Short-circuit protection circuit, short-circuit protection method and switching power supply for power converter

Abstract

Disclosed are a short-circuit protection circuit, a short-circuit protection method and a switching power supply of a power converter. The power converter comprises a transformer and a main power tube connected with a primary winding of the transformer. The short-circuit protection circuit comprises: a soft start unit, which provides a drive voltage rising from zero with a controlled slope to a control end of the main power tube; a comparison unit, which compares a first sampling voltage representing a current flowing through the main power tube with a preset protection threshold voltage; and a logic control unit, which, when the drive voltage reaches a first voltage greater than a threshold voltage of the main power tube and the sampling voltage is greater than or equal to the protection threshold voltage, indicates that a short-circuit winding exists in the transformer, and turns off the main power tube for short-circuit protection. The short-circuit detection after the drive voltage exceeds the threshold voltage can avoid the period in which the Miller platform is located, does not need to set a shielding time, avoids misjudgment of the sampling voltage, shortens the turn-on time of the main power tube, reduces the current of the main power tube and avoids tube breakdown.

Description

Technical Field

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

[0002] With the rapid development of electronic systems, the demand for high-power-density, high-efficiency switching power supply converters is increasing. Isolated switching power supplies have been widely researched and applied due to their superior characteristics. Most common isolated switching power supplies include a transformer. During production testing of power converters such as PD adapters, short circuits in the transformer windings often cause excessive current in the primary-side main power transistor, leading to a power failure upon power-up. Therefore, short circuits are typically detected by monitoring the voltage across a sampling resistor that characterizes the main power transistor current. Once the sampling voltage reaches the short-circuit protection threshold, the main power transistor is immediately shut down. However, because the main power transistor experiences a Miller plateau time upon initial conduction, the rapid drop in drain-source voltage during this period can cause a spike in the sampling voltage, falsely triggering short-circuit protection and shutting down the main power transistor.

[0003] To avoid accidental activation of short-circuit protection, traditional methods typically involve... Figure 1 Set a blocking time period as well. Figure 1 A schematic diagram of a prior art switching power supply setting a shielding time for short-circuit protection is shown, such as... Figure 1 As shown, after the drive voltage Vgate of the main power transistor is generated, a shielding time TLEB is set, which is approximately several hundred nanoseconds. Then, the voltage Vcs across the sampling resistor is detected. Once the sampled voltage Vcs reaches the protection threshold voltage Vcsscp, the main power transistor is immediately turned off. Because the drive voltage Vgate generates a Miller plateau time near the threshold voltage Vth, the shielding time TLEB is usually set relatively long to allow sufficient margin and avoid misjudgment caused by voltage spikes in the sampled voltage Vcs during the Miller plateau time. However, when the transformer short-circuits, it is approximately as if the input voltage is directly applied to the leakage inductance. Especially for small-capacity leakage inductors, the peak current will surge rapidly during the long shielding time. Excessive peak current can cause the small-power MOSFET to quickly enter the saturation region, leading to a breakdown due to instantaneous heat accumulation. Even if it doesn't break down, the large energy stored in the leakage inductance at the moment of turn-off can cause the power transistor to break down. Furthermore, since the shielding time is related to the transformer, the parasitic capacitance of the main power transistor, and the drive current of the main power transistor, it is difficult to shorten precisely. Therefore, setting the shielding time duration can bring considerable difficulties to circuit use, especially since traditional short-circuit detection schemes are difficult to balance reliable protection and false triggering suppression in low-power flyback applications. Summary of the Invention

[0004] To address the aforementioned technical problems, this application provides a short-circuit protection circuit, a short-circuit protection method, and a switching power supply for a power converter.

[0005] According to one aspect of the present invention, a short-circuit protection circuit for a power converter is provided. The power converter includes a transformer and a main power transistor connected to the primary winding of the transformer. The short-circuit protection circuit includes: a soft-start unit that provides a drive voltage to the control terminal of the main power transistor, causing the drive voltage to rise from zero with a controlled slope; a comparison unit that compares a first sampling voltage with a preset protection threshold voltage, the first sampling voltage representing the current flowing through the main power transistor; and a logic control unit that generates a turn-off signal to turn off the main power transistor when it detects that the drive voltage reaches a first voltage and the first sampling voltage is greater than or equal to the protection threshold voltage. The first voltage is greater than the threshold voltage of the main power transistor, and the first sampling voltage being greater than or equal to the protection threshold voltage indicates that a short circuit has occurred in the winding of the transformer.

[0006] Optionally, the soft-start unit causes the drive voltage to rise from zero to a second voltage with a non-fixed slope during a first controlled period, wherein the second voltage is greater than or equal to the first voltage.

[0007] Optionally, the difference between the first voltage and the threshold voltage of the main power transistor is greater than 0.3V and does not exceed 2V.

[0008] Optionally, the soft-start unit causes the drive voltage to rise at a first slope from the second voltage during a second controlled period, and the average growth rate of the drive voltage during the second controlled period is greater than the average growth rate of the drive voltage during the period from the threshold voltage of the main power transistor to the second voltage.

[0009] Optionally, the soft-start unit converts the received reference voltage into the drive voltage and adjusts the slope of the drive voltage according to the slope of the reference voltage to perform soft-start control on the main power transistor.

[0010] Optionally, the short-circuit protection circuit further includes: a reference voltage generating circuit, wherein the reference voltage provided by the reference voltage generating circuit has an initial value greater than zero, and causes the reference voltage to rise from the initial value to a third voltage during a first controlled period, and to rise from the third voltage at a second slope during a second controlled period, wherein the average growth rate of the reference voltage during the second controlled period is greater than the average growth rate of the reference voltage during the first controlled period.

[0011] Optionally, the soft-start unit includes: an operational amplifier, the inverting input of which receives a second sampled voltage representing the drive voltage, and the non-inverting input of which receives the reference voltage; a pull-up current source, the control terminal of which is connected to the output terminal of the operational amplifier, and the output terminal of which is connected to the control terminal of the main power transistor; and a gate capacitor, the first terminal of which is connected to the output terminal of the pull-up current source, and the second terminal of which is grounded, the drive voltage being provided from the first terminal of the gate capacitor, wherein the operational amplifier amplifies the difference between the two input terminals to adjust the magnitude of the charging current provided by the pull-up current source to the gate capacitor.

[0012] Optionally, when the driving voltage is equal to the threshold voltage of the main power transistor, the second sampling voltage is greater than the initial value of the reference voltage.

[0013] Optionally, the short-circuit protection circuit further includes: a startup time adjustment unit connected to the input terminal of the soft-start unit, which adjusts at least the first time when the drive voltage rises from zero to the first voltage to adjust the startup time of the main power transistor. The startup time includes a first controlled period and a second controlled period. The startup time adjustment unit makes the first time of the main power transistor in steady state greater than its first time in short-circuit protection state, and the first sampled voltage in steady state is always less than the protection threshold voltage.

[0014] Optionally, the start-up time adjustment unit is connected to the supply terminal of the reference voltage and adjusts the rise slope of the reference voltage under different states to at least adjust the first time. The start-up time adjustment unit makes the rise slope of the reference voltage in the first time under the short-circuit protection state greater than the rise slope in the first time under the steady state.

[0015] Optionally, the start-up time adjustment unit includes: a first capacitor connected between the reference voltage supply terminal and the ground terminal; a second capacitor, the first terminal of the second capacitor being connected to the reference voltage supply terminal via a switching switch, and the second terminal of the second capacitor being grounded, wherein the switching switch remains open for at least a first time under short-circuit protection state and remains closed during the start-up time under steady state, or the switching switch remains open during the start-up time under short-circuit protection state and remains closed for at least a first time under steady state.

[0016] According to another aspect of the present invention, a short-circuit protection method for a power converter is provided, the power converter including a transformer and a main power transistor connected to the primary winding of the transformer, wherein the short-circuit protection method includes: providing a drive voltage to a control terminal of the main power transistor, causing the drive voltage to rise from zero with a controlled slope; comparing a first sampling voltage with a preset protection threshold voltage, the first sampling voltage representing the current flowing through the main power transistor; and generating a turn-off signal to turn off the main power transistor when it is detected that the drive voltage reaches a first voltage and the first sampling voltage is greater than the protection threshold voltage, wherein the first voltage is greater than or equal to the threshold voltage of the main power transistor, and the first sampling voltage being greater than or equal to the protection threshold voltage indicates that a short circuit exists in the transformer winding.

[0017] According to another aspect of the present invention, a switching power supply is provided, including a power converter, the power converter including a transformer and a main power transistor connected to the primary winding of the transformer, wherein the switching power supply further includes: the short-circuit protection circuit of the power converter described above, the short-circuit protection circuit of the power converter being used to control the operating state of the main power transistor.

[0018] The short-circuit protection circuit, short-circuit protection method, and switching power supply for a power converter provided by this invention gradually increase the drive voltage with a controlled slope through a soft-start unit. This controllable change in drive voltage makes it easier to detect the first voltage above the threshold voltage. Therefore, after the drive voltage reaches the first voltage, a shutdown signal is generated only when the first sampled voltage reaches the protection threshold voltage, turning off the main power transistor for short-circuit protection. By determining whether the first sampled voltage exceeds the protection threshold voltage after the drive voltage exceeds it, the spike in the first sampled voltage within the Miller plateau can be completely avoided. Short-circuit states can be accurately detected without setting a shielding time and without being affected by the Miller plateau. Furthermore, starting short-circuit protection detection just after the drive voltage exceeds the threshold voltage shortens the turn-on time of the main power transistor during a short circuit, avoiding heat accumulation and breakdown caused by the transistor entering the saturation region.

[0019] Furthermore, by gradually increasing the drive voltage from zero to a second voltage with a decreasing slope during the first controlled period, where the second voltage is greater than the first voltage, the change in drive voltage from the threshold voltage to the second voltage is smooth. Therefore, the value of the drive voltage is easily detected, and the moment when the drive voltage just exceeds the threshold voltage can be accurately obtained. This simplifies the acquisition of the first voltage, eliminating the need for a large time margin or a shielding time, thus resolving a series of problems caused by shielding time. Subsequently, during the second controlled period, the drive voltage increases with a larger first slope, ensuring that the set voltage is quickly reached in the absence of a winding short circuit, allowing the main power transistor to turn on stably. Moreover, since the voltage rises to a maximum of the second voltage during the first controlled period, in the event of a short circuit protection failure, the main power transistor can be turned off starting near a lower second voltage, significantly shortening the turn-off time and further reducing the rise in peak current.

[0020] Furthermore, by setting a reference voltage and adjusting the slope of the drive voltage according to the slope of the reference voltage, the drive voltage follows the changes in the reference voltage. Thus, by setting the waveform of the reference voltage through the circuit, the rising waveform of the drive voltage can be accurately controlled, improving the control accuracy of the short-circuit protection. The reference voltage has an initial value greater than zero, allowing the drive voltage to gradually rise from zero, saving turn-on time.

[0021] Furthermore, a startup time adjustment unit is incorporated to control the slope of the reference voltage under different states. This ensures that, at least in the first period of the startup time, the rising slope of the reference voltage under short-circuit protection is greater than that under steady-state conditions. Consequently, the startup time of the main power transistor in steady state is longer than that under short-circuit protection, resulting in better EMI performance. By setting a switch to switch the slope between different states, the power converter can be applied to various scenarios, expanding its applicability and enhancing its versatility.

[0022] 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

[0023] Figure 1 A schematic diagram of setting the shielding time for short-circuit protection in a prior art switching power supply is shown;

[0024] Figure 2 A schematic circuit diagram of a switching power supply provided according to an embodiment of the present invention is shown;

[0025] Figure 3 A schematic circuit diagram of a short-circuit protection circuit for a power converter according to a first embodiment of the present invention is shown.

[0026] Figure 4 It shows that according to Figure 3 A schematic diagram of the waveforms of each signal in steady state of the short-circuit protection circuit in the embodiment;

[0027] Figure 5 It shows that according to Figure 3 A schematic diagram of the waveforms of each signal when the short-circuit protection circuit of the embodiment performs short-circuit protection;

[0028] Figure 6 A schematic flowchart of a short-circuit protection method for a power converter according to an embodiment of the present invention is shown;

[0029] Figure 7 A schematic circuit diagram of a short-circuit protection circuit for a power converter according to a second embodiment of the present invention is shown.

[0030] Figure 8 It shows that according to Figure 7 The waveform diagram of the reference voltage of the short-circuit protection circuit in the embodiment under short-circuit protection state and steady state is shown. Detailed Implementation

[0031] 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.

[0032] Figure 2 A schematic circuit diagram of a switching power supply provided according to an embodiment of the present invention is shown.

[0033] like Figure 2As shown, the switching power supply 10 involved in this invention is an isolated topology, which can cover various power converters such as push-pull, flyback, and resonant. The following description uses a flyback converter as an example. The flyback converter includes a main circuit and a primary-side control circuit 20. The main circuit includes a primary-side circuit, a secondary-side circuit, and a transformer T. The transformer T includes a primary winding Np and a secondary winding Ns that are coupled together. In the primary-side circuit, the AC source AC forms the input power supply Vin after passing through the rectifier bridge BD. The main power transistor Q1 can be, for example, a MOSFET, or a GaN transistor, SiC transistor, etc. The source of the main power transistor Q1 is grounded through a sampling resistor Rcs, the drain is connected to the primary winding Np, and the gate is connected to the primary-side control circuit 20, which controls the conduction and turn-off of the main power transistor Q1 according to the drive signal Vdr. A diode D1 and a capacitor C0 are connected between the drain of the main power transistor Q1 and the input power supply Vin. A resistor Ri is connected in parallel across the capacitor C0. A filter capacitor Ci is also connected between the input power supply Vin and ground. In the secondary-side circuit, the anode of diode D3 is connected to the opposite terminal of the secondary winding Ns, and the cathode of diode D3 is grounded through the output capacitor Co. The load resistor RL is connected in parallel across the output capacitor Co. The flyback converter may also include an auxiliary winding Na, which is coupled to the primary winding Np and the secondary winding Ns. Resistors Ru and Rp are connected in series across Na. A node voltage Vs is provided at the common node of resistors Ru and Rp. This node voltage Vs characterizes the output voltage Vout across the output capacitor Co.

[0034] The primary-side control circuit 20 includes multiple pins, such as the DRV pin, VS pin, CS pin, VDD pin, HV pin, and GND pin. The HV pin receives the bus voltage, the GND pin is grounded, and the VS pin receives the node voltage Vs. The CS pin is connected to the common node of the sampling resistor Rcs and the main power transistor Q1 through a resistor Rt, and is used to obtain the first sampling voltage Vcs on the sampling resistor Rcs. Alternatively, the first sampling voltage Vcs can be obtained using the on-resistance of the main power transistor Q1 itself, or a sampling resistor not directly connected to the main power transistor Q1 can be used. Regardless of the method, the first sampling voltage Vcs represents the current flowing through the main power transistor Q1. The DRV pin is connected to the gate of the main power transistor Q1 and controls the operating state of the main power transistor Q1 according to the drive signal Vdr. The corresponding terminal of the auxiliary winding Na is connected to the VDD pin through a diode D2 and a capacitor C to provide power.

[0035] In this embodiment, the primary-side control circuit 20 includes a short-circuit protection circuit 100 for the power converter. This short-circuit protection circuit 100 is connected to the CS pin and the DRV pin, receives the first sampling voltage Vcs, and provides a drive signal Vdr to the control terminal (i.e., the gate) of the main power transistor Q1. The short-circuit protection circuit 100 is used to control the operating state of the main power transistor Q1 and perform short-circuit protection.

[0036] The following section describes the short-circuit protection method and circuit of the power converter in this application, using specific circuit examples.

[0037] Figure 3 A schematic circuit diagram of a short-circuit protection circuit for a power converter according to a first embodiment of the present invention is shown. Figure 4 It shows that according to Figure 3 A schematic diagram of the waveforms of each signal in steady state of the short-circuit protection circuit in the embodiment.

[0038] Combination Figure 3 and Figure 4 The short-circuit protection circuit 100 of this embodiment includes a soft-start unit 110, a comparison unit 120, a logic control unit 130, a selection circuit 140, and a reference voltage generation circuit 150. The soft-start unit 110 provides a drive voltage Vgate to the control terminal of the main power transistor Q1, which rises from zero with a controlled slope. During a first controlled period, the drive voltage Vgate slowly climbs from zero with a non-fixed slope until it reaches a second voltage Vth2 higher than the threshold voltage Vth of the main power transistor Q1. Then, in a second controlled period, the soft-start unit 110 causes the drive voltage Vgate to continue rising from the second voltage Vth2 with a first slope until it reaches the maximum value of the drive voltage. This first slope can be a fixed slope or a non-fixed slope. During the first controlled period, the drive voltage Vgate first rises from zero to the threshold voltage Vth, experiences a Miller plateau for a period of time, and then continues to rise from the threshold voltage Vth to the second voltage Vth2. Furthermore, during the first controlled period, the moment when the drive voltage Vgate reaches the first voltage Vth1 is monitored. The first voltage Vth1 is slightly higher than the threshold voltage Vth, and the second voltage Vth2 is greater than or equal to the first voltage Vth1. The average growth rate of the drive voltage Vgate during the second controlled period is greater than the average growth rate of the drive voltage Vgate during the time period from the threshold voltage Vth of the main power transistor Q1 to the second voltage Vth2. That is, during the second controlled period, the drive voltage Vgate continues to rise rapidly with a larger slope, thereby suppressing EMI and shortening the overall soft-start time. The first and second controlled periods can be regarded as the overall startup time of the main power transistor Q1.

[0039] Furthermore, the aforementioned slope control is achieved through a reference voltage Vref. The soft-start unit 110 converts the received reference voltage Vref into a drive voltage Vgate, and adjusts the slope of the drive voltage Vgate according to the slope of the reference voltage Vref to perform soft-start control on the main power transistor Q1. The reference voltage Vref can be obtained by a reference voltage generation circuit 150. The initial value Vset of the reference voltage Vref provided by the reference voltage generation circuit 150 is greater than zero, and the reference voltage Vref slowly rises from the initial value Vset to the third voltage Vth2 at a small slope during the first controlled period. This small slope can be a fixed slope or a non-fixed slope. During the second controlled period, it continues to rise from the third voltage Vth2 at a larger second slope. The average growth rate of the reference voltage Vref during the second controlled period is greater than the average growth rate of the reference voltage Vref during the first controlled period.

[0040] In one embodiment, during a first controlled period, both the reference voltage Vref and the drive voltage Vgate rise slowly with a small, non-fixed slope, while during a second controlled period, both rise with a large, fixed slope. That is, the slope changes of the drive voltage Vgate and the reference voltage Vref exhibit the same trend in both controlled periods.

[0041] Specifically, Figure 3In this design, the soft-start unit 110 includes a voltage divider network, an operational amplifier EA, a pull-up current source, and a gate capacitor Cgs. The pull-up current source is implemented, for example, as a transistor, denoted by transistor M1. The voltage divider network includes a first resistor R1 and a second resistor R2 connected in series. The inverting input of the operational amplifier EA is coupled to the common node of the first resistor R1 and the second resistor R2, meaning that the inverting input of the operational amplifier EA receives a second sampled voltage representing the drive voltage Vgate, and the non-inverting input of the operational amplifier EA receives a reference voltage Vref. The control terminal of the transistor M1 is connected to the output terminal of the operational amplifier EA. The drain of the transistor M1 receives the supply voltage VCC, and the source is connected to the first resistor R1. The first terminal of the gate capacitor Cgs is connected to the common node of the transistor M1 and the first resistor R1, and the second terminal is grounded. The drive voltage Vgate is provided from the first terminal of the gate capacitor Cgs. The gate capacitor Cgs can be the parasitic capacitance of the main power transistor Q1 itself, or it can be the superposition of the parasitic capacitance of the main power transistor Q1 itself and an additionally provided capacitance. Operational amplifier EA amplifies the difference between its two inputs to adjust the charging current supplied by transistor M1 to the gate capacitor Cgs. The pull-up current source provides charging current based on the output of operational amplifier EA, charging the gate capacitor Cgs to generate a drive voltage Vgate. This drive voltage Vgate is divided by the first resistor R1 and the second resistor R2 to obtain the second sampling voltage VR2. VR2 represents the magnitude of the drive voltage Vgate and is input to the inverting input of operational amplifier EA, thus forming negative feedback. The voltage difference between the two inputs of operational amplifier EA is ΔV = Vref - VR2. A larger ΔV results in a larger current flowing out of transistor M1, and a smaller ΔV results in a smaller current flowing out of transistor M1, thereby adjusting the slope of the drive voltage Vgate. Furthermore, error amplifier EA causes the voltage VR2 at the intermediate node of the voltage divider network to follow the reference voltage Vref. Since VR2 is also a voltage divider voltage of the drive voltage Vgate, the drive voltage Vgate also follows the reference voltage Vref, and their waveforms show the same trend. Of course, in some embodiments, the voltage divider network can be removed, and the driving voltage Vgate on the gate capacitor Cgs can be directly input to the inverting input of the operational amplifier EA. Alternatively, only the second resistor R2 can be retained, or other methods can be used to obtain the second sampling voltage.

[0042] like Figure 4As shown, to make the drive voltage Vgate controllable, the reference voltage Vref can be set according to the actual situation, changing with the conduction time, and has a certain initial value Vset to ensure that the operational amplifier EA can work normally at the beginning. That is, at time t1, the reference voltage generation circuit 150 first outputs an initial value Vset greater than zero. The initial value Vset can also be less than the value of the second sampling voltage corresponding to when the drive voltage Vgate reaches the threshold voltage Vth. The initial value Vset causes the pull-up current source to generate a charging current, and the drive voltage Vgate starts to rise from zero. Then, in the first controlled period, it slowly rises to the second voltage Vth2 with a small slope. The time period t1-t5 is the first controlled period. In this period, the rising slope of the reference voltage Vref can gradually decrease, and the reference voltage Vref presents a curve with a very small slope. The reference voltage Vref slowly rises from the initial value Vset to the third voltage Vth3. Then the drive voltage Vgate also rises with a small slope. At time t2, the driving voltage Vgate rises to the threshold voltage Vth. The time interval t2-t3 is approximately the Miller plateau time. After time t3, the driving voltage Vgate continues to rise slowly until it reaches the first voltage Vth1 at time t4. This first voltage Vth1 is a value slightly larger than the threshold voltage Vth. For example, the difference between the first voltage Vth1 and the threshold voltage Vth of the main power transistor is greater than 0.3V and does not exceed 2V. In practical applications, this range can be adjusted according to circuit requirements, and a difference value can be taken from the adjusted range. Taking a difference of 1V as an example, the first voltage Vth1 = Vth + 1V. Then, during the time interval t4-t5, the driving voltage Vgate continues to rise until it reaches the second voltage Vth2 at time t5. Correspondingly, the third voltage Vth2 and the second voltage Vth2 have a certain proportional relationship. The driving voltage Vgate follows the reference voltage Vref and has a certain delay. Afterwards, during the second controlled period, the reference voltage Vref continues to rise with a larger second slope, rapidly rising from the third voltage Vth3 to its maximum value. During the second controlled period corresponding to t5-t6, if the system is in steady state and no short-circuit protection is triggered, the drive voltage Vgate rapidly rises from the second voltage Vth2 to its maximum value with a fixed, relatively large first slope. During the period t6-t7, both the reference voltage Vref and the drive voltage Vgate remain at their maximum values.

[0043] Further, see also Figure 3The comparison unit 120 compares the acquired first sampled voltage Vcs with a preset protection threshold voltage Vcsscp. After detecting that the drive voltage Vgate reaches the first voltage Vth1 and the first sampled voltage Vcs is greater than or equal to the protection threshold voltage Vcsscp, the logic control unit 130 generates a turn-off signal Voff, turning off the main power transistor Q1. When the first sampled voltage Vcs is greater than or equal to the protection threshold voltage Vcsscp, it indicates that a short-circuited winding exists in the transformer. This short-circuited winding can be any one of the primary winding Np, the secondary winding Ns, and the auxiliary winding Na. The selection circuit 140 receives the drive voltage Vgate and the turn-off signal Voff to generate a drive signal Vdr to control the turn-on and turn-off of the main power transistor Q1. Specifically, the comparison unit 120 includes a first comparator COM1 and a second comparator COM2. The non-inverting input and inverting input of the first comparator COM1 receive the drive voltage Vgate and the first voltage Vth1, respectively, and compare their magnitudes. The non-inverting and inverting inputs of the second comparator COM2 receive the first sampled voltage Vcs and the protection threshold voltage Vcsscp, respectively, and compare their magnitudes. That is, if the first sampled voltage Vcs is detected to be greater than or equal to the protection threshold voltage Vcsscp after time t4, the main power transistor Q1 needs to be turned off.

[0044] There are many ways to implement the logic control unit 130. For example, it can include a NAND gate U1, whose two inputs are connected to the outputs of the first comparator COM1 and the second comparator COM2, respectively. The output of the NAND gate U1 provides a turn-off signal Voff. Alternatively, the logic control unit 130 can also include an AND gate and a switching transistor connected between the selection circuit 140 and ground. The control terminal of the switching transistor is connected to the output of the AND gate. When the AND gate output is high, the switching transistor is turned on, providing the turn-off signal Voff. Other implementations of the logic control unit 130 are also possible, which will not be listed here.

[0045] See Figure 4 At time t4, the drive voltage Vgate reaches the first voltage Vth1. At this time, the first sampling voltage Vcs is compared with the protection threshold voltage Vcsscp. Since there is no winding short circuit in the steady state, no high voltage spikes are generated. Therefore, the first sampling voltage Vcs is always less than the protection threshold voltage Vcsscp, and the logic control unit 130 does not generate a turn-off signal Voff. So from time t4 onwards, the drive voltage Vgate can continue to rise following the reference voltage Vref. It reaches its maximum voltage at time t6, and then remains at the maximum voltage during the time interval t6-t7. Starting from time t7, the drive voltage Vgate decreases, turning off the main power transistor Q1, and the first sampling voltage Vcs decreases.

[0046] Figure 5 It shows that according to Figure 3 The waveform diagram of each signal when the short-circuit protection circuit of the embodiment performs short-circuit protection.

[0047] Figure 5 The waveforms of various signals are shown when the transformer has a short-circuited winding. Figure 3 and Figure 5 At time t1, the drive voltage Vgate reaches the threshold voltage Vth. The period from t1 to t2 is the Miller plateau time, during which the drain-source voltage Vds of the main power transistor Q1 drops rapidly. During the period from t2 to t3, the drain-source voltage Vds remains at a low negative voltage. At time t3, the drive voltage Vgate is detected to have reached the first voltage Vth1. Then, at time t4, the first sampled voltage Vcs is detected to have reached the protection threshold voltage Vcsscp, triggering short-circuit protection and turning off the main power transistor Q1. The drive voltage Vgate then begins to drop. The entire startup time of the main power transistor Q1 is only about 40 nanoseconds, far less than the shielding time TLEB. Furthermore, after triggering short-circuit protection, due to soft-start, the drive voltage Vgate is still at a low level. Starting the turn-off from a lower voltage effectively shortens the pull-down time (dt=Cgs*dVgate / i) under the same pull-down current, further reducing the rise in peak current.

[0048] Figure 6 A schematic flowchart of a short-circuit protection method for a power converter according to an embodiment of the present invention is shown.

[0049] The short-circuit protection method of this power converter is used in the aforementioned switching power supply and short-circuit protection circuit. (See also...) Figure 6 The short-circuit protection method for the power converter in this embodiment includes, for example, the following steps:

[0050] In step S101, a drive voltage is provided to the control terminal of the main power transistor, causing the drive voltage to rise from zero with a controlled slope.

[0051] In step S102, the magnitude of the first sampling voltage is compared with the preset protection threshold voltage. The first sampling voltage represents the current flowing through the main power transistor.

[0052] In step S103, when the driving voltage reaches the first voltage and the first sampling voltage is greater than the protection threshold voltage, a turn-off signal is generated to turn off the main power transistor.

[0053] Furthermore, when the first voltage is greater than the threshold voltage of the main power transistor, and the first sampled voltage is greater than or equal to the protection threshold voltage, it indicates the presence of a short-circuit winding in the transformer. The drive voltage rises from zero to a second voltage with a gradually decreasing slope during the first controlled period, and the second voltage is greater than or equal to the first voltage. The short-circuit protection method of this embodiment is used for the above-mentioned... Figures 2-5 The switching power supply and short-circuit protection circuit shown above can be referred to, and the relevant content will not be repeated here.

[0054] Figure 7 A schematic circuit diagram of a short-circuit protection circuit for a power converter according to a second embodiment of the present invention is shown. Figure 8 It shows that according to Figure 7 The waveform diagram of the reference voltage of the short-circuit protection circuit in the embodiment under short-circuit protection state and steady state is shown.

[0055] In the short-circuit protection circuit of the first embodiment described above, the overall startup time of the drive voltage Vgate is relatively short. However, to ensure good EMI performance, it is generally undesirable for the main power transistor Q1 to turn on too quickly; that is, it is desirable for the startup time of the main power transistor Q1 to reach several hundred nanoseconds. But when a winding short circuit occurs, it is desirable to shorten the startup time to avoid excessive losses near the Miller plateau leading to thermal breakdown. Therefore, in this embodiment, an additional startup time adjustment unit 160 is added based on the first embodiment to realize the switching function of startup time between steady state and short-circuit protection state.

[0056] like Figure 7 As shown, in this embodiment, the reference voltage protection circuit 150 includes a current source A1, which is used to provide a reference current Iref, thereby obtaining the following... Figure 4 The reference voltage Vref is shown. The startup time adjustment unit 160 is connected to the input terminal of the soft-start unit 110 and the output terminal of the current source A1. It adjusts at least the first time required for the drive voltage Vgate to rise from zero to the first voltage Vth1, thereby adjusting the startup time of the main power transistor Q1. This startup time includes a first controlled period and a second controlled period. Furthermore, the startup time adjustment unit 160 ensures that the first time of the main power transistor Q1 in steady state is greater than its first time in short-circuit protection state, thus making the startup time in steady state greater than the startup time in short-circuit protection state. Also, in steady state, the first sampled voltage Vcs is always less than the protection threshold voltage Vcsscp, and short-circuit protection will not be triggered. Further, the startup time adjustment unit 160 is connected to the supply terminal of the reference voltage Vref, and adjusts the rising slope of the reference voltage Vref in different states to at least adjust the first time. This ensures that, at least in the first time, the startup time adjustment unit 160 ensures that the rising slope of the reference voltage Vref in short-circuit protection state is greater than the rising slope in steady state.

[0057] like Figure 8 As shown, taking the example where the rise slope of the reference voltage Vref in any controlled period under short-circuit protection is greater than that under steady-state conditions, the first and second controlled periods under short-circuit protection are both shortened, resulting in an overall shortened start-up time. However, the initial value of the reference voltage Vref is the same, Vset, and the magnitude of the third voltage Vth3 reached by the reference voltage Vref at the end of the first controlled period is also the same.

[0058] Figure 7 A specific example of a startup time adjustment unit 160 is given, which includes a first capacitor Ck1, a second capacitor Ck2, and a switching switch K1. The first capacitor Ck1 is connected between the supply terminal of the reference voltage Vref (the output terminal of the current source A1) and the ground terminal. The first terminal of the second capacitor Ck2 is connected to the supply terminal of the reference voltage Vref through the switching switch K1, and the second terminal of the second capacitor Ck2 is grounded. The switching switch K1 remains open for at least the first time under short-circuit protection conditions and remains closed during the startup time under steady-state conditions. That is, the switching switch K1 is closed by default during the startup time and remains closed when the system is in steady state. At this time, the slope of the reference voltage is dVref / dt=Iref / (C1+C2), where Iref is the reference current provided by the current source A1. Therefore, Vref rises slowly, and the startup time of the main power transistor Q1 is relatively long. When the system enters short-circuit protection mode, the switch K1 is open for at least the first short period. At this time, the slope of the reference voltage is dVref / dt = Iref / C2, and the reference voltage Vref rises faster than in steady state. This shortens the startup time of the main power transistor Q1, reducing the turn-on loss during short-circuit protection. Alternatively, the switch K1 can be set to open by default, remaining open during the startup time in short-circuit protection mode and closed for at least the first short period in steady state. This will also result in a longer startup time for the main power transistor in steady state compared to the startup time in short-circuit protection mode.

[0059] Accordingly, the short-circuit protection method of this embodiment is in Figure 6 Based on this, it can also include: ensuring that the first time of the main power transistor in steady state is greater than its first time in short-circuit protection state. For details, please refer to [link to specific principles]. Figures 7-8 The description will not be repeated here.

[0060] 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.

[0061] 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 here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

[0062] 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 short-circuit protection circuit for a power converter, the power converter comprising a transformer and a main power transistor connected to the primary winding of the transformer, wherein, The short-circuit protection circuit includes: The soft-start unit provides a drive voltage to the control terminal of the main power transistor, causing the drive voltage to rise from zero with a controlled slope. The comparison unit compares the magnitude of the first sampling voltage with the preset protection threshold voltage, wherein the first sampling voltage represents the current flowing through the main power transistor; The logic control unit generates a shutdown signal when it detects that the drive voltage reaches a first voltage and the first sampled voltage is greater than or equal to the protection threshold voltage, thereby turning off the main power transistor. Wherein, the first voltage is greater than the threshold voltage of the main power transistor, and the first sampling voltage being greater than or equal to the protection threshold voltage indicates that there is a short-circuit winding in the transformer.

2. The short-circuit protection circuit according to claim 1, wherein, The soft-start unit causes the drive voltage to rise from zero to a second voltage with a non-fixed slope during a first controlled period, wherein the second voltage is greater than or equal to the first voltage.

3. The short-circuit protection circuit according to claim 1, wherein, The difference between the first voltage and the threshold voltage of the main power transistor is greater than 0.3V and does not exceed 2V.

4. The short-circuit protection circuit according to claim 2, wherein, The soft-start unit causes the drive voltage to rise at a first slope from the second voltage during the second controlled period, and the average growth rate of the drive voltage during the second controlled period is greater than the average growth rate of the drive voltage during the period from the threshold voltage of the main power transistor to the second voltage.

5. The short-circuit protection circuit according to claim 1, wherein, The soft-start unit converts the received reference voltage into the drive voltage and adjusts the slope of the drive voltage according to the slope of the reference voltage to perform soft-start control on the main power transistor.

6. The short-circuit protection circuit according to claim 5 further includes: A reference voltage generating circuit provides an initial value of a reference voltage that is greater than zero, and causes the reference voltage to rise from the initial value to a third voltage during a first controlled period, and to rise from the third voltage at a second slope during a second controlled period, wherein the average growth rate of the reference voltage during the second controlled period is greater than the average growth rate of the reference voltage during the first controlled period.

7. The short-circuit protection circuit according to claim 5, wherein, The soft-start unit includes: An operational amplifier, wherein the inverting input of the operational amplifier receives a second sampled voltage characterizing the driving voltage, and the non-inverting input of the operational amplifier receives the reference voltage; A pull-up current source, the control terminal of which is connected to the output terminal of the operational amplifier, and the output terminal of which is connected to the control terminal of the main power transistor; and A gate capacitor, the first terminal of which is connected to the output terminal of the pull-up current source, and the second terminal of which is grounded, provides the driving voltage from the first terminal of the gate capacitor. The operational amplifier amplifies the difference between the two input terminals to adjust the magnitude of the charging current provided by the pull-up current source to the gate capacitor.

8. The short-circuit protection circuit according to claim 7, wherein, When the driving voltage is equal to the threshold voltage of the main power transistor, the second sampling voltage is greater than the initial value of the reference voltage.

9. The short-circuit protection circuit according to claim 6 further includes: A startup time adjustment unit, connected to the input terminal of the soft-start unit, adjusts at least the first time it takes for the drive voltage to rise from zero to the first voltage, thereby adjusting the startup time of the main power transistor. The startup time includes a first controlled period and a second controlled period. The startup time adjustment unit ensures that the first time of the main power transistor in steady state is greater than its first time in short-circuit protection state, and that the first sampled voltage in steady state is always less than the protection threshold voltage.

10. The short-circuit protection circuit according to claim 9, wherein, The start-up time adjustment unit is connected to the reference voltage supply terminal and adjusts the rise slope of the reference voltage under different states to at least adjust the first time. The start-up time adjustment unit ensures that the rise slope of the reference voltage during the first time period under the short-circuit protection state is greater than the rise slope during the first time period under the steady state.

11. The short-circuit protection circuit according to claim 10, wherein, The start-up time adjustment unit includes: The first capacitor is connected between the terminal providing the reference voltage and the ground terminal; The second capacitor has its first terminal connected to the reference voltage supply terminal via a switching switch, and its second terminal grounded. Wherein, the switching switch remains open for at least a first time during the short-circuit protection state and remains closed during the start-up time of the steady state, or the switching switch remains open during the start-up time of the short-circuit protection state and remains closed for at least a first time during the steady state.

12. A short-circuit protection method for a power converter, the power converter comprising a transformer and a main power transistor connected to the primary winding of the transformer, wherein, The short-circuit protection method includes: A drive voltage is provided to the control terminal of the main power transistor, causing the drive voltage to rise from zero with a controlled slope. The magnitude of the first sampling voltage is compared with the preset protection threshold voltage, where the first sampling voltage represents the current flowing through the main power transistor; When the drive voltage reaches a first voltage and the first sampled voltage is greater than the protection threshold voltage, a shutdown signal is generated to turn off the main power transistor. Wherein, the first voltage is greater than or equal to the threshold voltage of the main power transistor, and the first sampling voltage being greater than or equal to the protection threshold voltage indicates that there is a short-circuit winding in the transformer.

13. A switching power supply, comprising a power converter, the power converter including a transformer and a main power transistor connected to the primary winding of the transformer, wherein, The switching power supply also includes: According to any one of the power converters in claims 1-11, the short-circuit protection circuit of the power converter is used to control the operating state of the main power transistor.

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