Switching transistor isolated driving circuit and double-transistor forward converter

Abstract

The application discloses a switch tube isolation type driving circuit and a double-tube forward converter, and belongs to the technical field of power electronics. The switch tube isolation type driving circuit comprises a primary driving circuit, a driving transformer, a first switch tube driving circuit and a second switch tube driving circuit; the primary driving circuit receives a pulse width modulation signal and drives a primary winding; two secondary windings are respectively connected with two switch tube driving circuits to drive the gate and the source of a first switch tube and a second switch tube respectively. Through isolation driving, in combination with active pull-down turn-off and small duty ratio excitation maintenance, the problems of driving waveform distortion and unreliable turn-off under small duty ratio or light load working conditions can be improved.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, and in particular to a switching transistor isolated drive circuit and a two-transistor forward converter. Background Technology

[0002] In high-frequency, high-efficiency power conversion applications, the two-transistor forward converter is a common energy conversion topology. A two-transistor forward converter typically includes a pair of cooperating main power switches, using a high-frequency transformer for energy transfer. It can achieve core reset without a dedicated reset winding, while also reducing voltage stress on the switching devices. The driving schemes for the switching transistors of a two-transistor forward converter mainly include isolated driving and half-bridge driving chips. Isolated driving offers advantages such as good isolation performance, strong anti-interference capability, suitability for high-voltage, high-power applications, and natural synchronization of the two driving signals, thus it is widely used in driving the switching transistors of two-transistor forward converters. Traditional isolated driving schemes often use a DC blocking capacitor in series on the primary side to prevent transformer saturation, but this method is prone to problems such as driving waveform distortion and asymmetry in the duty cycles of the two driving signals. Furthermore, the turn-off of the isolated driving circuit usually relies on the reverse voltage generated after the primary winding is de-energized: the reverse voltage is large when the duty cycle is large, making turn-off relatively easy; however, under small duty cycles or light load conditions, the reverse voltage is insufficient, reducing the reliability of the switch turn-off and posing a risk of device damage. Summary of the Invention

[0003] The technical problem to be solved by this application is to improve the distortion of the drive waveform and the asymmetry of the duty cycle of the two paths caused by the structure such as the primary side DC blocking capacitor in traditional isolated drive, and to improve the reliability of the switch turn-off under small duty cycle or light load conditions.

[0004] To achieve the above objectives, in a first aspect, this application provides a switch-isolated driving circuit for driving a first switch and a second switch of a dual-transistor forward converter. The switch-isolated driving circuit includes a primary driving circuit, a driving transformer, a first switch driving circuit, and a second switch driving circuit. The input terminal of the primary driving circuit is used to receive a pulse width modulation signal, and the output terminal of the primary driving circuit is connected to the primary winding of the driving transformer. The driving transformer includes two secondary windings, which are respectively connected to the input terminals of the first switch driving circuit and the second switch driving circuit. The output terminal of the first switch driving circuit is connected to the gate and source of the first switch, and the output terminal of the second switch driving circuit is connected to the gate and source of the second switch.

[0005] In some embodiments, the first switch driving circuit includes a first turn-off driving unit, which is used to drive the first switch to turn off when the corresponding secondary winding outputs a reverse driving voltage, and includes a first switching device connected between the gate (G1) and the source (S1) of the first switch. When the first switching device is turned on, it pulls down the gate (G1) potential of the first switch to its source (S1) potential. The primary drive circuit includes a first inductor connected in series with the primary winding of the drive transformer. The first inductor is used to maintain the excitation current of the primary winding when the duty cycle of the pulse width modulation signal is lower than a preset value.

[0006] In some embodiments, the first switch driving circuit includes a first turn-on driving unit, which is used to drive the first switch to turn on when a positive drive voltage is output from the corresponding secondary winding. The first conduction drive unit includes a first resistor and a first diode. The first resistor is connected between the first end of the corresponding secondary winding and the gate of the first switching transistor, and the first diode is connected between the second end of the corresponding secondary winding and the source of the first switching transistor.

[0007] In some embodiments, the first switching device is a first MOSFET, the drain of the first MOSFET is connected to the gate of the first switching transistor through a second resistor, the source of the first MOSFET is connected to the source of the first switching transistor, and the gate of the first MOSFET is connected to the second end of the corresponding secondary winding through a third resistor.

[0008] In some embodiments, the first switch driving circuit further includes a first voltage clamping unit connected between the gate and the source of the first MOS transistor, for limiting the voltage between the gate and the source of the first MOS transistor. The first voltage clamping unit includes a fourth resistor, a first transient voltage suppression diode, and a second diode connected in parallel.

[0009] In some embodiments, the first switch driving circuit further includes a second voltage clamping unit connected between the gate and the source of the first switch, for limiting the voltage between the gate and the source of the first switch. The second voltage clamping unit includes a fifth resistor and a second transient voltage suppression diode connected in parallel.

[0010] In some embodiments, the second switch driving circuit has the same structure as the first switch driving circuit and is symmetrically arranged relative to the driving transformer.

[0011] In some embodiments, the primary driving circuit includes a totem pole driving unit, which includes a first transistor and a second transistor. The bases of both the first and second transistors are connected to an eleventh resistor to receive pulse width modulation signals. The emitters of the first and second transistors are connected to form a totem pole output node. The collector of the first transistor is connected to a driving power supply through a thirteenth resistor, and the collector of the second transistor is connected to a reference ground potential. A fourteenth resistor is connected between the collector and the base of the second transistor, and a fifth diode is connected between the collector and the emitter of the second transistor. The first transistor is an NPN transistor, and the second transistor is a PNP transistor.

[0012] In some embodiments, the primary drive circuit further includes a primary switching unit, which includes a fifth MOS transistor. The gate of the fifth MOS transistor is connected to the totem pole output node through a fifteenth resistor. The totem pole output node is also connected to one end of the primary winding of the drive transformer through a twelfth resistor. The drain of the fifth MOS transistor is used to connect to the other end of the primary winding of the drive transformer. The source of the fifth MOS transistor is connected to a reference ground potential. A sixteenth resistor is connected between the gate and the source of the fifth MOS transistor.

[0013] In some embodiments, the drain of the fifth MOS transistor is connected to the primary winding of the drive transformer via the first inductor; when the duty cycle of the pulse width modulation signal is lower than a preset value, the first inductor is used to maintain the magnetizing current of the primary winding of the drive transformer.

[0014] In some embodiments, the primary drive circuit further includes a voltage absorption unit, which includes a sixth diode, a seventeenth resistor, and a first capacitor. The sixth diode is connected between one end of the seventeenth resistor and the drain of the fifth MOS transistor, the other end of the seventeenth resistor is connected to the drive power supply, and the first capacitor is connected in parallel with the seventeenth resistor.

[0015] In some embodiments, the drive transformer includes a primary winding and two secondary windings, wherein the turns ratio of the primary winding to the two secondary windings is 1:1:1.

[0016] Secondly, this application also provides a dual-transistor forward converter, including a control chip, a first switching transistor, a second switching transistor, and a switching transistor isolated drive circuit as described in any of the above embodiments. The control chip is used to input a pulse width modulation signal to the switching transistor isolated drive circuit, and the switching transistor isolated drive circuit is used to control the first switching transistor and the second switching transistor to be turned on or off according to the pulse width modulation signal.

[0017] The technical solution provided in this application achieves isolated driving through a drive transformer, enabling better synchronization of the two switching transistor drive signals. Maintaining the primary excitation current under low duty cycle conditions through the first inductor helps reduce drive waveform distortion; the active pull-down turn-off structure on the secondary side pulls the gate potential of the main power switch down to the source potential, which helps improve turn-off reliability under low duty cycle or light load conditions. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a system schematic diagram of the switch-isolated drive circuit according to an embodiment of this application; Figure 2 This is a circuit diagram of a switch-isolated drive circuit according to an embodiment of this application; Figure 3 This is a schematic diagram of the primary drive circuit in an embodiment of this application; Figure 4 This is a schematic diagram of the structure of the first switch driving circuit and the second switch driving circuit according to an embodiment of this application. Explanation of icon numbers: 10. Primary drive circuit; 11. Totem pole drive unit; 12. Primary switching unit; 13. Voltage absorption unit; 20. Drive transformer; 30. First switching transistor drive circuit; 31. First turn-on drive unit; 32. First turn-off drive unit; 33. First voltage clamping unit; 34. Second voltage clamping unit; 40. Second switching transistor drive circuit; 41. Second turn-on drive unit; 42. Second turn-off drive unit; 43. Third voltage clamping unit; 44. Fourth voltage clamping unit; U, Control chip; G1, Gate of the first switch transistor; S1, Source of the first switch transistor; G2, Gate of the second switch transistor; S2, Source of the second switch transistor; L1, First inductor; Q1, First MOSFET; Q2, Second MOSFET; Q3, First transistor; Q4, Second transistor; Q5, Fifth MOSFET; R1 to R17, Resistors; D1 to D6, Diodes; T1 to T4, Transient voltage suppression diodes; C1, First capacitor; Vdrive, Drive power supply; GND, Reference ground potential; BYQ1, Drive transformer body.

[0020] The realization of the objectives, functional features and advantages of the embodiments of this application will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0021] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0022] Furthermore, descriptions involving "first," "second," etc., in the embodiments of this application are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, features defined with "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. If the combination of technical solutions is contradictory or impossible to implement, such a combination of technical solutions should be considered non-existent and not within the scope of protection claimed in this application.

[0023] As a prelude to the embodiments of this application, a related technology for isolated drive of a two-transistor forward converter is introduced. A two-transistor forward converter typically includes a pair of main power switches and a high-frequency transformer. The isolated drive scheme uses the drive transformer to isolate the control side from the power side and simultaneously provide drive signals to both switches. Traditional schemes often use a DC blocking capacitor in series on the primary side to suppress magnetic saturation, but this easily causes distortion of the drive waveform and asymmetry of the two duty cycles; during turn-off, it mainly relies on the reverse voltage formed by the primary side de-energization, which is insufficient under small duty cycles or light loads, resulting in decreased turn-off reliability. In view of this, the embodiments of this application provide a switch-isolated drive circuit and a two-transistor forward converter, which improve the quality of the drive waveform and enhance turn-off reliability by means of isolated drive structure, primary-side excitation maintenance, and secondary-side active pull-down turn-off, thereby solving at least one of the above-mentioned technical problems.

[0024] Please see Figure 1 , Figure 1 This is a system schematic diagram of a switch-isolated drive circuit according to an embodiment of this application. This application proposes a switch-isolated drive circuit for driving the first and second switches of a two-transistor forward converter. Figure 1As shown, the control chip U outputs a pulse width modulation signal to the primary drive circuit 10; the primary drive circuit 10 drives the primary winding of the drive transformer 20; the drive transformer 20 includes two secondary windings, which are respectively connected to the first switch drive circuit 30 and the second switch drive circuit 40; the first switch drive circuit 30 and the second switch drive circuit 40 output to the gate and source of the corresponding switch in the two-transistor forward converter, respectively. Thus, the control side and the power side are electrically isolated through the drive transformer 20, and the two drive paths are naturally synchronized.

[0025] Please see Figure 2 , Figure 2 This is a circuit diagram of the isolated drive circuit for the switching transistor according to an embodiment of this application. Figure 2 Based on the system architecture, an example connection relationship of the primary drive circuit 10, drive transformer 20, first switch drive circuit 30, and second switch drive circuit 40 is given. The primary drive circuit 10 includes a totem pole drive unit 11, a primary switch unit 12, and a voltage absorption unit 13, etc.; the drive transformer 20 (shown as BYQ1 in the figure) has one primary winding and two secondary windings; the first switch drive circuit 30 includes a first turn-on drive unit 31, a first turn-off drive unit 32, a first voltage clamping unit 33, and a second voltage clamping unit 34, and its output terminal is connected to the gate G1 and source S1 of the first switch; the second switch drive circuit 40 has the same structure as the first switch drive circuit 30 and is symmetrically arranged, and its output terminal is connected to the gate G2 and source S2 of the second switch.

[0026] Please see Figure 3 , Figure 3 This is a schematic diagram of the primary drive circuit according to an embodiment of this application, used to more clearly illustrate the primary-side power and control path; and can be combined with Figure 2Understand its position in the overall circuit. The pulse width modulation signal is input to the totem pole driver unit 11 via the eleventh resistor R11. The totem pole driver unit 11 includes a first transistor Q3 (NPN) and a second transistor Q4 (PNP), whose bases are connected and their emitters are connected to form the totem pole output node; the collector of the first transistor Q3 is connected to the drive power supply Vdrive via the thirteenth resistor R13, and the collector of the second transistor Q4 is connected to the reference ground potential GND; a fourteenth resistor R14 is provided between the collector and base of the second transistor Q4, and a fifth diode D5 is provided between the collector and emitter to improve level switching and clamping. The totem pole output node is connected to one end of the primary winding of the drive transformer via the twelfth resistor R12, and connected to the gate of the fifth MOSFET Q5 via the fifteenth resistor R15; a sixteenth resistor R16 is connected between the gate and source of the fifth MOSFET Q5, and the source of the fifth MOSFET Q5 is connected to GND. When the pulse width modulation signal is high, the totem pole output is pulled up, the fifth MOSFET Q5 is turned on, and a positive drive voltage is formed across the primary winding of the driving transformer; when the pulse width modulation signal is low, the totem pole output is pulled down, the fifth MOSFET Q5 is turned off, and a reverse drive voltage is formed in the primary winding due to the maintenance of the excitation current.

[0027] Continue reading Figure 3 and combined Figure 2 A first inductor L1 is connected in series between the drain of the fifth MOSFET Q5 and the primary winding of the drive transformer 20. The first inductor L1 is used to maintain the excitation current of the primary winding when the duty cycle of the pulse width modulation signal is lower than a preset value, thereby helping to avoid distortion or aberration of the drive signal. The voltage absorption unit 13 includes a sixth diode D6, a seventeenth resistor R17, and a first capacitor C1. The sixth diode D6 is connected between one end of the seventeenth resistor R17 and the drain of the fifth MOSFET Q5, and the other end of the seventeenth resistor R17 is connected to the drive power supply Vdrive. The first capacitor C1 is connected in parallel with the seventeenth resistor R17 to suppress the primary side voltage spike during level transition. The primary drive circuit 10 thus converts the pulse width modulation signal output by the control chip U into a primary drive voltage corresponding to its phase.

[0028] Please continue reading. Figure 2 The drive transformer 20 includes a primary winding and two secondary windings. The turns ratio of the primary winding to the two secondary windings can be 1:1:1 to ensure that the secondary drive voltage is consistent with the primary drive voltage in amplitude and phase. The two secondary windings provide isolated drive voltages to the first switch drive circuit 30 and the second switch drive circuit 40, respectively.

[0029] Please see Figure 4 , Figure 4 This is a schematic diagram of the structure of the first switch driving circuit and the second switch driving circuit according to an embodiment of this application; and can be combined with Figure 2 Understand the connection relationship between the secondary drive and the gate-source of the main power switch. Taking the first switch drive circuit 30 as an example: when the corresponding secondary winding outputs a positive drive voltage, the first turn-on drive unit 31 forms a positive loop through the first resistor R1 and the first diode D1, providing a turn-on drive between the gate G1 and the source S1 of the first switch; when the corresponding secondary winding outputs a reverse drive voltage, the first MOS transistor Q1 in the first turn-off drive unit 32 is connected to the second terminal of the corresponding secondary winding through the third resistor R3 to obtain gate drive and turn on, and pulls down the potential of the gate G1 of the first switch to the potential of the source S1 through the second resistor R2, thereby realizing active turn-off.

[0030] Continue reading Figure 4 In some embodiments, the first voltage clamping unit 33 is connected between the gate and source of the first MOSFET Q1, and may include a fourth resistor R4 connected in parallel, a first transient voltage suppression diode T1, and a second diode D2, to limit the gate-source voltage of Q1 and reduce the risk of overvoltage damage. The second voltage clamping unit 34 is connected between the gate G1 and source S1 of the first switch, and may include a fifth resistor R5 connected in parallel and a second transient voltage suppression diode T2, to limit the gate-source voltage of the main power switch. This clamping configuration helps improve the withstand capability of the drive circuit under surge or abnormal voltage conditions.

[0031] Furthermore, the second switch driver circuit 40 has the same structure as the first switch driver circuit 30 and is symmetrically arranged relative to the drive transformer 20. For example, the second switch driver circuit 40 may include a second turn-on drive unit 41, a second turn-off drive unit 42, a third voltage clamping unit 43, and a fourth voltage clamping unit 44. The connection relationships of its internal resistors, diodes, MOSFETs, and transient voltage suppression diodes are the same as those of the corresponding units in the first path. Its output terminal is connected to the gate G2 and source S2 of the second switch. Thus, the upper and lower switch drive paths are symmetrical, which helps maintain the consistency of the two drive signals.

[0032] Combination Figures 1 to 4 An example of its operation is described below: When the control chip U outputs a high-level pulse width modulation signal, the fifth MOSFET Q5 on the primary side is turned on, and the forward voltage of the secondary winding turns on the first and second switches via the turn-on drive unit. When the control chip U outputs a low-level pulse width modulation signal, the fifth MOSFET Q5 is turned off, and the primary excitation current (which can be maintained by the first inductor L1 at a small duty cycle) causes a reverse voltage in the secondary winding, turning on the MOSFET in the turn-off drive unit and pulling the gate of the corresponding main power switch down to the source, thus completing the turn-off. Compared with the traditional scheme relying on DC blocking capacitors, the embodiments of this application are beneficial in reducing waveform distortion; compared with simply relying on a weak reverse voltage for passive turn-off, active pull-down is beneficial in improving the turn-off reliability under small duty cycles or light loads.

[0033] This application also proposes a two-transistor forward converter, including a control chip U, a first switch, a second switch, and a switch-isolated drive circuit as described in any of the above embodiments. The control chip U is used to input a pulse width modulation signal to the switch-isolated drive circuit, which controls the first and second switches to be turned on or off according to the signal. Since this two-transistor forward converter adopts the above-described switch-isolated drive circuit technology, it at least has the corresponding beneficial effects of the above-described technology, which will not be elaborated further here.

[0034] It should be noted that the MOSFET in the above embodiments can be replaced by other power switching devices with controllable switching capabilities, such as insulated-gate bipolar transistors; the transistor can also be replaced by a driver stage topology with similar push-pull drive capabilities. The specific winding process, core material, and winding endpoint numbering of the drive transformer can be adjusted according to the engineering implementation, as long as one primary winding corresponds to two secondary windings and supplies power to two symmetrical drive circuits.

[0035] In the above embodiments, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions in other embodiments.

[0036] The isolated drive circuit and dual-transistor forward converter provided in the embodiments of this application have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of this application. The description of the above embodiments is only for the purpose of helping to understand the core ideas of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A switch-isolated drive circuit for driving the first and second switching transistors of a two-transistor forward converter, characterized in that, The isolated drive circuit for switching transistors includes a primary drive circuit (10), a drive transformer (20), a first switch drive circuit (30), and a second switch drive circuit (40). The input terminal of the primary driving circuit (10) is used to receive the pulse width modulation signal, and the output terminal of the primary driving circuit (10) is connected to the primary winding of the driving transformer (20). The driving transformer (20) includes two secondary windings, which are respectively connected to the input terminals of the first switching transistor driving circuit (30) and the second switching transistor driving circuit (40). The output terminal of the first switching transistor driving circuit (30) is connected to the gate (G1) and source (S1) of the first switching transistor, and the output terminal of the second switching transistor driving circuit (40) is connected to the gate (G2) and source (S2) of the second switching transistor.

2. The switch-isolated drive circuit according to claim 1, characterized in that, The first switching transistor driving circuit (30) includes a first turn-off driving unit (32), which is used to drive the first switching transistor to turn off when the corresponding secondary winding outputs a reverse driving voltage, and includes a first switching device connected between the gate (G1) and the source (S1) of the first switching transistor. When the first switching device is turned on, it pulls down the gate (G1) potential of the first switching transistor to its source (S1) potential. The primary drive circuit (10) includes a first inductor (L1) connected in series with the primary winding of the drive transformer (20). The first inductor (L1) is used to maintain the excitation current of the primary winding when the duty cycle of the pulse width modulation signal is lower than a preset value.

3. The switch-isolated drive circuit according to claim 1 or 2, characterized in that, The first switching transistor driving circuit (30) includes a first conduction driving unit (31), which is used to drive the first switching transistor to conduct when the corresponding secondary winding outputs a positive driving voltage; The first conduction drive unit (31) includes a first resistor (R1) and a first diode (D1). The first resistor (R1) is connected between the first end of the corresponding secondary winding and the gate (G1) of the first switch, and the first diode (D1) is connected between the second end of the corresponding secondary winding and the source (S1) of the first switch.

4. The switch-isolated drive circuit according to claim 2, characterized in that, The first switching device is a first MOSFET (Q1). The drain of the first MOSFET (Q1) is connected to the gate (G1) of the first switching transistor through a second resistor (R2). The source of the first MOSFET (Q1) is connected to the source (S1) of the first switching transistor. The gate of the first MOSFET (Q1) is connected to the second end of the corresponding secondary winding through a third resistor (R3).

5. The switch-isolated drive circuit according to claim 4, characterized in that, The first switching transistor driving circuit (30) further includes a first voltage clamping unit (33) connected between the gate and the source of the first MOS transistor (Q1) for limiting the voltage between the gate and the source of the first MOS transistor (Q1); The first voltage clamping unit (33) includes a fourth resistor (R4), a first transient voltage suppression diode (T1), and a second diode (D2) connected in parallel.

6. The switch-isolated drive circuit according to claim 1, characterized in that, The first switching transistor driving circuit (30) further includes a second voltage clamping unit (34) connected between the gate (G1) and the source (S1) of the first switching transistor, for limiting the voltage between the gate (G1) and the source (S1) of the first switching transistor; The second voltage clamping unit (34) includes a fifth resistor (R5) and a second transient voltage suppressor diode (T2) connected in parallel.

7. The switch-isolated drive circuit according to claim 2 or 5, characterized in that, The second switch drive circuit (40) has the same structure as the first switch drive circuit (30) and is symmetrically arranged relative to the drive transformer (20).

8. The switch-isolated drive circuit according to claim 2, characterized in that, The primary drive circuit (10) includes a totem pole drive unit (11), which includes a first transistor (Q3) and a second transistor (Q4). The bases of the first transistor (Q3) and the second transistor (Q4) are connected to receive pulse width modulation signals via an eleventh resistor (R11). The emitters of the first transistor (Q3) and the second transistor (Q4) are connected to form a totem pole output node. The collector of the first transistor (Q3) is connected to the drive power supply (Vdrive) via a thirteenth resistor (R13). The collector of the second transistor (Q4) is connected to the reference ground potential (GND). A fourteenth resistor (R14) is connected between the collector and the base of the second transistor (Q4). A fifth diode (D5) is connected between the collector and the emitter of the second transistor (Q4). The first transistor (Q3) is an NPN transistor, and the second transistor (Q4) is a PNP transistor.

9. The switch-isolated drive circuit according to claim 8, characterized in that, The primary drive circuit (10) further includes a primary switching unit (12), which includes a fifth MOS transistor (Q5). The gate of the fifth MOS transistor (Q5) is connected to the totem pole output node through a fifteenth resistor (R15). The totem pole output node is also connected to one end of the primary winding of the drive transformer (20) through a twelfth resistor (R12). The drain of the fifth MOS transistor (Q5) is used to connect to the other end of the primary winding of the drive transformer (20). The source of the fifth MOS transistor (Q5) is connected to the reference ground potential (GND). A sixteenth resistor (R16) is connected between the gate and the source of the fifth MOS transistor (Q5).

10. The switch-isolated drive circuit according to claim 9, characterized in that, The drain of the fifth MOS transistor (Q5) is connected to the primary winding of the drive transformer (20) by the first inductor (L1). When the duty cycle of the pulse width modulation signal is lower than a preset value, the first inductor (L1) is used to maintain the excitation current of the primary winding of the drive transformer (20).

11. The switch-transistor isolated drive circuit according to claim 9, characterized in that, The primary drive circuit (10) further includes a voltage absorption unit (13), which includes a sixth diode (D6), a seventeenth resistor (R17), and a first capacitor (C1). The sixth diode (D6) is connected between one end of the seventeenth resistor (R17) and the drain of the fifth MOS transistor (Q5). The other end of the seventeenth resistor (R17) is connected to the drive power supply (Vdrive). The first capacitor (C1) is connected in parallel with the seventeenth resistor (R17).

12. The switch-isolated drive circuit according to claim 1, characterized in that, The drive transformer (20) includes a primary winding and two secondary windings, wherein the turns ratio of the primary winding to the two secondary windings is 1:1:

1.

13. A two-transistor forward converter, characterized in that, The device includes a control chip (U), a first switch, a second switch, and a switch-isolated drive circuit as described in any one of claims 1 to 12. The control chip (U) is used to input a pulse width modulation signal to the switch-isolated drive circuit, and the switch-isolated drive circuit is used to control the first switch and the second switch to be turned on or off according to the pulse width modulation signal.

Images