Overvoltage protection circuit of switching power supply and switching power supply

Abstract

The invention discloses an overvoltage protection circuit of a switching power supply and the switching power supply, the switching power supply comprises a main power tube and a freewheeling tube, and the main power tube enables the switching power supply to provide an output voltage for a load according to an input voltage through periodic connection and disconnection. The overvoltage protection circuit comprises a sampling and holding unit for sampling an input voltage within a follow current time of a follow current tube and then holding a sampled signal to obtain a sampling and holding signal; and the overvoltage control unit is used for performing overvoltage protection on the switching power supply according to the duty ratio, the sampling holding signal and the characterization voltage of the overvoltage protection threshold. The overvoltage protection circuit provided by the invention samples the input voltage during the follow current period, so that the information of the output voltage is obtained according to the sampled input voltage and the duty ratio, overvoltage protection is carried out when overvoltage is output, and the overvoltage protection circuit is applicable to the specific situation that the main power tube in the switching power supply is connected with the input voltage or the reference ground, and is wide in adaptability and low in cost.

Description

Technical Field

[0001] The present application relates to the field of power electronics technology, and in particular to an overvoltage protection circuit of a switching power supply and a switching power supply. Background Art

[0002] A power converter is a power supply that uses a switch tube to control the charging and discharging process of an energy storage element to supply power, and controls the on-time ratio of the switch tube to maintain a stable output voltage and / or output current. Common power converters mostly use topologies such as Boost, Buck, and Buck-Boost.

[0003] Figure 1a and Figure 1b The schematic diagrams of a conventional switching power supply with a main power tube connected to an input voltage and its overvoltage protection circuit and a switching power supply with a main power tube connected to a reference ground and its overvoltage protection circuit are shown respectively. Figure 1a As shown, the switching power supply 10 (for example, using Buck topology) includes a rectifier bridge 11, a main power tube M0, a freewheeling diode D0, an inductor L0, an input capacitor Ci and an output capacitor Co. The rectifier bridge 11 rectifies the AC power supply and outputs it as an input voltage Vin. The main power tube M0 and the inductor L0 are connected in series between the input end and the output end. The input capacitor Ci is connected between the input end and the reference ground of the switching power supply (set at point A), and the output capacitor Co is connected between the output end and the reference ground. The anode of the freewheeling diode D0 is grounded, and the cathode is connected to the common node B of the main power tube M0 and the inductor L0. The reference ground potential of its overvoltage protection circuit 20 is the potential of the node B of the main power tube and the inductor. The overvoltage protection circuit 20 includes an output sampling unit 21 and an overvoltage control unit 23. The output sampling unit 21 directly samples the output voltage Vout, and the sampled voltage is input to the overvoltage control unit 23. The overvoltage control unit 23 generates an overvoltage protection signal OVP according to the comparison result of the sampled voltage of the output voltage Vout and the internally set overvoltage protection threshold.

[0004] Figure 1b The buck switching power supply shown in the figure has a topology similar to Figure 1a Similar, but different, Figure 1bIn the circuit, the input end and the output end are directly connected, and the main power tube M0 is connected to the reference ground of the switching power supply (set at point A). The reference ground potential of its overvoltage protection circuit 20 is the reference ground potential of the switching power supply. The overvoltage protection circuit 20 includes an input sampling unit 22 and an overvoltage control unit 23. The input sampling unit 21 samples the input voltage Vin, and the sampled voltage is input to the overvoltage control unit 23. The overvoltage control unit 23 calculates the representative voltage of the output voltage according to the sampled voltage of the input voltage Vin and the duty cycle D, and then compares it with the internally set overvoltage protection threshold to generate an overvoltage protection signal OVP. Alternatively, the input sampling unit 21 samples the output voltage through the auxiliary winding, and the overvoltage control unit 23 compares the sampled output voltage with the overvoltage protection threshold to generate an overvoltage protection signal OVP.

[0005] The same is true for switching power supplies of other topologies. The overvoltage protection circuits of the switching power supply in which the main power tube is connected to the input voltage and the switching power supply in which the main power tube is connected to the reference ground are different. Currently, there is no overvoltage protection circuit that is compatible with the above two switching power supplies. Therefore, the above two switching power supplies currently need to separately manufacture overvoltage protection circuits and corresponding chips, which is costly, time-consuming, and has complicated circuit design, which is not conducive to mass production. Summary of the invention

[0006] In order to solve the above technical problems, the present application provides an overvoltage protection circuit of a switching power supply and a switching power supply to solve the problems in the prior art.

[0007] According to one aspect of the present invention, an overvoltage protection circuit of a switching power supply is provided, wherein the switching power supply comprises a main power tube and a freewheeling tube, wherein the overvoltage protection circuit comprises: a sampling and holding unit, which samples an input voltage during the freewheeling time of the freewheeling tube, and then holds the sampled signal to obtain a sampling and holding signal; and an overvoltage control unit, which performs overvoltage protection on the switching power supply according to a duty cycle, the sampling and holding signal and a voltage representing an overvoltage protection threshold.

[0008] Optionally, when the main power tube is connected to the input voltage, the reference ground potential of the overvoltage protection circuit is the potential of the connection point between the main power tube and the inductor in the switching power supply; when the main power tube is connected to the reference ground of the switching power supply, the reference ground potential of the overvoltage protection circuit is equal to the reference ground potential of the switching power supply.

[0009] Optionally, the switching power supply further includes a drive control circuit and a zero-crossing detection circuit, wherein the drive control circuit generates a switch drive signal to control the on and off of the main power tube, and the zero-crossing detection circuit outputs a zero-crossing detection signal at the zero-crossing point of the inductor current, wherein the sampling and holding unit determines the freewheeling time according to the switch drive signal and the zero-crossing detection signal, and starts to hold the sampled signal upon receiving the zero-crossing detection signal to obtain the sampling and holding signal.

[0010] Optionally, the overvoltage control unit operates on the duty cycle and the sampling and holding signal and the characterizing signal of the overvoltage protection threshold based on a first functional relationship to obtain a trigger condition, and performs overvoltage protection when the switching power supply meets the trigger condition; wherein the trigger condition represents that the output voltage is greater than the overvoltage protection threshold, and the first functional relationship represents the relationship between the input voltage, the output voltage and the duty cycle in the switching power supply.

[0011] Optionally, the duty cycle includes a conduction duty cycle and / or a freewheeling duty cycle, the ratio of the conduction time to the sum of the conduction time and the freewheeling time is the conduction duty cycle, and the ratio of the freewheeling time to the sum of the conduction time and the freewheeling time is the freewheeling duty cycle; when the switching power supply is a step-down switching power supply, the trigger condition is: Vins×Ds>Vref, Vins is the sampling and holding signal, Ds is the conduction duty cycle, Vref is the characterization voltage of the overvoltage protection threshold, and the product of the sampling and holding signal and the conduction duty cycle represents the output voltage; when the switching power supply is a step-up and step-down switching power supply, the trigger condition is: Vins×Ds / Ds'>Vref, Vins is the sampling and holding signal, Ds is the conduction duty cycle, Ds' is the freewheeling duty cycle, Vref is the characterization voltage of the overvoltage protection threshold, and the product of the sampling and holding signal and the conduction duty cycle and the ratio of the freewheeling duty cycle represent the output voltage,

[0012] The overvoltage control unit obtains the on-duty ratio and the freewheeling duty ratio according to the freewheeling time and the on-time of the main power tube.

[0013] Optionally, the sampling and holding unit includes: a resistor voltage divider network, which obtains a divided voltage proportional to the input voltage; a first capacitor, wherein the first end of the first capacitor is connected to the output end of the resistor voltage divider network, the second end of the first capacitor is connected to the ground end, and the first capacitor samples and holds the divided voltage; a first switch, wherein the first end of the first switch is connected to the output end of the resistor voltage divider network, the second end of the first switch is connected to the first end of the first capacitor, and the first switch is turned on during the freewheeling time of the freewheeling tube.

[0014] Optionally, the sampling and holding unit also includes: a third resistor connected between the second end of the first switch and the first end of the first capacitor; and a voltage follower, wherein the positive input end of the voltage follower is connected to a common node of the third resistor and the first capacitor, and the reverse input end of the voltage follower is connected to the output end of the voltage follower.

[0015] Optionally, the switching power supply is a step-down switching power supply, and the overvoltage control unit includes: a second switch, a first end of the second switch receives the sampling and holding signal; a third switch, a first end of the third switch is connected to the second end of the second switch, and the second end of the third switch is connected to the ground; a first RC filter network is connected between the first end of the third switch and the ground, and includes at least a second capacitor and a fourth resistor; and a first comparator, a first input end of the first comparator is connected to the output end of the first RC filter network, a second input end of the first comparator receives the voltage representing the overvoltage protection threshold, and an output end of the first comparator outputs an overvoltage control signal, wherein the second switch is turned on only during the on-time, and the third switch is turned on only during the freewheeling time.

[0016] Optionally, the switching power supply is a step-down switching power supply, and the overvoltage control unit includes: a first voltage-controlled current source, connected to the first end of the fourth capacitor through a sixth switch and a seventh switch, the control end of the first voltage-controlled current source receives the sampling and holding signal, the first end of the sixth switch is connected to the output end of the first voltage-controlled current source, the first end of the seventh switch is connected to the second end of the sixth switch, the second end of the seventh switch is connected to the first end of the fourth capacitor, and the second end of the fourth capacitor is connected to the ground end; a second voltage-controlled current source, connected between the second end of the sixth switch and the ground end, the control end of the second voltage-controlled current source receives the characterization voltage of the overvoltage protection threshold, a second comparator, the first input end of the second comparator is connected to the first end of the fourth capacitor, the second input end of the second comparator receives zero voltage, and the output end of the second comparator outputs the overvoltage control signal; wherein, the sixth switch is turned on only during the on-time, and the seventh switch is turned on during the on-time and the freewheeling time.

[0017] Optionally, the overvoltage control unit also includes: a fifth capacitor, a first end of the fifth capacitor is connected to the second end of the sixth switch, and the second end of the fifth capacitor is connected to the ground; and an eighth switch, a first end of the eighth switch is connected to the first end of the seventh switch, and the second end of the eighth switch is connected to the ground, and the eighth switch is turned on after the freewheeling time ends to release the charge on the fifth capacitor.

[0018] Optionally, the switching power supply is a buck-boost switching power supply, and the overvoltage control unit includes: a second switch, a first end of the second switch receives the sampling and holding signal; a third switch, a first end of the third switch is connected to the second end of the second switch, and the second end of the third switch is connected to the ground; a first RC filter network is connected between the first end of the third switch and the ground, and includes a second capacitor and a fourth resistor; a fourth switch, a first end of the fourth switch receives the characterizing voltage of the overvoltage protection threshold; a fifth switch, a first end of the fifth switch is connected to the second end of the fourth switch, and the second end of the fifth switch is connected to the ground; and a second RC filter network is connected between the first end of the fifth switch and the ground, and includes at least a third capacitor and a fifth resistor; a first comparator, a first input end of the first comparator is connected to the output end of the first RC filter network, a second input end of the first comparator is connected to the output end of the second RC filter network, and the output end of the first comparator outputs the overvoltage control signal, wherein the fourth switch is only turned on during the freewheeling time, and the fifth switch is only turned on during the on-time.

[0019] Optionally, the switching power supply is a buck-boost switching power supply, and the overvoltage control unit includes: a first voltage-controlled current source, connected to the first end of the fourth capacitor through a sixth switch and a seventh switch, the control end of the first voltage-controlled current source receives the sampling and holding signal, the first end of the sixth switch is connected to the output end of the first voltage-controlled current source, the first end of the seventh switch is connected to the second end of the sixth switch, the second end of the seventh switch is connected to the first end of the fourth capacitor, and the second end of the fourth capacitor is connected to the ground end; a second voltage-controlled current source, connected between the second end of the sixth switch and the ground end, the control end of the second voltage-controlled current source receives the characterization voltage of the overvoltage protection threshold, a second comparator, the first input end of the second comparator is connected to the first end of the fourth capacitor, the second input end of the second comparator receives zero voltage, and the output end of the second comparator outputs an overvoltage control signal; and a ninth switch, connected between the output end of the second voltage-controlled current source and the ground end, and the ninth switch is only turned on during the freewheeling time of the freewheeling tube.

[0020] According to another aspect of the present invention, a switching power supply is provided, comprising: the above-mentioned overvoltage protection circuit, wherein the overvoltage protection circuit performs overvoltage protection on the switching power supply when outputting a valid overvoltage control signal.

[0021] Optionally, when the main power tube in the switching power supply is connected to the input voltage, the reference ground potential of the overvoltage protection circuit is the potential of the connection point between the main power tube and the inductor in the switching power supply; when the main power tube is connected to the reference ground of the switching power supply, the reference ground potential of the overvoltage protection circuit is equal to the reference ground potential of the switching power supply.

[0022] The beneficial effects of the present invention include at least:

[0023] The overvoltage protection circuit of the switching power supply and the switching power supply provided by the present invention samples the input voltage during the freewheeling time of the freewheeling tube, then holds the sampled and held signal to obtain the relationship between the input voltage and the output voltage of the main power tube according to the topological structure. Thus, the switching power supply is protected from overvoltage by utilizing the sampled and held signal, the duty cycle, and the voltage representing the overvoltage protection threshold. Thus, the overvoltage protection circuit can be used regardless of whether the main power tube in the switching power supply is connected to the input terminal or the reference ground. For a certain topological structure, only one overvoltage protection circuit or corresponding chip needs to be designed and manufactured, and it can be used simultaneously in the above two switching power supplies. Thus, the overvoltage protection circuit has a wide range of applications, can greatly reduce the circuit design and manufacturing costs, is convenient for mass production, and has a good overvoltage protection effect.

[0024] Furthermore, the zero-crossing detection circuit outputs a zero-crossing detection signal at the zero-crossing point of the inductor current, thereby accurately limiting the freewheeling time of the freewheeling tube. The sampling signal of the input voltage is maintained at the zero-crossing point of the inductor current to obtain a sampling and holding signal, so that a stable and accurate sampling and holding signal of the input voltage can be obtained. In addition, the representative voltage of the output voltage is calculated based on the sampling and holding signal of the previous switching cycle and the conduction time and freewheeling time of the current switching cycle. The sampling and holding unit is independent of the overvoltage control unit, and the obtained sampling and holding signal is stable, less affected by the outside world, and has small fluctuations. The obtained representative voltage of the output voltage is less different from the actual output voltage, and the accuracy of the overvoltage protection is high.

[0025] Furthermore, an overvoltage control unit with different circuit structures can be designed according to the relationship between the input voltage, the output voltage and the duty cycle under the same topology. It can be in the form of a combination of a switch and a filter circuit, or in the form of a combination of a voltage-controlled current source, a switch and a capacitor, etc. The embodiments are rich in variety, the circuit structures are diversified, and the scope of application is wide.

[0026] It should be noted that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1a and Figure 1b Schematic diagrams of a conventional switching power supply with a main power tube connected to an input voltage and its overvoltage protection circuit and a switching power supply with a main power tube connected to a reference ground and its overvoltage protection circuit are shown respectively;

[0028] Figure 2a and Figure 2bSchematic diagrams of a step-down switching power supply and an overvoltage protection circuit thereof in which a main power tube is connected to an input voltage and a step-down switching power supply and an overvoltage protection circuit thereof in which a main power tube is connected to a reference ground according to an embodiment of the present invention are respectively shown;

[0029] Figure 3 A schematic circuit block diagram of an overvoltage protection circuit of a step-down switching power supply according to a first embodiment of the present invention is shown;

[0030] Figure 4 A schematic circuit block diagram of an overvoltage protection circuit of a step-down switching power supply according to a second embodiment of the present invention is shown;

[0031] Figure 5a and Figure 5b Schematic diagrams of a buck-boost switching power supply and an overvoltage protection circuit thereof in which a main power tube is connected to an input voltage and a buck-boost switching power supply and an overvoltage protection circuit thereof in which a main power tube is connected to a reference ground according to an embodiment of the present invention are respectively shown;

[0032] Figure 6 A schematic circuit block diagram of an overvoltage protection circuit of a buck-boost switching power supply according to a first embodiment of the present invention is shown;

[0033] Figure 7 A schematic circuit block diagram of an overvoltage protection circuit of a buck-boost switching power supply according to a second embodiment of the present invention is shown;

[0034] Figure 8 A schematic flow chart of an overvoltage protection method for a switching power supply according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0035] In order to facilitate the understanding of the present invention, the present invention will be described more fully below with reference to the relevant drawings. The preferred embodiments of the present invention are shown in the drawings. However, the present invention can be implemented in different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0036] Figure 2a and Figure 2b Schematic diagrams of a switching power supply and an overvoltage protection circuit thereof in which the main power tube is connected to an input voltage and a switching power supply and an overvoltage protection circuit thereof in which the main power tube is connected to a reference ground according to the present invention are respectively shown.

[0037] like Figure 2aAs shown, the switching power supply of this embodiment includes a power converter 100 and a control circuit 200, and the power converter 100 includes: a rectifier bridge 101, a main power tube M1, a freewheeling tube, an inductor L1, an input capacitor Ci and an output capacitor Co, the main power tube M1 is connected to the input end, receives the input voltage, and the reference ground of the power converter 100 is set at node A. The power converter 100 takes the buck topology (Buck) as an example, and the freewheeling tube includes a freewheeling diode or a synchronous rectifier tube, etc., and the freewheeling diode D1 is used as an example here. The rectifier bridge 101 is connected between the live wire L and the neutral wire N, and generates an input voltage Vin at the input end after rectifying the AC power supply, the input capacitor Ci is connected between the input end and the reference ground of the power converter 100, the main power tube M1 and the inductor L1 are connected in series between the input end and the output end, and the output capacitor Co is connected between the output end and the reference ground of the power converter 100. The input capacitor Ci at the input end is connected to the drain end of the main power tube M1, and the common node of the source end of the main power tube M1 and the inductor L1 is node B. The anode of the freewheeling diode D1 is connected to the reference ground, and the cathode is connected to the common node (node ​​B) of the main power tube M1 and the inductor L1. The load 30 is connected to both ends of the output capacitor Co, and the output voltage Vout is generated on the load 30. The control circuit 200 is connected to the control end of the main power tube M1, and is used to output the switch drive signal GATE to control the conduction and shutdown of the main power tube M1. The control circuit 200 controls the freewheeling diode D1 to turn off during the conduction period of the main power tube M1, the inductor L1 stores energy, and the inductor current increases; the control circuit 200 controls the main power tube M1 to turn off during the shutdown period, and the freewheeling diode D1 is turned on, and the electric energy stored in the inductor L1 is provided to the load 30, so that an output current is generated on the load, and the inductor current decreases. The main power tube M1 is turned on and off once for a switching cycle. In a switching cycle, the inductor current first rises and then falls. Through the conduction and shutdown of multiple switching cycles, the required output voltage Vout is provided to the load 30. The power converter 100 operates in BCM (critical conduction) or DCM (discontinuous conduction) mode. In a switching cycle, the inductor current first rises and then falls. Adjacent switching cycles are separated by a certain dead time. For example, during the period when the switch drive signal GATE is at a high level, the inductor current rises. During the period when the main power tube M1 is turned on, the time period from the beginning of the main power tube M1 to the beginning of the main power tube M1 is the conduction time of the main power tube M1; during the period when the switch drive signal GATE is at a low level, the inductor current falls. The time period from the beginning of the main power tube M1 turning off to the inductor current reaching the zero crossing point is the freewheeling time of the freewheeling tube. During this time period, the main power tube M1 is turned off. In the following embodiments, the conduction time of the main power tube M1 and the freewheeling time of the freewheeling tube also refer to this rule.

[0038] Furthermore, the control circuit 200 of the embodiment of the present invention includes an overvoltage protection circuit 210, a drive control circuit 240 and a zero-crossing detection circuit 250. The drive control circuit 240 is used to generate a switch drive signal GATE to control the on and off of the main power tube M1, so that the power converter 100 provides an output current to the load 30 according to the input voltage Vin, so as to generate an output voltage Vout on the load 30. The zero-crossing detection circuit 250 is used to detect a signal representing the current on the inductor L1, that is, to detect the inductor current, and output a zero-crossing detection signal ZCD at the zero-crossing point of the inductor current. The zero-crossing detection circuit 250, for example, directly detects the current on the inductor L1, or determines the magnitude of the current on the inductor L1 by detecting the drain terminal voltage or the gate terminal voltage of the main power tube M1. The overvoltage protection circuit 210 is used to output a valid overvoltage control signal OVP when the output voltage Vout exceeds a preset overvoltage protection threshold value Vref. The drive control circuit 240 receives the zero-crossing detection signal ZCD and the overvoltage control signal OVP, and generates a switch drive signal GATE accordingly. The drive control circuit 240 also controls the main power tube M1 to disconnect when receiving a valid overvoltage control signal OVP. Of course, the control circuit 200 of this embodiment may also include other circuits and modules, such as an overtemperature detection circuit, an undervoltage protection circuit, etc., which are not described here. The overvoltage protection circuit 210 may be a chip with multiple pins, and the ground GND pin of the chip is connected to the node B, that is, the reference ground potential of the overvoltage protection circuit 210 is the potential of the common node B of the main power tube M1 and the inductor L1. The overvoltage protection circuit 210 includes a sampling and holding unit 220 and an overvoltage control unit 230. The sampling and holding unit 220 samples and holds the input voltage Vin to obtain a sampling and holding signal Vins, and the overvoltage control unit 230 performs overvoltage protection on the switching power supply according to the duty cycle, the sampling and holding signal Vins and the characterizing voltage of the overvoltage protection threshold, and outputs a valid overvoltage control signal OVP when the overvoltage protection is triggered. Specifically, the overvoltage control unit 230 operates the duty cycle, the sampling and holding signal, and the overvoltage protection threshold based on the first functional relationship to obtain a trigger condition, and performs overvoltage protection when the switching power supply meets the trigger condition. The trigger condition represents that the output voltage is greater than the overvoltage protection threshold, and the first functional relationship represents the relationship between the input voltage, the output voltage, and the duty cycle in the switching power supply.

[0039] like Figure 2b As shown, the switching power supply includes a power converter 100 and a control circuit 200. Taking the buck topology (Buck) as an example, the power converter 100 also includes a rectifier bridge 101, a main power tube M1, a freewheeling tube (taking the freewheeling diode D1 as an example), an inductor L1, an input capacitor Ci and an output capacitor Co. The main power tube M1 is connected to the reference ground of the power converter 100, and the reference ground of the power converter 100 is set at node A. The rectifier bridge 101, the input capacitor Ci and the output capacitor Co are all connected to the reference ground of the power converter 100. Figure 2a The embodiment is the same, except that Figure 2b In the power converter 100, the main power tube M1 and the inductor L1 are located on the reference ground side. The main power tube M1 and the inductor L1 are still connected in series, the anode of the freewheeling diode D1 is connected to the common node of the main power tube M1 and the inductor L1, and the cathode of the freewheeling diode D1 is connected to the input terminal. Figure 2b The working principle of the power converter 100 is Figure 2a The working principle of the power converter is the same as that of the control circuit 200 of this embodiment, which will not be described in detail here. Figure 2a The control circuit 200 of the embodiment is completely identical, or at least the overvoltage protection circuit 210 in the control circuit 200 is completely identical, wherein the reference ground potential of the overvoltage protection circuit 210 is equal to the reference ground potential of the power converter 100. That is, the overvoltage protection circuit 210 can simultaneously adapt to the switching power supply in which the main power tube is connected to the input voltage and the switching power supply in which the main power tube is connected to the reference ground.

[0040] Combination Figure 2a and Figure 2b The overvoltage protection circuit 210 of this embodiment, which is applicable to both the switching power supply in which the main power tube is connected to the input voltage and the switching power supply in which the main power tube is connected to the reference ground, includes a sampling and holding unit 220 and an overvoltage control unit 230. Figure 2a The ground GND pin of the overvoltage protection circuit 210 has the same potential as the node B. Figure 2b The ground GND pin of the overvoltage protection circuit 210 has the same potential as the reference ground of the switching power supply, that is, node A. Figure 2a When the main power tube M1 is turned on, the potential of the ground pin GND of the overvoltage protection circuit 210 is the input voltage, and the input voltage cannot be sampled at this time. Figure 2a and Figure 2b In the two switching power supplies shown, in this embodiment, the sampling and holding unit 220 samples the input voltage Vin within the freewheeling time of the freewheeling tube (or during the freewheeling period of the freewheeling tube).

[0041] Specifically, the sampling and holding unit 220 receives the switch driving signal GATE to sample the input voltage Vin within the freewheeling time of the freewheeling tube, and then holds the sampled signal to obtain the sampled and held signal Vins. Preferably, the sampling and holding unit 220 also receives the zero-crossing detection signal ZCD, and starts to hold the sampled signal of the input voltage upon receiving the zero-crossing detection signal ZCD to obtain the sampled and held signal Vins. That is, the sampling and holding unit 220 determines the on-time of the main power tube M1 and the freewheeling time of the freewheeling tube according to the switch driving signal GATE and the zero-crossing detection signal ZCD. The time from the main power tube M1 being turned off to receiving the zero-crossing detection signal ZCD is the freewheeling time of the freewheeling tube. Sampling is performed within the freewheeling time, and the sampled signal is held at the end of the freewheeling time.

[0042] Specifically, the overvoltage control unit 230 operates the duty cycle and the sampling and holding signal and the characterization signal of the overvoltage protection threshold based on the first functional relationship to obtain the trigger condition, and performs overvoltage protection when the switching power supply meets the trigger condition. Among them, the trigger condition represents that the output voltage is greater than the overvoltage protection threshold, and the first functional relationship represents the relationship between the input voltage, the output voltage and the duty cycle in the switching power supply, and the first functional relationship is related to the topological structure of the switching power supply. Specifically, in this embodiment, taking the buck topology as an example, the duty cycle may include the conduction duty cycle. In the buck topology, the trigger condition is: Vins×Ds>Vref, Vins is the sampling and holding signal, Ds is the conduction duty cycle, Vref is the characterization voltage of the overvoltage protection threshold, and the product of the sampling and holding signal and the conduction duty cycle (Vins×Ds) represents the output voltage. In this embodiment, the overvoltage control unit 230 obtains the freewheeling time Toff of the freewheeling tube and the conduction time Ton of the main power tube, and obtains the conduction duty cycle accordingly. For example, the freewheeling time Toff of the freewheeling tube and the on-time Ton of the main power tube are obtained by receiving the switch drive signal GATE and the zero-crossing detection signal ZCD, and the sum of the on-time Ton and the freewheeling time Toff is the total time, that is, T=Ton+Toff, and the ratio of the on-time Ton to the sum of the on-time and the freewheeling time T is the on-duty ratio, Ds=Ton / T. Of course, other methods can also be used to obtain the on-duty ratio, such as using the volt-second balance law to obtain the on-duty ratio according to the slope of the inductor current in the on-time period and the freewheeling time period, which will not be described in detail here.

[0043] Figure 3 A schematic circuit block diagram of an overvoltage protection circuit of a step-down switching power supply according to a first embodiment of the present invention is shown.

[0044] like Figure 3 As shown, the overvoltage protection circuit 210 can be adapted to Figure 2a and Figure 2bThe power converter 100 shown in the Buck topology structure performs overvoltage protection when the product of the sampling and holding signal and the on-duty cycle is greater than the characterization voltage of the overvoltage protection threshold. In this embodiment, the overvoltage protection circuit 210 obtains the characterization voltage of the output voltage according to the product of the sampling and holding signal and the on-duty cycle, and compares the characterization voltage of the output voltage with the characterization voltage of the overvoltage protection threshold to perform overvoltage protection. Specifically, the sampling and holding unit 220 includes a resistor voltage divider network 221, a first switch S1, a first capacitor C1 and a first switch control unit 222. The resistor voltage divider network 221 includes a first resistor R1 and a second resistor R2 connected in series, which are used to obtain a voltage divider proportional to the input voltage Vin. The middle node of the first resistor R1 and the second resistor R2 is the output end of the resistor voltage divider network 221, and the corresponding sampling coefficient is K=R2 / (R1+R2). The first end of the first capacitor C1 is connected to the output end of the resistor voltage divider network 221, and the second end of the first capacitor C1 is connected to the ground end, which can be the GND pin of the overvoltage protection circuit 210. The first capacitor C1 samples and holds the divided voltage to obtain a sampled and held signal Vins, Vins=K×Vin=Vin×R2 / (R1+R2). The first end of the first switch S1 is connected to the output end of the resistor divider network 221, the second end of the first switch S1 is connected to the first end of the first capacitor C1, the control end of the first switch S1 is connected to the first switch control unit 222, and the first switch control signal KS1 output by the first switch control unit 222 controls the conduction and disconnection. The first switch control unit 222 includes a first RS trigger U1 and a NOT gate (or inverter) U2, the input end of the NOT gate U2 receives the switch drive signal GATE, the output end outputs the inverted signal GATEB of the switch drive signal, and is connected to the set end S of the first RS trigger U1, the reset end R of the first RS trigger U1 receives the zero crossing detection signal ZCD, and the output end Q outputs the first switch control signal KS1. The first RS trigger U1 is set according to the inverting signal GATEB of the switch drive signal, and is reset according to the zero-crossing detection signal ZCD, so as to control the first switch S1 to be turned on during the setting period, which is the freewheeling time of the freewheeling tube. Further, the sampling and holding unit 220 may also include a third resistor R3, which is connected between the second end of the first switch S1 and the first end of the first capacitor C1. The sampling and holding unit 220 may also include a voltage follower U3, the positive input end of the voltage follower U3 is connected to the common node of the third resistor R3 and the first capacitor C1, and the reverse input end of the voltage follower U3 is connected to the output end of the voltage follower U3.

[0045] In this embodiment, the overvoltage control unit 230 includes a second switch S2, a third switch S3, a first RC filter network and a first comparator COM1. The first end of the second switch S2 is connected to the output end of the voltage follower U3 and receives the sample and hold signal Vins. The first end of the third switch S3 is connected to the second end of the second switch S2, and the second end of the third switch S3 is connected to the ground end. The first RC filter network is connected between the first end of the third switch S3 and the ground end. The first RC filter network can be a single-stage filter network or a multi-stage filter network, and at least includes a second capacitor C2 and a fourth resistor R4. The first input end of the first comparator COM1 is connected to the output end of the first RC filter network and receives the voltage Vo1. The second input end of the first comparator COM1 receives the voltage K×Vref representing the overvoltage protection threshold Vref, where K is the sampling coefficient, K=R2 / (R1+R2). The output end of the first comparator COM1 outputs the overvoltage control signal OVP. The second switch S2 is turned on only during the on-time Ton, and the third switch S3 is turned on only during the freewheeling time Toff of the freewheeling tube. Then, during the freewheeling time of the freewheeling tube or during the freewheeling period, the switch drive signal GATE is invalid, the first RS trigger U1 is set, KS1 is high, the first switch S1 is turned on, and the input voltage Vin is sampled. After the inductor current passes through zero, the zero-crossing detection signal ZCD is high, the first RS trigger U1 is reset, KS1 is low, the first switch S1 is disconnected, and the sampled signal is maintained as the sampling and holding signal Vins by the first capacitor C1 and the voltage follower U3. In the next switching cycle, during the period when the switch drive signal GATE is high, the second switch S2 is turned on, the first switch S1 and the third switch S3 are disconnected, and the first RC filter network is charged according to the sampling and holding signal Vins. Due to the existence of the voltage follower U3, the sampling and holding signal Vins remains unchanged during charging. After the switch drive signal GATE becomes low, the second switch S2 is turned off, and the KS1 signal controls the third switch S3 to turn on, and the first RC filter network is discharged. Then, after entering the steady state, the first RC filter network output Output voltage Vo1 is The product of the sampling and holding signal Vins and the on-duty cycle Ds of the switching power supply is Vins×Ds. Vo1=Vins×Ds=K×Vin×Ds. If Vo1>K×Vref, the first comparator COM1 outputs a valid overvoltage control signal OVP. Among them, Vo1>K×Vref, that is, K×Vin×Ds>K×Vref, and in the step-down switching power supply, Vout=Vin×Ds, so Vo1>K×Vref means that the output voltage is greater than the overvoltage protection threshold, and it is considered that the trigger condition is met, and overvoltage protection is performed. Taking the first input end of the first comparator COM1 as the positive input end and the second input end as the reverse input end as an example, the overvoltage control signal OVP is valid when it is at a high level; taking the first input end of the first comparator COM1 as the reverse input end and the second input end as the positive input end as an example, the overvoltage control signal OVP is valid when it is at a low level. When the switching cycle of charging and discharging the first RC filter network is the current switching cycle, the switching cycle of sampling and holding the input voltage is the previous switching cycle.

[0046] Figure 4 A schematic circuit block diagram of an overvoltage protection circuit of a step-down switching power supply according to a second embodiment of the present invention is shown.

[0047] like Figure 4 As shown, the overvoltage protection circuit 210 of this embodiment can also be adapted to Figure 2a and Figure 2b The power converter 100 performs overvoltage protection when the product of the sampling and holding signal and the on-duty cycle is greater than the voltage representing the overvoltage protection threshold. In this embodiment, the overvoltage protection circuit 210 performs overvoltage protection based on the voltage representing the difference between the first volt-second product and the second volt-second product and the zero voltage. The first volt-second product represents the product of the input voltage and the on-time, and the second volt-second product represents the product of the sum of the freewheeling time and the on-time and the overvoltage protection threshold. Specifically, in this embodiment, the sampling and holding unit 220 includes a resistor divider network 221, a first switch S1, a first capacitor C1 and a first switch control unit 222. The circuit connection method and working principle of these components are the same as Figure 3The same, no further description is given here. The difference is that, in this embodiment, the overvoltage control unit 230 includes: a first voltage-controlled current source A1, a second voltage-controlled current source A2, a sixth switch S6, a seventh switch S7, a fourth capacitor C4, a second comparator COM2, an AND gate U5, and a second RS trigger U4. The first input end of the second comparator COM2 receives a voltage Vo3 representing the difference between the first volt-second product and the second volt-second product, the second input end receives a zero voltage V0, and the output end outputs an overvoltage control signal OVP; the first end of the fourth capacitor C4 is connected to the first input end of the second comparator COM2 to provide a voltage Vo3, and the second end of the fourth capacitor C4 is connected to the ground end. The first voltage-controlled current source A1 is connected to the first end of the fourth capacitor C4 through the sixth switch S6 and the seventh switch S7, and the control end of the first voltage-controlled current source A1 is connected to the first end of the first capacitor C1 to receive the sampling and holding signal Vins, that is, K×Vin. The first end of the sixth switch S6 is connected to the output end of the first voltage-controlled current source A1. The first end of the seventh switch S7 is connected to the second end of the sixth switch S6, and the second end of the seventh switch S7 is connected to the first end of the fourth capacitor C4. The second voltage-controlled current source A2 is connected between the second end of the sixth switch S6 and the ground end, and the control end of the second voltage-controlled current source A2 receives a voltage representing the overvoltage protection threshold Vref, that is, K×Vref. The set end S and the reset end R of the second RS trigger U4 receive the switch drive signal GATE and the zero-crossing detection signal ZCD, respectively, and the output end Q and the output inverting end Q -The second switch control signal KS2 and the inverted signal KS2B of the second switch control signal are output respectively. The sixth switch S6 is controlled by the switch drive signal GATE, and the seventh switch S7 is controlled by the second switch control signal KS2. The sixth switch S6 is turned on only during the on-time Ton of the main power tube, and the seventh switch S7 is turned on during both the on-time Ton of the main power tube and the freewheeling time Toff of the freewheeling tube, and the seventh switch S7 is turned off after the second RS trigger U4 receives the zero-crossing detection signal ZCD. In this embodiment, the second comparator COM2 is before the AND gate U5, and one input end of the AND gate U5 is connected to the output end of the second comparator COM2, and the other input end receives the inverted signal KS2B of the second switch control signal, and the output end outputs the overvoltage control signal OVP. In other embodiments, the second comparator COM2 can also be after the AND gate U5, and one input end of the AND gate U5 is connected to the first end of the fourth capacitor C4 to receive the Vo3 voltage, the other input end receives the KS2B signal, and the output end is connected to the first input end of the second comparator COM2, the second input end of the second comparator COM2 receives the zero voltage V0, and the output end provides the overvoltage control signal OVP. The first input end and the second input end of the second comparator COM2 can be selected as the forward input end or the reverse input end according to actual needs. Thus, during the conduction time of the main power tube M1, the sixth switch S6 is turned on, the seventh switch S7 is turned on, the first voltage-controlled current source A1 charges the fourth capacitor C4, and the second voltage-controlled current source A2 discharges the fourth capacitor C4; during the freewheeling time of the freewheeling tube, the sixth switch S6 is disconnected, the seventh switch S7 is still turned on, and the second voltage-controlled current source A2 continues to discharge the fourth capacitor C4. After the inductor current passes through zero, the seventh switch S7 is disconnected, and the fourth capacitor C4 provides a voltage Vo3 to the second comparator COM2, where Vo3 is Vins×Ton-K×Vref×(Ton+Toff). When Vo3 is greater than zero voltage (Vo3>V0), Vins×Ton>K×Vref×(Ton+Toff), that is, Vins×Ds>K×Vref. Similarly, in the step-down switching power supply, Vins×Ds>K×Vref indicates that the output voltage is greater than the overvoltage protection threshold, and it is considered that the trigger condition is met, and the overvoltage protection is performed, so the second comparator COM2 and the AND gate U5 cooperate to output a valid overvoltage control signal OVP.

[0048] In a preferred embodiment, the overvoltage control unit 230 may further include a fifth capacitor C5 and an eighth switch S8. The first end of the fifth capacitor C5 is connected to the second end of the sixth switch S6, and the second end of the fifth capacitor C5 is connected to the ground terminal. The first end of the eighth switch S8 is connected to the first end of the seventh switch S7, and the second end of the eighth switch S8 is connected to the ground terminal. The eighth switch S8 is controlled by the inverted signal KS2B of the second switch control signal and is turned on when the second RS flip-flop U4 receives the zero-crossing detection signal ZCD to discharge the charge on the fifth capacitor C5. The switching power supply has a buck topology, so that within the conduction time of the main power transistor M1, the sixth switch S6 is turned on, the seventh switch S7 is turned on, and the eighth switch S8 is turned off. The first voltage-controlled current source A1 charges the fourth capacitor C4 and the fifth capacitor C5 with a current proportional to the sample-and-hold signal Vins, and the second voltage-controlled current source A2 discharges the fourth capacitor C4 and the fifth capacitor C5 with a current proportional to the overvoltage protection threshold Vref; within the freewheeling time of the freewheeling diode, the sixth switch S6 is turned off, the seventh switch S7 remains turned on, and the eighth switch S8 remains turned off, and the second voltage-controlled current source A2 continues to discharge the fourth capacitor C4 and the fifth capacitor C5. After the inductor current crosses zero, the seventh switch S7 is turned off and the eighth switch S8 is turned on to discharge the fifth capacitor C5, while the voltage of the fourth capacitor C4 is held to provide the voltage Vo3 for overvoltage judgment. When the voltage Vo3 is greater than the zero voltage (Vo3 > V0), it indicates that the output voltage Vout is greater than the overvoltage protection threshold Vref (i.e., Vins×Ton - Vref×(Ton + Toff) > 0, Vins×Ds > K×Vref, K×Vout > K×Vref, Vout > Vref), and an effective overvoltage control signal OVP is output. At the same time, the voltage of the fifth capacitor C5 is discharged to zero. Since there is no process of discharging the fourth capacitor C4 to the ground, the capacitance value of the fourth capacitor is set to be much smaller than that of the fifth capacitor C5, that is, C4 << C5. Therefore, when the sixth switch S6 is turned on, the voltage on the fourth capacitor C4 is clamped by the fifth capacitor C5 to prevent the voltage on the fourth capacitor C4 from affecting the initial value of the voltage on the fifth capacitor C5 when the sixth switch S6 is turned on next time. In addition, the proportionality coefficients of the first voltage-controlled current source A1 and the second voltage-controlled current source A2 are equal.

[0049] In other embodiments, based on the trigger condition of overvoltage protection: Vins×Ds>Vref, the trigger condition can be deformed and the circuit structure can be adjusted based on this to achieve overvoltage protection. For example, during the conduction time of the main power tube and the freewheeling time of the freewheeling tube, a current proportional to K×Vref is used to charge the fourth capacitor C4 and the fifth capacitor C5, and during the conduction time of the main power tube, a current proportional to Vins is used to discharge the fourth capacitor C4 and the fifth capacitor C5, and the obtained Vo3 is K×Vref×(Ton+Toff)-Vins×Ton, and then the voltage value is compared with the zero voltage to perform overvoltage protection. Alternatively, overvoltage protection can be performed based on the first volt-second product Vins×Ton and the second volt-second product K×Vref×(Ton+Toff). At this time, the circuit structure also needs to be adjusted. During the conduction time of the main power tube and the freewheeling time of the freewheeling tube, a current proportional to K×Vref is used to charge the fourth capacitor C4, and during the conduction time of the main power tube, a current proportional to Vins is used to charge a capacitor with a capacitance equal to the fourth capacitor C4, and the two integral voltages are compared to perform overvoltage protection. The specific circuit forms are various and will not be elaborated here.

[0050] Figure 5a and Figure 5b Schematic diagrams of a buck-boost switching power supply and an overvoltage protection circuit thereof in which a power tube is connected to an input voltage and a buck-boost switching power supply and an overvoltage protection circuit thereof in which a main power tube is connected to a reference ground according to the present invention are respectively shown.

[0051] like Figure 5aAs shown, the switching power supply of this embodiment includes a power converter 300 and a control circuit 400, and the power converter 300 includes: a rectifier bridge 301, a main power tube M2, a freewheeling tube, an inductor L2, an input capacitor Ci and an output capacitor Co, and the main power tube M2 is connected to the input end to receive the input voltage Vin. The power converter 300 takes the buck-boost topology as an example, and the freewheeling tube includes a freewheeling diode or a synchronous rectifier tube, etc., and the freewheeling diode D1 is used as an example here. The rectifier bridge 301 is connected between the live wire L and the neutral wire N, and the input voltage Vin is generated at the input end after the AC power is rectified, and the input capacitor Ci is connected between the input end and the reference ground of the power converter 300, the main power tube M2 and the freewheeling diode D2 are connected in series between the input end and the output end, and the output capacitor Co is connected between the output end and the reference ground of the power converter 300. The common node of the source end of the main power tube M2 and the cathode of the freewheeling diode D2 is node B, and the reference ground of the power converter 300 is set at node A. The inductor L2 is connected between node B and the reference ground (node ​​A). The load 30 is connected across the output capacitor Co, and the output voltage Vout is generated on the load 30. The control circuit 400 is connected to the control end of the main power tube M2, and is used to output the switch drive signal GATE to control the conduction and shutdown of the main power tube M2. The power converter 300 operates in BCM (critical conduction) or DCM (discontinuous conduction) mode, and adjacent switching cycles are separated by a certain dead time. The working principle is the same as Figure 2a and Figure 2b The on-time Ton of the main power tube M2 and the freewheeling time Toff of the freewheeling tube are the same as those in the above embodiment and will not be described in detail here.

[0052] The control circuit 400 of this embodiment is Figure 2aSimilarly, the control circuit 400 also includes an overvoltage protection circuit 410, a drive control circuit 440 and a zero-crossing detection circuit 450. The drive control circuit 440 is used to generate a switch drive signal GATE to control the on and off of the main power tube M2. The zero-crossing detection circuit 450 is used to detect information representing the current on the inductor L2, and output a zero-crossing detection signal ZCD at the zero-crossing point of the inductor current. The zero-crossing detection circuit 450, for example, directly detects the current on the inductor L2, or determines the magnitude of the current on the inductor L2 by detecting the drain terminal voltage or gate terminal voltage of the main power tube M2. The overvoltage protection circuit 410 is used to output a valid overvoltage control signal OVP when the output voltage Vout exceeds the overvoltage protection threshold Vref. The drive control circuit 440 receives the zero-crossing detection signal ZCD and the overvoltage control signal OVP, and generates a switch drive signal GATE accordingly. The drive control circuit 440 also controls the main power tube M2 to disconnect when receiving a valid overvoltage control signal OVP. The overvoltage protection circuit 410 may be a chip with multiple pins, the ground GND pin of the chip has the same potential as the node B, and during the conduction period of the main power tube M2, the potential of the GND pin is equal to the input voltage, and the input voltage cannot be sampled. The overvoltage protection circuit 410 includes a sampling and holding unit 420 and an overvoltage control unit 430. The sampling and holding unit 420 samples and holds the input voltage Vin to obtain a sampling and holding signal Vins, and the overvoltage control unit 430 performs overvoltage protection on the switching power supply according to the duty cycle, the sampling and holding signal Vins, and the voltage representing the overvoltage protection threshold, and outputs a valid overvoltage control signal OVP when the overvoltage protection is triggered. Specifically, the overvoltage control unit 230 calculates the duty cycle and the sampling and holding signal, and the overvoltage protection threshold based on the first functional relationship to obtain the trigger condition, and performs overvoltage protection when the switching power supply meets the trigger condition. Among them, the trigger condition represents that the output voltage is greater than the overvoltage protection threshold, the first functional relationship represents the relationship between the input voltage, the output voltage and the duty cycle in the switching power supply, and the first functional relationship is related to the topological structure of the switching power supply.

[0053] like Figure 5b As shown, the switching power supply includes a power converter 300 and a control circuit 400. Taking the buck topology (Buck) as an example, the power converter 300 also includes a rectifier bridge 301, a main power tube M2, a freewheeling tube (taking the freewheeling diode D2 as an example), an inductor L2, an input capacitor Ci and an output capacitor Co. The main power tube M2 is connected to the reference ground of the power converter 300. The rectifier bridge 301, the inductor L2, the input capacitor Ci and the output capacitor Co are all connected to the reference ground of the power converter 300. Figure 5a The embodiment is the same, except that Figure 5b In the power converter 300, the main power tube M2 and the freewheeling diode D2 are located on the reference ground side. The common node of the source of the main power tube M2 and the input capacitor Ci is node A, and the reference ground of the power converter 300 is set at node A. Figure 5b The working principle of the power converter 300 is Figure 5a The working principle of the power converter is the same as that of the power converter, which will not be described here. The control circuit 400 of this embodiment can also be used with Figure 5a The control circuit 400 of the embodiment is completely the same, or at least the overvoltage protection circuit 410 in the control circuit 400 is completely the same, and the reference ground potential of the overvoltage protection circuit 410 is equal to the reference ground potential of the power converter 300. That is, the overvoltage protection circuit 410 can simultaneously adapt to the switching power supply of the main power tube connected to the input voltage and the switching power supply of the main power tube connected to the reference ground.

[0054] Combination Figure 5a and Figure 5b The overvoltage protection circuit 410 of the present embodiment, which is applicable to both of the two switching power supplies shown in the figure, includes a sampling and holding unit 420 and an overvoltage control unit 430. Figure 5a The ground GND pin of the overvoltage protection circuit 410 and Figure 5b The connection position of the ground GND pin of the overvoltage protection circuit 410 is different. When the main power tube M2 is turned on Figure 5a The sampling and holding unit 420 in the embodiment cannot sample the input voltage. Therefore, in order to be compatible with the above two switching power supplies, the sampling and holding unit 420 also samples the input voltage Vin during the freewheeling time of the freewheeling tube (or during the freewheeling period of the freewheeling tube).

[0055] Specifically, the sampling and holding unit 420 receives the switch drive signal GATE to sample the input voltage Vin within the freewheeling time of the freewheeling tube, and then holds the sampled signal to obtain the sampled and held signal Vins. The sampling and holding unit 420 also receives the zero-crossing detection signal ZCD, and starts to hold the sampling signal of the input voltage when receiving the zero-crossing detection signal ZCD to obtain the sampled and held signal Vins. The overvoltage control unit 430 calculates the duty cycle and the sampling and holding signal, and the characterization signal of the overvoltage protection threshold based on the first functional relationship to obtain the trigger condition, and performs overvoltage protection when the switching power supply meets the trigger condition. Among them, the trigger condition represents that the output voltage is greater than the overvoltage protection threshold, and the first functional relationship represents the relationship between the input voltage, the output voltage and the duty cycle in the switching power supply. Taking the buck-boost topology as an example, the duty cycle may include the on-duty cycle and the freewheeling duty cycle. In the buck-boost topology, the trigger condition is: Vins×Ds / Ds'>Vref, Vins is the sampling and holding signal, Ds is the on-duty cycle, Ds' is the freewheeling duty cycle, Vref is the voltage representing the overvoltage protection threshold, and the product of the sampling and holding signal and the on-duty cycle and the ratio of the freewheeling duty cycle (Vins×Ds / Ds') represents the output voltage. In this embodiment, the overvoltage control unit 430 obtains the freewheeling time Toff of the freewheeling tube and the on-time Ton of the main power tube, and obtains the on-duty cycle and the freewheeling duty cycle accordingly. For example, the freewheeling time Toff of the freewheeling tube and the on-time Ton of the main power tube are obtained by receiving the switch drive signal GATE and the zero crossing detection signal ZCD. The sum of the on-time Ton and the freewheeling time Toff is the total time, that is, T=Ton+Toff, then the ratio of the on-time Ton to the sum of the on-time and the freewheeling time T is the on-duty ratio, Ds=Ton / T. The ratio of the freewheeling time Toff to the sum of the on-time and the freewheeling time T is the freewheeling duty ratio, Ds'=Toff / T, and Ds'=1-Ds.

[0056] Figure 6 A schematic circuit block diagram of an overvoltage protection circuit of a buck-boost switching power supply according to a first embodiment of the present invention is shown.

[0057] like Figure 6 As shown, the overvoltage protection circuit 410 can also be adapted to Figure 5a and Figure 5bThe power converter 300 performs overvoltage protection when the ratio of the product of the sampling and holding signal and the on-duty cycle to the freewheeling duty cycle is greater than the voltage representing the overvoltage protection threshold. In this embodiment, the overvoltage protection circuit 410 performs overvoltage protection based on the first volt-second product and the third volt-second product. The first volt-second product represents the product of the input voltage and the on-time, and the third volt-second product represents the product of the overvoltage protection threshold and the freewheeling time. Specifically, the sampling and holding unit 420 includes a resistor divider network 421, a first switch S1, a first capacitor C1, a third resistor R3, a voltage follower U3 and a first switch control unit 422. The resistor divider network 421 includes a first resistor R1 and a second resistor R2 connected in series, and the first switch control unit 422 includes a first RS trigger U1 and a NOT gate U2, and the sampling coefficient is K=R2 / (R1+R2). The sampling and holding unit 420 of this embodiment and Figure 3 The sampling and holding unit 220 of the embodiment is completely the same and will not be described again here. The overvoltage control unit 430 includes a second switch S2, a third switch S3, a first RC filter network and a first comparator COM1. The first RC filter network is a single-stage or multi-stage network and includes at least a second capacitor C2 and a fourth resistor R4. The second switch S2, the third switch S3, the first RC filter network and the first comparator COM1 are all connected to Figure 3 The embodiments are completely consistent and will not be described again. The voltage Vo1 received by the first input terminal of the first comparator COM1 is the product of the sample-and-hold signal Vins and the conduction duty cycle Ds, that is, Vo1 = Vins × Ds.

[0058] Since the switching power supply of this embodiment is a buck-boost topology, the overvoltage control unit 430 further includes a fourth switch S4, a fifth switch S5, and a second RC filter network on the basis of the above components. The first end of the fourth switch S4 receives a voltage representing the overvoltage protection threshold Vref, i.e., K×Vref, the first end of the fifth switch S5 is connected to the second end of the fourth switch S4, and the second end of the fifth switch S5 is connected to the ground. The ground ends are all GND pins of the chip. The second RC filter network is connected between the first end and the ground end of the fifth switch S5, the second RC filter network is a single-stage or multi-stage filter network, at least including a third capacitor C3 and a fifth resistor R5, and the output end of the second RC filter network outputs a voltage Vo2. The fourth switch S4 is controlled by the first switch control signal KS1, the fifth switch S5 is controlled by the switch drive signal GATE, the fourth switch S4 is only turned on during the freewheeling time of the freewheeling tube, and the fifth switch S5 is only turned on during the on-time of the main power tube M2.

[0059] In this embodiment, during the freewheeling time of the freewheeling tube, the switch drive signal GATE is invalid, the first RS trigger U1 is set, KS1 is high, the first switch S1 is turned on, and the input voltage Vin is sampled. After the inductor current passes through zero, the zero-crossing detection signal ZCD is high, the first RS trigger U1 is reset, KS1 is low, the first switch S1 is turned off, and the sampled signal is maintained as the sample-hold signal Vins by the first capacitor C1 and the voltage follower U3, Vins = K × Vin. In the next switching cycle, during the period when the switch drive signal GATE is high, the second switch S2 and the fifth switch S5 are turned on, the fourth switch S4, the first switch S1 and the third switch S3 are turned off, and the first RC filter network is charged according to the sample-hold signal Vins, and the second RC filter network is discharged to the ground. Due to the existence of the voltage follower U3, the sample-hold signal Vins remains unchanged during charging. After the switch drive signal GATE becomes low level, the second switch S2 and the fifth switch S5 are turned off, the KS1 signal controls the third switch S3 and the fourth switch S4 to be turned on, and the overvoltage protection threshold Vref is used to charge the second RC filter network, and the first RC filter network is discharged to the ground. After entering the steady state, the voltage Vo1 provided by the first RC filter network is Vins×Ds, and the voltage Vo2 provided by the second RC filter network is K×Vref×Ds'. Then when the voltage Vo1 is greater than the voltage Vo2, it is Vins×Ds>K×Vref×Ds', that is, K×Vin×Ds / Ds'>K×Vref, and in the buck-boost switching power supply Vout=Vin×Ds / Ds', then Vo1>Vo2 means that the output voltage Vout is greater than the overvoltage protection threshold Vref, and it is considered that the trigger condition is met, and overvoltage protection is performed. Therefore, the first comparator COM1 outputs a valid overvoltage control signal OVP to trigger overvoltage protection.

[0060] Figure 7 A schematic circuit block diagram of an overvoltage protection circuit of a buck-boost switching power supply according to a second embodiment of the present invention is shown.

[0061] like Figure 7 As shown, the overvoltage protection circuit 410 of this embodiment can also be adapted to Figure 5a and Figure 5bThe power converter 300 performs overvoltage protection when the ratio of the product of the sampling and holding signal and the on-duty cycle to the freewheeling duty cycle is greater than the voltage representing the overvoltage protection threshold. In this embodiment, the overvoltage protection circuit 410 performs overvoltage protection based on the voltage representing the difference between the first volt-second product and the third volt-second product and the zero voltage. The first volt-second product represents the product of the input voltage and the on-time, and the third volt-second product represents the product of the overvoltage protection threshold and the freewheeling time. Specifically, in this embodiment, the sampling and holding unit 420 includes a resistor divider network 421, a first switch S1, a third resistor R3, a first capacitor C1 and a first switch control unit 422. The circuit connection method and working principle of these components are the same as Figure 6 The difference is that in this embodiment, the overvoltage control unit 430 includes: a first voltage-controlled current source A1, a second voltage-controlled current source A2, a sixth switch S6, a seventh switch S7, an eighth switch S8, a fourth capacitor C4, a fifth capacitor C5, a second comparator COM2, an AND gate U5 and a second RS trigger U4. The connection relationship between these components and the circuit working principle are the same as Figure 4 The embodiment is the same and will not be described here. Since the switching power supply is a buck-boost topology, the overvoltage control unit 430 is Figure 4 The overvoltage control unit 230 of the embodiment is slightly different in that the overvoltage control unit 430 of this embodiment also includes a ninth switch S9, which is connected between the output terminal of the second voltage-controlled current source A2 and the ground terminal. The ninth switch S9 is controlled by the first switch control signal KS1 and is turned on only during the freewheeling time of the freewheeling tube.

[0062] For the overvoltage protection circuit 410, during the conduction period of the main power transistor M2, the sixth switch S6 is turned on, the seventh switch S7 is turned on, and the eighth switch S8 and the ninth switch S9 are turned off. The first voltage-controlled current source A1 charges the fourth capacitor C4 and the fifth capacitor C5 with a current proportional to the sample-and-hold signal Vins; during the off period of the main power transistor M1, the sixth switch S6 is turned off, the seventh switch S7 remains conducting, the eighth switch S8 remains off, and the ninth switch S9 is turned on. The second voltage-controlled current source A2 discharges the fourth capacitor C4 and the fifth capacitor C5 with a current proportional to the characterization voltage (K×Vref) of the overvoltage protection threshold. After the inductor current crosses zero, both the seventh switch S7 and the ninth switch S9 are turned off, the eighth switch S8 is turned on, discharging the fifth capacitor C5, while the voltage of the fourth capacitor C4 is held and provides the voltage Vo3 to the second comparator COM2 for overvoltage determination. Therefore, the voltage Vo3 provided at the first terminal of the fourth capacitor C4 is the difference between the integrated voltage of the sample-and-hold signal Vins during the on-time Ton and the integrated voltage of the characterization voltage K×Vref of the overvoltage protection threshold during the freewheeling time Toff of the freewheeling diode (Vins×Ton - K×Vref×Toff). When the voltage Vo3 is greater than the zero voltage (Vo3 > V0), Vins×Ton - K×Vref×Toff > 0, Vins×Ds×T > K×Vref×Ds’×T, Vins×Ds / Ds’ > K×Vref. Similarly, in a buck-boost switching power supply, Vo3 > V0 indicates that the output voltage Vout is greater than the overvoltage protection threshold Vref, and it is considered that the trigger condition is satisfied for overvoltage protection. Therefore, the second comparator COM2 and the AND gate U5 cooperate to output a valid overvoltage control signal OVP. At the same time, the voltage of the fifth capacitor C5 is released to zero. The capacitance value of the fourth capacitor is set to be much smaller than that of the fifth capacitor C5, i.e., C4 << C5. Therefore, when the sixth switch S6 is conducting, the voltage on the fourth capacitor C4 is clamped by the fifth capacitor C5 to avoid affecting the initial value of the voltage on the fifth capacitor C5 when the sixth switch S6 conducts next time. In addition, the proportionality coefficients of the first voltage-controlled current source A1 and the second voltage-controlled current source A2 are equal.

[0063] The above only introduces the overvoltage protection circuit of the switching power supply compatible with the main power tube connected to the input voltage and the main power tube connected to the reference ground by taking the buck topology (Buck) and the buck-boost topology (Buck-boost) as examples, but in practical applications, the above overvoltage protection circuit can also be applied to other topologies, such as the boost topology (Boost) and the flyback topology (Flyback). The relationship between the output voltage Vout, duty cycle and input voltage Vin of the switching power supplies of different topologies is different, that is, the overvoltage trigger conditions of different switching power supplies are different. Of course, it is also necessary to change the circuit structure of the overvoltage control unit so that it can adapt to the overvoltage trigger conditions of the corresponding topology. The specific circuit structure will not be described one by one here. Accordingly, the overvoltage protection circuit designed and manufactured using the above ideas should fall within the protection scope of the present invention.

[0064] It should be noted that a person skilled in the art can obtain the information of the on-duty cycle and the freewheeling duty cycle by other means, such as obtaining the on-duty cycle and the freewheeling duty cycle according to the changing slope of the inductor current. The technology of obtaining the duty cycle according to the changing slope is already a mature prior art and will not be elaborated here. After obtaining the information of the on-duty cycle and the freewheeling duty cycle according to the changing slope, it is only necessary to set a multiplier and a divider to process the sampling and holding signal, the on-duty cycle and the freewheeling duty cycle of the input voltage to obtain the characterization signal of the output voltage, so as to compare and perform overvoltage protection. The circuit structure is also a mature technology and is only briefly described here without detailed explanation.

[0065] Figure 8 A schematic flow chart of an overvoltage protection method for a switching power supply according to an embodiment of the present invention is shown.

[0066] The present invention provides an overvoltage protection method for a switching power supply. Figure 2a-Figure 7 The overvoltage protection circuit executes the overvoltage protection method to simultaneously adapt the switching power supply whose input terminal receives the input voltage connected to the main power tube and the switching power supply whose reference ground is connected to the main power tube, thereby solving the problems of the prior art. The overvoltage protection method of this embodiment specifically includes the following steps:

[0067] In step S101, the input voltage is sampled within the freewheeling time of the freewheeling tube, and the sampled signal is then held to obtain a sample-and-hold signal.

[0068] In step S102, overvoltage protection is performed on the switching power supply according to the duty cycle, the sample and hold signal, and the voltage representing the overvoltage protection threshold.

[0069] In this step, the overvoltage protection of the switching power supply is performed according to the duty cycle, the sampling and holding signal, and the voltage representing the overvoltage protection threshold, including:

[0070] Based on the first functional relationship, the duty cycle and the sampling and holding signal and the overvoltage protection threshold are operated to obtain the trigger condition, and the overvoltage protection is performed when the switching power supply meets the trigger condition. The trigger condition represents that the output voltage is greater than the overvoltage protection threshold, and the first functional relationship represents the relationship between the input voltage, the output voltage and the duty cycle in the switching power supply.

[0071] The overvoltage protection method of this embodiment is implemented based on the overvoltage protection circuit of the switching power supply described in the above embodiment, and the specific circuit and working principle are not described in detail here.

[0072] In summary, the overvoltage protection circuit and overvoltage protection method of the switching power supply and the switching power supply provided by the present invention sample the input voltage during the freewheeling time of the freewheeling tube, then hold it to obtain the sampling and holding signal, and then obtain the relationship between the input voltage and the output voltage of the main power tube according to the topological structure. Thus, the switching power supply is protected from overvoltage by using the sampling and holding signal, the duty cycle, and the voltage representing the overvoltage protection threshold. Therefore, the overvoltage protection circuit can be used regardless of whether the main power tube in the switching power supply is connected to the input voltage or the reference ground. For a certain topological structure, only one overvoltage protection circuit or corresponding chip needs to be designed and manufactured, and it can be used simultaneously in the above-mentioned switching power supply. Therefore, the overvoltage protection circuit has a wide range of applications, can greatly reduce the circuit design and manufacturing costs, is convenient for mass production, and has a good overvoltage protection effect.

[0073] Furthermore, the zero-crossing detection circuit outputs a zero-crossing detection signal at the zero-crossing point of the inductor current, thereby accurately limiting the freewheeling time of the freewheeling tube. The sampling signal of the input voltage is maintained at the zero-crossing point of the inductor current to obtain a sampling and holding signal, so that a stable and accurate sampling and holding signal of the input voltage can be obtained. In addition, the representative voltage of the output voltage is calculated based on the sampling and holding signal of the previous switching cycle and the conduction time and freewheeling time of the current switching cycle. The sampling and holding unit is independent of the overvoltage control unit, and the obtained sampling and holding signal is stable, less affected by the outside world, and has small fluctuations. The obtained representative voltage of the output voltage is less different from the actual output voltage, and the accuracy of the overvoltage protection is high.

[0074] Furthermore, an overvoltage control unit with different circuit structures can be designed according to the relationship between the input voltage, the output voltage and the duty cycle under the same topology. It can be in the form of a combination of a switch and a filter circuit, or in the form of a combination of a voltage-controlled current source, a switch and a capacitor, etc. The embodiments are rich in variety, the circuit structures are diversified, and the scope of application is wide.

[0075] It should be noted that the numerical values ​​in this article are only used for exemplary description. In other embodiments of the present invention, other numerical values ​​can also be sampled to implement this solution. The specific settings should be reasonable according to the actual situation, and the present invention does not limit this.

[0076] Finally, it should be noted that: Obviously, the above embodiments are only examples for clearly explaining the present invention, and are not intended to limit the implementation methods. For ordinary technicians in the relevant field, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from this are still within the scope of protection of the present invention.

[0077] It should also be understood that the terms and expressions used herein are for description only, and one or more embodiments of this specification should not be limited to these terms and expressions. The use of these terms and expressions does not mean to exclude any equivalent features of the illustrations and descriptions (or parts thereof), and it should be recognized that various modifications that may exist should also be included in the scope of the claims. Other modifications, changes and substitutions may also exist. Accordingly, the claims should be deemed to cover all such equivalents.

Claims

1. An overvoltage protection circuit for a switching power supply, the switching power supply comprising a main power tube and a freewheeling tube, wherein: The overvoltage protection circuit comprises: A sampling and holding unit, which samples the input voltage during the freewheeling time of the freewheeling tube, and then holds the sampled signal to obtain a sampling and holding signal; and An overvoltage control unit performs overvoltage protection on the switching power supply according to the duty cycle, the sampling and holding signal and a voltage representing an overvoltage protection threshold.

2. The overvoltage protection circuit according to claim 1, wherein: When the main power tube is connected to the input voltage, the reference ground potential of the overvoltage protection circuit is the potential of the connection point between the main power tube and the inductor in the switching power supply; when the main power tube is connected to the reference ground of the switching power supply, the reference ground potential of the overvoltage protection circuit is equal to the reference ground potential of the switching power supply.

3. The overvoltage protection circuit according to claim 1, wherein: The switching power supply further includes a drive control circuit and a zero-crossing detection circuit. The drive control circuit generates a switch drive signal to control the on and off of the main power tube. The zero-crossing detection circuit outputs a zero-crossing detection signal at the zero-crossing point of the inductor current. The sampling and holding unit determines the freewheeling time according to the switch driving signal and the zero-crossing detection signal, and starts to hold the sampled signal when receiving the zero-crossing detection signal to obtain the sampling and holding signal.

4. The overvoltage protection circuit according to claim 1, wherein: The overvoltage control unit calculates the duty cycle, the sampling and holding signal, and the characterization signal of the overvoltage protection threshold based on the first functional relationship to obtain a trigger condition, and performs overvoltage protection when the switching power supply meets the trigger condition; The trigger condition represents that the output voltage is greater than an overvoltage protection threshold, and the first functional relationship represents the relationship between the input voltage, the output voltage and the duty cycle in the switching power supply.

5. The overvoltage protection circuit according to claim 4, wherein: The duty cycle includes a conduction duty cycle and / or a freewheeling duty cycle, the proportion of the conduction time to the sum of the conduction time and the freewheeling time is the conduction duty cycle, and the proportion of the freewheeling time to the sum of the conduction time and the freewheeling time is the freewheeling duty cycle; When the switching power supply is a step-down switching power supply, the trigger condition is: Vins×Ds>Vref, Vins is the sampling and holding signal, Ds is the conduction duty cycle, Vref is the representative voltage of the overvoltage protection threshold, and the product of the sampling and holding signal and the conduction duty cycle represents the output voltage; When the switching power supply is a buck-boost switching power supply, the trigger condition is: Vins×Ds / Ds'>Vref, Vins is the sampling and holding signal, Ds is the conduction duty cycle, Ds' is the freewheeling duty cycle, Vref is the representative voltage of the overvoltage protection threshold, and the product of the sampling and holding signal and the conduction duty cycle and the ratio of the freewheeling duty cycle represent the output voltage. The overvoltage control unit obtains the on-duty ratio and the freewheeling duty ratio according to the freewheeling time and the on-time of the main power tube.

6. The overvoltage protection circuit according to claim 1, wherein: The sampling and holding unit comprises: A resistor voltage divider network is used to obtain a divided voltage proportional to the input voltage; A first capacitor, wherein a first end of the first capacitor is connected to an output end of the resistor voltage divider network, a second end of the first capacitor is connected to a ground end, and the first capacitor samples and holds the divided voltage; A first switch, wherein a first end of the first switch is connected to an output end of the resistor divider network, a second end of the first switch is connected to a first end of the first capacitor, and the first switch is turned on during a freewheeling time of the freewheeling tube.

7. The overvoltage protection circuit according to claim 6, wherein: The sampling and holding unit also includes: a third resistor connected between the second end of the first switch and the first end of the first capacitor; and A voltage follower, wherein the positive input terminal of the voltage follower is connected to a common node of the third resistor and the first capacitor, and the negative input terminal of the voltage follower is connected to the output terminal of the voltage follower.

8. The overvoltage protection circuit according to claim 1, 4 or 5, wherein: The switching power supply is a step-down switching power supply, and the overvoltage control unit includes: a second switch, wherein a first end of the second switch receives the sample-and-hold signal; a third switch, wherein a first end of the third switch is connected to a second end of the second switch, and a second end of the third switch is connected to a ground end; a first RC filter network, connected between the first end of the third switch and the ground end, and comprising at least a second capacitor and a fourth resistor; and a first comparator, wherein a first input terminal of the first comparator is connected to an output terminal of the first RC filter network, a second input terminal of the first comparator receives a voltage representing the overvoltage protection threshold, and an output terminal of the first comparator outputs an overvoltage control signal, The second switch is turned on only during the on-time, and the third switch is turned on only during the freewheeling time.

9. The overvoltage protection circuit according to claim 1, 4 or 5, wherein: The switching power supply is a step-down switching power supply, and the overvoltage control unit includes: a first voltage-controlled current source, connected to a first end of a fourth capacitor through a sixth switch and a seventh switch, a control end of the first voltage-controlled current source receiving the sample-and-hold signal, a first end of the sixth switch connected to an output end of the first voltage-controlled current source, a first end of the seventh switch connected to a second end of the sixth switch, a second end of the seventh switch connected to a first end of the fourth capacitor, and a second end of the fourth capacitor connected to a ground end; A second voltage-controlled current source is connected between the second end of the sixth switch and the ground end, and a control end of the second voltage-controlled current source receives a voltage representing the overvoltage protection threshold. a second comparator, wherein a first input terminal of the second comparator is connected to the first terminal of the fourth capacitor, a second input terminal of the second comparator receives a zero voltage, and an output terminal of the second comparator outputs an overvoltage control signal; The sixth switch is turned on only during the on-time, and the seventh switch is turned on during both the on-time and the freewheeling time.

10. The overvoltage protection circuit according to claim 9, wherein: The overvoltage control unit also includes: a fifth capacitor, wherein a first end of the fifth capacitor is connected to the second end of the sixth switch, and a second end of the fifth capacitor is connected to the ground; and an eighth switch, wherein a first end of the eighth switch is connected to the first end of the seventh switch, a second end of the eighth switch is connected to the ground, and the eighth switch is turned on after the freewheeling time ends to release the charge on the fifth capacitor.

11. The overvoltage protection circuit according to claim 1, 4 or 5, wherein: The switching power supply is a buck-boost switching power supply, and the overvoltage control unit comprises: a second switch, wherein a first end of the second switch receives the sample-and-hold signal; a third switch, wherein a first end of the third switch is connected to a second end of the second switch, and a second end of the third switch is connected to a ground end; A first RC filter network, connected between the first end of the third switch and the ground end, comprising a second capacitor and a fourth resistor; a fourth switch, wherein a first end of the fourth switch receives a voltage representative of the overvoltage protection threshold; a fifth switch, wherein a first end of the fifth switch is connected to a second end of the fourth switch, and a second end of the fifth switch is connected to a ground end; and A second RC filter network, connected between the first end of the fifth switch and the ground end, comprising at least a third capacitor and a fifth resistor; a first comparator, wherein a first input terminal of the first comparator is connected to an output terminal of the first RC filter network, a second input terminal of the first comparator is connected to an output terminal of the second RC filter network, and an output terminal of the first comparator outputs an overvoltage control signal, The fourth switch is turned on only during the freewheeling time, and the fifth switch is turned on only during the on-time.

12. The overvoltage protection circuit according to claim 1, 4 or 5, wherein: The switching power supply is a buck-boost switching power supply, and the overvoltage control unit comprises: a first voltage-controlled current source, connected to a first end of a fourth capacitor through a sixth switch and a seventh switch, a control end of the first voltage-controlled current source receiving the sample-and-hold signal, a first end of the sixth switch connected to an output end of the first voltage-controlled current source, a first end of the seventh switch connected to a second end of the sixth switch, a second end of the seventh switch connected to a first end of the fourth capacitor, and a second end of the fourth capacitor connected to a ground end; A second voltage-controlled current source is connected between the second end of the sixth switch and the ground end, and a control end of the second voltage-controlled current source receives a voltage representing the overvoltage protection threshold. a second comparator, wherein a first input terminal of the second comparator is connected to the first terminal of the fourth capacitor, a second input terminal of the second comparator receives a zero voltage, and an output terminal of the second comparator outputs an overvoltage control signal; and A ninth switch is connected between the output terminal of the second voltage-controlled current source and the ground terminal, and the ninth switch is turned on only during the freewheeling time of the freewheeling tube.

13. A switching power supply comprising: According to the overvoltage protection circuit according to any one of claims 1 to 12, when the overvoltage protection circuit outputs a valid overvoltage control signal, the switching power supply is protected against overvoltage.

14. The switching power supply according to claim 13, wherein: When the main power tube in the switching power supply is connected to the input voltage, the reference ground potential of the overvoltage protection circuit is the potential of the connection point between the main power tube and the inductor in the switching power supply; when the main power tube is connected to the reference ground of the switching power supply, the reference ground potential of the overvoltage protection circuit is equal to the reference ground potential of the switching power supply.

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