Power convertor and controlling method thereof and controller

Abstract

A power converter includes a transformer, a resonant circuit, a switching circuit, and an auxiliary circuit. The transformer includes a primary side winding disposed on a primary side of the transformer, a magnetic inductor coupled to the primary side winding, and a secondary side winding that is disposed on a secondary side of the transformer and coupled to the primary side winding. The resonant circuit couples to the primary side winding. The switching circuit includes a first switch and a second switch coupled in series with the first switch. The resonant circuit is coupled between the first switch and the second switch. The auxiliary circuit at the primary side includes an auxiliary winding, an auxiliary switch coupled to the auxiliary winding, and an auxiliary capacitor connected in series with the auxiliary switch. The auxiliary switch is switched off and the magnetic inductor is discharged so that the auxiliary capacitor is charged. The auxiliary switch is switched on so that the auxiliary capacitor charges the magnetic inductor to generate a magnetic inductor current, and the magnetic inductor current switches on a parasitic diode of the first switch.

Description

Technical Field

[0001] This case relates to a power converter, and more particularly to a power converter suitable for achieving zero voltage switching in discontinuous resonant mode, and a controller for controlling the power converter. Prior Technology

[0002] As consumer electronics products increasingly demand higher power and smaller size from their adapters, switching power converters, with their high efficiency and small size, have replaced linear regulators as the mainstream technology in consumer electronics applications.

[0003] Traditional switching power converters typically use metal-oxide-semiconductor field-effect transistors (MOSFETs) as the switching elements for pulse width modulation (PWM). This is because MOSFETs have characteristics such as fast switching speed and low loss. However, if the parasitic capacitance of the MOSFET's output is not fully discharged when the switch is turned on, a voltage will appear between the drain and source of the MOSFET. This voltage will cause a loss due to the current flowing through the MOSFET's source; this loss is called the MOSFET's switching loss.

[0004] To reduce switching losses, traditional asymmetric half-bridge flyback converters typically continue to conduct the lower arm switch to reverse charge the magnetizing inductor when the magnetizing inductor discharges to 0 amperes (A), making the magnetizing inductor current reversed. This reverse current is then used to discharge the parasitic capacitance of the upper arm switch, achieving zero voltage switching. This mode is called continuous resonant mode.

[0005] While this continuous resonant mode works well under full-load, high-voltage conditions, the peak current of the magnetizing inductor decreases as the load or output voltage drops. This leads to excessively high operating frequencies, and the circulating energy of the resonant tank also limits the overall efficiency of the converter. Therefore, asymmetric half-bridge flyback converters typically use discontinuous resonant mode with discontinuous magnetizing inductor current when the load decreases or the output voltage is low. However, since the switching in discontinuous resonant mode is completed when the magnetizing inductor discharges to zero, there is no reverse magnetizing current, making zero-voltage switching impossible and resulting in additional switching losses. Therefore, how to provide power conversion to solve these problems is an important issue in this field. Summary of the Invention

[0006] A power converter includes a transformer, a resonant circuit, a switching circuit, and an auxiliary circuit. The transformer includes a primary winding disposed on the primary side of the transformer, a magnetizing inductor connected in parallel with the primary winding, and a secondary winding disposed on the secondary side of the transformer and coupled to the primary winding. The resonant circuit couples the primary winding. The switching circuit includes a first switch and a second switch connected in series with the first switch, wherein the resonant circuit is coupled between the first switch and the second switch. The auxiliary circuit disposed on the primary side includes an auxiliary winding, an auxiliary switch coupled to the auxiliary winding, and an auxiliary capacitor connected in series with the auxiliary switch. When the auxiliary switch is open, the magnetizing inductor discharges to charge the auxiliary capacitor; when the auxiliary switch is closed, the auxiliary capacitor charges the magnetizing inductor, generating a magnetizing current. This magnetizing current turns on the parasitic diode of the first switch.

[0007] A control method for a power converter, wherein the power converter includes a switching circuit having a first switch and a second switch, a transformer having a primary winding, a magnetizing inductor, and a secondary winding, a resonant circuit coupled between the primary winding and the switching circuit, and an auxiliary circuit having an auxiliary winding, an auxiliary switch, and an auxiliary capacitor, wherein the auxiliary circuit and the primary winding are located on the same side of the transformer. The control method includes turning on the second switch and charging the auxiliary capacitor for a first period by discharging the magnetizing inductor; in a second period after the first period, turning off the second switch and turning on the auxiliary switch, causing the auxiliary capacitor to charge the magnetizing inductor and generate a magnetizing current; and in a third period after the second period, turning off the auxiliary switch and turning on the parasitic diode of the first switch by the magnetizing current flowing through the first switch.

[0008] A controller is provided for controlling a power converter, wherein the power converter includes a switching circuit having a first switch and a second switch, a transformer having a primary winding, a magnetizing inductor, and a secondary winding, a resonant circuit coupled between the primary winding and the switching circuit, and an auxiliary circuit having an auxiliary winding, an auxiliary switch, and an auxiliary capacitor, wherein the auxiliary circuit and the primary winding are located on the same side of the transformer. The controller is configured to: turn on the second switch and charge the auxiliary capacitor for a first period by discharging the magnetizing inductor; in a second period following the first period, turn off the second switch and turn on the auxiliary switch, causing the auxiliary capacitor to charge the magnetizing inductor and generate a magnetizing current; and in a third period following the second period, turn off the auxiliary switch and turn on the first switch after the magnetizing current flows through the parasitic diode of the first switch. Simple Explanation of the Diagram

[0009] To make the above and other objects, features, advantages and embodiments of this disclosure more apparent and understandable, the accompanying drawings are described below: Figure 1 is a schematic diagram of a power converter and controller according to an embodiment of the present disclosure. Figure 2 is a flowchart illustrating the control method of the power converter of Figure 1 according to an embodiment of this case. Figure 3 is a waveform diagram of a plurality of signals of a power converter according to an embodiment of the present invention. Figures 4 through 10 are schematic diagrams illustrating the operation of the power converter of Figure 1 according to an embodiment of this case during various periods. Figure 11 is a schematic diagram of a power converter according to another embodiment of the present disclosure. Implementation

[0010] The following detailed description provides examples with reference to the accompanying drawings. However, the provided examples are not intended to limit the scope of this disclosure, and the description of the structural operation is not intended to limit the order of execution. Any structure resulting from the recombination of elements and producing a device with equivalent functionality is within the scope of this disclosure. Furthermore, the illustrations are for illustrative purposes only and are not drawn to their original dimensions. For ease of understanding, the same or similar elements will be labeled with the same symbols in the following description.

[0011] The terms "coupled," "coupled," or "connected" as used in this article may refer to two or more components making direct physical or electrical contact with each other, or making indirect physical or electrical contact with each other, or to two or more components operating or moving together.

[0012] In this article, the term "circuit" is used to refer to an object consisting of one or more transistors and / or one or more active and passive components connected in a certain manner to process signals.

[0013] Unless otherwise specified, the terms used throughout this specification and the claims generally have their ordinary meaning in the context of the art, the disclosure, and the specific content. Furthermore, the terms "comprising," "including," "having," "containing," etc., used herein are open-ended terms, meaning "including but not limited to." Additionally, the term "and / or" as used herein includes any one or more of the related listed items and all combinations thereof.

[0014] Please refer to Figure 1, which is a schematic diagram of a power converter 100 and a controller 160 according to an embodiment of the present disclosure. As shown in Figure 1, the power converter 100 includes a switching circuit 110, a resonant circuit 120, an auxiliary circuit 130, a transformer TX, a diode DO, and a capacitor CO. In some embodiments, the power converter 100 couples a load RL and generates an output voltage VO to the load RL based on the input voltage V PFC received by the switching circuit 110. In some embodiments, the diode DO may be replaced by a metal oxide semiconductor field-effect transistor (MOSFET). In some embodiments, the controller 160 generates signals to control the operation of the power converter and may be a microprocessor or any suitable integrated circuit.

[0015] In some embodiments, the transformer TX includes a primary winding N1, a magnetizing inductor Lm, and a secondary winding N2. The primary winding N1 is located on the primary side 140 of the transformer TX, and the secondary winding N2 is located on the secondary side 150 of the transformer TX. As shown in Figure 1, the switching circuit 110, the resonant circuit 120, and the auxiliary circuit 130 are located on the primary side 140. The diode DO, the capacitor CO, and the load RL are located on the secondary side 150.

[0016] In some embodiments, the switching circuit 110 includes a switch S1 and a switch S2. Switch S1 includes a parasitic capacitance C1 and a parasitic diode D1, and switches on or off in response to a signal Vgs_HS received at its gate. Switch S2 includes a parasitic capacitance C2 and a parasitic diode D2, and switches on or off in response to a signal Vgs_LS received at its gate. Switch S1 is coupled between the positive input terminal of the input voltage VpFC and node n1 between switches S1 and S2. Switch S2 is coupled between node n1 and ground.

[0017] In some embodiments, the resonant circuit 120 includes an inductor Lr and a capacitor Cr. The inductor Lr is coupled between the (circular) dot end of the primary winding N1 and node n1, and is coupled at the dot end of the primary winding N1 to one end of the magnetizing inductor Lm. The capacitor Cr is coupled between the non-dot end of the primary winding N1 and ground, and is coupled at the non-dot end of the primary winding N1 to the other end of the magnetizing inductor Lm. In some embodiments, the inductor Lr is the leakage inductance of the transformer TX or an independent inductor.

[0018] In some embodiments, the auxiliary circuit 130 includes an auxiliary switch Sa, an auxiliary capacitor Cb, and an auxiliary winding Na. The auxiliary switch Sa includes an auxiliary parasitic capacitor Ca and a parasitic diode Da, and switches on or off in response to a signal Vgs_Sa received at its gate terminal. The auxiliary switch Sa is coupled between the (circular) dot terminal and the ground terminal of the auxiliary winding Na. The auxiliary capacitor Cb is coupled between the non-dot terminal and the ground terminal of the auxiliary winding Na. In some embodiments, the capacitance value of parasitic capacitor C1 is less than the capacitance value of auxiliary capacitor Cb (also referred to as the "energy storage capacitance value"). In some embodiments, the capacitance value of parasitic capacitor C2 is less than the capacitance value of auxiliary capacitor Cb. In some embodiments, the capacitance value of auxiliary parasitic capacitor Ca is less than the capacitance value of parasitic capacitor C1. In some embodiments, the capacitance value of auxiliary parasitic capacitor Ca is less than the capacitance value of parasitic capacitor C2.

[0019] In the secondary side coupling relationship at 150°, the positive terminal of diode DO is coupled to one end of capacitor CO to ground, and the negative terminal of diode DO is coupled to the (round) dot terminal of secondary winding N2. The other end of capacitor CO is coupled to the non-dot terminal of secondary winding N2.

[0020] Please refer to Figures 2, 3, and 4 through 10. Figure 2 is a flowchart illustrating the control method 200 of the power converter 100 in Figure 1 according to an embodiment of this invention. Figure 3 is a waveform diagram 300 of a plurality of signals of the power converter 100 according to an embodiment of this invention. Figures 4 through 10 are schematic diagrams illustrating the operation of the power converter 100 in Figure 1 during various periods according to an embodiment of this invention. In some embodiments, the controller 160 executes the control method 200 to control the operation of the power converter 100.

[0021] According to step S210, during the period from time point t1 to t2, as shown in Figures 3 and 4, switch S1 turns on in response to a high-potential signal Vgs_HS, and switches S2 and auxiliary switches Sa turn off in response to low-potential signals Vgs_LS and Vgs_Sa, respectively. Node n1 has a voltage VHB, which is pulled up in response to the input voltage VPFC, charging inductor Lr and magnetizing inductor Lm, causing the current ILr flowing through inductor Lr and magnetizing inductor Lm and the magnetizing current ILm to increase. Magnetizing inductor Lm discharges parasitic capacitance C1 and charges capacitor Cr through magnetizing current ILm. In some embodiments, when the magnetizing current ILm rises to a set value, signal Vgs_HS turns low to disconnect switch S1.

[0022] Furthermore, in the embodiment shown in Figure 4, the diode DO is in an open state in response to its positive terminal being grounded and its negative terminal being at a low potential. Specifically, the terminals of the primary winding N1 and the secondary winding N2 are at high potentials. Therefore, the negative terminal of the diode DO, which is coupled to the terminal of the secondary winding N2, is at a high potential, while the positive terminal of the diode DO is grounded, causing the diode DO to be open.

[0023] According to step S220, during the period from time t2 to t3, as shown in Figure 5, switch S1 opens in response to the low-potential signal Vgs_HS, while switch S2, auxiliary switch Sa, and diode D0 remain open. The magnetizing current ILm discharges the parasitic capacitance C2. When the parasitic capacitance C2 has completely discharged, the magnetizing current ILm flows through the conducting parasitic diode D2, making the switching loss of switch S2 almost zero. Then, the signal Vgs_LS goes high to turn on switch S2.

[0024] According to step S230, during the period from time t3 to t4, as shown in Figure 6, switch S2 turns on in response to a high-potential signal Vgs_LS, while switch S1 and auxiliary switch Sa remain off. Capacitor Cr discharges, generating a current flowing through the primary winding N1, thus transferring energy from the primary winding N1 to the secondary winding N2. In response to the energy transfer from the primary winding N1 to the secondary winding N2, diode DO turns on, and the secondary winding N2 generates a current Io. Furthermore, energy from the primary winding N1 is also transferred to the auxiliary winding Na to generate a current Ia. Current Ia flows through the parasitic diode Da, charging the auxiliary capacitor Cb.

[0025] According to step S240, during the period from time t4 to t5 (which can be considered the "energy release period"), as shown in Figure 7, switch S2 opens in response to the low-potential signal Vgs_LS, and switch S1 and auxiliary switch Sa remain open. The current ILr flowing through inductor Lr and magnetizing inductor Lm, and the magnetizing current ILm, continue to decrease until the current value is zero (in other words, the magnetizing inductor Lm releases energy to zero). During the energy release period, no energy is transferred from the primary winding N1 to the auxiliary winding Na, so diode DO is open.

[0026] According to step S250, during the period from time t5 to t6 (which can be considered the "resonance period"), as shown in Figure 8, switches S1, S2, auxiliary switch Sa, and diode DO remain open. The magnetizing inductor Lm, parasitic capacitances C1 and C2 generate a resonance with a resonant period. When the resonance reaches a sufficient number of periods and the current flowing through inductor Lr (ILr) and the current flowing through magnetizing inductor Lm (ILm) are both zero, the signal Vgs_Sa goes high to turn on the auxiliary switch Sa. In some embodiments, the number of resonant periods is positively correlated with the output voltage VO.

[0027] According to step S260, during the period from time t6 to t7, as shown in Figure 9, the auxiliary switch Sa turns on in response to the high-potential signal Vgs_Sa, while switches S1, S2, and diode DO remain off. The auxiliary capacitor Cb releases energy to generate a reverse current Iar, which stores reverse energy in the magnetizing inductor Lm, causing the magnetizing current ILm to reach the reverse current value IMAGneg. In some embodiments, the reverse current value IMAGneg corresponds to the current value that can completely discharge the parasitic capacitor C1 and completely charge the parasitic capacitor C2. The reverse current value IMAGneg is based on formula (1): …(1) Coss s1 represents the capacitance value of parasitic capacitance C1, Coss s2 represents the capacitance value of parasitic capacitance C2, VPFC represents the input voltage VPFC, and Lm represents the inductance value of magnetizing inductance Lm. In some embodiments, when the magnetizing current ILm reaches the reverse current value MACAGneg, the signal Vgs_Sa goes low to disconnect the auxiliary switch Sa.

[0028] As described above, since the operation of the auxiliary switch Sa is used to store energy in the magnetizing inductor Lm, in some embodiments, the transistor size of the auxiliary switch Sa is smaller than that of the transistor size of the switch S1. In other words, the capacitance value of the auxiliary parasitic capacitance Ca is lower than that of the parasitic capacitance C1, which also reduces the drive loss Pgate of the auxiliary switch Sa. The drive loss Pgate of the auxiliary switch Sa is based on formula (2): …(2) Ciss represents the capacitance value of the auxiliary parasitic capacitance Ca, V gate represents the gate voltage value of the auxiliary switch Sa, and f sw represents the operating frequency of the auxiliary switch Sa.

[0029] In some embodiments, the transistor size of the auxiliary switch Sa is smaller than the transistor size of the switch S2.

[0030] In some embodiments, the voltage stress of the auxiliary switch Sa is proportional to the ratio of the number of turns in the primary winding N1 to the number of turns in the auxiliary winding Na. For example, the voltage stress at a turn ratio of 200% (20 turns in the primary winding N1 and 10 turns in the auxiliary winding Na) is half that at a turn ratio of 100% (10 turns in the primary winding N1 and 10 turns in the auxiliary winding Na).

[0031] In some embodiments, the switching loss Psw of the auxiliary switch Sa is based on formula (3): …(3) Id represents the drain current value of auxiliary switch Sa, Vds represents the cross voltage between the drain and source of auxiliary switch Sa, ton and toff represent the start-up time and turn-off time of auxiliary switch Sa, respectively, and fsw represents the operating frequency.

[0032] In some embodiments, the period from time point t6 to t7 is shorter than the period from time point t3 to t4.

[0033] According to step S270, during the period from time t7 to t8, as shown in Figure 10, the auxiliary switch Sa opens in response to the low-potential signal Vgs_Sa, while switch S2 and diode D0 remain open. The magnetizing current ILm discharges the parasitic capacitance C1 as current ILr. When the parasitic capacitance C1 has completely discharged, current ILm flows through the conducting parasitic diode D1, causing switch S1 to turn on with almost zero switching losses, achieving the effect of zero voltage switching (ZVS). Then, the signal Vgs_HS turns high to turn on switch S1.

[0034] Please refer to Figure 11, which is a schematic diagram of a power converter 400 according to an embodiment of this disclosure. Compared to the power converter 100 in Figure 1, an inductor Lr is coupled between the point terminal of the primary winding N1 and the positive input terminal of the input voltage V PFC. A capacitor Cr is coupled between the non-point terminal of the primary winding N1 and node n1.

[0035] In summary, the power converter and its control method disclosed herein enable the power converter to achieve zero-voltage switching even in discontinuous resonant mode, thereby reducing switching losses in the power converter.

[0036] Although the present disclosure has been described above with reference to embodiments, it is not intended to limit the present disclosure. Any person skilled in the art may make various modifications and alterations without departing from the spirit and scope of the present disclosure. Therefore, the scope of protection of the present disclosure shall be determined by the claims attached.

[0037] 100: Power Converter 110: Switching circuit 120: Resonant Circuit 130: Auxiliary Circuit 140: Primary Side 150: Secondary side 160: Controller 200: Control method of power converter 300: Waveform diagram of multiple signals 400: Power Converter C 1: Parasitic capacitance C 2: Parasitic capacitance C a: Auxiliary parasitic capacitance C b: Auxiliary capacitor CO: Capacitor C r: Capacitor D 1: Parasitic Dipolar Body D 2: Parasitic Dipolar Body D a: Parasitic Dipolar DO: Dipolar I a: current I ar: Reverse current I Lm: Excitation current I Lr: Current I o: current L m: Magnetizing inductance L r: Inductance N 1: Primary winding N2: Secondary winding N a: Auxiliary winding n 1: node RL: Load S1: Switch S2: Switch S a: Auxiliary switch t: time t1: Time point t2: Time point t3: Time point t 4: Time point t 5: Time point t 6: Time point t 7: Time point t 8: Time point TX: Transformer V a: Voltage V gs_HS: Signal V gs_LS: signal V gs_Sa: signal VHB: Voltage VO: Output voltage V PFC: Input Voltage S210: Steps S220: Steps S230: Steps S240: Steps S250: Steps S260: Steps S270: Steps

Claims

1. A power converter, comprising: A transformer includes: a primary winding disposed on the primary side of the transformer; a magnetizing inductor connected in parallel with the primary winding; and a secondary winding disposed on the primary side of the transformer and coupled to the primary winding; a resonant circuit coupled to the primary winding; a switching circuit including: a first switch; and a second switch connected in series with the first switch, wherein the resonant circuit is coupled between the first switch and the second switch; and an auxiliary circuit disposed on the primary side, including: an auxiliary winding; an auxiliary switch coupled to the auxiliary winding; and an auxiliary capacitor connected in series with the auxiliary switch, wherein the auxiliary switch is turned off and the magnetizing inductor discharges to charge the auxiliary capacitor; wherein the auxiliary switch is turned on, causing the auxiliary capacitor to charge the magnetizing inductor and generate a magnetizing current, the magnetizing current causing a parasitic diode of the first switch to conduct.

2. The power converter as claimed in claim 1, wherein the first switch includes a first parasitic capacitor, the capacitance of which is less than the energy storage capacitance of the auxiliary capacitor.

3. The power converter as claimed in claim 2, wherein the second switch includes a second parasitic capacitor, the capacitance of which is less than the energy storage capacitor value.

4. The power converter as claimed in claim 3, wherein an input voltage charges the magnetizing inductor when the first switch is turned on and the second switch is turned off.

5. The power converter as claimed in claim 4, wherein when the first switch is open and the second switch is on, energy is transferred from the primary winding to the secondary winding.

6. The power converter as claimed in claim 4, wherein when the first switch is open and the second switch is open, and the magnetizing inductor is de-energized to zero, the magnetizing inductor, the first parasitic capacitance, and the second parasitic capacitance generate a resonance having a resonant period.

7. The power converter as claimed in claim 6, wherein the resonant period is positively correlated with an output voltage value on the secondary side.

8. The power converter as claimed in claim 3, wherein an excitation current value of the excitation current is calculated according to the following formula: where IMAGneg is the excitation current value, VPFC is an input voltage value, Coss s1 is the capacitance value of the first parasitic capacitor, Coss s2 is the capacitance value of the second parasitic capacitor, and Lm is an excitation inductance value of the excitation inductor.

9. The power converter as claimed in claim 2, wherein the auxiliary switch includes an auxiliary parasitic capacitor whose capacitance value is less than that of the first parasitic capacitor.

10. The power converter as claimed in claim 9, wherein a drive loss P gate of the auxiliary switch is calculated according to the following formula: where Ciss is the capacitance value of the auxiliary parasitic capacitor, V gate is a gate voltage of the auxiliary switch, and f sw is an operating frequency.

11. The power converter as claimed in claim 1, wherein a voltage stress of the auxiliary switch is proportional to the ratio of the number of turns of the primary winding to the number of turns of the auxiliary winding.

12. The power converter as claimed in claim 1, wherein a first end of the auxiliary capacitor is coupled to a first end of the auxiliary winding, a second end of the auxiliary capacitor is coupled to a first end of the auxiliary switch, and a second end of the auxiliary switch is coupled to a second end of the auxiliary winding.

13. The power converter as claimed in claim 1, wherein the auxiliary switch is a transistor of a first size and the first switch is a transistor of a second size, wherein the first size is smaller than the second size.

14. The power converter as claimed in claim 13, wherein the second switch is a transistor of a third size, the third size being larger than the first size.

15. A control method for a power converter, wherein the power converter includes a switching circuit having a first switch and a second switch, a transformer having a primary winding, a magnetizing inductor, and a secondary winding, a resonant circuit coupled between the primary winding and the switching circuit, and an auxiliary circuit having an auxiliary winding, an auxiliary switch, and an auxiliary capacitor, wherein the auxiliary circuit and the primary winding are disposed on the same side of the transformer, wherein the control method includes: The second switch is turned on, and the auxiliary capacitor is charged for a first period by discharging the magnetizing inductor; In a second period following the first period, the second switch is turned off and the auxiliary switch is turned on, causing the auxiliary capacitor to charge the magnetizing inductor and generate a magnetizing current; and in a third period following the second period, the auxiliary switch is turned off, and the magnetizing current flows through the first switch to turn on a parasitic diode of the first switch.

16. The control method as described in claim 15, further comprising: During a release period between the first period and the second period, the first switch and the second switch are disconnected to release the magnetizing inductor; And during a resonance period after the energy release period and before the second period, a resonance with a resonance period is generated by the magnetizing inductor, a first parasitic capacitance of the first switch, and a second parasitic capacitance of the second switch.

17. The control method as described in claim 16, wherein at the end of the resonance period, a current flowing through the magnetizing inductor is zero.

18. The control method as described in claim 15, wherein the second period is shorter than the first period.

19. The control method as described in claim 15, wherein the ratio of the number of turns of the primary winding to the number of turns of the auxiliary winding is greater than 1.

20. The control method as described in claim 15, further comprising: During the first period, the first switch is turned off and the second switch is turned on, so that the energy of the primary winding is transferred to the secondary winding.

21. A controller for controlling a power converter, wherein the power converter includes a switching circuit having a first switch and a second switch, a transformer having a primary winding, a magnetizing inductor, and a secondary winding, a resonant circuit coupled between the primary winding and the switching circuit, and an auxiliary circuit having an auxiliary winding, an auxiliary switch, and an auxiliary capacitor, wherein the auxiliary circuit and the primary winding are disposed on the same side of the transformer, wherein the controller is configured to: turn on the second switch and charge the auxiliary capacitor by discharging the magnetizing inductor for a first period; in a second period following the first period, turn off the second switch and turn on the auxiliary switch, causing the auxiliary capacitor to charge the magnetizing inductor to generate a magnetizing current; and in a third period following the second period, turn off the auxiliary switch and turn on the first switch by the magnetizing current flowing through the first switch, turning on a parasitic diode of the first switch.

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