Switching protection method, protection circuit and flyback switching power supply for flyback converter

Abstract

Disclosed are a switch protection method and a protection circuit for a flyback converter and a flyback switching power supply, which perform saturation protection on a first switch tube of a primary side. The switch protection method comprises: during conduction of the first switch tube, sampling predetermined parameters of the flyback converter at the same sampling interval time; taking an average value of values of the first m samplings as a first sampling signal and a value of the m+i sampling as a second sampling signal, i starting from 1 and increasing; comparing each second sampling signal with the first sampling signal in turn; when the value of the second sampling signal is less than the product of a comparison coefficient and the first sampling signal, outputting a protection signal, turning off the first switch tube, and indicating that the first switch tube reaches a saturation state, wherein the comparison coefficient is greater than 0 and less than 1, the protection signal indicates that the first switch tube reaches the saturation state, the source-drain voltage of the first switch tube does not need to be sampled, and the first switch tube can be turned off in time when the switch tube reaches saturation, thereby avoiding damage to the switch tube caused by excessive loss due to saturation of the switch tube.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, specifically to a switching protection method, protection circuit, and flyback switching power supply for a flyback converter. Background Technology

[0002] With the rapid development of electronic systems, people have increasingly higher demands for high power density and high efficiency switching power supply converters. Flyback switching power supplies, forward switching power supplies, dual-clamp ZVS converters, and so on have been widely studied and applied due to their excellent characteristics such as high efficiency, full-range soft switching, and adaptability to high switching frequencies.

[0003] A common flyback switching power supply topology includes a transformer, a primary winding, and a secondary winding. The main power transistor is connected to the primary winding, and the secondary rectifier is connected to the secondary winding. Both the primary and secondary windings control the switching transistors' on / off states through their respective control circuits. In current-controlled power supplies, a peak current is typically set to limit the peak current of the transformer's primary winding and the switching transistor. Currently, GaN transistors are commonly used as primary or secondary switching transistors. However, GaN transistors have a smaller saturation current compared to Si transistors. When the saturation current specification of the GaN transistor is selected to be relatively small, the saturation current of some switching transistors may even be smaller than the set peak current. Under transient or dynamic operating conditions, the GaN transistor may quickly reach saturation, causing an increase in the transistor's drain-source voltage, a sharp increase in switching losses, and ultimately, overheating and damage. To avoid damage caused by transistor saturation, high-voltage devices are usually added to the primary-side control chip to detect the source-drain voltage across the switching transistor and control its on or off state. This results in excessively high device costs and waste of high-voltage components. Summary of the Invention

[0004] To address the aforementioned technical problems, this application provides a switching protection method, a switching protection circuit, and a flyback switching power supply for a flyback converter, which promptly detects when the power transistor reaches saturation and shuts it off, thereby resolving the problems in the prior art.

[0005] According to a first aspect of the present invention, a switching protection method for a flyback converter is provided for saturation protection of a first switch on the primary side of the flyback converter, wherein the flyback converter includes a primary winding and a secondary winding, and the first switch is grounded through a sampling resistor, the switching protection method comprising:

[0006] During the conduction period of the first switch, predetermined parameters of the flyback converter are sampled at the same sampling interval.

[0007] The average value of the first m samples is taken as the first sample signal, and the value of the (m+i)th sample is taken as the second sample signal, with i increasing from 1.

[0008] Each of the second sampled signals and the first sampled signal is compared sequentially;

[0009] When the value of the second sampled signal is less than the product of the comparison coefficient and the first sampled signal, a protection signal is output to control the first switch to turn off.

[0010] The comparison coefficient is greater than 0 and less than 1.

[0011] Optionally, the switch protection method further includes:

[0012] An initial turn-on interval is set, and the first m samples are completed within the initial turn-on interval. The (m+i)th sample is performed outside the initial turn-on interval. The initial turn-on interval is a predetermined time period after the first switch has been turned on for a period of time.

[0013] Optionally, the flyback converter includes an auxiliary winding, and the predetermined parameter is the current flowing through the auxiliary winding, the magnitude of which characterizes the magnitude of the source-drain voltage across the first switching transistor.

[0014] Optionally, the predetermined parameter is the slope of the sampling voltage across the sampling resistor, and the magnitude of the slope of the sampling voltage characterizes the rate of change of the primary inductor current.

[0015] Optionally, the protection signal indicates that the source-drain voltage of the first switch increases during the conduction period, and the first switch is saturated.

[0016] Optionally, when the predetermined parameter is the current flowing through the auxiliary winding, the sampling point is the connection point of the first and second resistors connected in series to the auxiliary winding, and the voltage at the sampling point is clamped to a low voltage.

[0017] Optionally, when the predetermined parameter is the slope of the sampling voltage across the sampling resistor, the sampling point is the connection point between the sampling resistor and the first switching transistor, and the sampling voltage is the voltage value after filtering across the sampling resistor.

[0018] According to a second aspect of the present invention, a switching protection circuit for a flyback converter is provided for saturation protection of a first switch on the primary side of the flyback converter, wherein the flyback converter includes a primary winding and a secondary winding, the first switch is grounded through a sampling resistor, and the switching protection circuit is connected between the sampling resistor and the first switch, the switching protection circuit comprising:

[0019] The sampling module samples the predetermined parameters of the flyback converter at the same sampling interval during the conduction period of the first switch, and takes the average value of the first m samples as the first sampling signal, and takes the value of the (m+i)th sample as the second sampling signal, where i increases from 1.

[0020] The comparison module is connected to the sampling module and compares each of the second sampling signals with the first sampling signal in turn. When the value of the second sampling signal is less than the product of the comparison coefficient and the first sampling signal, a protection signal is output.

[0021] The drive module, connected to the comparator module and the control terminal of the first switching transistor, receives the protection signal and outputs a drive signal to control the first switching transistor to turn off.

[0022] The comparison coefficient is greater than 0 and less than 1.

[0023] Optionally, the sampling module completes the first m samplings within the initial turn-on interval, and performs the (m+i)th sampling outside the initial turn-on interval, wherein the initial turn-on interval is a predetermined time period after the first switch has been turned on for a period of time.

[0024] Optionally, the flyback converter includes an auxiliary winding.

[0025] The predetermined parameter sampled by the sampling module from the first sampling point is the current flowing through the auxiliary winding. The magnitude of the current flowing through the auxiliary winding represents the magnitude of the source-drain voltage across the first switching transistor.

[0026] Optionally, the switch protection circuit further includes:

[0027] The second switching transistor has its source connected to the junction of the first and second resistors connected in series in the auxiliary winding, its drain grounded, and its control terminal receiving the same drive signal as the first switching transistor.

[0028] Optionally, during the conduction of the first switch, the second switch is turned on, clamping the voltage at the connection point of the first resistor and the second resistor to a low voltage, and the connection point of the first resistor and the second resistor is the first sampling point.

[0029] Optionally, the predetermined parameter sampled by the sampling module from the second sampling point is the slope of the sampling voltage across the sampling resistor, and the magnitude of the slope of the sampling voltage characterizes the rate of change of the primary inductor current.

[0030] Optionally, a filter is connected at the connection point between the sampling resistor and the first switching transistor, and the connection point between the filter and the sampling module is the second sampling point.

[0031] According to a third aspect of the present invention, a flyback switching power supply is provided, comprising a transformer, a primary winding, a secondary winding, and an auxiliary winding, wherein the auxiliary winding is coupled to the secondary winding or the primary winding, a first switching transistor is connected to the primary winding, the first switching transistor is grounded through a sampling resistor, and the flyback switching power supply further comprises: a switching protection circuit of the flyback converter, the switching protection circuit of the flyback converter being used to control the conduction state of the first switching transistor and to provide saturation protection for the first switching transistor.

[0032] The present invention provides a switching protection method, a switching protection circuit, and a flyback switching power supply for a flyback converter. By sampling predetermined parameters of the flyback converter multiple times, the average value of the first m samples is used as the first sampling signal, and the sampling data from the (m+1)th sample onwards is used as the second sampling data. When the second sampling data is less than the product of the first sampling data and the comparison coefficient, the first switching transistor is considered to have reached saturation, and a protection signal is output to turn it off. This enables rapid detection of the saturation state of the switching transistor and timely shutdown of the first switching transistor, preventing overheating damage caused by power transistor saturation and increasing the reliability of the system.

[0033] Furthermore, by using the current of the auxiliary winding or the voltage slope of the sampling resistor as a predetermined parameter, which characterizes the source-drain voltage or the inductor current on the primary side of the first switching transistor, the switch protection circuit can indirectly obtain the source-drain voltage across the first switching transistor by sampling the predetermined parameter without the need for a high-voltage chip or high-voltage device, thus turning off the switching transistor in time. This simplifies the circuit structure and chip design of the switch protection circuit and the switching power supply, and reduces costs.

[0034] It should be noted that the above general description and the following detailed description are exemplary and explanatory only, and do not limit the present invention. Attached Figure Description

[0035] Figure 1a A schematic circuit diagram of a flyback switching power supply is shown, which detects the saturation state of a transistor using a high-voltage device.

[0036] Figure 1b Another schematic circuit diagram of a flyback switching power supply is shown, which detects the saturation state of a transistor using a high-voltage device.

[0037] Figure 2 A schematic circuit diagram of a switching protection circuit for a flyback switching power supply and a flyback converter according to an embodiment of the present invention is shown.

[0038] Figure 3 A schematic flowchart of a switching protection method for a flyback converter according to an embodiment of the present invention is shown;

[0039] Figure 4a and Figure 4b Schematic waveforms of various signals of the flyback switching power supply according to the first embodiment of the present invention are shown when the first switching transistor is not saturated and when the first switching transistor is saturated.

[0040] Figure 5 The diagram shows schematic waveforms of various signals of a flyback switching power supply according to a first embodiment of the present invention under a switching protection method using a flyback converter.

[0041] Figure 6a and Figure 6b Schematic waveforms of various signals of the flyback switching power supply according to the second embodiment of the present invention are shown when the first switching transistor is not saturated and when the first switching transistor is saturated.

[0042] Figure 6c The diagram shows schematic waveforms of various signals of a flyback switching power supply according to a second embodiment of the present invention under a switching protection method using a flyback converter. Detailed Implementation

[0043] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in various forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0044] Figure 1a A schematic circuit diagram of a flyback switching power supply is shown, which detects the saturation state of a transistor using a high-voltage device. Figure 1b Another schematic circuit diagram of a flyback switching power supply is shown, which detects the saturation state of a transistor using a high-voltage device.

[0045] like Figure 1a and 1b As shown, the flyback switching power supply includes a primary winding Np, a secondary winding Ns, an auxiliary winding Naux, and a transformer T. The gate of the first switching transistor Q1 is connected to the primary-side control chip 100. The first switching transistor Q1 is turned on and off according to the drive signal of the DRV pin of the primary-side control chip 100. The source of the first switching transistor Q1 is grounded through a resistor Rcs, and the drain is connected to the primary winding Np. The AC power supply provides a DC input voltage Vin to the primary side after passing through a power filter and a rectifier bridge. A diode D0 and a capacitor C0 are connected between the drain of the first switching transistor Q1 and the input power supply Vin. A resistor R0 is connected in parallel across the capacitor C0. A capacitor C1 is also connected between the rectifier bridge and ground. The secondary winding Ns is grounded through a diode D1 and a capacitor C2. The auxiliary winding Naux is connected in series with a resistor Rup and a resistor Rdown. The other end of the resistor Rdown is grounded.

[0046] Furthermore, the primary-side control chip 100 includes multiple pins. The VS pin is connected to the junction of resistors Rup and Rdown to sample the voltage or current on the auxiliary winding. The Drain pin is connected to the drain of the first switching transistor Q1, and the drain voltage of Q1 is detected by the high-voltage device inside the control chip 100. The GND pin is connected between resistor Rcs and ground to receive the ground voltage. The FB pin detects the output voltage through a feedback circuit, which includes capacitor C4. The DRV pin controls the output drive signal according to the internal circuit to drive the first switching transistor Q1 to turn on or off. The CS pin is connected between resistor Rcs and the source of Q1 through resistor R1. A capacitor C3 is also connected at the junction of the CS pin and resistor R1. The other end of capacitor C3 is grounded, and the CS pin samples the voltage on resistor Rcs.

[0047] exist Figure 1a In the primary-side control chip 100, there is a switching transistor Q0. Its drain is connected to the power supply terminal through resistor R2 to receive the power supply voltage VCC, and its source is grounded. The drive signal from the DRV pin is input to the control terminal of the switching transistor Q0 after passing through inverter U1. The drain of the switching transistor Q0 is also connected to the Drain pin through diode D2 to sample the drain voltage of the switching transistor Q1. Furthermore, the drain of the switching transistor Q0 is also connected to the positive input terminal of comparator U0. The inverting input terminal of comparator U0 receives the reference voltage Vref. Comparator U0 compares the drain voltage of the switching transistor Q1 with the reference voltage. When the drain voltage exceeds the reference voltage, it outputs a protection signal Vpro. Then, the switching transistor Q1 is turned off through the DRV pin to avoid Q1 saturation, which would cause excessive losses and damage to the switching transistor.

[0048] exist Figure 1b In the primary-side control chip 100, there is a switching transistor Q0. The drain of the switching transistor Q0 is connected to the Drain pin to sample the drain voltage of the switching transistor Q1. The source is grounded through resistor R3. The control terminal receives the same drive signal as the switching transistor Q1. Furthermore, the positive input terminal of the comparator U2 receives the sampled drain voltage, and the inverting input terminal is connected to the reference voltage Vref. When the drain voltage of Q1 is greater than the reference voltage, an effective protection signal Vpro is output. Then, the switching transistor Q1 is turned off through the DRV pin to avoid Q1 saturation and excessive heat loss that could damage the switching transistor.

[0049] Figure 1a and Figure 1bIn the current circuit, the source-drain voltage of Q1 is sampled through the Drain pin. When the switching transistor is saturated, the voltage is very high. To prevent damage to the chip, the switching transistors Q0 in the primary-side control chip 100 are all high-voltage devices, and each requires sampling the source-drain voltage of Q1 through an additional Drain pin. This results in high chip manufacturing complexity, cumbersome manufacturing process, and high cost. Therefore, this invention provides a control circuit and control method that does not require high-voltage devices or a Drain pin, and indirectly detects the saturation state of the switching transistor Q1 to promptly turn off the switching transistor Q1, thus conveniently and quickly achieving protection against power transistor saturation. The following detailed description is provided with reference to specific embodiments.

[0050] Figure 2 A schematic circuit diagram of a switching protection circuit for a flyback switching power supply and a flyback converter according to an embodiment of the present invention is shown.

[0051] like Figure 2 As shown, the flyback switching power supply in this embodiment is... Figure 1a and Figure 1b The structure is the same as that of a flyback switching power supply, the only difference being that the switching protection circuit 200 in this embodiment differs from the control chip 100. This embodiment is similar to... Figures 1a-1b The similarities between the embodiments will not be repeated. The switch protection circuit 200 in this embodiment is used to control the conduction and turn-off of the first switch Q1 and to provide saturation protection for the first switch Q1. The switch protection circuit 200 is connected to the control electrode of the first switch Q1 and includes: a second switch Q2, a sampling module 201, a comparison module 202 and a driving module 203. During the conduction period of the first switch Q1, the sampling module 201 samples the predetermined parameters of the flyback converter at the same sampling interval, and takes the average value of the first m samples as the first sampling signal, and takes the value of the (m+i)th sample as the second sampling signal, where i increases from 1; the comparison module 202 is connected to the sampling module 201, and compares each second sampling signal with the first sampling signal in sequence. When the value of the second sampling signal is less than the product of the comparison coefficient and the first sampling signal, the protection signal Vpro is output; the drive module 203 is connected to the comparison module 202 and the control terminal of the first switch Q1, receives the protection signal Vpro and outputs the drive signal Vdr to control the first switch Q1 to turn off. The predetermined parameters represent the inductance current of the primary winding or the source-drain voltage across the first switch, and the comparison coefficient is greater than 0 and less than 1.

[0052] In this embodiment, resistor Rcs is a sampling resistor. The first end of Rcs is connected to the source of the first switching transistor Q1, and the second end is grounded. When the first switching transistor Q1 is turned on, current flows through the sampling resistor Rcs. The first end of Rcs is also connected to resistor R1, and the second end of R1 is connected to the CS pin of the switch protection circuit 200. A capacitor C3 is also connected between the second end of R1 and ground. Capacitor C3 and resistor R1 form a high-pass filter, filtering the voltage across the sampling resistor Rcs before it is acquired via the CS pin. This CS pin is the second sampling point, through which the voltage across the sampling resistor Rcs can be sampled. That is, the second sampling point can be considered as the connection point between the sampling resistor Rcs and the first switching transistor Q1. The sampled voltage is the filtered voltage value across the sampling resistor Rcs. The predetermined parameter is the slope of the sampled voltage across the sampling resistor Rcs, and the magnitude of this slope characterizes the rate of change of the primary inductor current.

[0053] Furthermore, the source of the second switch Q2 is connected to the junction of the first resistor Rup and the second resistor Rdown connected in series in the auxiliary winding Naux (i.e., the VC pin). The drain of the second switch Q2 is grounded, and its control terminal receives the same drive signal as the first switch Q1. The drive signals for both the first switch Q1 and the second switch Q2 are output from the DRV pin of the switch protection circuit 200. That is, the output of the drive module 203 is connected to the DRV pin, outputting drive signals to the first switch Q1 and the second switch Q2. The first switch Q1 and the second switch Q2 are in the same conduction state. Furthermore, the GND pin of the switch protection circuit 200 is grounded, and the FB pin receives the feedback signal of the output voltage through the feedback circuit.

[0054] When the first switch Q1 on the primary side is turned on under the control of the drive signal, the second switch Q2 is also turned on. Since the auxiliary winding Naux is coupled to the primary winding Np, the voltage Vaux across the auxiliary winding follows the voltage of the primary winding Np. The drain of the second switch Q2 is grounded, and the source is connected to the same-name terminal of the auxiliary winding through the first resistor Rup. The voltage of the same-name terminal of the auxiliary winding is negative. Therefore, when the second switch Q2 is turned on, the direction of the current I_Rup can be considered to be from the second switch Q2 to the first resistor Rup. Taking this as the positive direction of the current, the voltage of the VS pin at the connection point of the first resistor Rup and the second resistor Rdown is clamped to a low voltage or approximately 0V. Taking the VS pin as the first sampling point, the current I_Rup flowing through the auxiliary winding is sampled. The magnitude of the current I_Rup flowing through the auxiliary winding represents the magnitude of the source-drain voltage across the first switch.

[0055] In this embodiment, the sampling module 201 is connected to the first sampling point or the second sampling point, and samples the current at the VS pin through the first path L1, that is, the current I_Rup flowing through the auxiliary winding; or samples the slope of the sampling voltage of the sampling resistor Rcs at the CS pin through the second path L2.

[0056] Moreover, an initial turn-on interval T is set in advance. Since the signal waveforms are unstable when the first switch Q1 just conducts and spike signals will be generated, the selection of the initial turn-on interval T needs to avoid this period of time. Then the initial turn-on interval T is a predetermined period of time after the first switch Q1 conducts for a period of time; and when the first switch Q1 is in the unsaturated state, the waveforms of each signal are relatively stable. Therefore, the initial turn-on interval T is also a very short period of time after the first switch Q1 conducts, that is, it ensures a conduction time in the unsaturated state of the first switch Q1 and avoids the influence of the spike current at the starting moment. Then, T can be, for example, 200 ns - 800 ns after conduction. <0oo0140>

[0057] Within this initial turn-on interval, the sampling module 201 samples the predetermined parameters of the flyback converter at the first sampling point or the second sampling point at the same sampling interval time. When the predetermined parameter is the current I_Rup flowing through the auxiliary winding, it represents the magnitude of the source-drain voltage across the first switch Q1. When the predetermined parameter is the slope of the sampling voltage Vcs, it represents the magnitude of the current on the first switch Q1 and also represents the magnitude of the inductance current on the primary winding. Moreover, the sampling module 201 completes the first m samplings within the initial turn-on interval T and takes the average value as a reference value, and then performs the (m + i)-th sampling outside the initial turn-on interval T, where i starts to increase from 1, and the (m + i)-th sampling is the actual sampling value. The comparison module 202 sequentially compares the result of the (m + i)-th sampling with the average value of the results of the first m samplings, that is, compares the actual sampling value with the reference value. For example, the results of the first m samplings are used as the first sampling signal, and the result of the (m + 1)-th sampling is used as the second sampling signal. If the second sampling signal is less than the product of the first sampling signal and k, it is considered that the first switch Q1 is saturated, and a protection signal Vpro is output to the drive module 203, where k is a comparison coefficient and 0 < k < 1. When the result of the (m + 1)-th sampling is not less than the product of the results of the first m samplings and k, i is incremented by 1, and the result of the (m + 2)-th sampling is used as the second sampling signal to be compared with the first sampling signal until the second sampling signal is less than the product of the first sampling signal and k, and then the protection signal Vpro is output. The drive module 203 outputs a drive signal Vdr according to this protection signal Vpro to turn off the first switch Q1. The protection signal Vpro indicates that the source-drain voltage of the first switch Q1 increases during conduction and the first switch Q1 is saturated. Of course, the drive module 203 can also output a drive signal to the first switch Q1 to make it conduct according to other circuit structures. Here, the generation circuit of the drive signal for making the switch Q1 conduct is not introduced in detail.

[0058] Figure 3 A schematic flowchart of a switching protection method for a flyback converter according to an embodiment of the present invention is shown.

[0059] The switching protection method of this flyback converter is based on Figure 2 The flyback switching power supply and switching protection circuit 200 are implemented, see [link / reference]. Figure 2 and Figure 3 The switching protection method for the flyback converter in this embodiment includes, for example, the following steps:

[0060] In step S101, during the first switch's conduction period, predetermined parameters of the flyback converter are sampled at the same sampling interval.

[0061] In this step, the first switch Q1 is turned on, and the second switch Q2 is also turned on. The sampling module 201 samples the predetermined parameters at the same sampling interval.

[0062] When the predetermined parameter is the slope of the sampling voltage Vcs across the sampling resistor Rcs, the sampling module 201 samples the slope of the sampling voltage Vcs across the sampling resistor Rcs at the same sampling interval through the second sampling point. The predetermined parameter, the slope of the sampling voltage Vcs, characterizes the inductor current on the primary side.

[0063] In a further embodiment, the predetermined parameter is the current I_Rup flowing through the auxiliary winding. The sampling module 201 samples the current I_Rup flowing through the auxiliary winding at the same sampling interval through the first sampling point. The predetermined parameter, the current I_Rup flowing through the auxiliary winding, characterizes the source-drain voltage across the first switching transistor Q1.

[0064] Furthermore, the switch protection method also includes: setting an initial switching interval T, completing the first m samplings within the initial switching interval T, taking the average value as a reference value, and performing the (m+i)th sampling outside the initial switching interval T as the measured value, where m can be a value set according to actual needs, such as 20 times.

[0065] In step S102, the average value of the first m samples is taken as the first sampling signal, and the value of the (m+i)th sample is taken as the second sampling signal, with i increasing from 1.

[0066] In this step, the average value of the first m samples is used as the first sampling signal, i.e., the reference value, while the value of the (m+i)th sampling signal is used as the second sampling signal, i.e., multiple measured values.

[0067] In step S103, each second sampled signal is compared with the first sampled signal in turn.

[0068] In this step, the first sample signal is compared with the (m+1)th sample signal as the second sample signal. If the (m+1)th sample signal does not meet the condition, the (m+2)th sample signal is used as the second sample signal, and so on.

[0069] In step S104, when the value of the second sampling signal is less than the product of the comparison coefficient and the first sampling signal, a protection signal is output to control the first switch to turn off.

[0070] In this step, when the value of the second sampling signal is less than the product of the first sampling signal and the comparison coefficient, the first switch Q1 is considered to be saturated. At this time, the protection signal Vpro is output. The protection signal Vpro indicates that the source-drain voltage of the first switch Q1 gradually increases during the conduction period and gradually tends to saturate.

[0071] In this embodiment, when the predetermined parameter is the current I_Rup flowing through the auxiliary winding, the sampling point is the connection point of the first resistor Rup and the second resistor Rdown connected in series with the auxiliary winding, i.e., the VS pin, and the voltage at the sampling point is clamped to a low voltage. When the predetermined parameter is the slope of the sampling voltage Vcs across the sampling resistor Rcs, the sampling point is the connection point between the sampling resistor Rcs and the first switching transistor Q1, and the sampling voltage Vcs is the filtered voltage value across the sampling resistor Rcs. Acquiring these two predetermined parameters is relatively convenient, and the saturation state of the first switching transistor can be characterized by changes in these predetermined parameters.

[0072] The switch protection circuit and method of this invention, by setting an initial turn-on interval T, collects m predetermined parameters within the initial turn-on interval T, and takes the average value as the first sampling signal. The predetermined parameters sampled after the initial turn-on interval T are used as the second sampling signal. The first sampling signal represents the source-drain voltage of Q1 when it is not saturated. When it is not saturated, the source-drain voltage of Q1 is basically stable, and the fluctuation of the value of the first m sampling signals is small. When the second sampling signal is less than k times the first sampling signal, it indicates that the deviation between the second sampling signal and the value of the previous m sampling signals is large. At this time, it can be considered that the first switch Q1 is saturated. The first switch Q1 is turned off in time by the protection signal to avoid the switch saturation causing excessive losses and damage to the switch. Only a low-voltage second switch Q2 is set in the switch protection circuit 200. The turn-off control of the first switch Q1 can be realized by sampling at two sampling points. There is no need to set high-voltage devices inside the switch protection circuit, nor is there a need to set a drain pin, which saves the manufacturing cost of the primary-side control chip and simplifies the manufacturing process.

[0073] Figure 4a and Figure 4bSchematic waveforms of various signals of the flyback switching power supply according to the first embodiment of the present invention are shown when the first switching transistor is not saturated and when the first switching transistor is saturated.

[0074] Figure 4a and Figure 4b The waveform diagrams shown are those of various signals in a flyback switching power supply when the predetermined parameters are the current I_Rup flowing through the auxiliary winding. These mainly include the drive signal of the first switching transistor Q1, the source-drain voltage Vds of the first switching transistor Q1, the voltage Vaux across the auxiliary winding, and the waveform of the current I_Rup flowing through the auxiliary winding. Combined with... Figures 2-4b During the conduction of the primary-side switch Q1, the voltage at the VS pin is clamped to 0V, and the current I_Rup flowing out of the VS pin is detected. When the first switch Q1 is on and not saturated, the voltage across it is negligible. Therefore, the voltage Vaux across the auxiliary winding Naux is approximately Vin*Na / Np, where Na is the number of turns in the auxiliary winding and Np is the number of turns in the primary winding. Correspondingly, when the first switch Q1 is not saturated, I_Rup = (Vin*Na / Np) / Rup.

[0075] See Figure 4a During the period t00-t01, the first switch Q1 is on, i.e., the primary side conduction period Ti. The drive signal of the first switch Q1 is high, and the source-drain voltage Vds is negligible. The voltage across the auxiliary winding is approximately Vin*Na / Np. Since the voltage polarity of the same terminals is the same, the voltage Vaux across the auxiliary winding is a stable negative voltage, and the current I_Rup flowing out of the VS pin is also a stable value. The current in the positive direction is represented as positive. During the period t01-t02, the first switch Q1 on the primary side is off, which is the secondary side conduction period To. The drive signal of the first switch Q1 is low, and the source-drain voltage Vds is a stable value. The voltage Vaux across the auxiliary winding Naux follows the secondary winding, and the current I_Rup flowing through the auxiliary winding is almost zero.

[0076] And in Figure 4b During the primary-side turn-on period Ti, when the first switch Q1 saturates, the voltage Vds across the source and drain of the first switch Q1 gradually increases and eventually stabilizes, approaching the input voltage Vin. Simultaneously, the voltage Vaux across the auxiliary winding gradually increases from a negative value to approximately zero, and the current I_Rup flowing through the auxiliary winding gradually decreases to approximately zero. During the secondary-side turn-on period To, the waveforms of each signal are similar to... Figure 4a The same applies. In this embodiment, the initial activation interval T is selected during the period when the signals Ti are stable during the primary side activation.

[0077] Figure 5The schematic waveform diagram of each signal of the flyback switching power supply according to the first embodiment of the present invention under the switching protection method applied to the flyback converter is shown;

[0078] As Figure 5 shown, according to the switching protection method of the flyback converter of the present invention, at time t1, the waveforms of each signal start to change, and the time period T1 between t0 and t1 is set as the initial turn-on interval. The initial turn-on interval can be a period of time within this T1 time period. For example, a short period of time after the minimum conduction time of Q1. The average value of I_Rup detected within this T1 time period is I_Rup1, that is, the first sampling signal. After this T1 time period, I_Rup is detected as I_Rup2, that is, the second sampling signal. If I_Rup2 < k * I_Rup1 (0 < k < 1), it is determined that the first switching transistor Q1 is saturated, and the switching transistor is immediately turned off. For example, at time t2, it is detected that I_Rup2 < k * I_Rup1, and at this time, the first switching transistor Q1 is turned off. Compared with Figure 4a , the turn-off time is advanced from t01 to t2, avoiding excessive thermal loss caused by the saturation of the switching transistor and damaging the transistor.

[0079] Figure 6a and Figure 6b respectively show the schematic waveform diagrams of each signal of the flyback switching power supply according to the second embodiment of the present invention when the first switching transistor is not saturated and when the first switching transistor is saturated.

[0080] Figure 6a and Figure 6b The waveform diagrams shown are the waveform diagrams of each signal of the flyback switching power supply when the predetermined parameter is the slope K of the sampling voltage Vcs, mainly including the drive signal of the first switching transistor Q1, the source-drain voltage Vds of the first switching transistor Q1, and the waveform of the sampling voltage Vcs. Combining Figures 2-3 and Figures 6a-6b , during the period Ti when the primary switching transistor Q1 is conducting, the source-drain voltage across the first switching transistor Q1 is stable, and the current flowing through Q1 gradually increases, then the voltage Vcs collected by the CS pin rises linearly.

[0081] Refer to Figure 6a , the time period from t00 to t03 is the period Ti when the primary switching transistor Q1 is conducting. The drive signal of the first switching transistor Q1 is at a high level, the source-drain voltage Vds is approximately 0, and the sampling voltage Vcs rises linearly. The rising slope K1 is: K1 = Vin * Rcs / Lm, where Rcs is the resistance value of the sampling resistor and Lm is the inductance of the transformer excitation inductance. At time t03, the secondary side conducts and the primary side is turned off. The drive signal of the first switching transistor Q1 is at a low level, the source-drain voltage Vds is at a high level, and the sampling voltage Vds is approximately 0.

[0082] Refer to Figure 6b, during the conduction period Ti of the primary side switching transistor, if the first switching transistor Q1 is saturated, that is, when its current reaches the saturation current, the source-drain voltage Vds gradually increases and finally stabilizes, approaching Vin, and the sampling voltage Vds随之上升最后达到稳定, so the rising slope of Vcs will become slower. After the first switching transistor Q1 is saturated, the rising slope K2 of the sampling voltage Vcs is: K2 = (Vin - Vds) * Rcs / Lm, and the time period t03 - t04 is the secondary side conduction interval To.

[0083] Figure 6c FIG. shows the schematic waveform diagrams of various signals of the flyback switching power supply according to the second embodiment of the present invention under the switching protection method applied to the flyback converter.

[0084] As Figure 6c shown, according to the switching protection method of the flyback converter of the present invention, when the signals change at time t3, the time period t0 - t3 is set as the initial turn-on interval T2. The initial turn-on interval can also be a period of time within the time period t0 - t3. The average slope of the sampling voltage Vcs is detected to be K1 within T2. After T2, the rising slope of the sampling voltage Vcs is detected to be K2. If at time t4, K2 < k * K1 (0 < k < 1), it is determined that the first switching transistor Q1 is saturated, and the switching transistor is immediately turned off. Compared with Figure 6a , the turn-off time of the first switching transistor Q1 is advanced from t03 to t4, so that the switching transistor is turned off when it is saturated, avoiding excessive loss caused by the saturation of the switching transistor and causing damage to the switching transistor.

[0085] According to the above waveform diagram, it can be seen that through the switching protection circuit and switching protection method of the present invention, there is no need to add a high-voltage Drain pin and a high-voltage sampling device in the switching protection circuit. The saturation state of the switching transistor can be detected by the auxiliary winding detection method or the slope detection method of the sampling voltage (the slope can be obtained according to dv / dt), and protection can be made in time, increasing the reliability of the system.

[0086] In summary, the switching protection method, switching protection circuit and flyback switching power supply provided by the present invention sample the predetermined parameters of the flyback converter multiple times, and use the average value of the first m samplings as the first sampling signal, and the sampling data after the (m + 1)th time as the second sampling data. When the second sampling data is less than the product of the first sampling data and the comparison coefficient, it is considered that the first switching transistor reaches the saturation state, and a protection signal is output to turn it off, so as to quickly detect the saturation state of the switching transistor and turn off the first switching transistor in time, avoiding excessive loss caused by the saturation of the switching transistor and causing damage to the switching transistor, and increasing the reliability of the system.

[0087] Furthermore, by using the current of the auxiliary winding or the voltage slope of the sampling resistor as a predetermined parameter, which characterizes the source-drain voltage or the inductor current on the primary side of the first switching transistor, the switch protection circuit can indirectly obtain the source-drain voltage across the first switching transistor by sampling the predetermined parameter without the need for a high-voltage chip or high-voltage device, thus turning off the switching transistor in time. This simplifies the circuit structure and chip design of the switch protection circuit and the switching power supply, and reduces costs.

[0088] It should be noted that the numerical values ​​in this article are for illustrative purposes only. In other embodiments of the present invention, other numerical values ​​may be sampled to implement this solution. The specific values ​​should be reasonably set according to the actual situation, and the present invention does not limit them.

[0089] Finally, it should be noted that the above embodiments are merely examples for clearly illustrating the present invention and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

[0090] It should also be understood that the terminology and expressions used herein are for descriptive purposes only, and one or more embodiments described herein should not be limited to these terms and expressions. The use of these terms and expressions does not exclude any illustrative and descriptive equivalent features (or parts thereof), and it should be recognized that various modifications that may exist should also be included within the scope of the claims. Other modifications, variations, and substitutions may also exist. Accordingly, the claims should be considered to cover all such equivalents.

Claims

1. A switching protection method for a flyback converter, used to provide saturation protection for the first switch on the primary side of the flyback converter, wherein, The flyback converter includes a primary winding and a secondary winding, the first switch is grounded through a sampling resistor, and the switch protection method includes: During the conduction period of the first switch, predetermined parameters of the flyback converter are sampled at the same sampling interval. An initial turn-on interval is set, and the first m samples are completed within the initial turn-on interval. The (m+i)th sample is performed outside the initial turn-on interval. The initial turn-on interval is a predetermined time period after the first switch has been turned on for a period of time. The average value of the first m samples is taken as the first sample signal, and the value of the (m+i)th sample is taken as the second sample signal, with i increasing from 1. Each of the second sampled signals and the first sampled signal is compared sequentially; When the value of the second sampled signal is less than the product of the comparison coefficient and the first sampled signal, a protection signal is output to control the first switch to turn off. Wherein, the comparison coefficient is greater than 0 and less than 1, the flyback converter further includes an auxiliary winding, the predetermined parameter is the current flowing through the auxiliary winding, and the magnitude of the current flowing through the auxiliary winding characterizes the magnitude of the source-drain voltage across the first switching transistor.

2. The switch protection method according to claim 1, wherein, The protection signal indicates that the source-drain voltage of the first switch increases during the conduction period, and the first switch is saturated.

3. The switch protection method according to claim 1, wherein, When the predetermined parameter is the current flowing through the auxiliary winding, the sampling point is the connection point of the first and second resistors connected in series to the auxiliary winding, and the voltage at the sampling point is clamped to a low voltage.

4. A switching protection method for a flyback converter, used to provide saturation protection for the first switch on the primary side of the flyback converter, wherein, The flyback converter includes a primary winding and a secondary winding, the first switch is grounded through a sampling resistor, and the switch protection method includes: During the conduction period of the first switch, predetermined parameters of the flyback converter are sampled at the same sampling interval. An initial turn-on interval is set, and the first m samples are completed within the initial turn-on interval. The (m+i)th sample is performed outside the initial turn-on interval. The initial turn-on interval is a predetermined time period after the first switch has been turned on for a period of time. The average value of the first m samples is taken as the first sample signal, and the value of the (m+i)th sample is taken as the second sample signal, with i increasing from 1. Each of the second sampled signals and the first sampled signal is compared sequentially; When the value of the second sampled signal is less than the product of the comparison coefficient and the first sampled signal, a protection signal is output to control the first switch to turn off. Wherein, the comparison coefficient is greater than 0 and less than 1, the predetermined parameter is the slope of the sampling voltage on the sampling resistor, and the magnitude of the slope of the sampling voltage characterizes the rate of change of the primary inductor current.

5. The switch protection method according to claim 4, wherein, The protection signal indicates that the source-drain voltage of the first switch increases during the conduction period, and the first switch is saturated.

6. The switch protection method according to claim 4, wherein, When the predetermined parameter is the slope of the sampling voltage across the sampling resistor, the sampling point is the connection point between the sampling resistor and the first switching transistor, and the sampling voltage is the voltage value after filtering across the sampling resistor.

7. A switching protection circuit for a flyback converter, used to provide saturation protection for the first switch on the primary side of the flyback converter, wherein, The flyback converter includes a primary winding and a secondary winding. The first switching transistor is grounded through a sampling resistor. The switching protection circuit is connected between the sampling resistor and the first switching transistor. The switching protection circuit includes: The sampling module samples predetermined parameters of the flyback converter at the same sampling interval during the conduction period of the first switch, and completes the first m samplings within the initial turn-on interval, and performs the (m+i)th sampling outside the initial turn-on interval, where the initial turn-on interval is a predetermined time period after the first switch has been conducted for a certain period of time; the sampling module also takes the average value of the first m samples as the first sampling signal, and the value of the (m+i)th sample as the second sampling signal, where i increases from 1; The comparison module is connected to the sampling module and compares each of the second sampling signals with the first sampling signal in turn. When the value of the second sampling signal is less than the product of the comparison coefficient and the first sampling signal, a protection signal is output. The drive module, connected to the comparator module and the control terminal of the first switching transistor, receives the protection signal and outputs a drive signal to control the first switching transistor to turn off. The comparison coefficient is greater than 0 and less than 1, and the flyback converter also includes an auxiliary winding. The predetermined parameter sampled by the sampling module from the first sampling point is the current flowing through the auxiliary winding, and the magnitude of the current flowing through the auxiliary winding represents the magnitude of the source-drain voltage across the first switching transistor.

8. The switch protection circuit according to claim 7 further includes: The second switching transistor has its source connected to the junction of the first and second resistors connected in series in the auxiliary winding, its drain grounded, and its control terminal receiving the same drive signal as the first switching transistor.

9. The switch protection circuit according to claim 8, wherein, During the period when the first switch is turned on, the second switch is turned on, clamping the voltage at the connection point of the first resistor and the second resistor to a low voltage, and the connection point of the first resistor and the second resistor is the first sampling point.

10. A switching protection circuit for a flyback converter, used to provide saturation protection for the first switch on the primary side of the flyback converter, wherein, The flyback converter includes a primary winding and a secondary winding. The first switching transistor is grounded through a sampling resistor. The switching protection circuit is connected between the sampling resistor and the first switching transistor. The switching protection circuit includes: The sampling module samples predetermined parameters of the flyback converter at the same sampling interval during the conduction period of the first switch, and completes the first m samplings within the initial turn-on interval, and performs the (m+i)th sampling outside the initial turn-on interval, where the initial turn-on interval is a predetermined time period after the first switch has been conducted for a certain period of time; the sampling module also takes the average value of the first m samples as the first sampling signal, and the value of the (m+i)th sample as the second sampling signal, where i increases from 1; The comparison module is connected to the sampling module and compares each of the second sampling signals with the first sampling signal in turn. When the value of the second sampling signal is less than the product of the comparison coefficient and the first sampling signal, a protection signal is output. The drive module, connected to the comparator module and the control terminal of the first switching transistor, receives the protection signal and outputs a drive signal to control the first switching transistor to turn off. Wherein, the comparison coefficient is greater than 0 and less than 1, and the predetermined parameter sampled by the sampling module from the second sampling point is the slope of the sampling voltage on the sampling resistor, and the magnitude of the slope of the sampling voltage characterizes the rate of change of the primary inductor current.

11. The switch protection circuit according to claim 10, wherein, A filter is connected at the connection point between the sampling resistor and the first switching transistor, and the connection point between the filter and the sampling module is the second sampling point.

12. A flyback switching power supply, comprising a transformer, a primary winding, a secondary winding, and an auxiliary winding, wherein the auxiliary winding is coupled to the secondary winding or the primary winding, a first switching transistor is connected to the primary winding, and the first switching transistor is grounded through a sampling resistor; the flyback switching power supply further comprises: The switching protection circuit of the flyback converter according to any one of claims 7-9 is used to control the conduction state of the first switching transistor and to provide saturation protection for the first switching transistor.

13. A flyback switching power supply, comprising a transformer, a primary winding, and a secondary winding, wherein the secondary winding is coupled to the primary winding, a first switching transistor is connected to the primary winding, the first switching transistor is grounded through a sampling resistor, and the flyback switching power supply further comprises: The switching protection circuit of the flyback converter according to any one of claims 10-11 is used to control the conduction state of the first switching transistor and to provide saturation protection for the first switching transistor.

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