JP2002010634A - Switching power supply device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a voltage stress on a load element in a DC/DC converter which makes a synchronous rectification of its switching voltage by lowering its output voltage by stopping or controlling its switching circuit in high-speed when the output voltage extraordinarily rises. SOLUTION: An output signal Vo.v is transmitted when an overvoltage output detection circuit 18 detects an overvoltage output. With this a rectifier control circuit 14 immediately lowers an output voltage Vo by stopping its rectifier function by cutting off FET 5, 6 of a rectifier circuit 4 by an output signal VD. Further a PMW control circuit 15 continuously suspends a drive signal Vg of a main switching element 3 by a signal VL from a latch circuit 19 which receives the output signal Vo.v. With this a switching operation of a switching circuit 22 is stopped, an AC voltage supply to the secondary side of a transformer 2 is suspended, and time delay from overvoltage detection to output voltage lowering control is gone.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC-DC converter for synchronously rectifying an AC voltage generated by a switching element such as an FET, and more particularly to detecting an output overvoltage of the DC-DC converter and applying the detected overvoltage to a load. The present invention relates to a switching power supply device having a function of stopping the supply of power.

[0002]

2. Description of the Related Art A switching power supply of this type is provided with an output voltage protection circuit in a switching power supply comprising a switching circuit and a synchronous rectification circuit, as disclosed in, for example, Japanese Patent Application Laid-Open No. HEI 8-19251. Things are known. In this technology, the output voltage protection circuit monitors the duration of the control pulse of the main switching element, determines that the output becomes overvoltage if the control pulse exceeds a predetermined duration, and sends a stop signal to the main switching element. Feeding and switching operation are stopped. Further, Japanese Unexamined Patent Publication No.
In the non-isolated synchronous rectification type DC-DC converter without a switching circuit and a transformer, if an overvoltage of the output is detected, a commutation transistor (a so-called main switching element is turned on when the main switching element is off). A technique has been reported in which an output voltage is reduced by forcibly turning on a transistor that flows commutation energy) to short-circuit between terminals.

FIG. 3 is an example of a circuit diagram of a synchronous rectification type switching power supply according to the prior art. In the figure, a switching power supply 38 includes a switching circuit 40 including a DC power supply 23, a transformer 25, and a main switching element 24, resistors 26 and 27, and a rectifying FET2.
A rectifier circuit 28 of a synchronous rectification system including the flywheel diodes 9 and 30 and a flywheel diode 31, a smoothing filter 32 including a choke coil 33 and a capacitor 34 for smoothing a DC voltage, an error amplifier 35, and a PWM control circuit 37
And an output overvoltage detection circuit 36. The switching power supply 38 is connected to a load 39 such as a computer.

Next, a switching power supply 3 shown in FIG.
8 will be described. Main switching element 24
Performs a switching operation according to a control signal Vg output from the PWM control circuit 37, generates an AC voltage on the primary side of the transformer 25, and supplies a predetermined AC voltage to the secondary side. Then, when the AC voltage supplied to the secondary side of the transformer 25 is a positive voltage (that is, when the dot mark on the secondary side shown in the drawing has a positive potential), the rectifying FET 3
The gate of 0 is turned on. Further, when the AC voltage supplied to the secondary side is a voltage in the negative direction (that is, when the dot mark on the secondary side has a negative potential), the rectifying FET 2
The gate of No. 9 is turned on. In this way, by controlling ON / OFF of the respective FETs 29 and 30,
AC voltage is synchronously rectified. The rectified DC voltage is DC-smoothed by a smoothing filter 32 including a choke coil 33 and a capacitor 34, and the load 3
9 is supplied with a DC output voltage Vo.

[0005] The error amplifier 35 monitors the DC-smoothed output voltage Vo, and sends a feedback signal to the PWM control circuit 37 so that the output voltage is always constant. Then, the PWM control circuit 37 outputs the main switching element 2 based on the input feedback signal.
4 to control the pulse width of the control signal Vg to generate a stabilized constant voltage output voltage Vo. Further, the output overvoltage detection circuit 36 monitors the DC-smoothed output voltage Vo, similarly to the error amplifier 35. For example, it is assumed that the output voltage of the switching power supply 38 rises for some reason, and the output voltage Vo reaches the detection threshold of the output overvoltage detection circuit 36. Then, the output overvoltage detection circuit 36 that has detected that the output voltage Vo has risen sends out a detection signal Vdet to the PWM control circuit 37. And PWM
The control circuit 37 uses the input detection signal Vdet to
The control signal Vg to the main switching element 24 is stopped, and the switching operation of the main switching element 24 is stopped.
Alternatively, the duty (that is, the ON width) of the control signal Vg to the main switching element 24 is reduced to reduce the output voltage Vg.
In some cases, control is performed so as to suppress the rise of o.

[0006]

However, according to the above-mentioned conventional overvoltage protection method including the above-mentioned prior publication, even when the overvoltage protection circuit is activated when the output voltage is abnormally increased, the load on the load side of a computer or the like is reduced. There is a possibility that a voltage stress more than necessary is applied to the circuit. The reason is that when the output overvoltage detection circuit in the switching power supply detects an abnormal rise in the output voltage and then tries to stop the switching operation of the switching circuit, the destination of the detection signal of the output overvoltage detection circuit is PWM. The control circuit, and the signal source for stopping the switching operation is PW
It depends on being in the M control circuit.

That is, the operation order of the output voltage suppression mode at the time of output overvoltage is as follows: output overvoltage detection → command for stopping the drive signal of the main switching element from the PWM control circuit →
Stop operation of main switching element (or voltage drop control)
→ The operation of the synchronous rectifier circuit stops → The output voltage drops. That is, since a delay time occurs between the detection of the output overvoltage of the operation and the decrease of the output voltage of the operation, the output voltage value when the output voltage actually starts to decrease by the overvoltage control is detected by the output overvoltage detection circuit. The output voltage value is higher than the output voltage value at the time. Therefore, there occurs a problem that an overvoltage exceeding the control by the output overvoltage detection circuit is applied to the circuit element on the load side. According to the technology disclosed in Japanese Patent Application Laid-Open No. H11-187651, an overvoltage is output to forcibly turn on a commutation transistor to short-circuit between terminals to prevent overvoltage. There is a possibility that current stress may be applied to the element.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a DC / DC converter for performing synchronous rectification of a switching voltage, in which the switching circuit is stopped at a high speed when the output voltage rises abnormally. Another object of the present invention is to control the output voltage so as to prevent the voltage stress on the load element.

[0009]

In order to solve the above-mentioned problems, a switching power supply of the present invention comprises a switching circuit for generating an AC voltage from a DC voltage by a switching element, and an AC voltage generated by the switching circuit. In a switching power supply device including a synchronous rectification circuit that converts a DC voltage into a DC voltage by synchronous rectification, and an overvoltage protection circuit that reduces the output voltage when the output voltage is overvoltage, the overvoltage protection circuit detects an overvoltage of the output voltage. At this time, the operation of the switching circuit is stopped and the operation of the synchronous rectifier circuit is stopped at the same time.

That is, according to the switching power supply of the present invention, the switching circuit and the synchronous rectifier circuit are simultaneously stopped immediately when the output voltage becomes excessive. For this reason, there is no delay time from the detection of the overvoltage to the stop of the switching circuit to the reduction of the output voltage, so that the output voltage does not exceed the overvoltage threshold instantaneously. Therefore, there is no possibility that the circuit element on the load side receives a voltage stress due to the overvoltage of the output voltage.

Further, the switching power supply of the present invention comprises:
In the above invention, the switching circuit includes switching control means for sending a switching signal for controlling ON / OFF width of the switching element, and the synchronous rectification circuit controls the rectification timing of the rectifying element for synchronous rectification. Rectification control means for transmitting the synchronous rectification signal from the rectification control means at the same time as stopping the transmission of the switching signal from the switching control means when the overvoltage of the output voltage is detected. Signal stopping means for causing the signal to stop.

That is, according to the switching power supply of the present invention, when the switching power supply is operating normally, the switching circuit and the synchronous rectification circuit function normally and send a constant voltage to the output. In an overvoltage state, the operation of the switching element of the switching circuit is stopped, and the cutoff control of the rectifying element of the synchronous rectifier circuit is performed, so that the output voltage immediately decreases.
Therefore, the response speed of the protection operation at the time of the output overvoltage is increased, and the abnormally high voltage is not applied to the load element.

Further, the switching power supply of the present invention comprises:
In the above invention, the overvoltage protection circuit, when detecting an overvoltage of the output voltage, includes a detection state holding unit that holds a detection state of the overvoltage, the switching control unit, based on a command signal from the detection state holding unit, The transmission of the switching signal is continuously stopped.

That is, according to the switching power supply of the present invention, if the output voltage exceeds the overvoltage threshold even for a moment, this state is maintained and the operation of the switching circuit is stopped continuously. Therefore, it is possible to avoid an unstable control state in which the operation / stop of the switching circuit is chattered due to an instantaneous increase in the output voltage.

Further, the switching power supply of the present invention comprises:
In the above invention, the overvoltage protection circuit includes a mask time unit that does not detect an overvoltage of the output voltage until a predetermined time from the start of the switching power supply, and the overvoltage protection circuit includes an overshoot voltage at the start of the switching power supply. Thus, the operation of the switching circuit and the synchronous rectifier circuit is not stopped.

That is, according to the switching power supply of the present invention, even if an overshoot voltage is generated at the time of starting the switching power supply, there is no possibility that the overvoltage protection operation is activated and the switching circuit is stopped. Therefore, in an operation such as a normal start / stop operation, the operation can be performed with the same usability as the conventional switching power supply device.
In each of the above-mentioned inventions, the rectifying element constituting the synchronous rectifying circuit is preferably an FET.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of a switching power supply according to the present invention will be described below in detail with reference to the drawings. FIG. 1 is a circuit diagram of a synchronous rectification type switching power supply device according to an embodiment of the present invention. Referring to FIG. 1, a switching power supply device 21 according to the present invention includes a switching circuit 22, a rectifier circuit 4 of a synchronous rectification system, a smoothing filter 10, and various control circuits 41, and supplies power to a load 39 such as a computer. .

The switching circuit 22 includes the DC power supply 1 and the F
The rectifier circuit 4 includes a main switching element 3 such as an ET and a transformer 2 for converting a desired AC voltage.
It comprises synchronous rectification FETs 5 and 6, resistors 7 and 8, and a flywheel diode 9. The smoothing filter 10 for smoothing the DC voltage includes a choke coil 1.
1 and a capacitor 12.

Further, the various control circuits 41 monitor the output voltage Vo smoothed by the smoothing filter 10, send an output voltage correction signal to the PWM control circuit 15, and provide a control signal to the rectification control circuit 14. An error amplifier 13 having a sending function, a rectification control circuit 14 for controlling the gates of rectification FETs 5 and 6 of the rectification circuit 4, a PWM control circuit 15 for controlling the switching of the main switching element 3, and monitoring the output voltage Vo When the output voltage Vo rises to an abnormal voltage, an overvoltage detection signal is sent to the rectification control circuit 14, the latch circuit 19, and the ALM circuit 20. The output overvoltage detection circuit 17 includes the timer circuit 16 and the output voltage detection circuit 17. A detection circuit 18 and a PWM circuit for detecting an output overvoltage
Latch circuit 19 for sending a circuit stop signal to control circuit 15
And an ALM circuit 20 for notifying an alarm to the outside of the switching power supply 21 when an output overvoltage is detected.

Next, the switching power supply 2 shown in FIG.
The operation 1 will be described with reference to a time chart waveform shown in FIG. The PWM control circuit 15 starts operating by an arbitrary voltage supplied from the DC power supply 1, controls the switching operation of the main switching element 3, and
It is known as a general switching power supply circuit method that an AC voltage is generated on the secondary side of the power supply circuit, and a desired DC output voltage Vo is obtained by feedback control of the error amplifier 13 via the rectifier circuit 4 and the smoothing filter 10. Therefore, a detailed description of the operation is omitted.

In FIG. 2, the horizontal axis represents the passage of time,
Each of the waveforms (a) to (g) represents the waveform of each part indicated by the symbol in FIG. That is, (a) is the output voltage Vo, (b) is the output signal VFB of the error amplifier 13,
(C) is the output signal VD of the rectification control circuit 14, (d) is the output signal Vo.v of the output overvoltage detection circuit 18, and (e) is AL.
The output signal VALM of the M circuit 20 and (f)
(G) represents the output signal Vg of the PWM control circuit 15. Therefore, in the following description, in order to facilitate comparison with FIG.
Symbols such as (a), (b), and (c) are given so that the waveform can be specified.

First, when the DC power supply 1 is turned on and the output voltage Vo (a) increases from time t0, the error amplifier 13 sends a feedback signal to the PWM control circuit 15 so as to send a maximum pulse. Send out. At this time, the output voltage V
o (a) is a rising state, and the rectification control circuit 14
Means that the input signal VFB (b) from the error amplifier 13 is VF
Since the gate stop control of the rectification FETs 5 and 6 is not performed unless the current reaches B ″, the output signal VD (c) of the rectification control circuit 14 outputs An H-level signal is transmitted so that stop control is not performed on each gate of the FETs 5 and 6.

When the output voltage Vo (a) rises, an overshoot phenomenon in which the output voltage Vo (a) exceeds the set output voltage Va due to a delay in the transmission of the feedback signal from the error amplifier 13 to the PWM control circuit 15 occurs. I do. However, at time t1, the output signal VFB (b) of the error amplifier 13 goes beyond the range of the triangular wave, which is the reference pulse width source of the PWM control circuit 15, and shifts to the region of VFB ″. The rectification control circuit 14 to which the output VFB '' has been input changes the output signal VD (c) to L.
Drop to level. As a result, the rectification control circuit 14
The gates of the rectifying FETs 5 and 6 are controlled, and the FETs 5 and F
Since the ET 6 is cut off, the rectifying circuit 4 stops the rectifying function, and the output voltage Vo (a) decreases rapidly.

Then, at time t2, the output DC voltage Vo
When (a) reaches the set output voltage Va, the output signal VFB ″ (b) of the error amplifier 13 shifts within the range of the triangular wave and outputs a level signal of VFB ′. Then, the rectification control circuit 14 outputs the output signal VFB ′ (b) of the error amplifier 13.
Is detected, and its own output signal VD (c) is detected.
Is shifted from the L level to the H level, and the cutoff state of the rectifying FETs 5 and 6 is eliminated. Thereby, the rectifier circuit 4 restarts the synchronous rectification operation and supplies the output voltage Vo (a) stabilized by the feedback control of the error amplifier 13. Therefore, even if the output voltage Vo overshoots at the time of startup, the error amplifier 13
The response speed is increased by stopping the FETs 5 and 6 in the rectifier circuit 4 depending on the level of the output signal VFB (b), so that the overshoot of the output voltage Vo can be suppressed small.

The timer circuit 16 in the output overvoltage detecting circuit 18 is set so that the output voltage detecting circuit 17 does not operate within the time t2. That is, the output signal Vo.v (d) of the output overvoltage detection circuit 18 keeps the alarm mask time tmask until a predetermined time beyond the time t2.
Is provided, so that the output voltage detection circuit 17 does not detect an overvoltage due to the overshoot voltage at the start of the output voltage.

At time t2, the error amplifier 13 detects the output voltage Vo, and outputs the output correction signal to the PWM control circuit 15 so that the output voltage becomes the set output voltage.
And the switching circuit supplies a stable output voltage. At this time, the output signal VFB (b) of the error amplifier 13 is controlled within the range of VFB ', and the output signal VD (c) of the rectification control circuit 14 receives the output signal VFB' (b) from the error amplifier 13. As a result, the H level is output, and the rectifying FETs 5 and 6 perform the rectifying operation by the AC voltage generated on the secondary side of the voltage conversion transformer 2.

At time t3, the output voltage Vo
When (a) rises and exceeds the range of the triangular wave at time t4, the output signal VFB '(b) of the error amplifier 13 becomes PWM
A signal (VFB ″) for reducing power transmission to the secondary side of the transformer 2 is sent to the control circuit 15. At this time, the rectification control circuit 14 identifies the VFB ″ signal, inverts the level of the output signal VD (c) of the rectification control circuit 14 from the H level to the L level, and controls the gates of the rectification FETs 5 and 6. The FETs 5 and 6 are cut off.
As a result, the rectification circuit 4 loses the rectification function by turning off the rectification FETs 5 and 6, and the output DC voltage Vo
(A) decreases.

Next, at time t5, the output signal (b) of the error amplifier 13 which has been VFB ″ shifts to the region of VFb ′ within the range of the triangular wave, so that the rectification control circuit 14 has an error. The output signal VFB 'of the amplifier 13 is input, the output signal VD (c) of the rectification control circuit 14 becomes H level, and the cutoff state of each gate of the rectification FETs 5 and 6 is eliminated. As a result, the synchronous rectification operation is restarted, so that the operation is performed in a direction to be controlled and stabilized at the set output voltage Vo (a).

Next, at time t6, the PWM control circuit 15 does not operate normally due to a failure in the internal circuit of the switching power supply 21 and the output voltage Vo (a).
Is assumed to start rising. In this case, the output voltage V
o (a) continues to rise, and at time t7, the output voltage V
When o (a) reaches the overvoltage detection value (detection threshold) Vdet of the output overvoltage detection circuit 18, the output overvoltage detection circuit 18
Detects an output abnormality and sends out an output signal Vo.v (d).

Then, the rectification control circuit 14 receives the output signal Vo.v (d) from the output overvoltage detection circuit 18 and
The level of the output signal VD (c) is inverted from H level to L level. Further, the ALM circuit 20 receives the output signal Vo.v (d) from the output overvoltage detection circuit 18, outputs the output signal VALM (e), and sends an alarm signal to the outside. Further, the latch circuit 19 includes the output overvoltage detection circuit 1.
8 and outputs the output signal VL (f) to the PWM control circuit 15. Then, the PWM control circuit 15 receiving the output signal VL (f)
Sends an output signal Vg (g) for stopping the switching operation of the main switching element 3.

That is, at the time t7, the rectification control circuit 14, to which the Vo.v signal (d) is input from the output overvoltage detection circuit 18, inverts the level of the output signal VD (c) to L level and performs rectification. Rectifying FET 5 and FE in circuit 4
Since the gate of T6 is controlled to cut off the FETs 5 and 6, the output voltage Vo (a) immediately drops due to the rectification function being lost. Similarly, the ALM circuit 20 to which the Vo.v signal (d) is input from the output overvoltage detection circuit 18 outputs the VALM signal (e) as an alarm signal from the switching power supply device 21 to the outside, and outputs the Vo.v signal. (D)
The signal level of the VALM signal (e) is held as an alarm signal by the input of the signal.

Further, the latch circuit 19 also outputs the latch signal VL (f) by inputting the Vo.v signal (d) from the output overvoltage detecting circuit 18 and outputs the latch signal VL (f).
Is received, the PWM control circuit 15 stops the drive signal Vg (g) of the main switching element 3. by this,
The switching operation of the switching circuit 22 stops,
No AC voltage is supplied to the secondary side of the transformer 2.
Also, the signal level of the VL signal (f) from the latch circuit 19 is fixed by the input of the Vo.v signal (d) from the output overvoltage detection circuit 18 as in the case of the VALM signal.

As described above, the output voltage Vo (a) decreases. At time t8, the output voltage Vo (a) falls below the overvoltage detection value (detection threshold) Vdet of the output overvoltage detection circuit 18, and the output overvoltage detection circuit 18 Even if the output Vo.v signal (d) is inverted to the H level, the P.P.
Since the operation of the WM control circuit 15 is stopped by the input of the latch signal VL (f), the output voltage Vo (a) is not restored. Further, an alarm signal VALM (e) notifying the occurrence of an abnormality to the outside of the switching power supply 21
Is output from the ALM circuit, so that the situation can be confirmed and a predetermined response can be taken.

[0034]

As described above, the switching power supply circuit of the present invention uses the output voltage detection circuit, and the control for stopping the gate of the FET in the synchronous rectification circuit directly by the output voltage level of this detection circuit. It is carried out. Therefore, when the output voltage abnormally rises due to a circuit failure or the like, the response until the operation of the synchronous rectifier circuit stops without any delay due to signal transmission as in the related art becomes faster. Voltage stress can be reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a synchronous rectification type switching power supply device according to an embodiment of the present invention.

FIG. 2 is a time chart waveform showing an operation of the switching power supply device shown in FIG.

FIG. 3 is an example of a circuit diagram of a synchronous rectification type switching power supply device according to the related art.

[Description of Signs] 1,23 DC power supply 2,25 Transformer 3,24 Main switching element (FET) 4,28 Rectifier circuit 5,6,29,30 FET 7,8,26,27 Resistance 9,31 Flywheel diode 10, 32 Smoothing filter 11, 33 Choke coil 12, 34 Capacitor 13, 35 Error amplifier 14 Rectification control circuit 15, 37 PWM control circuit 16 Timer circuit 17 Output voltage detection circuit 18, 36 Output overvoltage detection circuit 19 Latch circuit 20 ALM circuit 21, 38 switching power supply device 22, 40 switching circuit 39 load 41 various control circuits

Claims (5)

[Claims] 1. A switching circuit for generating an AC voltage from a DC voltage by a switching element, a synchronous rectifier circuit for converting an AC voltage generated by the switching circuit into a DC voltage by synchronous rectification, and a synchronous rectifier circuit when an output voltage is overvoltage. An overvoltage protection circuit that reduces an output voltage, wherein the overvoltage protection circuit stops the operation of the switching circuit and detects the operation of the synchronous rectification circuit when detecting the overvoltage of the output voltage. A switching power supply characterized by being stopped. 2. The switching circuit includes switching control means for transmitting a switching signal for controlling ON / OFF width of the switching element, and the synchronous rectification circuit controls rectification timing of a rectifying element for synchronous rectification. A rectification control unit for transmitting a synchronous rectification signal; wherein the overvoltage protection circuit stops transmitting a switching signal from the switching control unit when detecting an overvoltage of the output voltage, and simultaneously outputs a synchronous signal from the rectification control unit. The switching power supply device according to claim 1, further comprising a signal stopping unit that stops sending the rectified signal. 3. The overvoltage protection circuit includes a detection state holding unit that holds an overvoltage detection state when the overvoltage protection circuit detects an overvoltage of an output voltage, and the switching control unit includes a detection state holding unit. 3. The switching power supply according to claim 2, wherein the transmission of the switching signal is continuously stopped based on the command signal from the switching power supply. 4. The overvoltage protection circuit includes a mask time unit that does not detect an overvoltage of an output voltage until a predetermined time from the start of the switching power supply, and the overvoltage protection circuit includes an overvoltage at the time of startup of the switching power supply. The switching power supply according to any one of claims 1 to 3, wherein the operation of the switching circuit and the synchronous rectification circuit is not stopped by the shoot voltage. 5. The switching power supply according to claim 1, wherein the rectifying element that performs synchronous rectification is an FET.