An isolated active clamp ZVS SEPIC AC-DC converter

Abstract

The application discloses an isolated active clamp ZVS SEPIC AC-DC converter, comprising a main power circuit and a control circuit. The main power circuit comprises a boost inductor, a power frequency diode, a high-frequency switch tube, an inter-stage capacitor, an active clamp branch, an isolation transformer, a secondary side synchronous rectifier and an output capacitor; the control circuit comprises a voltage sampling circuit, a current sampling circuit, a logic gate circuit and a single-chip microcomputer. The converter controls the working states of the high-frequency switch tube and the clamp switch tube according to the input voltage polarity, and realizes power factor correction in a continuous current mode. The application cancels the traditional input rectifier bridge, reduces the number of semiconductor devices in the current path; the active clamp branch is used to recover the leakage inductance and resonance energy, suppress the switching voltage peak, and assist the switch tube to realize zero-voltage turn-on, thereby reducing the switching loss and electromagnetic interference, improving the converter efficiency, and being suitable for high-frequency, high-power density isolated AC-DC power supply.

Description

Technical Field

[0001] This invention relates to the field of AC-DC conversion and power factor correction technology for power conversion devices, and in particular to a bridgeless isolated SEPIC PFC converter with active clamping. Background Technology

[0002] In single-phase AC-DC power supplies, a basic AC-DC conversion can be achieved using a diode rectifier bridge and a large-capacity DC bus capacitor. However, this scheme results in a pulsed input current, leading to problems such as low power factor, high input current harmonic content, and pollution of the AC power grid. To meet the requirements of low harmonics and high power factor, a power factor correction circuit is typically introduced at the front end.

[0003] Existing isolated PFC power supplies mostly employ a two-stage structure, where the front-stage PFC circuit shapes the input current, and the rear-stage isolated DC-DC converter provides electrical isolation and output voltage regulation. This two-stage approach involves numerous control components and devices, requiring energy to undergo two conversions, which limits system efficiency and power density. The isolated SEPIC PFC converter, however, can simultaneously achieve buck-boost, isolation, and power factor correction within a single stage, making it suitable for high-power-density power supply applications.

[0004] However, traditional isolated SEPIC PFC converters with rectifier bridges have at least several semiconductor devices conducting in the input current path, resulting in significant conduction losses. Simultaneously, the leakage inductance of the isolation transformer easily generates high voltage spikes when the main switch is turned off, increasing voltage stress on the switch and causing electromagnetic interference. While traditional RCD passive snubber circuits or clamping circuits can limit voltage spikes, they dissipate leakage inductance energy across resistors, reducing system efficiency.

[0005] Therefore, there is a need for an isolated PFC converter that can reduce rectifier bridge conduction losses, recover leakage inductance energy, suppress voltage spikes in the switching transistors, and achieve soft switching. Summary of the Invention

[0006] The purpose of this invention is to provide a bridgeless, actively clamped, isolated SEPIC PFC converter to solve the problems of high conduction loss, difficulty in recovering leakage inductance energy from the isolation transformer, and large voltage spikes and switching losses in existing isolated PFC converters with rectifier bridges.

[0007] The technical solution to achieve the purpose of this invention is as follows: an isolated active clamp ZVS SEPIC AC-DC converter, comprising a main power circuit and a control circuit, wherein the main power circuit includes an AC input terminal V in Boost inductor L bThe circuit consists of a first power frequency diode D1, a second power frequency diode D2, an interstage capacitor C, an isolation transformer, a secondary rectifier unit, an output filter unit, an active clamping branch, and a bridgeless high-frequency switching branch; the active clamping branch includes a clamping capacitor C. r The circuit includes a clamping switch S3; the bridgeless high-frequency switching branch includes a first high-frequency switch S1 and a second high-frequency switch S2; the control circuit includes an input / output voltage sampling circuit and a current sampling circuit. The control circuit controls the conduction state of the first high-frequency switch S1, the second high-frequency switch S2 and the clamping switch S3 according to the polarity of the input voltage, so that the converter can achieve power factor correction and isolated DC output in continuous current mode.

[0008] Compared with the prior art, the significant advantages of this invention are: (1) the input rectifier bridge is eliminated, the number of semiconductor devices in the input current path is reduced, and the conduction loss is reduced; (2) the active clamping branch can recover the leakage inductance energy of the transformer and suppress the switching voltage spike; (3) the main switch can achieve zero voltage turn-on in a wide operating range, reducing switching loss and electromagnetic interference; (4) it is suitable for high-frequency and miniaturized isolated PFC power supplies. Attached Figure Description

[0009] Figure 1 This is a schematic diagram of the circuit topology of a bridgeless isolated SEPIC PFC converter with active clamping according to the present invention.

[0010] Figure 2 This is a schematic diagram of the key current, voltage, and drive waveforms of the converter of the present invention during one high-frequency switching cycle.

[0011] Figure 3 is a schematic diagram of the five operating modes of the converter of the present invention in the positive half-cycle continuous current mode. Figures (a) to (e) correspond to modes one to five, respectively. In the figure, the black element represents the branch that works in the mode, the red element represents the branch that is disconnected in the mode, and the blue element represents the element that is always on in each mode.

[0012] Figure 4 This diagram illustrates the relationship between the zero-voltage turn-on range and the equivalent resonant inductance value under different output power conditions.

[0013] Figure 5 This is a block diagram of the average current control of the converter of the present invention.

[0014] Figure 6 The above are simulated waveforms of the input voltage, boost inductor current, magnetizing inductor current, and equivalent resonant inductor current during the power frequency cycle in this embodiment of the invention.

[0015] Figure 7 This is a simulation waveform diagram of the high-frequency switching cycle in an embodiment of the present invention. Detailed Implementation

[0016] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the following embodiments are only for illustrating the present invention and are not intended to limit the scope of protection of the present invention; those skilled in the art can make equivalent substitutions or adjustments to the circuit parameters, device types and control implementation methods without departing from the technical concept of the present invention.

[0017] Combination Figures 1-7 The present invention discloses an isolated active clamp ZVS SEPIC AC-DC converter, comprising a main power circuit and a control circuit, wherein the main power circuit includes an AC input terminal V in Boost inductor L b The circuit consists of a first power frequency diode D1, a second power frequency diode D2, an interstage capacitor C, an isolation transformer, a secondary rectifier unit, an output filter unit, an active clamping branch, and a bridgeless high-frequency switching branch; the active clamping branch includes a clamping capacitor C. r The circuit includes a clamping switch S3; the bridgeless high-frequency switching branch includes a first high-frequency switch S1 and a second high-frequency switch S2; the control circuit includes an input / output voltage sampling circuit and a current sampling circuit, which control the conduction state of the first high-frequency switch S1, the second high-frequency switch S2 and the clamping switch S3 according to the polarity of the input voltage, so that the converter can achieve power factor correction and isolated DC output in continuous current mode.

[0018] Boost inductor L b Connected to AC input terminal V in Between the bridgeless high-frequency switch branch; the first power frequency diode D1 and the second power frequency diode D2 are respectively the AC input terminals V in The positive and negative half-cycles of the generated AC input voltage provide low-frequency current paths; the interstage capacitor C is connected between the drain of the first high-frequency switch S1 and the primary side of the isolation transformer to transfer energy and block the DC component. m and equivalent resonant inductance L r The secondary-side rectifier unit includes a secondary-side synchronous rectifier diode S. SR The output filtering unit includes an output capacitor C. o The secondary-side synchronous rectifier tube S SR With output capacitor C o The connection forms an isolated DC output terminal.

[0019] AC input terminal v in The first terminal is connected to the boost inductor L b The first terminal is connected to the boost inductor L. b The second terminal is connected between the source of the first high-frequency switch S1 and the drain of the second high-frequency switch S2. The AC input terminal vin The second end is connected between the anode of the first power frequency diode D1 and the cathode of the second power frequency diode D2; the cathode of the first power frequency diode D1 is connected to the drain of the first high-frequency switch S1 and the first end of the interstage capacitor C, and the anode of the second power frequency diode D2 is connected to the source of the second high-frequency switch S2 and the AC input terminal V. in The second terminal; the first high-frequency switch S1 and the second high-frequency switch S2 are connected in series to form a bridgeless high-frequency switch branch, wherein the source of the first high-frequency switch S1 is connected to the drain of the second high-frequency switch S2, the drain of the first high-frequency switch S1 is connected to the interstage capacitor C, and the source of the second high-frequency switch S2 is connected to the AC input terminal V. in The second terminal; the second terminal of the interstage capacitor C is connected to the equivalent resonant inductor L. r The first terminal and the clamping capacitor C r The first terminal, the equivalent resonant inductance L r The second end is connected to the first end of the primary winding of the isolation transformer, and the second end of the primary winding of the isolation transformer is connected to the source of the second high-frequency switching transistor S2; the clamping capacitor C r The second terminal is connected to the drain of the clamping switch S3, and the source of the clamping switch S3 is connected to the source of the second high-frequency switch S2, so that the clamping capacitor C r The clamping switch S3 is connected in series to form an active clamping branch, and the active clamping branch is connected to the equivalent resonant inductor L. r The primary winding of the isolation transformer is connected in parallel with the series branch formed by the primary winding; one end of the secondary winding of the isolation transformer is connected to the secondary synchronous rectifier tube S. SR The input terminal of the secondary-side synchronous rectifier S SR The output terminal of the capacitor is connected to the positive terminal, and the other end of the secondary winding of the isolation transformer is connected to the negative terminal; the output capacitor C o The two ends of the capacitor are connected to the positive and negative terminals of the output, respectively. The load is connected between the positive and negative terminals of the output, so that the output capacitor C... o It is connected in parallel with the load to form an isolated DC output terminal.

[0020] The output voltage sampling circuit is connected to the output terminal of the output filter unit, that is, connected to the output capacitor C. o The two ends are used to collect the output voltage V at the output terminal of the isolation converter. o The input voltage sampling circuit is connected to the AC input terminal V. in The two ends are used to collect the input voltage v vin The current sampling circuit is connected in series in the boost inductor branch to collect the boost inductor current i. Lb The input voltage sampling circuit is connected to the AC input terminal V.in The two ends are used to collect the input voltage v vin The control circuit is based on the input voltage v vin .

[0021] The active clamping branch receives leakage inductance energy from the isolation transformer and equivalent resonant inductance L during the high-frequency switch turn-off period. r Energy is fed back to the power transfer process to limit voltage spikes across the first high-frequency switch S1 or the second high-frequency switch S2.

[0022] When the AC input voltage is in the positive half-cycle, the first high-frequency switch S1 remains on, and the second high-frequency switch S2 and the clamping switch S3 are complementary in conduction; when the AC input voltage is in the negative half-cycle, the second high-frequency switch S2 remains on, and the first high-frequency switch S1 and the clamping switch S3 are complementary in conduction.

[0023] The clamping switch S3 achieves soft switching during the resonance process of the active clamping branch. During the positive half-cycle of the input voltage, the junction capacitance of the second high-frequency switch S2 is discharged and the junction capacitance of the clamping switch S3 is charged through the boost inductor current and the equivalent resonant inductor current during the dead time, enabling the second high-frequency switch S2 to achieve zero-voltage turn-on. During the negative half-cycle of the input voltage, the junction capacitance of the first high-frequency switch S1 is discharged and the junction capacitance of the clamping switch S3 is charged through the boost inductor current and the equivalent resonant inductor current during the dead time, enabling the first high-frequency switch S1 to achieve zero-voltage turn-on. Since the clamping switch S3 is a naturally soft-switching switch, the ZVS of the second high-frequency switch S2 is mainly considered during the positive half-cycle, and the ZVS of the first high-frequency switch S1 is mainly considered during the negative half-cycle.

[0024] The ZVS condition for the converter is:

[0025] ,

[0026] Among them, f r f is the resonant frequency. s Where is the switching frequency, and M is the output voltage to input voltage transfer ratio. Where S is the duty cycle of the clamping switch S3, and D is the duty cycle of the high-frequency switch. .

[0027] ,

[0028] Among them, L Lr For the equivalent resonant inductance L r The sensitivity value, C Cr Clamping capacitor C r The capacitance value.

[0029] For ease of explanation, assume that the output voltage ripple is relatively small compared to the DC component, the switching frequency is much higher than the input voltage frequency, and the input voltage can be approximated as constant within one high-frequency switching cycle.

[0030] During the positive half-cycle of the input voltage, the first high-frequency switch S1 remains on, while the second high-frequency switch S2 and the clamping switch S3 conduct complementary high-frequency signals. During the negative half-cycle of the input voltage, the second high-frequency switch S2 remains on, while the first high-frequency switch S1 and the clamping switch S3 conduct complementary high-frequency signals. Since the positive and negative half-cycles are symmetrical, the following explanation uses the positive half-cycle as an example to illustrate its operating mode.

[0031] like Figure 2 As shown in Figure 3, when the converter operates in continuous current mode, it can be divided into five modes within one high-frequency switching cycle.

[0032] Mode 1: First high-frequency switch S1 and second high-frequency switch S2 are turned on, clamping switch S3 is turned off, and the input voltage v vin Added to boost inductor L b At both ends, the boost inductor current i Lb Linear rise; the voltage across the interstage capacitor C acts on the magnetizing inductor L m and equivalent resonant inductance L r Magnetizing inductor current i Lm and equivalent resonant inductor current i Lr Linear increase. The current path in this mode is: boost inductor current i Lb From the AC input terminal v in The current begins to flow through the boost inductor L b After the second high-frequency switch S2 and the second power frequency diode D2 return to the AC input terminal V... in Equivalent resonant inductor current i Lr The current flows through the interstage capacitor C, the first high-frequency switch S1, the second high-frequency switch S2, and the magnetizing inductor L. m At this point, the primary and secondary sides are disconnected, and the output capacitor provides energy to the load.

[0033] During this stage, the boost inductor L b Current The expression is:

[0034] (1),

[0035] Among them, V m ω represents the input voltage amplitude, ω represents the angular frequency, t represents time, and t0 represents the initial moment of the mode.

[0036] Magnetizing inductor current i Lm and equivalent resonant inductor current i Lr The expression is:

[0037] (2),

[0038] in, This represents the interstage capacitor voltage.

[0039] Mode 2: The second high-frequency switch S2 is turned off, and the boost inductor current i Lb and equivalent resonant inductor current i Lr The junction capacitance of the second high-frequency switch S2 is charged, and the junction capacitance of the clamping switch S3 is discharged. The equivalent resonant inductance L r Resonance occurs with the relevant junction capacitance, creating conditions for the soft switching of the clamping switch S3. The current path in this mode is: boost inductor current i Lb From the AC input terminal v in The current begins to flow through the boost inductor L b The junction capacitance of the second high-frequency switch S2, the second power frequency diode D2, and then return to the AC input terminal V. in Equivalent resonant inductor current i Lr Energy is transferred to the secondary side via the interstage capacitor C, the junction capacitance of the first high-frequency switch S1 and the second high-frequency switch S2, and the isolation transformer; the secondary side current i s Flow through the secondary side synchronous rectifier tube S SR The junction capacitance provides energy to the output capacitor and the load.

[0040] The expression for resonant current is:

[0041] (3),

[0042] Among them, I Lm_max I represents the maximum value of the magnetizing inductor current. Lb_max C represents the maximum value of the boost inductor current. oss C represents the parasitic capacitance of the switching transistor. pj V represents the parasitic capacitance of the synchronous rectifier diode equivalent to the primary side. Cr The voltage across the clamping capacitor represents the average voltage, t1 represents the initial moment of mode two, and ω... r denoted by ω, which represents the resonant angular frequency of mode two, and ω represents the transformer turns ratio.

[0043] (4),

[0044] Mode 3: The first high-frequency switch S1 and the clamping switch S3 are turned on, the second high-frequency switch S2 is turned off, the secondary synchronous rectifier is turned on, and the magnetizing inductor L... m Clamped by the output voltage conversion value; boost inductor L b The voltage across the terminals is the input voltage plus the interstage capacitor C and the clamping capacitor C. rThe voltage difference causes the boost inductor current to decrease; the equivalent resonant inductance L r With clamping capacitor C r Energy exchange occurs. The current path in this mode is: boost inductor current i Lb From the AC input terminal v in The current begins to flow through the boost inductor L b The circuit consists of the first high-frequency switch S1, interstage capacitor C, active clamp branch and transformer parallel branch, second power frequency diode D2, and then returns to the AC input terminal V. in Equivalent resonant inductor current i Lr Flow through clamping capacitor C r The clamping switch S3 and the isolation transformer transfer energy to the secondary side; the secondary current i s Flow through the secondary side synchronous rectifier tube S SR It provides energy to the output capacitor and the load.

[0045] Equivalent resonant inductor current i Lr It can be described as:

[0046] (5),

[0047] Among them, v Cr t2 represents the voltage across the clamping capacitor, t2 represents the initial moment of mode three, and ω represents the resonant angular frequency of mode three. r1 :

[0048] (6),

[0049] Characteristic impedance Z1:

[0050] (7),

[0051] The expressions for the boost inductor current and the magnetizing inductor current during this stage are:

[0052] (8),

[0053] (9),

[0054] Mode 4: Clamping switch S3 is turned off. At this time, the boost inductor current i Lb With the equivalent resonant inductor current i Lr The difference discharges the junction capacitance of the second high-frequency switch S2 and charges the junction capacitance of the clamping switch S3, resulting in an equivalent resonant inductance L. r Resonance with the junction capacitance ensures that the second high-frequency switch S2 has zero-voltage turn-on conditions before the next mode is turned on. The current path for this mode is: boost inductor current i Lb From the AC input terminal v in The current begins to flow through the boost inductor Lb The junction capacitance of the second high-frequency switch S2, the second power frequency diode D2, and then return to the AC input terminal V. in Equivalent resonant inductor current i Lr Flow through clamping capacitor C r The junction capacitance of the clamping switch S3 and the isolation transformer transfer energy to the secondary side; the secondary current i s Flow through the secondary side synchronous rectifier tube S SR It provides energy to the output capacitor and the load.

[0055] The expression for resonant current is:

[0056] (10),

[0057] Among them, I Lm_t3 This represents the value of the magnetizing inductor current at time t3, where t3 represents the initial time of mode four. This indicates the minimum current of the boost inductor.

[0058] Mode 5: The first high-frequency switch S1 and the second high-frequency switch S2 are turned on, the clamping switch S3 is turned off, the secondary synchronous rectifier is still turned on, and the input voltage is applied to the boost inductor L. b The voltage across the interstage capacitor C and the reduced output voltage together act on the equivalent resonant inductor L. r Magnetizing inductance L m The output voltage continues to be clamped by the equivalent resonant inductor current i. Lr With the excitation inductor current i Lm When the current values ​​are equal, the mode ends and the next switching cycle begins. The current path for this mode is: boost inductor current i Lb From the AC input terminal v in The current begins to flow through the boost inductor L b After the second high-frequency switch S2 and the second power frequency diode D2 return to the AC input terminal V... in Equivalent resonant inductor current i Lr Energy is transferred to the secondary side via the interstage capacitor C, the first high-frequency switch S1, the second high-frequency switch S2, and the isolation transformer; the secondary side current i s Flow through the secondary side synchronous rectifier tube S SR It provides energy to the output capacitor and the load.

[0059] (11),

[0060] (12),

[0061] (13),

[0062] Where represents the initial time of mode 5 t4.

[0063] As can be seen from the above modes, clamping switch S3 can achieve soft switching in the active clamping branch. During the positive half-cycle, the junction capacitance of the second high-frequency switch S2 is charged and discharged mainly through the boost inductor current and the equivalent resonant inductor current to achieve zero-voltage turn-on of S2. During the negative half-cycle, the same mechanism is used to achieve zero-voltage turn-on of the first high-frequency switch S1. The zero-voltage turn-on range is related to the equivalent resonant inductance L. r Clamping capacitor C r Output power is related to the power frequency phase angle, such as Figure 4 As shown, when the equivalent resonant inductance value is appropriate, the converter can obtain a wide soft-switching range within half a power frequency cycle.

[0064] like Figure 5 As shown, the control strategy of this invention employs average current control. The control circuit controls the output voltage V based on the acquired voltage. o With voltage control reference signal V o_ref The comparison is performed, and the result is adjusted by a proportional-integral (PI) controller to obtain the voltage loop output signal A; the input voltage v vin The effective value signal B of the input voltage is obtained by the effective value calculation module RMS; This indicates that the voltage loop output signal A and the squared signal of the effective value of the input voltage B are compared. 2 Perform proportional calculations to obtain the current control reference signal i. in_ref The current control reference signal i in_ref With boost inductor current i Lb The comparison error signal is adjusted by the proportional controller P to generate complementary control signal PWM1 and control signal Q3 for the clamping switch; input voltage v vin The output value after polarity determination by comparator comp, along with the PWM1 signal, enters the logic circuit to output the control signal Q2 of the second high-frequency switch S2 and the control signal Q1 of the first high-frequency switch S1.

[0065] Example

[0066] This embodiment provides a bridgeless isolated SEPIC PFC converter with active clamping, featuring a 220VAC / 50Hz input, 48V DC output, and 240W output power. Its main parameters are shown in Table 1.

[0067] Table 1 Main parameters of the converter

[0068]

[0069] Based on the parameters in Table 1, a simulation model was built on the MATLAB / Simulink platform. Figure 6This shows the voltage value v at the AC input terminal during one power frequency cycle. vin Boost inductor current i Lb Magnetizing inductor current i Lm and equivalent resonant inductor current i Lr The waveform. (From) Figure 6 It can be seen that under average current control, the boost inductor current can follow the input voltage in a sinusoidal manner, indicating that the converter has good power factor correction capability.

[0070] Figure 7 The high-frequency switching cycle waveform of the converter operating in continuous current mode is shown. The trend of inductor current variation in the figure is consistent with the analysis of the five operating modes mentioned above, proving that the topology and its control strategy can achieve the desired current transfer and soft switching process.

[0071] The above description is merely a preferred embodiment of the present invention and should not be construed as limiting the scope of protection of the present invention. Any equivalent substitutions, modifications, or refinements made to circuit connections, device models, controller implementations, or parameter values ​​within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An isolated active clamp ZVS SEPIC AC-DC converter, characterized in that: It includes a main power circuit and a control circuit, wherein the main power circuit includes an AC input terminal V in Boost inductor L b The circuit consists of a first power frequency diode D1, a second power frequency diode D2, an interstage capacitor C, an isolation transformer, a secondary rectifier unit, an output filter unit, an active clamping branch, and a bridgeless high-frequency switching branch; the active clamping branch includes a clamping capacitor C. r The circuit includes a clamping switch S3; the bridgeless high-frequency switching branch includes a first high-frequency switch S1 and a second high-frequency switch S2; the control circuit includes an input voltage sampling circuit, an output voltage sampling circuit, and a current sampling circuit. The control circuit controls the conduction state of the first high-frequency switch S1, the second high-frequency switch S2, and the clamping switch S3 according to the polarity of the input voltage, so that the converter can achieve power factor correction and isolated DC output in continuous current mode.

2. The isolated active clamp ZVS SEPIC AC-DC converter according to claim 1, characterized in that: Boost inductor L b Connected to AC input terminal V in Between the bridgeless high-frequency switch branch; the first power frequency diode D1 and the second power frequency diode D2 are respectively the AC input terminals V in The positive and negative half-cycles of the generated AC input voltage provide a low-frequency current path; the interstage capacitor C is connected between the drain of the first high-frequency switch S1 and the primary side of the isolation transformer to transfer energy and block the DC component.

3. The isolated active clamp ZVS SEPIC AC-DC converter according to claim 2, characterized in that: The isolation transformer includes the primary magnetizing inductance L. m and equivalent resonant inductance L r The secondary-side rectifier unit includes a secondary-side synchronous rectifier diode S. SR The output filtering unit includes an output capacitor C. o The secondary-side synchronous rectifier tube S SR With output capacitor C o The connection forms an isolated DC output terminal.

4. The isolated active clamp ZVS SEPIC AC-DC converter according to claim 3, characterized in that: AC input terminal v in The first terminal is connected to the boost inductor L b The first terminal is connected to the boost inductor L. b The second terminal is connected between the source of the first high-frequency switch S1 and the drain of the second high-frequency switch S2. The AC input terminal v in The second end is connected between the anode of the first power frequency diode D1 and the cathode of the second power frequency diode D2; The cathode of the first power frequency diode D1 is connected to the drain of the first high-frequency switch S1 and the first terminal of the interstage capacitor C. The anode of the second power frequency diode D2 is connected to the source of the second high-frequency switch S2 and the AC input terminal V. in The second end; The first high-frequency switch S1 and the second high-frequency switch S2 are connected in series to form a bridgeless high-frequency switching branch. The source of the first high-frequency switch S1 is connected to the drain of the second high-frequency switch S2. The drain of the first high-frequency switch S1 is connected to the interstage capacitor C. The source of the second high-frequency switch S2 is connected to the AC input terminal V. in The second end; The second terminal of the interstage capacitor C is connected to the equivalent resonant inductor L. r The first terminal and the clamping capacitor C r The first terminal, the equivalent resonant inductance L r The second end is connected to the first end of the primary winding of the isolation transformer, and the second end of the primary winding of the isolation transformer is connected to the source of the second high-frequency switching transistor S2. The clamping capacitor C r The second terminal is connected to the drain of the clamping switch S3, and the source of the clamping switch S3 is connected to the source of the second high-frequency switch S2, so that the clamping capacitor C r The clamping switch S3 is connected in series to form an active clamping branch, which is connected to the equivalent resonant inductor L. r It is connected in parallel with the series branch formed by the primary winding of the isolation transformer; One end of the secondary winding of the isolation transformer is connected to the secondary synchronous rectifier tube S. SR The input terminal of the secondary-side synchronous rectifier S SR The output terminal is connected to the positive output terminal, and the other end of the secondary winding of the isolation transformer is connected to the negative output terminal. The output capacitor C o The two ends of the capacitor are connected to the positive and negative terminals of the output, respectively. The load is connected between the positive and negative terminals of the output, so that the output capacitor C... o Connected in parallel with the load to form an isolated DC output terminal; The output voltage sampling circuit is connected to the output terminal of the output filter unit, that is, connected to the output capacitor C. o The two ends are used to collect the output voltage V at the output terminal of the isolation converter. o The input voltage sampling circuit is connected to the AC input terminal V. in The two ends are used to collect the input voltage v vin The current sampling circuit is connected in series in the boost inductor branch to collect the boost inductor current i. Lb The input voltage sampling circuit is connected to the AC input terminal V. in The two ends are used to collect the input voltage v vin The control circuit is based on the input voltage v vin .

5. An isolated active clamp ZVS SEPIC AC-DC converter according to claim 4, characterized in that: The active clamping branch receives leakage inductance energy from the isolation transformer and equivalent resonant inductance L during the high-frequency switch turn-off period. r Energy is fed back to the power transfer process to limit voltage spikes across the first high-frequency switch S1 or the second high-frequency switch S2.

6. The isolated active clamp ZVS SEPIC AC-DC converter according to claim 5, characterized in that, When the AC input voltage is in the positive half-cycle, the first high-frequency switch S1 remains on, and the second high-frequency switch S2 and the clamping switch S3 are complementary in conduction; when the AC input voltage is in the negative half-cycle, the second high-frequency switch S2 remains on, and the first high-frequency switch S1 and the clamping switch S3 are complementary in conduction.

7. The isolated active clamp ZVS SEPIC AC-DC converter according to claim 5, characterized in that, Within one high-frequency switching cycle of the positive half-cycle, the converter sequentially includes five operating modes: Mode 1: The first high-frequency switch S1 and the second high-frequency switch S2 are turned on, and the clamping switch S3 is turned off. Mode 2: The second high-frequency switch S2 is turned off, and the equivalent resonant inductance L... r Resonance with the junction capacitance of the switching transistor; Mode 3: The first high-frequency switch S1 and the clamping switch S3 are turned on, and the clamping capacitor C... r With equivalent resonant inductance L r Participating in energy transfer; Mode 4: Clamping switch S3 is turned off, equivalent resonant inductance L r Resonance with the junction capacitance of the switching transistor; Mode 5: The first high-frequency switch S1 and the second high-frequency switch S2 are turned on, and the secondary rectifier unit continues to be turned on until the equivalent resonant inductor current is equal to the magnetizing inductor current.

8. An isolated active clamp ZVS SEPIC AC-DC converter according to claim 7, characterized in that, The control circuit uses the acquired output voltage V o With voltage control reference signal V o_ref The comparison is performed, and the result is adjusted by a proportional-integral (PI) controller to obtain the voltage loop output signal A; the input voltage v vin The effective value signal B of the input voltage is obtained by the effective value calculation module RMS; This indicates that the voltage loop output signal A and the squared signal of the effective value of the input voltage B are compared. 2 Perform proportional calculations to obtain the current control reference signal i. in_ref The current control reference signal i in_ref With boost inductor current i Lb The comparison error signal is adjusted by the proportional controller P to generate complementary control signal PWM1 and control signal Q3 for the clamping switch; input voltage v vin The output value after polarity determination by comparator comp, along with the PWM1 signal, enters the logic circuit to output the control signal Q2 of the second high-frequency switch S2 and the control signal Q1 of the first high-frequency switch S1.

9. An isolated active clamp ZVS SEPIC AC-DC converter according to claim 8, characterized in that: The clamping switch S3 achieves soft switching during the resonance process of the active clamping branch. During the positive half-cycle of the input voltage, the junction capacitance of the second high-frequency switch S2 is discharged and the junction capacitance of the clamping switch S3 is charged through the boost inductor current and the equivalent resonant inductor current during the dead time, enabling the second high-frequency switch S2 to achieve zero-voltage turn-on. During the negative half-cycle of the input voltage, the junction capacitance of the first high-frequency switch S1 is discharged and the junction capacitance of the clamping switch S3 is charged through the boost inductor current and the equivalent resonant inductor current during the dead time, enabling the first high-frequency switch S1 to achieve zero-voltage turn-on. Since the clamping switch S3 is a naturally soft-switching transistor, the ZVS of the second high-frequency switch S2 is mainly considered during the positive half-cycle, and the ZVS of the first high-frequency switch S1 is mainly considered during the negative half-cycle. The ZVS condition of the converter is: ， Among them, f r f is the resonant frequency. s Where is the switching frequency, and M is the output voltage to input voltage transfer ratio. Where S is the duty cycle of the clamping switch S3, and D is the duty cycle of the high-frequency switch. ; ， Among them, L Lr For the equivalent resonant inductance L r The sensitivity value, C Cr Clamping capacitor C r The capacitance value.

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