CN108123429B

CN108123429B - Overvoltage protection circuit

Info

Publication number: CN108123429B
Application number: CN201611112578.1A
Authority: CN
Inventors: 许志源
Original assignee: Yarongyuan Science And Technology Shenzhen Co Ltd
Current assignee: Yarongyuan Science And Technology Shenzhen Co Ltd
Priority date: 2016-11-30
Filing date: 2016-11-30
Publication date: 2019-12-27
Anticipated expiration: 2036-11-30
Also published as: CN108123429A

Abstract

The invention provides an overvoltage protection circuit, which can distinguish whether an overvoltage state of output voltage of a switching type power supply device is caused by internal feedback failure or caused by an external power supply, can utilize the time difference of starting overvoltage protection to prevent a temporary and harmless external power supply from influencing the normal work of the switching type power supply device, and can also stop the operation of the switching type power supply device in real time to achieve protection when the internal feedback of the switching type power supply device fails.

Description

Overvoltage protection circuit

Technical Field

The present invention relates to an overvoltage protection circuit, and more particularly, to an overvoltage protection circuit suitable for a switching power supply device.

Background

A general Switching power supply (Switching power supply) uses an overvoltage protection circuit to control a Pulse Width Modulation (PWM) control circuit in the Switching power supply to stop outputting a PWM signal when an output terminal of the Switching power supply exhibits an overvoltage, so as to reduce an output voltage of the output terminal, thereby performing overvoltage protection on an internal circuit of the Switching power supply and an internal circuit of an external system of the output terminal, and preventing any one of the two from being damaged.

The output terminal will present overvoltage due to two conditions, one of which is caused by the failure of the internal circuit of the switching power supply device, such as the failure of the feedback circuit; the other is the back emf feedback caused by the external system, such as the external system with a motor, during deceleration. However, the conventional overvoltage protection circuit cannot distinguish the two circuits, so that when a user operates the external system normally to cause back electromotive force feedback, the conventional overvoltage protection circuit still controls the switching power supply device to immediately reduce the magnitude of the output voltage thereof to perform overvoltage protection, which causes the external system to be shut down and cannot be used normally, thereby causing a user to be troubled.

Disclosure of Invention

It is an object of the present invention to provide an overvoltage protection circuit which can distinguish between overvoltages caused by the two conditions and perform overvoltage protection in two different ways.

The invention provides an overvoltage protection circuit which is suitable for a switching type power supply device. The overvoltage protection device can distinguish whether the overvoltage state of the output voltage of the switching type power supply device is caused by internal feedback failure or caused by an external power supply by utilizing the time difference of starting the overvoltage protection, so that the situation that the switching type power supply device normally works is avoided being influenced by the temporary and harmless external power supply, and the switching type power supply device can be stopped to act in real time to achieve protection when the internal feedback of the switching type power supply device fails.

In order to make the aforementioned and other objects, features and advantages of the invention comprehensible, preferred embodiments accompanied with figures are described in detail below.

Drawings

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

Detailed Description

Fig. 1 is a schematic diagram of a coupling relationship of an overvoltage protection circuit. In fig. 1, the switching Power supply apparatus 100 includes a noise filter 110, an ac-dc conversion circuit 120, a Power stage (Power stage)130, a transformer 140, a rectification circuit 150, an energy storage unit 160, a feedback circuit 170, and a signal isolation unit 180. The primary winding of the transformer 140 may 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 may be an electromagnetic interference filter, and the user may decide whether to employ the noise filter 110.

In addition, the ac-dc conversion circuit 120 may be a bridge rectifier. The power stage 130 has a PWM control circuit 132 and a power transistor 134 as a switch. The power transistor 134 can be connected in series with the primary winding of the transformer 140, and whether to allow current to pass through the primary winding can be determined by controlling the On/off state (On/off state) of the power transistor 134. The PWM control circuit 132 is configured to generate a PWM signal and output the PWM signal to the control terminal of the power transistor 134, so as to control the switching frequency of the power transistor 134 between the on state and the off state.

The output of the secondary winding of the transformer 140 is rectified and filtered by the rectifying circuit 150 and the energy storage unit 160, respectively, and then is provided as the output voltage VOUT of the switching power supply 100 to an external system (not shown) from the output terminal 190. Furthermore, the rectifying circuit 150 and the energy storage unit 160 may be adopted according to design requirements. In addition, the signal isolation unit 180 may be an optical coupler (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 duty cycle (duty cycle) of the PWM signal accordingly. Therefore, when the output terminal 190 of the switching power supply 100 exhibits an overvoltage, the PWM control circuit 132 can lower the output voltage VOUT according to the feedback signal transmitted from the signal isolation unit 180 for performing the 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 resistor 210 has a first resistance value, and the resistor 220 has a second resistance value, which is greater than the first resistance value. 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. The reference potential SGND is a ground potential.

The switch unit 240 has a first terminal 241, a second terminal 242 and a third terminal 243, the second terminal 242 is coupled to the other terminal of the impedance 210, and the first terminal 241 is coupled to one terminal of the impedance 220. The over-voltage detecting unit 250 is coupled to the output terminal 190 of the switching power supply apparatus 100, the first terminal 241 of the switch unit 240, and the other end of the impedance 220, and is configured to receive the output voltage VOUT at the output terminal 190 and determine whether the output voltage VOUT exceeds a 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 switching unit 240, the fourth terminal 271 is coupled to one terminal of the secondary side coil of the transformer of the switching power supply apparatus 100 for detecting whether the voltage output by the secondary side coil reaches a second predetermined value, and if yes, the fifth terminal 278-5 and the reference potential SGND are turned on, and if the overvoltage detection unit 250 outputs a positive voltage, the switching unit 240 is controlled to turn on 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 the over-voltage protection trigger signal TRI according to the voltage magnitude stored in the energy storage unit 230, so as to control the PWM control circuit 132 to stop outputting the PWM signal to the power transistor 134 by using the over-voltage protection trigger signal TRI.

In this case, the impedances 210, 220 and the energy storage unit 230 can be resistors and capacitors, respectively. Next, detailed embodiments of the rest of the overvoltage protection circuit 200 will be described. As shown in fig. 1, the overvoltage detection unit 250 includes a voltage divider circuit 252 and a comparator 254. The voltage divider 252 is coupled between the output terminal 190 of the switching power supply apparatus 100 and the reference voltage SGND, and generates a voltage dividing signal according to the voltage at the output terminal 190. The comparator 254 has a positive input terminal for receiving the divided signal of the voltage divider 252, a negative input terminal for receiving the reference voltage Vref, and an output terminal coupled to the first terminal 241 of the switch unit 240 and the impedance 220. The voltage divider circuit 252 may be an impedance 252-1 and an impedance 252-2, wherein one terminal of the impedance 252-1 is coupled to the output terminal 190, one terminal of the impedance 252-2 is coupled to the other terminal of the impedance 252-1, and the other terminal of the impedance 252-2 is coupled to the reference voltage SGND. Both impedances 252-1 and 252-2 may be resistors.

The switch unit 240 includes a PNP transistor 244, a resistor 245 and a resistor 246. The PNP transistor 244 has an emitter, a base and a collector, and the base thereof is coupled to the fifth terminal 278-5 of the voltage sampling unit 270 through the impedance 245 and is determined to be turned on or not.

The voltage sampling unit 270 includes a diode 273, an energy storage unit 274, a voltage divider 276 and a voltage control switch 278. The anode of the diode 273 is coupled to one end of the secondary winding 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 divider 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, wherein 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 terminal R is configured to receive the voltage-dividing signal generated by the voltage-dividing circuit 276. When the voltage at the reference terminal R of the voltage-controlled switch 278 reaches the third predetermined value, the voltage-controlled switch 278 makes the fifth terminal 278-5 and the sixth terminal 278-6 turn on. The voltage divider 276 may be an impedance 276-1 and 276-2. Wherein one terminal of the impedance 276-1 is coupled to the cathode of the diode 273, one terminal of the impedance 276-2 is coupled to the other terminal of the impedance 276-1, and the other terminal of the impedance 276-2 is coupled to the reference potential SGND.

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

Next, a detailed operation manner of the overvoltage protection circuit 200 will be described. First, the operation of the voltage protection circuit 200 in the case that the internal circuit of the switching power supply 100 fails and the output terminal 190 thereof exhibits overvoltage will be described. Referring to fig. 1 again, when the output voltage VOUT at the output terminal 190 exceeds a first predetermined value, such that the voltage of the divided signal generated by the voltage divider 252 is greater than the voltage of the reference voltage Vref, the output of the comparator 254 is in positive saturation.

As mentioned above, the overvoltage appearing at the output terminal 190 is caused by a failure of an internal circuit of the switching power supply apparatus 100, for example, a failure of the feedback circuit 170. Then the feedback circuit 170 cannot generate a feedback signal, so the PWM control circuit 132 cannot adjust the duty cycle of the PWM signal according to the feedback signal transmitted from the signal isolation unit 180, and thus the output voltage VOUT cannot be reduced, and the voltage output by the Secondary winding (Secondary winding) will be continuously increased.

When the voltage output from the secondary winding is continuously increased to reach the second default value and the voltage of the voltage-dividing signal generated by the voltage-dividing circuit 276 reaches the third default value, the fifth terminal 278-5 and the sixth terminal 278-6 of the voltage-controlled switch 278 are turned on. Therefore, the third terminal 243 of the switch unit 240 is electrically connected to the reference potential SGND and 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 from the comparator 254 is selected to go through 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 quickly saturated to conduct the diode 282, so that the signal transmitting portion 284 can quickly generate the coupling signal. After receiving the coupling signal, the signal receiving portion 286 may generate the over-voltage protection trigger signal TRI to the PWM control circuit 132, so that the PWM control circuit 132 may immediately reduce the output voltage VOUT by controlling the operation of the power transistor 134 for over-voltage protection.

The operation of the voltage protection circuit 200 in the event that the external system causes the output terminal 190 to exhibit an overvoltage will be described. Referring to fig. 1 again, when the output voltage VOUT at the output terminal 190 exceeds a first predetermined value, and the voltage of the divided signal generated by the voltage divider 252 is greater than the voltage of the reference voltage Vref, the output of the comparator 254 is in positive saturation.

In view of the above, the overvoltage appearing at the output terminal 190 is caused by the external system, such as the back emf feedback caused by the external system with a motor during deceleration. Therefore, the feedback circuit 170 will normally generate the feedback signal, so that the PWM control circuit 132 can adjust the duty cycle of the PWM signal according to the feedback signal transmitted from the signal isolation unit 180, thereby reducing the output voltage VOUT and lowering the voltage output by the Secondary winding (Secondary winding).

When the voltage output from the secondary winding continuously decreases and cannot reach the second default value, and the voltage of the divided voltage signal generated by the voltage divider 276 cannot reach the third default value, the fifth terminal 278-5 and the sixth terminal 278-6 of the voltage-controlled switch 278 cannot be turned on. Therefore, the PNP transistor 244 is turned Off (Off state) and the first end 241 and the second end 242 of the switch unit 240 cannot be turned on. Therefore, the current output from the comparator 254 can only charge the energy storage unit 230 through the impedance 220.

Since the resistance of the resistor 220 is much larger than the resistance of the path provided by the PNP transistor 244, the charging time of the energy storage unit 230 is prolonged, and the diode 282 is turned on after a delay. Since the signal transmitting portion 284 must wait until the diode 282 is turned on to generate the coupling signal, the signal receiving portion 286 receives the coupling signal after delaying the period of time to generate the over-voltage protection trigger signal TRI to the PWM control circuit 132. Therefore, the PWM control circuit 132 can control the operation of the power transistor 134 to lower the output voltage VOUT after the delay time, so as to perform the over-voltage protection. In other words, in this case, the switching power supply 100 does not immediately perform the overvoltage protection, which results in the external system not operating normally, but performs the overvoltage protection after a delay. Of course, it should be understood by those skilled in the art that the delay time can be adjusted by changing the resistance of the impedance 220, and can also be adjusted by changing the capacitance of the energy storage unit 230.

In one embodiment, the impedance 210 is coupled between the first terminal 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, the details are not repeated here.

In summary, in the overvoltage protection circuit of the invention, the determination result of the voltage sampling unit can be used to reflect whether the output terminal of the switching power supply device exhibits an overvoltage due to a failure of an internal circuit of the switching power supply device or due to an external system. 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 charge the energy storage unit by allowing the current to pass through the impedance or charge the energy storage unit by allowing the current to pass through the path. The resistance value of the path is smaller than that of the impedance, so that the energy storage unit can have two different charging times under two different overvoltage conditions, and the time for generating the overvoltage protection trigger signal by the signal isolation unit under the two different overvoltage conditions is different. By means of such control, the overvoltage protection circuit of the invention can immediately generate the overvoltage protection trigger signal to control the switching type power supply device to perform overvoltage protection when the internal circuit of the switching type power supply device fails, and delay the time of generating the overvoltage protection trigger signal when the external system causes overvoltage, so that the switching type power supply device cannot immediately perform overvoltage protection to cause the external system to fail to operate normally.

Although the present invention has been described with reference to the preferred embodiments, it should be understood that various changes and modifications can be made therein by those skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

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.