TW202145692A - Self-driving power supply based on resonance energy recycling - Google Patents

Abstract

A power supply includes two transformers, two resonance voltage supply circuits, a discharging circuit, a power switch and a rectifier switch. The first transformer is configured to convert an input voltage into an output voltage. The two resonance voltage supply circuits disposed on the primary side of the two transformers are configured to provide resonance voltage energy associated with the ON/OFF status of the power switch. The second transformer is configured to transfer the resonance voltage energy to the secondary side of the two transformers for controlling the rectifier switch accordingly. The discharging circuit is disposed on the secondary side for providing a discharging path for the rectifier switch when turned off.

Description

Power supply with self-sustained drive design with resonant energy recovery

The present invention relates to a power supply with self-sustained driving design of resonant energy recovery, in particular to a power supply that can take into account power conversion efficiency, component safety and output voltage stability.

Different components in a computer system require different operating voltages, so a power supply is generally used to convert alternating current (AC) indoor power into direct current (DC) by means of voltage transformation, rectification and filtering to drive different zeroes. components. A power supply in a traditional flyback architecture uses a power switch to control the primary-side path of the transformer and an output diode to control the secondary-side path of the transformer. When the power switch is turned on, the input electric energy will be converted and the magnetic energy will be stored in the transformer. At this time, the output diode of the reverse bias voltage will isolate the output path; when the power switch is turned off, the energy stored in the transformer will be output through the forward bias voltage. The diode is released to the output terminal, and the power output is smoothed by an output capacitor.

Due to the large power loss of the output diode, another prior art power supply uses a rectifier switch to control the secondary-side path of the transformer, thereby improving the power conversion efficiency. However, this structure requires an additional driving IC to control the rectifier switch synchronously, so that the rectifier switch is turned off when the power switch is turned on, and turned on when the power switch is turned off. The above-mentioned driving integrated circuit is arranged on the secondary side, and the high temperature or high frequency switching operation of the power supply can easily shorten the life of the driving integrated circuit, and once damaged, the power supply will not be able to output smoothly. In addition, the non-ideal characteristics of the rectifier switch (such as parasitic capacitance) will store power when it is turned on. The stored power will affect the output stability of the power supply, and in severe cases, the power supply may not be able to output smoothly.

Therefore, there is a need for a power supply that can take into account power conversion efficiency, device safety and output voltage stability.

The present invention provides a power supply with resonant energy recovery self-driving design, which includes a first transformer, a first switch, a second transformer, a second switch, a first resonant voltage supply circuit, and a second Resonant voltage supply circuit. The first transformer includes a first primary side winding and a first secondary side winding for converting an input voltage into an output voltage. The first switch includes a first terminal coupled to the input voltage; a second terminal coupled to a first ground potential; and a control terminal for receiving a first control signal. The second transformer includes a second primary side winding and a second secondary side winding for converting a first resonant voltage energy or a second resonant voltage energy into a second control signal. The second switch includes a first end coupled to the first secondary side winding; a second end selectively coupled to a second ground potential; and a control end for receiving the second control signal. The first resonant voltage supply circuit is coupled to the first switch and the second secondary side winding for resonating with the parasitic capacitance of the first switch to provide the first resonant voltage energy. The second resonant voltage supply circuit is coupled to the first switch and the first primary side winding for dividing the voltage across the first switch to provide the second resonant voltage energy.

FIG. 1 is a functional block diagram of a power supply 100 with resonant energy recovery self-driving design according to an embodiment of the present invention. The power supply 100 includes a main transformer TR1, an auxiliary transformer TR2, a magnetizing inductor Lm, a power switch Q1, a rectifier switch Q2, an output capacitor C _OUT , a first resonant voltage supply circuit 10, and a second resonant voltage A supply circuit 20, and a discharge circuit 30. The power supply 100 can convert an input voltage V _IN supplied by commercial power into an output voltage V _{OUT to} drive a load (not shown in FIG. 1 ).

FIG. 2 is a schematic diagram of the implementation of the power supply 100 according to the embodiment of the present invention. The main transformer TR1 includes a primary side winding (represented by the number of turns NP1) and a secondary side winding (represented by the number of turns NS1). The primary side winding NP1 is coupled to the input voltage V _IN , and the secondary side winding NS1 is coupled to the output terminal of the power supply 100 through the rectifier switch Q2. In the operation of the main transformer TR1, the relationship of the related voltages is V _IN /V _OUT =NP1/NS1. In boost applications, the number of turns NS1 of the secondary winding is greater than the number of turns NP1 of the primary winding; in buck applications, the number of turns NS1 of the secondary winding is smaller than the number of turns NP1 of the primary winding. In an embodiment of the present invention, the ratio of the values of NP1 and NS1 may be 36:6. However, the number of turns NP1 of the primary winding and the number of turns NS1 of the secondary winding in the main transformer TR1 do not limit the scope of the present invention.

The auxiliary transformer TR2 includes a primary side winding (represented by the number of turns NP2) and a secondary side winding (represented by the number of turns NS2). The primary side winding NP2 is coupled to the power switch Q1 through the resonant voltage supply circuit 10 and the resonant energy supply circuit 20, and the secondary side winding NS2 is coupled to the control terminal of the rectifier switch Q2. The auxiliary transformer TR2 is used to sense the energy related to the on/off state of the power switch Q1, and accordingly provide a control signal GD2 to control the on/off state of the auxiliary switch Q2 synchronously, so that the auxiliary switch Q2 is turned on when the power switch Q1 is turned on. is turned off, and is turned on when the power switch Q1 is turned off.

In the main transformer TR1 and the auxiliary transformer TR2 of the present invention, the primary side windings NP1~NP2 and the secondary side windings NS1~NS2 are wound on the same magnetic core 15, wherein the primary side winding NP1 and the secondary side winding NS1 form a voltage induction unit, while the primary side winding NP2 and the secondary side winding NS2 form a voltage induction unit. In an embodiment of the present invention, the ratio of the values of NP1, NS1, NP2, and NS2 may be 36:6:6:4. However, the number of turns of the primary side windings NP1-NP2 and the secondary windings in the main transformer TR1 and the auxiliary transformer TR2 The number of turns NS1 to NS2 of the side windings does not limit the scope of the present invention.

The magnetizing inductance Lm and the power switch Q1 are disposed on the primary side of the main transformer TR, and are connected in series between the input voltage V _IN and a ground potential GND1, wherein the magnetizing inductance Lm is connected in parallel with the primary side winding NP1 of the main transformer TR1. The first terminal of the power switch Q1 is coupled to the input voltage V _IN through the magnetizing inductor Lm, the second terminal is coupled to the ground potential GND1, and the control terminal is used for receiving a control signal GD1. The control signal GD1 can be provided by a pulse width modulation (PWM) integrated circuit, which can be a pulse signal with a specific duty cycle, so that the power switch Q1 can be selectively turned on or off. The parasitic capacitance between the first terminal and the second terminal of the _{power switch Q1 is represented by C OSS1} , the voltage across the first terminal and the second terminal of the _{power switch Q1 is represented by V DS1} , and the control terminal of the power switch Q1 and the first terminal are represented by V DS1. The bias voltage between the two terminals is represented _{by V GS1.}

The first terminal of the rectifier switch Q2 is coupled to the secondary side winding NS1 of the main transformer TR1, the second terminal is selectively coupled to the ground potential GND through the discharge circuit 30, and the control terminal is used for receiving the control signal GD2. The above-mentioned control signal GD2 can be provided by the secondary winding NS2 of the auxiliary transformer TR2. The parasitic capacitance between the first terminal and the second terminal of the _{rectifier switch Q2 is represented by C OSS2} , the cross-voltage between the first terminal and the second terminal of the _{rectifier switch Q2 is represented by V DS2} , and the control terminal of the rectifier switch Q2 and the second terminal are represented by V DS2. The bias voltage between the two terminals is represented _{by V GS2.}

The resonant voltage supply circuit 10 is disposed on the primary side of the main transformer TR1 and the auxiliary transformer TR2, and includes a diode D1 and a resonant inductor Lx. The anode of the diode D1 is coupled to the first terminal of the power switch Q1, the cathode of the diode D1 is coupled to the primary side winding NP2 of the auxiliary transformer TR2 and the resonant inductor Lx, and the resonant inductor Lx is connected in parallel with the primary of the auxiliary transformer TR2 side winding NP2.

The resonant energy supply circuit 20 is disposed on the primary side of the main transformer TR1 and the auxiliary transformer TR2, and includes an auxiliary switch Q3, voltage dividing resistors R1-R2, a storage capacitor C1, and a diode D2. The first end of the auxiliary switch Q3 is coupled to the first end of the power switch Q1 through the voltage dividing resistor R1, the second end is coupled to the primary side winding NP2 of the auxiliary transformer TR2 and is coupled to the ground potential GND1 through the voltage dividing resistor R2, The control terminal is coupled to the ground potential GND through the storage capacitor C1. The stored energy of the storage capacitor C1 can provide a control signal GD3 to the control terminal of the auxiliary switch Q3, so that the auxiliary switch Q3 can be selectively turned on or off. The anode of the diode D2 is coupled to the primary side winding NP2 of the auxiliary transformer TR2, the cathode of the diode D1 is coupled between the control terminal of the auxiliary switch Q3 and the energy storage capacitor C1, and the resistor R2 is coupled to the auxiliary switch Q3 between the second terminal and the ground potential GND1.

The discharge circuit 30 is disposed on the secondary side of the main transformer TR1 and the auxiliary transformer TR2, and includes a diode D3 and a vibration-snubber resistor Rx. The anode of the diode D3 is coupled to the second terminal of the rectifier switch Q2, and the cathode of the diode D1 is coupled to the secondary winding NS2 of the auxiliary transformer TR2, and is coupled to a ground potential GND2 through the snubber resistor Rx .

FIG. 3 is a schematic diagram of related signals during operation of the power supply 100 according to the embodiment of the present invention. As known to those with ordinary knowledge in the related art, the operation of an electronic device sequentially includes an off state, a semi-activated state (also referred to as a transient state), and a fully activated state (also referred to as a steady state). For the purpose of illustration, FIG. 3 divides the operation of the power supply 100 into five states S0-S4 for illustration, wherein the state S0 is the initial non-power-on state when the AC mains is not powered on, and the AC mains just leaves the standby state after the AC mains is powered on. The states S1-S3 entered from the power-on state S0 are the semi-starting state, and the state S4 is the fully-starting state after the AC mains is powered on for a period of time. After the power supply 100 enters the state S4 for the first time, it will then operate in a steady state. To be more specific, the power supply 100 will only enter the transient state S1 once after power-on, and then will switch in the order of the state S2 , the state S3 and the state S4 in sequence. Table 1 below shows the state of each element in the power supply 100 in states S0-S4. Power switch Q1 Rectifier switch Q2 Auxiliary switch Q3 Diode D1 Diode D2 Diode D3 state S0 deadline deadline deadline deadline deadline deadline State S1 turn on deadline deadline deadline deadline deadline State S2 deadline deadline deadline turn on deadline deadline state S3 deadline turn on turn on deadline turn on deadline State S4 turn on deadline deadline deadline deadline turn on Table I

As shown in FIG. 3 and Table 1, in the state S0, the power supply 100 is not connected to the power supply, at this time, the power switch Q1, the rectifier switch Q2, and the auxiliary switch Q3 are all turned off (V _GS1 =V _DS1 =V _GS2 = V _DS2 =0), the diodes D1 ˜ D3 are also turned off, so the power supply 100 has no voltage output.

In the states S1-S4 after the power supply 100 is connected to the power supply, the potential of the control terminal of the power switch Q1 will change with the control signal GD1, that is to say, _{the waveform of the bias voltage V GS1} and the control signal GD0 have the same duty cycle. The states S1 and S4 correspond to the period during which the control signal GD1 and the bias voltage V _GS1 have the enabling potential, and the states S2 and S3 correspond to the period during which the control signal GD1 and the bias voltage V _GS1 have the disabling potential.

As shown in FIG. 3 and Table 1, after the power supply is connected and the bias voltage V _GS1 has an enabling potential, the power supply 100 will first enter the half-start state S1, at which time the power switch Q1 will be turned on to establish The input loop makes the magnetizing inductor Lm begin to store the energy of the _{input voltage V IN.} Since the first terminal of the power switch Q1 is pulled to the ground potential GND (V _DS1 =0) when the power switch Q1 is turned on, the diode D1 in the resonant voltage supply circuit 10 is turned off. At this time, the primary side winding NP2 of the auxiliary transformer TR2 is turned off. There is no induced energy in the secondary side winding NS2, so the rectifier switch Q2 and the diodes D2~D3 are also cut off.

For the states S2 and S3 when the power supply 100 is connected to the power supply and the bias voltage V _GS1 has a disabling potential, FIG. 4 is a schematic diagram of an equivalent circuit of the power supply 100 when the power supply 100 operates in the state S2 according to the embodiment of the present invention. FIG. 5 is a schematic diagram of an equivalent circuit of the power supply 100 operating in the state S3 according to the embodiment of the present invention. For the state S4 when the power supply 100 is powered on and the bias voltage V _GS1 has an enabling potential, FIG. 6 is a schematic diagram of an equivalent circuit of the power supply 100 operating in the state S4 according to the embodiment of the present invention.

As shown in FIGS. 3 to 4 and Table 1, when the bias voltage V _GS1 is switched from the enable potential to the disable potential after the power is connected, the power supply 100 will switch from the half-on state S1 to the half-on state In state S2, the power switch Q1 is turned off and the diode D1 is turned on due to the forward bias. In this case, the parasitic capacitance C _{OSS1 of the} power switch Q1 will resonate with the resonant inductance Lx via the diode D1 in the resonant voltage supply circuit 10 , thereby generating a resonant voltage energy to increase the voltage across the power switch Q1 _VDS1 . The above-mentioned resonant voltage energy is then induced by the primary side winding NP2 of the auxiliary transformer TR2 to the secondary side winding NS2 to provide the control signal GD2, thereby pulling up the bias voltage V _{GS2 of the} rectifier switch Q2. In the state S2, _{the value of the bias voltage V GS2} is not enough to turn on the rectifier switch Q2, so the value of the _{voltage V DS2 across the switch Q2 is -(V NS1} +V _OUT ), where V _{NS1 is} the secondary winding of the main transformer TR1 Overvoltage on NS1. At this time, the diode D2 of the resonance energy supply circuit 20 and the diode D3 of the discharge circuit 30 are also turned off. Under this condition, through the voltage sensing and conduction diode D1 of the auxiliary transformer TR2, the snubber resistor Rx of the discharge circuit 30, the parasitic capacitance C _{OSS1 of the} power switch Q1 and the resonant inductance Lx will form a resistor-inductor-capacitance (RLC) Snubber circuit. When the resonant energy induced from the primary side winding NP2 of the auxiliary transformer TR2 to the secondary side winding NS2 is too large, the above RLC snubber circuit can provide overvoltage protection to avoid the damage of the rectifier switch Q2.

As shown in Figures 3 and 5 and Table 1, in the state S2, when the auxiliary transformer TR2 induces sufficient resonant voltage energy from the primary side winding NP2 to the secondary side winding NS2, the bias voltage V _GS2 will reach enough to turn on the rectifier switch Q2. At this time, the power supply 100 will switch from the state S2 of the half-start state to the state S3 of the half-start state, and the turned-on rectifier switch Q2 will establish an output end loop, so that the parasitic capacitance C _{OSS2 of the} rectifier switch Q2 and the output capacitance C _OUT begins to be charged, and the voltage V _DS2 across the rectifier switch Q2 is limited to a forward voltage V _F by the internal energy of the parasitic capacitor C _OSS2 . Under this condition, the voltage energy on the resonant inductance Lx will be greater than the voltage energy on the parasitic capacitance C _OSS1 of the power switch Q1 , so the diode D1 in the resonant voltage supply circuit 10 will be turned off, and the resonant energy supply circuit 20 will be turned off. The diode D2 in the circuit is turned on, so that the voltage energy on the resonant inductor Lx can be transferred to the storage capacitor C1. When the control signal GD3 provided by the energy storage capacitor C1 is sufficient to turn on the auxiliary switch Q3, _{the voltage energy on the parasitic capacitor C OSS1} of the power switch Q1 will be divided by the voltage divider resistors R1 and R2, and a voltage divider will be established on the voltage divider resistor R2. voltages V _R. The divided voltage V _R will be sensed by the auxiliary transformer TR2 from the primary side to the secondary side winding NP2, NS2, and further allows the rectifying switch Q2 is continuously maintained in the conducting state.

As shown in Figs. 3 and 6 and Table 1, when the bias voltage V _{GS1 is} switched to the enabling level again after going through the states S0-S3, the power supply 100 will enter the state S4, at which time the power switch Q1 will be activated again. On, the energy of the input voltage V _IN will be induced from the primary side winding NP1 of the main transformer TR1 to the secondary side winding NS1, while the primary side winding NP2 and the secondary side winding NS2 of the auxiliary transformer TR2 have no induced energy, so the rectifier switch Q2 will was cut off again. At this time, the voltage energy previously stored in the parasitic capacitance C _OSS2 of the rectifier switch Q2 in the state S3 will be rapidly discharged to the ground potential GND2 through the diode D3 and the snubber resistor Rx of the discharge circuit 30 , so the output voltage will not be affected stability of V _OUT.

In the present invention, the value of the magnetizing inductance Lm can be 500μH (error ±5%), _{the value of the parasitic capacitance C OSS1} of the power switch Q1 can be 100pF (error ± 20%), the value of the _{parasitic capacitance C OSS2 of the rectifier switch Q2 can be} 100pF (error ±20%), the value of voltage dividing resistor R1 can be 19KΩ (error ±1%), the value of voltage dividing resistor R2 can be 1KΩ (error ±1%), the value of _{energy storage capacitor C OUT can be 47μF} (error ±5%), the value of resonant inductance Lx can be 28μH (error ±5%), the value of snubber resistor Rx can be 26Ω (error ±5%), and the value of _{output capacitor C OUT can be 680μF (error ±5%)} ±10%). However, the implementation of the above elements does not limit the scope of the present invention.

In the embodiment of the present invention, the power switch Q1, the rectifier switch Q2 and the auxiliary switch Q3 may be metal-oxide-semiconductor field-effect transistors (MOSFETs), bipolar junction transistors ( bipolar junction transistor, BJT), or other components with similar functions. For N-type transistors, the enable potential is high and the disable potential is low; for P-type transistors, the enable potential is low and the disable potential is high. However, the types of the power switch Q1, the rectifier switch Q2 and the auxiliary switch Q3 do not limit the scope of the present invention.

The first transformer can convert the input voltage to an output voltage. The two resonant voltage supply circuits arranged on the primary side can provide the resonant voltage energy in the on/off state of the relevant power, and then are induced to the secondary side by the second transformer to control the rectifier switch synchronously. The discharge circuit is arranged on the secondary side to provide a discharge path when the rectifier switch is turned off.

To sum up, the power supply of the present invention can achieve the function of resonant energy recovery and self-sufficiency driving. The two resonant voltage supply circuits arranged on the primary side are used to provide the resonant voltage energy in the on/off state of the relevant power, and then the above-mentioned primary The side resonant voltage energy is transmitted to the secondary side to provide a switch control signal, thereby synchronously controlling the power switch on the primary side and the rectifier switch on the secondary side. Compared with the prior art, the rectifier switch is controlled by a driving integrated circuit arranged on the secondary side, the resonant voltage supply circuit of the present invention is not easily damaged due to high temperature or high frequency switching, and when the secondary side rectifier switch is turned off A discharge path can be provided. Therefore, the power supply of the present invention can take into account the power conversion efficiency, component safety and output voltage stability. The above descriptions are only preferred embodiments of the present invention, and all equivalent changes and modifications made according to the scope of the patent application of the present invention shall fall within the scope of the present invention.

10, 20: Resonant voltage supply circuit 15: Magnetic core 30: Discharge circuit C _OUT : Output capacitor TR1: Main transformer TR2: Auxiliary transformer NP1, NP2: Primary side winding and number of turns NS1, NS2: Secondary side winding and number of turns Lm: magnetizing inductance Lx: resonant inductance R1, R2: voltage dividing resistor Rx: snubber resistor D1~D3: diode Q1: power switch Q2: rectifier switch Q3: auxiliary switch C _OSS1 , C _OSS2 : parasitic capacitance C1: storage Capacitance V _IN : Input voltage V _OUT : Output voltage V _DS1 , V _DS2 : Cross voltage V _GS1 , V _GS2 : Bias voltage GND1 , GND2 : Ground potential GD1 , GD2 : Control signal

FIG. 1 is a functional block diagram of a power supply with resonant energy recovery self-sustaining drive design according to an embodiment of the present invention. FIG. 2 is a schematic diagram of an implementation manner of a power supply according to an embodiment of the present invention. FIG. 3 is a schematic diagram of related signals during operation of a power supply according to an embodiment of the present invention. FIG. 4 is a schematic diagram of an equivalent circuit of a power supply operating in a specific state according to an embodiment of the present invention. FIG. 5 is a schematic diagram of an equivalent circuit of a power supply operating in a specific state according to an embodiment of the present invention. FIG. 6 is a schematic diagram of an equivalent circuit of a power supply operating in a specific state according to an embodiment of the present invention.

10, 20: Resonant voltage supply circuit

15: Magnetic core

30: Discharge circuit

C _OUT : output capacitor

TR1: Main Transformer

TR2: Auxiliary Transformer

NP1, NP2: Primary side winding and number of turns

NS1, NS2: Secondary side winding and number of turns

Lm: magnetizing inductance

Lx: resonant inductance

R1, R2: divider resistors

Rx: Snubber resistor

D1~D3: Diode

Q1: Power switch

Q2: Rectifier switch

Q3: Auxiliary switch

C _OSS1 , C _OSS2 : parasitic capacitance

C1: Storage capacitor

V _IN : Input voltage

V _OUT : output voltage

V _DS1 , V _DS2 : Overvoltage

V _GS1 , V _GS2 : Bias voltage

GND1, GND2: ground potential

GD1, GD2: control signal

Claims (10)

A power supply with resonant energy recovery self-driving design, comprising: a first transformer comprising a first primary side winding and a first secondary side winding for converting an input voltage into an output voltage; a first switch comprising: a first end, coupled to the input voltage; a second end coupled to a first ground potential; and a control terminal for receiving a first control signal; a second transformer comprising a second primary side winding and a second secondary side winding for converting a first resonant voltage energy or a second resonant voltage energy into a second control signal; a second switch comprising: a first end coupled to the first secondary side winding; a second terminal selectively coupled to a second ground potential; and a control terminal for receiving the second control signal; a first resonant voltage supply circuit, coupled to the first switch and the second secondary side winding, for resonating with the parasitic capacitance of the first switch to provide the first resonant voltage energy; and A second resonant voltage supply circuit, coupled to the first switch and the first primary side winding, is used for dividing the voltage across the first switch to provide the second resonant voltage energy. The power supply of claim 1, wherein the first resonant voltage supply circuit comprises: a first diode comprising: an anode coupled to the first end of the first switch; and a cathode coupled to the second primary side winding of the second transformer; A resonant inductor is coupled between the cathode of the first diode and the first ground potential, and is connected in parallel with the second primary side winding of the second transformer. The power supply of claim 1, wherein the second resonant voltage supply circuit comprises: a first voltage dividing resistor and a second voltage dividing resistor; an energy storage capacitor; a third switch comprising: a first end coupled to the first end of the first switch through the first voltage dividing resistor; a second end coupled to the second primary side winding of the second transformer and coupled to the first ground potential through the second voltage dividing resistor; and a control terminal coupled to the first ground potential through the energy storage capacitor; and a second diode comprising: an anode coupled to the second primary side winding of the second transformer; and A cathode is coupled between the control terminal of the third switch and the energy storage capacitor. The power supply of claim 1, further comprising a magnetizing inductor connected in parallel with the first primary side winding of the first transformer. The power supply of claim 1, further comprising a discharge circuit comprising: A third diode comprising: an anode coupled to the second end of the second switch; and a cathode coupled to the second primary side winding of the second transformer; and A damping resistor is coupled between the cathode of the third diode and the second ground potential. The power supply of claim 1, wherein the first primary side winding, the first secondary side winding, the second primary side winding and the second secondary side winding are wound on the same magnetic core. The power supply of claim 1, further comprising an output capacitor connected in parallel between the first secondary side winding and the second ground potential. The power supply of claim 1, wherein: The first resonant voltage supply circuit is further used for providing the first resonant voltage energy within a first period of time; The second transformer is further used for converting the first resonant voltage energy into the second control signal during the first period; and During the first period, both the first switch and the second switch are turned off. The power supply of claim 8, wherein: The second resonant voltage supply circuit is further used for providing the second resonant voltage energy in a second time period following the first time period; The second transformer is further used for converting the first resonant voltage energy into the second control signal during the second period; and During the second period, the first switch is turned off and the second switch is turned on. The power supply of claim 8, further comprising a discharge circuit for discharging energy stored in the parasitic capacitance of the second switch to the second ground potential during a third time period following the second time period , wherein the first switch is turned on and the second switch is turned off during the third period.

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