Isolated switching converter and energy recycle circiut and method thereof

Abstract

An energy recovery circuit for an isolation circuit is disclosed. The isolation circuit includes a transformer, a primary-side switch, and a secondary-side synchronous rectifier switch. The energy recovery circuit includes: an energy recovery branch comprising a clamping capacitor, an auxiliary switch, and a current sensing resistor connected in series, wherein a first terminal of the clamping capacitor is coupled to a first terminal of the primary winding, a second terminal of the clamping capacitor is coupled to a first terminal of the auxiliary switch, a second terminal of the auxiliary switch is coupled to a first terminal of the current sensing resistor, a second terminal of the current sensing resistor is coupled to a second terminal of the primary winding, and a recovery control integrated circuit including a first terminal, a second terminal, and a third terminal, wherein the first terminal is coupled to a second terminal of the primary winding, the second terminal is coupled to a first terminal of the current sensing resistor to receive a current sensing signal, and the third terminal is coupled to a control terminal of the auxiliary switch. During the discharge of the clamping capacitor, the voltage between the control terminal and the second terminal of the auxiliary switch is reduced.

Description

Isolation circuit and energy recovery circuit and method thereof The present invention relates to an electronic circuit, and in particular to an energy recovery circuit for an isolation circuit and an energy recovery method thereof. Common clamping circuits in isolation circuits include RCD clamping and active clamping. Figure 1 shows a conventional isolation circuit 100 using RCD clamping. As shown in Figure 1, the RCD clamping is implemented by an RCD snubber 10, which includes a clamping capacitor Csn, a snubber resistor Rsn, and a diode Dsn. When the primary control signal GP turns off the primary switch QP coupled to the primary winding Np, the leakage inductance of the transformer T transfers its leakage inductance energy to charge the clamping capacitor Csn through the diode Dsn. After the charging process of the clamping capacitor Csn is completed, the energy stored in the clamping capacitor Csn is consumed by the snubber resistor Rsn. Although the voltage spike of the isolation circuit 100 can be suppressed as a result, the efficiency of the isolation circuit 100 is reduced because the leakage inductance energy is consumed rather than recovered. Active clamping overcomes the shortcomings of RCD clamping. Figure 2 shows a conventional isolation circuit 200 using active clamping. When the primary switch QP is on, the auxiliary switch QH remains off. Current flows through the primary winding Np and primary switch QP to the primary reference ground, storing energy in the transformer T and leakage inductor Lk. When the primary switch QP is off, the leakage inductor Lk of the transformer T charges the clamping capacitor Csn through the body diode of the auxiliary switch QH, transferring the leakage inductance energy and storing it in the clamping capacitor Csn. The auxiliary switch QH is then turned on, and the energy stored in the clamping capacitor Csn is not dissipated but recovered through the auxiliary switch QH and transformer T. The addition of the auxiliary switch QH provides a bidirectional current path, which not only suppresses the voltage spike on the primary switch QP but also helps recover the energy dissipated by the RCD clamp in Figure 1 and fully releases this recovered energy to the secondary output, thereby improving the efficiency of the isolation circuit 200. However, in some applications, when the current oscillation after the auxiliary switch tube QH is turned off is transmitted to the secondary side, it will cause the secondary side synchronous rectifier tube to turn off prematurely, thereby reducing the reliability and efficiency of the isolation circuit. In order to solve one or more of the aforementioned problems, the present invention provides an isolation circuit, an energy recovery circuit, and an energy recovery method to prevent voltage spikes on the primary side switch while recovering leakage inductance energy and further preventing the secondary side synchronous rectification switch from shutting down prematurely. According to one embodiment of the present invention, an energy recovery circuit for an isolation circuit is provided. The isolation circuit includes a transformer having a primary winding and a secondary winding, a primary switch coupled to the second end of the primary winding, and a secondary synchronous rectification switch coupled to the secondary winding. The energy recovery circuit includes an energy recovery branch and an integrated circuit. The energy recovery branch is coupled between the first and second ends of the primary winding and includes a clamping capacitor, an auxiliary switch, and a current sensing resistor coupled in series. The first end of the clamping capacitor is coupled to the first end of the primary winding, the second end of the clamping capacitor is coupled to the first end of the auxiliary switch, the second end of the auxiliary switch is coupled to the first end of the current sensing resistor, and the second end of the current sensing resistor is coupled to the second end of the primary winding. The integrated circuit has a first terminal, a second terminal, and a third terminal, wherein the first terminal is coupled to the second end of the primary winding, the second terminal is coupled to the first end of the current sensing resistor to receive a current sensing signal representing the current flowing through the energy recovery branch, and the third terminal is coupled to the control end of the auxiliary switch to control the on and off of the auxiliary switch, wherein the voltage between the control end and the second end of the auxiliary switch is reduced during the discharge of the clamping capacitor. According to another embodiment of the present invention, an isolation circuit is provided, including a transformer having a primary winding and a secondary winding, a primary switch coupled to the primary winding, a secondary synchronous rectification switch coupled to the secondary winding, and the energy recovery circuit as described above. According to another embodiment of the present invention, a method for recovering energy in an isolation circuit is provided. The isolation circuit includes a transformer having a primary winding and a secondary winding, a primary switch coupled to the primary winding, and a secondary synchronous rectification switch coupled to the secondary winding. The method includes: coupling an energy recovery branch in parallel with the primary winding, wherein the energy recovery branch includes a clamping capacitor, an auxiliary switch, and a current detection resistor coupled in series; coupling to a connection point between the primary switch and the primary winding through a first terminal of an integrated circuit; receiving a current detection signal representing a current flowing through the energy recovery branch through a second terminal of the integrated circuit; coupling to a control terminal of the auxiliary switch through a third terminal of the integrated circuit to control the on and off of the auxiliary switch; and reducing the gate-source voltage of the auxiliary switch during the discharge of the clamping capacitor. According to embodiments of the present invention, the energy recovery circuit proposed herein can increase the efficiency of an isolation circuit. The integrated circuit of the present invention can be optionally combined with any primary-side controller to form an isolation circuit. While maintaining high efficiency, it can prevent premature shutdown of the secondary-side synchronous rectification switch, thereby improving the reliability of the isolation circuit. The following describes various embodiments of the present invention. In the following description, certain specific details, such as exemplary circuits and exemplary values ​​for these circuit components, are provided to provide a detailed understanding of the embodiments. However, those skilled in the art will appreciate that one or more of these specific details are not necessarily required to practice the present invention, and that other methods, components, materials, etc. may be used to practice the present invention. In other instances, well-known structures, materials, processes, or operations are not described or shown in detail to avoid obscuring the present invention. Throughout the specification and claims, terms such as "left", "right", "inside", "outside", "front", "back", "up", "on", "above", "under", "lower", "below", "under" and similar terms are used for descriptive purposes only and are not used to describe permanently fixed relative relationships. It should be understood that these terms are interchangeable under appropriate circumstances, so that the embodiments of the technology described herein can, for example, operate in other orientations than shown or otherwise described herein. Terms such as "coupled" and "connected" described herein are defined as connecting directly or indirectly, whether electrically or non-electrically. Terms such as "a", "the" and "said" include the plural. The phrase "in one embodiment" used herein does not necessarily refer to the same embodiment, but may also be the same embodiment. Those skilled in the art should understand that the meaning of the above terms does not limit these terms, but is merely used to provide illustrative examples for these terms. The present invention can be applied to any isolation circuit. In the following detailed description, for the sake of brevity, only a flyback circuit is used as an example to explain the specific working principle of the present invention. Figure 3 shows a schematic diagram of an isolation circuit 300 according to an embodiment of the present invention. The isolation circuit 300 includes a transformer T, a primary switch QP, a primary control circuit 10, an integrated circuit 20, an energy recovery branch 30, and an output circuit 40. The transformer T has a primary winding Np and a secondary winding Ns. The transformer T is coupled to and receives an input voltage Vin and generates an output voltage Vout for the load via the secondary synchronous rectification switch SR and output capacitor Cout of the output circuit 40. The primary switch QP is coupled between the second end of the primary winding Np and the primary reference ground and is controlled by a primary drive signal GP provided by the primary control circuit 10. The energy recovery branch 30 includes a clamping capacitor Csn, an auxiliary switch QH, and a current sense resistor Rcs connected in series. In the embodiment of Figure 3 , the energy recovery branch 30 is coupled in parallel with the primary winding Np. Specifically, the first terminal of the clamping capacitor Csn is coupled to the first terminal of the primary winding Np, and the second terminal of the clamping capacitor Csn is coupled to the first terminal of the auxiliary switch QH. The second terminal of the auxiliary switch QH is coupled to the first terminal of the current sensing resistor Rcs, and the second terminal of the current sensing resistor Rcs is coupled to the second terminal of the primary winding Np. The integrated circuit 20 has multiple terminals, including a power supply terminal VCC, a voltage regulator terminal VDD, a ground terminal VSS, a control terminal VG, a current sense terminal CS, and a set terminal SET. The power supply terminal VCC receives the power supply voltage VPR from the primary control circuit 10 via a diode D0 to power the integrated circuit 20. The power supply voltage VPR is determined to be on or off based on the logic state of the power supply voltage VPR relative to the ground terminal VSS. The power supply voltage VPR received by the power supply terminal VCC is converted into a regulated voltage signal and output at the voltage regulator terminal VDD. The voltage regulator terminal VDD is coupled to the ground terminal VSS via a capacitor C0. The ground terminal VSS is coupled to the primary switch QP and the second terminal of the primary winding Np, i.e., the connection point between the primary winding Np and the primary switch Qp. The current sense terminal CS is coupled to the first terminal of the current sense resistor Rcs to receive a current sense signal VCS representing the current flowing through the energy recovery branch 30. The control terminal VG is coupled to the control terminal of the auxiliary switch QH to control the on and off of the auxiliary switch QH. The SET terminal is coupled to an external resistor Rset to set a maximum on-time threshold of the auxiliary switch QH. As shown in Figure 3, when the primary switch QP is on, the auxiliary switch QH remains off. Current flows through the primary winding Np and primary switch QP, storing energy in the transformer T and leakage inductor Lk. When the primary switch QP is off, the leakage inductor Lk of the transformer T transfers energy to the clamping capacitor Csn and the secondary output. Current flows from the second end of the primary winding Np through the current sensing circuit Rcs and the body diode of the auxiliary switch QH, charging the clamping capacitor Csn. Once current flows through the body diode of the auxiliary switch QH, the auxiliary switch QH is quickly turned on. The energy stored in the clamping capacitor Csn is then released to the secondary output through the transformer T. The clamping capacitor Csn discharges, and current flows from the auxiliary switch QH through the current sense resistor Rcs and the leakage inductor Lk. During the discharge of the clamping capacitor Csn, the voltage between the control terminal and the second terminal of the auxiliary switch QH decreases, preventing current oscillations from causing premature shutdown of the secondary synchronous rectification switch SR. Specifically, the operating principles of the embodiments of the present invention are explained with reference to Figures 4 to 6. 4 shows a block diagram of an integrated circuit 20A for use in an isolation circuit 300 according to an embodiment of the present invention. As shown in FIG4 , the integrated circuit 20A includes a turn-on control circuit 201 , a turn-off control circuit 202 , and a gate driver 203 . In the embodiment shown in FIG4 , the conduction control circuit 201 is configured to compare the current detection signal VCS with the conduction threshold voltage VCS_ON and generate a conduction control signal DRV_ON based on the comparison result. As shown in FIG4 , the conduction control circuit 201 includes a comparator CMP1. The comparator CMP1 has an inverting input, a non-inverting input, and an output. The inverting input of the comparator CMP1 is coupled to the current detection terminal CS to receive the current detection signal VCS representing the current flowing through the energy recovery branch 30. The non-inverting input of the comparator CMP1 is coupled to the conduction threshold voltage VCS_ON, and the conduction control signal DRV_ON is provided at the output. In one embodiment, the conduction threshold voltage VCS_ON is -20 mV. In response to the current detection signal VCS decreasing to less than the conduction threshold voltage VCS_ON, the conduction control signal DRV_ON switches from a low voltage level to an active high voltage level, turning on the auxiliary switch QH. In the embodiment shown in FIG4 , the shutdown control circuit 202 is configured to compare the current detection signal VCS with the shutdown threshold voltage VCS_ZCD and generate a shutdown control signal DRV_OFF based on the comparison result. As shown in FIG4 , the shutdown control circuit 202 includes a comparator CMP2 and a falling-edge one-shot circuit 221. The comparator CMP2 has an inverting input, a non-inverting input, and an output. The non-inverting input of the comparator CMP2 is coupled to the current detection terminal CS to receive the current detection signal VCS, and the inverting input is coupled to the shutdown threshold voltage VCS_ZCD. The falling-edge one-shot circuit 221 has an input and an output. The input is coupled to the output of the comparator CMP2, and the output generates a falling-edge-triggered pulse signal as the shutdown control signal DRV_OFF and provides it to the output. In one embodiment, the shutdown threshold voltage VCS_ZCD is a value close to zero but greater than zero. In response to the current detection signal VCS decreasing to be less than the turn-off threshold voltage VCS_OFF, the turn-off control signal DRV_OFF is switched from a high level to a low level, and the auxiliary switch QH is turned off. The gate driver 203 has a first input terminal, a second input terminal, and an output terminal. The first input terminal is coupled to the on-control circuit 201 to receive the on-control signal DRV_ON, and the second input terminal is coupled to the off-control circuit 202 to receive the off-control signal DRV_OFF. Based on the on-control signal DRV_ON and the off-control signal DRV_OFF, the gate driver 203 provides a drive control signal GH at the output terminal to the control terminal VG of the integrated circuit 20A. In one embodiment, the gate driver 203 has a first driving phase and a second driving phase. In the first driving phase, the gate driver 203 controls the gate voltage VG_QH of the auxiliary switch QH to maintain at a first voltage. When the gate driver 203 switches from the first driving phase to the second driving phase, the gate voltage VG_QH of the auxiliary switch QH is pulled down to reduce the voltage between the control terminal and the second terminal of the auxiliary switch QH (e.g., the gate-source voltage VGS_QH of the auxiliary switch QH). In the embodiment shown in FIG4 , the integrated circuit 20A further includes a first comparison circuit 204. The first comparison circuit 204 is configured to determine whether to enable the gate driver 203 to switch to the second driving stage. As shown in FIG4 , the first comparison circuit 204 includes a comparator CMP3. The comparator CMP3 has an inverting input terminal, a non-inverting input terminal, and an output terminal. The non-inverting input terminal of the comparator CMP3 is coupled to the current detection terminal CS to receive the current detection signal VCS, the inverting input terminal is coupled to the reference threshold voltage Vref, and the first comparison signal COP1 is provided at the output terminal. In response to the current detection signal VCS increasing to the reference threshold voltage Vref, the first comparison signal COP1 is switched from a low voltage level to a high voltage level, and the second driving stage of the gate driver 203 is enabled, and the gate driver 203 enters the second driving stage. The gate driver 203 further includes a third input terminal coupled to the output terminal of the first comparison circuit 204 to receive the first comparison signal COP1. In the embodiment shown in FIG4 , the gate driver 203 includes a first charging path 21 , a second discharging path 22 , and a third discharging path 23 . Specifically, in response to the active level of the turn-on control signal DRV_ON, i.e., in response to the current detection signal VCS decreasing below the turn-on threshold voltage VCS_ON, the first charging path 21 is turned on to provide a charging path from a supply voltage VS to the control terminal VG of the integrated circuit 20A. The gate driver 203 enters a first driving phase, and the gate voltage VG_QH of the auxiliary switch QH increases and is maintained at a first voltage. In response to the active level of the first comparison signal COP1, i.e., in response to the current detection signal VCS increasing to the reference threshold voltage Vref, the first discharge path 22 is turned on, providing a discharge path from the control terminal VG to the ground terminal VSS. The gate driver 203 switches from the first driving phase to the second driving phase, and the voltage VG_QH at the control terminal of the auxiliary switch QH gradually decreases. In response to the active potential of the shutdown control signal DRV_OFF, that is, in response to the current detection signal VCS decreasing to less than the shutdown threshold voltage VCS_ZCD, the second discharge path 23 is turned on to provide a fast discharge path from the control terminal VG to the ground terminal VSS, and the auxiliary switch QH is turned off. In the embodiment shown in FIG4 , the first charging path 21 includes a charging current source IS1 and a first charging transistor S1 connected in series between the supply voltage VS and the control terminal VG. In other embodiments, the first charging path 21 may include multiple series branches of charging current sources and charging transistors to provide charging paths with varying drive capabilities. The first discharging path 22 includes a discharging current source IS2 and a first discharging transistor S2 connected in series between the control terminal VG and the ground terminal VSS. The second discharging path 23 includes a second discharging transistor S3 connected in series between the control terminal VG and the ground terminal VSS. In the embodiment shown in FIG4 , the second discharging path 23 has a greater discharge rate than the first discharging path 22. In the embodiment shown in FIG4 , the gate driver 203 controls the on / off switching of the first charging transistor S1 based on the on-control signal DRV_ON, controls the on / off switching of the first discharging transistor S2 based on the first comparison signal COP1, and controls the second discharging transistor S3 based on the off-control signal DRV_OFF. For example, turning on the first charging transistor S1 also turns on the auxiliary switch QH. When the first discharge transistor S2 is turned on, the control terminal of the auxiliary switch QH is electrically connected to the ground terminal VSS via the discharge current source IS2, causing the gate voltage of the auxiliary switch QH to drop. When the second discharge transistor S3 is turned on, the gate of the auxiliary switch QH is directly electrically connected to the ground terminal VSS, rapidly reducing the gate voltage of the auxiliary switch QH to zero potential of the integrated circuit 20A. FIG5 illustrates the operating waveforms of the conventional isolation circuit 200 shown in FIG2 . As shown in FIG5 , from top to bottom, the waveforms are: the voltage Vsw at the connection point between the primary winding Np and the primary switch Qp; the current sense signal VCS1 representing the current flowing through the clamping capacitor Csn; the current iLm flowing through the magnetizing inductor Lm; the current iSR1 flowing through the secondary synchronous rectifier switch SR; the gate voltage VG_QH of the auxiliary switch QH; and the gate voltage VG_SR of the secondary synchronous rectifier switch SR. It should be noted that in the schematic diagram shown in FIG5 , the waveform of the voltage Vsw at the connection point is drawn with the primary reference ground of the isolation circuit 200 as the reference ground potential. The current detection signal VCS1, the current iLm flowing through the excitation inductor Lm, and the gate voltage VG_QH of the auxiliary switch QH are drawn with the connection point between the primary winding Np and the primary switch Qp as the reference ground potential. The current iSR1 flowing through the secondary synchronous rectifier switch SR and the gate voltage VG_SR of the secondary synchronous rectifier switch SR are drawn with the secondary reference ground of the isolation circuit 200 as the reference ground potential. The current iSR1 flowing through the secondary synchronous rectifier switch SR comes from two sources: the magnetizing inductance Lm of the transformer T and the clamping capacitor Csn. As shown in Figure 5, after the primary switch QP turns off, the secondary synchronous rectifier switch SR turns on, and the current iLm flowing through the magnetizing inductance Lm begins to decrease linearly. Furthermore, after the primary switch QP turns off, current flows from the second end of the primary winding Np through the body diode of the auxiliary switch QH and the clamping capacitor Csn. Subsequently, the auxiliary switch QH turns on, as indicated by point 101 in Figure 5. When the current sense signal VCS1 decreases to the shutdown threshold voltage VCS_ZCD, the auxiliary switch QH turns off, as indicated by point 102 in Figure 5. The current flowing through the clamping capacitor Csn (current sense signal VCS1) first increases and then decreases. Subsequently, the clamping capacitor Csn continues to charge and discharge, causing the current sense signal VCS1 to oscillate. Consequently, the energy transferred to the secondary side, and the current iSR1, also oscillates. The dashed line in the VG_SR waveform in Figure 5 represents the gate voltage of the secondary synchronous rectifier switch SR under normal conditions. Because the secondary current iSR1 oscillates downward, causing it to fall below a certain threshold voltage prematurely, the system determines that the secondary current iSR1 crosses zero prematurely, and the secondary synchronous rectifier switch SR is prematurely turned off, as indicated by point 103 in Figure 5 . To prevent premature shutdown of the secondary synchronous rectifier switch SR, the inventors have proposed various embodiments of the present invention. FIG6 illustrates operating waveforms of the isolation circuit 300 shown in FIG4 according to an embodiment of the present invention. As shown in FIG6 , from top to bottom, the waveforms are: the voltage Vsw at the connection point between the primary winding Np and the primary switch Qp; the gate voltage VG_QH of the auxiliary switch QH; the current sense signal VCS representing the current flowing through the energy recovery branch 30; the current iLm flowing through the magnetizing inductor Lm; the current iSR flowing through the secondary synchronous rectifier switch SR; and the gate voltage VG_SR of the secondary synchronous rectifier switch SR. It should be noted that in the schematic diagram shown in FIG6 , the waveform of the voltage Vsw at the connection point is drawn with the primary reference ground of the isolation circuit 300 as the reference ground potential. The current detection signal VCS, the current iLm flowing through the excitation inductor Lm, and the gate voltage VG_QH of the auxiliary switch QH are drawn with the ground terminal VSS as the reference ground potential. The current iSR1 flowing through the secondary synchronous rectifier switch SR and the gate voltage VG_SR of the secondary synchronous rectifier switch SR are drawn with the secondary reference ground of the isolation circuit 300 as the reference ground potential. In the embodiment shown in FIG6 , after the primary switch QP is turned off, the auxiliary switch QH is turned on when the current detection signal VCS decreases to the turn-on threshold voltage VCS_ON, as indicated by point 101 in FIG6 . The gate driver 203 enters the first driving phase, with the gate voltage VG_QH of the auxiliary switch QH maintained at a first voltage. When the current detection signal VCS increases to the reference threshold voltage Vref, the gate driver 203 switches from the first driving phase to the second driving phase, as indicated by point 104 in FIG6 . The gate voltage VG_QH of the auxiliary switch QH decreases, thereby also decreasing the gate-source voltage VGS_QH of the auxiliary switch QH. Current oscillations flowing through the energy recovery branch 30 are suppressed. When the current detection signal VCS decreases to the turn-off threshold voltage VCS_ZCD, the auxiliary switch QH is turned off. Thereafter, the magnetizing current iLm continues to decrease linearly. Since the current oscillation flowing through the energy recovery branch 30 is suppressed, the secondary current iSR no longer oscillates. The secondary synchronous rectification switch SR is not turned off until the secondary current iSR actually crosses zero, as shown by point 105 in FIG6 . FIG7 shows a block diagram of an integrated circuit 20B for use in an isolation circuit 400 according to yet another embodiment of the present invention. As shown in FIG7 , a gate driver 203A includes a first input terminal for receiving a turn-on control signal DRV_ON, a second input terminal for receiving a turn-off control signal DRV_OFF, a third input terminal for receiving a first comparison signal COP1, an output terminal for providing a drive control signal GH, a first charging path 21A from a supply voltage VS to a control terminal VG, a first discharging path 22A from the control terminal VG to a ground terminal VSS, and a second discharging path 23A from the control terminal VG to a ground terminal VSS. In the embodiment shown in FIG7 , the first charging path 21A includes a gate resistor RG1 and a first charging transistor S1 connected in series. The first discharging path 22A includes a gate resistor RG2 and a first discharging transistor S2 connected in series. The second discharging path 23A includes a gate resistor RG3 and a second discharging transistor S3 connected in series. Gate resistors RG1-RG3 serve as gate drive condition setting units, adjusting the turn-on and turn-off speeds of the auxiliary switch QH. For example, if the gate resistance of RG3 is smaller than that of RG2, the turn-off speed of the auxiliary switch QH in the second discharge path 23A will be greater than that in the first discharge path 22A. In the embodiment shown in FIG7 , the second discharge path 23A further includes an acceleration capacitor C1 connected in parallel with the gate resistor RG3 to accelerate the turn-off of the auxiliary switch QH. In response to the active voltage of the on-control signal DRV_ON, the first charging transistor S1 is turned on, the first charging path 21A is turned on, and the auxiliary switch QH is turned on. In response to the active voltage of the first comparison signal COP1, the first discharging transistor S2 is turned on, the first discharging path 22A is turned on, and the gate voltage of the auxiliary switch QH is pulled low. In response to the active voltage of the off-control signal DRV_OFF, the second discharging transistor S3 is turned on, the second discharging path 23A is turned on, and the auxiliary switch QH is quickly turned off. FIG8 shows a block diagram of an integrated circuit 20C for use in an isolation circuit 400 according to yet another embodiment of the present invention. Compared to the integrated circuit 20A shown in FIG4 , the integrated circuit 20C shown in FIG8 further includes a maximum on-time control circuit 205 . Compared to the gate driver 203 shown in FIG4 , the gate driver 203B shown in FIG8 further includes an OR circuit. In the embodiment shown in FIG8 , the maximum on-time control circuit 205 has a first input, a second input, and an output. The first input is coupled to the control terminal VG or the conduction control circuit 201 to receive the drive control signal GH or the conduction control signal DRV_ON. The second input is coupled to the set terminal SET, and the output provides a maximum on-time control signal OFF1. The OR circuit OR has a first input, a second input, and an output. The first input receives the off-control signal DRV_OFF, the second input receives the maximum on-time control signal OFF1, and the output is coupled to the control terminal of the second discharge transistor S3. The maximum on-time control circuit 205 begins timing after the auxiliary switch QH is turned on. After a maximum on-time threshold has passed, the maximum on-time control circuit 205 generates the maximum on-time control signal OFF1 at its output. The OR circuit OR controls the second discharge path to conduct, rapidly turning off the auxiliary switch QH to ensure that the maximum on-time of the auxiliary switch QH does not exceed the maximum on-time threshold. The maximum on-time threshold is set by coupling the resistor Rset to the setting terminal SET. Figure 9 shows a schematic circuit diagram of an isolation circuit 400 according to another embodiment of the present invention. Compared to the energy recovery branch 30 shown in Figure 4 , the energy recovery branch 30A shown in Figure 9 further includes a resistor R1 coupled between the second terminal of the auxiliary switch QH and the first terminal of the current sensing resistor Rcs. One embodiment of the present invention is now described using Figure 10 . Figure 10 illustrates the operating principle of an isolation circuit 400 according to an embodiment of the present invention. As shown in Figure (a), when the primary switch QP is turned off, current ics flows from the second end of the primary winding Np through the current sensing circuit Rcs, resistor R1, and the body diode of the auxiliary switch QH to charge the clamping capacitor Csn. Once current ics flows through the body diode of the auxiliary switch QH, it quickly turns on the auxiliary switch QH. At this point, the gate-source voltage of the auxiliary switch QH is expressed using Equation (1): (1) Subsequently, as shown in Figure (b), during the discharge of the clamp capacitor Csn, the current ics reverses direction and flows from the auxiliary switch QH through the resistor R1, the current sense resistor Rcs, and the leakage inductor Lk. At this time, the gate-source voltage of the auxiliary switch QH is expressed by formula (2): (2) Equations (1) and (2) show that during the discharge period of the clamping capacitor Csn, the gate-source voltage of the auxiliary switch QH is reduced. The oscillation of the voltage Vsw at the connection point between the primary winding Np and the primary switch QP is also reduced, thereby preventing premature shutdown of the secondary synchronous rectifier switch SR due to current oscillation. Specifically, the effects of the embodiment of the present invention are illustrated with reference to FIG11. Figure 11 shows three operating waveforms of the isolation circuit 400 according to an embodiment of the present invention. As shown in Figure 11, from left to right, the operating conditions of the isolation circuit 400 are shown when R1 = 0Ω, R1 = 2Ω, and R1 = 4Ω. When R0 = 0Ω, the voltage Vsw at the connection point oscillates the most, and the secondary synchronous rectifier switch SR is turned off prematurely. As the resistance value of resistor R1 increases, for example, to R1 = 4Ω, the voltage oscillation of the voltage Vsw at the connection point is significantly suppressed, ensuring that the secondary synchronous rectifier switch SR turns off normally. FIG12 shows a flow chart of an energy recovery method 600 for an isolation circuit according to an embodiment of the present invention. The isolation circuit includes a transformer having a primary winding and a secondary winding, a primary switch coupled to the primary winding, and a secondary synchronous rectification switch coupled to the secondary winding. The method 600 includes steps 601-605. In step 601, an energy recovery branch is coupled in parallel with the primary winding, wherein the energy recovery branch includes a clamping capacitor, an auxiliary switch, and a current detection resistor coupled in series in sequence. In one embodiment, the energy recovery branch further includes a first resistor coupled between the auxiliary switch and the current detection resistor. In step 602 , a first terminal of an integrated circuit is coupled to a connection point between a primary switch and a primary winding. In step 603 , a current detection signal representing the current flowing through the energy recovery branch is received through the second terminal of the integrated circuit. In step 604 , the third terminal of the integrated circuit is coupled to the control terminal of the auxiliary switch to control the on / off state of the auxiliary switch. In step 605, the gate-source voltage of the auxiliary switch is reduced during the discharge of the clamp capacitor. In one embodiment, during the first driving phase, the gate voltage of the auxiliary switch is controlled to maintain a first voltage, and when switching to the second driving phase, the gate voltage of the auxiliary switch is pulled down. The switching from the first driving phase to the second driving phase occurs when the current detection signal increases to a value greater than a reference threshold voltage. In this document, relative terms such as first, second, and similar terms are used only to distinguish between multiple entities or actions and do not limit or imply any actual relationship or order between such entities or actions. Ordinal numbers such as "first," "second," and "third" simply indicate different ones of the numbers and do not limit or imply any order unless otherwise defined in the claims. Unless otherwise defined in the claims, the order of the text in the claims does not imply that the process steps must be executed in a chronological or logical order. The order of the steps may be interchanged or adjusted without departing from the scope of the present invention, without conflicting with the description of the claims, and without being logically reasonable. Those skilled in the art will appreciate that the present invention is not limited to the specific illustrations and descriptions herein. Rather, the scope of the present invention is defined by the claims and encompasses all combinations and subcombinations of the various features described above, as well as variations and modifications of such features that would be apparent to those skilled in the art upon review of the foregoing description and that are not prior art. 100: Isolation circuit 10: RCD buffer / primary side control circuit Csn: Clamping capacitor Rsn: Buffer resistor Dsn: Diode GP: Primary side control signal / primary side drive signal Np: Primary winding QP: Primary side switch T: Transformer 200: Isolation circuit QH: Auxiliary switch / auxiliary switch tube Lk: Leakage inductance 300: Isolation circuit 20: Integrated circuit 30: Energy recovery branch 40: Output circuit Ns: Secondary winding Vin: Input voltage Vout: Output voltage SR: Secondary side synchronous rectification switch Cout: Output capacitor Rcs: Current detection resistor VCC: Power supply terminal VDD: Voltage regulator terminal VSS: Ground terminal VG: Control terminal CS: Current detection terminal SET: Setting terminal. D0: diode VPR: power supply voltage C0: capacitor VCS: current detection signal Rset: external resistor / resistor 20A: integrated circuit 201: on-control circuit 202: off-control circuit 203: gate driver VCS_ON: on-threshold voltage DRV_ON: on-control signals CMP1, CMP2, CMP3: comparator VCS_ZCD: off-threshold voltage DRV_OFF: off-control signal 221: falling edge one-shot circuit GH: drive control signal VG_QH: gate voltage Vref: reference threshold voltage COP1: first comparison signal 204: first comparison circuit 21: first charging path 22: second discharging path 23: third discharging path VS: power supply voltage IS2: discharge current source S1: first charging transistor S2: first discharging transistor Body S3: Second Discharge Transistor Vsw: Voltage VCS1: Current Detection Signal Lm: Magnetizing Inductor iLm: Current iSR1: Current VG_SR: Gate Voltages 101, 102, 103, 104, 105: Point VGS_QH: Gate-Source Voltage 20B: Integrated Circuit 203A: Gate Driver 21A: First Charging Path 22A: First Discharging Path 23A: Second Discharging Path RG1, RG2, RG3: Gate Resistor C1: Acceleration Capacitor 400: Isolation Circuit 20C: Integrated Circuit 205: Maximum On-Time Control Circuit 203B: Gate Driver OR: OR Circuit SET: Setting Terminal OFF1: Maximum On-Time Control Signal 30A: Energy Recovery Branch R1: Resistor ics: Current 600: Method 601, 602, 603, 604, 605: Steps The present invention will be further understood with reference to the following detailed description and accompanying drawings, wherein like elements have like reference numerals. The accompanying drawings are for illustration purposes only and may only show a portion of the device and are not necessarily drawn to scale. [Figure 1] shows a conventional isolation circuit 100 using RCD clamping; [Figure 2] shows a conventional isolation circuit 200 using active clamping; [Figure 3] shows a schematic diagram of an isolation circuit 300 according to an embodiment of the present invention; [Figure 4] shows a circuit block diagram of an integrated circuit 20A for an isolation circuit 400 according to an embodiment of the present invention; [Figure 5] shows an operating waveform diagram of the isolation circuit 200 shown in Figure 2; [Figure 6] shows an operating waveform diagram of the isolation circuit 300 shown in Figure 4 according to an embodiment of the present invention; [Figure 7] shows a circuit block diagram of an integrated circuit 20B for the isolation circuit 300 according to another embodiment of the present invention; [Figure 8] shows a circuit block diagram of an integrated circuit 20C for the isolation circuit 300 according to another embodiment of the present invention; [Figure 9] shows a circuit schematic diagram of an isolation circuit 400 according to another embodiment of the present invention; [Figure 10] shows a schematic diagram of the operating principle of the isolation circuit 400 according to an embodiment of the present invention; [Figure 11] shows three operating waveform diagrams of the isolation circuit 400 according to an embodiment of the present invention; FIG12 shows a flow chart of an energy recovery method 600 for an isolation circuit according to an embodiment of the present invention. 10: RCD snubber / primary side control circuit 20: Integrated Circuits 30: Energy recovery branch 40: Output circuit 300: Isolation circuit C0: capacitor C1: Speed-up capacitor Cout: output capacitance CS: Current detection terminal Csn: Clamping capacitor D0: diode GH: drive control signal GP: Primary side control signal / primary side drive signal Lk: leakage inductance Lm: magnetizing inductance Np: Primary winding Ns: Secondary winding QH: Auxiliary switch / auxiliary switch tube QP: Primary switch Rcs: Current detection resistor Rset: External resistor / resistor SET: Setting terminal SR: Secondary side synchronous rectification switch T: Transformer VCC: power supply terminal VDD: voltage regulator terminal VG: Control terminal Vin: input voltage Vout: output voltage VPR: power supply voltage VSS: Ground terminal Vsw: voltage

Claims

1. An energy recovery circuit for an isolation circuit, the isolation circuit comprising a transformer having a primary winding and a secondary winding, a primary-side switch coupled to a second end of the primary winding, and a secondary-side synchronous rectifier switch coupled to the secondary winding, the energy recovery circuit comprising: The energy recovery branch, coupled between the first and second ends of the primary winding, includes a clamping capacitor, an auxiliary switch, and a current sensing resistor connected in series. The first end of the clamping capacitor is coupled to the first end of the primary winding, the second end of the clamping capacitor is coupled to the first end of the auxiliary switch, the second end of the auxiliary switch is coupled to the first end of the current sensing resistor, and the second end of the current sensing resistor is coupled to the second end of the primary winding. And an integrated circuit having a first terminal, a second terminal and a third terminal, wherein the first terminal is coupled to the second end of the primary winding, the second terminal is coupled to the first end of the current sensing resistor to receive a current sensing signal representing the current flowing through the energy recovery branch, and the third terminal is coupled to the control terminal of the auxiliary switch to control the on and off of the auxiliary switch, wherein the voltage between the control terminal of the auxiliary switch and the second terminal of the auxiliary switch is reduced during the discharge of the clamping capacitor.

2. The energy recovery circuit as described in claim 1, wherein the integrated circuit further comprises: The turn-on control circuit compares the current detection signal with the turn-on threshold voltage and generates a turn-on control signal based on the comparison result. The circuit includes a shutdown control circuit that compares the current detection signal with the shutdown threshold voltage and generates a shutdown control signal based on the comparison result; and a gate driver that receives the turn-on control signal and the shutdown control signal to provide a drive control signal to the third terminal of the integrated circuit. In the first drive phase, the gate driver controls the gate voltage of the auxiliary switch to remain at the first voltage. When the gate driver switches from the first drive phase to the second drive phase, the gate voltage of the auxiliary switch is pulled down.

3. The energy recovery circuit as claimed in claim 2, wherein the integrated circuit further includes a first comparison circuit that compares a current detection signal with a reference threshold voltage and generates a first comparison signal based on the comparison result to determine whether to switch the gate driver into a second driving stage.

4. The energy recovery circuit as claimed in claim 3, wherein the gate driver comprises: The first charging path from the supply voltage to the third terminal of the integrated circuit, wherein the first charging path is turned on in response to the effective potential of the turn-on control signal; A first discharge path from the third terminal of the integrated circuit to the first terminal of the integrated circuit, wherein the first discharge path is turned on in response to an effective potential of the first comparison signal; and a second discharge path from the third terminal of the integrated circuit to the first terminal of the integrated circuit, wherein the second discharge path is turned on in response to an effective potential of the turn-off control signal.

5. The energy recovery circuit as claimed in claim 4, wherein the first discharge path includes a current source and a first discharge transistor connected in series between the third terminal and the first terminal of the integrated circuit, and the second discharge path includes a second discharge transistor coupled between the third terminal and the first terminal of the integrated circuit.

6. The energy recovery circuit as claimed in claim 4, wherein the first discharge path includes a second gate resistor and a first discharge transistor connected in series between the third terminal and the first terminal of the integrated circuit, and the second discharge path includes a first circuit branch and a second discharge transistor connected in series between the third terminal and the first terminal of the integrated circuit, wherein the first circuit branch includes a third gate resistor and a capacitor coupled in parallel.

7. The energy recovery circuit as claimed in claim 2, wherein the integrated circuit further comprises: The maximum conduction time control circuit generates a maximum conduction time control signal to control the auxiliary switch to turn off when the conduction time of the auxiliary switch reaches the maximum conduction time threshold.

8. The energy recovery circuit as claimed in claim 1, further comprising a first resistor coupled between a second terminal of an auxiliary switch and a first terminal of a current sensing resistor.

9. An isolation circuit, comprising: A transformer has a primary winding and a secondary winding; The primary-side switch is coupled between the primary winding and the primary reference ground; The secondary-side synchronous rectifier switch is coupled to the secondary-side winding; And the energy recovery circuit as described in any one of claims 1 to 7.

10. A method for recovering energy using an isolation circuit, the isolation circuit comprising a transformer having a primary winding and a secondary winding, a primary-side switch coupled to the primary winding, and a secondary-side synchronous rectifier switch coupled to the secondary winding, the method comprising: The energy recovery branch is coupled in parallel with the primary winding, wherein the energy recovery branch includes a clamping capacitor, an auxiliary switch and a current sensing resistor coupled in series. The first terminal of the integrated circuit is coupled to the connection point between the primary switch and the primary winding; the second terminal of the integrated circuit receives a current detection signal representing the current flowing through the energy recovery branch; the third terminal of the integrated circuit is coupled to the control terminal of the auxiliary switch to control the on and off of the auxiliary switch; and to reduce the gate-source voltage of the auxiliary switch during the discharge of the clamping capacitor.

11. The method of claim 10, wherein in the first driving phase, the gate voltage of the auxiliary switch is maintained at a first voltage, and when switching to the second driving phase, the gate voltage of the auxiliary switch is pulled low.

12. The method as described in request item 10, wherein: When the current detection signal is detected to be greater than the reference threshold voltage, the system switches from the first driving stage to the second driving stage.

13. The method as described in claim 10, further comprising: In response to the detection that the current sensing signal is greater than the reference threshold voltage, a first discharge path is provided from the third terminal of the integrated circuit to the first terminal. And in response to the detection that the current detection signal is less than the turn-off threshold voltage, a second discharge path is provided from the third terminal of the integrated circuit to the first terminal.

14. The method of claim 10, wherein the energy recovery branch further includes a first resistor coupled between the auxiliary switch and the current sensing resistor.

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