Active clamping flyback converter and control method
By introducing specific control methods into the active clamp flyback converter, controlling the on- and off of the clamp switch, the problem of poor leakage inductance energy recovery in the prior art is solved, and more efficient energy recovery and loss reduction is achieved.
Patent Information
- Application Number
- CN202311516794.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2023-11-14
- Publication Date
- 2025-05-16
AI Technical Summary
The existing ACF control method has poor effect on the recovery of leakage inductance energy, resulting in large energy loss of flyback converters in power adapters.
By introducing a control method in the active clamp flyback converter, the specific steps include controlling the clamp switch to turn on when the transformer is in the demagnetization stage and controlling the clamp switch to turn off when the clamp capacitor and the leakage inductance resonant current of the transformer reach a preset current threshold.
This method improves the recovery effect of leakage inductance energy to at least a certain extent, and reduces the core loss and secondary diode rectification loss caused by negative excitation current.
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Figure CN120016806A_ABST
Abstract
Description
Technical Field
[0001] The present disclosure relates to the technical field of converters, and in particular to an active clamp flyback converter and a control method thereof. Background Art
[0002] Miniaturization and high power density have become the development trend of power adapters. In actual operation, due to the presence of leakage inductance, the flyback converter in the power adapter cannot transfer all the energy of the primary circuit to the secondary circuit, resulting in loss. In addition, the resonance between the leakage inductance of the primary circuit and the junction capacitance of the switch tube (such as MOS tube) of the primary circuit causes a high-frequency voltage spike to be generated at the drain of the main switch tube. In the process of product design, in order to reduce the voltage stress of the main switch tube, it is usually added as follows Figure 1 The RCD absorption circuit shown in the figure uses a resistor R p The leakage inductance energy is dissipated, resulting in leakage inductance energy being wasted. The active clamp flyback (ACF) circuit can recover part of the leakage inductance energy, but different ACF control methods have different effects on the recovery of leakage inductance energy. Therefore, a better ACF control method is needed.
[0003] It should be noted that the information disclosed in the above background technology section is only used to enhance the understanding of the background of the present disclosure, and therefore may include information that does not constitute the prior art known to ordinary technicians in the field. Summary of the invention
[0004] The present disclosure provides an active clamp flyback converter and a control method, which at least improve the recovery effect of leakage inductance energy to a certain extent, and improve the problem of poor recovery effect of leakage inductance energy in the existing ACF control method.
[0005] Other features and advantages of the present disclosure will become apparent from the following detailed description, or may be learned in part by the practice of the present disclosure.
[0006] According to one aspect of the present disclosure, a control method for an active clamped flyback converter is provided. The active clamped flyback converter includes:
[0007] A transformer, comprising a primary winding and a secondary winding coupled to each other;
[0008] A primary switch connected in series with the primary winding;
[0009] A clamping branch is connected in parallel to the primary winding or in parallel to the primary switch; the clamping branch includes a clamping switch and a clamping capacitor connected in series;
[0010] Control methods include:
[0011] When the input voltage of the active clamp flyback converter is less than the preset voltage threshold, or the input voltage of the active clamp flyback converter is less than the product of the output voltage of the active clamp flyback converter and the turns ratio of the transformer, the following steps are performed:
[0012] In the same switching cycle, when the transformer is in the demagnetization stage, the clamp switch is controlled to be turned on; and when the resonant current of the clamp capacitor and the leakage inductance of the transformer reaches a preset current threshold, the clamp switch is controlled to be turned off.
[0013] In one embodiment of the present disclosure, the active clamped flyback converter further includes a secondary circuit electrically connected to the secondary winding, wherein when the input voltage of the active clamped flyback converter is greater than or equal to a preset voltage threshold, and the input voltage of the active clamped flyback converter is greater than the product of the output voltage of the active clamped flyback converter and the turns ratio of the transformer, the control method includes:
[0014] In the same switching cycle, after the freewheeling phase of the secondary circuit ends, the clamp switch is controlled to be turned on; and before the primary switch is turned on, the clamp switch is controlled to be turned off.
[0015] In one embodiment of the present disclosure, when the input voltage of the active clamped flyback converter is greater than or equal to a preset voltage threshold, and the input voltage of the active clamped flyback converter is greater than the product of the output voltage of the active clamped flyback converter and the turns ratio of the transformer, the control method further includes:
[0016] In the same switching cycle, when the transformer is in the demagnetization stage, the clamp switch is controlled to be turned on; and when the resonant current of the clamp capacitor and the leakage inductance of the transformer reaches a preset current threshold, the clamp switch is controlled to be turned off.
[0017] In one embodiment of the present disclosure, after the freewheeling phase of the secondary circuit ends, controlling the clamp switch to turn on includes:
[0018] After the freewheeling phase of the secondary circuit ends, when the voltage across the primary switch oscillates to a peak value, the clamp switch is controlled to turn on.
[0019] In one embodiment of the present disclosure, before the primary switch is turned on, controlling the clamp switch to be turned off includes:
[0020] The conduction time is determined according to the input voltage of the active clamp flyback converter, the output voltage of the active clamp flyback converter, the equivalent capacitance of the primary switch, and the magnetizing inductance of the transformer;
[0021] Before the primary switch is turned on, and when the on time of the clamp switch reaches the on time, the clamp switch is controlled to be turned off.
[0022] In one embodiment of the present disclosure, after the freewheeling stage of the secondary circuit ends, the clamp switch is controlled to be turned on. When the primary switch is turned off and the current flowing through the secondary winding is a positive current, the secondary circuit is in the freewheeling stage.
[0023] In one embodiment of the present disclosure, the control method further includes: determining whether the transformer is in a demagnetization stage according to a current flowing through the magnetizing inductance of the primary winding.
[0024] In one embodiment of the present disclosure, determining whether the transformer is in a demagnetization stage according to the current flowing through the magnetizing inductance of the primary winding includes:
[0025] When the current flowing through the magnetizing inductance of the primary winding is positive and in a decreasing stage, the transformer is in the demagnetization stage.
[0026] In one embodiment of the present disclosure, when the input voltage of the active clamped flyback converter is less than a preset voltage threshold, or the input voltage of the active clamped flyback converter is less than the product of the output voltage of the active clamped flyback converter and the turns ratio of the transformer, the active clamped flyback converter operates in a single-pulse non-complementary mode.
[0027] In one embodiment of the present disclosure, the preset current threshold is 0A.
[0028] In one embodiment of the present disclosure, when the input voltage of the active clamped flyback converter is less than the product of the output voltage of the active clamped flyback converter and the turns ratio of the transformer, the voltage across the primary switch oscillates to 0V; otherwise, the voltage across the primary switch oscillates less than 0V.
[0029] In one embodiment of the present disclosure, the active clamp flyback converter further includes a secondary circuit, wherein the secondary circuit includes an output capacitor;
[0030] The control method also includes:
[0031] When the primary switch is turned off and the clamp switch is turned on, the clamp capacitor transfers energy to the output capacitor and / or an output load through the clamp switch.
[0032] According to another aspect of the present disclosure, there is provided an active clamp flyback converter, comprising:
[0033] A transformer, comprising a primary winding and a secondary winding coupled to each other;
[0034] A primary switch is connected in series with the primary winding;
[0035] A clamping branch is connected in parallel to the primary winding or the primary switch; the clamping branch includes a clamping switch and a clamping capacitor connected in series;
[0036] A control unit is connected to the primary switch and the clamp switch, wherein the control unit is configured to: when the input voltage of the active clamp flyback converter is less than a preset voltage threshold, or the input voltage of the active clamp flyback converter is less than the product of the output voltage of the active clamp flyback converter and the turns ratio of the transformer, control the clamp switch to turn on in the same switching cycle when the transformer is in a demagnetization stage; and control the clamp switch to turn off when the resonant current of the clamp capacitor and the leakage inductance of the transformer reaches a preset current threshold.
[0037] The active clamp flyback converter and control method provided by the embodiments of the present disclosure, when the input voltage of the active clamp flyback converter is less than a preset voltage threshold, or the input voltage of the active clamp flyback converter is less than the product of the output voltage of the active clamp flyback converter and the turns ratio of the transformer, in the same switching cycle, when the transformer is in the demagnetization stage, the clamp switch is controlled to be turned on, so that the core loss caused by the negative excitation current and the secondary diode rectification loss can be reduced. When the resonant current of the clamp capacitor and the leakage inductance of the transformer reaches the preset current threshold, the clamp switch is controlled to be turned off, which can reduce the turn-off loss of the clamp switch and the leakage inductance oscillation loss of the clamp capacitor and the transformer.
[0038] It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS
[0039] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present disclosure and, together with the description, serve to explain the principles of the present disclosure.
[0040] Obviously, the drawings described below are only some embodiments of the present disclosure, and a person skilled in the art can obtain other drawings based on these drawings without any creative work.
[0041] Figure 1 A circuit diagram of a conventional RCD clamped flyback converter is shown;
[0042] Figure 2 A circuit diagram of an active clamp flyback converter in an embodiment of the present disclosure is shown;
[0043] Figure 3 Show Figure 2 A specific circuit diagram of an active clamp flyback converter is shown in FIG.
[0044] Figure 4 A circuit diagram of another active clamp flyback converter in an embodiment of the present disclosure is shown;
[0045] Figure 5A typical working waveform of a conventional complementary control active clamp is shown;
[0046] Figure 6 A waveform of a conventional non-complementary active clamping device operating under low voltage and heavy load is shown;
[0047] Figure 7 The working waveform of an active clamp flyback converter in an embodiment of the present disclosure is shown;
[0048] Figure 8 Another working waveform of an active clamp flyback converter in an embodiment of the present disclosure is shown;
[0049] Fig. 9 The working waveform of another active clamp flyback converter in the embodiment of the present disclosure is shown;
[0050] Fig.10 The working waveform of another active clamp flyback converter in the embodiment of the present disclosure is shown;
[0051] Fig.11 A structural block diagram of an electronic device in an embodiment of the present disclosure is shown. DETAILED DESCRIPTION
[0052] Example embodiments will be described more fully hereinafter with reference to the accompanying drawings.
[0053] It should be noted that example embodiments may be implemented in many forms and should not be construed as being limited to the examples set forth herein.
[0054] In the embodiments of the present disclosure, the terms “first”, “second” and “third” are used for descriptive purposes only and should not be understood as indicating or implying relative importance.
[0055] The term "and / or" in this disclosure is only a description of the association relationship of associated objects, indicating that there may be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.
[0056] The active clamp flyback converter can realize the recovery of leakage inductance energy and ZVS of the main switch, and further improve the product efficiency and power density. Active clamping is divided into complementary active clamping and non-complementary active clamping. Among them, non-complementary active clamping is favored because of its high efficiency at light load and high voltage input full load.
[0057] Figure 5 The typical operating waveform of a conventional complementary control active clamp flyback converter is shown in FIG. Figure 3 Take the circuit diagram shown as an example. Figure 5Dri_L is used to represent the working waveform of the primary switch LS, Dri_H is used to represent the working waveform of the clamp switch H_S, V DS It is used to represent the voltage between the drain and source of the primary switch L_S, I P Used to indicate the input current of the transformer, I C Used to indicate the clamping branch current, I S Used to represent the secondary current, I m Used to indicate the excitation current. Figure 5 As shown, the primary switch L_S and the active clamp switch H_S work in a complementary manner. Specifically, in a switching cycle, when the primary switch L_S is turned off, the clamp switch H_S is turned on; when the primary switch L_S is turned on, the clamp switch H_S is turned off. Of course, there may be a short dead time between the two to prevent the primary switch L_S from being turned on when the clamp switch H_S is not completely turned off, so that the two are directly connected, causing damage to the active clamp flyback converter.
[0058] The difference between the non-complementary active clamping and the complementary active clamping is that in a switching cycle, the control of the primary switch L_S and the clamping switch H_S in the non-complementary active clamping flyback converter is not complementary. The control of the clamping switch H_S is, for example, to turn on the clamping switch H_S briefly before the primary switch L_S is turned on.
[0059] The conventional control method of the non-complementary active clamp flyback converter includes, for example, when the input voltage is low and the load is heavy, the active clamp flyback converter operates in the critical continuous mode, and its waveform can be as follows: Figure 6 As shown, the excitation current I m Indicated by the dotted line. As soon as the secondary circuit freewheeling phase ends, the clamp switch H_S is turned on, but this brings additional core loss. In addition, because the on-time of the clamp switch H_S changes with the input voltage V in Therefore, when the clamp switch H_S is turned off under low input voltage conditions, the resonant current is relatively large, resulting in relatively large turn-off losses, and the leakage inductance L k of energy will be wasted.
[0060] In actual implementation, the leakage inductance recovery efficiency of the above low-voltage heavy-load is not very ideal. The inventors found that the main reason for the unsatisfactory recovery efficiency is that the clamp switch H_S is turned on immediately after the freewheeling phase, which brings additional core loss, and the hard shutdown of the clamp switch H_S also brings losses.
[0061] To solve the above problems, an active clamp flyback converter is provided in an embodiment of the present disclosure, and the control unit is configured to execute the control method of the active clamp flyback converter provided in the embodiment of the present disclosure. The control method of the active clamp flyback converter provided in the present disclosure can further improve the leakage inductance recovery efficiency of the non-complementary active clamp by optimizing the conduction time of the primary switch L_S and the clamp switch H_S.
[0062] like Figure 2-4 As shown, an embodiment of the present disclosure provides an active clamp flyback converter, which includes a transformer, a primary switch L_S and a clamp branch; wherein the transformer includes a primary winding and a secondary winding coupled to each other; the primary switch L_S is connected in series to the primary winding; the clamp branch is connected in parallel to the primary winding or to the primary switch L_S, and the clamp branch includes a clamp switch H_S and a clamp capacitor C connected in series. C .
[0063] With the above Figure 1 Through the resistor R p The dissipation of leakage inductance energy is different. The active clamped flyback converter provided in the embodiment of the present disclosure can transfer the leakage inductance energy to the output end of the active clamped flyback converter by controlling the on-off of the clamp switch H_S, thereby realizing the recovery of the leakage inductance energy.
[0064] Figure 2 The circuit of an active clamp flyback converter provided by an embodiment of the present disclosure is shown, wherein the clamp branch is connected in parallel to the primary winding, and the clamp branch includes a clamp switch H_S and a clamp capacitor C connected in series. C .
[0065] Figure 3 Show Figure 2 The active clamp flyback converter is shown in FIG. 1 , which specifically shows the transformer leakage inductance L k and magnetizing inductance L m , and the circuit also shows a secondary circuit. The secondary circuit is electrically connected to the secondary winding, and is used to convert the energy transmitted by the secondary winding into an output voltage V o output to the load.
[0066] Figure 4 A circuit of another active clamp flyback converter provided by an embodiment of the present disclosure is shown. Figure 3 The active clamp flyback converter shown is similar, except that the clamping branch is connected in parallel to the primary switch L_S.
[0067] It should be noted that Figure 2 , Figure 3 and Figure 4Only basic components of the active clamped flyback converter are shown. According to actual needs, the active clamped flyback converter provided by the embodiment of the present disclosure may also include other components.
[0068] like Figure 2-Figure 4 The active clamp flyback converter shown in FIG. 1 may further include an output capacitor C o ; Output capacitor C o It can be used for filtering, or for storing the clamp capacitor C when the primary switch L_S is turned off and the clamp switch H_S is turned on. C Part of the energy transferred via the clamp switch H_S, such as Figure 7-Figure 9 Stage three t2~t3.
[0069] like Figure 2-Figure 4 In the active clamp flyback converter shown, the secondary circuit may further be provided with a secondary switch SR. It should be noted that the control process of the secondary switch SR is not discussed in the control method of the active clamp flyback converter provided in the embodiment of the present disclosure. Unless otherwise specified, the secondary switch SR is in an on state.
[0070] like Figure 2-Figure 4 The active clamp flyback converter shown in FIG. 1 may further include a control unit (not shown). The control unit is connected to the primary switch L_S and the clamp switch H_S, and is used to control the on-off of the primary switch L_S and the on-off of the clamp switch H_S. The primary switch L_S and the clamp switch H_S may be MOS tubes, and the control unit may control the on-off of the primary switch L_S and the on-off of the clamp switch H_S through a pulse signal.
[0071] The control method of the active clamp flyback converter provided by the embodiment of the present disclosure is described in detail below.
[0072] The control method of the active clamp flyback converter provided in the embodiment of the present disclosure can be applied to Figure 2 , Figure 3 or Figure 4 The active clamp flyback converter shown.
[0073] It should be noted that the execution body of the control method of the active clamp flyback converter may be the control unit in the above embodiment.
[0074] In some embodiments, the control method of the active clamp flyback converter provided in the embodiment of the present disclosure includes: when the input voltage V of the active clamp flyback converter is in is less than the preset voltage threshold, or the input voltage V inWhen the voltage V0 of the active clamp flyback converter is less than the product of the output voltage V0 of the active clamp flyback converter and the turns ratio n of the transformer, the following steps are performed: in the same switching cycle, when the transformer is in the demagnetization stage, the clamp switch H_S is controlled to be turned on; and when the clamp capacitor C C and the transformer leakage inductance L k When the resonant current reaches a preset current threshold, the clamp switch H_S is controlled to be turned off.
[0075] In the above embodiment, the preset voltage threshold can be used to distinguish the input voltage V in For high voltage or low voltage, to simplify the description, in the following text, when the input voltage V in If the input voltage is less than the preset voltage threshold, it is called low voltage. in An input voltage greater than or equal to a preset voltage threshold is called a high voltage. In some embodiments, the preset voltage threshold is, for example, 140VAC.
[0076] The control unit can obtain the input voltage V in and output voltage V o , then according to the input voltage V in and the preset voltage threshold to determine the input voltage V in Is it low voltage? If the input voltage V in For low voltage, or V in <n*V o , the above control scheme is executed, that is, in the same switching cycle, when the transformer is in the demagnetization stage, the clamp switch H_S is controlled to be turned on; and when the clamp capacitor C C and the transformer leakage inductance L k When the resonant current reaches the preset current threshold, the clamp switch H_S is controlled to turn off. in <n*V o When the voltage across the primary switch L_S can naturally oscillate to 0V, for example, when the primary switch L_S is a MOS tube, the voltage between its drain and source V DS It can oscillate naturally to 0V.
[0077] Whether the transformer is in the demagnetization stage is related to the current flowing through the transformer. The control unit can determine whether the transformer is in the demagnetization stage according to the change of the current flowing through the transformer.
[0078] In some embodiments, the demagnetization stage can be based on the magnetizing inductance L flowing through the primary winding. m The current (such as Figure 7-Figure 9 I m ), that is, the control method may further include determining the magnetizing inductance L flowing through the primary winding. m The current I m , determine whether the transformer is in the demagnetization stage.
[0079] Specifically, in some embodiments, according to the magnetizing inductance L flowing through the primary winding m The current I m , to determine whether the transformer is in the demagnetization stage, it can be when the magnetizing inductance L flowing through the primary winding m The current I m When the current is positive and decreasing (such as Figure 7-Figure 9 t0~t4), where the current I is defined m From the magnetizing inductance L m When the current flows from the upper terminal A to the lower terminal B, it is a positive current, and the transformer is in the demagnetization stage. For the specific period of the demagnetization stage, please refer to the description of the working waveform of the active clamp flyback converter below.
[0080] In some embodiments, when the active clamp flyback converter input voltage V in is less than the preset voltage threshold, or the input voltage V in When the input voltage is lower than the product of the output voltage Vo of the active clamp flyback converter and the turns ratio n of the transformer, the active clamp flyback converter operates in a single pulse non-complementary mode. in <n*V o When the control unit controls the clamp switch HS to be turned on only once in the same switching cycle based on the above control method, such as Figure 7 , Figure 8 The operating waveform Dri_H of the clamp switch H_S is shown.
[0081] In some embodiments, the preset current threshold may be a value close to 0. As an example, the preset current threshold may range from 0 to 0.2 A. As another example, the preset current threshold may be 0 A. When the preset current threshold is 0 A, when the clamping capacitor C C and the transformer leakage inductance L k When the resonant current is 0A, the control unit turns off the clamp switch HS, thereby improving the recovery efficiency of the leakage inductance and reducing the turn-off loss of the clamp switch HS and the leakage inductance oscillation loss.
[0082] The following is combined with Figure 2-4 And attached Figure 7 Description in <n*V o , the working waveform of the active clamp flyback converter, at this time the input voltage V in can be less than the preset voltage threshold, of course the input voltage V in It may also be greater than or equal to a preset voltage threshold, that is, the input voltage may be a low voltage or a high voltage.
[0083] Phase 1 t0~t1: At t0, the primary switch L_S is turned off, and the primary current I p The output capacitor (also known as the junction capacitor) of the primary switch L_S is charged and the output capacitor (also known as the junction capacitor) of the clamp switch H_S is discharged. When the voltage V DS Rising to the input voltage V in Plus the clamping capacitor C C When the voltage is high, the clamp switch H_S is turned on, V DS is clamped, the leakage inductance L k To the clamp capacitor C C Charging, leakage inductance L k Energy is stored in the clamp capacitor C C Inside. Primary current I p Rapidly decreases. At time t1, the primary current I p Drops to 0, the secondary current I s Reached maximum value.
[0084] Phase 2 t1-t2: Secondary current I at time t1 s It starts to drop and transfer energy to the load at the output. At t2 in the demagnetization phase, the clamp switch H_S is turned on.
[0085] Phase 3 t2~t3: At t2, the clamp switch H_S is turned on, and the primary magnetizing inductor L m The voltage across the two ends is clamped by the secondary circuit output, and the clamping capacitor C C and transformer leakage inductance L k Start to resonate, clamp capacitor C C Energy begins to transfer to the secondary circuit. After half a resonant cycle, the resonant current reaches 0A at time t3, and the clamp switch H_S is controlled to turn off. At this time, the leakage inductance L k Energy is recovered, and the turn-off loss is small at this time, and the leakage inductance L k No oscillation will be generated, and leakage inductance loss is reduced. It should be noted that the clamp switch H_S can be turned on at any time during the demagnetization stage.
[0086] Phase 4 t3~t4: At this time, the secondary winding continues to demagnetize until the secondary current Is drops to 0 at time t4.
[0087] Phase 5 t4~t5: Starting from t4, the primary magnetizing inductance L m It resonates with the junction capacitance of the primary switch L_S, the clamp switch H_S and the secondary switch tube SR. DS Start from V in +n*V o Start oscillating, the oscillation amplitude is n*V o , because in this embodiment V in <n*V o , at time t5 VDS Oscillates to 0V, at which time the primary switch L_S is turned on to achieve ZVS.
[0088] Phase 6 t5~t6(t0): At t5, the primary switch L_S is turned on, and the primary voltage of the transformer is V in , transformer excitation current I m And the primary current I p The secondary switch SR is turned off. At t0, the primary current I p When the preset value is reached, it shuts down.
[0089] like Figure 7 As shown, when V in <n*V o When the primary switch L_S is closed, the clamp switch H_S is turned on during the transformer demagnetization stage (t0~t4). m Clamped to n*V by the secondary circuit o , leakage inductance L k and the clamping capacitor C C Resonance, the clamping capacitor C C The energy is transferred to the secondary circuit. When the resonant current reaches 0, the clamp switch H_S is turned off to reduce the oscillation of the leakage inductance and the hard turn-off loss.
[0090] Combine the following Figure 2-4 And attached Figure 8 This indicates that the input voltage is low and V in >n*V o The working waveform of the active clamp flyback converter when .
[0091] Phase 1 t0~t1: At t0, the primary switch L_S is turned off, and the primary current I p The output capacitor (also known as the junction capacitor) of the primary switch L_S is charged and the output capacitor (also known as the junction capacitor) of the clamp switch H_S is discharged. When the voltage V DS Rising to the input voltage V in Plus the clamping capacitor C C When the voltage is high, the clamp switch H_S is turned on, V DS is clamped, the leakage inductance L k To the clamp capacitor C C Charging, leakage inductance L k Energy is stored in the clamp capacitor C C Inside. Primary current I p Rapidly decrease. At time t1, the primary current drops to 0, and the secondary current I s Reached maximum value.
[0092] Phase 2 t1-t2: Secondary current I at time t1 sIt starts to drop and transfer energy to the load at the output. At t2 in the demagnetization phase, the clamp switch H_S is turned on.
[0093] Phase 3 t2~t3: At t2, the clamp switch H_S is turned on, and the primary magnetizing inductor L m The voltage across the two ends is clamped by the secondary circuit output, and the clamping capacitor C C and transformer leakage inductance L k Start to resonate, clamp capacitor C C Energy begins to transfer to the secondary circuit. After half a resonant cycle, the resonant current reaches 0A at time t3, and the clamp switch H_S is controlled to turn off. At this time, the leakage inductance L k Energy is recovered, and the turn-off loss is small at this time, and the leakage inductance L k No oscillation will be generated, and leakage inductance loss will be reduced. It should be noted that the clamp switch H_S can be turned on at any time during demagnetization.
[0094] Phase 4 t3~t4: At this time, the secondary winding continues to demagnetize until the secondary current Is drops to 0 at time t4.
[0095] Phase 5 t4~t5: Starting from t4, the primary magnetizing inductance L m It resonates with the junction capacitance of the primary switch L_S, the clamp switch H_S and the secondary switch SR. DS Start from V in +n*V o Start oscillating, the oscillation amplitude is n*V o , because in this embodiment V in >n*V o , at time t5 V DS The oscillation to the bottom cannot oscillate to 0V. At this time, the primary switch L_S is turned on. Here, the hard turn-on voltage of the primary switch L_S is V in -n*V o , because V in It is relatively small, so the loss is relatively small.
[0096] Phase 6 t5~t6(t0): At t5, the primary switch L_S is turned on, and the primary voltage of the transformer is V in , transformer excitation current I m and the primary current I p The secondary switch SR is turned off when the current increases linearly. At t6, the primary current reaches the preset value and is turned off.
[0097] like Figure 8 As shown, when the input voltage is low and V in >n*V o When V DS It cannot completely oscillate to 0, but the input voltage V inFor low voltage, the hard switching loss is relatively small, so the clamp switch H_S can be turned on during the transformer demagnetization stage (t0~t4) to release the clamp capacitor C C energy, thereby reducing the additional negative excitation current, reducing the core loss and conduction loss.
[0098] In some embodiments, the active clamp flyback converter further includes a secondary circuit, and the secondary circuit is electrically connected to the secondary winding. in is greater than or equal to the preset voltage threshold, and the input voltage V in Greater than the output voltage V of the active clamp flyback converter o and the turns ratio of the transformer, that is, the input voltage V in is high voltage, and V in >n*V o When the secondary circuit is switched off, the following steps can be performed: In the same switching cycle, after the secondary circuit freewheeling phase ends (such as Fig. 9 After the t4 moment, Fig.10 After the t2 moment), the clamp switch H_S is controlled to be turned on; and before the primary switch L_S is turned on (such as Fig. 9 Before time t6, Fig.10 Before the moment t4, the clamp switch H_S is controlled to be turned off. Fig.10 In the embodiment shown, after the freewheeling phase ends, the clamp switch H_S is at the oscillation peak (eg Fig.10 The clamp switch H_S is turned on for a specific time at time t3 to feed back the clamp energy and generate a negative excitation current to achieve ZVS. In this embodiment, the clamp switch H_S is turned on only once in the same switching cycle. For the specific time period, please refer to the description of the working waveform of the active clamp flyback converter below.
[0099] In some embodiments, Fig. 9 As shown, when the input voltage V in is greater than or equal to the preset voltage threshold, and the input voltage V in Greater than the output voltage V of the active clamp flyback converter o and the turns ratio of the transformer, that is, the input voltage V in is high voltage, and V in >n*V o When the secondary circuit is in the freewheeling phase (e.g. Fig. 9 After the moment t4 of the switching operation, the clamp switch H_S is controlled to be turned on; and before the primary switch L_S is turned on (such as Fig. 9Before the time t6 of the current flow, the clamp switch H_S is controlled to be turned off. In this embodiment, after the freewheeling phase ends, the clamp switch H_S is at the oscillation peak (such as Fig. 9 At t5 (at the moment of t5), it is turned on for a specific time to feed back the clamping energy and generate a negative excitation current to achieve ZVS. In addition, the following steps are performed: In the same switching cycle, when the transformer is in the demagnetization stage (such as Fig. 9 When the resonant current of the clamping capacitor and the leakage inductance of the transformer reaches a preset current threshold (such as t0-t4), the clamping switch H_S is controlled to be turned on; and when the resonant current of the clamping capacitor and the leakage inductance of the transformer reaches a preset current threshold (such as Fig. 9 At time t3, the clamp switch H_S is controlled to be turned off. In this embodiment, the clamp switch H_S is turned on twice in the same switching cycle. The specific time periods of the two times are described in the following description of the working waveform diagram of the active clamp flyback converter.
[0100] In the above Fig. 9 and Fig.10 In the embodiment shown, after the freewheeling phase of the secondary circuit ends, the clamp switch H_S is controlled to be turned on. Specifically, after the freewheeling phase of the secondary circuit ends, when the voltage across the primary switch L_S oscillates to a peak value, the clamp switch H_S is controlled to be turned on. Fig. 9 t0~t4, Fig.10 For details of t0-t2, the specific on-time period of the clamp switch H_S, see the description of the working waveform of the active clamp flyback converter below.
[0101] In some embodiments, before the primary switch L_S is turned on, the clamp switch H_S is controlled to be turned off, including: according to the input voltage V in , the output voltage of the active clamp flyback converter, the equivalent capacitance of the primary switch L_S, and the magnetizing inductance of the transformer are used to determine the conduction time; before the primary switch L_S is turned on, and when the on time of the clamp switch H_S reaches the on time, the clamp switch H_S is controlled to be turned off.
[0102] In some embodiments, the on-time value is positively correlated with the input voltage of the active clamped flyback converter, the on-time value is negatively correlated with the output voltage of the active clamped flyback converter, the on-time value is positively correlated with the equivalent capacitance of the primary switch, and the on-time value is positively correlated with the magnetizing inductance of the transformer.
[0103] In some embodiments, the on-time may be determined by the following formula:
[0104]
[0105] Among them, t on_H Indicates the conduction time, V inrepresents the input voltage of the active clamp flyback converter, V o represents the output voltage of the active clamp flyback converter, C eq Represents the equivalent capacitance of the primary switch, L m represents the magnetizing inductance of the transformer, and n represents the turns ratio of the transformer.
[0106] Specifically, in some embodiments, according to the current I flowing through the secondary winding s , determine whether the secondary circuit is in the freewheeling stage, which can be when the primary switch L_S is in the off state and the current I flowing through the secondary winding s When the current is positive (such as Fig. 9 t0~t4, such as Fig.10 t0~t2), where the current I is defined s When the current flows from the lower end C of the secondary winding to the upper end D, it is a positive current, and the secondary circuit is in the freewheeling stage. For the specific period of the freewheeling stage, please refer to the description of the working waveform of the active clamp flyback converter below.
[0107] In the embodiment of the present disclosure, the input voltage V in is high voltage, and V in >n*V o The frequency reduction is controlled by controlling the time from the end of the freewheeling phase to the opening of the clamp switch H_S, thereby optimizing the efficiency of high voltage input and light load.
[0108] Combine the following Figure 2-Figure 4 And attached Fig.10 Description Input voltage V in is high voltage, and V in >n*V o This is the first working waveform of the active clamp flyback converter.
[0109] Phase 1 t0~t1: At t0, the primary switch L_S is turned off, and the primary current I p The output capacitance (also known as the junction capacitance) of the primary switch L_S is charged and the output capacitance (also known as the junction capacitance) of the clamp switch H_S is discharged. The voltage V DS Rising to the input voltage V in Plus the clamping capacitor C C Voltage, primary current I p Rapidly decreases. At time t1, the primary current I p Drops to 0, the secondary current I s Reached maximum value.
[0110] Phase 2 t1-t2: Secondary current I at time t1 s It starts to decrease and transfer energy to the load at the output end until the secondary current I sDown to 0.
[0111] Phase 3 t2~t3: Starting from t2, the primary magnetizing inductance L m It resonates with the junction capacitance of the primary switch L_S, the clamp switch H_S and the secondary switch SR. DS Start from V in +n*V o Start oscillating, the oscillation amplitude is n*V o After a set time, V DS When the voltage oscillates to the peak value, the clamp switch H_S is turned on.
[0112] Phase 4 t3~t4: At t3, the clamp switch H_S is turned on to generate a negative excitation current. After a preset time, the clamp switch H_S is turned off at t4, which helps the primary switch L_S achieve ZVS and reduce turn-on loss.
[0113] Phase 5 t4~t5(t0): At t4, the primary switch L_S ZVS is turned on, and the primary voltage of the transformer is V in , transformer excitation current I m And the primary current I p The secondary switch SR is turned off. At t5 (t0), the primary current I p When the preset value is reached, it shuts down.
[0114] like Fig. 9 As shown, when the input voltage V in is high voltage, and V in >n*V o The clamp switch H_S is turned on once after the freewheeling phase ends to generate a negative excitation current to achieve ZVS of the primary switch L_S. This can reduce the current of the clamp switch H_S, reduce the turn-off loss of the clamp switch H_S, and reduce the leakage inductance oscillation loss.
[0115] Combine the following Figure 2-Figure 4 And attached Fig. 9 Description Input voltage V in is high voltage, and V in >n*V o When , the second working waveform of the active clamp flyback converter is shown.
[0116] Phase 1 t0~t1: At t0, the primary switch L_S is turned off, and the primary current I p The output capacitor (also known as the junction capacitor) of the primary switch L_S is charged and the output capacitor (also known as the junction capacitor) of the clamp switch H_S is discharged. When the voltage V DS Rising to the input voltage V in Plus the clamping capacitor C CWhen the voltage is high, the clamp switch H_S is turned on, V DS is clamped, the leakage inductance L k To the clamp capacitor C C Charging, leakage inductance L k Energy is stored in the clamp capacitor C C Inside. Primary current I p Rapidly decreases. At time t1, the primary current I p Drops to 0, the secondary current I s Reached maximum value.
[0117] Phase 2 t1-t2: Secondary current I at time t1 s It starts to drop and transfer energy to the load at the output. At t2 in the demagnetization phase, the clamp switch H_S is turned on.
[0118] Phase 3 t2~t3: At t2, the clamp switch H_S is turned on, and the primary magnetizing inductor L m The voltage across the two ends is clamped by the secondary circuit output, and the clamping capacitor C C and transformer leakage inductance L k Start to resonate, clamp capacitor C C Energy begins to transfer to the secondary circuit. After half a resonant cycle, the resonant current reaches 0A at time t3, and the clamp switch H_S is controlled to turn off. At this time, the leakage inductance L k Energy is recovered, and the turn-off loss is small at this time, and the leakage inductance L k No oscillation will occur and leakage inductance loss will be reduced.
[0119] Phase 4 t3~t4: At this time, the secondary winding continues to demagnetize until the secondary current I s Down to 0.
[0120] Phase 5 t4~t5: Starting from t4, the primary magnetizing inductance L m It resonates with the junction capacitance of the primary switch L_S, the clamp switch H_S and the secondary switch SR. DS Start from V in +n*V o Start oscillating, the oscillation amplitude is n*V o After a set time, V DS When the voltage oscillates to the peak value, the clamp switch H_S is turned on again.
[0121] Phase 6 t5~t6: At t5, the clamp switch H_S is turned on, generating a negative excitation current. After a preset time, the clamp switch H_S is turned off at t6, which helps the primary switch L_S achieve ZVS and reduce the turn-on loss. Because the clamp switch H_S has been turned on once in the freewheeling phase to release the energy of the clamp capacitor. After the freewheeling phase, the current after the clamp switch H_S is turned on is greatly reduced, which is conducive to reducing the turn-off loss of the clamp switch H_S and the leakage inductance oscillation loss.
[0122] Phase 7 t6~t7(t0): At t6, the primary switch L_S ZVS is turned on, and the primary voltage of the transformer is V in , transformer excitation current I m And the primary current I p The secondary switch SR is turned off. At t7 (t0), the primary current I p When the preset value is reached, it shuts down.
[0123] like Fig. 9 As shown, when the input voltage V in is high voltage, and V in >n*V o The clamp switch H_S is first turned on once in the demagnetization stage to release the leakage inductance energy, and then turned off when the resonant current is close to 0 A. After the freewheeling stage, the reverse excitation is turned on again to achieve ZVS of the primary switch L_S, which can reduce the current of the second opening of the clamp switch H_S, reduce the turn-off loss of the clamp switch H_S and the leakage inductance oscillation loss.
[0124] The control method of the active clamp flyback converter provided in the embodiment of the present disclosure is as follows: when the input voltage is low voltage, or V in <n*V o When the transformer is demagnetized, the clamp switch H_S is turned on, and the clamp capacitor C c When the leakage inductance resonant current is close to 0A, the clamp switch H_S is turned off. This can reduce the core loss caused by the negative excitation current and the secondary switch rectification loss. In addition, the clamp capacitor C c When the leakage inductance resonant current is close to 0A, the clamp switch H_S is turned off to reduce the turn-off loss of the clamp switch H_S and the leakage inductance oscillation loss. The efficiency improvement is more obvious when the leakage inductance is relatively large. in >n*V o The clamp switch H_S is turned on after the freewheeling phase ends and works in non-complementary discontinuous mode, which effectively reduces the frequency, reduces switching losses and improves the average efficiency.
[0125] like Fig.11As shown, the embodiment of the present disclosure also provides an electronic device 1100, including a processor 1101, a memory 1102, and a program or instruction stored in the memory 1102 and executable on the processor 1101. When the program or instruction is executed by the processor 1101, each process of the control method embodiment of the above-mentioned active clamp flyback converter is implemented, and the same technical effect can be achieved. To avoid repetition, it will not be described here.
[0126] Other embodiments of the disclosure will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein.
[0127] The present disclosure is intended to cover any variation, use or adaptation of the present disclosure, which follows the general principles of the present disclosure and includes common knowledge or customary technical means in the technical field not disclosed in the present disclosure. The description and examples are to be regarded as exemplary only, and the true scope and spirit of the present disclosure are indicated by the appended claims.
Claims
1. A control method for an active clamp flyback converter, characterized in that: The active clamp flyback converter comprises: A transformer, comprising a primary winding and a secondary winding coupled to each other; A primary switch connected in series with the primary winding; A clamping branch connected in parallel to the primary winding or in parallel to the primary switch; the clamping branch comprises a clamping switch and a clamping capacitor connected in series; The control method comprises: When the input voltage of the active clamped flyback converter is less than a preset voltage threshold, or the input voltage of the active clamped flyback converter is less than the product of the output voltage of the active clamped flyback converter and the turns ratio of the transformer, the following steps are performed: In the same switching cycle, when the transformer is in the demagnetization stage, the clamp switch is controlled to be turned on; and when the resonant current of the clamp capacitor and the leakage inductance of the transformer reaches a preset current threshold, the clamp switch is controlled to be turned off.
2. The control method according to claim 1, characterized in that: The active clamped flyback converter further includes a secondary circuit, the secondary circuit being electrically connected to the secondary winding, wherein when the input voltage of the active clamped flyback converter is greater than or equal to the preset voltage threshold, and the input voltage of the active clamped flyback converter is greater than the product of the output voltage of the active clamped flyback converter and the turns ratio of the transformer, the control method includes: In the same switching cycle, after the freewheeling phase of the secondary circuit ends, the clamp switch is controlled to be turned on; and before the primary switch is turned on, the clamp switch is controlled to be turned off.
3. The control method according to claim 2, characterized in that: When the input voltage of the active clamped flyback converter is greater than or equal to the preset voltage threshold, and the input voltage of the active clamped flyback converter is greater than the product of the output voltage of the active clamped flyback converter and the turns ratio of the transformer, the control method further includes: In the same switching cycle, when the transformer is in the demagnetization stage, the clamp switch is controlled to be turned on; and when the resonant current of the clamp capacitor and the leakage inductance of the transformer reaches the preset current threshold, the clamp switch is controlled to be turned off.
4. The control method according to claim 2 or 3, characterized in that: After the freewheeling phase of the secondary circuit ends, controlling the clamp switch to turn on includes: After the freewheeling phase of the secondary circuit ends, when the voltage across the primary switch oscillates to a peak value, the clamp switch is controlled to be turned on.
5. The control method according to claim 2 or 3, characterized in that: Before the primary switch is turned on, controlling the clamp switch to be turned off includes: Determining the conduction time according to the input voltage of the active clamp flyback converter, the output voltage of the active clamp flyback converter, the equivalent capacitance of the primary switch, and the magnetizing inductance of the transformer; Before the primary switch is turned on, and when the on time of the clamp switch reaches the on time, the clamp switch is controlled to be turned off.
6. The control method according to claim 2 or 3, characterized in that: After the freewheeling stage of the secondary circuit ends, the clamp switch is controlled to be turned on. When the primary switch is turned off and the current flowing through the secondary winding is a positive current, the secondary circuit is in the freewheeling stage.
7. The control method according to claim 1 or 3, characterized in that: The control method further comprises: Whether the transformer is in the demagnetization stage is determined according to the current flowing through the magnetizing inductance of the primary winding.
8. The control method according to claim 7, characterized in that: Determining whether the transformer is in the demagnetization stage according to the current flowing through the magnetizing inductance of the primary winding includes: When the current flowing through the magnetizing inductance of the primary winding is a positive current and shows a decreasing trend, the transformer is in the demagnetization stage.
9. The control method according to claim 1, characterized in that: When the input voltage of the active clamped flyback converter is less than the preset voltage threshold, or the input voltage of the active clamped flyback converter is less than the product of the output voltage of the active clamped flyback converter and the turns ratio of the transformer, the active clamped flyback converter operates in a single-pulse non-complementary mode.
10. The control method according to claim 1 or 3, characterized in that: The preset current threshold is 0A.
11. The control method according to claim 1, characterized in that: When the input voltage of the active clamped flyback converter is less than the product of the output voltage of the active clamped flyback converter and the turns ratio of the transformer, the voltage across the primary switch oscillates to 0V; otherwise, the voltage across the primary switch oscillates less than 0V.
12. The control method according to any one of claims 1 to 11, characterized in that: The active clamp flyback converter further includes a secondary circuit, wherein the secondary circuit includes an output capacitor; The control method further comprises: When the primary switch is turned off and the clamp switch is turned on, the clamp capacitor transfers energy to the output capacitor and / or an output load through the clamp switch.
13. An active clamp flyback converter, characterized in that: include: A transformer, comprising a primary winding and a secondary winding coupled to each other; A primary switch, connected in series to the primary winding; A clamping branch, connected in parallel to the primary winding or in parallel to the primary switch; the clamping branch comprises a clamping switch and a clamping capacitor connected in series; a control unit connected to the primary switch and the clamp switch, wherein the control unit is configured to: when the input voltage of the active clamp flyback converter is less than a preset voltage threshold, or the input voltage of the active clamp flyback converter is less than the product of the output voltage of the active clamp flyback converter and the turns ratio of the transformer, control the clamp switch to turn on when the transformer is in a demagnetization stage within the same switching cycle; And, when the resonant current of the clamping capacitor and the leakage inductance of the transformer reaches a preset current threshold, the clamping switch is controlled to be turned off.