JP2002136130A - Switching power device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a switching power device which prevents generation of surge voltages and an undershoot phenomenon when its operation stops, and is capable of soft starting. SOLUTION: This switching power device PWM-controls a primary-side main switch 4 by a control pulse signal which corresponds to an output voltage, and is fitted with a control circuit 14, which generates a control pulse signal whose time ratio becomes gradually smaller, when the operation of the switching power device stops.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply.

[0002]

2. Description of the Related Art FIG. 4 is a circuit diagram showing an example of a conventional switching power supply. The switching power supply has input terminals 1 and 2, a smoothing capacitor 3, and a main switch M
A primary-side drive circuit including the OSFET 4 and the primary winding of the transformer 5, a secondary winding of the transformer 5, a rectifying switch MOSFET 6, a return switch MOSFET 7, a choke coil 8, an output capacitor 9, and output terminals 10 and 1.
1, a synchronous rectification type forward switching power supply device comprising a secondary-side output circuit including voltage dividing resistors 12, 13 and a control circuit 14. In the configuration described above, the main switch MOSFET 4 is PWM-controlled by the control circuit 14 so that the output voltage obtained from the output terminals 10 and 11 is stabilized.

In such a switching power supply device, when the operation is stopped at time t11 as shown in FIG.
Applied to the gate of the FET 6 and rectifying switch MOSF
The gate-source voltage Vgs of ET6 increases. Therefore, the rectification switch MOSFET 6 is turned on, the secondary winding of the transformer 5 is excited, and when the transformer 5 is saturated, the internal resistance of the rectification switch MOSFET 6 causes the drain-source voltage Vds of the rectification switch MOSFET 6 to increase. To rise. Eventually, the drain-source voltage Vds becomes the MOPS for the reflux switch at time t12.
When the gate-source voltage Vgs of the FET 7 exceeds the threshold level Vth, the freewheeling switch MOSFE
T7 turns on. At the same time, rectifier switch MOS
FET 6 is turned off.

Next, when the gate voltage of the freewheeling MOSFET 7 is discharged and becomes lower than the threshold level Vth, the freewheeling switch MOSFET 7 is turned off, and energy from the output is applied to the gate of the rectifying switch MOSFET 7 again.

[0005] This self-excited oscillation phenomenon is repeated until the output energy is exhausted. At this time, a surge voltage (see FIG. 5) exceeding the withstand voltage of the synchronous rectifiers (MOSFETs 6 and 7) is generated, which may damage the synchronous rectifiers.

FIG. 6 is a circuit diagram showing another example of a conventional switching power supply. In FIG. 6, FIG.
In addition to the conventional configuration shown in FIG.
The switch element 15 is connected to the gate circuit of ET6. Then, the switch element 15 is replaced with a rectifying switch MOS.
It is controlled so that it is turned on when the FET 6 is operating (on), and is turned off when the operation of the switching power supply is stopped. As described above, by controlling the switch element 15, the rectifying switch MOSFET 6 maintains the off state when the operation is stopped, so that generation of a surge voltage due to the self-excited oscillation phenomenon in FIG. 4 can be suppressed.

FIG. 7 is a circuit diagram showing still another example of the conventional switching power supply. In FIG. 7, in addition to the configuration of the conventional example shown in FIG. 3, a switch element 16 and a voltage source 17 are connected to the gate circuit of the freewheel switch MOSFET 7. And the switch element 16
Is controlled to be off when the freewheeling switch MOSFET 7 is operating (on) and on when the switching power supply is not operating. By controlling the switch element 16 in this manner, the freewheeling switch MOSFET 7 maintains the ON state when the operation is stopped, so that the generation of a surge voltage due to the self-excited oscillation phenomenon in FIG. 4 can be suppressed.

[0008]

However, FIG. 6 and FIG.
In the device shown in (1), there is a problem that the number of components such as a switch element and a voltage source increases, and the cost increases.
Further, as shown in FIG.
When the self-oscillation phenomenon is prevented by turning on the SFET 7, the output energy is
May cause an undershoot phenomenon in which the output voltage becomes negative. Then, due to the undershoot phenomenon, a reverse voltage is applied to the component to exert an adverse effect, and in the worst case, there is a problem that the component may be destroyed.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a switching power supply which can prevent a surge voltage and an undershoot phenomenon from occurring when the operation is stopped, and can be soft-started at the time of startup.

[0010]

In view of the above-mentioned object,
The switching power supply of the present invention performs PWM control of the primary side main switch with a control pulse signal corresponding to the output voltage. A control circuit is provided for generating a control pulse signal whose duty ratio gradually decreases when the operation of the switching power supply is stopped.

Thus, when the operation of the switching power supply is stopped, the self-excited oscillation phenomenon of the synchronous rectifier due to the output energy can be prevented, and the output voltage undershoot can be prevented.

According to another aspect of the present invention, in addition to the switching power supply of the above-described invention, the control circuit changes the triangular wave generating means, the output voltage detecting means for detecting the output voltage of the switching power supply, and when the operation is stopped. Pulse width limiting means for generating a pulse width limiting signal, a triangular wave signal from the triangular wave generating means, a detection signal from the output voltage detecting means, and a pulse width limiting signal from the pulse width limiting means,
And control pulse signal generating means for generating a control pulse signal. The control pulse signal generating means, based on a comparison between the triangular wave signal from the triangular wave generating means and the detection signal from the output voltage detecting means during normal operation of the switching power supply,
A control pulse signal having a predetermined duty ratio is generated, and when the operation of the switching power supply is stopped, the duty ratio gradually decreases based on a comparison between the triangular wave signal from the triangular wave generating means and the pulse width limiting signal from the pulse width limiting means. Generate a control pulse signal.

Thus, a control pulse signal having a predetermined duty ratio can be generated during normal operation of the switching power supply device, and a control pulse having a gradually decreasing duty ratio can be generated when operation is stopped.

According to another aspect of the present invention, in addition to the switching power supply of the above-described aspect, the pulse width limiting means includes a first resistor and a second resistor connected in series, and a pulse resistor connected in parallel to the first resistor. A time constant circuit comprising a series connection of a capacitor and a third resistor, and a series connection of a semiconductor switch element and a fourth resistor connected in parallel to the capacitor. The semiconductor switch element is supplied with an operation stop signal of the switching power supply to its control electrode and turned on, so that the pulse width that changes when the operation is stopped from the series connection point of the first and second resistors of the time constant circuit is stopped. The limiting signal is supplied to the control pulse signal generating means.

Thus, based on the discharging operation of the time constant circuit, it is possible to generate a control pulse having a gradually decreasing duty ratio when the operation of the switching power supply is stopped.

According to another aspect of the present invention, in addition to the switching power supply of the above-described invention, the time constant circuit functions as a soft start means when the switching power supply is activated. As a result, when the operation of the switching power supply is stopped, the self-excited oscillation phenomenon of the synchronous rectifying element due to the output energy can be prevented, and the undershoot of the output voltage can be prevented. can do.

[0017]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of a switching power supply unit according to the present invention. In FIG. 1, the same components as those of the conventional circuit diagrams of FIGS.
The description is given with the same reference numerals. This switching power supply device is a synchronous rectification type forward switching power supply having the same configuration as the conventional circuit diagram, but is characterized by the configuration of the control circuit 14.

The control circuit 14 changes as an output voltage detecting means for detecting an output voltage at the output terminals 10 and 11 as an output voltage detecting means, and as a pulse width limiting means when the switching power supply starts and stops. A pulse width limiting circuit 19 for generating a pulse width limiting signal;
A control pulse signal as a control pulse signal generating means to which a triangular wave signal from a triangular wave generator 32 as a triangular wave generating means, a detection signal from an output voltage detecting circuit 18 and a pulse width limiting signal from a pulse width limiting circuit 19 are input. And a generation circuit 20.

The output voltage detection circuit 18 is connected to the output terminals 10,
11 and an error amplification circuit 2
1, a photo-coupler 21 having a photo-diode 21a connected to an output, a photo-transistor 22b of the photo-coupler 22 connected to + Vcc, a resistor 23 connected between the photo-transistor 22 and the ground, It is composed of

The pulse width limiting circuit 19 includes a series connection of a resistor 25 as a first resistor and a resistor 26 as a second resistor, and a capacitor 2 connected in parallel to the resistor 25.
A time constant circuit composed of a series connection of a resistor 7 and a resistor 28;
And a series connection of a transistor 30 as a semiconductor switch element and a resistor 29 as a fourth resistor connected in parallel to the capacitor 27. A control terminal 31 to which an operation stop signal of the switching power supply is supplied is connected to a control electrode, that is, a base of the transistor 30.

The control pulse signal generating circuit 20 comprises a triangular wave generator 32 as a triangular wave generating means and a three-input type comparator 33. Comparator 3
3 is connected to the phototransistor 22 b of the photocoupler 22 in the output voltage detection circuit 18.
And the resistor 23. Comparator 3
The third positive input terminal 3 is connected to a series connection point of the resistors 25 and 26 of the pulse width limiting circuit 19. The negative input terminal of the comparator 33 is connected to the triangular wave generator 3
2 are connected.

Next, the operation of the switching power supply having the above-described configuration will be described with reference to the circuit diagram shown in FIG. 1 and the signal timing charts of the respective parts of FIG. 1 shown in FIGS.

First, the switching power supply will be described with reference to FIGS. 1 and 2. When a DC input voltage is applied to the input terminals 1 and 2 at time t1 by starting, the voltage of the + Vcc power supply rises. Accordingly, the time constant circuit of the pulse width limiting circuit 19 operates. At this time, the transistor 30 is supplied with a high-level signal to the control terminal 31 and is in an off state. As a result, the pulse width limiting signal supplied to the first plus input terminal of the comparator 33 of the control pulse signal generation circuit 20, that is, the voltage at the serial connection point of the resistors 25 and 26 becomes the waveform B of FIG. As shown in (1), the voltage rises first to a divided voltage determined by the parallel resistance value of the resistors 25 and 28 and the resistance value of the resistor 26. Next, this voltage gradually decreases by the charging time constant of the capacitor 27 and the resistor 28, and finally reaches a divided voltage determined by the resistance value of the resistor 25 and the resistance value of the resistor 26.

On the other hand, the triangular wave generator 32 generates a triangular wave signal shown by a waveform A in FIG. However, since the output voltage of the switching power supply circuit has not yet risen, the output voltage detection circuit 18 is not operating, and the second positive input terminal of the comparator 33 has a waveform C shown in FIG. As described above, the output voltage detection circuit 18
Is supplied with a low-level voltage determined by the resistors 23 and 24 of FIG.

Therefore, the comparator 33 outputs the waveform A
2C is compared with the pulse width limiting signal of waveform B, and as its output, a control pulse signal whose pulse width is narrow at first but gradually widens is generated as shown in FIG. It is supplied to the main switch MOSFET 4 of the side drive circuit. This controls the main switch MOSFET 4 to turn on and off.

As described above, when the switching power supply is started, the time constant circuit of the pulse width limiting circuit 19 functions as a well-known soft start means, and the main switch MOSFET 4 controls the control pulse signal whose duty ratio gradually increases. Is controlled by Therefore, the output voltage of the output terminals 10 and 11 of the secondary side output circuit gradually increases, so that a problem due to an inrush current or the like is eliminated.

Next, the output voltage of the output terminals 10 and 11 of the secondary side output circuit is increased by the turn-on and turn-off control of the MOSFET 4 for the main switch, and the output voltage detection circuit 18 determines the output voltage in advance by the error detection circuit 21. When an error from the output voltage V0 is detected, the photocoupler 2
Current flows through the second photodiode 22a to emit light, and the phototransistor 22b is turned on. Thereby, the detection signal of the output voltage detection circuit 18 applied to the second plus input terminal of the comparator 33 is determined by the on-resistance of the phototransistor 22b and the parallel resistance of the resistor 24 and the resistance of the resistor 23. Up to voltage. The voltage after this rise is higher than the pulse width limit signal shown in waveform B.

Therefore, after the output voltage rises, the comparator 33 compares the detection signal of the output voltage detection circuit 18 of the waveform C with the triangular wave signal of the waveform A instead of the pulse width limitation signal of the waveform B. Then, the comparator 33 generates a control pulse signal having a predetermined duty ratio as its output, performs PWM control of the main switch MOSFET 4 so that the output voltage V0 is stabilized, and the switching power supply enters a normal operation state. .

Next, the operation of the switching power supply when the operation is stopped will be described with reference to FIGS. FIG.
At time t3, for example, when the operation of the switching power supply device is instructed to stop and no DC input voltage is supplied to the input terminals 1 and 2, the output terminals 10 and 10
11, the detection signal from the output voltage detection circuit 18 also decreases as shown in FIG. 3B, and the duty ratio of the control pulse signal output from the control pulse signal generation circuit 20 is increased. Thus, an attempt is made to maintain the output voltage V0.

However, when the operation is stopped, in summary, the comparator 33 replaces the detection signal from the output voltage detection circuit 18 with the pulse width limitation signal from the pulse width limitation circuit 19 and the triangular wave signal from the triangle wave generator 32. Based on
Operate to generate a control pulse signal.

Hereinafter, the operation when the operation is stopped will be described in detail. If the DC input voltage is not supplied to the input terminals 1 and 2 by the operation stop instruction at the time t3, the input monitoring circuit (not shown) detects that the DC input voltage has been lost.
A low-level operation stop signal is output from the input monitoring circuit and supplied to the control terminal 31 of the pulse width limiting circuit 19. As a result, the transistor 30 is turned on, the resistor 29 is connected in parallel to the capacitor 27 of the time constant circuit, and the charge of the capacitor 27 is
Is discharged with the time constant determined by

Therefore, as shown in FIG. 3B, the pulse width limiting signal of the pulse width limiting circuit 19 gradually increases based on the above-mentioned discharge time constant, and becomes higher than the detection signal of the output voltage detecting circuit 18. , Finally the resistor 29 and the resistor 2
The voltage settles down to the divided voltage determined by the resistance value of the resistor 26 and the resistance value of the parallel connection circuit of the series connection body 8 and the resistor 25.

Therefore, after the pulse width limitation signal of the pulse width limitation circuit 19 becomes higher than the detection signal of the output voltage detection circuit 18 shown in the waveform C, the comparator 33 replaces the detection signal with the waveform B of the waveform B. The pulse width limiting signal is compared with the triangular wave signal of waveform A. And comparator 3
3 generates, as its output, a control pulse signal whose duty ratio gradually decreases, and outputs a MOSFET for a main switch.
4 is subjected to PWM control. As a result, as shown in FIG. 3A, the output voltage of the switching power supply gradually decreases from the time t3 when the operation stop is instructed, and at time t4, the supply of the control pulse signal stops, and the output voltage becomes zero. Become.

As described above, when the switching power supply is started, the time ratio of the control pulse signal is gradually increased by the soft start means by the time constant circuit of the pulse width limiting circuit, so that the soft start is performed. Problems due to inrush current and the like can be prevented.

Also, by simply adding the resistor 29 and the semiconductor switch element 30 to the above-mentioned soft start means, when the operation of the switching power supply is stopped, the output voltage is reduced by gradually reducing the duty ratio of the control pulse signal. After gradually lowering the energy from the output, the control pulse signal is stopped.
As a result, no ON voltage is generated at the gate of the rectifying switch MOSFET 6, and the generation of a surge voltage due to self-excited switching between the MOSFETs 6 and 7 of the synchronous rectifier can be suppressed. Also, the undershoot phenomenon in which the output voltage becomes negative can be suppressed.

As described above, the embodiment of the present invention has been described. However, the present invention is not limited to this, and various modifications and applications are possible.

For example, in the above embodiment, the pulse width limiting circuit 19 is composed of the resistors 25, 26, 28, 29, the capacitor 27, and the transistor 30,
Another configuration that performs the same operation may be used.

[0039]

According to the present invention, the self-oscillation phenomenon of the synchronous rectifier caused by the output energy when the operation of the switching power supply is stopped is reduced by using the MOSF of the synchronous rectifier as in the prior art.
There is no need to connect a semiconductor switch element to the ET gate circuit, and this can be prevented by adding a small number of components. Further, since the operation is stopped after the output energy becomes small, the undershoot phenomenon in which the output voltage decreases to the negative side can be prevented.

Further, it is possible to prevent a problem due to an inrush current or the like at the time of starting the switching power supply device.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a switching power supply according to the present invention.

2 (a), 2 (b) and 2 (c) are signal timing charts of respective parts at the time of startup in the switching power supply device of FIG.

3 (a), 3 (b) and 3 (c) are signal timing charts of respective units when operation of the switching power supply device of FIG. 1 is stopped.

FIG. 4 is a circuit diagram showing an example of a conventional switching power supply device.

5 is a signal timing chart of each unit when operation of the switching power supply device of FIG. 5 is stopped.

FIG. 6 is a circuit diagram showing another example of a conventional switching power supply device.

FIG. 7 is a circuit diagram showing still another example of a conventional switching power supply device.

[Explanation of symbols]

Reference Signs List 4 MOSFET for main switch (primary side main switch) 6 MOSFET for rectification switch 7 MOSFET for reflux switch 10 Output terminal 11 Output terminal 14 Control circuit 18 Output voltage detection circuit (output voltage detection means) 19 Pulse width limitation circuit (Pulse width limitation) Means) 20 Control pulse signal generation circuit (control pulse signal generation means) 25 Resistance (first resistance; part of time constant circuit) 26 Resistance (second resistance; part of time constant circuit) 27 Capacitor (time constant) (Part of the circuit) 28 Resistance (third resistance; part of time constant circuit) 29 Resistance (fourth resistance) 30 Transistor (semiconductor switch element) 31 Control terminal 32 Triangular wave generator (triangular wave generating means) 33 Comparator

Continuation of the front page (72) Inventor Yoshifumi Shimizu 1-11-15 Higashi Gotanda Shinagawa-ku, Tokyo Radio building Densai Lambda Co., Ltd. F term (reference) 5H730 AA20 BB23 DD04 EE13 FD01 FF02 FF19 FG05 XC09 XC14

Claims (4)

[Claims] 1. A switching power supply device for performing PWM control of a primary side main switch by a control pulse signal according to an output voltage, wherein the control pulse signal whose duty ratio gradually decreases when the operation of the switching power supply device is stopped is generated. A switching power supply device comprising: 2. The control circuit, comprising: a triangular wave generating unit; an output voltage detecting unit detecting an output voltage of the switching power supply; a pulse width limiting unit generating a pulse width limiting signal that changes when the operation is stopped; Control pulse signal generating means for receiving the triangular wave signal from the triangular wave generating means, the detection signal from the output voltage detecting means, and the pulse width limiting signal from the pulse width limiting means, and generating the control pulse signal. A control pulse signal having a predetermined duty ratio based on a comparison between a triangular wave signal from the triangular wave generating means and a detection signal from the output voltage detecting means during a normal operation of the switching power supply; When the operation of the switching power supply is stopped, the triangular wave signal from the triangular wave generating means and the pulse Based on the comparison of the pulse width limiting signal from limiting means, the switching power supply device according to claim 1, wherein generating a control pulse signal duty ratio is reduced gradually. 3. The pulse width limiting means includes a series connection of a first resistor and a second resistor, and a series connection of a capacitor and a third resistor connected in parallel to the first resistor. A time constant circuit, and a series connection of a semiconductor switch element and a fourth resistor connected in parallel to the capacitor. The semiconductor switch element has a control electrode to which an operation stop signal of the switching power supply is supplied. To turn on the first constant of the time constant circuit.
3. The switching power supply device according to claim 2, wherein a pulse width limiting signal that changes at the time of stopping the operation is supplied to the control pulse signal generating means from a series connection point of the resistor and the second resistor. 4. The switching power supply according to claim 3, wherein the time constant circuit functions as a soft start unit when the switching power supply is started.