltage side switching element, and a voltage and a current for each of the input and output of the voltage conversion circuit A control method for causing the voltage conversion circuit to perform step-down conversion by the control unit in a voltage conversion device including a first detector and a second detector for detecting the above and a control unit for turning on / off the high-voltage side switching element. The high-voltage side switch for a first time according to the length of the resonance period of the resonance current by the first inductor and the capacitor When the switching element is turned on, the conversion efficiency of the voltage conversion circuit is calculated based on the detection results of the first and second detectors, and a second time different from the first time is specified and specified. When the high-voltage side switching element is turned on for only 2 hours, the conversion efficiency of the voltage conversion circuit is calculated based on the detection results of the first and second detectors, and only the first time and the specified second time The conversion efficiencies calculated when the high-voltage side switching elements are turned on are compared, and an on-time for turning on the high-voltage side switching elements is determined based on the comparison result.

  A computer program according to an aspect of the present invention includes a first inductor, a high-voltage side switching element and a low-voltage side switching element connected in series via the first inductor, and the first inductor and the low-voltage side switching element. A voltage conversion circuit having a second inductor having one end connected to a connection point and a capacitor connected in parallel to the low-voltage side switching element, and a voltage conversion circuit for detecting a voltage and a current for each of an input and an output of the voltage conversion circuit A computer program for causing the voltage conversion circuit to perform step-down conversion in the voltage conversion device including a first detector, a second detector, and a control unit for turning on and off the high-voltage side switching element, wherein the control Calculating the conversion efficiency of the voltage conversion circuit based on the detection results of the first and second detectors. An efficiency calculating unit, a specifying unit for specifying a second time different from the first time according to the length of the resonance period of the resonance current by the first inductor and the capacitor, the specifying by the first time and the specifying unit When the high-voltage side switching element is turned on only for the second time, the efficiency comparison unit that compares the conversion efficiencies calculated by the efficiency calculation unit, and the high-voltage side switching element is turned on based on the comparison result of the efficiency comparison unit. It functions as an on-time determining unit that determines an on-time to be performed.

  In addition, this application is implement | achieved as a step-down control method of the voltage converter and voltage converter circuit which each have such a characteristic process part and step, or makes a computer perform the step corresponding to this characteristic process part For example, a part or all of the voltage conversion device can be realized as a semiconductor integrated circuit, or can be realized as another system including the voltage conversion device.

  Based on the above, when so-called soft switching is performed using LC resonance, it is possible to suitably reduce the loss of the switching element even when the resonance period varies.

It is a block diagram which shows the structural example of the voltage converter which concerns on embodiment. It is a timing chart which shows an example of the waveform of each part at the time of the pressure | voltage fall operation of a voltage converter circuit. It is explanatory drawing which shows an example of the state D1 in the pressure | voltage fall operation | movement of a voltage converter circuit. It is explanatory drawing which shows an example of the state D2 in the pressure | voltage fall operation | movement of a voltage converter circuit. It is explanatory drawing which shows an example of the state D3 in the pressure | voltage fall operation | movement of a voltage converter circuit. It is explanatory drawing which shows an example of the state D4 in the pressure | voltage fall operation | movement of a voltage converter circuit. It is a timing chart which shows typically the waveform of each part after a voltage conversion circuit passes through state D4, changes to state D3 through states D1 and D2. It is a graph which shows the example of the current waveform of SW element which changes according to the fluctuation | variation of a resonance period. It is a graph which illustrates the characteristic of the conversion efficiency with respect to output current about a voltage conversion circuit. It is a graph which shows the fluctuation characteristic of the ON time of SW element with respect to output current and ambient temperature. It is a flowchart of the main routine which shows the process sequence of CPU which calculates the ON time which should actually turn on SW element. It is a flowchart of the main routine which shows the process sequence of CPU which calculates the ON time which should actually turn on SW element. It is a flowchart which shows the process sequence of CPU which concerns on the subroutine of conversion for 1 period. It is a flowchart which shows the process sequence of CPU which concerns on the subroutine of efficiency calculation.

[Description of Embodiment of the Present Invention]
First, embodiments of the present invention will be listed and described. Moreover, you may combine arbitrarily at least one part of embodiment described below.

(1) A voltage converter according to an aspect of the present invention includes a first inductor, a high-voltage side switching element and a low-voltage side switching element connected in series via the first inductor, the first inductor, and the low-voltage A voltage conversion circuit having a second inductor having one end connected to a connection point of the side switching element, and a capacitor connected in parallel to the low voltage side switching element, and the voltage by turning on / off the high voltage side switching element A voltage conversion device including a control unit that performs step-down conversion in a conversion circuit, the first detector detecting a voltage and a current input to the voltage conversion circuit, and a voltage and a current output from the voltage conversion circuit And a control unit that calculates a conversion efficiency of the voltage conversion circuit based on detection results of the first and second detectors. An efficiency calculating unit, a specifying unit for specifying a second time different from the first time according to the length of the resonance period of the resonance current by the first inductor and the capacitor, and specifying by the first time and the specifying unit When the high-voltage side switching element is turned on only for the second time, the efficiency comparison unit that compares the conversion efficiencies calculated by the efficiency calculation unit, and the high-voltage side switching element based on the comparison result of the efficiency comparison unit. An on-time determining unit that determines an on-time to be turned on.

(10) A step-down control method for a voltage conversion circuit according to an aspect of the present invention includes a first inductor, a high-voltage side switching element and a low-voltage side switching element connected in series via the first inductor, and the first A voltage conversion circuit having a second inductor having one end connected to a connection point between the inductor and the low-voltage side switching element, and a capacitor connected in parallel to the low-voltage side switching element, and each of an input and an output of the voltage conversion circuit A control method for causing the voltage conversion circuit to perform step-down conversion in the voltage conversion device including a first detector and a second detector that detect voltage and current, and a control unit that turns on and off the high-voltage side switching element. The high-voltage side for the first time corresponding to the length of the resonance period of the resonance current by the first inductor and the capacitor. When the switching element is turned on, the conversion efficiency of the voltage conversion circuit is calculated based on the detection results of the first and second detectors, and a second time different from the first time is specified and specified. When the high-voltage side switching element is turned on for only 2 hours, the conversion efficiency of the voltage conversion circuit is calculated based on the detection results of the first and second detectors, and only the first time and the specified second time The conversion efficiencies calculated when the high-voltage side switching elements are turned on are compared, and an on-time for turning on the high-voltage side switching elements is determined based on the comparison result.

(11) A computer program according to an aspect of the present invention includes a first inductor, a high-voltage side switching element and a low-voltage side switching element connected in series via the first inductor, the first inductor and the low-voltage side A voltage conversion circuit having a second inductor having one end connected to a connection point of the switching element and a capacitor connected in parallel to the low-voltage side switching element, and a voltage and a current for each of an input and an output of the voltage conversion circuit A computer program for causing the voltage conversion circuit to perform step-down conversion in the control unit in a voltage conversion device including a first detector and a second detector to detect, and a control unit for turning on and off the high-voltage side switching element. The control unit is configured to convert the voltage conversion circuit based on the detection results of the first and second detectors. An efficiency calculating unit that calculates the second time different from the first time according to the length of the resonance period of the resonance current by the first inductor and the capacitor, the first time and the specifying unit When the high-voltage side switching element is turned on for the specified second time, an efficiency comparison unit that compares the conversion efficiencies calculated by the efficiency calculation unit, and the high-voltage side switching element based on the comparison result of the efficiency comparison unit Is made to function as an on-time determining unit that determines an on-time to be turned on.

  In this aspect, in the voltage conversion circuit, the high-voltage side switching element, the first inductor, and the low-voltage side switching element having capacitors connected to both ends are connected in this order. One end of the second inductor is connected to the connection point of the side switching element. When the control unit turns on / off the high-voltage side switching element, the voltage input to the high-voltage side of the voltage conversion circuit is stepped down and output to the low-voltage side.

  The control unit calculates the first conversion efficiency of the voltage conversion circuit when the high-voltage side switching element is turned on for the first time corresponding to the length of the resonance period of the resonance current by the first inductor and the capacitor, When the second time different from the first time is specified and the high-voltage side switching element is turned on for the second time, the second conversion efficiency of the voltage conversion circuit is calculated. The control unit further determines the time when the high-voltage side switching element is turned on when the conversion efficiency is higher in order to solve the problem of the present application based on the result of comparing the first and second conversion efficiencies. Is determined as the on-time to be turned on. As a result, the actual conversion efficiency of the voltage conversion circuit is higher than when the high-voltage side switching element is turned on for the calculated on-time.

(2) When the control unit specifies the second time longer than the first time by the specifying unit, the conversion efficiency is lower when the comparison result by the efficiency comparison unit is turned on only for the second time. In some cases, it is preferable that the specifying unit specifies a second time shorter than the first time.

  In this aspect, the second time longer than the first time is specified first, and the second conversion efficiency of the voltage conversion circuit is calculated. As a result of comparing the second conversion efficiency calculated by making the second time longer than the first time and the above-described first conversion efficiency, if the second conversion efficiency is lower, A second time shorter than the first time is specified. This increases the probability that the second conversion efficiency calculated next is higher than the first conversion efficiency.

(3) The control unit calculates the conversion efficiency by the efficiency calculation unit and compares the conversion efficiency by the efficiency comparison unit every time the specifying unit specifies the second time to change in time series. Is preferred.

  In this aspect, the second time different from the first time is changed in time series, and the second conversion efficiency of the voltage conversion circuit is calculated each time. As a result of comparing the second conversion efficiency calculated in accordance with the second time different in time series as described above and the first conversion efficiency described above, the high-voltage side switching element is turned on when the conversion efficiency is the highest. The determined time is determined as the on-time when the high-voltage side switching element is actually to be turned on. As a result, an ON time during which the actual conversion efficiency of the voltage conversion circuit is highest is searched.

(4) The control unit stores the conversion efficiency calculated in advance for the voltage conversion circuit in association with the output current, and the first storage based on the current detected by the second detector. It is determined whether or not the conversion efficiency calculated by the efficiency calculation unit is lower than a predetermined value when the high-voltage side switching element is turned on for the second time specified by the specification unit than the conversion efficiency read from the unit It is preferable that the determination unit further specifies the second time to be further changed when the determination unit determines that the second time is low.

  In this aspect, the ideal conversion efficiency calculated in advance for the voltage conversion circuit is stored in the first storage unit in association with the output current as the calculation condition. When executing the voltage conversion, the second time is sequentially specified, and the conversion efficiency is calculated when the high-voltage side switching element is turned on for the specified second time. When the conversion efficiency calculated here is lower than the conversion efficiency read from the first storage unit based on the detected output current by a predetermined value or more, the second time is further changed. Thereby, the search for a suitable on-time is continued until the difference between the measured conversion efficiency of the voltage conversion circuit and the ideal conversion efficiency becomes smaller than a predetermined value.

(5) It is preferable that the specifying unit specifies the second time by adding or subtracting a time obtained by multiplying a predetermined minute time by an integer with respect to the first time.

  In this aspect, since the second time is specified by adding or subtracting a time obtained by multiplying the predetermined minute time by an integer to the first time, the second time is finely adjusted around the first time. The

(6) It is preferable that the specifying unit specifies the second time within a predetermined range set in advance.

  In this aspect, the second time specified differently from the first time is set within a predetermined range. This prevents the on-time from being inappropriately determined when the resonance period varies due to a factor different from the assumed variation factor.

(7) The apparatus further includes a temperature sensor that detects an ambient temperature of the voltage conversion circuit, and the control unit corresponds to a difference obtained by subtracting the first time from the ON time to an ambient temperature of the voltage conversion circuit and an output current. Whether or not a difference is stored in a storage area of the second storage unit corresponding to the second storage unit for storing and the current detected by the second detector and the ambient temperature detected by the temperature sensor A storage determination unit that determines whether the difference is stored in the storage determination unit, and a time obtained by adding the difference read from the storage area to the first time is determined as a second time. The difference which memorize | stores the difference which subtracted the said 1st time from the ON time determined by the said ON time determination part in the said storage area, when it determines with the ON time determination part and the said memory | storage determination part not being memorize | stored With memory It is preferred to have the.

  In this aspect, when the conversion efficiency difference corresponding to the detected output current and the ambient temperature is stored in the second storage unit when the voltage conversion is executed, the difference read from the second storage unit is the first difference. Add to 1 hour to determine on-time. On the other hand, when the difference in conversion efficiency is not stored in the second storage unit, the second time and the conversion efficiency are sequentially specified and calculated, and when the on-time is determined, the first time is determined from the determined on-time. The subtracted difference is stored in the second storage unit in association with the detected output current and ambient temperature. Thereby, when the ON time determined in the past according to the output current and the ambient temperature is stored in the second storage unit, the calculation of the second time and the conversion efficiency becomes unnecessary. As the time elapses, the contents stored in the second storage unit become enriched.

(8) It is preferable that the control unit further includes an estimation unit that estimates a time from when the high-voltage side switching element is turned on until the resonance current becomes a minimum, and sets the first time.

  In this aspect, since the time from when the high-voltage side switching element is turned on to the time when it is estimated that the resonance current will be minimized is the first time, the resonance period is an ideal period. At the center, an on-time when the conversion efficiency is higher is searched.

(9) It is preferable that the estimation unit calculates the first time based on the following equation.
Tmi = (Lr1 / Vi) {Io-Vi (1-D) D / (2Lr2fs)}
+ (3/4) Tr
However,
Tmi: the first time Lr1: the inductance of the first inductor Vi: the input voltage of the voltage conversion circuit Io: the output current of the voltage conversion circuit D: the target duty ratio Lr2 of the high-voltage side switching element: the second inductor Inductance (Lr2 >> Lr1)
fs: switching frequency of the high-voltage side switching element Tr: 2π√ (Lr1Cr1)
Cr1: Capacitance of the capacitor

  In this aspect, the time from when the high-voltage side switching element is turned on until the resonance current becomes minimum is estimated based on the equation.

[Details of the embodiment of the present invention]
Specific examples of a voltage conversion device, a voltage conversion circuit step-down control method, and a computer program according to embodiments of the present invention will be described below with reference to the drawings. In addition, this invention is not limited to these illustrations, is shown by the claim, and intends that all the changes within the meaning and range equivalent to a claim are included. In addition, the technical features described in each embodiment can be combined with each other.

(Embodiment)
FIG. 1 is a block diagram illustrating a configuration example of the voltage conversion device according to the embodiment. The voltage conversion apparatus includes a voltage conversion circuit 1 and a control unit 5 that controls voltage conversion by the voltage conversion circuit 1. The voltage conversion circuit 1 steps down the voltage inputted from the outside via the high-voltage side terminals H and G and outputs it from the low-voltage side terminals L and G. A capacitor 41 is connected between the terminals H and G, and a relatively high-voltage battery 2 such as a lithium ion battery is connected to the outside. A capacitor 42 is connected between the terminals L and G, and a relatively low-voltage battery 3 such as a lead storage battery is connected to the outside. The voltage converter can perform a boosting operation of boosting a voltage input from the outside via terminals L and G and outputting the boosted voltage from terminals H and G in parallel.

  The voltage conversion device further includes a voltage detector V1 that detects a voltage input to the voltage conversion circuit 1 by dividing the voltage using two resistors, and a current detector A1 that detects a current input to the voltage conversion circuit 1. The voltage detector V2 that detects the voltage output from the voltage conversion circuit 1 by dividing it with two resistors, the current detector A2 that detects the current output from the voltage conversion circuit 1, and the voltage conversion circuit 1 A temperature sensor Th1 for detecting the ambient temperature. The detection results of the voltage detector V1, the current detector A1, the voltage detector V2, the current detector A2, and the temperature sensor Th1 are given to the control unit 5.

  The voltage detector V1 and the current detector A1 correspond to the first detector. The voltage detector V2 and the current detector A2 correspond to the second detector. The voltage detectors V1 and V2 are not limited to those that divide the voltage to be detected by a resistor. The current detectors A1 and A2 include, for example, a shunt resistor and a differential amplifier that amplifies a voltage generated when a current flows through the shunt resistor, but are not limited thereto.

  The voltage conversion circuit 1 includes a first inductor L1, and a high-voltage side switching element Q1 and a low-voltage side switching element Q2 (hereinafter, high-voltage side and low-voltage side) connected in series between the terminals H and G via the first inductor L1. A switching element is also simply referred to as a SW element), a second inductor L2 having one end connected to a connection point between the first inductor L1 and the SW element Q2 (hereinafter, the first and second inductors are simply referred to as inductors), and a SW element Q2. And a capacitor C1 connected to both ends thereof. The other end of the inductor L2 is connected to the terminal L. The SW element Q2 may be replaced with a diode whose anode is connected to the terminal G. The temperature sensor Th1 is preferably arranged as close as possible to the inductor L1, the inductor L2, and the capacitor C1.

  The SW element Q <b> 1 has a drain connected to the terminal H and a gate connected to the control unit 5. The SW element Q <b> 2 has a source connected to the terminal G and a gate connected to the control unit 5. The SW elements Q1 and Q2 here are N-channel MOSFETs (Metal Oxide Field Effect Transistors), but are not limited to this, for example, bipolar transistors, IGBTs (Insulated Gate Bipolar Transistors), etc. The switching element may be used.

  The voltage conversion circuit 1 further includes a SW element Q3 having a drain connected to a connection point between the SW element Q1 and the inductor L1. The SW element Q <b> 3 has a source connected to the terminal G and a gate connected to the control unit 5. In the present embodiment, the SW element Q3 may be replaced with a diode whose anode is connected to the terminal G.

  The control unit 5 is a central processing unit (CPU) 51 that controls the SW elements Q1 and Q2, and a nonvolatile memory such as a flash memory, an EPROM (Erasable Programmable Read Only Memory), and an EEPROM (Electrically EPROM). ROM 52 using RAM, RAM 53 using rewritable memory such as DRAM (Dynamic Random Access Memory), SRAM (Static Random Access Memory), etc., and timer 54 for measuring time. The CPU 51, ROM 52, RAM 53, and timer 54 are connected to each other by a bus. The RAM 53 includes a first storage unit 531 and a second storage unit 532 which will be described later.

  The control unit 5 further A / D converts the detection results of the voltage detector V1 and the current detector A1, and the drive circuit 55 that generates an ON signal for turning on the SW elements Q1, Q2, and Q3 and applies the signal to the gate. An A / D converter 56 and an A / D converter 57 for A / D converting the detection results of the voltage detector V2, the current detector A2, and the temperature sensor Th1 are provided, all of which are connected to the CPU 51 by a bus. Has been. The A / D converter 57 also A / D converts the voltage of the capacitor C1. The drive signal applied to the gate of the SW element Q1 is, for example, a PFM (Pulse Frequency Modulation) signal. However, any other signal may be used as long as the ratio of the on time to the total on / off time, that is, the duty ratio is defined. It may be a signal.

  The CPU 51 controls the operation of each unit according to a control program stored in advance in the ROM 52 and performs processes such as input / output and calculation. A computer program that defines the procedure of each process performed by the CPU 51 may be loaded in advance into the RAM 53 using a means (not shown), and the loaded computer program may be executed by the CPU 51. You may comprise a wear circuit.

  Specifically, the control unit 5 turns on / off the SW element Q1 at appropriate times, detects the voltage of the capacitor C1 (that is, the drain voltage of the SW element Q2), and based on the detection results, the SW element Q2 Control to turn on and off at appropriate times. The control unit 5 also takes in the detection results of the voltage detector V1 and the current detector A2, and calculates the length of a commutation period, the resonance period, and the ON time and OFF time of the SW element Q1 described later. The controller 5 further calculates the input power from the detection results of the voltage detector V1 and the current detector A1, calculates the output power from the detection results of the voltage detector V2 and the current detector A2, and determines these calculation results. Based on this, the conversion efficiency of the voltage conversion circuit 1 is calculated.

  FIG. 2 is a timing chart showing an example of the waveform of each part during the step-down operation of the voltage conversion circuit 1. The five timing charts shown in FIG. 2 all have the same time axis (t) as the horizontal axis, and the voltage of the SW element Q1, the inductor L1, the capacitor C1, the SW element Q2, and the inductor L2 in order from the top of the figure. A waveform and a current waveform are shown. The thick solid line in the figure indicates the voltage, and the thin solid line indicates the current. Particularly for the SW elements Q1 and Q2, the control voltage applied to the gate is indicated by a broken line. In the present embodiment, the direction from the terminal H to the terminal L is the current direction of the inductor L1, the inductor L2, and the SW element Q1. The direction from one end of the inductor L2 toward the terminal G is the current direction of the capacitor C1, and the opposite direction is the current direction of the SW element Q2.

  Hereinafter, the step-down operation of the voltage conversion circuit 1 in the states D1 to D4 illustrated in FIG. 2 will be described for each state. States D1 to D4 correspond to one switching cycle. In the explanatory diagrams from FIG. 3 to FIG. 6, illustration of the SW element Q3 is omitted. FIG. 3 is an explanatory diagram illustrating an example of the state D1 in the step-down operation of the voltage conversion circuit 1. State D1 corresponds to the first mode. The control unit 5 operates the voltage conversion circuit 1 in the first mode in which a sinusoidal resonance current flows through the inductor L1 and the capacitor C1 by turning on the SW element Q1 with the SW element Q2 turned off. In the first mode, a current flows from the terminal H via the SW element Q1 and the inductor L1, and a current flows from the terminal L via the inductor L2.

  In the state D1 shown in FIG. 3, while the voltage of the capacitor C1 rising in a sine wave is lower than the voltage between the terminals H and G, the current of the inductor L1 continues to increase in a sine wave, and the voltage of the capacitor C1 is increased to the terminals H and G. After becoming higher than the voltage between, the current of the inductor L1 starts to decrease. During this time, the voltage of the inductor L1 continues to decrease in a sine wave shape. During the state D1, the voltage of the capacitor C1 continues to rise with time.

  FIG. 4 is an explanatory diagram illustrating an example of the state D2 in the step-down operation of the voltage conversion circuit 1. State D2 also corresponds to the first mode. When the direction of the resonance current flowing in the capacitor C1 is reversed in the state D1 shown in FIG. 3 and the voltage of the capacitor C1 changes from rising to lowering, the voltage conversion circuit 1 becomes the state D2. In the state D2, the discharge current of the capacitor C1 flows to the terminal L side through the inductor L2. While the voltage of the capacitor C1 is higher than the voltage between the terminals H and G, the current of the inductor L1 continues to decrease.

  FIG. 5 is an explanatory diagram illustrating an example of the state D3 in the step-down operation of the voltage conversion circuit 1. State D3 corresponds to the second mode. In the state D2 shown in FIG. 4, when the sinusoidal current flowing through the SW element Q1 and the inductor L1 is minimized or the direction of the current is reversed and the current flows through the parasitic diode, the control unit 5 switches the SW element Q1. By turning off, the voltage conversion circuit 1 enters the state D3. As a result, the SW element Q1 is turned off when the current of the SW element Q1 is minimal or while the voltage of the SW element Q1 is close to 0V, so that the switching loss of the SW element Q1 is reduced. In the state D3, initially, the return current of the inductor L2 flows through the capacitor C1.

  When the control unit 5 turns off the SW element Q1 before the current of the inductor L1 becomes close to zero in the state D2, a surge voltage may be generated in the inductor L1. On the other hand, in the present embodiment, the SW element Q3 is connected between the connection point of the SW element Q1 and the inductor L1 and the terminal G. For this reason, the current flowing through the inductor L1 before the SW element Q1 is turned off continues to flow through the parasitic diode of the SW element Q3, and the surge voltage is suppressed. The control unit 5 may positively turn on the SW element Q3 from when the SW element Q1 is turned off to an appropriate time.

  FIG. 6 is an explanatory diagram illustrating an example of the state D4 in the voltage step-down operation of the voltage conversion circuit 1. In the state D3 shown in FIG. 5, after the voltage of the capacitor C1 and the SW element Q2 becomes lower than the predetermined voltage threshold, the control unit 5 turns on the SW element Q2, whereby the voltage conversion circuit 1 becomes the state D4. State D4 also corresponds to the second mode. As a result, the return current of the inductor L2 flows through the SW element Q2 having a low on-resistance. Further, since the SW element Q2 is turned on while the voltage of the SW element Q2 is relatively low, the switching loss of the SW element Q2 is reduced.

  In the state D4, when an appropriate time corresponding to the step-down ratio of the output voltage to the terminals L and G with respect to the input voltage from the terminals H and G has elapsed, the control unit 5 turns on the SW element Q1 after turning off the SW element Q2. As a result, the voltage conversion circuit 1 enters the state D1 shown in FIG. In this way, the state transition from state D1 to state D4 is repeated.

  Next, the above-described states D1 and D2 and the operation of the voltage conversion circuit 1 before and after that will be described in more detail. FIG. 7 is a timing chart schematically showing the waveforms of the respective parts from when the voltage conversion circuit 1 exits from the state D4 to when it switches to the state D3 through the states D1 and D2. The five timing charts shown in FIG. 7 all have the same time axis (t) as the horizontal axis, and in order from the top in the figure, the ON period of the SW element Q1, the effective ON period of the SW element Q1, The current of the element Q1, the current of the SW element Q2 (including the current of the parasitic diode), and the current of the inductor L2 are shown. The scale of the vertical axis is not necessarily the same. In particular, regarding the current of the SW element Q1, the case where the output current Io is relatively large and the case where it is small are indicated by a solid line and a one-dot chain line, respectively. Io is an average value of the output current of the voltage conversion circuit 1. ΔI is the amplitude of the ripple current included in the output current. The time from time t1 to t3 is the ON time Tmi of the SW element Q1.

  A period from time t1 to t3 shown on the horizontal axis in FIG. 7 corresponds to states D1 and D2 of the voltage conversion circuit 1, and a period after time t3 corresponds to states D3 and D4. The period from the state D1 through D2, D3, D4 to the next switching cycle state D1 corresponds to the current switching cycle. A period from time t0 to time t1 before time t1 represents a transitional state from state D4 to D1 in the previous switching cycle. During this time, the control unit 5 may have already turned off the SW element Q2, or may remain on until just before time t1. When the SW element Q2 is turned off, the current of the inductor L2, that is, the return current flowing from the source to the drain of the SW element Q2 in FIG. 6 described above, flows to the parasitic diode of the SW element Q2. The current in the inductor L2 gradually decreases from time t0 to time t1.

  The absolute value of the voltage of the SW element Q2 in the transient state is the on-voltage of the SW element Q2 or the on-voltage of the parasitic diode. That is, since the absolute value of the voltage of the SW element Q2 from time t0 to t1 can be regarded as substantially 0V, the voltage conversion circuit 1 transitions to the state D1 when the control unit 5 turns on the SW element Q1 at time t1. In this case, a current having a slope of dI / dt = Vi / Lr1 flows through the SW element Q1 and the inductor L1. However, Vi is an input voltage between the terminals H and G, and Lr1 is an inductance of the inductor L1.

  The return current of the inductor L2 flowing in the SW element Q2 is gradually replaced with the current having the above-described gradient that has started flowing in the SW element Q1 and the inductor L1 at time t1, so that the current of the SW element Q2 is changed from time t1 to time t2. In the meantime, it decreases linearly and becomes zero at time t2. As a result, during the period from time t1 to t2, the current of the inductor L2 gradually decreases with the same slope as the current flowing from time t0 to t1. Between time t1 and t2, since the return current of the inductor L2 corresponds to a commutation period in which the return current of the SW element Q2 is commutated from the SW element Q2, and the current of the inductor L2 continues to decrease, the time Ti during this time is It does not contribute to the duty ratio that determines the step-down ratio of the voltage conversion circuit 1. Therefore, the effective on-time of the SW element Q1 is a time Ton from time t2 to t3.

  After the commutation period, a sinusoidal resonance current flows through the inductor L1 and the capacitor C1 through the SW element Q1 from time t2 to time t3. The current of the inductor L1, that is, the current of the SW element Q1, becomes minimum at time t3 when indicated by a solid line, and becomes zero at time ta before time t3 when indicated by a one-dot chain line. In either case, the SW element Q1 is turned off at time t3. While the SW element Q1 is on, energy is injected into the inductor L2, and the current of the inductor L2 increases by ΔI. Thereafter, the current of the inductor L2 decreases by ΔI until the state D1 of the next switching period is reached.

  In FIG. 7, the current of the SW element Q1 is shown by a broken line when it is assumed that the SW element Q1 is kept on after time t3. In this case, if the time when the current of the SW element Q1 becomes the same as that at the time t2 is t4, the time from the time t2 to the time t4 corresponds to the resonance period Tr, and the time from the time t2 to the time t3, that is, the SW element The effective on-time Ton of Q1 corresponds to 3/4 of the resonance period Tr. The resonance period Tr is expressed by the following formula (1).

Tr = 2π√ (Lr1Cr1) (1)
However,
Lr1: Inductor Cr1 inductance Cr1: Capacitor C1 capacitance

  The current of SW element Q1 indicated by the alternate long and short dash line is less than or equal to zero from time ta to tb, and the loss of SW element Q1 is substantially zero at any point during this time, regardless of the SW element Q1 being turned off. It becomes. Therefore, in the present embodiment, the SW element Q1 is turned off at time t3 when the resonance current flowing through the SW element Q1 is minimized regardless of the magnitude of the output current Io.

  As described above, a current having a slope of dI / dt = Vi / Lr1 flows through the SW element Q1 from time t1 to time t2. The current It2 of the SW element Q1 at time t2 (“(It2)”, which is less than “It2” when indicated by the alternate long and short dash line, is expressed as “It2” in any case below) of the inductor L2 Equal to the current. Since the current of the inductor L2 at this time is Io−ΔI / 2, the following equation (2) is established.

It2 = Io−ΔI / 2
= (Vi / Lr1) Ti (2)
However,
Io: average value of output current ΔI: ripple current of output current of voltage conversion circuit 1 (peak to peak)
Vi: input voltage Lr1 of voltage conversion circuit 1: inductance of inductor L1 Ti: time from time t1 to t2 (length of commutation period)

  On the other hand, when the inductance of the inductor L1 is negligible with respect to the inductance of the inductor L2, the input / output voltage of the voltage conversion circuit 1 is expressed by the following equations (3) and (4) if the influence of the resonance current is ignored. ).

Vi−Vo = Lr2ΔI / Ton (3)
Vo / Vi = D (4)
However,
Vi: input voltage of voltage conversion circuit 1 Vo: output voltage Lr2 of voltage conversion circuit 1: inductance of inductor L2 ΔI: ripple current Ton of output current of voltage conversion circuit 1: effective on-time of SW element Q1 D: SW Effective duty ratio of switching element Q1

  Here, Ton is obtained by multiplying the switching period, which is the reciprocal of the switching frequency fs of the SW element Q1, by D, and Ton corresponds to (3/4) Tr as described above. Therefore, fs is expressed by the following formula (5 ). Furthermore, Ton of Formula (3) is represented by the following Formula (6) using fs.

fs = D / {(3/4) Tr} (5)
Ton = D (1 / fs) ... (6)
However,
fs: switching frequency of the SW element Q1 D: effective duty ratio of switching of the SW element Q1 Tr: resonance period Ton: effective on-time of the SW element Q1

  By applying Equations (4) and (6) to Equation (3) to eliminate Vo and Ton, the following Equation (7) is obtained.

ΔI = (Vi / Lr2) (1-D) D / fs (7)

  Next, by substituting equation (7) into equation (2), the following equation (8) is obtained.

Ti = (Lr1 / Vi) {Io-Vi (1-D) D / (2Lr2fs)} (8)

  On the other hand, the time from when the SW element Q1 is turned on until the current flowing through the SW element Q1 becomes the minimum, that is, the ON time Tmi of the SW element Q1 is the time obtained by adding Ton to Ti, and as described above, Ton is Tr Therefore, Tmi is expressed by the following equation (9) using the right side of equation (8) and (3/4) Tr. Thus, if Ti is calculated, it is possible to calculate the on-time Tmi, which is the time during which the SW element Q1 should be turned on, very easily.

Tmi = (Lr1 / Vi) {Io-Vi (1-D) D / (2Lr2fs)}
+ (3/4) Tr (9)

  Since Vi is detected by the voltage detector V1, Io is detected by the current detector A2, and Lr2 and Tr are constants, the effective duty ratio D, that is, the target duty ratio is substituted into the equation (9). The on-time Tmi is calculated by substituting fs calculated by Expression (5) into Expression (9). And the OFF time Toff of SW element Q1 can be determined by the following formula | equation (10).

Toff = 1 / fs-Ton
= 1 / fs- (3/4) Tr (10)
However,
fs: switching frequency of SW element Q1 Ton: effective on-time of SW element Q1 Tr: resonance period

  As described above, the calculated on-time Tmi of the SW element Q1 is calculated by the equation (9), but the circuit constants of the circuit elements such as the inductor L1 and the capacitor C1 fluctuate according to their own temperature change and the like. The on-time Tac for actually turning on the SW element Q1 is deviated from Tmi. FIG. 8 is a graph showing an example of the current waveform of the SW element Q1 that changes in accordance with the fluctuation of the resonance period. The horizontal axis of the figure represents time (t), and the unit is μs. The vertical axis represents the current of the SW element Q1, and the unit is A. A curve indicated by a solid line in the figure shows a current waveform when the resonance period is an ideal period corresponding to the equation (9). The curves indicated by the broken line and the alternate long and short dash line indicate current waveforms when the resonance period varies by −20% and + 20% with respect to the ideal period.

  Even when the resonance period is shorter than the ideal period, even if the resonance period is shorter than the ideal period, even if it is turned on for the calculated on-time Tmi from when the SW element Q1 is turned on to when the resonance current is estimated to be minimal. The result is that the time Tmi is too long. If the resonance period is longer than the ideal period, the on-time Tmi is too short. As described with reference to FIG. 7, when the resonance period is an ideal period and the SW element Q1 is turned on for the calculated on-time Tmi, the voltage of the SW element Q1 is minimal or close to 0V. Since the SW element Q1 is turned off, the switching loss of the SW element Q1 is minimized. On the other hand, when the SW element Q1 is turned on for the calculated on-time Tmi even though the resonance period fluctuates with respect to the ideal period, the switching loss of the SW element Q1 is larger than the above minimum value. The probability of becoming high. In other words, generally, an on-time value that minimizes the switching loss of the SW element Q1 exists as a value different from the calculated on-time Tmi.

  Therefore, in the present embodiment, the ON time for turning on the SW element Q1 (hereinafter simply referred to as ON time) corresponds to the calculated ON time Tmi (first time corresponding to the length of the resonance period: The conversion efficiency of the voltage conversion circuit 1 is compared and determined in a case where the time is an integral multiple of a predetermined minute time (hereinafter also referred to as a second time) than Tmi. In this case, it is preferable to compare and determine the conversion efficiency each time the absolute value is increased in time series by setting the integer value to be positive or negative. For example, the minute time may be equal to the minimum unit of the ON signal generated by the drive circuit 55. If the determination result indicates that the conversion efficiency is reduced, the integer value may be inverted. Then, the on-time Tac that indicates that the conversion efficiency becomes larger and preferably maximizes according to the determination result is determined as the on-time Tac that should actually turn on the SW element Q1.

  By the way, as shown by the one-dot chain line and the broken line in FIG. 8, when the resonance period fluctuates to the same extent in both positive and negative directions with respect to the ideal period, the SW element Q1 of the SW element Q1 is based on the calculated on-time Tmi. When the on-time is changed, the change in current is larger when the resonance period fluctuates in the negative direction. That is, the change in the conversion efficiency of the voltage conversion circuit 1 is larger because the change in the switching loss of the SW element Q1 is larger in the case of the broken line having a shorter resonance period than the change in the ON time of the SW element Q1. This makes it easy to determine the direction in which the on-time should be changed. Therefore, it is preferable that the integer value has a positive initial value. When the on-time of SW element Q1 is longer or shorter than the calculated on-time Tmi, it is preferable to change the on-time within the range of the previously calculated minimum and maximum values of on-time.

  Hereinafter, the minimum value and the maximum value of the on-time calculated by Expression (9) will be described. Ti and Tr included in the right side of Expression (9) for calculating the on-time Tmi are expressed by Expression (8) and Expression (1), respectively. The values of Lr1, Lr2, Cr1, Vi, and Io included in the right side of Expression (1) and Expression (8) are the following Expression (11), Expression (12), Expression (13), Expression (14), and Expression, respectively. It is assumed that the value is within the range represented by (15). In this case, the minimum value TiMin and maximum value TiMax of Ti, and the minimum value TrMin and maximum value TrMax of Tr are expressed by the following expressions (16) and (17), and expressions (18) and (19). expressed.

Lr1Min ≦ Lr1 ≦ Lr1Max (11)
Lr2Min ≦ Lr2 ≦ Lr2Max (12)
CrMin ≦ Cr ≦ CrMax (13)
ViMin ≦ Vi ≦ ViMax (14)
IoMin ≦ Io ≦ IoMax (15)
TiMin = (Lr1Min / ViMax)
X {IoMin-ViMax (1-D) D / (2Lr2Minfs)} (16)
TiMax = (Lr1Max / ViMin)
X {IoMax-ViMin (1-D) D / (2Lr2Maxfs)} (17)
TrMin = 2π√ (Lr1MinCr1Max) (18)
TrMax = 2π√ (Lr1MaxCr1Min) (19)

  Since the right side of the equation (9) is Ti + (3/4) Tr, the upper limit value or the lower limit value expressed by the equations from the equation (11) to the equation (15) is used. When Tmi is calculated, the value of Tmi is a value within the range represented by the following equation (20). Expression (20) is expressed as Expression (21) using the minimum value Min and the maximum value Max of the on-time.

TiMin + (3/4) TrMin ≦ Tmi ≦ TiMax + (3/4) TrMax (20)
Min ≦ Tmi ≦ Max (21)

  When changing the ON time of the SW element Q1, if the minimum value of the conversion efficiency cannot be found even if the ON time is changed to the minimum value Min or the maximum value Max, the resonance period varies due to an unexpected factor. Since the possibility is great, it is preferable to carry out a separate abnormality process.

  Next, the relationship between the conversion efficiency of the voltage conversion circuit 1 and the output current when the resonance period is an ideal period will be described. FIG. 9 is a chart illustrating characteristics of conversion efficiency with respect to output current for the voltage conversion circuit 1. Actually, a table corresponding to the table of FIG. 9 is stored in the first storage unit 531 in the RAM 53. This table may be a two-dimensional table showing characteristics of conversion efficiency with respect to output current and ambient temperature. FIG. 9 shows that the ideal conversion efficiency is 80%, 85%, 90%, 95%, 90%, 90%, 85% when the output current is changed from 10 A to 80 A in increments of 10 A. It is.

  Since the voltage conversion circuit 1 performs PFM control, the on-time of the SW element Q1 is substantially constant, and the current flowing through the SW element Q1 varies depending on the magnitude of the output current. For this reason, as the output current increases, the loss due to the ON resistance of the SW element Q1 and the switching loss increase, and the conversion efficiency tends to decrease. On the other hand, as the output current becomes smaller, the loss in the circuit portion other than the SW element Q1 cannot be ignored, and the conversion efficiency tends to decrease. Therefore, the conversion efficiency becomes relatively large in a state where the output current flows appropriately.

  Since FIG. 9 shows the conversion efficiency when the resonance period is an ideal period, the conversion efficiency calculated according to the change in the ON time of the SW element Q1 is read from FIG. 9 according to the output current. If the conversion efficiency substantially coincides with the conversion efficiency, it is considered that the conversion efficiency corresponding to the output current reaches a maximum. In the example shown in FIG. 9, for example, if the difference between the calculated conversion efficiency and the conversion efficiency that can be read from the table of FIG.

  Next, the relationship between the output current and ambient temperature and the variation in the ON time of the SW element Q1 will be described. FIG. 10 is a chart showing fluctuation characteristics of the ON time of the SW element Q1 with respect to the output current and the ambient temperature. Actually, a table corresponding to the table of FIG. 10 is stored in the second storage unit 532 in the RAM 53. In FIG. 10, when the output current is changed from 10 A to 80 A in increments of 10 A and the ambient temperature is changed from 0 ° C. to 60 ° C. in increments of 20 ° C., the on-time Tac to actually turn on the SW element Q 1 is Values that vary with respect to the calculated on-time Tmi are shown in ns. Black circles in the figure indicate that some numerical value is stored. Of these, actual numerical values (+12, +2, -5) are described for representative portions. Φ indicates that no numerical value is stored.

  As described above, it is basically necessary to calculate the on-time Tac for the SW element Q1, but when the difference between the on-time Tac and the on-time Tmi is already stored in the table shown in FIG. The on-time Tac can be calculated by adding the difference read by the proportional distribution to the on-time Tmi. On the other hand, when no difference is stored in the table shown in FIG. 10, the on-time Tac is calculated as usual, and the difference with respect to the on-time Tmi is written in the table of FIG. 10 in association with the output current and the ambient temperature at that time. That's fine. In this way, the stored contents of the table shown in FIG. 10 can be enriched. Note that the contents of the table of FIG. 10 may be temporarily deleted, and calibration may be performed in which the difference between the on time Tac and the on time Tmi is stored for each output current and each ambient temperature.

  Below, operation | movement of the control part 5 mentioned above is demonstrated using the flowchart which shows it. The processing shown below is executed by the CPU 51 in accordance with a control program stored in advance in the ROM 52. 11 and 12 are flowcharts of the main routine showing the processing procedure of the CPU 51 for calculating the on-time Tac for which the SW element Q1 should be actually turned on. FIG. 13 is a flowchart showing the processing procedure of the CPU 51 related to the one-cycle conversion subroutine, and FIG. 14 is a flowchart showing the processing procedure of the CPU 51 related to the efficiency calculation subroutine. In the subroutine shown in FIG. 13, the SW element Q1 is turned on / off for one switching cycle. In the subroutine shown in FIG. 14, the conversion efficiency of the voltage conversion circuit 1 is calculated.

  The main routine shown in FIG. 11 is started in a timely manner when the voltage converter performs step-down conversion. However, in order to calculate the conversion efficiency as accurately as possible, it is preferable that the conversion efficiency is started when the fluctuation of the output current is relatively small. The timing in the figure is performed using the timer 54. The storage flag in the figure is stored in the RAM 53 with an initial value of 0. In the figure, n is a counter and is stored in the RAM 53 with an initial value of 0. ΔT and N in the figure are positive fixed values. Note that the target duty ratio is calculated in a timely manner in other processes.

  When the main routine of FIG. 11 is started, the CPU 51 converts the input voltage detection result by the voltage detector V1 by the A / D converter 56 (S11: hereinafter, by the A / D converters 56 and 57). Listed without conversion.) The CPU 51 further captures the detection result of the output current by the current detector A2 and stores it in the RAM 53 for later reference (S12), and calculates the on-time Tmi, that is, the first time by the equation (9) ( This corresponds to the estimation unit) and is stored in the RAM 53 for later reference (S13). In the calculation of the first time, the input voltage Vi detected by the voltage detector V1, the output current Io detected by the current detector A2, the target duty ratio (D), and the switching frequency fs calculated by the equation (5). And the resonance period Tr calculated by the equation (1) and other known constants are used.

  Next, the CPU 51 takes in the detection result of the ambient temperature by the temperature sensor Th1 and stores it in the RAM 53 for later reference (S14), and then the difference corresponding to the output current and the ambient temperature at this time is the second storage unit 532. (S15: equivalent to a storage determination unit). When the difference is stored in the second storage unit 532 (S15: YES), the CPU 51 determines the on-time Tac by adding the difference read from the second storage unit 532 to the calculated first time (S16: 11 corresponds to the second on-time determining unit), and the processing in FIG.

  On the other hand, when the difference is not stored in the second storage unit 532 (S15: NO), the CPU 51 stores the difference according to the ON time Tac calculated later to indicate that the difference should be stored in the second storage unit 532. The flag is set to 1 (S17). Next, the CPU 51 sets the calculated first time as the on-time Tac at this time (S18), and uses the first time stored in the RAM 53 as an argument (S19) to call and execute a subroutine related to one cycle conversion (S20). . The CPU 51 further calls and executes a subroutine relating to efficiency calculation (S21), and stores the calculated first conversion efficiency (hereinafter simply referred to as efficiency) in the RAM 53 for later reference (S22).

  Moving on to FIG. 12, the CPU 51 sets a new argument by adding a predetermined minute time ΔT to the argument at the time of the previous subroutine call (S31: corresponding to a specific unit), and the argument is turned on with the minimum value Min of the on time. It is determined whether or not the time is within the range of the maximum value Max (corresponding to a predetermined range), that is, whether or not the second time is Min and Max (S32). Repetitively executing the process of step S31 includes adding a time obtained by multiplying ΔT by an integer to the first time that is the initial value of the argument to specify the second time, and changing the integer value to 1, 2, 3,・ ・ Equivalent to changing the time series. When the argument is not within the above range (S32: NO), the CPU 51 terminates the calculation of the on-time Tac and moves the process to step S43 to be described later. In step S32, it may be determined whether the argument is larger than Min and smaller than Max.

  On the other hand, if the argument is within the above range (S32: YES), the CPU 51 calls and executes a subroutine related to one-cycle conversion based on the calculated argument (S33). Next, the CPU 51 calls and executes a subroutine related to efficiency calculation (S34), and determines whether or not the second conversion efficiency calculated in the subroutine (hereinafter simply referred to as efficiency) is higher than the efficiency stored in the RAM 53. (S35: equivalent to an efficiency comparison unit). When the calculated efficiency is higher than the stored efficiency (S35: YES), the CPU 51 stores the calculated efficiency in the RAM 53 (S36) and updates it, and sets the argument at this time as the on time Tac (S37: on time) The counter n is incremented by 1 (corresponding to the determination unit) (S38).

  Thereafter, the CPU 51 captures the detection result of the output current by the current detector A2 (S39) or reads out and acquires the output current stored in the RAM 53, and stores it in the first storage unit 531 corresponding to the acquired output current. The efficiency thus read is read (S40). Next, the CPU 51 calculates an efficiency difference by subtracting the efficiency calculated in the subroutine from the read efficiency (S41), and determines whether the calculated efficiency difference is equal to or greater than a predetermined value (for example, 2.5%). (S42). If the efficiency difference is greater than or equal to the predetermined value (S42: YES), the CPU 51 moves the process to step S31 in order to further add ΔT to the on-time as an argument.

  On the other hand, when the efficiency difference is not equal to or greater than the predetermined value (S42: NO), that is, when the calculated efficiency substantially matches the ideal efficiency, the CPU 51 determines whether or not the storage flag is set to 1 ( S43). When the storage flag is not set to 1 (S43: NO), the CPU 51 determines the on-time at that time as the on-time Tac and ends the processing of FIG.

  On the other hand, when the storage flag is set to 1 (S43: YES), that is, when the corresponding difference is not stored in the second storage unit 532, the CPU 51 calculates the first calculated from the determined on-time Tac. The difference is calculated by subtracting the time (S44). Next, the CPU 51 stores the calculated difference in the second storage unit 532 in association with the output current and the ambient temperature at that time (S45: equivalent to the difference storage unit), and ends the process of FIG. The output current obtained in step S39 is used as the output current in this case. The ambient temperature stored in step S14 is used as the ambient temperature.

  When the calculated efficiency is not higher than the stored efficiency in step S35 described above (S35: NO), the CPU 51 determines whether or not the minute time ΔT is positive (S46), and is negative instead of positive. (S46: NO), assuming that the sign of ΔT has already been inverted, the process proceeds to step S43. On the other hand, when ΔT is positive (S46: YES), the CPU 51 determines whether or not the counter n is N (for example, 3) or more (S47). This is to determine whether or not the process of specifying the second time by adding ΔT to the first time in time series is executed N times or more.

  If the counter n is greater than or equal to N (S47: YES), the CPU 51 determines that the maximum point of efficiency has been detected and moves the process to step S43. On the other hand, if the counter n is not greater than or equal to N (S47: NO), the CPU 51 reverses the sign of ΔT (S48), assuming that the efficiency is lowered by increasing the ON time (S48), and uses the first time as an argument ( S49) The process moves to step S31. As a result, it is possible to detect the maximum value of the efficiency by decreasing the on-time in a time series. After the sign of ΔT is inverted in step S48, the process of step S31 is repeatedly executed by subtracting the time obtained by multiplying the absolute value of ΔT by an integer from the first time to specify the second time by an integer. This is equivalent to changing the value of 1 in a time series as 1, 2, 3,.

  Turning to FIG. 13, the names of the SW elements Q1 and Q2 are described without being omitted here. When a subroutine related to conversion for one cycle is called, the CPU 51 turns off the low voltage side switching element Q2 (S51) and then turns on the high voltage side switching element Q1 (S52). As a result, the voltage conversion circuit 1 operates in the first mode, and resonance occurs between the inductor L1 and the capacitor C1.

  Next, the CPU 51 starts measuring the on-time by the timer 54 (S53), and determines whether or not the time given as an argument has elapsed (S54). When the time does not elapse (S54: NO), the CPU 51 stands by until the time elapses.

  When the argument time has elapsed (S54: YES), the CPU 51 turns off the high-voltage side switching element Q1 (S55), and operates the voltage conversion circuit 1 in the second mode. This causes the return current of the inductor L2 to flow through the capacitor C1 or the low-voltage side switching element Q2.

  Next, the CPU 51 calculates the off time Toff by substituting the switching frequency fs calculated by the equation (5) into the equation (10) (S56). Then, the CPU 51 starts counting the off time of the high-voltage side switching element Q1 by counting down the calculated off time Toff (S57). When calculating the off-time Toff using the equations (5) and (10), the effective on-time Ton (that is, (3/4) Tr) is known in addition to the target duty ratio D. is necessary. Here, considering that Ti described above is sufficiently smaller than Ton, the on-time of the high-voltage side switching element Q1 given as an argument is substituted into Ton and is substituted into equations (5) and (10). Even if there is some error in the OFF time Toff of the calculation result, the output voltage finally converges to the target voltage by the feedback control as the entire voltage converter.

  Thereafter, the CPU 51 determines whether or not the off time has elapsed (S58). If the off time has elapsed (S58: YES), the CPU 51 returns to the called main routine. On the other hand, when the off time has not elapsed (S58: NO), the CPU 51 detects the voltage of the low-voltage side switching element Q2, and determines whether or not the detected voltage is lower than a predetermined voltage threshold (S59).

  When the detected voltage is not lower than the predetermined voltage threshold (S59: NO), the CPU 51 shifts the processing to step S58. On the other hand, when the detected voltage is lower than the predetermined voltage threshold value (S59: YES), the CPU 51 turns on the low voltage side switching element Q2 (S60), and moves the process to step S58. The voltage threshold value here may be a positive value, or may be zero or a negative value less than or equal to zero. If the low voltage side switching element Q2 is a simple diode, steps S59 and S60 may be skipped.

  Turning to FIG. 14, when a subroutine related to efficiency calculation is called, the CPU 51 captures the detection result of the input voltage by the voltage detector V1 (S71), and further captures the detection result of the input current by the current detector A1. (S72) The input power is calculated (S73). Next, the CPU 51 fetches the detection result of the output voltage by the voltage detector V2 (S74), and further fetches the detection result of the output current by the current detector A2 (S75) to calculate the output power (S76). Thereafter, the CPU 51 calculates the conversion efficiency by dividing the calculated output power by the input power (S77: equivalent to the efficiency calculation unit), and returns to the called routine.

  In the flowchart of the main routine described above, the minute time ΔT is added to the on time for each switching period, but the average efficiency is calculated for the same on time over a plurality of periods, and the calculated efficiency is stored. The efficiency may be compared and determined in step S35. The plurality of cycles in this case is preferably determined in consideration of the delay time of the feedback system due to the capacitor C42, for example.

  Further, the ON time of the SW element Q1 may be limited to a time different from the calculated on time Tmi once in a plurality of cycles. Further, the minute time ΔT to be added to the on-time is not set to a constant value, for example, the initial value of the absolute value of ΔT is relatively large, and the absolute value of ΔT is decreased as time passes. Also good.

  Further, in the flowchart of the main routine described above, ΔT is first added to the first time and then ΔT is subtracted from the first time. However, the order of this addition and subtraction may be reversed.

  Further, in the flowchart of the subroutine relating to the conversion for one cycle described above, the off time is calculated in step S56. However, without calculating the off time, the argument time (on time) is divided by the target duty to calculate the cycle. It may be calculated. In this case, it is only necessary to start counting the calculated cycle either before or after step S53, skip step S57, and determine whether or not the cycle has elapsed in step S58.

  As described above, according to the present embodiment, in the voltage conversion circuit 1, the SW element Q1, the inductor L1, and the SW element Q2 having the capacitor C1 connected at both ends are connected in this order, and the inductor L1. One end of the inductor L2 is connected to the connection point of the SW element Q2. When the control unit 5 turns on / off the SW element Q1, the voltage input to the voltage conversion circuit 1 via the terminals H and G is stepped down and output from the terminals L and G.

  The control unit 5 calculates the first conversion efficiency of the voltage conversion circuit 1 when the SW element Q1 is turned on only for a first time corresponding to the length of the resonance period of the resonance current by the inductor L1 and the capacitor C1. When the second time different from the first time is specified and the SW element Q1 is turned on only for the second time, the second conversion efficiency of the voltage conversion circuit 1 is calculated, and the first and second conversion efficiencies are calculated. As a result of comparison, the time when the SW element Q1 is turned on when the conversion efficiency is higher is determined as the on-time Tac when the SW element Q1 should actually be turned on. As a result, the actual conversion efficiency of the voltage conversion circuit 1 is higher than when the SW element Q1 is turned on for the calculated on-time Tmi. Therefore, when so-called soft switching is performed using LC resonance, it is possible to prevent deterioration in conversion efficiency even when the resonance period varies.

  Further, according to the present embodiment, the second time longer than the first time is specified first, and the second conversion efficiency of the voltage conversion circuit 1 is calculated. As a result of comparing the second conversion efficiency calculated by making the second time longer than the first time and the above-described first conversion efficiency, if the second conversion efficiency is lower, A second time shorter than the first time is specified. This increases the probability that the second conversion efficiency calculated next is higher than the first conversion efficiency.

  Furthermore, according to the present embodiment, the second time different from the first time is changed in time series, and the second conversion efficiency of the voltage conversion circuit 1 is calculated each time. As a result of comparing the second conversion efficiency calculated according to the second time different in time series as described above and the first conversion efficiency, the SW element Q1 is turned on when the conversion efficiency is the highest. The time is determined as the on-time Tac in which the SW element Q1 should be actually turned on. Thereby, it is possible to search for the on-time when the actual conversion efficiency of the voltage conversion circuit 1 is the highest.

  Furthermore, according to the present embodiment, the ideal conversion efficiency calculated in advance for the voltage conversion circuit 1 is stored in the first storage unit 531 in association with the output current as the calculation condition. When performing the voltage conversion, the second conversion efficiency is calculated when the second time is sequentially specified and the SW element Q1 is turned on for the specified second time. If the calculated second conversion efficiency is lower than the conversion efficiency read from the first storage unit 531 based on the detected output current by a predetermined value or more, the second time is further changed. Thereby, it becomes possible to continue searching for a suitable on-time until the difference between the measured conversion efficiency of the voltage conversion circuit 1 and the ideal conversion efficiency becomes smaller than a predetermined value. If the calculated second conversion efficiency is not lower than the conversion efficiency read from the first storage unit 531 by a predetermined value or more, a suitable on-time Tac is determined before detecting the maximum value of the conversion efficiency. Is possible.

  Further, according to the present embodiment, the second time is specified with the first time as the center in order to specify the second time by adding or subtracting the time obtained by multiplying the predetermined minute time ΔT by an integer to the first time. Fine adjustment is possible.

  Furthermore, according to the present embodiment, the second time specified differently from the first time is set to fall within a preset range between the minimum value Min of the on-time and the maximum value Max of the on-time. This makes it possible to prevent the on-time Tac from being inappropriately determined when the resonance period varies due to a factor different from the assumed variation factor.

  Furthermore, according to the present embodiment, when the conversion of the conversion efficiency corresponding to the detected output current and the ambient temperature is stored in the second storage unit 532 when performing the voltage conversion, the difference is read from the second storage unit 532. The on-time Tac is determined by adding the difference to the first time. On the other hand, if the difference in conversion efficiency is not stored in the second storage unit 532, the second time and the conversion efficiency are sequentially specified and calculated, and when the on time Tac is determined, the second time and the conversion efficiency are determined from the determined on time Tac. The difference obtained by subtracting 1 hour is stored in the second storage unit 532 in association with the detected output current and ambient temperature. Thereby, when the ON time Tac determined in the past according to the output current and the ambient temperature is stored in the second storage unit 532, it is possible to eliminate the calculation of the second time and the conversion efficiency. Furthermore, the contents stored in the second storage unit 532 can be enriched over time.

  Furthermore, according to the present embodiment, the time from when the SW element Q1 is turned on to the time when it is estimated that the resonance current will be minimized is set as the first time, and therefore the resonance period is an ideal period. It is possible to search for the on-time Tac at which the conversion efficiency is higher, centering on.

  Furthermore, according to the present embodiment, it is possible to estimate the time Tmi from when the SW element Q1 is turned on until the resonance current flowing through the SW element Q1 is minimized based on the equation (9).

DESCRIPTION OF SYMBOLS 1 Voltage conversion circuit 41, 42 Capacitor 5 Control part 51 CPU
52 ROM
53 RAM
531 1st memory | storage part 532 2nd memory | storage part 54 Timer 55 Drive circuit 56, 57 A / D converter Q1 High voltage | pressure side switching element (SW element)
Q2 Low voltage side switching element (SW element)
Q3 SW element L1 First inductor (inductor)
L2 Second inductor (inductor)
C1 Capacitor V1, V2 Voltage detector A1, A2 Current detector Th1 Temperature sensor H, L, G terminals

Claims (11)

A first inductor, a high-voltage side switching element and a low-voltage side switching element connected in series via the first inductor, and a second inductor having one end connected to a connection point of the first inductor and the low-voltage side switching element And a voltage conversion circuit having a capacitor connected in parallel to the low-voltage side switching element, and a voltage conversion device comprising a control unit for turning on / off the high-voltage side switching element and performing a step-down conversion to the voltage conversion circuit. And
A first detector for detecting a voltage and a current input to the voltage conversion circuit;
A second detector for detecting the voltage and current output from the voltage conversion circuit;
The controller is
An efficiency calculation unit that calculates the conversion efficiency of the voltage conversion circuit based on the detection results of the first and second detectors;
A specifying unit that specifies a second time different from the first time according to a length of a resonance period of a resonance current by the first inductor and the capacitor;
An efficiency comparing unit that compares the conversion efficiencies calculated by the efficiency calculating unit when the high-voltage side switching element is turned on only for the first time and the second time specified by the specifying unit;
An on-time determination unit that determines an on-time for turning on the high-voltage side switching element based on a comparison result of the efficiency comparison unit.   When the control unit specifies a second time longer than the first time in the specifying unit, the result of the comparison in the efficiency comparison unit is lower in conversion efficiency when turned on only for the second time, The voltage conversion device according to claim 1, wherein the specifying unit specifies a second time shorter than the first time.   The control unit calculates conversion efficiency by the efficiency calculation unit and compares the conversion efficiency by the efficiency comparison unit every time the specifying unit specifies the second time to change in time series. 2. The voltage converter according to 2. The controller is
A first storage unit that stores conversion efficiency calculated in advance for the voltage conversion circuit in association with an output current;
When the high-voltage side switching element is turned on for the second time specified by the specifying unit rather than the conversion efficiency read from the first storage unit based on the current detected by the second detector, the efficiency calculating unit And a determination unit for determining whether the calculated conversion efficiency is lower than a predetermined value,
The voltage conversion device according to claim 3, wherein when the determination unit determines that the second time is low, the specifying unit specifies the second time to be further changed.   5. The voltage conversion device according to claim 1, wherein the specifying unit specifies the second time by adding or subtracting a time obtained by multiplying a predetermined minute time by an integer with respect to the first time. 6. .   The voltage conversion device according to claim 1, wherein the specifying unit specifies the second time within a predetermined range set in advance. A temperature sensor for detecting an ambient temperature of the voltage conversion circuit;
The controller is
A second storage unit for storing a difference obtained by subtracting the first time from the on-time in association with an ambient temperature of the voltage conversion circuit and an output current;
A storage determination unit that determines whether or not a difference is stored in a storage area of the second storage unit corresponding to the current detected by the second detector and the ambient temperature detected by the temperature sensor;
A second on-time determination unit that determines a time obtained by adding the difference read from the storage area to the first time as an on-time when the storage determination unit determines that the difference is stored;
A difference storage unit that stores, in the storage area, a difference obtained by subtracting the first time from the on-time determined by the on-time determination unit when the storage determination unit determines that the difference is not stored. The voltage converter according to any one of claims 1 to 6.   8. The control unit according to claim 1, further comprising: an estimation unit configured to estimate a time from when the high-voltage side switching element is turned on to when the resonance current becomes a minimum, to be the first time. The voltage converter described in 1. The voltage conversion device according to claim 8, wherein the estimation unit calculates the first time based on the following equation.
Tmi = (Lr1 / Vi) {Io-Vi (1-D) D / (2Lr2fs)}
+ (3/4) Tr
However,
Tmi: the first time Lr1: the inductance of the first inductor Vi: the input voltage of the voltage conversion circuit Io: the output current of the voltage conversion circuit D: the target duty ratio Lr2 of the high-voltage side switching element: the second inductor Inductance (Lr2 >> Lr1)
fs: switching frequency of the high-voltage side switching element Tr: 2π√ (Lr1Cr1)
Cr1: Capacitance of the capacitor A first inductor, a high-voltage side switching element and a low-voltage side switching element connected in series via the first inductor, and a second inductor having one end connected to a connection point of the first inductor and the low-voltage side switching element A voltage conversion circuit having a capacitor connected in parallel to the low-voltage side switching element, a first detector and a second detector for detecting a voltage and a current for each of an input and an output of the voltage conversion circuit, and the above In the voltage conversion device including a control unit for turning on / off the high-voltage side switching element, the control unit causes the voltage conversion circuit to perform step-down conversion,
When the high-voltage side switching element is turned on for a first time corresponding to the length of the resonance period of the resonance current by the first inductor and the capacitor, the voltage is determined based on the detection result of the first and second detectors. Calculate the conversion efficiency of the conversion circuit,
Specifying a second time different from the first time;
When the high-voltage side switching element is turned on for the specified second time, the conversion efficiency of the voltage conversion circuit is calculated based on the detection results of the first and second detectors,
Comparing the conversion efficiencies respectively calculated when the high-voltage side switching element is turned on only for the first time and the specified second time;
A step-down control method for a voltage conversion circuit, wherein an on-time for turning on the high-voltage switching element is determined based on a comparison result. A first inductor, a high-voltage side switching element and a low-voltage side switching element connected in series via the first inductor, and a second inductor having one end connected to a connection point of the first inductor and the low-voltage side switching element A voltage conversion circuit having a capacitor connected in parallel to the low-voltage side switching element, a first detector and a second detector for detecting a voltage and a current for each of an input and an output of the voltage conversion circuit, and the above A computer program for causing the voltage conversion circuit to perform step-down conversion in the control unit in a voltage conversion device including a control unit for turning on / off a high-voltage side switching element,
The control unit
An efficiency calculation unit that calculates the conversion efficiency of the voltage conversion circuit based on the detection results of the first and second detectors;
A specifying unit that specifies a second time different from the first time according to a length of a resonance period of a resonance current by the first inductor and the capacitor;
An efficiency comparing unit that compares the conversion efficiencies calculated by the efficiency calculating unit when the high-voltage side switching element is turned on for the first time and the second time specified by the specifying unit, and a comparison of the efficiency comparing units A computer program that functions as an on-time determining unit that determines an on-time to turn on the high-voltage side switching element based on a result.

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