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 generates an output voltage according to an input voltage through periodic connection and disconnection of a main power tube. The overvoltage protection circuit comprises a voltage sampling unit which samples node voltage on a sampling node of the switching power supply to obtain node sampling voltage, and the node voltage is the sum of the product of output voltage and coefficient and input voltage; and the overvoltage control unit is used for performing overvoltage protection on the switching power supply according to the conduction duty ratio, the node sampling voltage and the characterization voltage of the overvoltage protection threshold. According to the overvoltage protection method, a proper sampling point is selected to enable the sampling voltage to be related to the output voltage and the input voltage at the same time, then the node voltage is sampled to calculate the output voltage, overvoltage protection is performed, sampling is simple, the calculated amount is small, errors are small, the overvoltage protection effect is good, an auxiliary winding is not needed, and the system cost is low.

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 Schematic diagrams of a conventional buck-boost switching power supply and its overvoltage protection circuit and a flyback switching power supply and its overvoltage protection circuit are shown respectively. Figure 1a As shown, the switching power supply 10 (for example, using Buck-boost 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 input capacitor Ci is connected between the input terminal and the reference ground, the inductor L0 and the main power tube M0 are connected in series between the input terminal and the reference ground, the anode of the freewheeling diode D0 is connected to the common node of the main power tube M0 and the inductor L0, and the output capacitor Co is connected between the output terminal and the cathode of the freewheeling diode D0. The load 30 is connected in parallel to both ends of the output capacitor Co. The overvoltage protection circuit 20 of the switching power supply includes an input sampling unit 21 and an overvoltage control unit 22. The input sampling unit 21 directly samples the input voltage Vin, and the sampled voltage is input to the overvoltage control unit 22. The overvoltage control unit 22 calculates the output voltage according to the input voltage Vin and the acquired duty cycle D and freewheeling duty cycle D', and then generates an overvoltage protection signal OVP by comparing the result with the internally set overvoltage protection threshold.

[0004] Figure 1b It is a flyback switching power supply, and its topology is similar to Figure 1a Similar, but different, Figure 1b In the circuit, the transformer T0 is used to replace the inductor current L0. The transformer T0 includes a primary winding Np connected to the main power tube M0 and a secondary winding Ns connected to the freewheeling diode D0. The output voltage cannot be sampled directly, so the output voltage can be sampled through the auxiliary winding Na and the resistors Ru and Rp connected to the auxiliary winding Na. The overvoltage protection circuit 20 compares the sampled voltage with the internally set overvoltage protection threshold to generate an overvoltage protection signal OVP.

[0005] For the above-mentioned switching power supply, a lot of sampling information needs to be obtained during overvoltage protection, or the sampling needs to rely on the auxiliary winding, and the sampling accuracy is low. The calculation of the output voltage is complex, the cost is high, it is time-consuming, the circuit design is cumbersome, and the system error is large. 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 an input end of the switching power supply receives an input voltage, the switching power supply comprises a main power tube and a freewheeling tube, and an output voltage is generated at an output end by periodically turning on and off the main power tube, wherein the overvoltage protection circuit comprises: a voltage sampling unit, which samples a node voltage on a sampling node of the switching power supply to obtain a node sampling voltage; an overvoltage control unit, which performs overvoltage protection on the switching power supply according to a conduction duty cycle, the node sampling voltage and a voltage representing an overvoltage protection threshold, wherein the node voltage is the sum of the product of the output voltage and a coefficient and the input voltage; and the conduction duty cycle is the ratio of the conduction time of the main power tube to the sum of the conduction time of the main power tube and the freewheeling time of the freewheeling tube.

[0008] Optionally, the overvoltage control unit operates on the on-duty cycle, the sampling and holding signal, and the voltage representing the overvoltage protection threshold based on the relationship among the output voltage, input voltage, and on-duty cycle of the switching power supply, so as to perform overvoltage protection when a trigger condition is met, wherein the trigger condition represents that the output voltage is greater than a preset overvoltage protection threshold.

[0009] Optionally, the overvoltage control unit performs overvoltage protection when Vnos×Ds>Vref, wherein Vnos is the node sampling voltage, Ds is the on-duty cycle, and Vref is a voltage representing the overvoltage protection threshold.

[0010] Optionally, the switching power supply also includes an inductor, the inductor and the main power tube are connected in series between the input end and the reference ground, one end of the freewheeling tube is connected to a common node of the inductor and the main power tube, the other end of the freewheeling tube is connected to the output end, the sampling node is one end of the freewheeling tube connected to the output end, and the coefficient is 1.

[0011] Optionally, the switching power supply also includes a transformer, the primary winding of the transformer is connected to the main power tube, the secondary winding of the transformer is connected to the freewheeling tube, the sampling node is one end of the main power tube connected to the primary winding, and the coefficient is N, where N is the turns ratio of the primary winding to the secondary winding.

[0012] Optionally, the voltage sampling unit samples the node voltage within the freewheeling time of the freewheeling tube, and maintains the sampled signal within a time period outside the freewheeling time of the freewheeling tube to obtain the node sampling voltage.

[0013] Optionally, the voltage sampling unit includes: a voltage divider circuit, which obtains the divided voltage of the node voltage; a first capacitor, wherein the first end of the first capacitor is connected to the output end of the voltage divider circuit, 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 voltage divider circuit, the second end of the first switch is connected to the first end of the first capacitor, the first switch is turned on during the freewheeling time of the freewheeling tube, and is turned off in a time period outside the freewheeling time of the freewheeling tube.

[0014] Optionally, the overvoltage control unit calculates the product of the node sampling voltage and the on-duty cycle to obtain a representative voltage of the output voltage, and compares the representative voltage of the output voltage with the representative voltage of the overvoltage protection threshold to perform overvoltage protection.

[0015] Optionally, the overvoltage control unit includes: a voltage follower, a positive input terminal of the voltage follower receives the node sampling voltage, and a reverse input terminal and an output terminal of the voltage follower are connected; a filtering network, an output terminal outputs a voltage representing the output voltage; a second switch, a first end of the second switch is connected to the output terminal of the voltage follower, and a second end of the second switch is connected to the input terminal of the filtering network; and a third switch, a first end of the third switch is connected to the second end of the second switch, and a second end of the third switch is connected to the ground terminal, 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 overvoltage control unit charges the energy storage element according to the node sampling voltage and discharges the energy storage element according to the voltage representing the overvoltage protection threshold during the on-time, and discharges the energy storage element according to the voltage representing the overvoltage protection threshold during the freewheeling time to generate a third value representing the difference between the first value and the second value, and compares the third value with the zero voltage to perform overvoltage protection;

[0017] The first value is the product of the node sampling voltage and the conduction time, and the second value is the product of the sum of the conduction time and the freewheeling time and the overvoltage protection threshold representative voltage.

[0018] Optionally, the energy storage element includes a third capacitor, and the overvoltage control unit also includes: a first voltage-controlled current source, whose control end receives the node sampling voltage to generate a first current proportional to the node sampling voltage, and charges the third capacitor during the on-time; and a second voltage-controlled current source, whose control end receives the characterization voltage of the overvoltage protection threshold to generate a second current proportional to the characterization voltage of the overvoltage protection threshold, and discharges the third capacitor during the on-time and the freewheeling time; a fourth switch, whose first end is connected to the output end of the first voltage-controlled current source, and whose second end is connected to the input end of the second voltage-controlled current source; and a fifth switch, whose first end is connected to the second end of the fourth switch, whose second end is connected to the first end of the third capacitor, whose second end is connected to the ground, and whose third value is obtained according to the voltage of the third capacitor.

[0019] Optionally, the overvoltage control unit also includes: a fourth capacitor, a first end of the fourth capacitor is connected to the first end of the fifth switch, and a second end of the fourth capacitor is connected to the ground; and a sixth switch, a first end of the sixth switch is connected to the first end of the fifth switch, and a second end of the sixth switch is connected to the ground, and the sixth switch is turned on after the freewheeling time ends to release the charge on the fourth capacitor.

[0020] Optionally, the overvoltage control unit discharges the energy storage element according to the node sampling voltage and charges the energy storage element according to the voltage representative of the overvoltage protection threshold during the on-time, and charges the energy storage element according to the voltage representative of the overvoltage protection threshold during the freewheeling time to generate a fourth value representative of the difference between the second value and the first value, and compares the fourth value with the zero voltage to perform overvoltage protection;

[0021] The first value is the product of the node sampling voltage and the conduction time, and the second value is the product of the sum of the conduction time and the freewheeling time and the overvoltage protection threshold representative voltage.

[0022] Optionally, the overvoltage control unit obtains a first value representing the product of the node sampling voltage and the conduction time of the main power tube according to a multiplication operation, and the overvoltage control unit also performs a multiplication operation to obtain a second value representing the product of the sum of the conduction time and the freewheeling time and the overvoltage protection threshold characterizing voltage, and the overvoltage control unit compares the first value and the second value to perform overvoltage protection; or, the overvoltage control unit calculates the difference between the first value and the second value, and compares the difference between the first value and the second value with zero voltage to perform overvoltage protection; or, the overvoltage control unit calculates the difference between the second value and the first value, and compares the difference between the second value and the first value with zero voltage to perform overvoltage protection.

[0023] Optionally, the switching power supply also includes a drive control circuit and a zero-crossing detection circuit, the drive control circuit generates a switch drive signal to control the conduction state 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, and the overvoltage control unit receives the switch drive signal and the zero-crossing detection signal to obtain the freewheeling time of the freewheeling tube according to the switch drive signal and the zero-crossing detection signal, and obtain the conduction time of the main power tube according to the switch drive signal.

[0024] According to another aspect of the present invention, a switching power supply is provided, the switching power supply comprising a power converter and a control circuit, the power converter comprising a main power tube and a freewheeling tube, wherein the control circuit comprises: a drive control circuit, generating a switch drive signal to control the on and off of the main power tube, so that the power converter generates an output voltage at an output end according to an input voltage at an input end; and 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.

[0025] The beneficial effects of the present invention include at least:

[0026] The overvoltage protection circuit of the switching power supply and the switching power supply provided by the present invention samples the node voltage at the sampling node of the switching power supply to obtain the node sampling voltage, and then performs overvoltage protection according to the conduction duty cycle, the node sampling voltage and the characterization voltage of the overvoltage protection threshold. The node voltage is the sum of the product of the output voltage and the coefficient and the input voltage, so that the node sampling voltage containing the information of the input voltage and the output voltage can be sampled at one sampling point, and the output voltage can be easily calculated according to the node sampling voltage. The required sampling and calculation amount is small, the circuit structure is simple, the response is fast, the system error is small, and no auxiliary winding is required, saving chip area and cost. Further, the relationship between the input voltage, the output voltage and the conduction duty cycle is obtained according to the topological structure, and the representative voltage of the sampling and holding signal, the conduction duty cycle and the overvoltage protection threshold is used for calculation based on the relationship, so that when the calculation result characterizes that the output voltage is greater than the preset overvoltage protection threshold, it is considered that the trigger condition is met and overvoltage protection is performed. Therefore, the output voltage can be characterized according to the node sampling voltage, and the trigger condition of the overvoltage protection is accurately obtained, the overvoltage protection has high accuracy, and the system reliability is high. Moreover, the circuit designs corresponding to the operations are diverse, have good adaptability, require less calculations, have small system errors, and have low circuit costs.

[0027] Furthermore, the zero-crossing detection circuit detects the zero-crossing point of the inductor current to output a zero-crossing detection signal, so that the freewheeling time of the freewheeling tube can be accurately limited, making the voltage sampling and the calculation of the output voltage more accurate. Moreover, for the flyback switching power supply, by sampling the signal during the freewheeling time of the freewheeling tube and maintaining the signal in the time period outside the freewheeling time of the freewheeling tube, the node sampling voltage is obtained, and stable and accurate information on the input voltage and output voltage can be obtained. The voltage sampling unit and the overvoltage control unit are independent of each other, and the obtained node sampling voltage is stable, less affected by the outside world, and has small fluctuations. The obtained output voltage representation voltage is slightly different from the actual output voltage, and the overvoltage protection has high accuracy.

[0028] Furthermore, an overvoltage control unit with different circuit structures can be designed according to the relationship between the input voltage, output voltage, and on-duty cycle under the same topology. It can be in the form of a combination of switches and filter circuits, or in the form of a voltage-controlled current source, a switch, and a capacitor combination, etc. The embodiments are rich in variety, the circuit structures are diversified, and the scope of application is wide.

[0029] 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

[0030] Figure 1a and Figure 1b Schematic diagrams of a conventional buck-boost switching power supply and its overvoltage protection circuit and a flyback switching power supply and its overvoltage protection circuit are shown respectively;

[0031] Figure 2 A schematic diagram of a buck-boost switching power supply and an overvoltage protection circuit thereof according to an embodiment of the present invention is shown;

[0032] Figure 3 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 4 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 5 A schematic diagram of a flyback switching power supply and an overvoltage protection circuit thereof according to an embodiment of the present invention is shown;

[0035] Figure 6 A schematic circuit block diagram of an overvoltage protection circuit of a flyback switching power supply according to a first embodiment of the present invention is shown;

[0036] Figure 7 A schematic circuit block diagram of an overvoltage protection circuit of a flyback switching power supply according to a second embodiment of the present invention is shown. DETAILED DESCRIPTION

[0037] 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.

[0038] Figure 2 A schematic diagram of a buck-boost switching power supply and an overvoltage protection circuit thereof according to an embodiment of the present invention is shown.

[0039] like Figure 2As shown, the switching power supply of this embodiment includes a power converter 100 and a control circuit 200. The power converter 100 is a step-up / down converter, including a main power tube M1, a freewheeling tube, an inductor L1, an input capacitor Ci and an output capacitor Co. The power converter 100 takes a buck topology (Buck-boost) as an example. The input end of the power converter 100 receives an input voltage Vin, the input capacitor Ci is connected between the input end and the reference ground, and the inductor L1 and the main power tube M1 are connected in series between the input end and the reference ground. The freewheeling tube can be a freewheeling diode or a synchronous rectifier. Here, the freewheeling diode D1 is taken as an example. The anode of the freewheeling diode D1 is connected to the common node of the main power tube M1 and the inductor L1, the cathode of the freewheeling diode D1 is connected to the output end, and the output capacitor Co is connected between the input end (one end of the inductor L1) and the cathode of the freewheeling diode D1. The voltage across the output capacitor Co is the output voltage Vout, and the common node between the output capacitor Co and the cathode of the freewheeling diode D1 is the node A. The control circuit 200 is connected to the control terminal of the main power tube M1 and is used to output a switch driving signal GATE to control the on and off of the main power tube M1 .

[0040] The control circuit 200 controls the freewheeling diode D1 to turn off during the conduction of the main power tube M1, the inductor L1 stores energy, and the inductor current increases; after the control circuit 200 controls the main power tube M1 to turn off, the freewheeling diode D1 is turned on, and the electric energy stored in the inductor L1 is provided to the output end. When the output end is connected to a load, an output current can be 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 increases and then decreases. The required output voltage Vout is provided at the output end through the conduction and shutdown of multiple switching cycles. The power converter 100 operates in BCM (critical conduction) or DCM (discontinuous conduction) mode. In a switching cycle, the inductor current first increases and then decreases, and adjacent switching cycles are separated by a certain dead time. For example, when the switch drive signal GATE is at a high level, the inductor current rises, and during the conduction period of the main power tube M1, the time period from the start of the conduction to the start of the shutdown of the main power tube M1 is the conduction time of the main power tube M1; when the switch drive signal GATE is at a low level, the inductor current decreases, and the zero-crossing detection signal ZCD is generated when the inductor current drops to the zero-crossing point. The time period from the main power tube M1 being turned off according to the switch drive signal GATE 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 and the freewheeling tube is turned on. The time period from the zero-crossing point of the inductor current to the next turn-on of the main power tube M1 is the dead time. 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.

[0041] Further, the control circuit 200 of the embodiment of the present invention includes an overvoltage protection circuit 210, a drive control circuit 250 and a zero-crossing detection circuit 260. The drive control circuit 250 is used to generate a switch drive signal GATE to control the conduction and shutdown of the main power tube M1, so that the power converter 100 generates an output voltage Vout at the output end according to the input voltage Vin at the input end. The zero-crossing detection circuit 260 is used to detect a signal representing the current on the inductor L1, and output a zero-crossing detection signal ZCD at the zero-crossing point of the inductor current. The overvoltage protection circuit 210 samples the node voltage Vno on the sampling node of the switching power supply to obtain the node sampling voltage Vnos, and then performs overvoltage protection on the switching power supply according to the node sampling voltage Vnos and the overvoltage protection threshold voltage Vref. When the output voltage Vout is greater than the preset overvoltage protection threshold Vth, an effective overvoltage control signal OVP is output to trigger overvoltage protection. The overvoltage protection threshold voltage Vref is used to represent the overvoltage protection threshold Vth, and is in a certain proportional relationship with the overvoltage protection threshold Vth. The drive control circuit 250 is also used to receive the zero-crossing detection signal ZCD and the overvoltage control signal OVP to generate the switch drive signal GATE. The drive control circuit 250 controls the main power tube M1 to disconnect when receiving the 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.

[0042] In this embodiment, the overvoltage protection circuit 210 includes a voltage sampling unit 220 and an overvoltage control unit 230. The voltage sampling unit 220 samples the node voltage Vno of the sampling node (node ​​A) of the switching power supply to obtain a node sampling voltage Vnos proportional to the node voltage Vno, and the node voltage Vno is the sum of the product of the output voltage Vout and the coefficient K and the input voltage Vin, that is, Vno=K×Vout+Vin. For this buck-boost topology, the coefficient K is 1, that is, Vno=Vout+Vin. The overvoltage control unit 230 performs overvoltage protection on the switching power supply according to the on-duty cycle Ds, the node sampling voltage Vnos and the voltage Vref representing the overvoltage protection threshold. The on-duty cycle Ds is the ratio of the on-time Ton of the main power tube to the sum of the on-time of the main power tube and the freewheeling time of the freewheeling tube (T=Ton+Toff). For example, the on-time Ton of the main power tube M1 and the freewheeling time Toff of the freewheeling tube are obtained by receiving the switch drive signal GATE and the zero-crossing detection signal ZCD, thereby obtaining the on-duty cycle Ds. Specifically, the overvoltage control unit 230 also calculates the on-duty cycle Ds, the sampling and holding signal Vnos and the voltage Vref representing the overvoltage protection threshold based on the relationship between the output voltage Vout, the input voltage Vin and the on-duty cycle Ds corresponding to the topological structure of the switching power supply, so as to perform overvoltage protection when the trigger condition is met, and the trigger condition represents that the output voltage Vout is greater than the preset overvoltage protection threshold Vth. For the buck-boost topology, the relationship between the output voltage Vout and the input voltage Vin is: Vout×(1-Ds)=Vin×Ds, which is transformed into: Vout=(Vin+Vout)×Ds, Ds is the on-duty cycle, so the output voltage can be represented by the product of the node sampling voltage Vnos and the on-duty cycle Ds or its transformation. With Vref as the characterizing voltage of the overvoltage protection threshold, the characterizing voltage Vref of the overvoltage protection threshold is proportional to the preset overvoltage protection threshold Vth. When the condition Vnos×Ds>Vref is satisfied, it is considered that Vout>Vth, triggering the overvoltage protection, so the triggering condition can be Vnos×Ds>Vref. Vnos×Ds>Vref can also have many variations, for example, Vnos×Ds×T>Vref×T, Vnos×Ton-Vref×T>0, etc., which can be regarded as the triggering condition Vnos×Ds>Vref being satisfied or satisfied. Therefore, no matter what circuit form the overvoltage protection unit 230 adopts, the overvoltage protection can be triggered as long as the condition Vnos×Ds>Vref is satisfied.

[0043] In this embodiment, a switching power supply and its overvoltage protection circuit 210 are introduced by taking a buck-boost topology as an example. However, in actual applications, the overvoltage protection circuit 210 of this embodiment can also be adapted to switching power supplies of other topologies at the same time, and different circuit forms of overvoltage control units 230 can be provided for different topologies, which are described in detail below.

[0044] Figure 3 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.

[0045] like Figure 3 As shown, the overvoltage protection circuit 210 is suitable for Figure 2 The power converter 100. The overvoltage control unit 230 obtains the on-time Ton of the main power tube M1 and the freewheeling time Toff of the freewheeling tube according to the switch drive signal GATE and the zero-crossing detection signal ZCD, thereby obtaining the on-duty cycle Ds of the corresponding switching power supply. In this embodiment, the overvoltage control unit 230 obtains the relationship between the output voltage Vout, the on-duty cycle Ds and the input voltage Vin according to the topological structure of the switching power supply, so as to calculate the product of the on-duty cycle Ds and the node sampling voltage Vnos to obtain the representative voltage Vo1 of the output voltage, and then uses the product of the preset overvoltage protection threshold Vth and the proportional coefficient K1 as the representative voltage of the overvoltage protection threshold, and Vref represents the representative voltage of the overvoltage protection threshold, then Vref=K1×Vth. The overvoltage protection unit 230 also compares the representative voltage Vo1 of the output voltage with the representative voltage Vref of the overvoltage protection threshold to perform overvoltage protection.

[0046] Specifically, the voltage sampling unit 220 includes a first resistor R1 and a second resistor R2 connected in series, which are used to obtain the divided voltage of the node voltage Vno, that is, the node sampling voltage Vnos. The middle node of the first resistor R1 and the second resistor R2 is the output end, then Vnos=Vno×R2 / (R1+R2)=Vno×K1, and the proportionality coefficient in this embodiment can be K1. Therefore, the voltage sampling unit 220 continuously samples during the entire switching cycle of the switching power supply, that is, the node sampling voltage Vnos can be obtained at any time during the switching cycle. The overvoltage control unit 230 includes an electric energy supply unit 231, an energy storage unit 232, a switch unit 233 and a comparison unit 235. The electric energy supply unit 231 receives the node sampling voltage Vnos to provide electric energy to the energy storage unit 232, such as providing a charging voltage or a charging current. The switch unit 233 is connected between the electric energy supply unit 231 and the energy storage unit 232, and controls the charging time of the energy storage unit 232 according to its own conduction time, so as to achieve the effect of adjusting the representative voltage Vo1 of the output voltage. The comparison unit 235 performs overvoltage protection according to the characteristic voltage Vo1 of the output voltage and the characteristic voltage Vref of the overvoltage protection threshold. The overvoltage control unit 230, for example, uses the node sampling voltage Vnos to charge the energy storage element during the conduction time of the main power tube, and uses the ground voltage GND to discharge the energy storage element during the freewheeling time of the freewheeling tube to obtain the product of the conduction duty cycle Ds and the node sampling voltage Vnos. In this embodiment, the overvoltage control unit 230 may also include a switch control unit 234 for controlling the conduction and disconnection of the switch unit 233. The power supply unit 231, for example, includes a voltage follower U3, and the energy storage unit 232, that is, the energy storage element, for example, includes a single-stage or multi-stage filter network, and the filter network may be an RC filter network or an LC filter network, etc. Here, the RC filter network is taken as an example. The switch unit 233 includes a second switch S2 and a third switch S3, and the comparison unit 235 includes a first comparator COM1. The positive input terminal of the voltage follower U3 is connected to the middle node of the first resistor R1 and the second resistor R2, and receives the node sampling voltage Vnos. The reverse input terminal of the voltage follower U3 is connected to the output terminal. The first terminal of the second switch S2 is connected to the output terminal of the voltage follower U3, and receives the node sampling voltage Vnos. The first terminal of the third switch S3 is connected to the second terminal of the second switch S2, and the second terminal of the third switch S3 is connected to the ground terminal. The RC filter network is connected between the first terminal of the third switch S3 and the ground terminal. The 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 third resistor R3. The first input terminal of the first comparator COM1 is connected to the output terminal of the RC filter network, and receives the representative voltage Vo1 of the output voltage, that is, the product of the on-duty cycle Ds and the node sampling voltage Vnos.The second input terminal of the first comparator COM1 receives the voltage Vref representing the overvoltage protection threshold, Vref=Vth×K1=Vth×R2 / (R1+R2), and the output terminal of the first comparator COM1 outputs the overvoltage control signal OVP. In this embodiment, the second switch S2 is only turned on during the conduction time of the main power tube, and the third switch S3 is only turned on during the freewheeling time of the freewheeling tube. Figure 3 The switching power supply applicable to the overvoltage protection circuit 210 is a buck-boost topology structure, and the corresponding relationship between the output voltage Vout, the on-duty cycle Ds and the input voltage Vin is: Vout = (Vin + Vout) × Ds = Vno × Ds = Vnos × Ds / K1, so Vnos × Ds (Vno × Ds × K1) can be used as the representative voltage Vo1 of the output voltage, and by comparing Vnos × Ds with Vref, it can be known whether the output voltage exceeds the overvoltage protection threshold.

[0047] The switch control unit 234 includes an RS trigger U1 and a NOT gate U2 (not shown in the figure), 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 RS trigger U1, the reset end R of the RS trigger U1 receives the zero-crossing detection signal ZCD, and the output end Q outputs the first switch control signal KS1. The RS trigger U1 is set according to the inverted signal GATEB of the switch drive signal, and 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 time period when the main power tube M1 is turned off. Then, during the period when the main power tube M1 is turned off and the switch drive signal GATE is invalid, the RS trigger U1 is set, KS1 is high, the first switch S1 is turned on, and the node voltage Vno is sampled. After the inductor current passes through zero, the zero-crossing detection signal ZCD is high, the RS trigger U1 is reset, KS1 is low, the first switch S1 is turned off, and the sampled signal is maintained as the node sampling voltage Vnos 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 turned off, and the RC filter network is charged according to the node sampling voltage Vnos. Due to the existence of the voltage follower U3, the node sampling voltage Vnos 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. After entering the steady state, the first RC filter network generates a voltage Vo1 representing the output voltage. Then the representative voltage Vo1 of the output voltage is the product of the node sampling voltage Vnos and the on-duty cycle Ds of the switching power supply Vnos×Ds, Vo1=Vnos×Ds. When Vo1>Vref(K1×Vth), Vno×K1×Ds>K1×Vth, Vno×Ds>Vth, (Vin+Vout)×Ds>Vth, so Vout>Vth, that is, when the representative voltage Vo1 of the output voltage is greater than the preset overvoltage protection threshold Vth, the output voltage Vout is greater than the overvoltage protection threshold Vth, and it is considered that the trigger condition is met, and overvoltage protection is performed, and the first comparator COM1 outputs a valid overvoltage control signal OVP. When the switching cycle of charging and discharging the RC filter network is the current switching cycle, the switching cycle of sampling and holding the input voltage is the previous switching cycle.

[0048] Figure 4 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.

[0049] like Figure 4 As shown, the overvoltage protection circuit 210 of this embodiment is also applicable to Figure 2The power converter 100. The overvoltage control unit 230 thereof calculates a third value representing the difference between the first value and the second value, or calculates a fourth value representing the difference between the second value and the first value, and compares the third value with zero voltage for overvoltage protection, or compares the fourth value with zero voltage for overvoltage protection. Specifically, the overvoltage control unit 230 can calculate the third value or the fourth value according to the freewheeling time Toff of the freewheeling tube, the on-time Ton of the main power tube, the sum T of the on-time of the main power tube and the freewheeling time of the freewheeling tube, the node sampling voltage Vnos and the preset overvoltage protection threshold Vth. When the output voltage Vout is greater than the overvoltage protection threshold Vth, (Vout+Vin)×Ds>Vth, (Vout+Vin)×Ds×T-Vth×T>0, that is, (Vout+Vin)×Ton-Vth×T>0, (Vout+Vin)×K1×Ton-Vth×K1×T>0, that is, Vnos×Ton-Vth×K1×T>0, or Vth×K1×T-Vnos×Ton<0. Therefore, the product Vnos×Ton of the node sampling voltage Vnos and the conduction time Ton of the main power tube is taken as the first value, and the product T of the sum of the conduction time of the main power tube and the freewheeling time of the freewheeling tube and the overvoltage protection threshold voltage Vref is taken as the second value, and the overvoltage protection threshold voltage Vref is the product of the preset overvoltage protection threshold Vth and the proportional coefficient K1. The overvoltage control unit 230 is used to calculate a third value representing the difference between the first value and the second value (Vnos×Ton-Vth×T×K1), or a fourth value representing the difference between the second value and the first value (Vth×T×K1-Vnos×Ton), and then compare the third value or the fourth value with the zero voltage V0 to perform overvoltage protection.

[0050] Then, the overvoltage control unit 230 charges the energy storage element according to the node sampling voltage Vnos and discharges the energy storage element according to the representative voltage Vref of the overvoltage protection threshold during the conduction time of the main power tube, and discharges the energy storage element according to the representative voltage Vref of the overvoltage protection threshold during the freewheeling time of the freewheeling tube. The representative voltage Vref of the overvoltage protection threshold is the product of the overvoltage protection threshold Vth and the proportional coefficient K1, so as to output a third value representing the difference between the first value and the second value (Vnos×Ton-Vth×T×K1), and the third value is proportional to Vnos×Ton-Vth×T×K1. Alternatively, the overvoltage control unit 230 discharges the energy storage element according to the node sampling voltage Vnos and charges the energy storage element according to the voltage Vref representing the overvoltage protection threshold during the conduction time of the main power tube, and charges the energy storage element according to the voltage Vref representing the overvoltage protection threshold during the freewheeling time of the freewheeling tube, so as to output a fourth value representing the difference between the second value and the first value, and the fourth value is proportional to Vth×T×K1-Vnos×Ton. The following is an example of the calculation of the third value representing the difference between the first value and the second value by the overvoltage control unit 230.

[0051] Specifically, in this embodiment, the voltage sampling unit 220 and Figure 3The overvoltage control unit 230 also includes an energy supply unit 231, an energy storage unit 232, a switch unit 233, a switch control unit 234 and a comparison unit 235. The energy storage unit 232, i.e., the energy storage element, includes at least a third capacitor C3, and the energy supply unit 231 includes a first voltage-controlled current source A1 and a second voltage-controlled current source A2. The switch unit 233 includes a fourth switch S4 and a fifth switch S5, the switch control unit 234 includes an RS trigger U4, and the comparison unit 235 includes a second comparator COM2 and an AND gate U5. One input end of the AND gate U5 receives the third value Vo2, the other input end receives the inverted signal KS2B of the second switch control signal output by the RS trigger U4, 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, the output end of the second comparator COM2 outputs the overvoltage control signal OVP, and the first input end and the second input end of the second comparator COM2 can be a positive input end and a reverse input end, respectively. Of course, the second comparator COM2 can also be swapped with the AND gate U5, and the second comparator COM2 inputs the comparison result of the third value Vo2 and the zero voltage V0 into the AND gate U5, which is not introduced in detail here. Further, the first end of the third capacitor C3 is connected to an input end of the AND gate U5, and the third value Vo2 representing the difference between the first value and the second value is obtained according to the voltage of the third capacitor C3, and the second end of the third capacitor C3 is connected to the ground end. The first voltage-controlled current source A1 is connected to the first end of the third capacitor C3 through the fourth switch S4 and the fifth switch S5, and the control end of the first voltage-controlled current source A1 is connected to the common node of the first resistor R1 and the second resistor R2, and receives the node sampling voltage Vnos. The first end of the fourth switch S4 is connected to the output end of the first voltage-controlled current source A1. 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 first end of the third capacitor C3. The second voltage-controlled current source A2 is connected between the second end of the fourth switch S4 and the ground end, and the control end of the second voltage-controlled current source A2 receives the voltage K×Vth corresponding to the product of the overvoltage protection threshold Vth and the proportional coefficient K1. The set end S and the reset end R of the 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 fourth switch S4 is controlled by the switch drive signal GATE, and the fifth switch S5 is controlled by the second switch control signal KS2. The fourth switch S4 is turned on only during the on-time of the main power tube, and the fifth switch S5 is turned on during both the on-time and the freewheeling time of the main power tube, and the fifth switch S5 is turned off after the RS trigger U4 receives the zero-crossing detection signal ZCD. 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 second switch control signal KS2, and the output end outputs the overvoltage control signal OVP. Thus, during the conduction time of the main power tube M1, the fourth switch S4 is turned on, the fifth switch S5 is turned on, the first voltage-controlled current source A1 charges the third capacitor C3 according to the first current proportional to the node sampling voltage Vnos, and the second voltage-controlled current source A2 discharges the third capacitor C3 according to the second current proportional to the product of the preset overvoltage protection threshold Vth and the proportionality coefficient K1, and the transconductance of the first voltage-controlled current source A1 and the second voltage-controlled current source A2 are set equal. During the freewheeling time of the freewheeling tube, the fourth switch S4 is turned off, the fifth switch S5 is still turned on, and the second voltage-controlled current source A2 continues to discharge the third capacitor C3. After the inductor current passes through zero, the fifth switch S5 is turned off, and the third capacitor C3 provides the second comparator COM2 with a third value Vo2 representing the difference between the first value and the second value to perform overvoltage judgment. The third value Vo2=g m / C×(Vnos×Ton-Vth×T×K1), where g m is the transconductance of the first voltage-controlled current source A1 and the second voltage-controlled current source A2, C is the capacitance of the energy storage element, (Vnos×Ton-Vth×T×K1) is the difference between the first value and the second value, g m / C is a positive value and the third value Vo2 is greater than the zero voltage V0 (Vo2>V0), then Vnos×Ton>Vth×K1(Ton+Toff), that is, (Vout+Vin)×Ds×K1×T-Vth×K1×T>0, so (Vout+Vin)×Ds>Vth. Since Vout=(Vin+Vout)×Ds, Vout>Vth, it is considered that the trigger condition is met, and overvoltage protection is performed. The second comparator COM2 outputs a valid overvoltage control signal OVP.

[0052] In a preferred embodiment, the energy storage unit 232 may further include a fourth capacitor C4 and a sixth switch S6. The first terminal of the fourth capacitor C4 is connected to the second terminal of the fourth switch S4, and the second terminal of the fourth capacitor C4 is connected to the ground terminal. The first terminal of the sixth switch S6 is connected to the first terminal of the fifth switch S5, and the second terminal of the sixth switch S6 is connected to the ground terminal. The sixth switch S6 is controlled by the inverted signal KS2B of the second switch control signal and is turned on when the RS flip-flop U4 receives the zero-crossing detection signal ZCD to discharge the charge on the fourth capacitor C4. The switching power supply has a buck-boost topology. Thus, during the conduction time of the main power transistor M1, the fourth switch S4 is turned on, the fifth switch S5 is turned on, and the sixth switch S6 is turned off. The first voltage-controlled current source A1 charges the fourth capacitor C4 and the third capacitor C3 with a current proportional to the node sampling voltage Vnos, and the second voltage-controlled current source A2 discharges the fourth capacitor C4 and the third capacitor C3 with a current proportional to the product K1×Vth of the preset overvoltage protection threshold Vth and the proportionality coefficient K1; during the freewheeling time of the freewheeling diode, the fourth switch S4 is turned off, the fifth switch S5 remains on, and the sixth switch S6 remains off. The second voltage-controlled current source A2 continues to discharge the fourth capacitor C4 and the third capacitor C3. After the inductor current crosses zero, the fifth switch S5 is turned off, the sixth switch S6 is turned on, and the fourth capacitor C4 is discharged, while the voltage of the third capacitor C3 is maintained to provide a third value Vo2 representing the difference between the first value and the second value for overvoltage judgment. When the third value Vo2 is greater than the zero voltage V0 (Vo2>V0), then Vnos×Ton-Vth×K1×(Ton+Toff)>0, Vout>Vth), and the second comparator COM2 outputs a valid overvoltage control signal OVP. At the same time, the voltage of the fourth capacitor C4 is discharged to zero. Since there is no process of discharging the third capacitor C3 to the ground, the capacitance value of the third capacitor C3 is set to be much smaller than the capacitance value of the fourth capacitor C4, that is, C3<<C4. Therefore, when the fourth switch S4 is turned on, the voltage on the third capacitor C3 is clamped by the fourth capacitor C4 to prevent the voltage on the third capacitor C3 from affecting the initial value of the voltage of the fourth capacitor C4 when the fourth switch S4 is turned on again.

[0053] In another embodiment, the overvoltage control unit 230 may also use different circuit units to perform multiplication operations to obtain a first value representing the product of the node sampling voltage and the conduction time and a second value representing the product of the sum of the conduction time and the freewheeling time and the overvoltage protection threshold voltage, and then use a comparator to compare the first value and the second value, output the comparison result, and perform overvoltage protection according to the comparison result. For example, the overvoltage control unit 230 performs multiplication operations based on the node sampling voltage Vnos and the conduction time Ton to obtain a first value representing the product of the node sampling voltage and the conduction time, and also performs multiplication operations based on the sum of the conduction time and the freewheeling time T (T=Ton+Toff) and the overvoltage protection threshold voltage (K1×Vth) to obtain a second value representing the product of the sum of the conduction time and the freewheeling time and the overvoltage protection threshold voltage. The first value is input to the first input terminal of the comparator, and the second value is input to the second input terminal of the comparator. For example, when the first value is greater than the second value, the comparator outputs a valid level signal, and when the first value is less than the second value, the comparator outputs an invalid level signal. That is, the overvoltage control unit 230 calculates the first value of the product Vnos×Ton representing the node sampling voltage Vnos and the conduction time Ton of the main power tube, and the overvoltage control unit 230 also calculates the second value of the product (Vth×T×K1) representing the sum of the conduction time of the main power tube and the freewheeling time of the freewheeling tube and the voltage representing the overvoltage protection threshold. The overvoltage control unit 230 also compares the first value and the second value to output an overvoltage control signal to perform overvoltage protection. Alternatively, the first value and the second value can be calculated separately, and then the difference between the first value and the second value is calculated, and the overvoltage protection is performed by comparing with the zero voltage. Alternatively, the first value and the second value can be calculated separately, and then the difference between the second value and the first value is calculated, and the difference is compared with the zero voltage for overvoltage protection.

[0054] In summary, in the above-mentioned multiple embodiments, Vos=(Vin+Vout)×K1×Ton / T=Vnos×Ton / T is used to characterize the output voltage, that is, Vnos×Ton / (Ton+Toff) is calculated. In the prior art, when the input voltage is sampled to calculate the output voltage, Vo=Vin×Ds / (1-Ds). Since the sampling of the freewheeling time of the freewheeling tube depends on the detection of ZCD, that is, the result of (1-Ds) depends on the detection of ZCD, the calculation accuracy of the output voltage is poor. In this embodiment, Vnos×Ton / (Ton+Toff) is calculated. Although the sampling of Toff also depends on the detection of ZCD, due to the existence of Ton, the influence of ZCD is reduced, and the sampling and calculation accuracy of the output voltage is improved. Compared with the prior art, the error is very small and the accuracy is very high.

[0055] Figure 5 A schematic diagram of a flyback switching power supply and an overvoltage protection circuit thereof according to an embodiment of the present invention is shown.

[0056] like Figure 5 As 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 main power tube M1, a freewheeling tube, a transformer T1, an input capacitor Ci and an output capacitor Co. The power converter 300 takes a flyback topology as an example, the input end of the power converter 300 receives an input voltage Vin, the input capacitor Ci is connected between the input end and the reference ground, the transformer T1 includes a primary winding Np and a secondary winding Ns, and the main power tube M1 is connected between the primary winding Np and the reference ground. The freewheeling tube can be a freewheeling diode or a synchronous rectifier tube. Here, the freewheeling diode D1 is taken as an example, the anode of the freewheeling diode D1 is connected to the secondary winding Ns, and the cathode is connected to the output end, one end of the output capacitor Co is connected to the secondary winding Ns, and the other end is connected to the cathode of the freewheeling diode D1, and the voltage across the output capacitor Co is the output voltage Vout. The node where the main power tube M1 is connected to one end of the primary winding Np is node A, which is the sampling node of the switching power supply, and the voltage on node A is the node voltage Vno. The control circuit 400 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 power converter 300 operates in BCM (critical conduction) or DCM (discontinuous conduction) mode. The transformer T1 of this embodiment can be equivalent to the above-mentioned inductor L1, so that the working principle of the power converter 300 is the same as that of the inductor L1. Figure 2 The definitions of the on-time Ton of the main power tube M1 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.

[0057] The control circuit 400 of this embodiment is Figure 2Similar to the control circuit 200, the control circuit 400 also includes an overvoltage protection circuit 410, a drive control circuit 450 and a zero-crossing detection circuit 460. The drive control circuit 450 is used to generate a switch drive signal GATE to control the conduction and shutdown of the main power tube M1. The zero-crossing detection circuit 460 is used to detect the current on the secondary winding Ns, which is equivalent to the above-mentioned inductor current, so that the zero-crossing detection circuit 460 outputs a zero-crossing detection signal ZCD at the zero-crossing point of the inductor current. The overvoltage protection circuit 410 samples the node voltage Vno on the sampling node of the switching power supply to obtain the node sampling voltage Vnos, and then performs overvoltage protection on the switching power supply according to the node sampling voltage Vnos and the voltage Vref representing the overvoltage protection threshold, and outputs a valid overvoltage control signal OVP when the output voltage Vout is greater than the preset overvoltage protection threshold Vth. The drive control circuit 450 is also used to receive the zero-crossing detection signal ZCD and the overvoltage control signal OVP to generate the switch drive signal GATE. The drive control circuit 450 controls the main power tube M1 to disconnect when receiving the valid overvoltage control signal OVP. The overvoltage protection circuit 410 also includes a voltage sampling unit 420 and an overvoltage control unit 430. The voltage sampling unit 420 samples and holds the node voltage Vno to obtain the node sampling voltage Vnos. The overvoltage control unit 430 performs overvoltage protection on the switching power supply according to the on-duty cycle Ds, the node sampling voltage Vnos and the overvoltage protection threshold representative voltage Vref. The on-duty cycle Ds is the ratio of the on-time Ton of the main power tube to the sum of the on-time of the main power tube and the freewheeling time of the freewheeling tube (T=Ton+Toff). The representative voltage Vref of the overvoltage protection threshold is used to represent the overvoltage protection threshold Vth, and is in a certain proportional relationship with the overvoltage protection threshold Vth.

[0058] exist Figure 5In the embodiment, when the main power tube M1 is turned on, the node A is grounded, which will affect the sampling of the node voltage Vno. Therefore, in the present embodiment, the voltage sampling unit 420 samples the node voltage Vno during the period when the main power tube M1 is turned off. Specifically, the voltage sampling unit 420 receives the switch drive signal GATE to sample the node voltage Vno during the freewheeling time of the freewheeling tube, and then maintains the sampled signal in a time period outside the freewheeling time of the freewheeling tube to obtain the node sampling voltage Vnos. Preferably, the voltage sampling unit 420 also receives the zero crossing detection signal ZCD to start maintaining the sampling signal of the node voltage when the zero crossing detection signal ZCD is received to obtain the node sampling voltage Vnos. That is, the conduction time of the main power tube M1 and the freewheeling time of the freewheeling tube can be well defined according to the switch drive signal GATE and the zero crossing detection signal ZCD. The time from the start of the main power tube M1 being turned off to the receipt of the zero crossing detection signal ZCD is the freewheeling time of the freewheeling tube. Sampling is performed during this time period, and the sampled signal is maintained at the end of the freewheeling time. The node voltage Vno is the sum of the product of the output voltage Vout and the coefficient K and the input voltage Vin, that is, Vno = K × Vout + Vin. For this flyback topology, the coefficient K is N, N is the turns ratio of the primary winding Np and the secondary winding Ns, that is, Vno = N × Vout + Vin, and the node sampling voltage Vnos is proportional to the node voltage Vno.

[0059] Specifically, the overvoltage control unit 430 also calculates the on-duty ratio Ds, the sampling and holding signal Vnos, and the voltage Vref representing the overvoltage protection threshold based on the relationship between the output voltage Vout, the input voltage Vin, and the on-duty ratio Ds corresponding to the topological structure of the switching power supply, so as to perform overvoltage protection when the trigger condition is met, and the trigger condition represents that the output voltage Vout is greater than the preset overvoltage protection threshold Vth. In this embodiment, the overvoltage control unit 430 obtains the voltage representing the output voltage of the current switching cycle based on the node sampling voltage Vnos obtained in the previous switching cycle and the freewheeling time Toff and the on-time Ton of the freewheeling tube in the current switching cycle, and then performs overvoltage protection. For the flyback topology, the relationship between the output voltage Vout and the input voltage Vin is: N×Vout×(1-Ds)=Vin×Ds, which is transformed into: N×Vout=(Vin+N×Vout)×Ds=Vno×Ds, where Ds is the on-duty cycle, so the output voltage can be characterized by the product of the node sampling voltage Vnos and the on-duty cycle Ds or its transformation. That is, Vos=(Vin+N×Vout)×Ton / T is used to characterize the output voltage, and Vref is used as the voltage characterizing the overvoltage protection threshold. When the condition Vnos×Ds>Vref is met, it is considered that Vout>Vth, triggering the overvoltage protection, so the triggering condition can be Vnos×Ds>Vref, which is transformed into Vnos×Ds×T>Vref×T, Vnos×Ton-Vref×T>0, etc., which can all be regarded as the condition Vnos×Ds>Vref being met. Therefore, in the overvoltage control unit 430, when the trigger condition Vnos×Ds>Vref is met, it is considered that Vout>Vth, and the overvoltage protection is triggered.

[0060] Figure 6 A schematic circuit block diagram of an overvoltage protection circuit of a flyback switching power supply according to a first embodiment of the present invention is shown.

[0061] like Figure 6 As shown, the overvoltage protection circuit 410 can be adapted Figure 5The power converter 300. The overvoltage control unit 430 obtains the on-time Ton of the main power tube M1 and the freewheeling time Toff of the freewheeling tube according to the switch drive signal GATE and the zero-crossing detection signal ZCD to obtain the on-duty cycle Ds of the corresponding switching power supply. In this embodiment, the overvoltage control unit 430 obtains the relationship between the output voltage Vout, the on-duty cycle Ds and the input voltage Vin according to the topological structure of the switching power supply to calculate the product of the on-duty cycle Ds and the node sampling voltage Vnos to obtain the representative voltage Vo1 of the output voltage, and then uses the product of the preset overvoltage protection threshold Vth and the proportional coefficient K2 as the representative voltage of the overvoltage protection threshold, and Vref represents the representative voltage of the overvoltage protection threshold, then Vref=K2×Vth. The overvoltage protection unit 230 also compares the representative voltage Vo1 of the output voltage with the representative voltage Vref of the overvoltage protection threshold to perform overvoltage protection.

[0062] Specifically, the voltage sampling unit 420 includes a voltage divider circuit 421, a first switch S1, a first capacitor C1 and a first switch control unit 422. The voltage divider circuit 421 includes a first resistor R1 and a second resistor R2 connected in series, which are used to obtain a divided voltage of the node voltage Vno, and the middle node between the first resistor R1 and the second resistor R2 is the output end of the voltage divider circuit 421. The first end of the first capacitor C1 is connected to the output end of the voltage divider circuit 421, 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 410. The first capacitor C1 samples and holds the divided voltage to obtain the node sampling voltage Vnos, then Vnos=Vno×R2 / (R1+R2)=Vno×K1, and the proportional coefficient K2 in this embodiment can be K1×N, N is the turns ratio, that is, N=Np / Ns. The first end of the first switch S1 is connected to the output end of the voltage divider circuit 221, the second end of the first switch S1 is connected to the first end of the first capacitor C1, and the control end of the first switch S1 is connected to the first switch control unit 422, and the first switch control signal KS1 output by the first switch control unit 422 controls the conduction and disconnection. The first switch control unit 422 includes an 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 RS trigger U1, the reset end R of the RS trigger U1 receives the zero-crossing detection signal ZCD, and the output end Q outputs the first switch control signal KS1. The RS trigger U1 is set according to the inverted signal GATEB of the switch drive signal, and 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, and the setting period is the time period of the freewheeling time of the freewheeling tube. Furthermore, the voltage sampling unit 420 may further include a fourth resistor R4 , and the fourth resistor R4 is connected between the second end of the first switch S1 and the first end of the first capacitor C1 .

[0063] The overvoltage control unit 430 includes an electric energy supply unit 431, an energy storage unit 432, a switch unit 433 and a comparison unit 435. The electric energy supply unit 431 receives the node sampling voltage Vnos to provide electric energy to the energy storage unit 432, such as providing a charging voltage or a charging current. The switch unit 433 is connected between the electric energy supply unit 431 and the energy storage unit 432, and controls the charging time of the energy storage unit 432 according to its own conduction time, so as to achieve the effect of adjusting the representative voltage Vo1 of the output voltage. The comparison unit 435 performs overvoltage protection according to the representative voltage Vo1 of the output voltage and the representative voltage Vref of the overvoltage protection threshold. For example, the overvoltage control unit 430 uses the node sampling voltage Vnos to charge the energy storage element during the conduction time of the main power tube, and uses the ground voltage GND to discharge the energy storage element during the freewheeling time of the freewheeling tube to obtain the product of the conduction duty cycle Ds and the node sampling voltage Vnos. In this embodiment, the power supply unit 431 includes, for example, a voltage follower U3, the energy storage unit 432 includes, for example, a single-stage or multi-stage filter network, the filter network includes an RC network or an LC network, and here the RC filter network is taken as an example, the RC filter network at least includes a third resistor R3 and a second capacitor C2, the switch unit 433 includes a second switch S2 and a third switch S3, the third switch S3 is turned on and off by the first switch control signal KS1, and the comparison unit 435 includes a first comparator COM1. The voltage follower U3, the second switch S2, the third switch S3, the first RC filter network and the first comparator COM1 of the overvoltage control unit 430 of this embodiment are all the same as Figure 3The embodiment is completely consistent, and its output terminal generates a representative voltage Vo1 of the output voltage, that is, the product of the on-duty cycle Ds and the node sampling voltage Vnos. The specific circuit principle is not repeated here. The representative voltage Vo1 of the output voltage received by the first input terminal of the first comparator COM1 is the product of the node sampling voltage Vnos and the on-duty cycle Ds of the switching power supply, that is, Vnos×Ds=Vno×Ds×R2 / (R1+R2)=Vno×Ds×K1, and the representative voltage Vref of the overvoltage protection threshold received by the second input terminal of the first comparator COM1 is the product of the overvoltage protection threshold Vth and the proportional coefficient K2, Vref=Vth×K2=Vth×N×R2 / (R1+R2), and the output terminal of the first comparator COM1 outputs the overvoltage control signal OVP. When the output voltage representative voltage Vo1 is greater than the overvoltage protection threshold representative voltage Vref, Vo1>Vref, Vno×K1×Ds>K1×N×Vth, Vno×Ds>N×Vth, (Vin+N×Vout)×Ds>N×Vth, so N×Vout>N×Vth, Vout>Vth, that is, when the output voltage representative voltage Vo1 is greater than the preset overvoltage protection threshold Vth, the output voltage Vout is greater than the overvoltage protection threshold Vth, it is considered that the trigger condition is met, overvoltage protection is performed, and an effective overvoltage control signal OVP is output. The working principle of the circuit will not be repeated here.

[0064] Figure 7 A schematic circuit block diagram of an overvoltage protection circuit of a flyback switching power supply according to a second embodiment of the present invention is shown.

[0065] like Figure 7 As shown, the overvoltage protection circuit 410 of this embodiment can also be applied to Figure 5 The voltage sampling unit 420 can be connected with the power converter 300. Figure 6The voltage sampling unit 420 of the embodiment is completely consistent and will not be described in detail here. The overvoltage control unit 430 thereof calculates a third value representing the difference between the first value and the second value, or calculates a fourth value representing the difference between the second value and the first value, and compares the third value with zero voltage for overvoltage protection, or compares the fourth value with zero voltage for overvoltage protection. Specifically, the overvoltage control unit 430 can calculate a third value representing the difference between the first value and the second value, or a fourth value representing the difference between the second value and the first value, based on the freewheeling time Toff of the freewheeling tube, the on-time Ton of the main power tube, the sum T of the on-time of the main power tube and the freewheeling time of the freewheeling tube, the node sampling voltage Vnos and the preset overvoltage protection threshold Vth. When the output voltage Vout is greater than the overvoltage protection threshold Vth, (N×Vout+Vin)×Ds>N×Vth, (N×Vout+Vin)×Ds×T-Vth×N×T>0, that is, (N×Vout+Vin)×Ton-Vth×N×T>0, Vno×Ton-Vth×N×T>0, Vno×K1×Ton-Vth×N×K1×T>0, that is, Vnos×Ton-Vth×K2×T>0, or Vth×K2×T-Vnos×Ton<0. Therefore, the product Vnos×Ton of the node sampling voltage Vnos and the conduction time Ton of the main power tube is taken as the first value, and the sum T of the conduction time of the main power tube and the follow-up time of the follow-up tube and the product T×Vth×K2 of the preset overvoltage protection threshold Vth and the proportional coefficient K2 is taken as the second value. The overvoltage control unit 430 is used to calculate a third value representing the difference between the first value and the second value (Vnos×Ton-Vth×T×K2), or a fourth value representing the difference between the second value and the first value (Vth×T×K2-Vnos×Ton), and then compare the third value or the fourth value with the zero voltage V0 to perform overvoltage protection.

[0066] Then, the overvoltage control unit 430 uses the node sampling voltage Vnos to charge the energy storage element and uses the overvoltage protection threshold voltage Vref to discharge the energy storage element during the conduction time of the main power tube, and uses the overvoltage protection threshold voltage Vref to discharge the energy storage element during the freewheeling time of the freewheeling tube, and the overvoltage protection threshold voltage Vref is the product of the overvoltage protection threshold Vth and the proportional coefficient K2, so as to obtain a third value representing the difference between the first value and the second value (Vnos×Ton-Vth×T×K2). Alternatively, the overvoltage control unit 430 uses the node sampling voltage Vnos to discharge the energy storage element and uses the overvoltage protection threshold voltage Vref to charge the energy storage element during the conduction time of the main power tube, and uses the overvoltage protection threshold voltage Vref to charge the energy storage element during the freewheeling time of the freewheeling tube, so as to obtain a fourth value representing the difference between the second value and the first value (Vth×T×K2-Vnos×Ton). The following is an example in which the overvoltage control unit 430 calculates the third value representing the difference between the first value and the second value.

[0067] Specifically, in this embodiment, the overvoltage control unit 430 includes an electric energy supply unit 431, an energy storage unit 432, a switch unit 433, a switch control unit 434 and a comparison unit 435. The energy storage unit 432, i.e., the energy storage element, includes at least a third capacitor C3, and the electric energy supply unit 431 includes a first voltage-controlled current source A1 and a second voltage-controlled current source A2. The switch unit 433 includes a fourth switch S4 and a fifth switch S5, and the switch unit 433 may also include a sixth switch S6 and a fourth capacitor C4, the switch control unit 434 includes an RS trigger U4, and the comparison unit 435 includes a second comparator COM2 and an AND gate U5. These components and Figure 4 The connection relationship between the components in the embodiment is consistent with the circuit principle and will not be repeated here.

[0068] In another embodiment, the overvoltage control unit 430 may also use different circuit units to perform multiplication operations to obtain a first value representing the product of the node sampling voltage and the conduction time and a second value representing the product of the sum of the conduction time and the freewheeling time and the voltage representing the overvoltage protection threshold, and then use a comparator to compare the first value and the second value, output the comparison result, and perform overvoltage protection according to the comparison result. For example, the overvoltage control unit 230 performs multiplication operations according to the node sampling voltage Vnos and the conduction time Ton to obtain a first value representing the product of the node sampling voltage and the conduction time, and also performs multiplication operations according to the sum of the conduction time and the freewheeling time T (T=Ton+Toff) and the voltage representing the overvoltage protection threshold (K2×Vth) to obtain a second value representing the product of the sum of the conduction time and the freewheeling time and the voltage representing the overvoltage protection threshold. The first value is input to the first input terminal of the comparator, and the second value is input to the second input terminal of the comparator. When the first value is greater than the second value, the comparator outputs a valid level signal, and when the first value is less than the second value, the comparator outputs an invalid level signal. That is, the overvoltage control unit 430 calculates the first value of the product Vnos×Ton representing the node sampling voltage Vnos and the conduction time Ton of the main power tube, and the overvoltage control unit 430 also calculates the second value of the product Vth×T×K2 representing the sum of the conduction time of the main power tube and the freewheeling time of the freewheeling tube and the voltage (K2×Vth) representing the overvoltage protection threshold. The overvoltage control unit 430 also compares the first value and the second value to output an overvoltage control signal to perform overvoltage protection. Alternatively, the first value and the second value can be calculated separately, and then the difference between the first value and the second value is calculated, and the overvoltage protection is performed by comparing with the zero voltage. Alternatively, the first value and the second value can be calculated separately, and then the difference between the second value and the first value is calculated, and the difference is compared with the zero voltage for overvoltage protection.

[0069] The above only introduces the overvoltage protection circuit of the switching power supply by taking the buck-boost topology and the flyback topology as examples, but in practical applications, the overvoltage protection circuit can also be applied to other topologies, such as the boost topology and the buck topology, etc. Of course, it is also necessary to change the circuit structure of the overvoltage control unit so that it can adapt to the relationship between the input voltage, output voltage and duty cycle 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.

[0070] 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.

[0071] Of course, the present invention can also provide an overvoltage protection method for a switching power supply, wherein the switching power supply includes a main power tube and a freewheeling tube, and the main power tube is periodically turned on and off so that the switching power supply generates an output voltage at the output end according to the input voltage at the input end. Figure 2-Figure 7 The overvoltage protection circuit of all embodiments, the overvoltage protection method specifically comprises the following steps:

[0072] In step S101, the node voltage on the sampling node of the switching power supply is sampled to obtain the node sampling voltage. In this step, the node voltage is the sum of the product of the output voltage and the coefficient and the input voltage.

[0073] In step S102, overvoltage protection is performed on the switch power supply according to the on-duty ratio, the node sampling voltage and the voltage representing the overvoltage protection threshold. The on-duty ratio is the ratio of the on-time of the main power tube to the sum of the on-time of the main power tube and the freewheeling time of the freewheeling tube.

[0074] 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.

[0075] In summary, the overvoltage protection circuit and overvoltage protection method of the switching power supply and the switching power supply provided by the embodiment of the present invention sample the node voltage at the sampling node of the switching power supply to obtain the node sampling voltage, and then perform overvoltage protection according to the conduction duty cycle, the node sampling voltage and the voltage representing the overvoltage protection threshold. The node voltage is the sum of the product of the output voltage and the coefficient and the input voltage, so that the node sampling voltage containing the information of the input voltage and the output voltage can be sampled at one sampling point, and the output voltage can be easily calculated according to the node sampling voltage. The required sampling and calculation amount is small, the circuit structure is simple, the response is fast, the system error is small, and no auxiliary winding is required, saving chip area and cost. Further, the relationship between the input voltage, the output voltage and the conduction duty cycle is obtained according to the topological structure, and the sampling and holding signal, the conduction duty cycle and the voltage representing the overvoltage protection threshold are used for calculation based on the relationship, so that when the calculation result represents that the output voltage is greater than the preset overvoltage protection threshold, it is considered that the trigger condition is met and overvoltage protection is performed. Therefore, the output voltage can be characterized according to the node sampling voltage, and the trigger condition of the overvoltage protection is accurately obtained, the overvoltage protection has high accuracy, and the system reliability is high. Moreover, the circuit designs corresponding to the operations are diverse, have good adaptability, require less calculations, have small system errors, and have low circuit costs.

[0076] Furthermore, the zero-crossing detection circuit detects the zero-crossing point of the inductor current to output a zero-crossing detection signal, so that the freewheeling time of the freewheeling tube can be accurately limited, making the voltage sampling and the calculation of the output voltage more accurate. Moreover, for the flyback switching power supply, by sampling the signal during the freewheeling time of the freewheeling tube and maintaining the signal in the time period outside the freewheeling time of the freewheeling tube, the node sampling voltage is obtained, and stable and accurate information on the input voltage and output voltage can be obtained. The voltage sampling unit and the overvoltage control unit are independent of each other, and the obtained node sampling voltage is stable, less affected by the outside world, and has small fluctuations. The obtained output voltage representation voltage is slightly different from the actual output voltage, and the overvoltage protection has high accuracy.

[0077] Furthermore, an overvoltage control unit with different circuit structures can be designed according to the relationship between the input voltage, output voltage, and on-duty cycle under the same topology. It can be in the form of a combination of switches and filter circuits, or in the form of a voltage-controlled current source, a switch, and a capacitor combination, etc. The embodiments are rich in variety, the circuit structures are diversified, and the scope of application is wide.

[0078] 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.

[0079] 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.

[0080] 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 of a switching power supply, wherein the input end of the switching power supply receives an input voltage, the switching power supply comprises a main power tube and a freewheeling tube, and an output voltage is generated at the output end by periodically turning on and off the main power tube, wherein: The overvoltage protection circuit comprises: A voltage sampling unit, sampling a node voltage on a sampling node of the switching power supply to obtain a node sampling voltage; An overvoltage control unit performs overvoltage protection on the switching power supply according to the on-duty cycle, the node sampling voltage and the voltage representing the overvoltage protection threshold, Wherein, the node voltage is the sum of the product of the output voltage and the coefficient and the input voltage; The on-duty ratio is the ratio of the on-time of the main power tube to the sum of the on-time of the main power tube and the freewheeling time of the freewheeling tube.

2. The overvoltage protection circuit according to claim 1, wherein: The overvoltage control unit calculates the on-duty cycle, the sampling and holding signal, and the voltage representing the overvoltage protection threshold based on the relationship between the output voltage, the input voltage, and the on-duty cycle of the switching power supply, so as to perform overvoltage protection when a trigger condition is met, wherein the trigger condition represents that the output voltage is greater than a preset overvoltage protection threshold.

3. The overvoltage protection circuit according to claim 2, wherein: The overvoltage control unit performs overvoltage protection when Vnos×Ds>Vref, wherein Vnos is the node sampling voltage, Ds is the on-duty cycle, and Vref is the representative voltage of the overvoltage protection threshold.

4. The overvoltage protection circuit according to any one of claims 1 to 3, wherein: The switching power supply also includes an inductor, which is connected in series with the main power tube between the input end and the reference ground, one end of the freewheeling tube is connected to the common node of the inductor and the main power tube, and the other end of the freewheeling tube is connected to the output end, the sampling node is one end of the freewheeling tube connected to the output end, and the coefficient is 1.

5. The overvoltage protection circuit according to any one of claims 1 to 3, wherein: The switching power supply also includes a transformer, the primary winding of the transformer is connected to the main power tube, the secondary winding of the transformer is connected to the freewheeling tube, the sampling node is one end of the main power tube connected to the primary winding, and the coefficient is N, where N is the turns ratio of the primary winding to the secondary winding.

6. The overvoltage protection circuit according to claim 5, wherein: The voltage sampling unit samples the node voltage during the freewheeling time of the freewheeling tube, and maintains the sampled signal during a time period outside the freewheeling time of the freewheeling tube to obtain the node sampling voltage.

7. The overvoltage protection circuit according to claim 6, wherein: The voltage sampling unit comprises: A voltage divider circuit, obtaining a divided voltage of the node voltage; a first capacitor, wherein a first end of the first capacitor is connected to the output end of the voltage divider circuit, a 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 a first end of the first switch is connected to the output end of the voltage divider circuit, a 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 and is turned off during a time period other than the freewheeling time of the freewheeling tube.

8. The overvoltage protection circuit according to claim 1, wherein: The overvoltage control unit calculates the product of the node sampling voltage and the on-duty cycle to obtain a representative voltage of the output voltage, and compares the representative voltage of the output voltage with the representative voltage of the overvoltage protection threshold to perform overvoltage protection.

9. The overvoltage protection circuit according to claim 8, wherein: The overvoltage control unit comprises: A voltage follower, wherein the positive input terminal of the voltage follower receives the node sampling voltage, and the negative input terminal of the voltage follower is connected to the output terminal; A filter network, wherein the output terminal outputs a voltage representative of the output voltage; a second switch, wherein a first end of the second switch is connected to an output end of the voltage follower, and a second end of the second switch is connected to an input end of the filter network; and 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; The second switch is turned on only during the on-time, and the third switch is turned on only during the freewheeling time.

10. The overvoltage protection circuit according to claim 1, wherein: The overvoltage control unit charges the energy storage element according to the node sampling voltage and discharges the energy storage element according to the voltage representing the overvoltage protection threshold value during the on-time, and discharges the energy storage element according to the voltage representing the overvoltage protection threshold value during the freewheeling time to generate a third value representing the difference between the first value and the second value, and compares the third value with the zero voltage to perform overvoltage protection; The first value is the product of the node sampling voltage and the conduction time, and the second value is the product of the sum of the conduction time and the freewheeling time and the overvoltage protection threshold representative voltage.

11. The overvoltage protection circuit according to claim 10, wherein: The energy storage element includes a third capacitor, and the overvoltage control unit further includes: A first voltage-controlled current source, a control end of which receives the node sampling voltage to generate a first current proportional to the node sampling voltage to charge the third capacitor during the on-time; and A second voltage-controlled current source, a control end of which receives a voltage representative of the overvoltage protection threshold to generate a second current proportional to the voltage representative of the overvoltage protection threshold, and discharges the third capacitor during the on-time and the freewheeling time; a fourth switch, wherein a first end of the fourth switch is connected to the output end of the first voltage-controlled current source, and a second end of the fourth switch is connected to the input end of the second voltage-controlled current source; a fifth switch, wherein a first end of the fifth switch is connected to a second end of the fourth switch, a second end of the fifth switch is connected to a first end of the third capacitor, and a second end of the third capacitor is connected to a ground end, The third value is obtained according to the voltage of the third capacitor.

12. The overvoltage protection circuit according to claim 11, wherein: The overvoltage control unit also includes: a fourth capacitor, wherein a first end of the fourth capacitor is connected to the first end of the fifth switch, and a second end of the fourth capacitor is connected to the ground end; and A sixth switch, wherein a first end of the sixth switch is connected to the first end of the fifth switch, a second end of the sixth switch is connected to the ground end, and the sixth switch is turned on after the freewheeling time ends to release the charge on the fourth capacitor.

13. The overvoltage protection circuit according to claim 1, wherein: The overvoltage control unit discharges the energy storage element according to the node sampling voltage and charges the energy storage element according to the voltage representing the overvoltage protection threshold value during the on-time, and charges the energy storage element according to the voltage representing the overvoltage protection threshold value during the freewheeling time to generate a fourth value representing the difference between the second value and the first value, and compares the fourth value with zero voltage to perform overvoltage protection; The first value is the product of the node sampling voltage and the conduction time, and the second value is the product of the sum of the conduction time and the freewheeling time and the overvoltage protection threshold representative voltage.

14. The overvoltage protection circuit according to claim 1, wherein: The overvoltage control unit performs a multiplication operation to obtain a first value representing the product of the node sampling voltage and the conduction time, and the overvoltage control unit further performs a multiplication operation to obtain a second value representing the product of the sum of the conduction time and the freewheeling time and the overvoltage protection threshold representative voltage, The overvoltage control unit compares the first value and the second value to perform overvoltage protection; Alternatively, the overvoltage control unit calculates a difference between the first value and the second value, and compares the difference between the first value and the second value with zero voltage to perform overvoltage protection; Alternatively, the overvoltage control unit calculates a difference between the second value and the first value, and compares the difference between the second value and the first value with zero voltage to perform overvoltage protection.

15. 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 conduction state 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 overvoltage control unit receives the switch drive signal and the zero-crossing detection signal to obtain the freewheeling time of the freewheeling tube according to the switch drive signal and the zero-crossing detection signal, and obtains the conduction time of the main power tube according to the switch drive signal.

16. A switching power supply, comprising a power converter and a control circuit, wherein the power converter comprises a main power tube and a freewheeling tube, wherein: The control circuit comprises: A drive control circuit generates a switch drive signal to control the on and off of the main power tube, so that the power converter generates an output voltage at the output end according to the input voltage at the input end; and According to the overvoltage protection circuit according to any one of claims 1 to 15, the overvoltage protection circuit performs overvoltage protection on the switching power supply when the overvoltage protection circuit outputs a valid overvoltage control signal.

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