Magnetic reset and leakage inductance voltage oscillation absorption circuit for forward converter

Abstract

The invention provides a magnetic reset and leakage inductance voltage oscillation absorption circuit for a forward converter, a third winding reset circuit and an RCD clamping reset circuit cooperate to reset a transformer, and a third auxiliary winding in the third winding reset circuit is connected in series with a first diode to be connected with two ends of an input power supply; and a second capacitor in the RCD clamping reset circuit is connected to the two ends of the active switch through a second diode. And the other end of the primary winding of the transformer is connected with the first resistor and the positive electrode of the input power supply. The secondary winding is connected with an output circuit and is connected with two ends of a load through a third diode. The RCD clamping reset circuit absorbs peak voltage of leakage inductance, the third auxiliary winding clamps the maximum voltage of a capacitor in the RCD buffer, and magnetic energy is actively reset at the same time. According to the circuit, the energy utilization rate of the system is improved to a certain extent while the leakage inductance peak voltage is absorbed.

Description

Technical Field

[0001] The invention belongs to the field of forward converters and relates to a magnetic reset and leakage inductance voltage oscillation absorption circuit for a forward converter. Background Art

[0002] The single-ended forward converter is widely used due to its simple circuit, low cost, good efficiency, low voltage stress, good isolation performance and dynamic response. In order to make the forward converter work stably, the transformer needs to be reset at the end of each cycle. At the same time, the switch voltage oscillation caused by leakage inductance needs to be solved to reduce the rated voltage requirement of the switch. This oscillation is caused by the energy stored in the winding leakage inductance when the switch is turned off, which charges the switch tube capacitance Cds, resulting in a very high voltage spike at both ends of the switch. After the leakage inductance energy is completely released, the voltage at both ends of the switch reaches a steady-state value.

[0003] In response to the above problems, commonly used reset methods include reverse current reset, DC reset, pulse reset, third winding reset, RCD clamp reset, resonant reset, active clamp reset, etc. The DC reset in the reverse current reset can utilize the full flux swing, but requires an additional reset power supply, and the continuous output current during the operation of the pulse power system results in large losses and low overall system efficiency. Unlike DC reset, the pulse reset method only works during the reset period, and has more advantages in energy utilization and system dynamic response rate, but it requires a complex control circuit. The third winding reset circuit is as follows: Figure 1 As shown in the figure, it is simple, efficient, mature and reliable, and can basically achieve lossless reset, so it is widely used. The above reverse current reset form is to demagnetize the primary winding through electromagnetic induction, so the energy stored in the transformer leakage inductance during the switch-on period is not restored. The RCD reset circuit structure is simple, as shown in the figure below. Figure 2 As shown in the figure, the circuit makes the leakage inductance energy first to the clamp capacitor C s Charging to suppress voltage spikes. However, the excitation energy and leakage inductance energy are all consumed in the clamping resistor, affecting the system efficiency; and if the clamping capacitor is not selected correctly, it will cause greater switch stress. Resonant reset technology can enable the switch tube to achieve soft switching, but the voltage stress of the switch tube is high. Active clamping technology can extend the duty cycle and is suitable for occasions with a wide input voltage range. Resonant reset, active clamping reset and other technologies that add active switches to recover excitation energy and absorb leakage inductance spike voltage have certain advantages in specific applications, but adding active switches will increase costs and complicate the drive circuit. These traditional reset circuits either have unsatisfactory leakage inductance spike voltage absorption capabilities and large losses, or have high circuit costs and complex drive circuits. Summary of the invention

[0004] 1. Technical problems to be solved: The traditional reset circuit has the problems of poor leakage inductance peak voltage absorption capability, large loss, high circuit cost and complex driving circuit.

[0005] 2. Technical solution: In order to solve the above problems, the present invention provides a magnetic reset and leakage inductance voltage oscillation absorption circuit for a forward converter, wherein the third winding reset circuit and the RCD clamp reset circuit cooperate to reset the transformer, wherein the third auxiliary winding in the third winding reset circuit is connected in series with the first diode to the two ends of the input power supply, and the RCD clamp reset circuit comprises a first resistor, a second capacitor, and a second diode, wherein the second capacitor is connected to the two ends of the active switch via the second diode. The transformer has a primary winding, a secondary winding, and a third auxiliary winding, wherein one end of the primary winding is connected to the active switch and one end connected to the second diode, and the other end is connected to one end of the first resistor and the positive electrode of the input power supply, wherein the secondary winding is connected to the output circuit and is connected to the two ends of the load via the third diode, and the four diodes are connected in parallel to the two ends of the load to flow the energy on the load side after the active switch is turned off.

[0006] Further, the turns ratio of the primary winding and the secondary winding is set to N:mN according to the required output voltage, wherein m and N are both integers greater than 0.

[0007] Further, the primary winding and the secondary winding are wound in the same direction, and the primary winding and the third auxiliary winding are wound in opposite directions.

[0008] Furthermore, the turns ratio of the primary winding and the third auxiliary winding is the same.

[0009] Furthermore, the active switch is used to control the excitation time of the primary excitation inductor, that is, the output pulse width.

[0010] Furthermore, the capacity of the second capacitor is greater than the output capacitance of the active switch.

[0011] Furthermore, the excess energy charged by the second capacitor in each cycle is discharged before the next cycle starts until the voltage of the second capacitor is equal to the input power supply voltage, thereby reducing the peak value of the clamping capacitor voltage.

[0012] Furthermore, when the magnetic flux changes slightly during the magnetization process, the third auxiliary winding is used as a backup reset, and the RCD clamp reset circuit resets the residual energy of the unidirectional transformer while suppressing the turn-off ringing and alleviating the voltage stress of the active switch; when the magnetic flux changes significantly during the magnetization process, when the RCD clamp reset circuit acts alone, the voltage of the second capacitor will exceed the withstand voltage of the active switch, so the third auxiliary winding inductively coupled with the primary winding of the transformer is used to clamp the maximum voltage of the second capacitor in the RCD clamp reset circuit, and at the same time, the third auxiliary winding N3 actively resets the magnetic energy.

[0013] Furthermore, the magnetization process with small flux change is divided into five working stages: t 1-1 -t 1-2 During the time period, the active switch is turned on, the circuit works in the forward mode, and the leakage inductance of the primary and secondary sides stores energy.

[0014] t 1-2 -t 1-3 During the time period, the active switch is turned off, the load current is gradually freewheeled by the fourth diode, the leakage energy charges the second capacitor, and the leakage inductance voltage spike is suppressed.

[0015] t 1-3 -t 1-4 During the time period, the leakage energy release ends, the charging speed of the second capacitor slows down until the charging current is 0, and the voltage of the second capacitor reaches a maximum value.

[0016] t 1-4 -t 1-5 During the time period, the second capacitor begins to discharge, the voltage amplitude of the second capacitor decreases, the first current is the sum of the second current and the third current, the third current gradually increases, and the second current gradually decreases to 0; at the end of this stage, the second current drops to 0, the second diode turns into the cut-off state, and the transformer is reset. The first current is the current flowing through the first resistor, and the direction is to flow through the first resistor to the positive end of the input power supply; the second current is the current flowing through the switch tube, and the direction is from the positive to the negative of the second diode; the third current is the current flowing through the second capacitor, and the direction is to flow through the second capacitor to the ground when the second capacitor is charged, and to flow through the second capacitor to the first resistor when discharging.

[0017] t 1-5 -t 1-6 During the time period, the second capacitor feeds back excess energy to the input power supply via the first resistor until the voltage of the second capacitor is equal to the voltage of the input power supply.

[0018] Furthermore, the magnetization process with large flux changes is divided into 7 working stages: t 2-1 -t 2-2 During the time period, the active switch is turned on, the circuit works in the forward mode, and the leakage inductance of the primary and secondary sides stores energy.

[0019] t 2-2 -t 2-3 During the time period, the active switch is turned off, the load current is gradually continued by the fourth diode, the leakage energy charges the second capacitor, and the leakage inductance voltage spike is suppressed.

[0020] t 2-2 -t 2-3 At the end of the time period, the reset energy charges the second capacitor to twice the input supply voltage.

[0021] t 2-3 -t 2-4 During the time period, the voltage of the second capacitor has been charged to twice the input power supply voltage, and when it continues to rise, the voltage across the primary winding = the voltage of the second capacitor Cs - the input power supply voltage is greater than the input power supply voltage, which causes the induced voltage of the third auxiliary winding to be greater than the input power supply voltage. At this time, the first diode connected in series with the third auxiliary winding N3 is turned on, and the third auxiliary winding gradually takes over the reset function of the RCD clamp reset circuit. The fourth current is quickly converted to the fifth current, and a third winding inductance - first diode - third winding leakage inductance - primary winding excitation inductance - primary winding leakage inductance - second diode - second capacitor loop is formed to charge the second capacitor, and the capacitor voltage increases slightly. In the excitation stage, the end of the primary winding connected to the positive pole of the power supply is positive, and the other end is negative; in the reset stage, the end of the primary winding connected to the positive pole of the power supply is negative, and the other end is positive. The voltages of the third auxiliary winding and the secondary winding are determined according to the same-name ends. The fourth current is the current flowing through the primary winding, and its direction is always from both ends of the input power supply to the active switch; the fifth current is the current flowing through the third auxiliary winding, and its direction is from the ground to the positive end of the input power supply.

[0022] t 2-4 -t 2-5 During the time period, the third auxiliary winding completely assumes the reset function of the RCD clamp reset circuit, the voltage across the second capacitor reaches a peak value, slightly exceeding 2 times the input power supply voltage, and the second diode enters a reverse cutoff state, causing the second capacitor to discharge through the first resistor.

[0023] t 2-5 -t 2-6 During the time period, the voltage amplitude of the second capacitor will decrease due to the discharge of the first resistor, but the residual magnetism will recharge the clamping capacitor to twice the input power supply voltage, causing the second capacitor voltage to oscillate slightly around twice the input power supply voltage, and the third auxiliary winding will maintain active reset at this stage until the residual magnetism is insufficient to recharge the second capacitor to twice the input power supply voltage, at which time the RCD clamp reset takes over the reset function.

[0024] t 2-6 -t 2-7 During the time period, the second capacitor begins to discharge, the voltage amplitude of the second capacitor decreases, the first current is the sum of the second current and the third current, the third current gradually increases, and the second current gradually decreases to 0; at the end of this stage, the second current drops to 0, the switch tube turns into the cut-off state, and the transformer is reset. The first current is the current flowing through the first resistor, and the direction is to flow through the first resistor to the positive end of the input power supply; the second current is the current flowing through the switch tube, and the direction is from the positive to the negative of the second diode; the third current is the current flowing through the second capacitor, and the direction is to flow through the second capacitor to the ground when the second capacitor is charged, and to flow through the second capacitor to the first resistor when discharging.

[0025] t 2-7 -t 2-8 During the time period, the second capacitor feeds back excess energy to the input power supply via the first resistor until the capacitor voltage is equal to the input power supply voltage.

[0026] 3. Beneficial effects: The present invention proposes a magnetic reset and leakage inductance voltage oscillation absorption circuit for a forward converter, that is, RCD clamping reset and third winding active reset sequentially cooperate. When a smaller excitation energy needs to be reset, the peak voltage is suppressed by the RCD buffer, and the energy in the excitation winding is absorbed to reset the transformer. When a larger excitation energy needs to be reset, the third winding inductively coupled with the primary winding of the transformer is used to clamp the maximum voltage of the capacitor in the RCD buffer to reduce the switch stress, and the third auxiliary winding actively resets the magnetic energy, thereby improving the energy utilization rate to a certain extent. This reset circuit has low cost and simple circuit. While using a passive circuit to solve the leakage inductance peak voltage and excessive switch stress, it also solves the problem of low reset energy utilization rate, so it is suitable for situations with a wide pulse width input range and high power output. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a schematic diagram of the third winding reset circuit of a commonly used reset circuit.

[0028] Figure 2 This is a schematic diagram of the commonly used reset circuit RCD clamp reset circuit.

[0029] Figure 3 It is a reset circuit with leakage inductance peak voltage absorption function for forward converter.

[0030] Figure 4 It is the equivalent circuit of a forward converter with a reset circuit.

[0031] Figure 5 This is a typical waveform diagram of the RCD clamp reset function alone.

[0032] Figure 6 This is a typical waveform diagram of the combined effect of RCD clamp reset and tertiary winding active reset.

[0033] Figure 7 It is a simulation circuit diagram of a forward converter with the reset circuit.

[0034] Figure 8 This is a simulation analysis of the RCD circuit's ability to suppress switch ringing and relieve switch tube voltage stress.

[0035] Fig. 9 This is a simulation diagram of the RCD clamp reset circuit and the third winding reset together.

[0036] Fig.10 It is a comparison of voltage stress under different reset modes, and an experimental verification of the RCD circuit's ability to suppress switch ringing and reduce switch tube voltage stress.

[0037] Fig.11 It is the RCD action waveform when the magnetic flux change is small during the magnetization process. It is an experimental verification of the RCD circuit's ability to suppress switch ringing and reduce the voltage stress of the switch tube.

[0038] Fig.12 It is an experimental verification of the RCD clamp reset circuit acting alone under the condition of large changes in magnetic flux during magnetization.

[0039] Fig.13 It is an experimental verification of the RCD clamp reset circuit and the tertiary winding working together under the condition of large changes in magnetic flux during magnetization. DETAILED DESCRIPTION

[0040] The present invention is described in detail below with reference to the accompanying drawings and embodiments. Figure 3 As shown, a magnetic reset and leakage inductance voltage oscillation absorption circuit for a forward converter, the third winding reset circuit and the RCD clamp reset circuit cooperate to reset the transformer, and the third auxiliary winding N3 in the third winding reset circuit is connected in series with the first diode D3 to the input power supply U in The RCD clamp reset circuit includes a first resistor R s , the second capacitor C s , the second diode D s , the second capacitor C s Through the second diode D s Connected to both ends of the active switch Q, so that the switching stress of the active switch Q is enveloped by the second capacitor C s The voltage range is within the range of , so the characteristic that the capacitor voltage cannot change suddenly can be used to suppress the voltage spike caused by leakage inductance.

[0041] The transformer has a primary winding N1, a secondary winding N2 and a third auxiliary winding N3. One end of the primary winding N1 is connected to an active switch Q, a second diode D s One end is connected to the first resistor R s One end and the input power supply U in The secondary winding N2 is connected to the output circuit, and connected to the two ends of the load through the third diode D1. The four diodes D2 are connected in parallel at both ends of the load to flow the energy on the load side after the active switch is turned off. s The second capacitor C s Provide a discharge channel, namely the second capacitor C s The excess energy is transferred through the first resistor R s Flows back across the input source until the capacitor voltage equals the input supply voltage.

[0042] The active switch Q is used to control the excitation time of the primary excitation inductance, that is, the output pulse width. During the conduction time of the active switch, power is effectively transferred to the load.

[0043] The turns ratio of the primary winding N1 and the secondary winding N2 can be set to N:mN according to the required output voltage, where m and N are both integers greater than 0. The primary winding N1 and the secondary winding N2 have the same winding direction, but the isolation between the primary ground and the secondary highest voltage level must be met during winding.

[0044] The turns ratio of the primary winding N1 and the third auxiliary winding N3 are the same to maintain a consistent voltage relationship. The primary winding N1 and the third auxiliary winding N3 are wound closely to improve coupling efficiency and reduce leakage inductance.

[0045] The second capacitor C s The capacity of the second capacitor C should be much larger than the output capacitance of the active switch Q to suppress the leakage inductance voltage spike. However, in order to quickly reset and reduce the second capacitor C s The maximum energy stored to reduce the first resistor R s Energy loss, so the second capacitor C s The capacity should not be too large.

[0046] The first resistor R s The main function is to provide a second capacitor C s Provides a discharge channel. The second capacitor C s The excess energy charged in each cycle is discharged to the second capacitor C before the next cycle starts. s The voltage is equal to the input power supply voltage to avoid redundant energy superposition and reduce the peak value of the clamping capacitor voltage.

[0047] In order to facilitate the analysis of the working principle of a single module, Figure 3 The circuit is equivalent to Figure 4 The circuit structure shown.

[0048] Where L m is the primary winding excitation inductance, L1 is the primary winding leakage inductance, D1 is the unidirectional transmission diode, D2 is the load freewheeling diode, L load and R load are load inductance and load resistance respectively, L2 is the secondary leakage inductance, L r is the third winding inductance, L3 is the third winding leakage inductance, and the first current I Rs is the current flowing through the first resistor R s The third current I Cs is the current flowing through the second capacitor C s The fourth current I m is the current flowing through the primary winding, the fifth current Ir is the current flowing through the third auxiliary winding N3. The reference direction of the above current is the direction of the arrow corresponding to the current in the figure. The first current I Rs The direction is through the first resistor R s Flows to the positive terminal of the input power supply, the second current I Rs The direction is from the second diode D s The third current I Cs The direction of the second capacitor C s When charging, it passes through the second capacitor C s Flows into the ground and discharges through the second capacitor C s Flowing into the first resistor R s , the fourth current I m The direction of the fifth current I r The direction is from ground to the positive terminal of the input power supply.

[0049] Figure 5 Shows Figure 4 The waveform of the RCD clamp reset acting alone when the flux change is small during the magnetization process. The RCD clamp capacitor absorbs the reset energy and plays a certain role in clamping the switch tube voltage. At the same time, it absorbs the energy stored in the leakage inductance of the multi-turn winding to avoid the switch tube's turn-off overvoltage.

[0050] It can be divided into 5 working stages: 1-1 -t 1-2 During the time period, the active switch Q is turned on, the circuit works in the forward mode, and the primary and secondary leakage inductances store energy.

[0051] t 1-2 -t 1-3 During this time period, the active switch Q is turned off, and the load current is gradually continued by the fourth diode D2, leaking energy to the second capacitor C s Charging, the leakage inductance voltage spike is suppressed.

[0052] t 1-3 -t 1-4 During this time period, the leakage energy release ends and the second capacitor C s The charging speed slows down until the charging current is 0, and the second capacitor C s The voltage reaches the maximum value.

[0053] t 1-4 -t 1-5 During this time period, the second capacitor C s Start discharging, the second capacitor C s The voltage amplitude decreases, and the first current I Rs The second current I s With the third current I Cs The sum of the third current ICs Gradually increases, the second current I s Gradually decreases to 0; at the end of this stage, the second current I s drops to 0, the second diode D s Enter the cut-off state and the transformer reset is completed.

[0054] t 1-5 -t 1-6 During this time period, the second capacitor C s The excess energy is transferred through the first resistor R s Feedback to the input power supply until the capacitor voltage is equal to the input power supply voltage.

[0055] Figure 6 Show Figure 4 The situation where the RCD clamp reset and the active reset of the third winding work together when the magnetic flux changes greatly during the magnetization process. First, the RCD circuit acts the same as before, absorbing energy and causing the clamp capacitor voltage to increase. When the clamp capacitor voltage reaches twice the input power supply U in When the voltage continues to increase, the voltage of the primary winding exceeds U in , resulting in the voltage of the third winding being greater than the input power supply U in The voltage makes the first diode D3 conduct, and the third winding begins to assume the function of RCD reset. When the energy required to reset the excitation winding is low and close to completion, the circuit reset switches from the third winding reset to the RCD clamp reset until the reset is completed.

[0056] It can be divided into seven working stages. 2-1 -t 2-2 During the time period, the active switch Q is turned on, the circuit works in the forward mode, and the leakage inductance of the primary and secondary sides stores energy.

[0057] t 2-2 -t 2-3 During the time period, the active switch Q is turned off, and the load current is gradually continued by the fourth diode D2, leaking energy to the second capacitor C s Charging, the leakage inductance voltage spike is suppressed.

[0058] t 2-2 -t 2-3 At the end of the time period, the reset energy charges the second capacitor to twice the input power supply U in Voltage.

[0059] t 2-3 -t 2-4 During this time period, the voltage of the second capacitor has been charged to twice the input power supply U in voltage, and continues to rise, the voltage U across the primary winding N1 p = Second capacitor Cs voltage U Cs -Input power supply Uin The voltage is greater than the input power U in voltage, which causes the induced voltage of the third auxiliary winding N3 to be greater than the input power supply U in voltage, at this time, the first diode D3 connected in series with the third auxiliary winding N3 is turned on, and the third auxiliary winding N3 gradually takes over the reset function of the RCD clamp reset circuit. The fourth current I m Rapidly convert to the fifth current I r , and forms the third winding inductance L r -First diode D3 -Third winding leakage inductance L3 -Primary winding excitation inductance L m -Primary winding leakage inductance L1 -Second diode D s - Second capacitor C s The loop is connected to the second capacitor C s During charging, the capacitor voltage increases slightly. In the excitation stage, the end of the primary winding connected to the positive pole of the power supply is positive, and the other end is negative; in the reset stage, the end of the primary winding connected to the positive pole of the power supply is negative, and the other end is positive. The voltages of the third auxiliary winding and the secondary winding are determined according to the same-named ends. The same-named ends refer to the terminals with the same polarity in the coils of each level wound on the transformer.

[0060] t 2-4 -t 2-5 During this time period, the third auxiliary winding N3 fully assumes the reset function of the RCD clamp reset circuit, and the second capacitor C s The voltage at both ends reaches a peak value, slightly more than 2 times the input power U in Voltage, second diode D s Entering the reverse cutoff state, the second capacitor C s Through the first resistor R s Discharge.

[0061] t 2-5 -t 2-6 During this period, the second capacitor C s The voltage amplitude depends on the first resistor R s The discharge will decrease, but the residual magnetism will recharge the clamp capacitor to twice the input power supply U in voltage, so that the second capacitor Cs voltage U Cs At 2 times the input power U in The voltage oscillates slightly around the input voltage, and the third auxiliary winding N3 will remain actively reset in this stage until the residual magnetism is insufficient to charge the second capacitor to twice the input power supply U in voltage, at which point the RCD clamp reset takes over the reset function.

[0062] t 2-6 -t 2-7 During this time period, the second capacitor C s Start discharging, the second capacitor Cs The voltage amplitude decreases, and the first current I Rs The second current I s With the third current I Cs The sum of the third current I Cs Gradually increases, the second current I s Gradually decreases to 0; at the end of this stage, the second current I s Drop to 0, switch tube D s The transformer is reset after the second current I s is the current flowing through the switch tube D s The current, the first current I Rs is the current flowing through the first resistor R s The third current I Cs is the current flowing through the second capacitor C s of current.

[0063] t 2-7 -t 2-8 During this time period, the second capacitor C s The excess energy is transferred through the first resistor R s Feedback to the input power supply until the capacitor voltage is equal to the input power supply voltage.

[0064] The present invention adopts a passive circuit and does not require complex drive control, so that the circuit structure is simple, the cost is low, and it is easy to implement. The RCD clamp reset circuit is used to absorb the peak voltage of the leakage inductance to protect the switch.

[0065] When a large excitation energy needs to be reset, the voltage of the second capacitor will increase without limit when the RCD clamp reset circuit acts alone. For this reason, a third auxiliary winding inductively coupled with the primary winding of the transformer is added to clamp the maximum voltage of the capacitor in the RCD buffer to reduce the switching stress; The third auxiliary winding clamps the capacitor voltage in the RCD buffer and actively resets the magnetic energy; the RCD clamp reset circuit resets the capacitor C s The excess energy is transferred to s Feedback to the input power supply until the capacitor voltage is equal to the input power supply voltage. Both measures improve energy utilization to a certain extent.

[0066] The simulation circuit built in Pspice is as follows Figure 7 As shown, it includes an RCD clamp reset circuit, a transformer, a third auxiliary winding and a first diode connected in series therewith, an active switch Q, a drive circuit, a primary and secondary leakage inductance of the transformer, a high-voltage diode, and a load-side freewheeling diode. The primary leakage inductance of the transformer is included in the transformer parameters, and the secondary leakage inductance of the transformer is connected to the secondary winding; the load is set to a 200Ω resistor. In the RCD clamp reset circuit, the first resistor R s Take 1kΩ, the second capacitor Cs Take 100nF, switch tube D s The reverse withstand voltage is the input power supply U in Voltage. The transformer mutual inductance K is 1, the transformer primary and secondary turns ratio is N:N, and they are wound in the same direction; the primary winding and reset winding turns ratio is N:N, and they are wound in opposite directions. The input power supply U in Voltage = 500V, energy storage capacitor is 1mF, and the on-time is adjustable. At this time, the reverse withstand voltage of the active switch Q should be greater than 2 times the input power supply U in Voltage, instantaneous current carrying capacity should be greater than the load pulse current. The reverse withstand voltage of the high-voltage diode is the input power supply U in Voltage, the reverse withstand voltage of the load side freewheeling diode is also the input power supply U in Voltage.

[0067] The RCD clamp reset circuit uses the principle that the capacitor voltage cannot change suddenly to suppress the turn-off ringing and overvoltage caused by leakage inductance. When the flux change during the magnetization process is small, the RCD clamp reset circuit can relieve a certain amount of switching pressure. To this end, the RCD clamp reset and the third winding reset are used to individually act on the switching stress of the MOSFET when the output pulse width is 5μs and the output is low flux change under a 200Ω load. The results are shown in Figure 2. Figure 8 It can be found that, compared with the passive reset using the third winding, the RCD clamp reset can suppress the voltage ringing and alleviate the switch tube voltage stress to a certain extent.

[0068] When the magnetic flux changes greatly during the magnetization process, if only RCD is used to reset the residual magnetism, the voltage across the RCD capacitor will continue to increase, even higher than twice the charging voltage. Fig. 9 As shown. The third winding inductively coupled with the primary winding of the transformer is used to clamp the maximum voltage of the capacitor in the RCD snubber to reduce the switching stress. At the same time, the third auxiliary winding actively resets the magnetic energy, which improves the energy utilization rate to a certain extent. Fig. 9 As shown in the small window on the lower right, this reset method can achieve reset while ensuring that the switch tube does not interrupt ringing and has low energy loss, which is of great significance for achieving wide pulse width and high power output.

[0069] In the experiment, when the magnetic flux changes little during the magnetization process, the device with the same simulation parameters is used; while when the magnetic flux changes greatly during the magnetization process, the voltage is proportionally reduced and the pulse width is proportionally enlarged due to the magnetic saturation condition of the transformer, while the other parameters remain unchanged, in order to verify the role of the reset circuit with leakage inductance spike voltage absorption function. Fig.10 and Fig.11 The clamping effect of RCD was verified when the magnetic flux change was small during the magnetization process, suppressing the ringing caused by the leakage inductance on the MOSFET switch stress when the switch was turned off. Fig.12 and Fig.13It is verified that when the magnetic flux changes greatly during the magnetization process, the RCD clamp reset is used to absorb the leakage inductance spike, and the third winding inductively coupled with the primary winding of the transformer is used to clamp the maximum voltage of the capacitor in the RCD buffer to reduce the switching stress, which will help limit the voltage between the drain and source of the MOSFET and protect the switch. At the same time, the third auxiliary winding actively resets the magnetic energy, which improves the energy utilization to a certain extent.

Claims

1. A magnetic reset and leakage inductance voltage oscillation absorption circuit for a forward converter, characterized in that: The third winding reset circuit and the RCD clamp reset circuit cooperate to reset the transformer. The third auxiliary winding (N3) in the third winding reset circuit is connected in series with the first diode (D3) to the input power supply (U in ) at both ends, the first capacitor (C) is connected to the input power supply (U in ) across the RCD clamp reset circuit, the first resistor (R s )、Second capacitor(C s )、The second diode (D s ), the second capacitor (C s ) through the second diode (D s ) is connected to both ends of the active switch (Q), the transformer has a primary winding (N1), a secondary winding (N2) and a third auxiliary winding (N3), one end of the primary winding (N1) is connected to the active switch (Q), the second diode (D s ) and the other end is connected to the first resistor (R s ) and the input power supply (U in ) positive electrode; the secondary winding (N2) is connected to the output circuit and connected to both ends of the load through the third diode (D1); a fourth diode (D2) is connected in parallel at both ends of the load to flow the energy on the load side after the active switch is turned off.

2. The magnetic reset and leakage inductance voltage oscillation absorption circuit for a forward converter according to claim 1, characterized in that: The turns ratio of the primary winding (N1) and the secondary winding (N2) is set to N:mN according to the required output voltage, wherein m and N are both integers greater than 0.

3. The magnetic reset and leakage inductance voltage oscillation absorption circuit for a forward converter according to claim 1, characterized in that: The primary winding (N1) and the secondary winding (N2) are wound in the same direction, and the primary winding and the third auxiliary winding (N3) are wound in opposite directions.

4. The magnetic reset and leakage inductance voltage oscillation absorption circuit for a forward converter according to claim 1, characterized in that: The primary winding (N1) and the third auxiliary winding (N3) have the same turn ratio.

5. The magnetic reset and leakage inductance voltage oscillation absorption circuit for a forward converter according to claim 1, characterized in that: The active switch (Q) is used to control the excitation time of the primary excitation inductor, that is, the output pulse width.

6. The magnetic reset and leakage inductance voltage oscillation absorption circuit for a forward converter according to claim 1, characterized in that: The second capacitor (C s ) is larger than the output capacitance of the active switch (Q).

7. The magnetic reset and leakage inductance voltage oscillation absorption circuit for a forward converter according to claim 1, characterized in that: The second capacitor (C s The excess energy charged in each cycle is discharged to the second capacitor (C s ) voltage is equal to the input power supply voltage to avoid redundant energy superposition and reduce the peak value of the clamping capacitor voltage.

8. The magnetic reset and leakage inductance voltage oscillation absorption circuit for a forward converter according to any one of claims 1 to 7, characterized in that: When the flux change during the magnetization process is small, the third auxiliary winding (N3) is used as a backup reset. The RCD clamp reset circuit resets the residual energy of the unidirectional transformer while suppressing the turn-off ringing and relieving the voltage stress of the active switch (Q). When the flux change during the magnetization process is large, the second capacitor (C s ) will exceed the withstand voltage of the active switch (Q), so the third auxiliary winding (N3) inductively coupled with the primary winding of the transformer is used to clamp the second capacitor (C s ) and at the same time the third auxiliary winding (N3) actively resets the magnetic energy.

9. The magnetic reset and leakage inductance voltage oscillation absorption circuit in a forward converter as claimed in claim 8, wherein: The magnetization process is divided into five working stages when the magnetic flux change is small: t 1-1 -t 1-2 During this time period, the active switch (Q) is turned on, the circuit works in the forward mode, and the leakage inductance of the primary and secondary sides stores energy; t 1-2 -t 1-3 During this time period, the active switch (Q) is turned off, and the load current is gradually continued by the fourth diode (D2), leaking energy to the second capacitor (C s ) charging, the leakage inductance voltage spike is suppressed; t 1-3 -t 1-4 During this time period, the leakage energy release ends and the second capacitor (C s ) The charging speed slows down until the charging current is 0, and the second capacitor (C s ) The voltage reaches the maximum value; t 1-4 -t 1-5 During this time period, the second capacitor (C s ) starts to discharge, and the second capacitor (C s ) voltage amplitude decreases, the first current (I Rs ) is the second current (I s ) and the third current (I Cs ) and the third current (I Cs ) gradually increases, and the second current (I s ) gradually decreases to 0; at the end of this stage, the second current (I s ) drops to 0, the second diode (D s ) turns into the cut-off state, and the transformer reset is completed; the first current (I Rs ) is the current flowing through the first resistor (R s ) and the direction is through the first resistor (R s ) flows to the positive terminal of the input power supply; The second current (I s ) is the current flowing through the switch tube (D s ) and the direction of the current is from the second diode (D s ) flows from positive to negative; the third current (I Cs ) is the current flowing through the second capacitor (C s ) and the direction of the current is the second capacitor (C s ) is charged through the second capacitor (C s ) flows into the ground and discharges through the second capacitor (C s ) flows into the first resistor (R s ); t 1-5 -t 1-6 During this time period, the second capacitor (C s ) transfers the excess energy through the first resistor (R s ) is fed back to the input power supply until the second capacitor (C s ) voltage is equal to the input supply voltage.

10. The magnetic reset and leakage inductance voltage oscillation absorption circuit in a forward converter as claimed in claim 6, wherein: The magnetization process with large flux changes is divided into 7 working stages: t 2-1 -t 2-2 During the time period, the active switch (Q) is turned on, the circuit works in the forward mode, and the leakage inductance of the primary and secondary sides stores energy; t 2-2 -t 2-3 During the time period, the active switch (Q) is turned off, and the load current is gradually continued by the fourth diode (D2), leaking energy to the second capacitor (C s ) charging, the leakage inductance voltage spike is suppressed; t 2-2 -t 2-3 At the end of the time period, the reset energy will be transferred to the second capacitor (C s ) is charged to twice the input power (U in )Voltage; t 2-3 -t 2-4 During this time period, the second capacitor (C s ) voltage has been charged to twice the input power (U in ) voltage and continues to increase, the voltage across the primary winding (N1) (U p ) = Second capacitor (Cs) voltage (U Cs )-Input power (U in ) voltage is greater than the input power supply (U in ) voltage, which causes the induced voltage of the third auxiliary winding (N3) to be greater than the input power supply (U in ) voltage, at this time, the first diode (D3) connected in series with the third auxiliary winding (N3) is turned on, and the third auxiliary winding (N3) gradually takes over the reset function of the RCD clamp reset circuit. The fourth current (I m ) quickly converts to the fifth current (I r ), and forms the third winding inductance (L r )-first diode (D3)-third winding leakage inductance (L3)-primary winding excitation inductance (L m )-primary winding leakage inductance (L1)-second diode (D s )-Second capacitor (C s ) loop to the second capacitor (C s ) is charged, and the capacitor voltage increases slightly. In the excitation stage, the end of the primary winding connected to the positive pole of the power supply is positive, and the other end is negative; in the reset stage, the end of the primary winding connected to the positive pole of the power supply is negative, and the other end is positive. The voltage of the third auxiliary winding and the secondary winding is determined according to the same end. The fourth current (I m ) is the current flowing through the primary winding, and the direction is always from the two ends of the input power supply to the active switch; The fifth current (I r ) is the current flowing through the third auxiliary winding (N3), and the direction is from ground to the positive terminal of the input power supply; t 2-4 -t 2-5 During this period, the third auxiliary winding (N3) fully assumes the reset function of the RCD clamp reset circuit, and the second capacitor (C s ) reaches a peak value, slightly more than twice the input power (U in ) voltage, the second diode (D s ) enters the reverse cutoff state, making the second capacitor (C s ) through the first resistor (R s ) discharge; t 2-5 -t 2-6 During this period, the second capacitor (C s ) voltage amplitude due to the first resistor (R s ) discharge will decrease, but the residual magnetism will recharge the clamp capacitor to twice the input power supply (U in ) voltage, so that the second capacitor (Cs) voltage (U Cs ) at 2 times the input power (U in ) voltage, and the third auxiliary winding (N3) will maintain active reset in this stage until the residual magnetism is insufficient to reset the second capacitor (C s ) is recharged to twice the input power (U in ) voltage, at which point the RCD clamp reset takes over the reset function; t 2-6 -t 2-7 During this time period, the second capacitor (C s ) starts to discharge, and the second capacitor (C s ) voltage amplitude decreases, the first current (I Rs ) is the second current (I s ) and the third current (I Cs ) and the third current (I Cs ) gradually increases, and the second current (I s ) gradually decreases to 0; at the end of this stage, the second current (I s ) drops to 0, the switch tube (D s ) turns into the cut-off state, and the transformer reset is completed. The first current (I Rs ) is the current flowing through the first resistor (R s ) and the direction is through the first resistor (R s ) flows to the positive terminal of the input power supply; The second current (I s ) is the current flowing through the switch tube (D s ) and the direction of the current is from the second diode (D s ) flows from positive to negative; the third current (I Cs ) is the current flowing through the second capacitor (C s ) and the direction of the current is the second capacitor (C s ) is charged through the second capacitor (C s ) flows into the ground and discharges through the second capacitor (C s ) flows into the first resistor (R s ); t 2-7 -t 2-8 During this time period, the second capacitor (C s ) transfers the excess energy through the first resistor (R s ) is fed back to the input power supply until the capacitor voltage is equal to the input power supply voltage.

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