Flyback circuit and control method for clamping switch suitable for flyback circuit

Abstract

The present application provides a flyback circuit and a control method for a clamp switch suitable for the flyback circuit, wherein the flyback circuit includes a transformer, a main switch, a clamp capacitor, a clamp switch and a secondary side rectifying unit. The transformer includes a primary winding and a secondary winding, the turns ratio of the primary winding to the secondary winding is K, and K is a positive number. The main switch is connected in series with the primary winding and then receives an input voltage. The clamp capacitor and the clamp switch are connected in series and then connected in parallel with the primary winding. The secondary side rectifying unit is connected in series with the secondary winding and then supplies an output voltage to a load. The control method includes: when the product of K and the output voltage is greater than or equal to the input voltage, controlling the clamp switch to be turned on M times within N consecutive switching cycles of the main switch, where 1 ≤ M < N, and M and N are integers.

Description

Technical Field

[0001] This case relates to flyback circuits and their control methods, particularly a flyback circuit and a control method for a clamping switch applicable to flyback circuits. Background Technology

[0002] In recent years, with the miniaturization of power adapters, the requirements for efficiency have become increasingly stringent.

[0003] When a power adapter uses a quasi-resonant flyback circuit, zero-voltage switching of the main switch can be achieved through resonance between the parasitic capacitance of the main switch and the magnetizing inductance of the transformer at low input voltage. However, zero-voltage switching cannot be achieved through resonance when the input voltage is high. To solve this problem, active clamping is usually used. Active clamping is divided into complementary active clamping and non-complementary active clamping. Non-complementary active clamping is favored due to its high efficiency under light load and high-voltage input full load. When using non-complementary active clamping, although zero-voltage switching of the main switch can be achieved without turning on the clamp switch at low input voltage, failure to turn on the clamp switch will lead to energy accumulation in the clamping capacitor and overvoltage. Therefore, the clamp switch must be turned on for a period of time under low input voltage. Since turning on the clamp switch at low input voltage is only to release excess energy on the clamping capacitor, the required on time of the clamp switch is very short. However, due to chip manufacturing limitations, the minimum on-time of each clamping switch is restricted, making it impossible to achieve the shortest time required to release energy from the clamping capacitor. Furthermore, in existing control methods, the clamping switch conducts in every switching cycle of the main switch, generating an additional negative current in the circuit. This leads to increased peak and RMS currents, resulting in increased core and conduction losses and reduced efficiency under low-voltage full-load conditions. Therefore, for flyback circuits using non-complementary active clamping, the main obstacle to improving efficiency lies in efficiency under low-voltage full-load conditions.

[0004] Therefore, the urgent need is to invent a flyback circuit that can improve upon the aforementioned known technologies and a control method for a clamping switch applicable to flyback circuits. Summary of the Invention

[0005] The purpose of this invention is to provide a flyback circuit and a control method for a clamping switch applicable to the flyback circuit. This method can reduce the total conduction time by reducing the number of times the clamping switch on the primary side is turned on, effectively reducing the additional negative current generated by the clamping switch being turned on, thereby reducing core loss and conduction loss, and improving efficiency at low voltage.

[0006] To achieve the above objectives, the present application provides a control method for a clamp switch suitable for a flyback circuit, wherein the flyback circuit includes a transformer, a main switch, a clamp capacitor, the clamp switch, and a secondary side rectification unit. The transformer includes a primary winding and a secondary winding, the turns ratio of the primary winding to the secondary winding is K, and K is a positive number. The main switch is connected in series with the primary winding and then receives an input voltage. The clamp capacitor and the clamp switch are connected in series and then connected in parallel with the primary winding. The secondary side rectification unit is connected in series with the secondary winding and then supplies an output voltage to a load. The control method includes: when the product of K and the output voltage is greater than or equal to the input voltage, controlling the clamp switch to be turned on M times in N consecutive switching cycles of the main switch, where 1≤M<N, and M and N are integers.

[0007] To achieve the above objectives, the present application further provides a flyback circuit, including a transformer, a main switch, a clamp capacitor, a clamp switch, and a secondary side rectification unit. The transformer includes a primary winding and a secondary winding, wherein the turns ratio of the primary winding to the secondary winding is K, and K is a positive number. The main switch is connected in series with the primary winding and then receives an input voltage. The clamp capacitor and the clamp switch are connected in series and then connected in parallel with the primary winding. The secondary side rectification unit is connected in series with the secondary winding and then supplies an output voltage to a load. When the product of K and the output voltage is greater than or equal to the input voltage, the clamp switch is turned on M times in N consecutive switching cycles of the main switch, where 1≤M<N, and M and N are integers. Brief Description of the Drawings

[0008] Figure 1 is a schematic diagram of the circuit structure of the flyback circuit according to the preferred embodiment of the present application;

[0009] Figure 2A and Figure 2B respectively illustrate the operating waveforms of the flyback circuit 1 when operating in the critical mode and the discontinuous mode;

[0010] Figure 3A and Figure 3B respectively illustrate Figure 2A and Figure 2B modified examples;

[0011] Figure 4 is Figure 2A a schematic diagram of waveforms in a partial time period;

[0012] Figure 5 is Figure 2B a schematic diagram of waveforms in a partial time period.

[0013]

Description of Symbols

[0014] 1: Flyback circuit

[0015] 11: Transformer

[0016] 111: Primary winding

[0017] 112: Secondary winding

[0018] C1: Clamping capacitor

[0019] S1: Clamping switch

[0020] S2: Main switch

[0021] 12: Secondary-side rectifier unit

[0022] Ip: Primary current

[0023] Is: Secondary current

[0024] IC1: Current flowing through the clamping capacitor

[0025] Vin: Input voltage

[0026] Vo: Output voltage

[0027] Lk: Leakage inductance

[0028] Vds: Voltage between the drain and source of main switch S2

[0029] Dri_S1, Dri_S2: Drive signals

[0030] Lm: Magnetizing inductance

[0031] TS2: On-time of the main switch

[0032] T12: On-time of the secondary rectifier unit

[0033] Tdem: Demagnetization detection time

[0034] TS1: On-time of clamping switch

[0035] Tdead: Dead Time

[0036] Tdelay: Delay time

[0037] Tblank: Oscillation Time

[0038] AUX: Auxiliary winding

[0039] Vaux: Auxiliary winding voltage Detailed Implementation

[0040] Some typical embodiments that embody the features and advantages of this invention will be described in detail in the following description. It should be understood that this invention can have various variations in different forms, all of which do not depart from the scope of this invention, and the descriptions and illustrations therein are for illustrative purposes only and not intended to limit this invention.

[0041] Figure 1is a schematic circuit structural diagram of a flyback circuit according to a preferred embodiment of the present case. As Figure 1 shown, the flyback circuit 1 includes a transformer 11, a clamp capacitor C1, a clamp switch S1, a main switch S2, and a secondary side rectification unit 12. In Figure 1 , the primary side current is Ip, the secondary side current is Is, and the current flowing through the clamp capacitor C1 is IC1. The transformer 11 includes a primary winding 111 and a secondary winding 112, where the turns ratio of the primary winding 111 to the secondary winding 112 is K, and K is a positive number. The main switch S2 is connected in series with the primary winding 111 to receive an input voltage Vin. The clamp switch S1 is connected in series with the clamp capacitor C1 and then connected in parallel with the primary winding 111. The secondary side rectification unit 12 is connected in series with the secondary winding 112 to provide an output voltage Vo to a load, wherein the secondary side rectification unit 12 may be, but is not limited to, an active switch. In addition, Figure 1 Lk is the leakage inductance of the transformer 11, and Lm is the magnetizing inductance of the transformer 11.

[0042] the control method suitable for the clamp switch S1 of the flyback circuit 1 is described in detail as follows.

[0043] The function of turning on the clamp switch S1 mainly lies in realizing zero-voltage turn-on of the main switch S2 and releasing the energy in the clamp capacitor C1. When the product of K and the output voltage Vo is greater than or equal to the input voltage Vin (that is, under the condition that the input voltage Vin is relatively low), zero-voltage turn-on of the main switch S2 can be realized even if the clamp switch S1 is not turned on, so the turn-on frequency and duration of the clamp switch S1 only need to be sufficient to release the energy in the clamp capacitor C1. Therefore, under this condition of relatively low input voltage Vin, the clamp switch S1 is controlled to be turned on M times in N consecutive switching cycles of the main switch S2, where 1≤M<N, and M and N are integers. Accordingly, the total turn-on time of the clamp switch S1 can be reduced by reducing the number of turn-on times of the clamp switch S1, which effectively reduces the additional negative current generated by the turn-on of the clamp switch S1, thereby reducing core loss and turn-on loss, so as to improve the efficiency at low input voltage.

[0044] In some embodiments, when the product of K and the output voltage Vo is greater than or equal to the input voltage Vin, the clamp switch S1 is controlled to be turned on only once in four consecutive switching cycles of the main switch S2, that is, M / N=1 / 4. In some embodiments, in order to reduce the additional negative current as much as possible on the premise of being sufficient to release the energy in the clamp capacitor C1, when at least one of the output voltage Vo, the input voltage Vin and K changes, the value of M / N can be adjusted by correspondingly changing M and / or N. For example, when the output voltage Vo increases, M / N can be correspondingly reduced.

[0045] Furthermore, when the product of K and the output voltage Vo is less than the input voltage Vin (i.e., under the condition that the input voltage Vin is relatively high), it is determined whether the absolute value of the difference between the product of K and the output voltage Vo and the input voltage Vin (i.e., |K*Vo-Vin|) is less than a default value. If the determination result is yes, then the clamping switch S1 is controlled to conduct M times in N consecutive switching cycles of the main switch S2; if the determination result is no, then the clamping switch S1 is controlled to conduct once in each switching cycle of the main switch S2.

[0046] As the load changes, the operating mode of the flyback circuit 1 will also change accordingly. In some embodiments, the flyback circuit 1 operates in critical mode (i.e., critical discontinuous mode) when the load is greater than or equal to a threshold, and operates in discontinuous mode when the load is less than the threshold. Figure 2A and Figure 2B The operating waveforms of flyback circuit 1 in critical mode and discontinuous mode are shown respectively. Figure 2A and Figure 2B In the diagram, Vds is the voltage across the main switch S2, Dri_S1 is the drive signal for the clamping switch S1, and Dri_S2 is the drive signal for the main switch S2. Figure 2A and Figure 2B In the example, clamping switch S1 conducts once within eight consecutive switching cycles of main switch S2, i.e., M / N = 1 / 8, but in practice, this is not the limitation. For example... Figure 3A and Figure 3B As shown, the clamping switch S1 can also be turned on once within four consecutive switching cycles of the main switch S2, i.e., M / N = 1 / 4. During the switching cycles when the clamping switch S1 is not turned on, the leakage inductance energy charges the clamping capacitor C1 to store the energy. When the clamping switch S1 is turned on, the energy in the clamping capacitor C1 is released, thereby recovering the leakage inductance energy.

[0047] Please see Figure 1 and Figure 4 ,in Figure 4 for Figure 2AThe diagram shows waveforms for a portion of the time period. When the flyback circuit 1 operates in critical mode, during the switching cycle when the clamping switch S1 is on (here, the switching cycle refers to the switching cycle of the main switch S2), firstly, the main switch S2 is turned on for a duration of TS2, during which energy is stored in the magnetizing inductor Lm. After the main switch S2 is turned off, the secondary rectifier unit 12 is turned on to demagnetize the transformer 11, with a duration of T12. After the secondary rectifier unit 12 is turned off, it is detected whether the transformer 11 has finished demagnetizing, with a detection time of Tdem. After detecting that the transformer 11 has finished demagnetizing, the clamping switch S1 is turned on to release the energy in the clamping capacitor C1, with a duration of TS1. After the clamping switch S1 is turned off, the switching cycle ends after the dead time Tdead between the clamping switch S1 and the main switch S2. Furthermore, during the switching cycle when the clamping switch S1 is not conducting, after the demagnetization of the transformer 11 is detected, the switching cycle ends after a delay time Tdelay.

[0048] Please see Figure 1 and Figure 5 ,in Figure 5 for Figure 2B The diagram shows waveforms for a portion of the time period. When the flyback circuit 1 operates in discontinuous mode, during the switching cycle when the clamping switch S1 is on (here, the switching cycle refers to the switching cycle of the main switch S2), firstly, the main switch S2 is on, simultaneously storing energy in the magnetizing inductor Lm. After the main switch S2 is off, the secondary rectifier unit 12 is on to demagnetize the transformer 11. After the secondary rectifier unit 12 is off (i.e., after the transformer 11 is demagnetized), the voltage Vds across the main switch S2 oscillates to achieve frequency reduction and thereby improve efficiency under light load. The peak value of the oscillation of voltage Vds decreases, and the length of the oscillation time Tblank of voltage Vds depends on the required frequency reduction amplitude. The clamping switch S1 is on when the voltage Vds across the main switch S2 oscillates to its peak to release the energy in the clamping capacitor C1. After the clamping switch S1 is off, the switching cycle ends after the dead time Tdead between the clamping switch S1 and the main switch S2. Furthermore, during the switching cycle when the clamping switch S1 is not conducting, after the voltage Vds across the main switch S2 oscillates for the required duration, the switching cycle ends after a delay time Tdelay.

[0049] In some embodiments, such as Figure 1 As shown, the flyback circuit 1 also includes an auxiliary winding AUX, and as... Figure 4 and Figure 5As shown, the voltage Vaux across the auxiliary winding AUX has the same waveform shape as the voltage Vds across the main switch S2, but the magnitude of voltage Vaux is smaller, making it easier to detect. Therefore, in practice, during detection (e.g., demagnetization detection), voltage Vaux can be detected instead of voltage Vds.

[0050] In summary, this invention provides a flyback circuit and a control method for a clamping switch applicable to the flyback circuit. It can reduce the total conduction time by reducing the number of times the clamping switch on the primary side is turned on, effectively reducing the additional negative current generated by the clamping switch being turned on, thereby reducing core loss and conduction loss, and improving efficiency at low voltage.

[0051] It should be noted that the above are merely preferred embodiments for illustrating this case, and this case is not limited to the described embodiments. The scope of this case is determined by the appended claims. Furthermore, this case may be modified in various ways by those skilled in the art, but all such modifications shall not depart from the scope of protection sought by the appended claims.

Claims

1. A control method for a clamping switch suitable for flyback circuits, characterized in that, A flyback circuit includes a transformer, a main switch, a clamp capacitor, a clamp switch, and a secondary side rectification unit. The transformer includes a primary winding and a secondary winding, the turns ratio of the primary winding to the secondary winding is K, and K is a positive number. The main switch is connected in series with the primary winding to receive an input voltage. The clamp capacitor and the clamp switch are connected in series and then connected in parallel with the primary winding. The secondary side rectification unit is connected in series with the secondary winding to provide an output voltage to a load. The control method includes: When the product of K and the output voltage is greater than or equal to the input voltage, the clamp switch is controlled to be turned on M times in N consecutive switching cycles of the main switch, where 1≤M<N, and M and N are integers.

2. The control method according to claim 1, characterized in that, The method further includes: adjusting M / N correspondingly when at least one of the output voltage, the input voltage and K changes.

3. The control method according to claim 1, characterized in that, The method further includes: when the product of K and the output voltage is less than the input voltage, determining whether the absolute value of the difference between the product and the input voltage is less than a default value. If the determination result is yes, the clamp switch is controlled to be turned on M times in N consecutive switching cycles of the main switch; if the determination result is no, the clamp switch is controlled to be turned on once in each switching cycle of the main switch.

4. The control method according to claim 1, characterized in that, The control method further includes: controlling the flyback circuit to operate in a critical mode when the load is greater than or equal to a threshold value, and controlling the flyback circuit to operate in a discontinuous mode when the load is less than the threshold value.

5. The control method according to claim 4, characterized in that, When the flyback circuit operates in the critical mode, the control method further includes: within the switching cycle where the clamp switch is turned on, controlling the clamp switch to be turned on after the transformer is demagnetized, so as to release the energy in the clamp capacitor.

6. The control method according to claim 4, characterized in that, When the flyback circuit operates in the discontinuous mode, the control method further includes: within the switching cycle where the clamp switch is turned on, controlling the clamp switch to be turned on when the voltage across the main switch oscillates to a peak, so as to release the energy in the clamp capacitor.

7. The control method according to claim 1, characterized in that, When the product of K and the output voltage is greater than or equal to the input voltage, the control method controls the clamp switch to be turned on only once in four consecutive switching cycles of the main switch.

8. A flyback circuit, characterized in that, Comprises: A transformer, including a primary winding and a secondary winding, wherein the turns ratio of the primary winding to the secondary winding is K, and K is a positive number; A main switch, connected in series with the primary winding to receive an input voltage; A clamp capacitor and a clamp switch, connected in series and then connected in parallel with the primary winding; and A secondary side rectification unit, connected in series with the secondary winding to provide an output voltage to a load, Wherein, when the product of K and the output voltage is greater than or equal to the input voltage, the clamp switch is turned on M times in N consecutive switching cycles of the main switch, where 1≤M<N, and M and N are integers.

9. The flyback circuit according to claim 8, characterized in that, When at least one of the output voltage, the input voltage and K changes, M / N is adjusted correspondingly.

10. The flyback circuit according to claim 8, characterized in that, When the product of K and the output voltage is less than the input voltage, if the absolute value of the difference between the product and the input voltage is less than a default value, the clamping switch will only be turned on once in multiple consecutive switching cycles of the main switch. If the absolute value of the difference between the product and the input voltage is greater than or equal to the default value, the clamping switch will be turned on once in each switching cycle of the main switch.

11. The flyback circuit according to claim 8, characterized in that, The flyback circuit operates in critical mode when the load is greater than or equal to a threshold, and operates in discontinuous mode when the load is less than the threshold.

12. The flyback circuit according to claim 11, characterized in that, When the flyback circuit operates in the critical mode, during the switching cycle in which the clamping switch is turned on, the clamping switch turns on after the transformer is demagnetized to release the energy in the clamping capacitor.

13. The flyback circuit according to claim 11, characterized in that, When the flyback circuit operates in the discontinuous mode, during the switching cycle in which the clamping switch is turned on, the clamping switch turns on when the voltage across the main switch oscillates to the peak, so as to release the energy in the clamping capacitor.

14. The flyback circuit according to claim 8, characterized in that, When the product of K and the output voltage is greater than or equal to the input voltage, the clamping switch is turned on only once in four consecutive switching cycles of the main switch.

Images