Forward active clamping driving system

Abstract

The utility model provides a forward active clamping driving system. A control circuit in the forward active clamping driving system comprises a main switch unit, an active clamping switch unit and an active clamping capacitor. The main control chip comprises a first pin and a plurality of second pins. The control end of the active clamping switch unit is electrically connected with the plurality of second pins through the clamping control circuit, the first end of the active clamping switch unit is electrically connected with the second polar plate of the active clamping capacitor, and the second end of the active clamping switch unit is electrically connected with the grounding end; and the clamping control circuit is used for controlling the active clamping switch unit to be switched on when the forward active clamping driving system is in a shutdown state. Thus, through the clamping control circuit, the active clamping switch unit can still be conducted when the system is in the shutdown state, and the situation that the active clamping switch unit is continuously turned off when the system is in the shutdown state, consequently, an active clamping capacitor is continuously charged, and the reliability of the main switch unit or the active clamping switch unit is affected is avoided.

Description

Technical Field

[0001] This utility model relates to the field of power management technology, and in particular to a forward active clamping drive system. Background Technology

[0002] In existing technology, when the main control chip enables or triggers the shutdown logic, the chip determines at the moment of shutdown that the main switch and the active clamp switch need to be shut down simultaneously. In the power-off state, the main switch and the active clamp switch remain off. The active clamp switch only has a unidirectional conduction path with its body diode, and the active clamp capacitor can only be charged, not discharged.

[0003] In a forward active clamp topology for synchronous rectification applications, when the chip has no PWM drive signal output, the primary and secondary sides of the transformer are not controlled by the main switch's on / off state. At the moment of power-off, the primary and secondary sides of the transformer undergo a free resonance process. During this free resonance process, the transformer's magnetizing current is commutated to the primary side through magnetic flux coupling, charging the active clamp capacitor. Due to circuit imperfections and conduction losses, the free resonance process is a decaying resonance with a gradually decreasing amplitude, having minimal impact on circuit reliability. However, when chip enable / disable logic is added, because the active clamp switch remains continuously off, the active clamp capacitor only undergoes a charging process without a discharging process. The voltage energy of the active clamp capacitor continuously accumulates, causing the voltage to continuously increase until it exceeds the breakdown voltage of the main switch and the active clamp switch. Discharge occurs through avalanche breakdown of the main switch and the active clamp switch, leading to repeated critical breakdowns of the main switch and the active clamp switch during repeated switching of the power system, ultimately resulting in thermal damage. Utility Model Content

[0004] This invention provides a forward active clamping drive system. Through the clamping control circuit, the active clamping switch unit can still be turned on when the system is in the off state, which avoids the active clamping switch unit being continuously turned off when the system is in the off state, causing the active clamping capacitor to be continuously charged, resulting in the main switch unit or the active clamping switch unit being broken down.

[0005] In a first aspect, this utility model provides a forward active clamping drive system, including a main control chip, a forward transformer, a control circuit, a self-driven synchronous rectifier circuit, and a clamping control circuit;

[0006] The control circuit is located on the primary side of the forward transformer, and the self-driven synchronous rectifier circuit is located on the secondary side of the forward transformer.

[0007] The control circuit includes a main switch unit, an active clamp switch unit, and an active clamp capacitor;

[0008] The main control chip includes a first pin and multiple second pins;

[0009] The control terminal of the main switch unit is electrically connected to the first pin, the first terminal of the main switch unit is connected to the primary side of the forward transformer via the first plate of the active clamping capacitor, and the second terminal of the main switch unit is electrically connected to the ground terminal.

[0010] The control terminal of the active clamping switch unit is electrically connected to multiple second pins through the clamping control circuit. The first terminal of the active clamping switch unit is electrically connected to the second plate of the active clamping capacitor, and the second terminal of the active clamping switch unit is electrically connected to the ground terminal. The clamping control circuit is used to control the active clamping switch unit to conduct when the forward active clamping drive system is in the off state.

[0011] Optionally, the plurality of the second pins include a first power supply pin, a second power supply pin, an enable determination pin, and a second output pin;

[0012] The clamping control circuit includes a first switch control unit, a second switch control unit, a third switch control unit, and a first diode;

[0013] The first terminal of the first switch control unit is electrically connected to the first power supply pin, the control terminal of the first switch control unit is electrically connected to the enable determination pin, and the second terminal of the first switch control unit is electrically connected to the control terminal of the second switch control unit.

[0014] The first terminal of the second switch control unit is electrically connected to the ground terminal, the second terminal of the second switch control unit is electrically connected to the first terminal of the first diode, and the second terminal of the first diode is electrically connected to the control terminal of the active clamping switch unit.

[0015] The first terminal of the third switch control unit is electrically connected to the second output pin, the second terminal of the third switch control unit is electrically connected to the second terminal of the first diode, and the control terminal of the third switch control unit is electrically connected to the second terminal of the second switch control unit and the second power supply pin, respectively.

[0016] Optionally, the clamping control circuit is used to control the first switch control unit and the second switch control unit to be turned on and the third switch control unit to be turned off according to the control signal of the second pin when the forward active clamping drive system is in the power-off state or the power-on state.

[0017] Optionally, the clamping control circuit is further configured to, when the forward active clamping drive system is in operation, control the first switch control unit and the second switch control unit to close, and control the third switch control unit to turn on, according to the control signal of the second pin.

[0018] Optionally, the first switch control unit includes a first transistor, a first resistor, and a second resistor;

[0019] The first end of the first transistor is electrically connected to the first power supply pin and the first end of the first resistor, respectively. The control end of the first transistor is electrically connected to the enable determination pin through the second resistor. The second end of the first transistor is electrically connected to the control end of the second switch control unit.

[0020] Optionally, the second switch control unit includes a first transistor and a third resistor;

[0021] The control terminal of the first transistor is electrically connected to the second terminal of the first switch control unit, the first terminal of the first transistor is electrically connected to the ground terminal, the second terminal of the first transistor is electrically connected to the first terminal of the first diode, the first terminal of the third resistor is electrically connected to the control terminal of the first transistor, and the second terminal of the third resistor is electrically connected to the first terminal of the first transistor.

[0022] Optionally, the third switch control unit includes a second transistor and a fourth resistor;

[0023] The control terminal of the second transistor is electrically connected to the first terminal of the fourth resistor and the first terminal of the first diode, the first terminal of the second transistor is electrically connected to the second output pin, and the second terminal of the second transistor is electrically connected to the second terminal of the first diode.

[0024] The second end of the fourth resistor is electrically connected to the second power supply pin.

[0025] Optionally, the main switch unit includes a main switch;

[0026] The active clamping switch unit includes an active clamping switch, a first capacitor, a second diode, and a fifth resistor;

[0027] The control terminal of the active clamp switch is electrically connected to the first plate of the first capacitor, the second terminal of the second diode, and the first terminal of the fifth resistor, respectively. The first terminal of the active clamp switch, the second terminal of the fifth resistor, and the first terminal of the second diode are all electrically connected to the ground terminal. The second terminal of the active clamp switch is electrically connected to the second plate of the active clamp capacitor.

[0028] The second plate of the first capacitor is electrically connected to the second terminal of the first diode.

[0029] Optionally, the main switch includes an NMOS transistor, and the active clamp switch includes a PMOS transistor.

[0030] Optionally, the self-driven synchronous rectifier circuit includes a first synchronous rectifier diode, a second synchronous rectifier diode, an inductor, and a second capacitor;

[0031] The secondary side of the forward transformer includes a first secondary side and a second secondary side;

[0032] The control terminal of the first synchronous rectifier is electrically connected to the first secondary side and the second terminal of the second synchronous rectifier, respectively. The first terminal of the first synchronous rectifier is electrically connected to the first terminal of the second synchronous rectifier and the first plate of the second capacitor, respectively. The second terminal of the first synchronous rectifier is electrically connected to the control terminal of the second synchronous rectifier and the second secondary side, respectively. The second terminal of the second synchronous rectifier is also electrically connected to the first terminal of the inductor, and the second terminal of the inductor is electrically connected to the second plate of the second capacitor and the output terminal of the forward active clamp drive system, respectively.

[0033] The primary winding of the forward transformer includes a first primary winding and a second primary winding. The first primary winding is electrically connected to the input terminal of the forward active clamp drive system, and the second primary winding is electrically connected to the first plate of the active clamp capacitor.

[0034] The forward active clamping drive system provided in this embodiment includes a main control chip, a forward transformer, a control circuit, a self-driven synchronous rectifier circuit, and a clamping control circuit. The control circuit is located on the primary side of the forward transformer, and the self-driven synchronous rectifier circuit is located on the secondary side of the forward transformer. The control circuit includes a main switching unit, an active clamping switch unit, and an active clamping capacitor. The main control chip includes a first pin and multiple second pins. The control terminal of the main switching unit is electrically connected to the first pin. The first terminal of the main switching unit and the first plate of the active clamping capacitor are connected to the primary side of the forward transformer, and the second terminal of the main switching unit is electrically connected to ground. The control terminal of the active clamping switch unit is electrically connected to multiple second pins through the clamping control circuit. The first terminal of the active clamping switch unit is electrically connected to the second plate of the active clamping capacitor, and the second terminal of the active clamping switch unit is electrically connected to ground. The clamping control circuit is used to control the active clamping switch unit to conduct when the forward active clamping drive system is in a powered-off state. In this way, the clamping control circuit enables the active clamping switch unit to remain on when the system is off, allowing the active clamping capacitor to charge and discharge when the system is off. This prevents the active clamping switch unit from being continuously off when the system is off, which would cause the active clamping capacitor to continuously charge and affect the reliability of the main switch unit or the active clamping switch unit. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the structure of a forward active clamping drive system provided in an embodiment of the present invention;

[0036] Figure 2 This is a schematic diagram of another forward active clamping drive system provided in this embodiment of the present invention;

[0037] Figure 3 This is a schematic diagram of another forward active clamping drive system provided in this embodiment of the utility model;

[0038] Figure 4 This is a schematic diagram of the driving timing of a forward active clamping drive system provided in an embodiment of this utility model. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of this utility model clearer, the technical solutions of this utility model will be fully described below with reference to the accompanying drawings of the embodiments of this utility model and through specific implementation methods. Obviously, the described embodiments are only some, not all, of the embodiments of this utility model. All other embodiments obtained by those skilled in the art based on the embodiments of this utility model without creative effort fall within the protection scope of this utility model.

[0040] Figure 1 This is a schematic diagram of a forward active clamping drive system provided in an embodiment of this utility model. See also... Figure 1 The forward active clamp drive system includes a main control chip 10, a forward transformer 20, a control circuit 30, a self-driven synchronous rectifier circuit 40, and a clamp control circuit 50. The control circuit 30 is located on the primary side of the forward transformer 20, and the self-driven synchronous rectifier circuit 40 is located on the secondary side of the forward transformer 20. The control circuit 30 includes a main switch unit 310, an active clamp switch unit 320, and an active clamp capacitor 330. The main control chip 10 includes a first pin 110 and multiple second pins 120. The control terminal of the main switch unit 310 is electrically connected to the first pin 110, the first terminal of the main switch unit 310 is connected to the first plate of the active clamp capacitor 330 on the primary side of the forward transformer 20, and the second terminal of the main switch unit 310 is electrically connected to the ground terminal GND. The control terminal of the active clamping switch unit 320 is electrically connected to multiple second pins 120 through the clamping control circuit 50. The first terminal of the active clamping switch unit 320 is electrically connected to the second plate of the active clamping capacitor 330, and the second terminal of the active clamping switch unit 320 is electrically connected to the ground terminal GND. The clamping control circuit 50 is used to control the active clamping switch unit 320 to conduct when the forward active clamping drive system is in the off state.

[0041] Specifically, such as Figure 1In the illustrated embodiment, the main control chip 10 can be a JWH3530 chip, but this embodiment of the present invention is not limited to this. In other embodiments, the main control chip 10 can also be a similar PWM main control chip. The control circuit 30 is located on the primary side of the forward transformer 20. The control circuit 30 includes a main switching unit 310, an active clamping switching unit 320, and an active clamping capacitor 330. The main switching unit 310 controls the switching on and off of the primary side of the forward transformer 20 through high-frequency switching action, thereby regulating the energy transfer on the secondary side. When the main switching unit 310 is on, the input voltage is applied to the primary side of the forward transformer 20, and energy is transferred to the secondary side through the primary side of the forward transformer 20. When the main switching unit 310 is off, it blocks the primary side current and stops energy transfer. The active clamping switching unit 320 is used to turn on when the main switch is off, transferring the stored energy to the active clamping capacitor 330 to avoid voltage spikes and simultaneously help the forward transformer 20 magnetically reset. The active clamping capacitor 330 absorbs this energy, acting as a buffer, and also participates in the soft-switching process to reduce switching losses. The main control chip 10 sends control signals to the control circuit 30 to control the operating state of the forward active clamping drive system. For example, the main control chip 10 includes a first pin 110, which includes a first output pin OUTM. The control terminal of the main switching unit 310 is electrically connected to the first pin 110. The first terminal of the main switching unit 310 is connected to the primary side of the forward transformer 20 via the first plate of the active clamping capacitor 330, and the second terminal of the main switching unit 310 is electrically connected to the ground terminal GND. Thus, the control signal from the first pin 110 controls the on / off state of the main switching unit 310.

[0042] The inventors discovered that in existing self-driven synchronous rectification schemes, the control terminal of the active clamping switch unit is directly electrically connected to a second output pin of the main control chip. Furthermore, the main control chip requires that both the active clamping switch unit and the main switch unit be synchronously turned off when the main control chip enables or triggers its shutdown logic. In this case, the active clamping switch unit only has a unidirectional conduction path via its body diode, and the active clamping capacitor can only charge, not discharge. Moreover, when the main control chip has no PWM drive signal output, the primary and secondary sides of the forward transformer are not controlled by the main switch unit. At the moment of power-off, the self-driven synchronous rectification circuit located on the secondary side of the forward transformer enters a free resonance process. During this free resonance process, the excitation current of the forward transformer is commutated to the primary side through magnetic flux coupling, charging the active clamping capacitor. Due to the non-ideal nature of the circuit and the presence of conduction losses, the free resonance process is a decaying resonance with a gradually decreasing amplitude, having a relatively small impact on circuit reliability. However, when chip enable / disable logic is added, because the active clamp switch is constantly off, the active clamp capacitor only undergoes a charging process without a discharging process. The voltage energy in the active clamp capacitor continuously accumulates, causing the voltage to continuously increase until it exceeds the breakdown voltage of the main switch and the active clamp switch. This results in discharge through avalanche breakdown of the main switch and the active clamp switch. Consequently, when the power system repeatedly switches on and off, the main switch and the active clamp switch repeatedly reach the critical breakdown point, ultimately leading to thermal damage.

[0043] Therefore, this embodiment of the present invention further includes a main control chip 10 comprising multiple second pins 120, and a clamping control circuit 50 is provided at the control terminals of the multiple second pins 120 and the active clamping switch unit 320. The clamping control circuit 50 is used to control the potential of the control terminal of the active clamping switch unit 320 based on the control signals output from the multiple second pins 120. Thus, at least when the forward active clamping drive system is in the off state, the active clamping switch unit 320 is controlled to be turned on. At this time, the active clamping capacitor 330 can be discharged through the active clamping switch unit 320. That is, when the forward active clamping drive system is in the off state, the active clamping capacitor 330 has both a charging process and a discharging process, thereby avoiding the active clamping switch unit from being continuously turned off when the system is in the off state, causing the active clamping capacitor to be continuously charged, which would affect the reliability of the main switch unit or the active clamping switch unit.

[0044] It is understandable that the main switch unit 310 includes an NMOS transistor and the active clamp switch unit 320 includes a PMOS transistor. Due to the limitations of their operation, the NMOS transistor of the main switch unit 310 and the PMOS transistor of the active clamp switch unit 320 cannot be turned on simultaneously during operation.

[0045] In summary, the forward active clamping drive system provided in this embodiment includes a main control chip, a forward transformer, a control circuit, a self-driven synchronous rectifier circuit, and a clamping control circuit. The control circuit is located on the primary side of the forward transformer, and the self-driven synchronous rectifier circuit is located on the secondary side of the forward transformer. The control circuit includes a main switching unit, an active clamping switch unit, and an active clamping capacitor. The main control chip includes a first pin and multiple second pins. The control terminal of the main switching unit is electrically connected to the first pin. The first terminal of the main switching unit and the first plate of the active clamping capacitor are connected to the primary side of the forward transformer, and the second terminal of the main switching unit is electrically connected to the ground terminal. The control terminal of the active clamping switch unit is electrically connected to multiple second pins through the clamping control circuit. The first terminal of the active clamping switch unit is electrically connected to the second plate of the active clamping capacitor, and the second terminal of the active clamping switch unit is electrically connected to the ground terminal. The clamping control circuit is used to control the active clamping switch unit to conduct when the forward active clamping drive system is in the off state. In this way, the clamping control circuit enables the active clamping switch unit to remain on when the system is off, allowing the active clamping capacitor to charge and discharge when the system is off. This prevents the active clamping switch unit from being continuously off when the system is off, which would cause the active clamping capacitor to continuously charge and affect the reliability of the main switch unit or the active clamping switch unit.

[0046] Optionally, based on the above embodiments, Figure 2 This is a schematic diagram of another forward active clamping drive system provided in an embodiment of this utility model. See also... Figure 2 The multiple second pins 120 include a first power supply pin REFA, a second power supply pin VCC, an enable determination pin FLT / SD, and a second output pin OUTA. The clamping control circuit 50 includes a first switch control unit 510, a second switch control unit 520, a third switch control unit 530, and a first diode D1. The first terminal of the first switch control unit 510 is electrically connected to the first power supply pin REFA, and its control terminal is electrically connected to the enable determination pin FLT / SD. The second terminal of the first switch control unit 510 is electrically connected to the control terminal of the second switch control unit 520. The first terminal of the second switch control unit 520 is electrically connected to ground GND, and its second terminal is electrically connected to the first terminal of the first diode D1. The second terminal of the first diode D1 is electrically connected to the control terminal of the active clamping switch unit 320. The first terminal of the third switch control unit 530 is electrically connected to the second output pin OUTA, and its second terminal is electrically connected to the second terminal of the second switch control unit 520 and the second power supply pin VCC, respectively.

[0047] Specifically, the clamping control circuit 50 is used to control the first switch control unit 510 and the second switch control unit 520 to turn on and the third switch control unit 530 to turn off when the forward active clamping drive system is in a power-off or power-on state, based on the control signal from the second pin 120. The clamping control circuit 50 is also used to control the first switch control unit 510 and the second switch control unit 520 to turn off and the third switch control unit 530 to turn on when the forward active clamping drive system is in a working state, based on the control signal from the second pin 120.

[0048] For example, when the forward active clamp drive system is in the startup state, the power supply signal provided by the second power supply pin VCC of the main control chip 10 is established before the enable determination signal. After the power supply signal provided by the second power supply pin VCC is established, the first power supply pin REFA normally outputs 5V. Since the enable determination signal is not established, the main control chip 10 determines that the startup condition is not met, and the signal output by the enable determination pin FLT / SD is low. At this time, the first switch control unit 510 and the second switch control unit 520 are turned on. Through the pull-down of the second switch control unit 520 and the first diode D1, the output of the clamp control circuit 50 is kept low (that is, the control terminal of the active clamp switch unit 320 is kept low). Due to the capacitor in the active clamp switch unit 320 (the active clamp switch unit 320 includes an active clamp switch and...), Since the capacitor at the control terminal of the active clamp switch is not charged, the voltage across the capacitor in the active clamp switch unit 320 is 0, the gate-source voltage of the active clamp switch is 0, and the active clamp switch remains off. Furthermore, due to the pull-down of the second switch control unit 520, the control terminal voltage of the third switch control unit 530 is 0, and the third switch control unit 530 is off. Thus, after power is established, because the third switch control unit 530 is off, even if the second output pin OUTA outputs a high level, the voltage output of the second power supply pin VCC will not be pulled down due to the pull-down of the third switch control unit 530, the first diode D1, and the second switch control unit 520, thereby preventing the power supply voltage from being pulled down. This ensures that the setting of the clamp control circuit 50 does not affect the normal operation of the forward active clamp drive system.

[0049] When the forward active clamp drive system is in operation, the first power supply pin REFA normally outputs 5V. After the enable condition is met, the signal output by the enable determination pin FLT / SD flips to a high level close to 5V. The first switch control unit 510 is turned off, the second switch control unit 520 is turned off, and the first diode D1 is cut off. This does not affect the control terminal voltage of the active clamp switch unit 320 or the control terminal voltage of the third switch control unit 530. Furthermore, the control terminal of the third switch control unit 530 and the first terminal of the first diode D1 are both connected to the 12V voltage of the second power supply pin VCC. Consequently, the third switch control unit 530 is turned on. At this time, when the second output pin OUTA outputs a high level, the high-level signal output by the second output pin OUTA is directly transmitted to the capacitor in the active clamping switch unit 320 to charge the capacitor. When the second output pin OUTA outputs a low level, the capacitor in the active clamping switch unit 320 is discharged through the third switch control unit 530. The negative voltage drives the active clamping switch unit 320 to conduct, demagnetizing the primary side of the forward transformer 20. This ensures that the setting of the clamping control circuit 50 does not affect the normal operation of the forward active clamping drive system.

[0050] When the forward active clamp drive system is in the power-off state, the power supply of the main control chip 10 remains normal, and the power supply pin REFA outputs 5V normally. Due to the undervoltage input during the power-off process, the enable judgment signal determines that the start-up condition is not met, and the signal output of the enable judgment pin FLT / SD flips to a low level. At this time, the first switch control unit 510 and the second switch control unit 520 are turned on. Through the pull-down of the second switch control unit 520 and the first diode D1, the output of the clamp control circuit 50 remains at a low level (i.e., the control terminal of the active clamp switch unit 320 remains at a low level). Since the capacitor in the active clamp switch unit 320 has been charged during normal operation, the voltage across the capacitor in the active clamp switch unit 320 is the drive voltage VCC-VG, where VG is the gate-source on-state voltage drop of the third switch control unit 530. The gate-source voltage of the active clamp switch in unit 320 is -(VCC-VG). The active clamp switch unit 320 is continuously turned on. Due to the pull-down of the second switch control unit 520, the control terminal voltage of the third switch control unit 530 is 0, and the third switch control unit 530 is turned off. The above operation process ensures that during the shutdown process, even if the second output pin OUTA outputs a shutdown signal (when the active clamp switch is a PMOS transistor, the shutdown signal is high level), the active clamp switch unit 320 can still be turned on. This allows the active clamp capacitor 330 to be charged and discharged when the system is in the shutdown state, thereby avoiding the active clamp switch unit 320 being continuously turned off when the system is in the shutdown state, which would cause the active clamp capacitor 330 to be continuously charged and affect the reliability of the main switch unit 310 or the active clamp switch unit 320.

[0051] Optionally, based on the above embodiments, Figure 3 This is a schematic diagram of another forward active clamping drive system provided in this embodiment of the present invention. Figure 4 This is a schematic diagram of the driving timing of a forward active clamping drive system provided in an embodiment of this utility model. See also... Figure 3 and Figure 4 The first switch control unit includes a first transistor M1, a first resistor R1, and a second resistor R2. The first terminal of the first transistor M1 is electrically connected to the power supply pin REFA and the first terminal of the first resistor R1. The control terminal of the first transistor M1 is electrically connected to the enable determination pin FLT / SD through the second resistor R2. The second terminal of the first transistor M1 is electrically connected to the control terminal of the second switch control unit (the first transistor M2). The second switch control unit includes a first transistor M2 and a third resistor R3. The control terminal of the first transistor M2 is electrically connected to the second terminal of the first switch control unit (the second terminal of the first transistor M1). The first terminal of the first transistor M2 is electrically connected to the ground terminal GND. The second terminal of the first transistor M2 is electrically connected to the first terminal of the first diode D1. The first terminal of the third resistor R3 is electrically connected to the control terminal of the first transistor M2, and the second terminal of the third resistor R3 is electrically connected to the first terminal of the first transistor M2.

[0052] The third switch control unit includes a second transistor M3 and a fourth resistor R4. The control terminal of the second transistor M3 is electrically connected to the first terminal of the fourth resistor R4 and the first terminal of the first diode D1, respectively. The first terminal of the second transistor M3 is electrically connected to the second output pin VCC, the second terminal of the second transistor M2 is electrically connected to the second terminal of the first diode D1, and the second terminal of the fourth resistor R4 is electrically connected to the second power supply pin VCC. In addition, the main switch unit includes a main switch Q1, and the active clamp switch unit includes an active clamp switch Q2, a first capacitor C1, a second diode D2, and a fifth resistor R5. The control terminal of the active clamp switch Q2 is electrically connected to the first plate of the first capacitor C1, the second terminal of the second diode D2, and the first terminal of the fifth resistor R5, respectively. The first terminals of the active clamp switch Q2, the fifth resistor R5, and the second diode D2 are all electrically connected to the ground terminal GND. The second terminal of the active clamp switch Q2 is electrically connected to the second plate of the active clamp capacitor 330, and the second plate of the first capacitor C1 is electrically connected to the second terminal of the first diode D1.

[0053] Based on the above connection relationships, Figure 3 The example is illustrated by using a main switch Q1 comprising an NMOS transistor and an active clamping switch Q2 comprising a PMOS transistor. Figure 4When the first output pin OUTM outputs a high level, the main switch Q1 is turned on; when the first output pin OUTM outputs a low level, the main switch Q1 is turned off. The control terminal of the active clamp switch Q2 is turned on when it is low and turned off when it is high.

[0054] For example, such as Figure 4 As shown, when the forward active clamp drive system is in the startup state, i.e., during time period t1, the power supply provided by the second power supply pin VCC of the main control chip 10 is established before the enable determination signal is established. After the power supply provided by the second power supply pin VCC is established, the first power supply pin REFA normally outputs 5V. Since the enable determination signal is not established, the main control chip 10 determines that the startup condition is not met, and the signal output by the enable determination pin FLT / SD remains low. At this time, the second resistor R2 is positive at the top and negative at the bottom, the base current flows through the first transistor M1, and the first transistor M1 is turned on. The gate of the first transistor M2 is the 5V voltage output by the power supply pin REFA, and the first transistor M2 is turned on. Through the first transistor M2 and the first diode D1, the voltage output to the second plate of the first capacitor C1 is ( When OUTA2 remains low, the voltage across the first capacitor C1 is 0 because the first capacitor C1 is not charged. The gate-source voltage of the active clamp switch Q2 is 0, so the active clamp switch Q2 remains off. Furthermore, due to the pull-down of the first transistor M2, the gate voltage of the second transistor M3 is 0, so the second transistor M3 is off. Thus, after the power supply is established, since the second transistor M3 is off, the branch of the second transistor M and the first diode D1 is disconnected. Therefore, even if the second output pin OUTA outputs a high level, the voltage output of the second power supply pin VCC will not be pulled down due to the pull-down of the first diode D1 and the first transistor M2, thereby avoiding the power supply voltage being pulled down and ensuring that the setting of the clamp control circuit 50 does not affect the normal operation of the forward active clamp drive system.

[0055] When the forward active clamp drive system is in operation, i.e. during time period t2, the power supply pin REFA outputs 5V normally. After the enable condition is met, the signal output by the enable pin FLT / SD flips to a high level close to 5V. The voltage across the second resistor R2 is 0, the first transistor M1 is turned off, the first transistor M2 is turned off, the first diode D1 is cut off, and the branch of the first diode D1 (i.e., the branch containing the first transistor M1, the first transistor M2, and the first diode D1) is floating. This does not affect the voltage (i.e., OUTA2) input to the second plate of the first capacitor C1 and the gate voltage of the second transistor M3. At this time, the control terminal of the second transistor M3 and the first terminal of the first diode D1 are both supplied with 12V voltage by the second power supply pin VCC. The second transistor M3 is turned on. The second transistor M3 is connected to the second output pin OUTA and the second plate (OUTA2) of the first capacitor C1. When the second output pin OUTA outputs a low level, the high level signal output by the second output pin OUTA is directly transmitted to the second plate (OUTA2) of the first capacitor C1 to charge the first capacitor C1. When the second output pin OUTA outputs a low level, the first capacitor C1 is discharged through the first diode D1 and the fifth resistor R5. That is, the active clamping switch Q2 is turned on by the negative voltage to demagnetize the primary side of the forward transformer 20, thereby ensuring that the setting of the clamping control circuit 50 does not affect the normal operation of the forward active clamping drive system.

[0056] When the forward active clamp drive system is in the off state, i.e., during time period t3, the power supply of the main control chip 10 remains normal, and the first power supply pin REFA outputs 5V normally. Due to the undervoltage input during the shutdown process, the enable judgment signal determines that the start-up condition is not met, and the signal output by the enable judgment pin FLT / SD flips to a low level. At this time, the second resistor R2 is positive at the top and negative at the bottom, the base current flows through the first transistor M1, and the first transistor M1 is turned on. The gate of the first transistor M2 is the 5V voltage output by the power supply pin REFA, and the first transistor M2 is turned on, through the first... Transistor M2 is pulled down by the first diode D1, and the voltage (OUTA2) output to the second plate of the first capacitor C1 remains low. Since the first capacitor C1 has been charged during normal operation, the voltage across the first capacitor C1 is the driving voltage VCC-VG. VG is the gate-source on-state voltage drop of the third switch control unit 530. The gate-source voltage of the active clamp switch Q2 is -(VCC-VG), and the active clamp switch Q2 is continuously turned on. Furthermore, due to the pull-down of the first transistor M2, the gate voltage of the second transistor M3 is 0, and the second transistor M3 is turned off. Thus, the above working process ensures that during the shutdown process, even if the second output pin OUTA outputs a shutdown signal (when the active clamp switch is a PMOS transistor, the shutdown signal is high level), the active clamp switch Q2 can still be turned on, allowing the active clamp capacitor 330 to be charged and discharged when the system is in the shutdown state. This avoids the active clamp switch Q2 being continuously turned off when the system is in the shutdown state, which would cause the active clamp capacitor 330 to be continuously charged and affect the reliability of the main switch unit 310 or the active clamp switch unit 320.

[0057] Optionally, based on the above embodiments, see also... Figure 3 The self-driven synchronous rectifier circuit includes a first synchronous rectifier tube M5, a second synchronous rectifier tube M6, an inductor L, and a second capacitor C2. The secondary side of the forward transformer 20 includes a first secondary side 210a and a second secondary side 210b. The control terminal of the first synchronous rectifier tube M5 is electrically connected to the second terminal of the first secondary side 210a and the second terminal of the second synchronous rectifier tube M6. The first terminal of the first synchronous rectifier tube M5 is electrically connected to the first terminal of the second synchronous rectifier tube M6 and the first plate of the second capacitor C2. The second terminal of the first synchronous rectifier tube M5 is electrically connected to the control terminal of the second synchronous rectifier tube M6 and the second secondary side 210b. The second terminal of the second synchronous rectifier tube M6 is also electrically connected to the first terminal of the inductor L. The second terminal of the inductor L is electrically connected to the second plate of the second capacitor C2 and the output terminal of the forward active clamp drive system. The primary side of the forward transformer 20 includes a first primary side 220a and a second primary side 220b. The first primary side 220a is electrically connected to the input terminal of the forward active clamp drive system, and the second primary side 220b is electrically connected to the first plate of the active clamp capacitor 330.

[0058] Specifically, such as Figure 3 As shown, when designing a self-driven synchronous rectification scheme, if the main control chip 10 has no PWM drive signal output, the primary and secondary sides of the forward transformer 20 are not controlled by the main switch Q1's on or off state. At the instant the power is turned off, there is unreleased energy on the second capacitor C2 and inductor L, causing them to enter a self-driven synchronous rectification oscillation state. Since the secondary side of the forward transformer 20, the first synchronous rectifier M5, and the second synchronous rectifier M6 are in the self-driven synchronous rectification circuit, the resonance process of the second capacitor C2 and inductor L will also excite and demagnetize the forward transformer 20. This process is called the free resonance process. When the first secondary side 210a is at a high level, the first synchronous rectifier M5 is turned on, and the second capacitor C2 and inductor L excite the secondary side of the forward transformer. When the resonant period is more than halfway through, the first secondary side 210a is at a low level, the first synchronous rectifier M5 is turned off, the second secondary side 210b rises to a high level, the second synchronous rectifier M6 is turned on, and the excitation current of the forward transformer is commutated to the primary side through magnetic flux coupling to charge the active clamping capacitor 330.

[0059] Note that the above description is merely a preferred embodiment of the present invention and the technical principles employed. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and various obvious changes, readjustments, combinations, and substitutions can be made without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in detail through the above embodiments, the present invention is not limited to the above embodiments. Many other equivalent embodiments may be included without departing from the concept of the present invention, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. A forward active clamping drive system, characterized in that, Includes main control chip, forward transformer, control circuit, self-driven synchronous rectifier circuit and clamping control circuit; The control circuit is located on the primary side of the forward transformer, and the self-driven synchronous rectifier circuit is located on the secondary side of the forward transformer. The control circuit includes a main switch unit, an active clamp switch unit, and an active clamp capacitor; The main control chip includes a first pin and multiple second pins; The control terminal of the main switch unit is electrically connected to the first pin, the first terminal of the main switch unit is connected to the primary side of the forward transformer via the first plate of the active clamping capacitor, and the second terminal of the main switch unit is electrically connected to the ground terminal. The control terminal of the active clamping switch unit is electrically connected to multiple second pins through the clamping control circuit. The first terminal of the active clamping switch unit is electrically connected to the second plate of the active clamping capacitor, and the second terminal of the active clamping switch unit is electrically connected to the ground terminal. The clamping control circuit is used to control the active clamping switch unit to conduct when the forward active clamping drive system is in the off state.

2. The forward active clamping drive system according to claim 1, characterized in that, The plurality of second pins include a first power supply pin, a second power supply pin, an enable determination pin, and a second output pin; The clamping control circuit includes a first switch control unit, a second switch control unit, a third switch control unit, and a first diode; The first terminal of the first switch control unit is electrically connected to the first power supply pin, the control terminal of the first switch control unit is electrically connected to the enable determination pin, and the second terminal of the first switch control unit is electrically connected to the control terminal of the second switch control unit. The first terminal of the second switch control unit is electrically connected to the ground terminal, the second terminal of the second switch control unit is electrically connected to the first terminal of the first diode, and the second terminal of the first diode is electrically connected to the control terminal of the active clamping switch unit. The first terminal of the third switch control unit is electrically connected to the second output pin, the second terminal of the third switch control unit is electrically connected to the second terminal of the first diode, and the control terminal of the third switch control unit is electrically connected to the second terminal of the second switch control unit and the second power supply pin, respectively.

3. The forward active clamping drive system according to claim 2, characterized in that, The clamping control circuit is used to control the first switch control unit and the second switch control unit to be turned on and the third switch control unit to be turned off, based on the control signal of the second pin, when the forward active clamping drive system is in a power-off state or a power-on state.

4. The forward active clamping drive system according to claim 2, characterized in that, The clamping control circuit is also used to control the first switch control unit and the second switch control unit to close and control the third switch control unit to turn on when the forward active clamping drive system is in operation, based on the control signal of the second pin.

5. The forward active clamping drive system according to claim 2, characterized in that, The first switch control unit includes a first transistor, a first resistor, and a second resistor; The first end of the first transistor is electrically connected to the first power supply pin and the first end of the first resistor, respectively. The control end of the first transistor is electrically connected to the enable determination pin through the second resistor. The second end of the first transistor is electrically connected to the control end of the second switch control unit.

6. The forward active clamping drive system according to claim 2, characterized in that, The second switch control unit includes a first transistor and a third resistor; The control terminal of the first transistor is electrically connected to the second terminal of the first switch control unit, the first terminal of the first transistor is electrically connected to the ground terminal, the second terminal of the first transistor is electrically connected to the first terminal of the first diode, the first terminal of the third resistor is electrically connected to the control terminal of the first transistor, and the second terminal of the third resistor is electrically connected to the first terminal of the first transistor.

7. The forward active clamping drive system according to claim 2, characterized in that, The third switch control unit includes a second transistor and a fourth resistor; The control terminal of the second transistor is electrically connected to the first terminal of the fourth resistor and the first terminal of the first diode, the first terminal of the second transistor is electrically connected to the second output pin, and the second terminal of the second transistor is electrically connected to the second terminal of the first diode. The second end of the fourth resistor is electrically connected to the second power supply pin.

8. The forward active clamping drive system according to claim 2, characterized in that, The main switch unit includes a main switch; The active clamping switch unit includes an active clamping switch, a first capacitor, a second diode, and a fifth resistor; The control terminal of the active clamp switch is electrically connected to the first plate of the first capacitor, the second terminal of the second diode, and the first terminal of the fifth resistor, respectively. The first terminal of the active clamp switch, the second terminal of the fifth resistor, and the first terminal of the second diode are all electrically connected to the ground terminal. The second terminal of the active clamp switch is electrically connected to the second plate of the active clamp capacitor. The second plate of the first capacitor is electrically connected to the second terminal of the first diode.

9. The forward active clamping drive system according to claim 8, characterized in that, The main switch includes an NMOS transistor, and the active clamp switch includes a PMOS transistor.

10. The forward active clamping drive system according to claim 1, characterized in that, The self-driven synchronous rectifier circuit includes a first synchronous rectifier diode, a second synchronous rectifier diode, an inductor, and a second capacitor; The secondary side of the forward transformer includes a first secondary side and a second secondary side; The control terminal of the first synchronous rectifier is electrically connected to the first secondary side and the second terminal of the second synchronous rectifier, respectively. The first terminal of the first synchronous rectifier is electrically connected to the first terminal of the second synchronous rectifier and the first plate of the second capacitor, respectively. The second terminal of the first synchronous rectifier is electrically connected to the control terminal of the second synchronous rectifier and the second secondary side, respectively. The second terminal of the second synchronous rectifier is also electrically connected to the first terminal of the inductor, and the second terminal of the inductor is electrically connected to the second plate of the second capacitor and the output terminal of the forward active clamp drive system, respectively. The primary winding of the forward transformer includes a first primary winding and a second primary winding; The first primary side is electrically connected to the input terminal of the forward active clamp drive system, and the second primary side is electrically connected to the first plate of the active clamp capacitor.

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