A flyback converter

Abstract

The present invention relates to the field of switching power supplies and discloses a flyback converter. Based on an existing trailing-edge non-complementary flyback active clamp circuit, an auxiliary winding and a rectifier are added to the primary side of the flyback converter. This circuit is used to transfer a portion of the transformer's energy to the clamp capacitor via the rectifier when the transformer transfers energy to the secondary side after the main power switch is turned off, thereby increasing the charging voltage of the clamp capacitor and enabling the clamp capacitor to store more energy. When the clamp switch is turned on, the energy of the clamp capacitor is transferred to the primary leakage inductance, thereby ensuring that the energy stored in the primary leakage inductance is sufficiently large. When the clamp switch is turned off, the leakage inductance current can resonate the junction capacitance voltage of the main power switch to zero, thereby achieving zero-voltage switching of the main power switch. While the present invention enables lossless absorption of leakage inductance energy, it also simplifies the control method for zero-voltage switching of the main power switch, enabling the main power switch to achieve zero-voltage switching in both continuous conduction mode and discontinuous conduction mode.

Description

Technical Field

[0001] The present invention relates to the field of switching power supplies, and in particular to the field of flyback converters. Background Art

[0002] In order to achieve lossless absorption of leakage inductance energy on the primary side of the flyback converter and zero voltage turn-on of the main power switch, an active clamping circuit is usually introduced. The existing flyback converter with an active clamping circuit on the primary side is as follows: Figure 1 As shown, the flyback converter includes a main power switch tube S1, a main power transformer T1 (where Lkp is the parasitic leakage inductance of the primary side Np of the transformer), an active clamping circuit consisting of a clamping switch tube S2 and a clamping capacitor C2, an output rectifier diode D1 and an output filter capacitor C1; the load Ro is connected in parallel between the positive and negative power outputs of the flyback converter.

[0003] The specific connection relationship of the flyback converter is as follows: one end of the primary winding of the main power transformer T1 is connected to the positive input of the power supply, the other end of the primary winding of the main power transformer T1 is connected to the drain of the main power switch tube S1, and the source of the main power switch tube S1 is grounded; one end of the secondary winding of the main power transformer T1 is connected to the anode of the output rectifier diode D1, the cathode of the output rectifier diode D1 is connected to one end of the output filter capacitor C1 and the positive output of the power supply, and the other end of the secondary winding of the main power transformer T1 is connected to the other end of the output filter capacitor C1 and the negative output of the power supply; one end of the primary winding of the main power transformer T1 and the other end of the secondary winding of the main power transformer T1 are the same-name terminals; one end of the clamping capacitor C2 is connected to one end of the primary winding of the main power transformer T1, the other end of the clamping capacitor C2 is connected to the drain of the clamping switch tube S2, and the source of the clamping switch tube S2 is connected to the other end of the primary winding of the main power transformer T1.

[0004] For lossless absorption of primary-side leakage inductance energy, the existing active clamping circuit can achieve this by controlling the on and off of the clamping switch S2. For the zero-voltage turn-on of the main power switch S1, a trailing-edge non-complementary control logic can be adopted, that is, the clamping switch S2 is turned on for a period of time at a certain moment before the main power switch S1 is turned on, and a dead time is reserved between the time when the clamping switch S2 drive signal is turned off and the time when the main power switch S1 is turned on. For the waveform diagram of the main power switch S1 and the clamping switch S2 using the trailing-edge non-complementary control logic, see Figure 2 , where G S1 The driving waveform of the main power switch S1, G S2 It is the driving waveform of the clamp switch tube S2.

[0005] At t0-t1, the main power switch tube S1 is turned on and begins to excite the transformer. During this period, the clamp switch tube S2 and the output rectifier diode D1 are in the off state, and the transformer does not transfer energy to the secondary side.

[0006] At time t1-t2, the main power switch tube S1 is turned off and the clamp switch tube S2 is not turned on yet. The primary side excitation inductance Np and the primary side leakage inductance Lkp charge the clamp capacitor C2. When the voltage across the clamp capacitor C2 exceeds a certain value, the excitation current will transfer energy to the secondary side and cannot continue to charge the clamp capacitor C2.

[0007] At time t2-t3, the clamp switch S2 is turned on, and the energy of the clamp capacitor C2 is transferred to the primary leakage inductor Lkp and the load Ro. The problem at this stage is that the charging energy of the clamp capacitor C2 is insufficient, so the energy provided to the primary leakage inductor Lkp during discharge cannot meet the ZVS requirement.

[0008] At time t3-t4, the clamp switch S2 is turned off and the main power switch S1 is not yet turned on. The current of the primary leakage inductor Lkp discharges the drain-source parasitic capacitance of the main power switch S1. The problem at this stage is that the energy of the primary leakage inductor Lkp is insufficient. In continuous mode, the voltage across the magnetizing inductor is clamped by the secondary output voltage and cannot participate in resonance. Therefore, the energy of the drain-source parasitic capacitance of the main power switch S1 cannot be fully released.

[0009] At time t4-t5, due to the above problem, ZVS cannot be achieved before the main power switch S1 is turned on.

[0010] The above existing trailing edge non-complementary control logic has the following two requirements:

[0011] 1. The dead time between the closing of the clamp switch S2 and the opening of the main power switch S1 is required to be small enough. After the clamp switch S2 is closed, if the main power switch S1 is not turned on in time, the leakage inductance current will reverse again, thus losing the soft switching characteristics. Usually, the dead time T d Select as:

[0012]

[0013] Among them, L kp is the primary leakage inductance, C ds_s1 The drain-source parasitic capacitance of the main power switch S1, C ds_s1 is the drain-source parasitic capacitance of the clamping switch tube S1.

[0014] Normally, the drain-source parasitic capacitance of the switch tube is in the nF level, and the primary leakage inductance L kp It also belongs to nH level, so the dead time T d It belongs to the ns level and is difficult to control.

[0015] 2. Primary leakage inductance L is required kpThe energy is large enough. The reverse energy stored in the leakage inductance must be greater than the energy stored in the output capacitor of the main power switch tube S1, that is:

[0016]

[0017] Among them I LK is the equivalent leakage inductance of the primary side, V ds_s1 The drain-source parasitic capacitance of the main power switch tube S1.

[0018] According to formula (2), the minimum leakage inductance is:

[0019]

[0020] Regarding the above two points, the active clamp flyback circuit using trailing-edge non-complementary control logic is divided into CCM and DCM modes for discussion:

[0021] (1) Discontinuous conduction mode (DCM mode): This usually occurs in high-voltage and light-load mode. In formula (3), V ds_s1 Larger and I Lkp Small, requiring leakage inductance L kp The value is large, but in DCM mode, since the primary excitation inductance participates in the resonance process of leakage inductance and output capacitance of main power switch tube S1, the primary leakage inductance L kp ’ =L kp +Primary inductance, the magnetizing inductance can assist the leakage inductance to complete the zero-voltage turn-on of the main power switch tube, and due to the participation of the primary excitation inductance, the dead time can be relatively increased.

[0022] (2) Continuous conduction mode (CCM mode): Since the primary excitation inductance is clamped by the output side, it cannot participate in the resonance process of the leakage inductance and the output capacitance of the main power switch tube S1. Therefore, it is necessary to satisfy formula (1) and formula (3) at the same time. That is, the dead time from the closing of the clamping switch tube S2 to the opening of the main power switch tube S1 is required to be small enough, and at the same time, the primary leakage inductance L of the transformer must be ensured. k Big enough.

[0023] In summary, in CCM mode, it is difficult or even impossible to achieve zero-voltage turn-on of the main power switch S1 using the existing flyback converter and trailing-edge non-complementary control logic.

[0024] The above information disclosed in the background technology section is only used to enhance the understanding of the background of the present application and therefore it may contain related technical information beyond the cognition of ordinary technicians in this field. Summary of the Invention

[0025] In view of this, the technical problem to be solved by the present invention is to propose a flyback converter, that is, on the basis of the original flyback converter, an auxiliary winding and a rectifier tube are added to the primary side of the transformer, and by transferring part of the energy of the transformer to the clamping capacitor, the energy storage capacity of the clamping capacitor is increased, thereby achieving ZVS of the main power switch tube in any state.

[0026] In order to solve the above technical problems, the technical solutions of the present invention are as follows:

[0027] A flyback converter comprises a primary circuit, a transformer, and a secondary circuit. The primary circuit comprises at least the primary winding of the transformer, a main power switch, and an active clamping circuit. The active clamping circuit comprises at least an active clamping switch and a clamping capacitor. The main power switch and the active clamping switch are controlled using trailing-edge non-complementary control logic. The converter is characterized in that a clamping capacitor charging circuit is further provided. The clamping capacitor charging circuit comprises an auxiliary winding and a rectifier connected in series with the auxiliary winding. The clamping capacitor charging circuit is configured to transfer the transformer's stored energy to the clamping capacitor via the auxiliary winding and the rectifier when the transformer transfers energy to the secondary circuit after the main power switch is turned off, thereby increasing the charging voltage of the clamping capacitor.

[0028] Preferably, one end of the primary winding of the transformer is connected to the positive input of the power supply, the other end of the primary winding of the transformer is connected to the drain of the main power switch tube, and the source of the main power switch tube is grounded; one end of the clamping capacitor is connected to one end of the primary winding of the transformer, the other end of the clamping capacitor is connected to the drain of the active clamping switch tube, and the source of the active clamping switch tube is connected to the other end of the primary winding of the transformer;

[0029] One end of the auxiliary winding is connected to the positive input terminal of the power supply, the other end of the auxiliary winding is connected to the anode of the rectifier tube, and the cathode of the rectifier tube is connected to the other end of the clamping capacitor.

[0030] Preferably, one end of the primary winding of the transformer is connected to the positive input of the power supply, the other end of the primary winding of the transformer is connected to the drain of the main power switch tube, and the source of the main power switch tube is grounded; one end of the clamping capacitor is connected to one end of the primary winding of the transformer, the other end of the clamping capacitor is connected to the drain of the active clamping switch tube, and the source of the active clamping switch tube is connected to the other end of the primary winding of the transformer;

[0031] The anode of the rectifier tube is connected to the positive terminal of the power input, the cathode of the rectifier tube is connected to one end of the auxiliary winding, and the other end of the auxiliary winding is connected to the other end of the clamping capacitor.

[0032] Preferably, the rectifier tube is a diode, a PN junction of a MOS tube or a PN junction of a transistor.

[0033] Preferably, a leakage inductance is connected in series between one end of the primary winding of the transformer and the positive input of the power supply, and the leakage inductance is a parasitic leakage inductance of the primary winding of the transformer or an external independent inductance.

[0034] The working principle of the present invention will be analyzed in detail in conjunction with specific embodiments and will not be elaborated here. The beneficial effects of the present invention compared with the prior art are as follows:

[0035] When the main power switch tube of the primary circuit is turned off and the transformer transfers energy to the secondary side, the auxiliary winding charges the clamping capacitor, thereby increasing the charging voltage of the clamping capacitor to ensure that the energy stored in the primary leakage inductance is large enough. This not only simplifies the control method of achieving ZVS of the main power switch tube of the primary circuit, but also enables the main power switch tube to achieve ZVS in any state. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 The schematic diagram of the existing flyback converter with an active clamp circuit on the primary side;

[0037] Figure 2 The present invention is a control logic diagram of a flyback converter with an active clamp circuit on the primary side;

[0038] Figure 3 is a schematic diagram of a first embodiment of a flyback converter according to the present invention;

[0039] Figure 4 FIG. 4 is a schematic diagram of a flyback converter according to a second embodiment of the present invention. DETAILED DESCRIPTION

[0040] The inventive concept of the present application is to add an auxiliary winding Nf and a rectifier tube D2 to the primary circuit of the flyback converter on the basis of the existing trailing-edge non-complementary flyback active clamp circuit, so that when the main power switch tube S1 is turned off and the transformer transfers energy to the secondary side, a part of the transformer energy is transferred to the clamping capacitor C2 through the auxiliary winding Nf and the rectifier tube D2, thereby increasing the charging voltage of the clamping capacitor C2, so that the clamping capacitor C2 can store more energy and ensure the leakage inductance L KP The stored energy is large enough that when the clamp switch S2 is turned off, the leakage inductance L KP The current can resonate the junction capacitance voltage of the main power switch tube S1 to zero, thereby achieving zero voltage turn-on of the main power switch tube S1.

[0041] The present invention makes the leakage inductance L KPWhile absorbing energy losslessly, the control method of zero-voltage turn-on of the main power switch tube S1 is simplified, so that the main power switch tube S1 can achieve zero-voltage turn-on regardless of continuous conduction mode or discontinuous conduction mode.

[0042] In order to enable those skilled in the art to better understand the present invention, the present invention is further described below in conjunction with a specific implementation circuit.

[0043] First embodiment

[0044] Figure 3 The schematic diagram of the first embodiment of the flyback converter of the present invention is shown. The flyback converter includes a primary circuit, a transformer T1, a secondary circuit and a clamping capacitor charging circuit. The primary circuit includes a primary winding N of the transformer T1. P , a main power switch tube S1 and an active clamping circuit, wherein the active clamping circuit includes an active clamping switch tube S2 and a clamping capacitor C2; the secondary loop has a rectifier tube D1 and a capacitor C1.

[0045] The primary winding N of transformer T1 P One end is connected to the positive input of the power supply VIN (where the leakage inductance L KP is the primary winding N of transformer T1 P The parasitic leakage inductance of the transformer T1 is N P The other end of the clamp capacitor C2 is connected to the drain of the main power switch tube S1, and the source of the main power switch tube S1 is grounded; one end of the clamp capacitor C2 is connected to the primary winding N of the transformer T1. P One end of the clamp capacitor C2 is connected to the drain of the active clamp switch S2, and the source of the active clamp switch S2 is connected to the primary winding N of the transformer T1. P The main power switch tube S1 and the active clamp switch tube S2 are controlled by trailing-edge non-complementary control logic.

[0046] The clamping capacitor charging circuit consists of an auxiliary winding Nf and a rectifier D2 connected in series with the auxiliary winding Nf. Specifically, one end of the auxiliary winding Nf is connected to the positive input of the power supply VIN, and the other end is connected to the anode of the rectifier D2. The cathode of the rectifier D2 is connected to the other end of the clamping capacitor C2. In this embodiment, the rectifier D2 is a diode.

[0047] The following combination Figure 2 The working principle of this embodiment is described as follows:

[0048] Among them G S1 The driving waveform of the main power switch tube S1, G S2 is the driving waveform of the clamp switch tube S2;

[0049] At time 0-t1, the main power switch S1 is turned on and begins to excite the transformer T1. During this period, the switch S2 and the rectifier D1 are in the off state. The transformer T1 does not transfer energy to the secondary circuit. At the same time, the rectifier D2 is also in the off state, and the auxiliary winding Nf does not transfer energy to the clamping capacitor C2.

[0050] At time t1-t2, the main power switch S1 is turned off and the clamp switch S2 is not turned on. At this time, the transformer T1 needs to freewheel, and the rectifier D1 is in the on state. The transformer T1 transfers energy to the secondary circuit. At the same time, the rectifier D2 is also in the on state. The auxiliary winding Nf transfers energy to the clamp capacitor C2. On the other hand, the leakage inductor Lkp also transfers energy to the clamp capacitor C2, thereby increasing the charging voltage of the clamp capacitor C2.

[0051] At time t2-t3, the clamp switch S2 is turned on, and the energy of the clamp capacitor C2 is transferred to the leakage inductor Lkp and the load Ro. Since the charging voltage of the clamp capacitor C2 is increased and the energy required by the load Ro is fixed, the energy of the leakage inductor Lkp increases.

[0052] At time t3-t4, the clamp switch S2 is turned off and the main power switch S1 has not yet turned on. If the flyback converter operates in DCM mode, the primary-side magnetizing inductance, leakage inductance Lkp, and output capacitance of the main power switch S1 (i.e., drain-source parasitic capacitance) resonate together to achieve ZVS of the main power switch S1. This is similar to a conventional flyback converter. During the resonance process, due to the addition of the magnetizing inductance, the drain-source parasitic capacitance of the main power switch S1 easily resonates to zero. If the flyback converter operates in CCM mode, the leakage inductance current discharges the drain-source parasitic capacitance of the main power switch S1. Since the leakage inductance energy is large enough at this time, it is sufficient to reverse after the voltage across the main power switch S1 drops to zero, thereby ensuring ZVS of the main power switch S1.

[0053] At this point, a working cycle ends, and then the above process is repeated.

[0054] Second embodiment

[0055] The second embodiment Figure 4 As shown, the difference from the first embodiment is that the connection method of the auxiliary winding Nf and the rectifier tube D2 is different. Specifically, the anode of the rectifier tube is connected to the positive terminal of the power input, the cathode of the rectifier tube is connected to one end of the auxiliary winding, and the other end of the auxiliary winding is connected to the other end of the clamping capacitor.

[0056] The working principle of this embodiment is the same as that of the first embodiment and will not be described in detail here.

[0057] It should be noted that the preferred embodiments of the present invention described above should not be regarded as limiting the present invention. A person skilled in the art may make several improvements and modifications without departing from the spirit and scope of the present invention. For example, the same-name terminal of the transformer T1 may be modified, the rectifier tube added to the flyback converter may be replaced with other devices that can achieve the same function, such as the PN junction of a MOS tube or the PN junction of a transistor, and the rectifier tube D1 may be modified to a synchronous rectifier mode. All changes to the circuits that achieve this function should also be regarded as within the scope of protection of the present invention. The scope of protection of the present invention should be based on the scope defined by the claims.

Claims

1. A flyback converter comprising a primary circuit, a transformer, and a secondary circuit, wherein the primary circuit comprises at least a primary winding of the transformer, a main power switch, and an active clamping circuit, wherein: The active clamping circuit includes at least an active clamping switch and a clamping capacitor. The main power switch and the active clamping switch are controlled using trailing-edge non-complementary control logic. The circuit is characterized in that a clamping capacitor charging circuit is further provided. The clamping capacitor charging circuit has an auxiliary winding and a rectifier connected in series with the auxiliary winding. The clamping capacitor charging circuit is configured to transfer the transformer's stored energy to the clamping capacitor via the auxiliary winding and the rectifier when the transformer transfers energy to the secondary circuit after the main power switch is turned off, thereby increasing the charging voltage of the clamping capacitor.

2. The flyback converter according to claim 1, wherein: One end of the primary winding of the transformer is connected to the positive input of the power supply, the other end of the primary winding of the transformer is connected to the drain of the main power switch tube, and the source of the main power switch tube is grounded; one end of the clamping capacitor is connected to one end of the primary winding of the transformer, the other end of the clamping capacitor is connected to the drain of the active clamping switch tube, and the source of the active clamping switch tube is connected to the other end of the primary winding of the transformer; One end of the auxiliary winding is connected to the positive input terminal of the power supply, the other end of the auxiliary winding is connected to the anode of the rectifier tube, and the cathode of the rectifier tube is connected to the other end of the clamping capacitor.

3. The flyback converter according to claim 1, wherein: One end of the primary winding of the transformer is connected to the positive input of the power supply, the other end of the primary winding of the transformer is connected to the drain of the main power switch tube, and the source of the main power switch tube is grounded; one end of the clamping capacitor is connected to one end of the primary winding of the transformer, the other end of the clamping capacitor is connected to the drain of the active clamping switch tube, and the source of the active clamping switch tube is connected to the other end of the primary winding of the transformer; The anode of the rectifier tube is connected to the positive terminal of the power input, the cathode of the rectifier tube is connected to one end of the auxiliary winding, and the other end of the auxiliary winding is connected to the other end of the clamping capacitor.

4. The flyback converter according to claim 1, wherein: The rectifier tube is a diode, a PN junction of a MOS tube or a PN junction of a triode.

5. The flyback converter according to claim 1, wherein: A leakage inductance is connected in series between one end of the primary winding of the transformer and the positive input of the power supply. The leakage inductance is a parasitic leakage inductance of the primary winding of the transformer or an external independent inductance.

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