CN108123429B - Overvoltage protection circuit - Google Patents

Abstract

The present invention proposes an overvoltage protection circuit, which can distinguish whether the overvoltage state of the output voltage of the switchable power supply device is caused by internal feedback failure or external power supply, and can use the time difference of starting overvoltage protection to avoid short-term and harmless The external power supply affects the normal operation of the switching power supply device, but when the internal feedback of the switching power supply device fails, the switching power supply device can be stopped in real time to achieve protection.

Description

Overvoltage Protection Circuit

technical field

The present invention relates to an overvoltage protection circuit, in particular to an overvoltage protection circuit suitable for a switching power supply device.

Background technique

A general switching power supply device (Switching power supply) will use an overvoltage protection circuit to control the Pulse Width Modulation (PWM) in the switching power supply device when the output terminal of the switching power supply device exhibits an overvoltage. Modulation, PWM) control circuit stops outputting the pulse width modulation signal, and then reduces the output voltage of the output terminal, so as to protect the internal circuit of the switching power supply device and the internal circuit of the external system of the output terminal from overvoltage, avoiding the two Either one of them is damaged.

The output end will exhibit overvoltage due to two situations. One is caused by the failure of the internal circuit of the switching power supply device, such as when the feedback circuit fails; the other is caused by an external system, such as a motor with a motor. The back EMF feedback caused by the external system during deceleration. However, the traditional overvoltage protection circuit cannot distinguish between the two, so that when the user operates the external system normally and causes back electromotive force feedback, the traditional overvoltage protection circuit will still control the switching power supply device to immediately reduce its output voltage The size of the overvoltage protection is implemented, which will cause the external system to shut down and cannot be used normally, causing users a lot of trouble.

Contents of the invention

The purpose of the present invention is to provide an overvoltage protection circuit, which can distinguish the overvoltage caused by the two situations, and adopt two different methods to perform overvoltage protection.

The invention proposes an overvoltage protection circuit, which is suitable for a switching power supply device. It can use the time difference of starting overvoltage protection to distinguish whether the output voltage overvoltage state of the switching power supply device is caused by internal feedback failure or external power supply, so as to avoid the short-term and harmless external power supply from affecting the normal operation of the switching power supply device. However, when the internal feedback of the switching power supply device fails, the operation of the switching power supply device can be stopped in real time to achieve protection.

In order to make the above and other objects, features and advantages of the present invention more comprehensible, preferred embodiments are described below in detail with accompanying drawings.

Description of drawings

FIG. 1 is a schematic diagram of a coupling relationship of an overvoltage protection circuit.

Detailed ways

FIG. 1 is a schematic diagram of a coupling relationship of an overvoltage protection circuit. In FIG. 1, the switching power supply device 100 includes a noise filter 110, an AC-DC conversion circuit 120, a power stage (Power stage) 130, a transformer 140, a rectifier circuit 150, an energy storage unit 160, a feedback circuit 170 and A signal isolation unit 180 . The primary coil of the transformer 140 can be coupled to the input voltage VIN through the power stage 130 , the AC-DC conversion circuit 120 and the noise filter 110 in sequence. The noise filter 110 can be an electromagnetic interference filter, and users can decide whether to use the noise filter 110 or not.

In addition, the AC-DC conversion circuit 120 can be a bridge rectifier. As for the power stage 130, it has a PWM control circuit 132 and a power transistor 134 used as a switch. The power transistor 134 can be connected in series with the primary side coil of the transformer 140 , and whether to allow current to pass through the primary side coil can be determined by controlling the on/off state of the power transistor 134 . The PWM control circuit 132 is used to generate a PWM signal and output the PWM signal to the control terminal of the power transistor 134 to control the switching frequency of the power transistor 134 between on and off states.

After the output of the secondary side coil of the transformer 140 is rectified and filtered by the rectifier circuit 150 and the energy storage unit 160, it can be used as the output voltage VOUT of the switching power supply device 100 and provided to the external system ( not shown). Furthermore, whether to use the rectifier circuit 150 and the energy storage unit 160 can also be determined according to design requirements. In addition, the signal isolation unit 180 can be an optocoupler (Photo coupler), which can transmit the feedback signal generated by the feedback circuit 170 to the PWM control circuit 132, so that the PWM control circuit 132 can adjust the PWM signal accordingly. Cycle (Dutycycle). Therefore, when the output terminal 190 of the switching power supply device 100 has an overvoltage, the PWM control circuit 132 can reduce the output voltage VOUT according to the feedback signal from the signal isolation unit 180 for overvoltage protection.

In addition, in FIG. 1 , the overvoltage protection circuit 200 includes an impedance 210 , an impedance 220 , an energy storage unit 230 , a switch unit 240 , an overvoltage detection unit 250 , a voltage sampling unit 270 and a signal isolation unit 280 . The impedance 210 has a first resistance, the impedance 220 has a second resistance, and the second resistance is greater than the first resistance. One end of the energy storage unit 230 is coupled to one end of the impedance 210 and one end of the impedance 220 , and the other end of the energy storage unit 230 is coupled to the reference potential SGND. Wherein, the reference potential SGND is the ground potential.

The switch unit 240 has a first end 241 , a second end 242 and a third end 243 , the second end 242 is coupled to the other end of the impedance 210 , and the first end 241 is coupled to one end of the impedance 220 . The overvoltage detection unit 250 is coupled to the output terminal 190 of the switching power supply device 100, the first terminal 241 of the switch unit 240, and the other end of the impedance 220 to receive the output voltage VOUT on the output terminal 190 and determine the output voltage Whether VOUT exceeds the first default value.

The voltage sampling unit 270 has a fourth terminal 271 and a fifth terminal 278-5, the fifth terminal 278-5 is coupled to the third terminal 243 of the switch unit 240, and the fourth terminal 271 is coupled to the transformer of the switching power supply device 100. One end of the secondary side coil is used to detect whether the output voltage of the secondary side coil reaches the second default value, and when it is judged to be yes, it will conduct the fifth end 278-5 and the reference potential SGND, and if it exceeds When the output of the voltage detection unit 250 is a positive voltage, the switch unit 240 can be controlled to conduct the electrical path between the first terminal 241 and the second terminal 242 .

The signal isolation unit 280 is coupled to the first energy storage unit 230 , the impedance 210 , the impedance 220 and the PWM control circuit 132 of the switching power supply device 100 , and determines whether to generate overvoltage protection according to the voltage stored in the energy storage unit 230 The trigger signal TRI is used to control the PWM control circuit 132 to stop outputting the PWM signal to the power transistor 134 by using the overvoltage protection trigger signal TRI.

In this example, the impedance 210, the impedance 220, and the energy storage unit 230 may be a resistor and a capacitor, respectively. Next, the detailed implementation of the rest of the overvoltage protection circuit 200 will be introduced first. As shown in FIG. 1 , the overvoltage detection unit 250 includes a voltage dividing circuit 252 and a comparator 254 . The voltage dividing circuit 252 is coupled between the output terminal 190 of the switching power supply device 100 and the reference potential SGND, and generates a voltage dividing signal according to the voltage of the output terminal 190 . The positive input terminal of the comparator 254 is used to receive the voltage division signal of the voltage dividing circuit 252 , the negative input terminal is used to receive the reference potential Vref, and its output is coupled to the first terminal 241 of the switch unit 240 and the impedance 220 . The voltage dividing circuit 252 can be impedances 252-1 and 252-2, wherein one end of the impedance 252-1 is coupled to the output terminal 190, one end of the impedance 252-2 is coupled to the other end of the impedance 252-1, and one end of the impedance 252-2 is coupled to the other end of the impedance 252-2. The other end is coupled to the reference potential SGND. Both the impedances 252-1 and 252-2 can be resistors.

The switch unit 240 includes a PNP transistor 244 , an impedance 245 and an impedance 246 . The PNP transistor 244 has an emitter, a base, and a collector, and its base is coupled to the fifth end 278 - 5 of the voltage sampling unit 270 through the impedance 245 to determine whether to conduct.

The voltage sampling unit 270 includes a diode 273 , an energy storage unit 274 , a voltage dividing circuit 276 and a voltage control switch 278 . The anode of the diode 273 is coupled to one end of the secondary coil of the transformer 140 . The energy storage unit 274 is coupled between the cathode of the diode 273 and the reference potential SGND. The voltage dividing circuit 276 is coupled between the cathode of the diode 273 and the reference potential SGND, and generates a voltage dividing signal according to the voltage stored in the energy storage unit 274 . The voltage-controlled switch 278 has a fifth terminal 278-5, a sixth terminal 278-6 and a reference terminal R, and the fifth terminal 278-5 is coupled to the switch unit 260, the sixth terminal 278-6 is coupled to the reference potential SGND, and the reference The terminal R is used for receiving the voltage division signal generated by the voltage division circuit 276 . When the voltage of the reference terminal R of the voltage-controlled switch 278 reaches the third default value, the voltage-controlled switch 278 makes the fifth terminal 278-5 and the sixth terminal 278-6 into a conduction state. The voltage dividing circuit 276 can be impedances 276-1 and 276-2. One end of the impedance 276-1 is coupled to the cathode of the diode 273, one end of the impedance 276-2 is coupled to the other end of the impedance 276-1, and the other end of the impedance 276-2 is coupled to the reference potential SGND.

The signal isolation unit 280 includes a diode 282 , a signal transmitting part 284 and a signal receiving part 286 . The anode of the diode 282 is coupled to the energy storage unit 230 and the impedance 220 . One end of the signal transmission part 284 is coupled to the cathode of the diode 282 , and the other end is coupled to the reference potential SGND. The signal transmission part 284 is used for generating coupling signals. One end of the signal receiving part 286 is coupled to the PWM control circuit 132 , and the other end is coupled to the reference potential PGND. The signal receiving part 286 is used for receiving the coupling signal and generating the overvoltage protection trigger signal TRI accordingly. Wherein, the signal transmitting part 284 includes a light emitting part which is an optical coupler, and the light emitting part is used to generate a light source as a coupling signal, and the signal receiving part 286 includes a light receiving part which is an optical coupler, and the light receiving part is used to receive The coupling signal formed by the light source. The diode 282 can be used or not depending on requirements.

Next, the detailed operation of the overvoltage protection circuit 200 will be introduced. Firstly, the operation of the voltage protection circuit 200 will be described when the internal circuit of the switching power supply device 100 fails and the output terminal 190 of the switching power supply device 100 exhibits overvoltage. Please refer to FIG. 1 again, when the output voltage VOUT on the output terminal 190 exceeds the first default value, so that the voltage of the divided signal generated by the voltage divider circuit 252 is greater than the voltage of the reference potential Vref, the output of the comparator 254 will appear. Positive saturation.

Based on the above, the overvoltage at the output terminal 190 is caused by the failure of the internal circuit of the switching power supply device 100 , for example, the failure of the feedback circuit 170 . Then the feedback circuit 170 will not be able to generate the feedback signal, so the PWM control circuit 132 will not be able to adjust the duty cycle of the PWM signal according to the feedback signal from the signal isolation unit 180, so the output voltage VOUT cannot be reduced, and the secondary The output voltage of the side coil (Secondary winding) will continue to increase.

When the voltage output by the secondary side coil continuously increases to reach the second default value, and then the voltage of the divided signal generated by the voltage divider circuit 276 reaches the third default value, the fifth terminal 278 of the voltage control switch 278 will be -5 forms a conduction state with the sixth terminal 278-6. Therefore, the third terminal 243 of the switch unit 240 is electrically connected to the reference potential SGND and is pulled down to the ground potential, so that the first terminal 241 and the second terminal 242 of the switch unit 240 are turned on. Since the resistance of the path provided by the PNP transistor 244 is much smaller than the resistance of the impedance 220 , the current output by the comparator 254 chooses the path provided by the PNP transistor 244 to charge the energy storage unit 230 .

According to the above description, the energy storage unit 230 can be fully charged quickly so that the diode 282 can be turned on, so that the signal transmission part 284 can quickly generate a coupled signal. After the signal receiving part 286 receives the coupling signal, it can generate an overvoltage protection trigger signal TRI to the PWM control circuit 132, so that the PWM control circuit 132 can immediately reduce the output voltage VOUT by controlling the operation of the power transistor 134, for overvoltage protection.

Next, the operation of the voltage protection circuit 200 will be described under the condition that the external system causes the output terminal 190 to exhibit overvoltage. Please refer to FIG. 1 again, when the output voltage VOUT on the output terminal 190 exceeds the first default value, so that the voltage of the divided signal generated by the voltage divider circuit 252 is greater than the voltage of the reference potential Vref, the output of the comparator 254 will appear. Positive saturation.

Based on the above, the overvoltage at the output terminal 190 is caused by the external system, for example, the back electromotive force feedback caused by the external system with a motor during deceleration. Therefore, the feedback circuit 170 will normally generate a feedback signal, so that the PWM control circuit 132 can adjust the duty cycle of the PWM signal according to the feedback signal from the signal isolation unit 180, thereby reducing the output voltage VOUT, and the secondary side coil ( Secondary winding) the output voltage will drop.

When the voltage output by the secondary side coil keeps dropping and fails to reach the second default value, and the voltage of the divided signal generated by the voltage divider circuit 276 cannot reach the third default value, the voltage control switch 278 cannot be activated. The fifth terminal 278-5 and the sixth terminal 278-6 form a conduction state. Therefore, the PNP transistor 244 will also be in an off state (Off state), so that the one end 241 and the second end 242 of the switch unit 240 cannot be turned on. Therefore, the current output by the comparator 254 can only charge the energy storage unit 230 through the impedance 220 .

Since the resistance value of the impedance 220 is much greater than the resistance value of the path provided by the PNP transistor 244 , the charging time of the energy storage unit 230 will be longer and the diode 282 will be turned on after a delay for a period of time. Since the signal transmitting part 284 must wait until the diode 282 is turned on to generate the coupling signal, the signal receiving part 286 can only receive the coupling signal after a delay of this period of time, so as to generate the overvoltage protection trigger signal TRI to the PWM control circuit 132 . It can be seen that the PWM control circuit 132 can reduce the output voltage VOUT by controlling the operation of the power transistor 134 after a delay of this period of time, so as to perform over-voltage protection. In other words, in this case, the switching power supply device 100 will not perform the overvoltage protection immediately to cause the external system to fail to operate normally, but will delay the overvoltage protection for a period of time. Of course, those skilled in the art should know that the delay time can be adjusted by changing the resistance value of the impedance 220 or by changing the capacitance value of the energy storage unit 230 .

In an embodiment, the impedance 210 is coupled between the first end 241 of the switch unit 240 and the emitter of the PNP transistor 244 . Since the operation of the circuit in this embodiment is the same as that of the circuit shown in FIG. 1 , it will not be repeated here.

To sum up, in the overvoltage protection circuit of the present invention, the judgment result of the voltage sampling unit can be used to reflect that if the output terminal of the switching power supply device exhibits overvoltage, it is caused by the failure of the internal circuit of the switching power supply device. Yes, it is still caused by the external system. In this way, the voltage sampling unit can determine whether to control the switch unit to provide a path connected in parallel with the impedance, so as to further determine whether to let the current pass through the impedance to charge the energy storage unit, or to let the current pass through the path to charge the energy storage unit . And because the resistance value of the above-mentioned path is smaller than the resistance value of the impedance, the energy storage unit can have two different charging times under two different overvoltage conditions, so that the signal isolation unit can be charged under two different overvoltage conditions. The time to generate the overvoltage protection trigger signal is also different. With such control, the overvoltage protection circuit of the present invention can immediately generate an overvoltage protection trigger signal to control the switchover power supply device to perform overvoltage protection when the internal circuit of the switchover power supply device fails, and cause overvoltage in the external system. The timing of generating the trigger signal for over-voltage protection is delayed when the voltage is low, so that the switching power supply device will not immediately perform over-voltage protection and cause the external system to fail to operate normally.

Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Anyone skilled in the art can make some changes and modifications without departing from the spirit and scope of the present invention. Therefore, this The protection scope of the invention shall be defined by the claims.

Claims (8)

1. An overvoltage protection circuit suitable for a switching power supply device, comprising:

a first impedance having a first resistance value;

a second impedance having a second resistance value, the second resistance value being greater than the first resistance value;

a first energy storage unit, one end of which is coupled to one end of the second impedance, and the other end of which is coupled to a first reference potential;

a switch unit having a first end, a second end and a third end, wherein the first end is coupled to one end of the second impedance;

an overvoltage detection unit, coupled to an output terminal of the switching power supply device, for receiving an output voltage at the output terminal, determining whether the output voltage exceeds a first default value, and outputting a positive voltage to the first terminal of the switch unit if the output voltage exceeds the first default value;

a voltage sampling unit having a fourth end and a fifth end, the fifth end being coupled to the third end of the switching unit, the fourth end being coupled to one end of a secondary side coil of a transformer for detecting whether a voltage output from the secondary side coil reaches a second default value, and conducting the fifth end and the first reference potential if the voltage output from the secondary side coil is judged to be positive, and controlling the switching unit to conduct an electrical path between the first end and the second end if the overvoltage detecting unit outputs the positive voltage; and

a signal isolation unit coupled to the first energy storage unit, the second impedance and a pulse width modulation control circuit, and determining whether to generate an overvoltage protection trigger signal according to the voltage magnitude stored in the first energy storage unit, so as to control the pulse width modulation control circuit to stop generating a pulse width modulation signal by using the overvoltage protection trigger signal.

2. The overvoltage protection circuit of claim 1, wherein one end of the first impedance is coupled to the first energy storage unit, and the other end of the first impedance is coupled to the second end of the switch unit.

3. The overvoltage protection circuit of claim 1, wherein said switching unit comprises:

a PNP transistor having an emitter, a base and a collector, wherein the emitter is coupled to the output of the over-voltage detection unit, the collector is coupled to the second terminal, and the base is coupled to the third terminal; and

and a third impedance coupled between the third terminal and the output terminal of the switching power supply device.

4. The overvoltage protection circuit of claim 3, wherein one end of said first impedance is coupled to said first end of said switch unit, and the other end is coupled to said emitter of said PNP transistor.

5. The overvoltage protection circuit of claim 1, wherein said overvoltage detection unit comprises:

a voltage dividing circuit coupled between the output terminal of the switching power supply device and the first reference potential and generating a voltage dividing signal according to the voltage of the output terminal; and

a comparator, the negative input end of which receives a second reference potential, the positive input end of which receives the voltage dividing signal, and the output of which is coupled to the first end of the switch unit and the other end of the second impedance.

6. The overvoltage protection circuit of claim 1, wherein said voltage sampling unit comprises:

a diode with its anode coupled to one end of the secondary winding;

the second energy storage unit is coupled between the cathode of the diode and the first reference potential;

the voltage division circuit is coupled between the cathode of the diode and the first reference potential and generates a voltage division signal according to the voltage magnitude stored by the second energy storage unit; and

a voltage-controlled switch having the fifth terminal, a sixth terminal and a reference terminal, wherein the sixth terminal is coupled to the first reference potential, the reference terminal receives the voltage-dividing signal, and when the voltage of the reference terminal reaches a third default value, the voltage-controlled switch turns on a path between the fifth terminal and the sixth terminal.

7. The overvoltage protection circuit of claim 1, wherein said signal isolation unit comprises:

a diode, the anode of which is coupled to the first energy storage unit and the second impedance;

a signal transmission part, one end of which is coupled to the cathode of the diode and the other end is coupled to the first reference potential, the signal transmission part is used for generating a coupling signal; and

a signal receiving part, one end of which is coupled with the pulse width modulation control circuit, and the other end is coupled with a second reference potential, the signal receiving part is used for receiving the coupling signal and generating the overvoltage protection triggering signal accordingly.

8. The overvoltage protection circuit of claim 7, wherein the signal transmitting portion comprises a light emitting portion which is an optical coupler for generating a light source as the coupling signal, and the signal receiving portion comprises a light receiving portion which is the optical coupler for receiving the coupling signal generated by the light source and generating the overvoltage protection trigger signal accordingly.