Adaptive slope resonant power converter and conversion control circuit and control method thereof

Abstract

The invention discloses an adaptive slope resonant power converter and a conversion control circuit and a control method thereof. The resonant power converter comprises an upper bridge switch and a lower bridge switch which are connected to form a half-bridge switching circuit, and the circuit is used for switching a transformer to generate an output voltage; the upper bridge driving circuit generates an upper bridge driving signal to drive the upper bridge switch according to the upper bridge control signal; the bias voltage is coupled to a bootstrap diode and a bootstrap capacitor, and supplies power to the upper bridge driving circuit from the bootstrap capacitor; wherein when the upper bridge switch needs to be turned on through soft switching, the upper bridge driving circuit generates an upper bridge driving signal at a fast slope; when soft switching is not used to turn on the upper bridge switch, the upper bridge driving circuit generates an upper bridge driving signal at a slow slope.

Description

Technical Field

[0001] The present invention relates to a resonant power converter, and in particular to a resonant power converter with an adaptive driving slope. The present invention also relates to a conversion control circuit and a control method for the resonant power converter. Background Art

[0002] Resonant power converters can provide soft switching to achieve high efficiency, high power density and low electromagnetic interference (EMI) performance. However, when the output voltage is still low at startup, the voltage of the resonant capacitor will not be sufficient to turn on the body diode of the upper bridge switch for soft switching, and the upper bridge switch may experience hard switching, which will cause high voltage stress on the power components and reduce the reliability of the power converter.

[0003] Compared with the above-mentioned prior art, the present invention discloses a control method and a conversion circuit, which adopts adaptive slope control to suppress voltage spikes caused by hard switching of a resonant power converter. Summary of the invention

[0004] In one aspect, the present invention provides a resonant power converter, comprising: an upper bridge switch and a lower bridge switch, coupled to each other to form a half-bridge switching circuit, for switching a transformer to generate an output voltage; an upper bridge driving circuit, for generating an upper bridge driving signal according to an upper bridge control signal to drive the upper bridge switch; a bias, connected to a bootstrap diode and a bootstrap capacitor, for providing a power supply from the bootstrap capacitor to the upper bridge driving circuit; wherein the upper bridge driving circuit generates the upper bridge driving signal with a fast slope to turn on the upper bridge switch when the upper bridge switch needs to perform soft switching; and generates the upper bridge driving signal with a slow slope to turn on the upper bridge switch when soft switching is not performed.

[0005] In one embodiment, the upper bridge driving circuit generates the upper bridge driving signal with the slow slope when the bias voltage is lower than a threshold voltage; and generates the upper bridge driving signal with the fast slope when the bias voltage is higher than the threshold voltage.

[0006] In one embodiment, the conduction of the lower bridge switch generates a circulating current, and the soft switching of the upper bridge switch is achieved when the lower bridge switch is turned off.

[0007] In one embodiment, the resonant power converter further includes a switching control circuit for generating the bias voltage, wherein the switching control circuit is used to generate an upper bridge control signal for controlling the upper bridge switch and a lower bridge control signal for controlling the lower bridge switch.

[0008] In one embodiment, the bias voltage is controlled to be lower than an under-voltage threshold voltage when the output voltage is lower than the threshold voltage, so that the upper bridge driving signal has the slow slope.

[0009] In one embodiment, during the startup period of the output voltage, the bias voltage is lower than the threshold voltage, so that the upper bridge driving signal has the slow slope.

[0010] In one embodiment, the upper bridge driving circuit controls a path resistance from the power supply to the upper bridge switch control terminal to have the first resistance value when the upper bridge switch needs to be turned on with the slow slope, and has a second resistance value when the upper bridge switch needs to be turned on with the fast slope, wherein the first resistance value is greater than the second resistance value.

[0011] In one embodiment, the upper bridge driving circuit includes a plurality of transistors coupled between the bias voltage and the control end of the upper bridge switch. When the upper bridge switch needs to be turned on with the slow slope, a first portion of the plurality of transistors is controlled to be turned on and have the first resistance value. When the upper bridge switch needs to be turned on with the fast slope, a second portion of the plurality of transistors is controlled to be turned on and have the second resistance value.

[0012] In one embodiment, the plurality of transistors include a first transistor and a second transistor, and when the upper bridge switch needs to be turned on with the slow slope, the first transistor is controlled to be turned on, and when the upper bridge switch needs to be turned on with the fast slope, the second transistor is controlled to be turned on, wherein the on-resistance of the first transistor is greater than the on-resistance of the second transistor.

[0013] In one embodiment, the upper bridge driving circuit further includes a resistor coupled to the first portion of the plurality of transistors, wherein the resistor is used to increase the path resistance when the upper bridge switch needs to be turned on with the slow slope.

[0014] In another aspect, the present invention provides a conversion control circuit for controlling a resonant power converter, wherein the resonant power converter includes: an upper bridge switch and a lower bridge switch, which are coupled to each other to form a half-bridge switching circuit, for switching a transformer to generate an output voltage, the conversion control circuit includes an upper bridge driving circuit, for generating an upper bridge driving signal according to an upper bridge control signal to drive the upper bridge switch; a switching control circuit, for generating a bias voltage, an upper bridge control signal and a lower bridge control signal according to the output voltage, wherein the bias voltage is coupled to a bootstrap diode and a bootstrap capacitor to provide a power supply for the bootstrap capacitor of the upper bridge driving circuit, and the lower bridge control signal is used to control the lower bridge switch; wherein the upper bridge driving circuit generates the upper bridge driving signal with a fast slope to turn on the upper bridge switch when the upper bridge switch needs to perform soft switching, and generates the upper bridge driving signal with a slow slope to turn on the upper bridge switch when the upper bridge switch does not perform soft switching.

[0015] In another aspect, the present invention provides a control method for controlling a resonant power converter. The resonant power converter includes an upper bridge switch and a lower bridge switch, which are coupled to form a half-bridge switching circuit for switching a transformer to generate an output voltage, and an upper bridge drive circuit for generating an upper bridge drive signal according to an upper bridge control signal to drive the upper bridge switch. The control method includes the following steps: generating a bias voltage, wherein the bias voltage is connected to a bootstrap diode and a bootstrap capacitor to provide the upper bridge drive circuit with power from the bootstrap capacitor; generating the upper bridge control signal and the lower bridge control signal according to the output voltage, wherein the lower bridge control signal is used to control the lower bridge switch; when the upper bridge switch needs to perform soft switching, controlling the upper bridge drive circuit to generate the upper bridge drive signal with a fast slope to turn on the upper bridge switch; and when the upper bridge switch does not perform soft switching, controlling the upper bridge drive circuit to generate the upper bridge drive signal with a slow slope to turn on the upper bridge switch.

[0016] In one embodiment, the step of generating the upper bridge drive signal includes: when the bias voltage is lower than a threshold voltage, controlling the upper bridge drive circuit to generate the upper bridge drive signal with the slow slope; when the bias voltage is higher than the threshold voltage, controlling the upper bridge drive circuit to generate the upper bridge drive signal with the fast slope.

[0017] In one embodiment, the step of generating the bias voltage includes: when the output voltage of the power converter is lower than an undervoltage threshold, controlling the bias voltage to be lower than the threshold voltage so that the upper bridge driving signal has the slow slope.

[0018] In one embodiment, the step of generating the bias voltage includes: controlling the bias voltage to be lower than a threshold voltage during the startup period of the output voltage, so that the upper bridge driving signal has the slow slope.

[0019] In one embodiment, the step of generating the upper bridge drive signal includes: when the upper bridge switch needs to be turned on at the slow slope, controlling a path resistance from the power supply to the control end of the upper bridge switch to have a first resistance value; when the upper bridge switch needs to be turned on at the fast slope, controlling the path resistance to have a second resistance value, wherein the first resistance value is greater than the second resistance value.

[0020] The following will be described in detail through specific embodiments to make it easier to understand the purpose, technical content, characteristics and effects achieved by the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A schematic diagram showing an embodiment of a resonant power converter according to the present invention.

[0022] Figure 2A and Figure 2BA schematic diagram showing an embodiment of an upper bridge driving circuit of a resonant power converter according to the present invention.

[0023] Figure 3 A schematic diagram showing an embodiment of a switching control circuit of a resonant power converter according to the present invention.

[0024] Explanation of symbols in the figure

[0025] 10: Transformer

[0026] 20: Resonant capacitor

[0027] 30: Upper bridge transistor

[0028] 40: Lower bridge transistor

[0029] 51, 52: Resistance

[0030] 56: Bootstrap diode

[0031] 57: Bootstrap capacitor

[0032] 60: Resistance

[0033] 70: Synchronous Rectification Transistor

[0034] 90: Optocoupler

[0035] 100: Secondary side controller

[0036] 200: Primary side controller

[0037] 300: Upper bridge drive circuit

[0038] 310: Comparator

[0039] 350: Transistor

[0040] 351: High impedance transistor

[0041] 355: High impedance resistor

[0042] 360: Transistor

[0043] 510: Potential Shifter

[0044] 515: Voltage Regulator

[0045] 520: Comparator with hysteresis

[0046] 530: Flip-flop

[0047] 550: Buffer

[0048] NA: Auxiliary winding

[0049] NP: Primary Winding

[0050] NS: Secondary winding

[0051] S1, S2: control signal

[0052] S11: upper bridge control signal

[0053] SG: Secondary side drive signal

[0054] SH: upper bridge drive signal

[0055] SL: lower bridge drive signal

[0056] TDS: During demagnetization

[0057] VCS: Current sensing signal

[0058] VD: voltage

[0059] VDD: voltage

[0060] VFB: Feedback signal

[0061] VNA: Auxiliary Winding Signal

[0062] VO: Output voltage

[0063] VTL: Threshold Voltage

[0064] VTUV: Threshold

[0065] VB: Bias voltage

[0066] VCB: offset bias voltage

[0067] VAUX: Signal DETAILED DESCRIPTION

[0068] The drawings in the present invention are all schematic, and are mainly intended to show the coupling relationship between various circuits and the relationship between various signal waveforms. The circuits, signal waveforms and frequencies are not drawn according to scale.

[0069] Figure 1The present invention represents a preferred embodiment of a resonant power converter. The half-bridge circuit is composed of an upper bridge transistor 30 (i.e., an upper bridge switch) and a lower bridge transistor 40 (i.e., a lower bridge switch). Connected to the output of the half-bridge circuit (i.e., a switching node LX) are a transformer 10 and a resonant capacitor 20 connected in series. The transformer 10 includes a primary winding NP, a secondary winding NS, and an auxiliary winding NA. The primary side controller 200 generates an upper bridge control signal S11 and a lower bridge drive signal SL. The upper bridge drive circuit 300 generates an upper bridge drive signal SH according to the upper bridge control signal S11. The upper bridge drive signal SH and the lower bridge drive signal SL are used to control the half-bridge circuit to switch the transformer 10, thereby generating an output voltage VO on the secondary side of the transformer 10. The resistor 60 is used to detect the primary side switching current IP of the transformer 10 and generate a current sensing signal VCS. The upper bridge drive signal SH and the lower bridge drive signal SL are generated according to the feedback signal VFB, which is determined by the output power of the power converter. Specifically, the secondary side controller 100 is connected to the output voltage VO to generate a feedback signal VFB. The feedback signal VFB is further transmitted to the primary side controller 200 through the optical coupler 90. The secondary side controller 100 also generates a secondary side drive signal SG for operating the synchronous rectification transistor 70 during the demagnetization period TDS of the transformer 10. During the switching period of the transformer 10, the auxiliary winding NA generates an auxiliary winding signal VNA. The resistors 51 and 52 attenuate the auxiliary winding signal VNA to generate a signal VAUX coupled to the primary side controller 200. The voltage level of the VAUX signal corresponds to the voltage level of the output voltage VO.

[0070] The upper bridge drive signal SH is used to excite the transformer 10 by turning on the transistor 30. During the demagnetization and resonance period of the transformer 10, the lower bridge drive signal SL turns on the transistor 40. The turning on the transistor 40 is also used to generate a circulating current in the transformer 10 to achieve zero voltage switching (ZVS) of the transistor 30. The voltage VCR of the resonant capacitor 20 is related to the output voltage VO.

[0071] However, in some cases, such as during startup, when the output voltage VO increases from a low voltage (e.g., zero voltage), the voltage VCR of the resonant capacitor 20 becomes too low to generate enough circulating current to achieve soft switching or ZVS of the transistor 30. If the slew rate of the gate voltage of the transistor 30 is high in this case, the hard switching of the transistor 30 will cause a high voltage spike on the synchronous rectifier transistor 70 during the switching of the transformer 10.

[0072] In one embodiment of the present invention, the primary side controller 200 is used to generate a bias voltage VB according to the voltage VD. In one embodiment, the voltage VDD is generated by rectifying the auxiliary winding signal VNA. The bias voltage VB is coupled to the bootstrap diode 56 and the bootstrap capacitor 57, and through the switching of the transistor 40, an offset bias voltage VCB provided to the upper bridge driving circuit 300 is generated on the bootstrap capacitor 57. In one embodiment, when it is intended to soft-switching the transistor 30, the upper bridge driving circuit 300 generates the upper bridge driving signal SH with a fast slope to turn on the transistor 30, which improves the switching efficiency. On the contrary, when soft switching is not used (or soft switching of the transistor 30 cannot be achieved due to the aforementioned reasons) to turn on the transistor 30, the upper bridge driving circuit 300 generates the upper bridge driving signal SH with a slow slope, which can prevent the above-mentioned transistor high voltage stress.

[0073] By generating bias voltages VB of different levels, the primary side controller 200 instructs the upper bridge driving circuit 300 to generate an upper bridge driving signal SH with a fast slope or a slow slope. Please note that the rising and / or falling slope (absolute value) of the upper bridge driving signal SH with a fast slope is higher than the rising and / or falling slope of the upper bridge driving signal SH with a slow slope. In one embodiment, when the bias source VCB is lower than the threshold voltage VTL, the upper bridge driving circuit 300 generates an upper bridge driving signal SH with a slow turn-on slope. On the other hand, when the bias source VCB exceeds the threshold voltage VTL, the upper bridge driving circuit 300 generates an upper bridge driving signal SH with a fast turn-on slope.

[0074] Figure 2A and Figure 2B FIG. 3 shows a preferred embodiment of the upper bridge driving circuit 300 according to the present invention. When the upper bridge control signal S11 is logic high (i.e., enabled), if the bias voltage VCB exceeds the threshold voltage VTL, the comparator 310 with hysteresis controls the transistor 350 to be turned on. On the contrary, if the bias voltage VCB is lower than the threshold voltage VTL, the comparator 310 controls the transistor 351 to be turned on. In one embodiment, the transistor 351 can be a high impedance transistor. In one embodiment, the on-resistance of the transistor 351 is higher than the on-resistance of the transistor 350. In order to further achieve a slow slope of the turn-on transistor 30, in one embodiment, a high impedance resistor 355 (such as Figure 2B The transistor 360 is connected in series with the transistor 351 to generate the upper bridge driving signal SH with a slow rate. The transistor 360 is used to turn off the upper bridge driving signal SH according to the upper bridge control signal S11.

[0075] Figure 3A preferred embodiment of a circuit for generating an upper bridge control signal S11, a lower bridge drive signal SL and a bias voltage VB according to the present invention is shown. In this embodiment, if the output voltage VO of the power converter is lower than the undervoltage threshold, the primary side controller 200 will generate a bias voltage VB lower than the threshold voltage VTL. The comparator 520 with hysteresis is used to reset the flip-flop 530 when the voltage nVO is lower than the threshold VTUV. The voltage nVO is sampled from the VAUX signal, and the voltage nVO is related to the voltage level of the output voltage VO. In addition, the power-on reset signal PRST is used to reset the flip-flop 530. The flip-flop 530 generates an H / L signal. When the flip-flop 530 is reset, the H / L signal controls the voltage regulator 515 to generate a bias voltage VB lower than the threshold voltage VTL. The upper bridge control signal S1 generates the upper bridge control signal S11 through the level shifter 510, and the lower bridge control signal S2 generates the lower bridge drive signal SL through the buffer 550.

[0076] On the other hand, when the voltage nVO exceeds the threshold VTUV, the voltage regulator 515 generates a bias voltage VB higher than the threshold voltage VTL.

[0077] The present invention has been described above with respect to the preferred embodiment, but the above description is only to make it easy for those skilled in the art to understand the content of the present invention, and is not intended to limit the scope of rights of the present invention. The various embodiments described are not limited to individual applications, but can also be applied in combination. For example, two or more embodiments can be used in combination, and part of the components in one embodiment can also be used to replace the corresponding components in another embodiment. In addition, under the same spirit of the present invention, those skilled in the art can think of various equivalent changes and various combinations. For example, the present invention refers to "processing or calculating or generating an output result according to a certain signal", which is not limited to the signal itself, but also includes, when necessary, converting the signal to voltage and current, current and voltage, and / or ratio conversion, and then processing or calculating the converted signal to generate an output result. It can be seen that under the same spirit of the present invention, those skilled in the art can think of various equivalent changes and various combinations, and there are many combinations, which are not listed here one by one. Therefore, the scope of the present invention should cover the above and all other equivalent changes.

Claims

1. A resonant power converter, comprising: An upper bridge switch and a lower bridge switch are coupled to each other to form a half-bridge switching circuit for switching a transformer to generate an output voltage; An upper bridge driving circuit, used for generating an upper bridge driving signal according to an upper bridge control signal to drive the upper bridge switch; a bias voltage connected to a bootstrap diode and a bootstrap capacitor to provide a power supply from the bootstrap capacitor to the upper bridge driving circuit; The upper bridge driving circuit generates the upper bridge driving signal with a fast slope to turn on the upper bridge switch when the upper bridge switch needs to perform soft switching; and generates the upper bridge driving signal with a slow slope to turn on the upper bridge switch when soft switching is not performed.

2. The resonant power converter according to claim 1, wherein: The upper bridge driving circuit generates the upper bridge driving signal with the slow slope when the bias voltage is lower than a threshold voltage; and generates the upper bridge driving signal with the fast slope when the bias voltage is higher than the threshold voltage.

3. The resonant power converter according to claim 1, wherein: The conduction of a lower bridge switch generates a circulating current, and when the lower bridge switch is turned off, soft switching of the upper bridge switch is achieved.

4. The resonant power converter according to claim 2, wherein: The invention also comprises a switching control circuit for generating the bias voltage. The switching control circuit is used to generate an upper bridge control signal for controlling the upper bridge switch and a lower bridge control signal for controlling the lower bridge switch.

5. The resonant power converter as claimed in claim 2, wherein: The bias voltage is controlled to be lower than an under-voltage threshold voltage when the output voltage is lower than the threshold voltage, so that the upper bridge driving signal has the slow slope.

6. The resonant power converter as claimed in claim 2, wherein: During the startup period of the output voltage, the bias voltage is lower than the threshold voltage, so that the upper bridge driving signal has the slow slope.

7. The resonant power converter of claim 1, wherein: The upper bridge drive circuit controls a path resistance from the power supply to the upper bridge switch control terminal to have the first resistance value when the upper bridge switch needs to be turned on with the slow slope, and has a second resistance value when the upper bridge switch needs to be turned on with the fast slope, wherein the first resistance value is greater than the second resistance value.

8. The resonant power converter as claimed in claim 7, wherein: The upper bridge driving circuit includes: A plurality of transistors are coupled between the bias voltage and the control terminal of the upper bridge switch, wherein when the upper bridge switch needs to be turned on with the slow slope, a first portion of the plurality of transistors is controlled to be turned on and have the first resistance value, and when the upper bridge switch needs to be turned on with the fast slope, a second portion of the plurality of transistors is controlled to be turned on and have the second resistance value.

9. The resonant power converter of claim 8, wherein: The multiple transistors include a first transistor and a second transistor, wherein when the upper bridge switch needs to be turned on with the slow slope, the first transistor is controlled to be turned on, wherein when the upper bridge switch needs to be turned on with the fast slope, the second transistor is controlled to be turned on, wherein the on-resistance of the first transistor is greater than the on-resistance of the second transistor.

10. The resonant power converter of claim 9, wherein: The upper bridge driving circuit further includes a resistor coupled to the first part of the plurality of transistors, wherein the resistor is used to increase the path resistance when the upper bridge switch needs to be turned on with the slow slope.

11. A switching control circuit for controlling a resonant power converter, wherein: The resonant power converter includes: an upper bridge switch and a lower bridge switch, which are coupled to each other to form a half-bridge switching circuit for switching a transformer to generate an output voltage. The conversion control circuit includes: An upper bridge driving circuit, used for generating an upper bridge driving signal according to an upper bridge control signal to drive the upper bridge switch; a switching control circuit for generating a bias voltage, an upper bridge control signal and a lower bridge control signal according to the output voltage, wherein the bias voltage is coupled to a bootstrap diode and a bootstrap capacitor to provide a power supply for the bootstrap capacitor of the upper bridge driving circuit, and the lower bridge control signal is used to control the lower bridge switch; The upper bridge driving circuit generates the upper bridge driving signal with a fast slope to turn on the upper bridge switch when the upper bridge switch needs to perform soft switching, and generates the upper bridge driving signal with a slow slope to turn on the upper bridge switch when the upper bridge switch does not perform soft switching.

12. The conversion control circuit according to claim 11, wherein: The upper bridge driving circuit generates the upper bridge driving signal with the slow slope when the bias voltage is lower than a threshold voltage; and generates the upper bridge driving signal with the fast slope when the bias voltage is higher than the threshold voltage.

13. The conversion control circuit according to claim 12, wherein: The bias voltage is controlled to be lower than an under-voltage threshold voltage when the output voltage of the power converter is lower than the under-voltage threshold voltage, so that the upper bridge driving signal has the slow slope.

14. The conversion control circuit according to claim 12, wherein: During the startup period of the output voltage, the bias voltage is controlled to be lower than the threshold voltage, so that the upper bridge driving signal has the slow slope.

15. The conversion control circuit according to claim 11, wherein: When the upper bridge switch needs to be turned on at the slow slope, the upper bridge drive circuit controls a path resistance from the power supply to the control end of the upper bridge switch to have a first resistance value, and when the upper bridge switch needs to be turned on at the fast slope, the path resistance has a second resistance value, wherein the first resistance value is greater than the second resistance value.

16. The conversion control circuit according to claim 15, wherein: The upper bridge driving circuit includes: A plurality of transistors are coupled between the bias voltage and the control terminal of the upper bridge switch, wherein when the upper bridge switch needs to be turned on with the slow slope, a first portion of the plurality of transistors is controlled to be turned on and have the first resistance value, and when the upper bridge switch needs to be turned on with the fast slope, a second portion of the plurality of transistors is controlled to be turned on and have the second resistance value.

17. The conversion control circuit according to claim 16, wherein: The multiple transistors include a first transistor and a second transistor, wherein when the upper bridge switch needs to be turned on with the slow slope, the first transistor is controlled to be turned on, wherein when the upper bridge switch needs to be turned on with the fast slope, the second transistor is controlled to be turned on, wherein the on-resistance of the first transistor is greater than the on-resistance of the second transistor.

18. The conversion control circuit according to claim 17, wherein: The upper bridge driving circuit further includes a resistor coupled to the first part of the plurality of transistors, wherein the resistor is used to increase the path resistance when the upper bridge switch needs to be turned on with the slow slope.

19. A method for controlling a resonant power converter, wherein: The resonant power converter includes: an upper bridge switch and a lower bridge switch, which are coupled to each other to form a half-bridge switching circuit, for switching a transformer to generate an output voltage, and an upper bridge driving circuit, for generating an upper bridge driving signal according to an upper bridge control signal to drive the upper bridge switch; the control method includes: Generate a bias voltage, wherein the bias voltage is connected to a bootstrap diode and a bootstrap capacitor, and provides power from the bootstrap capacitor to the upper bridge driving circuit; Generate the upper bridge control signal and the lower bridge control signal according to the output voltage, wherein the lower bridge control signal is used to control the lower bridge switch; When the upper bridge switch needs to perform soft switching, controlling the upper bridge driving circuit to generate the upper bridge driving signal with a fast slope to turn on the upper bridge switch; and When the upper bridge switch does not perform soft switching, the upper bridge driving circuit is controlled to generate the upper bridge driving signal with a slow slope to turn on the upper bridge switch.

20. The control method according to claim 19, wherein: The steps of generating the upper bridge driving signal include: When the bias voltage is lower than a threshold voltage, controlling the upper bridge driving circuit to generate the upper bridge driving signal at the slow slope; When the bias voltage is higher than the threshold voltage, the upper bridge driving circuit is controlled to generate the upper bridge driving signal with the fast slope.

21. The control method according to claim 20, wherein: The steps of generating the bias voltage include: When the output voltage of the power converter is lower than an under-voltage threshold, the bias voltage is controlled to be lower than the threshold voltage, so that the upper bridge driving signal has the slow slope.

22. The control method according to claim 20, wherein: The steps of generating the bias voltage include: During the startup period of the output voltage, the bias voltage is controlled to be lower than a threshold voltage so that the upper bridge driving signal has the slow slope.

23. The control method according to claim 19, wherein: The steps of generating the upper bridge driving signal include: When the upper bridge switch needs to be turned on at the slow slope, a path resistance from the power supply to the control terminal of the upper bridge switch is controlled to have a first resistance value; When the upper bridge switch needs to be turned on at the fast slope, the path resistor is controlled to have a second resistance value, wherein the first resistance value is greater than the second resistance value.

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