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

Abstract

An adaptive slope resonant power converter and its conversion control circuit and control method. The resonant power converter comprises: an upper bridge switch and a lower bridge switch, which are connected 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 to drive the upper bridge switch according to an upper bridge control signal; a bias voltage coupled to a bootstrap diode and a bootstrap capacitor to provide power for the upper bridge driving circuit from the bootstrap capacitor; wherein the upper bridge driving circuit generates the upper bridge driving signal at a fast slope when the upper bridge switch needs to be turned on by soft switching; and the upper bridge driving circuit generates the upper bridge driving signal at a slow slope when the upper bridge switch is turned on without using soft switching.

Description

Technical Field

[0001] This invention relates to a resonant power converter, and more particularly to a resonant power converter with an adaptive drive slope. The invention also relates to a conversion control circuit and control method for the resonant power converter. Background Technology

[0002] Resonant power converters offer soft switching for high efficiency, high power density, and low electromagnetic interference (EMI) performance. However, when the output voltage is still low at startup, the voltage across the resonant capacitor may be insufficient to turn on the body diode of the upper bridge switch for soft switching. The upper bridge switch may then experience hard switching, which can put high voltage stress on the power components and reduce the reliability of the power converter.

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

[0004] In one viewpoint, 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 drive circuit for generating an upper bridge drive signal to drive the upper bridge switch according to an upper bridge control signal; and 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 drive circuit; wherein the upper bridge drive circuit generates the upper bridge drive signal with a fast slope to turn on the upper bridge switch when the upper bridge switch needs to be soft-switched; and generates the upper bridge drive signal with a slow slope to turn on the upper bridge switch when no soft switching is required.

[0005] In one embodiment, the upper bridge drive circuit generates the upper bridge drive signal with the slow slope when the bias voltage is below a threshold voltage; and generates the upper bridge drive signal with the fast slope when the bias voltage is above 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. The switching control circuit generates an upper bridge control signal to control the upper bridge switch and a lower bridge control signal to control the lower bridge switch.

[0008] In one embodiment, the bias voltage is controlled to be below an undervoltage threshold voltage when the output voltage is below that threshold voltage, so that the upper bridge drive signal has the slow slope.

[0009] In one embodiment, the bias voltage is lower than the threshold voltage during the power-on period of the output voltage, causing the upper bridge drive signal to have the slow slope.

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

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

[0012] In one embodiment, the plurality of transistors includes a first transistor and a second transistor, wherein the first transistor is controlled to be turned on when the upper bridge switch needs to be turned on at the slow slope, and the second transistor is controlled to be turned on when the upper bridge switch needs to be turned on at the fast slope, 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 drive circuit further includes a resistor coupled to the first portion of the plurality of transistors, the resistor being used to increase the path resistance when the upper bridge switch needs to be turned on at the slow ramp rate.

[0014] In another viewpoint, 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 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 drive circuit for generating an upper bridge drive signal to drive the upper bridge switch according to an upper bridge control signal; and 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 to the bootstrap capacitor of the upper bridge drive circuit, and the lower bridge control signal is used to control the lower bridge switch; wherein the upper bridge drive circuit generates the upper bridge drive 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 drive 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 viewpoint, 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 coupled together 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 based on an upper-bridge control signal to drive the upper-bridge switch. The control method includes the following steps: generating a bias voltage connected to a bootstrap diode and a bootstrap capacitor to provide power to the upper-bridge drive circuit from the bootstrap capacitor; generating the upper-bridge control signal and the lower-bridge control signal based on the output voltage, wherein the lower-bridge control signal controls the lower-bridge switch; when the upper-bridge switch requires soft switching, controlling the upper-bridge drive circuit to generate the upper-bridge drive signal at a fast slope to turn on the upper-bridge switch; and when the upper-bridge switch does not require soft switching, controlling the upper-bridge drive circuit to generate the upper-bridge drive signal at 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 at the slow slope; and when the bias voltage is higher than the threshold voltage, controlling the upper bridge drive circuit to generate the upper bridge drive signal at 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 drive signal has the slow slope.

[0018] In one embodiment, the step of generating the bias voltage includes controlling the bias voltage below a threshold voltage during the power-on period of the output voltage, such that the upper bridge drive signal has the slow slope.

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

[0020] The following detailed description through specific embodiments will make it easier to understand the purpose, technical content, features and effects achieved by the present invention. Attached Figure Description

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

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

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

[0024] Explanation of symbols in the diagram

[0025] 10: Transformer

[0026] 20: Resonant capacitor

[0027] 30: Upper-bridge transistor

[0028] 40: Lower-bridge transistor

[0029] 51, 52: Resistors

[0030] 56: Bootstrap Diode

[0031] 57: Bootstrap capacitor

[0032] 60: Resistance

[0033] 70: Synchronous rectifier transistor

[0034] 90: Optical Coupler

[0035] 100: Secondary 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 offset device

[0044] 515: Voltage Regulator

[0045] 520: Comparator with hysteresis

[0046] 530: Flipper

[0047] 550: Buffer

[0048] NA: Auxiliary winding

[0049] NP: Primary winding

[0050] NS: Secondary winding

[0051] S1, S2: Control signals

[0052] S11: Bridge control signal

[0053] SG: Secondary-side drive signal

[0054] SH: Overpass 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

[0067] VAUX: Signal Detailed Implementation

[0068] The accompanying drawings in this 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 to scale.

[0069] Figure 1This represents a preferred embodiment of the resonant power converter of the present invention. The half-bridge circuit consists 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., the 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 based on 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. A 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 based on a 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 via optocoupler 90. The secondary-side controller 100 also generates a secondary-side drive signal SG to operate the synchronous rectifier transistor 70 during the demagnetization period (TDS) of the transformer 10. During transformer 10 switching, the auxiliary winding NA generates an auxiliary winding signal VNA. 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 transistor 30. During the demagnetization and resonance of the transformer 10, the lower bridge drive signal SL turns on transistor 40. Transistor 40 is also used to generate circulating current in the transformer 10 to achieve zero-voltage switching (ZVS) of transistor 30. The voltage VCR of the resonant capacitor 20 is related to the output voltage VO.

[0071] However, in certain situations, such as during power-on, 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 sufficient 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, 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 invention, the primary-side controller 200 generates a bias voltage VB based on a voltage VD. In one embodiment, the voltage VDD is generated by rectifying an auxiliary winding signal VNA. The bias voltage VB is coupled to a bootstrap diode 56 and a bootstrap capacitor 57, and through the switching of transistor 40, an offset bias voltage VCB is generated on the bootstrap capacitor 57 to provide to the upper bridge drive circuit 300. In one embodiment, when soft switching of transistor 30 is intended, the upper bridge drive circuit 300 generates an upper bridge drive signal SH with a fast slope to turn on transistor 30, which improves switching efficiency. Conversely, when soft switching is not used (or soft switching of transistor 30 cannot be achieved for the aforementioned reasons) to turn on transistor 30, the upper bridge drive circuit 300 generates an upper bridge drive signal SH with a slow slope, which prevents the aforementioned high voltage stress on the transistor.

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

[0074] Figure 2A and Figure 2B A preferred embodiment of the upper bridge drive circuit 300 according to the present invention is shown. When the upper bridge control signal S11 is logic high (i.e., in an enabled state), if the bias voltage VCB exceeds the threshold voltage VTL, the comparator 310 with hysteresis controls the transistor 350 to turn on. Conversely, if the bias voltage VCB is lower than the threshold voltage VTL, the comparator 310 controls the transistor 351 to turn on. In one embodiment, the transistor 351 may be a high-impedance transistor. In one embodiment, the on-impedance of the transistor 351 is higher than that of the transistor 350. To further achieve a slow slope for the transistor 30, in one embodiment, a high-impedance resistor 355 (e.g., Figure 2B (As shown) is connected in series with transistor 351 to generate a slow-rate upper-bridge drive signal SH. Transistor 360 is used to turn off the upper-bridge drive signal SH according to the upper-bridge control signal S11.

[0075] Figure 3A preferred embodiment of the circuitry for generating the upper bridge control signal S11, the lower bridge drive signal SL, and the 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 an undervoltage threshold, the primary-side controller 200 generates a bias voltage VB lower than the threshold voltage VTL. A 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 is related to the voltage level of the output voltage VO. Furthermore, a power-on reset signal PRST is used to reset the flip-flop 530. The flip-flop 530 generates an H / L signal, which, when the flip-flop 530 is reset, controls the voltage regulator 515 to generate a bias voltage VB lower than the threshold voltage VTL. The upper bridge control signal S11 is generated by the potential offset device 510, while the lower bridge control signal S2 is generated by the buffer 550 to produce the lower bridge drive signal SL.

[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 reference to preferred embodiments. However, the above description is only intended to facilitate understanding of the invention by those skilled in the art and is not intended to limit the scope of the invention. The described embodiments are not limited to individual application and can also be used in combination. For example, two or more embodiments can be used in combination, and some components of one embodiment can be used to replace corresponding components in another embodiment. Furthermore, within the same spirit of the invention, those skilled in the art can conceive of various equivalent changes and combinations. For example, the phrase "processing or calculating based on a signal or generating an output result" in the present invention is not limited to the signal itself, but also includes, when necessary, performing voltage-to-current conversion, current-to-voltage conversion, and / or proportional conversion on the signal, and then processing or calculating based on the converted signal to generate an output result. Therefore, within the same spirit of the invention, those skilled in the art can conceive of various equivalent changes and combinations, and there are many ways to combine them, which will not be listed here. 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 together to form a half-bridge switching circuit, which is used to switch a transformer to generate an output voltage. An upper bridge drive circuit is used to generate an upper bridge drive signal based on an upper bridge control signal to drive the upper bridge switch; A bias voltage, connected to a bootstrap diode and a bootstrap capacitor, provides a power supply from the bootstrap capacitor to the upper bridge drive circuit. The upper bridge drive circuit generates the upper bridge drive signal with a fast slope to turn on the upper bridge switch when the upper bridge switch needs to be soft-switched; and generates the upper bridge drive signal with a slow slope to turn on the upper bridge switch when the soft switching is not performed.

2. The resonant power converter as described in claim 1, wherein, The upper bridge drive circuit generates the upper bridge drive signal with the slow slope when the bias voltage is below a threshold voltage; and generates the upper bridge drive signal with the fast slope when the bias voltage is above the threshold voltage.

3. The resonant power converter as described in claim 1, wherein, When the lower bridge switch is turned on, a circulating current is generated. When the lower bridge switch is turned off, the upper bridge switch is soft-switched.

4. The resonant power converter as described in claim 2, wherein, It also includes a switching control circuit for generating the bias voltage, the switching control circuit for generating an upper bridge control signal to control the upper bridge switch and a lower bridge control signal to control the lower bridge switch.

5. The resonant power converter as described in claim 2, wherein, The bias voltage is controlled to be below a threshold voltage when the output voltage is below an undervoltage threshold, so that the upper bridge drive signal has the slow slope.

6. The resonant power converter as described in claim 2, wherein, During the power-on period of the output voltage, the bias voltage is lower than the threshold voltage, causing the upper bridge drive signal to have the slow slope.

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

8. The resonant power converter as described in claim 7, wherein, The upper bridge drive circuit includes: Multiple 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 at the slow slope, a first portion of the multiple transistors is controlled to be turned on and has the first resistance value, wherein when the upper bridge switch needs to be turned on at the fast slope, a second portion of the multiple transistors is controlled to be turned on and has the second resistance value.

9. The resonant power converter as described in claim 8, wherein, The plurality of transistors includes a first transistor and a second transistor, wherein the first transistor is controlled to be turned on when the upper bridge switch needs to be turned on at the slow slope, and wherein the second transistor is controlled to be turned on when the upper bridge switch needs to be turned on at the fast slope, wherein the on-resistance of the first transistor is greater than the on-resistance of the second transistor.

10. The resonant power converter as claimed in claim 9, wherein, The upper bridge drive circuit also includes a resistor coupled to the first portion of the plurality of transistors, which is used to increase the path resistance when the upper bridge switch needs to be turned on at the slow slope.

11. 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, coupled to each other to form a half-bridge switching circuit, used to switch a transformer to generate an output voltage; the conversion control circuit includes: An upper bridge drive circuit is used to generate an upper bridge drive signal based on an upper bridge control signal to drive the upper bridge switch; A switching control circuit is used to generate 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 drive circuit, and the lower bridge control signal is used to control the lower bridge switch. The upper bridge drive circuit generates the upper bridge drive signal with a fast slope to turn on the upper bridge switch when the upper bridge switch needs to perform a soft switch, and generates the upper bridge drive signal with a slow slope to turn on the upper bridge switch when the upper bridge switch does not perform a soft switch.

12. The conversion control circuit as described in claim 11, wherein, The upper bridge drive circuit generates the upper bridge drive signal with the slow slope when the bias voltage is below a threshold voltage; and generates the upper bridge drive signal with the fast slope when the bias voltage is above the threshold voltage.

13. The conversion control circuit as described in claim 12, wherein, The bias voltage is controlled to be below an undervoltage threshold when the output voltage of the power converter is below that threshold voltage, so that the upper bridge drive signal has the slow slope.

14. The conversion control circuit as described in claim 12, wherein, During the power-on period of the output voltage, the bias voltage is controlled to be below the threshold voltage, so that the upper bridge drive signal has the slow slope.

15. The conversion control circuit as described in 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 resistor from the power supply to the upper bridge switch control terminal to have a first resistance value, and when the upper bridge switch needs to be turned on at the fast slope, the path resistor has a second resistance value, wherein the first resistance value is greater than the second resistance value.

16. The conversion control circuit as described in claim 15, wherein, The upper bridge drive circuit includes: Multiple 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 at the slow slope, a first portion of the multiple transistors is controlled to be turned on and has the first resistance value, wherein when the upper bridge switch needs to be turned on at the fast slope, a second portion of the multiple transistors is controlled to be turned on and has the second resistance value.

17. The conversion control circuit as described in claim 16, wherein, The plurality of transistors includes a first transistor and a second transistor, wherein the first transistor is controlled to be turned on when the upper bridge switch needs to be turned on at the slow slope, and wherein the second transistor is controlled to be turned on when the upper bridge switch needs to be turned on at the fast slope, wherein the on-resistance of the first transistor is greater than the on-resistance of the second transistor.

18. The conversion control circuit as described in claim 17, wherein, The upper bridge drive circuit also includes a resistor coupled to the first portion of the plurality of transistors, which is used to increase the path resistance when the upper bridge switch needs to be turned on at the slow slope.

19. A control method for controlling a resonant power converter, wherein, The resonant power converter includes: an upper-bridge switch and a lower-bridge switch, coupled together 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 based on an upper-bridge control signal to drive the upper-bridge switch; the control method includes: A bias voltage is generated, wherein the bias voltage is connected to a bootstrap diode and a bootstrap capacitor, providing power from the bootstrap capacitor to the upper bridge drive circuit. The upper bridge control signal and the lower bridge control signal are generated based on the output voltage, wherein the lower bridge control signal is used to control the lower bridge switch; When the bridge switch needs to be soft-switched, the bridge drive circuit is controlled to generate the bridge drive signal at a fast ramp to turn on the bridge switch; and When the bridge switch does not perform soft switching, the bridge drive circuit is controlled to generate the bridge drive signal at a slow slope to turn on the bridge switch.

20. The control method as described in claim 19, wherein, The steps for generating the upper bridge drive signal include: When the bias voltage is lower than a threshold voltage, the upper bridge drive circuit is controlled to generate the upper bridge drive signal at the slow slope. When the bias voltage is higher than the threshold voltage, the upper bridge drive circuit is controlled to generate the upper bridge drive signal at the fast slope.

21. The control method as described in claim 20, wherein, The steps to generate this bias voltage include: When the output voltage of the power converter is lower than an undervoltage threshold, the bias voltage is controlled to be lower than the threshold voltage, so that the upper bridge drive signal has the slow slope.

22. The control method as described in claim 20, wherein, The steps to generate this bias voltage include: During the power-on period of the output voltage, the bias voltage is controlled to be below a threshold voltage, so that the upper bridge drive signal has the slow slope.

23. The control method as described in claim 19, wherein, The steps for generating the upper bridge drive signal include: When the upper bridge switch needs to be turned on at the slow slope, the 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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