JPH10323028A - DC-DC converter - Google Patents

Abstract

(57) [Summary] A switching frequency is reduced by accurately detecting a light load having a reduced load current from an error voltage. A triangular wave control circuit (15) changes a voltage gradient (dV / dt) of a triangular wave voltage (Vc) generated by a triangular wave generating circuit (10) so as to be substantially proportional to an input voltage (Vi), so that a load flowing continuously through a choke coil (4). In the state, the error voltage Ve is maintained constant by changing the voltage gradient of the triangular wave voltage Vc in proportion to the input voltage Vi. In a light load state where the current flowing through the choke is discontinuous, the output current I
The light load detection circuit 21 allows the error voltage Ve to be reduced to the second reference voltage Vr2.
Upon detecting the following, the oscillation frequency of the reference oscillator 9 is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a step-down DC-D
The present invention relates to a C-converter, and more particularly, to a DC-DC converter that performs control so as to reduce a switching frequency and reduce a loss at a light load when an output current to a load is small.

[0002]

2. Description of the Related Art Conventionally, as a DC-DC converter control circuit of this type, for example, there is a circuit shown in FIGS. In FIG. 9, a main circuit A for performing a step-down DC-DC conversion converts an input voltage Vi from an input voltage source 1 such as a battery into an F
A pulse output is converted into a pulse voltage by turning on and off the ET2, then rectified by the diode 3, smoothed by the choke coil 4 and the capacitor 5, and supplied to the output terminals 6a and 6b to supply a regulated output voltage Vo to the load.

The control circuit B first detects an error voltage Ve indicating an error between the first reference voltage Vr1 of the reference voltage source 8 and the output voltage Vo in order to obtain a specified output voltage by the error detection circuit 7. ing. Further, a reference oscillator 9 is provided,
A reference pulse signal having a period determined by the reference oscillation frequency fo is oscillated, and a triangular wave generating circuit 10 generates and charges a triangular wave voltage Vc by charging and discharging a capacitor 11.

The error voltage Ve and the triangular wave voltage Vc are input to a pulse width modulation circuit 12 using a comparator, and are converted into a pulse width modulation signal having a width corresponding to the error voltage. Is driven to control the output voltage Vo to a specified voltage determined by the first reference voltage. Further, a light load detection circuit 23 is provided in order to lower the switching frequency and reduce the loss when the output current to the load is small. The light load detection circuit 23 connects a current detection resistor 25 in series with the choke coil 4 of the main circuit A, inputs a current detection voltage Vs proportional to the load current to the light load detection circuit 23 using a comparator, and detects a light load. The oscillation frequency of the reference oscillator 9 is reduced by the comparison output from the reference voltage source 24 for detection when the voltage becomes equal to or lower than the reference voltage Vr3.

The main circuit A and the control circuit B of the DC-DC converter shown in FIG. 10 are basically the same as those shown in FIG. 9 except that the switching frequency is reduced at a light load when the output current is small to reduce the loss. The light load detection circuit 26 receives the error voltage Ve of the error detection circuit 7 and outputs the oscillation frequency of the reference oscillator 9 based on a comparison output when the reference voltage source 27 for detecting a light load becomes equal to or lower than the reference voltage Vr4. Is decreasing.

[0006]

However, such a conventional DC-DC converter, which reduces the loss by reducing the switching frequency at a light load where the output current to the load is small, is small.
The DC converter has the following problem. First, FIG.
In the DC-DC converter described above, as a method of detecting the output current, the current detection resistor 25 is inserted into a line through which the output current flows to detect a voltage drop. -There is a problem that the efficiency of the DC converter is deteriorated.

Further, the DC-DC converter of FIG. 10 detects the output current at the time of light load from the decrease of the error voltage Ve of the error detection circuit 7, but the error voltage Ve of the error detection circuit 7 is detected.
However, there is a problem that since the output voltage greatly changes depending on the input voltage Vi, a decrease in the output current due to a light load cannot be accurately detected based on the error voltage Ve. The present invention has been made in view of such a conventional problem, and provides a DC-DC converter capable of accurately detecting a light load in which a load current decreases from an error voltage and lowering a switching frequency. The purpose is to:

[0008]

In order to achieve the above object, a DC-DC converter according to the present invention is configured as follows.
First, the present invention provides a main circuit for supplying a load with a predetermined output voltage Vo, which is obtained by converting an input voltage into a pulse voltage by turning on and off a switching element and then rectifying and reducing the voltage to a load. An error detection circuit that detects an error from the voltage Vr1 and outputs an error voltage Ve, a reference oscillator that oscillates a reference pulse signal having a constant period (reference frequency fo), and a triangular wave generation circuit that generates a triangular wave voltage Vc synchronized with the reference pulse A step-down circuit comprising: a pulse width modulation circuit that compares an error voltage Ve with a triangular wave voltage Vc to generate a pulse width modulation signal having a width corresponding to the comparison result; and a drive circuit that drives a switching element by the pulse width modulation signal. Type DC-DC converter.

In the step-down DC-DC converter according to the present invention, the voltage gradient (dV / dt) of the triangular wave voltage Vc generated by the triangular wave generating circuit is determined by the input voltage Vi.
A triangular wave control circuit for changing the output voltage to be approximately proportional to the output voltage, and detecting that the error voltage Ve of the error detection circuit has become equal to or less than a predetermined second reference voltage Vr2 due to a decrease in the output current to the load, and detecting the oscillation frequency of the reference oscillator. And a light load detection circuit for reducing the load.

Here, the triangular wave control circuit operates in a load state where the current flowing through the choke coil of the main circuit is continuous.
The error voltage Ve is kept constant by changing the voltage gradient of the triangular wave voltage Vc in proportion to the input voltage Vi. On the other hand, in a light load state in which the current flowing through the choke is discontinuous, a decrease in the error voltage Ve due to a decrease in the output current Io is allowed, whereby the error voltage Ve detected by the light load detection circuit
Light load can be detected based on the

The light load detection circuit reduces the reference oscillation frequency of the reference oscillator to a range of 1/5 to 1/100 when detecting that the error voltage has become equal to or lower than the second reference voltage Vr2. As described above, the step-down DC-DC converter of the present invention has a triangular wave voltage V used for pulse width modulation in the load state where the current flowing through the choke coil is in a continuous state.
By changing the voltage gradient (dV / dt) of c in proportion to the input voltage Vi, the error voltage Ve can be kept constant even if the input voltage Vi fluctuates. Absent.

On the other hand, when the load is light, the current flowing through the choke coil becomes discontinuous, and as a result, the output current Io decreases. For this reason, the voltage Ve also decreases with a decrease in the output current Io. When the voltage Ve becomes equal to or lower than the second reference voltage AVr2 set in advance, it is accurately detected that the load has become light, and the oscillation frequency of the reference oscillator is reduced. be able to.

[0013]

FIG. 1 shows a step-down DC according to the present invention.
It is a circuit block diagram of an embodiment of a -DC converter.
1, the DC-DC converter of the present invention includes a main circuit A and a control circuit B. The main circuit A inputs the supply of the input voltage Vi from the input voltage source 1 such as a battery to the FET 2 as a switching element, converts the input voltage Vi into a pulse voltage by turning on and off the FET 2, and then rectifies the pulse voltage with the diode 3. Then, the input voltage Vi is finally smoothed by the choke coil 4 and the capacitor 5, and is converted into a specified output voltage Vo which is reduced and supplied to the load.

On the other hand, the control circuit B detects an error voltage Ve between the output voltage Vo of the main circuit A and the first reference voltage Vr1 set by the reference voltage source 8 by the error detection circuit 7. Further, a reference oscillator 9 is provided. A reference pulse signal of a reference cycle (reference oscillation frequency fo) from the reference oscillator 9 is input to the triangular wave generation circuit 10, and a reference pulse signal to the output stage capacitor 11 included in the triangular wave generation circuit 10 is provided. The triangular wave voltage Vc is generated by charging / discharging by the supply.

The error voltage Ve from the error detection circuit 7 and the triangular wave voltage Vc from the triangular wave generation circuit 10 are input to a PWM circuit (pulse width modulation circuit) 12 and have a pulse width of an on-duty corresponding to the error voltage Ve. It is converted into a pulse width modulation signal Vw. The pulse width modulation signal Vw from the PWM circuit 12 is supplied to the drive circuit 13, and turns on and off the FET 2 provided in the main circuit A.

According to the present invention, such a control circuit B is provided with a triangular wave control circuit 15 for the triangular wave generation circuit 10. The triangular wave control circuit 15 inputs the input voltage Vi to the main circuit A via the resistor 14, and changes the time gradient (dV / dt) of the triangular wave voltage generated by the triangular wave generation circuit 10 in proportion to the input voltage Vi. Thus, even if the input voltage Vi fluctuates, the output voltage Vo is not affected.

FIG. 2 is a circuit diagram showing a circuit embodiment of the triangular wave generation circuit 10 and the triangular wave control circuit 15 of FIG. 1 together with the PWM circuit 12 and the drive circuit 13. FIG. 3 shows signal waveforms at various parts in FIG. The reference oscillator 9 is shown in FIG.
As shown in (A), a reference pulse signal having a period T is output as a Q output. The triangular wave generating circuit 10 inputs a reference pulse signal from the Q output of the reference oscillator 16 to the base of the transistor 19, and drives the capacitor 11
To generate a triangular wave voltage Vc.

That is, when the Q output of the reference oscillator 9 is at L level, the transistor 19 is turned off, and the triangular wave control circuit 1
The capacitor 11 is charged with the charging current Ic flowing from the constant current source 18 provided for the capacitor 5. Upper limit value V of charging voltage at this time
max is limited by the Zener diode 21. When the Q output of the reference oscillator 9 goes high, the transistor 19
Is turned on, and the capacitor 11 is discharged. At this time, the lower limit value Vmin of the discharge voltage of the capacitor 11 is limited by the reference voltage source 20.

The triangular wave voltage Vc from the triangular wave generating circuit 10
Is compared with the error voltage Ve by the PWM circuit 12, and FIG.
(D) is converted into a pulse width modulation signal Vw having a pulse width corresponding to the error voltage Ve. The pulse width modulation signal Vw is input to the NAND circuit 22 of the drive circuit 13, and
The inverted logical product of the inverted Q output of the reference oscillator 9 shown in FIG. 3B is obtained, and the FET 2 is driven as the drive signal Vd having the on-time t as shown in FIG.

As a result, as shown in FIG. 3C, the triangular wave generating circuit 10 generates a triangular wave voltage Vc that changes between a lower limit value Vmin and an upper limit value Vmax in synchronization with the Q output of the reference oscillator 9. At this time, the voltage gradient of the triangular wave voltage Vc (dV /
dt) is controlled by the triangular wave control circuit 15. The triangular wave control circuit 15 controls the charging current Ic flowing through the capacitor 11 by the current source 18 so that Ic = K · Vi proportional to the input voltage Vi. That is, when the input voltage Vi increases, the charging current Ic increases, and the voltage gradient (dV / dt) of the triangular wave voltage Vc increases. When the input voltage Vi decreases, the charging current Ic decreases, and the voltage gradient (dV / dt) of the triangular wave voltage Vc decreases.

FIG. 4 shows pulse width modulation by extracting the error voltage Ve and the triangular wave voltage Vc input to the PWM circuit 12 of FIG. In FIG. 4, the reference cycle of the oscillation pulse of the reference oscillator 9 is T, and the triangular wave voltage Vc1 determined by the input voltage Vi at this time is a voltage gradient (dV /
dt). The PWM circuit 12 compares the error voltage Ve at that time with the triangular wave voltage Vc1 that changes with a voltage gradient (dc / dt) from the start timing of the reference period T, and turns on when the triangular wave voltage Vc1 is equal to or lower than the error voltage Ve. During the time t1, the drive circuit 1 as shown in FIG.
3, an L level output is generated, and the FET 2 is turned on as shown in FIG. 3E over the on time t1.

After the elapse of the ON time t1, the triangular wave voltage Vc1
Exceeds the error voltage Ve, the output of the PWM circuit 12 becomes L
The level is inverted from the level to the H level, and the FET2 is turned off for the remaining time (T-t1). Hereinafter, such t1
ON of FET2 over time and F over (T-t1) time
The turning off of ET2 is repeated every reference cycle T. Assuming that the input voltage Vi increases, the triangular wave control circuit 15 of FIG.
Charging current I for the capacitor 11 increased in proportion to
Flow c. For this reason, the voltage gradient (dV / dt) increases like the triangular wave voltage Vc2 indicated by the broken line in FIG.
Becomes shorter.

That is, when the input voltage Vi increases, the FET2
, The power supply to the output side is maintained constant. Therefore, even if the input voltage Vi fluctuates, the voltage gradient of the triangular wave voltage Vc by the triangular wave control circuit 15 (d
(V / dt), the output voltage Vo is kept constant. As a result, the error voltage Ve from the error detection circuit 7 is also kept constant.

The maintenance of the error voltage Ve by the control of the time gradient of the triangular wave voltage Vc in accordance with the fluctuation of the input voltage Vi by the triangular wave control circuit 15 as described above is based on the fact that the current I flowing through the choke coil 4 is controlled by the FET 2 as shown in FIG. Even if it fluctuates due to the on / off of the switching period T, it is established only when it is flowing continuously. FIG. 5 shows the current I flowing through the choke coil 4 in a load state in which the output current Io is changed to large, medium, and small. Until I = 0 at the boundary of the cycle T in which the output current Io is lowest, Choke coil 4
, A current is flowing continuously. On the other hand, when the output current Io further decreases due to a light load, the current I flowing through the choke 4 enters a discontinuous state in which no current flows in the latter half of the reference cycle T as shown in FIG.

When the current flowing through the choke coil 4 becomes discontinuous as shown in FIG. 6, the error voltage Ve from the error detection circuit 7 also decreases according to the output current Io. That is, when the current flowing through the choke coil 4 is in a discontinuous state, the output voltage Vo increases due to the decrease in the output current Io, and the error voltage Ve starts to decrease accordingly, and the PW
The M circuit 12 controls so as to shorten the ON time of the pulse width modulation signal Vw. As a result, the error voltage Ve also decreases according to the output current Io.

In the embodiment shown in FIG. 1, the error voltage Ve from the error detection circuit 7 is input to a light load detection circuit 16 using an amplifier, and a predetermined light load is detected by a reference voltage source 17. To the second reference Vr2 for
The oscillation frequency of the reference oscillator 9 is reduced according to the light load detection voltage Vf output from the light load detection circuit 16 when the voltage becomes equal to or lower than the reference voltage Vr2.

FIG. 7 shows the output current I in the embodiment of FIG.
FIG. 6 is a characteristic diagram of o and an error voltage Ve. Regarding the output current Io shown on the horizontal axis, when the smoothed current from the choke 4 is larger than the choke critical current Io1 at which the choke 4 becomes discontinuous, as in the case of the small output current Io in FIG. 5, the error voltage Ve is constant. However, the output current Io is equal to the choke critical current I
When it falls below o1, the error detection voltage Ve starts to decrease.

With respect to the error voltage Ve, a second reference voltage Vr2 for starting the decrease of the switching frequency is set corresponding to the frequency switching current Io2. When the output current Io falls below the frequency switching current I02, the light load detection voltage Vf from the light load detection circuit 16 in FIG.
Will increase as the number decreases. The reference oscillator 9
Voltage control is performed so as to decrease the reference oscillation frequency in proportion to the increase in the detection voltage Vf from the light load detection circuit 16.

FIG. 8 shows the reference oscillator 9 for the error voltage Ve.
The vertical axis indicates the normalized oscillation frequency (frequency ratio f) in which the reference oscillation frequency fo in the steady state when the output current Io is equal to or greater than the choke critical current Io1 is 1.0.
/ Fo). In FIG. 8, the error voltage Ve
Becomes lower than the second reference voltage Vr2 corresponding to the choke critical current Io1, the light load detection voltage Vf output from the light load detection circuit 16 of FIG. 1 to the reference oscillator 9 increases in accordance with the decrease in the error voltage Ve. Therefore, the reference oscillator 9 linearly lowers the oscillation frequency as the light load detection voltage Vf increases.

Here, the decrease in the oscillation frequency of the reference oscillator 9 is limited to a range of about 1/5 to 1/100 of the reference oscillation frequency 1.0, and in FIG. When the oscillation frequency drops to 1/10, it is kept constant at that position. At this time, the change in the error detection voltage Ve in FIG. 7 decreases at a constant gentle gradient while the oscillation frequency is linearly reduced according to the error voltage Ve in FIG.
If the decrease in the oscillation frequency is limited to a constant value and the output current further decreases, the characteristic will be sharply reduced toward the PWM triangular wave lower limit value min. As described above, the control for reducing the switching frequency in the light load detection state based on the error voltage Ve makes it possible to lengthen the switching cycle of the FET 2 and reduce the loss.

The present invention is not limited to the above-described embodiment, but can be appropriately modified without impairing the object.

[0032]

As described above, according to the present invention, in a load state where the current flowing through the choke coil is in a continuous state, the voltage gradient of the triangular wave voltage used for pulse width modulation is changed in proportion to the input voltage. As a result, even if the input voltage fluctuates, the error voltage becomes constant without being affected by the input voltage, and a malfunction such as erroneously detecting the light load state due to the fluctuation of the input voltage and lowering the switching frequency may be caused. It can be reliably prevented.

On the other hand, when the load is light, the current flowing through the choke coil becomes discontinuous. As a result, the error voltage decreases as the output current decreases. By detecting the light load state and lowering the switching frequency, the control for accurately detecting the light load state and lowering the switching frequency can be performed accurately.

For this reason, the loss at light load can be reduced by lowering the switching frequency, the power consumption at light load in a battery-driven device can be reduced, and the operating time by battery drive can be extended.

[Brief description of the drawings]

FIG. 1 is a circuit block diagram of an embodiment of the present invention.

FIG. 2 is a circuit diagram of a triangular wave generation circuit and a triangular wave control circuit of FIG.

FIG. 3 is a signal waveform diagram of each part in FIG. 2;

FIG. 4 is an explanatory diagram of pulse width modulation in which a voltage gradient of a triangular wave voltage is changed according to an input voltage.

FIG. 5 is an explanatory diagram of a load state in which a current continuously flows through a choke coil.

FIG. 6 is an explanatory view of a light load state in which a current flowing through a choke coil is discontinuous.

FIG. 7 is a characteristic diagram of an error voltage with respect to an output current according to the present invention.

FIG. 8 is an explanatory diagram of a control characteristic of an oscillation frequency to be reduced when a light load is detected by an error voltage according to the present invention.

FIG. 9 is a conventional circuit block diagram for detecting a light load state by a current detection resistor.

FIG. 10 is a conventional circuit block diagram for detecting a light load state from an error voltage.

[Explanation of symbols]

 A: Main circuit B: Control circuit 1: Input voltage source 2: FET (switching element) 3: Diode (rectifying element) 4: Choke coil 5: Smoothing capacitor 6a, 6b: Output terminal 7: Error detection circuit 8, 17, 20: Reference voltage source 9: Reference oscillator 10: Triangular wave generation circuit 11: Capacitor 12: Pulse width modulation circuit (PWM circuit) 13: Drive circuit 14: Resistance 15: Triangular wave control circuit 16: Light load detection circuit 18: Current source 19 : Transistor 21: Zener diode

Claims (3)

[Claims] 1. A main circuit for converting an input voltage into a pulse voltage by turning on and off a switching element, supplying a predetermined output voltage rectified and smoothed and stepped down to a load, and the output voltage and a predetermined first reference An error detection circuit that detects an error with respect to a voltage and outputs an error voltage; a reference oscillator that oscillates a reference pulse signal having a constant period; a triangular wave generation circuit that generates a triangular wave voltage synchronized with the reference pulse; And a pulse width modulation circuit that generates a pulse width modulation signal having a width corresponding to the comparison result, and a drive circuit that drives the switching element with the pulse width modulation signal. In the DC converter, a triangular wave control circuit that changes a voltage gradient of a triangular wave voltage generated by the triangular wave generation circuit so as to be substantially proportional to the input voltage. A light load detection circuit for detecting that the error voltage of the error detection circuit has become equal to or lower than a second predetermined reference voltage due to a decrease in output current to a load, and lowering the oscillation frequency of the reference oscillator. DC-DC characterized by that
converter. 2. The DC-DC converter according to claim 1, wherein the triangular wave control circuit converts a voltage gradient of the triangular wave voltage into an input voltage when a current flowing through a choke coil of the main circuit is continuous. The error voltage is maintained constant by changing the output voltage proportionally, and in a light load state in which the current flowing through the choke is discontinuous, the error voltage is allowed to decrease as the output current decreases. D
C-DC converter. 3. The DC-DC converter according to claim 1, wherein the light load detection circuit changes the reference oscillation frequency of the reference oscillator when detecting that the error voltage has become equal to or lower than the second reference voltage. A DC-DC converter characterized in that it is reduced to a range of 1/5 to 1/100.