JPH1080135A - AC-DC converter - Google Patents

Abstract

(57) Abstract: An inductor and a switching element are connected in series to an output side of a rectifier circuit, energy is supplied from the inductor as a current when the switching element is turned off, and the switching element is supplied after the current of the inductor has stopped flowing. In the AC-DC converter that turns on the power supply, there is a problem that an excessive current flows to the load and the output voltage at light load increases. A one-shot circuit is provided on the output side of a current sense amplifier for outputting an off command when a current of a switching element exceeds a threshold. A pulse is output to the flip-flop 4, and even if the current is released from the inductor L1 after the switching element 2 is turned off and the current becomes zero, the on-command from the zero current detector 3 is ignored by the pulse and the on-command is turned off. The timing is delayed, and during this time, the excess current is consumed by the load R0 to suppress an increase in the output voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AC-DC converter in which a chopper circuit is used to control a switching element so that an input current waveform is in proportion to a sine-wave voltage waveform of an AC power supply, thereby controlling a switching element to improve a power factor. Related to the device.

[0002]

2. Description of the Related Art As an AC-DC converter for converting an AC voltage into a DC voltage and supplying the DC voltage to a load, there is an apparatus using a boost chopper circuit. Since this type of converter switches at a high frequency, the input current waveform becomes the average value of each period of the switching current and the load becomes equivalent to a pure resistance, so that the power factor is improved and the peak value of the switching current is reduced. By changing it, the DC output voltage can be stabilized.

[0003] As a method of controlling the switching element,
A discontinuous type that releases the energy stored in the inductor to the load side when the switching element is off and turns on the switching element after the inductor energy has stopped flowing as a current (after the current stops flowing), and the inductor current There is a continuous type in which the switching element is turned on while the current is still flowing, and both have advantages and disadvantages. The present invention relates to the former discontinuous type, and a conventional example of this type of apparatus will be described with reference to FIG.

In FIG. 1, reference numeral 1 denotes a rectifier circuit comprising a diode bridge circuit, which performs full-wave rectification on an AC input which is a sine wave of an AC power supply, for example, a commercial power supply. A series circuit composed of an inductor L1, a diode D1 which is a one-way conduction element, and a smoothing capacitor C1 is connected between output terminals of the rectifier circuit 1. Also, a diode D1 and a smoothing capacitor C
A switching element 2 composed of, for example, an N-type field effect transistor (FET) and a resistor R1
Are connected in parallel with each other, thus forming a step-up chopper circuit. The capacitor C2 connected to the output side of the rectifier circuit 1 is for removing high-frequency ripple.

Next, a control circuit for controlling the switching element 2 will be described. This control circuit includes an under-voltage lockout (hereinafter referred to as “UVLO”) 11 for supplying a voltage to the inside of the control circuit. This U
The VLO 11 rises by the charged voltage of the capacitor C3 charged from the rectifier circuit 1 via the resistor R2, and functions as a voltage source for all circuits in the control circuit. Reference numeral 12 denotes a comparison voltage generation circuit that generates a comparison voltage Vref in the control circuit when the UVLO 11 rises.

An auxiliary secondary winding L is connected to the inductor L1.
2, one end of the auxiliary secondary winding L2 is connected to the positive input end of a zero current detector 3 composed of a comparator via a resistor R3, and the other end is connected to ground. . The resistor R4 and the transistor 10 are a clamp circuit of the zero current detector 3, and the diode D2 supplies the voltage generated in the auxiliary secondary winding L2 to the UVLO 21, and shares the power supply with the rectifier circuit 1.

The output side of the zero current detector 3 is RS
The flip-flop 4 is connected to a master input terminal and is connected to a slave input terminal via a delay circuit 41. The output side of this flip-flop 4 is an AND circuit 4
2, the gate control circuit 20 of the switching element 2
It is connected to the. The gate control circuit 20 includes a series circuit of NPN transistors 21 and 22 and resistors R5 and R6, and a connection point between the resistors R5 and R6 is connected to the gate of the switching element 2. The bases of the transistors 21 and 22 are connected to the output terminal and the inverted output terminal of the AND circuit 42, respectively.

On the other hand, point A on the input side of the zero current detector 3 is
When the current flowing through the inductor L1 is decreasing, the potential becomes high due to the induced voltage of the auxiliary secondary winding L2, and at other times, it becomes low. Therefore, the output of the zero current detector 3 indicates that the switching element 2 is turned off and the inductor L1
Becomes logic "1" when energy is released from the inductor, and becomes logic "0" when current stops flowing from the inductor L1. The flip-flop 4 is triggered when the signal from the zero current detector 3 falls from “1” to “0” and the AND circuit 42 switches the switching element 2
A signal of "1" corresponding to the ON command is given, the transistors 21 and 22 are turned on and off, respectively, and the switching element 2 is turned on. That is, the zero current detector 3, the delay circuit 41, and the flip-flop 4 constitute an ON command circuit 40 for the switching element 2.

One end of a resistor R1 connected in series to the switching element 2 is connected to one input terminal of a current sense amplifier 5 via a resistor R7, and the current sense amplifier 5 flows through the switching element 2. When the current exceeds the threshold value, that is, when the voltage across the resistor R1 exceeds the threshold value Vth, the output signal becomes “1”.
C4 is a capacitor. The output terminal of the current sense amplifier 5 is connected to the master input terminal of the flip-flop 4, and outputs "1" to change the output signal of the flip-flop 4 to "0". At this time, the transistor 21,
22 turns off and on, respectively, and the switching element 2 turns off. In this example, the current sense amplifier 5 and the flip-flop 4 constitute an off command circuit 50, and the flip-flop 4 also serves as a part of the on command circuit 40 and the off command circuit 50.

The threshold Vth of the current sense amplifier 5 has a magnitude corresponding to a signal obtained by multiplying the error between the output voltage V0 of the AC / DC converter and the reference voltage by the output voltage of the rectifier circuit 1. . That is, the output voltage V0 is divided and detected by the detection resistors R8 and R9, and an error between the detected voltage and the reference voltage Vref is extracted by the error amplifier 51, while the output voltage of the rectifier circuit 1 is detected by the detection resistors R10 and R9.
The voltage is divided by 11 and detected, and the detected voltage and the error are multiplied by a multiplier 52, and the multiplied value is set as a threshold Vth.

The overvoltage detector denoted by 53 outputs "1" when the output voltage V0 becomes excessive and supplies it to the master input terminal of the flip-flop 4, and the switching element 2
Is to turn off. Reference numeral 54 denotes a quick start circuit for preventing the charging of the capacitor C5 from being delayed. C6 is a capacitor.

Next, the operation of the AC / DC converter will be described. When AC power from the outlet is input to the rectifier circuit 1, the capacitor C3 is charged and the UVLO 11
Output rises and the whole system starts functioning. Since the switching operation of the switching element 2 does not start in the initial state, the timer 13 outputs the first ON command “1”.
To the AND circuit 42, thereby turning on and off the switching elements 21 and 22 to forcibly drive (turn on) the switching element 2. The timer 13 is a circuit for forcibly starting when the switching element 2 is not turned on for a certain period.

When the switching element 2 is turned on, a current flows from the rectifier circuit 1 to the switching element 2 through the primary side of the inductor L1, and energy is stored in the inductor L1. The amount of current flowing through the switching element 2 is determined by the threshold Vth of the current sense amplifier 5. This threshold Vth is set as a multiplication value of an error between the output voltage V0 of the AC-DC converter and the reference voltage and the voltage rectified by the rectifier circuit 1, and the current flowing through the switching element 2 is ( When the current detection value reaches the threshold Vth of the current sense amplifier 5, the output signal of the current sense amplifier 5 becomes "1" to reset the flip-flop 4, and the flip-flop 4 outputs "0" which is the OFF command. When a signal is output, the transistors 21 and 22 are turned off and on, respectively, and the switching element 2 is turned off.

At the moment when the switching element 2 is turned off, flyback occurs, and the energy stored in the inductor L1 is released as a current (to the load R0) to the output side of the device through the rectifier diode D1, and the output voltage V0 is reduced. To raise. At the same time, an electromotive force is generated in the auxiliary secondary winding L2, the potential at point A on the input side of the zero current detector 3 rises, and the output signal of the zero current detector 3 becomes "1". To the standby state.

When the current of the inductor L completely flows into the load R0, the potential at the point A falls and the output signal of the zero current detector 3 becomes "0". At this time, since the output signal of the current sense amplifier 5 is "0", the flip-flop 4 is set by the fall of the output of the zero current detector 3, and the signal of "1" which is the ON command from the flip-flop 4 is set. Is output, the transistors 21 and 22 are turned on and off, respectively, and the switching element 2 is turned on. The ON operation of the next switching element 2 starts after the current of the inductor L1 is completely cut off, so that the recovery loss of the diode D1 is eliminated.

Thereafter, similarly, the switching element 2 is turned on and off at a frequency sufficiently higher than the frequency of the AC input based on the ON command from the ON command circuit 40 and the OFF command from the OFF command circuit 50. FIG. 6 shows the switching element 2
The hatched area in the output voltage waveform of the rectifier circuit 1 is one of the time zones of several pulses of the switching element 2. The intervals between the gate pulses are set at equal intervals for convenience of illustration.

The threshold Vth of the current sense amplifier 5 of the off command circuit 50 is the same as the output voltage waveform of the rectifier circuit 1, and the closer the peak of the input voltage waveform is, the more the current flowing through the switching element 2 becomes. The current flows more than Vth, so that the input voltage waveform and the input current waveform become similar, and the phase difference between both waveforms disappears.

The threshold Vth is used for not only the function of following the input current waveform but also the output voltage V0.
There is also a role to stabilize. When the output voltage V0 changes, the error between the reference voltage and the detection voltage of V0 increases, so that the output of the error amplifier increases, and the threshold Vth changes as shown in FIG. As a result, the current value of the switching element 2 changes, and V0 is stabilized. FIG. 8 is a diagram in which the waveform of the current flowing through inductor L1 when the output voltage is relatively large, the ideal value of the peak value of the current, and the gate waveform of switching element 2 are associated with each other.

[0019]

However, the above-described device has a problem that the output voltage rises without stability at light load (when the output power is small). This is based on the fact that the detected value of the current of the switching element 2 exceeds the threshold value Vth, and there is a slight delay in the circuit from when the OFF command for the switching element 2 is output to when the switching element 2 is actually turned off. It is. Although it is ideal to design the response delay of the circuit as small as possible, there are restrictions such as the process and the current consumption of the IC, and the limit is determined.

That is, an extra current flows through the switching element 2 by the amount of the delay (delay time).
Excessive energy is stored in one. As a result, the current discharged from the inductor L1 to the load R0 after the switching element 2 is turned off exceeds the amount of current required by the load to maintain the output voltage V0 at the reference voltage, thereby causing an increase in V0. .

FIG. 9 shows this state. In the drawing, ion indicates a current when the switching element is on, ioff indicates a current when the switching element is off, ion indicates a current exceeding an ideal peak value according to the delay time, and ioff indicates an excess amount of energy stored in the inductor L1. It will flow extra according to.

By the way, when the output power is large, the off delay of the switching element 2 is a threshold voltage.
As shown in FIG. 8, the ratio of the current flowing during the delay is smaller than the ideal value, and the current consumption is large. Therefore, the rise of the output voltage V0 is relatively gentle,
When the output voltage V0 tries to rise beyond the reference voltage,
The threshold voltage Vth decreases accordingly, so that the output voltage V0 can be controlled stably even if a delay exists.

On the other hand, when the load is light, the threshold Vth becomes considerably low as shown in FIG. 10, so that the ratio of the current flowing during the delay to the ideal value increases even if the delay time is the same. . Therefore, the output voltage V0 rises sharply. Attempts to control the output voltage V0 by lowering Vth in order to suppress this increase. However, the load already keeps the output voltage V0 at the reference voltage only by the current flowing during the delay time. If the current consumption exceeds the required current, the output voltage V0 cannot be stabilized no matter how much Vth is reduced.

The present invention has been made under such circumstances, and it is an object of the present invention to provide an AC-DC converter capable of stabilizing an output voltage.

[0025]

SUMMARY OF THE INVENTION The present invention provides a rectifier circuit having an input connected to an AC power supply, an inductor connected to the output side of the rectifier circuit, and a one-way connected to the output side of the inductor. A series circuit of a conduction element and a smoothing capacitor, a switching element connected in series to the inductor and connected in parallel to the series circuit, and switched at a frequency higher than the frequency of the AC power supply; An ON command circuit that outputs an ON command for turning on the switching element, an OFF command circuit that outputs an OFF command for turning off the switching element,
The ON command circuit outputs an ON command when detecting a zero point of a current flowing through the inductor, and the OFF command circuit outputs a current flowing through the switching element by a voltage across the smoothing capacitor and an output voltage of the rectifying circuit. An AC-DC converter in which a switching command is output when a threshold value determined based on the above is output, and the switching element is turned on and off so that the input current waveform is proportional to the sine wave voltage waveform of the AC power supply. An on-command prohibition circuit that outputs an on-command prohibition signal for prohibiting output of an on-command for a predetermined period when an off command is output from the off-command circuit is provided. Is characterized in that the output of the ON command from the ON command circuit is prohibited. In this case, the on-command prohibition circuit can be configured to output a pulse having a fixed time width, which is an on-command prohibition signal.

The ON command when the output power is small is prohibited by the ON command prohibition signal, but the ON command when the output power is large is generated later than the time period during which the ON command prohibition signal is generated. It is preferable to set the generation period of the ON command inhibition signal so as not to be affected by the ON command inhibition signal. The small output power means that the output power is relatively small within the applicable range,
High output power means other than when the output power is low.

In the present invention, the ON command circuit outputs a ON command when the zero point of the current flowing through the inductor is detected, and outputs the ON command from the detection circuit with a delay for a predetermined period. And a delay circuit.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS AC according to an embodiment of the present invention
FIG. 1 shows the circuit of the DC converter. This embodiment differs from the circuit shown in FIG. 5 as a conventional example in that a one-shot circuit 6 is provided on the output side of the current sense amplifier 5, and the current sense amplifier 5 outputs a command "1" for turning off the switching element 2. When a signal is output, a pulse signal having a constant pulse width is output to the flip-flop 4 based on the OFF command. Since the other parts are the same as those of the circuit shown in FIG. 5, the operation related to this one-shot circuit 6 will be mainly described to avoid duplication of description.

As for the role of the one-shot circuit 6, after the switching element 2 is turned off, the current of the inductor L1 reaches zero, and the output of the zero current detector 3 of the on command circuit 40 changes from "1" to "0". In the case of a conventional circuit, a signal of "1", which is an ON command, is output from the flip-flop 4 based on this falling. After the switching element 2 is turned off, a pulse having a constant pulse width is output from the one-shot circuit 6. Is output, the falling of the zero current detector 3 is ignored, and the output of the signal of “1”, which is the ON command, from the flip-flop 4 is prohibited.

The operation of the circuit according to the embodiment of the present invention will be described. When AC power is supplied to the rectifier circuit 1, the system starts up and the switching element 2 starts to be turned on by the action of the timer 13 as described in the section of the related art. When the current of the switching element 2 increases and the detected value exceeds the threshold value Vth, the current sense amplifier 5 outputs a signal of "1" which is an off command. This signal enters a one-shot circuit 6 corresponding to an on-command prohibition circuit, and a pulse having a constant pulse width corresponding to the on-command prohibition signal is supplied from the one-shot circuit 6 to the master input terminal of the flip-flop 4.

The "1" signal from the one-shot circuit 6 also corresponds to an off command, and this off command is given as a "0" signal to the AND circuit 42 via the flip-flop 4, and the switching element 2 is turned off. become. FIG. 2 shows this state, and the gate signal of the switching element 2 falls slightly behind the rise of the output of the current sense amplifier 5 due to a delay on the circuit. At the moment when the switching element 2 is turned off, flyback occurs, and the energy stored in the inductor L1 flows to the load R0 as a current. Although the output of the current sense amplifier 5 falls immediately,
As shown by the dotted line, the signal of "1" is continuously input to the master input terminal of the flip-flop 4.

On the other hand, when the current of the inductor L1 completely flows to the load R0, the output of the zero current detector 3 falls at time t1 in FIG. 2, but at this time, the one-shot circuit 6 outputs "1".
Is input to the flip-flop 4. Therefore, even if the output of the zero current detector 3 falls, that is, the ON command is output from the zero current detector 3, the passage of the ON command through the flip-flop 4 is prohibited (the ON command is output from the flip-flop 4). The output of the signal of “1” is prohibited), and the switching element 2 is not turned on.

If the switching element 2 is not turned on after the current has completely flowed through the inductor L1, an electromotive force is generated in the auxiliary secondary winding L2 based on the fact that the current of the inductor L1 has become zero. As a result, an electromotive force is generated on the primary side, and a ringing occurs in which an electromotive force is generated on the secondary side. Therefore, even if the output of the zero current detector 3 falls, it rises again and then falls. This 2
Since the pulse of the one-shot circuit 6 has already fallen at the time t2 of the second fall, the inhibition of the output of the ON command is released, and the ON command (“0”) from the zero current detector 3 is released.
Is supplied to the AND circuit 42 as a signal of “1” through the flip-flop 4. As a result, the switching element 2 is turned on.

By the way, the time from when the switching element 2 is turned off until the current in the inductor L1 stops flowing is shorter when the output power is small (at light load) than when the output power is large. Therefore, the width of the pulse from the one-shot circuit 6 is set such that the output of the off command is prohibited when the load is light, but is set shorter than the fall time of the current of the inductor L1 when the output power is large. The one-shot circuit 6 itself does not act.

In the present invention, the width of the pulse from the one-shot circuit 6 may be set so as to prohibit the output of the OFF command even when the output power is large. In this case, the ON timing is delayed, Secondly, more current must be passed, resulting in a larger ripple in the output voltage, and therefore the above case is more advantageous.

FIG. 3 shows the current waveform of the inductor L1 when the output power is small, in correspondence with the gate signal of the switching element 2 and the one-shot pulse waveform. Although the peak value of the current is higher than the peak value of the ideal current, the ON time of the switching element 2 is forcibly delayed so that the OFF time period is long, that is, the inductor L1 is turned off after the switching element 2 is turned off. Even if the current reaches zero, the switching element 2 is turned off by the pulse width determined by the one-shot circuit 6 without immediately starting the next ON cycle, so that the excess accumulated in the inductor L1 is Consumed at R0. As a result, an increase in the output voltage V0 can be suppressed, and the output voltage V0 is stabilized.

FIG. 4 is a circuit diagram showing another embodiment of the present invention. This embodiment differs from the previous embodiment in that a one-shot circuit 6 is provided on the output side of current sense amplifier 5. Instead, zero current detector 3 and flip-flop 4
And the point provided between them. This circuit corresponds to the invention described in claim 4, and the one-shot circuit 6 corresponds to a delay circuit.

In this case, regardless of whether the load is light or not, the switching element 2 is delayed by a pulse width determined by the one-shot circuit 6 after the current flowing through the inductor L1 becomes zero after the switching element 2 is turned off. Is output, the excess energy of the inductor L1 is consumed by the load R0 during the off period, and the rise of the output voltage V0 is suppressed.

[0039]

As described above, according to the present invention, the effect that the output voltage is stabilized can be obtained.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an overall configuration of an embodiment of the present invention.

FIG. 2 is a time chart illustrating an operation of a main part according to the embodiment of the present invention.

FIG. 3 is a time chart showing a relationship among a current flowing through an inductor, a gate signal of a switching element, and a one-shot pulse in the embodiment of the present invention.

FIG. 4 is a circuit diagram showing an overall configuration of another embodiment of the present invention.

FIG. 5 is a circuit diagram showing a conventional AC-DC converter.

FIG. 6 is a time chart showing an operation of a main part in a conventional example.

FIG. 7 is a waveform diagram showing a threshold waveform of the current sense amplifier.

FIG. 8 is a time chart showing a relationship between a current flowing through an inductor and a gate signal of a switching element in a conventional example.

FIG. 9 is an explanatory diagram showing a current flow in a conventional example.

FIG. 10 is a time chart showing a relationship between a current flowing through an inductor and a gate signal of a switching element at a light load in a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Rectifier circuit L1 Inductor L2 Auxiliary secondary winding 2 Switching element 20 Gate control circuit D1 Rectifier diode C1 Smoothing capacitor R0 Load 3 Zero current detector 4 RS flip-flop 40 On command circuit 5 Current sense amplifier 51 Error amplifier 50 Off Command circuit 52 Multiplier 6 One shot circuit

[Procedure amendment]

[Submission date] November 7, 1996

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Name of invention

[Correction method] Change

[Correction contents]

[Title of the Invention] AC-DC converter

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location H03K 17/725 H03K 17/725 AB (72) Inventor Norihiro Miura 3-20 Minamiazabu, Minato-ku, Tokyo No. 1 Inside Nippon Motorola Co., Ltd.

Claims (4)

[Claims] A rectifier circuit connected to an input side of an AC power supply; an inductor connected to an output side of the rectifier circuit;
A series circuit of a one-way flow element and a smoothing capacitor connected to the output side of the inductor is connected in series to the inductor and connected in parallel to the series circuit. A switching element that is also switched at a high frequency, an ON command circuit that outputs an ON command for turning on the switching element, and an OFF command circuit that outputs an OFF command for turning off the switching element, The ON command circuit outputs an ON command when detecting a zero point of a current flowing through the inductor, and the OFF command circuit outputs a current flowing through the switching element based on a voltage across the smoothing capacitor and an output voltage of the rectifier circuit. An off command is output when the determined threshold is exceeded, and the input current waveform changes to the sine wave voltage waveform of the AC power supply. As an example, in an AC-DC converter in which a switching element is controlled to be turned on and off, when an off command is output from the off command circuit, an on command prohibition signal for prohibiting the output of the on command is output for a predetermined period. An AC-DC converter, comprising an ON command inhibiting circuit, wherein output of an ON command from the ON command circuit is inhibited while the ON command inhibiting signal is being output. 2. The AC-DC converter according to claim 1, wherein outputting the ON command inhibition signal for a predetermined period means outputting a pulse having a fixed time width. 3. The on command when the output power is low is prohibited by the on command prohibition signal, but the on command when the output power is high is generated after the time period during which the on command prohibition signal is generated. 3. The AC-DC converter according to claim 1, wherein a generation period of the ON command inhibition signal is set so as not to be affected by the ON command inhibition signal. A rectifier circuit connected to an AC power supply on an input side; an inductor connected to an output side of the rectifier circuit;
A DC circuit of a one-way element and a smoothing capacitor connected to the output side of the inductor, connected in series to the inductor and connected in parallel to the series circuit, and having a frequency higher than the frequency of the AC power supply. A switching element that is switched at a high frequency, an ON command circuit that outputs an ON command to turn on the switching element, and an OFF command circuit that outputs an OFF command to turn off the switching element, The on-command circuit outputs an on-command when detecting that the current of the inductor has stopped flowing, and the off-command circuit determines that the current flowing through the switching element is based on the voltage across the smoothing capacitor and the output voltage of the rectifier circuit. An off command is output when the determined threshold is exceeded, and the input current waveform is the sine wave voltage of the AC power supply. The switching element is turned on in proportion to the shape, off controlled by an AC - in DC converter, the on-command circuit includes a detection circuit for outputting an on command when detecting the zero point of the current flowing through the inductor,
And a delay circuit for delaying the ON command from the detection circuit for a certain period and outputting the ON command.