TWI721802B - Switching controller circuit and method for controlling flyback powr converter - Google Patents

Abstract

A switching control circuit for controlling a flyback power converter. The switching control circuit includes a power transformer, a primary side controller circuit and a secondary side controller circuit. The power transformer is coupled between an input voltage and an output voltage with an isolation manner. The primary side controller circuit controls a primary side switch in the flyback power converter. The secondary side controller circuit generates an SR (Synchronous Rectification) control signal to control an SR switch in the flyback power converter. The SR control signal includes an SR pulse and an SS (Soft Switching) pulse. The SR pulse controls the SR switch to be conductive in an SR period for synchronous rectification at the secondary side. The SS pulse controls the SR switch to be conductive in an SS period, whereby the primary side switch achieves soft switching.

Description

Switching control circuit and method for controlling flyback power supply circuit

The present invention relates to a switching control circuit, in particular to a switching control circuit for controlling a flyback power supply circuit. The invention also relates to a method for controlling a flyback power supply circuit.

The prior art related to the present invention includes: "K.-H. Chen, T.-J. Liang, Design of Quasi-resonant flyback converter control IC with DCM and CCM operation, 2014 International Power Electronics Conference, IEEE", "US8917068B2 , S. Chen, J. Jin, Quasi-resonant controlling and driving circuit and method for a flyback converter, 2014", "US 2011/0305048 A1, T.-Y. Yang, Y.-C. Su, C.- C. Lin, Active-Clamp Circuit for Quasi-Resonant Flyback Power Converter, 2011", "AA Saliva, Design Guide for QR Flyback Converter, Design Note DN 2013-01, Infineon Technologies North America Corp", "W. Yuan, etc ., "Novel Soft Switching Flyback Converter with Synchronous Rectification, IEEE IPEMC 2009", and "X. Huang, etc., A Novel Variable Frequency Soft Switching Method for Flyback Converter with Synchronous Rectifier, IEEE 2010".

Please refer to Figure 1A and Figure 1B at the same time. Figure 1A shows a prior art flyback power supply circuit (flyback power supply circuit 1). Figure 1B shows a flyback power supply corresponding to this prior art. Schematic diagram of the circuit's operating waveforms. The primary side control circuit 80 controls the primary side switch S1 to switch the power transformer 10 to generate the output voltage Vo, and the secondary side control circuit 90 generates the synchronous rectification control signal S2C to control the synchronous rectification switch S2 for secondary side synchronization. Rectify.

The disadvantage of the prior art shown in Figures 1A and 1B is that the synchronous rectifier switch S2 cannot be synchronized with the primary switch S1 on the primary side in real time and accurately, and the primary switch S1 does not perform soft switching (Soft Switching). In this case, the power conversion efficiency is poor.

Compared with the prior art shown in Figures 1A and 1B, the synchronous rectification switch S2 can be accurately synchronized with the primary side switch S1, and the flexible switching pulse of the synchronous rectification switch S2 allows the primary side switch S1 to switch during switching. Achieve flexible switching and effectively improve power conversion efficiency. In addition, the switching control circuit of the present invention can also adaptively determine the optimal conduction period and delay period of the flexible switching pulse wave according to different operation modes.

In one aspect, the present invention provides a switching control circuit for controlling a flyback power supply circuit to convert an input voltage to generate an output voltage. The switching control circuit includes: a power transformer with electrical characteristics It is coupled between the input voltage and the output voltage in an insulated manner; a primary side control circuit is used to generate a switching signal to control a primary side switch of the flyback power supply circuit to switch the A primary winding of the power transformer, wherein the primary winding is coupled to the input voltage; and a secondary control circuit for generating a synchronous rectification control signal to control one of the flyback power supply circuits Synchronous rectifying switch, and switching a secondary winding of the power transformer to generate the output voltage, wherein the synchronous rectification control signal has a synchronous rectifying (Synchronous Rectifying, SR) pulse and a soft switching (Soft Switching, SS) pulse The synchronous rectification pulse is used to control the synchronous rectification switch to conduct a synchronous rectification period to achieve secondary-side synchronous rectification, and the flexible switching pulse is used to control the synchronous rectification switch to conduct a flexible switching period to enable the primary The side switch achieves flexible switching; wherein the power transformer is magnetized when the primary side switch is turned on, and when the primary side switch is turned non-conductive, the energy obtained during the magnetization is transferred to the output voltage; wherein when the flyback When the power supply circuit is operated in the boundary conduction mode, when the power transformer turns on the synchronous rectifier switch by the synchronous rectification pulse to demagnetize, after the demagnetization of the secondary winding of the power transformer is completed, the secondary The side control circuit then continuously turns on the synchronous rectification switch by the flexible switching pulse, so that the primary-side switch achieves flexible switching when it is turned on next time, and the flexible switching pulse has the first conduction period; or when the return When the free-wheeling power supply circuit is operated in discontinuous conduction mode, the power transformer turns on the synchronous rectifier switch to demagnetize by the synchronous rectification pulse wave. After the demagnetization of the secondary winding of the power transformer is completed, the The secondary-side control circuit controls the synchronous rectifier switch to be non-conducting, and then the secondary-side control circuit turns on the synchronous rectifier switch again by the flexible switching pulse, so that the primary-side switch achieves flexible switching when it is turned on next time, The flexible switching pulse wave has a second conduction period.

In a preferred embodiment, the flexible switching pulse draws a negative current from the output voltage by turning on the secondary winding, thereby enabling the flexible switching of the primary switch when it is turned on next time.

In a preferred embodiment, the secondary-side control circuit detects the voltage related to the synchronous rectifier switch to detect the completion of the demagnetization of the secondary-side winding of the power transformer.

In a preferred embodiment, the switching control circuit further includes a signal shaping circuit for shaping the voltage related to the synchronous rectifier switch and providing it to the secondary side control circuit to detect the two parts of the power transformer. The demagnetization of the secondary winding is completed.

In a preferred embodiment, the primary-side control circuit detects the voltage related to the power transformer through an auxiliary winding of the power transformer to detect the completion of the demagnetization of the secondary-side winding of the power transformer.

In a preferred embodiment, the switching control circuit further includes a signal shaping circuit for shaping the voltage related to the power transformer and providing it to the primary control circuit to detect the secondary winding of the power transformer Demagnetization is complete.

In a preferred embodiment, the primary-side control circuit generates a clock signal for determining a highest switching frequency of the switching signal, wherein the clock signal is before the completion of the demagnetization of the secondary winding of the power transformer When generated, the primary side control circuit controls the primary side switch to turn on after the clock signal is delayed by a first delay period, and when the clock signal is generated after the demagnetization of the secondary winding of the power transformer is completed, The primary side control circuit controls the primary side switch to be turned on after the clock signal is delayed by a second delay period; wherein during the first delay period and the second delay period, the primary side switch is forbidden to be turned on; The first delay period is longer than the second delay period.

In a preferred embodiment, the secondary-side control circuit has a current threshold, and the secondary-side control circuit determines whether the secondary winding of the power transformer is based on the current flowing through the synchronous rectifier switch and the current threshold. The demagnetization is completed, and the current threshold is a settable value.

In a preferred embodiment, the first conduction period is longer than the second conduction period.

In a preferred embodiment, the switching control circuit further includes a signal transformer for transmitting the clock signal from the primary side control circuit to the secondary side.

A method for controlling a flyback power supply circuit to convert an input voltage to generate an output voltage, wherein a power transformer of the flyback power supply circuit is electrically insulated coupled to the input voltage And the output voltage; the method includes: generating a switching signal on the primary side of the flyback power supply circuit to control a primary side switch of the flyback power supply circuit to switch the power transformer A primary winding, wherein the primary winding is coupled to the input voltage; and a synchronous rectification control signal is generated on the secondary side of the flyback power supply circuit to control one of the flyback power supply circuits Synchronous rectifying switch, and switching a secondary winding of the power transformer to generate the output voltage, wherein the synchronous rectification control signal has a synchronous rectifying (Synchronous Rectifying, SR) pulse and a soft switching (Soft Switching, SS) pulse The synchronous rectification pulse is used to control the synchronous rectification switch to conduct a synchronous rectification period to achieve secondary-side synchronous rectification, and the flexible switching pulse is used to control the synchronous rectification switch to conduct a flexible switching period to enable the primary The side switch achieves flexible switching; wherein the power transformer is magnetized when the primary side switch is turned on, and when the primary side switch is turned non-conductive, the energy obtained during the magnetization is transferred to the output voltage; wherein when the flyback When the power supply circuit is operated in the boundary conduction mode, the power transformer turns on the synchronous rectifier switch to demagnetize by the synchronous rectification pulse wave. After the demagnetization of the secondary winding of the power transformer is completed, it is followed by The synchronous rectification switch is continuously turned on by the flexible switching pulse, so that the primary side switch achieves flexible switching when it is turned on next time, and the flexible switching pulse has a first conduction period; or when the flyback power supply circuit operates In discontinuous conduction mode, when the power transformer turns on the synchronous rectifier switch for demagnetization by the synchronous rectification pulse wave, after the demagnetization of the secondary winding of the power transformer is completed, the synchronous rectifier switch is controlled to be non-conducting Then, the secondary-side control circuit turns on the synchronous rectification switch again by the flexible switching pulse, so that the primary-side switch achieves flexible switching when the primary-side switch is turned on next time, and the flexible switching pulse has a second conduction period.

In a preferred embodiment, the method further includes: shaping the voltage related to the synchronous rectifier switch to detect the completion of the demagnetization of the secondary winding of the power transformer.

In a preferred embodiment, the step of detecting the completion of demagnetization of the secondary winding of the power transformer includes: detecting the voltage related to the power transformer through an auxiliary winding of the power transformer on the primary side, This is done by detecting the demagnetization of the secondary winding of the power transformer.

In a preferred embodiment, the method further includes: shaping the voltage related to the power transformer to detect the completion of the demagnetization of the secondary winding of the power transformer.

In a preferred embodiment, the method further includes: generating a clock signal on the primary side to determine a highest switching frequency of the switching signal, wherein when the clock signal is on the secondary winding of the power transformer When it is generated before the demagnetization is completed, the primary side switch is controlled to be turned on after the clock signal is delayed by a first delay period, and when the clock signal is generated after the demagnetization of the secondary winding of the power transformer is completed, The clock signal is delayed by a second delay period to control the primary side switch to turn on; wherein during the first delay period and the second delay period, the primary side switch is prohibited from turning on; wherein the first delay period Longer than the second delay period.

In a preferred embodiment, the step of determining whether the demagnetization of the secondary winding of the power transformer is completed includes: determining the secondary winding of the power transformer according to the current flowing through the synchronous rectifier switch and the current threshold Whether the demagnetization is completed, wherein the current threshold is a settable value.

In a preferred embodiment, the method further includes: using a signal transformer to transmit the clock signal from the primary side of the flyback power supply circuit to the secondary side of the flyback power supply circuit.

Detailed descriptions are given below by specific embodiments, so that it will be easier to understand the purpose, technical content, features, and effects of the present invention.

The drawings in the present invention are all schematic, and are mainly intended to show the coupling relationship between the circuits and the relationship between the signal waveforms. As for the circuits, signal waveforms, and frequencies, they are not drawn to scale.

Please refer to Figure 2A. Figure 2A shows an embodiment of the switching control circuit (switching control circuit 100) of the present invention. As shown in the figure, the switching control circuit 100 is used to control the flyback power supply circuit 1000 to switch The input voltage Vin generates the output voltage Vo, and provides power to the load circuit (not shown, well known to those with ordinary knowledge in the art, and will not be repeated here). The switching control circuit 100 includes a power transformer 10, a primary side control circuit 20 and a secondary side control circuit 30.

The power transformer 10 is electrically insulated and coupled between the input voltage Vin and the output voltage Vo. The primary switch S1 is coupled to the primary winding W1 of the power transformer 10, and the primary winding W1 is coupled to the input voltage Vin. The synchronous rectifier switch S2 and the secondary winding W2 of the power transformer 10 are connected in series between the output voltage Vo and the secondary ground node. In this embodiment, the synchronous rectifier switch S2 is coupled between the secondary winding W2 of the power transformer 10 and the secondary ground node. The synchronous rectifier switch S2 can also be coupled between the secondary winding W2 of the power transformer 10 and the output voltage Vo, as shown in the embodiment shown in FIG. 2B. To simplify the description, the following describes an embodiment in which the synchronous rectifier switch S2 is coupled between the secondary side winding W2 of the power transformer 10 and the secondary side ground node as shown in FIG. 2A. However, the same spirit can also be used. Applicable to the other form shown in Figure 2B above.

The primary control circuit 20 is used to generate a switching signal S1C, and the switching signal S1C is used to control the primary switch S1 to switch the primary winding W1 of the power transformer 10, wherein the primary winding W1 is coupled to the input voltage Vin. The secondary side control circuit 30 is used to generate a synchronous rectification control signal S2C to control the on and off of the synchronous rectification switch S2, and the secondary side winding W2 of the switching power transformer 10 generates an output voltage Vo. VDS1 is the voltage of the drain of the primary side switch S1, and VDS2 is the voltage of the first terminal of the synchronous rectification switch S2. In this embodiment, the first terminal of the synchronous rectifier switch S2 is the drain (current outflow terminal), and the second terminal of the synchronous rectifier switch S2 is the source (current inflow terminal). It should also be noted that in the embodiment in which the synchronous rectifier switch S2 is coupled between the secondary winding W2 of the power transformer 10 and the output voltage Vo, as shown in FIG. 2B, the synchronous rectifier switch S2 is One end is the source (current inflow end), and the second end of the synchronous rectifier switch S2 is the drain (current outflow end).

Please refer to FIG. 3 at the same time. FIG. 3 shows a waveform diagram corresponding to an embodiment of the switching control circuit of the present invention. In this embodiment, the flyback power supply circuit of the present invention operates in a discontinuous conduction mode (DCM-Discontinuous Conduction Mode). According to the present invention, the synchronous rectification control signal S2C has a synchronous rectification pulse wave PSR and a soft switching (Soft Switching, SS) pulse wave PSS. When the primary side switch S1 is turned on and then turned off again (such as t3 in Figure 3), the synchronization The rectified pulse wave PSR is used to control the synchronous rectification switch S2 to conduct a synchronous rectification period TSR to achieve synchronous rectification on the secondary side, wherein the synchronous rectification period TSR is substantially synchronized with the on-time of the induced current of the secondary winding W2, in other words, The synchronous rectification period TSR starts at the time (t3) when the secondary side winding W2 transfers energy from the primary side winding W1 to generate the secondary side current ISR (t3), and the synchronous rectification period TSR ends when the secondary side current ISR of the secondary side winding W2 drops When it is 0 (t4), the power conversion efficiency can be improved in this way. Where n is the ratio of the number of turns of the primary winding to the secondary winding.

Please continue to refer to Figure 3. On the other hand, the flexible switching pulse PSS is used to achieve the aforementioned flexible switching of the primary side switch S1. In detail, in this embodiment, when the load of the flyback power supply circuit 1000 is relatively light, that is, the load is not greater than a predetermined load threshold, and the flyback power supply circuit 1000 operates in discontinuous conduction mode When the power transformer 10 is magnetizing when the primary side switch S1 is turned on (magnetizing, t2-t3, Figure 3), and when the primary side switch S1 is turned non-conductive, the energy obtained during the magnetizing is transferred to the output Voltage Vo; when the synchronous rectification pulse PSR controls the synchronous rectification switch S2 to turn on, and the demagnetization of the secondary winding W2 of the power transformer 10 is completed (demagnetized, t4, Figure 3), the synchronous rectification switch S2 will be controlled first Turn on (t4-t5, Figure 3), and when the synchronous rectifier switch S2 is turned on again according to the flexible switching pulse PSS (such as t0 or t5 in Figure 3), the power transformer 10 will induce a negative effect on the secondary winding W2. When the secondary side current ISR is negative during the on-time TSS period (such as t0-t1), the secondary side current ISR will flow from the output capacitor Co. That is, the flexible switching pulse PSS draws a negative current (ie, a negative secondary current ISR) from the output voltage Vo by turning on the secondary winding W2, and transfers energy to the secondary winding W2. When the synchronous rectification switch S2 When the flexible switching pulse PSS is turned off again (such as t1), as shown in Figure 3, the power transformer 10 will induce a negative primary current Ip in the primary winding W1 during this period (such as t1-t2) , The negative primary current Ip can discharge the parasitic capacitance Cp of the primary switch S1, so that the drain voltage VDS1 of the primary switch S1 drops to a lower voltage, and the charge is recharged to the input capacitance through the primary winding W1 Ci, when the primary side switch S1 is turned on, the primary side switch S1 can achieve soft switching (SS-Soft Switching). In a preferred embodiment, the negative primary-side current Ip can discharge the parasitic capacitance Cp of the primary-side switch S1 to approximately 0V, so that the primary-side switch S1 can achieve zero voltage switching (ZVS-Zero Voltage Switching).

It should be noted that the aforementioned "load" refers to the power consumption of the load circuit, that is, the power consumed by the load circuit when the output voltage Vo provides power to the load circuit. The so-called "light load" (relatively small load) and "heavy load" (relatively large load) are separated by the aforementioned preset load threshold, which requires the design of individual flyback power supply circuits Parameters such as input voltage, output voltage, transformer inductance, etc. vary. For example, if the output power is greater than a preset load threshold for heavy load, and operating in boundary conduction mode, the output power is not greater than another or the same preset load threshold for light load, and operating in discontinuous conduction mode is in the art It is well known to those with general knowledge, so I won’t repeat it here. Of course, the so-called "light load" and "heavy load" can also be separated by the output current related to the output power in actual circuit applications, so I won't repeat them here.

In addition, the aforementioned "flexible switching" means that before the transistor (for example, corresponding to the primary side switch S1) is turned on, the residual voltage of the parasitic capacitance of the transistor is passed through a lossless discharge path (for example, corresponding to the The primary winding W1) discharges to a lower voltage, and recharges the charge to a non-destructive component (such as the input capacitor Ci), so that when the transistor is turned on, its drain-source voltage has been reduced to a lower voltage first Because the charge stored in the parasitic capacitance is not discharged by the on-resistance of the transistor during this process, the power conversion efficiency can be improved.

In addition, it should be noted that the parasitic effects of the circuit components or the matching between the components are not necessarily ideal. Therefore, although the parasitic capacitance Cp is to be discharged to 0V, it may not be accurately discharged to 0V in practice. It is only close to 0V, that is, according to the present invention, it is acceptable to have a certain degree of error between the voltage of the parasitic capacitance Cp after discharge and 0V due to the imperfections of the circuit, which means that the aforementioned discharge is "substantially" 0V. The meaning of this article is the same for other references to "generally".

Please refer to FIG. 4, which shows a waveform diagram of an embodiment of the flyback power supply circuit corresponding to the present invention. In this embodiment, the flyback power supply circuit of the present invention operates in the boundary conduction mode (BCM-Boundary Conduction Mode). This embodiment is similar to the embodiment in Fig. 3. The difference of this embodiment is that, as shown in Fig. 4, at the end of the synchronous rectification pulse PSR (the end of the synchronous rectification period TSR), that is, the secondary side When the current ISR drops to 0 (such as t4 in Figure 4), the synchronous rectification control signal S2C is simultaneously connected to the flexible switching pulse PSS' (such as t4-t5 in Figure 4). In other words, in this embodiment, the synchronous rectification The synchronous rectification pulse PSR of the control signal S2C is connected to the flexible switching pulse PSS', so that during the non-conduction period of the primary side switch, the external view of the synchronous rectification control signal S2C during the conduction period seems to have only one pulse, of which the flexible switching pulse The wave PSS' has an on-period TSS'. It is worth noting that, in a preferred embodiment, during the synchronous rectification pulse PSR, the secondary side current ISR is positive (in this embodiment, the output current is positive), and during the flexible switching pulse PSS', at least Part of the secondary side current ISR is negative (that is, negative current).

In one embodiment, the on-period TSS of the flexible switching pulse PSS' in the boundary conduction mode is longer than the on-period TSS of the flexible switching pulse PSS in the discontinuous conduction mode.

In the switching control circuit of the present invention, the completion of the demagnetization of the secondary winding W2 of the power transformer 10 can be detected by several different methods. In one embodiment, the secondary-side control circuit 30 detects the voltage related to the synchronous rectifier switch S2 to detect the completion of the demagnetization of the secondary-side winding W2 of the power transformer 10, that is, when the secondary-side current ISR reaches zero current. Please refer to Figure 5. Figure 5 shows a detailed waveform diagram corresponding to Figure 3. Taking Fig. 5 as an example, the secondary-side control circuit 30 detects the drain voltage VDS2 of the synchronous rectification switch S2. When the secondary-side current ISR is positive and the synchronous rectification switch S2 is turned on according to the synchronous rectification pulse PSR, The drain voltage VDS2 of the synchronous rectifier switch S2 is negative (such as t3-t4 in Figure 5), and when the demagnetization of the secondary winding W2 of the power transformer 10 is completed, that is, the secondary current ISR drops from positive to 0 At this time, the secondary side control circuit 30 can increase the drain voltage VDS2 of the synchronous rectifier switch S2 from a negative value to 0 to detect that the secondary winding W2 of the power transformer 10 is demagnetized. It should be noted that the so-called "detection of completion of the demagnetization of the secondary winding of the power transformer" refers to determining the end time point of the demagnetization procedure according to relevant parameters, that is, when the secondary current ISR reaches zero current, For the sake of brevity, it is indicated by "Detecting the completion of the demagnetization of the secondary winding of the power transformer", the same below.

Please refer to Fig. 5, Fig. 6A and Fig. 6B at the same time. Fig. 6A and Fig. 6B show two embodiments of the secondary side control circuit in the switching control circuit of the present invention (the secondary side control circuit 30A and 30B ).

As shown in Fig. 6A, in one embodiment, the secondary-side control circuit 30A includes a current comparator 31A for comparing the secondary-side current ISR with the current threshold Ith_ZC, and generating it to indicate that the secondary-side current ISR is 0 The current signal SRZC can be used to indicate that the demagnetization of the secondary winding W2 of the power transformer 10 is completed, that is, when the secondary current ISR reaches zero current. In an embodiment, the current threshold Ith_ZC is a settable value. Specifically, the current threshold Ith_ZC can be set to a threshold close to zero. In one embodiment, the current threshold Ith_ZC can be set to a threshold close to zero but greater than zero.

As shown in Figure 6B, in one embodiment, the secondary-side control circuit 30B includes a voltage comparator 31B for comparing the signal related to the secondary-side current ISR with the current threshold Vth_ZC to generate The signal SRZC whose side current ISR is 0 current can be used to indicate that the secondary winding W2 of the power transformer 10 is demagnetized. The signal related to the secondary side current ISR is, for example, but not limited to, the drain voltage of the aforementioned synchronous rectifier switch S2 VDS2. In an embodiment, the current threshold Vth_ZC is a settable value. Specifically, the current threshold Vth_ZC can be set to a threshold close to zero. In one embodiment, the current threshold Vth_ZC can be set to a threshold close to zero but less than zero.

In other embodiments, the primary side control circuit can also be used to detect the completion of the demagnetization of the secondary winding W2 of the power transformer 10. Please refer to Figure 7. Figure 7 shows an embodiment of the switching control circuit (switching control circuit 107) of the present invention. In this embodiment, the primary side control circuit 20' is formed by the auxiliary winding W3 of the power transformer 10' Detect the voltage related to the power transformer 10' to detect the completion of the demagnetization of the secondary winding W2 of the power transformer 10', that is, when the secondary current ISR reaches zero current. In another embodiment, the primary-side control circuit 20' can detect the drain voltage VDS1 of the primary-side switch S1 and detect the voltage related to the power transformer 10' to detect the secondary winding W2 of the power transformer 10' Demagnetization is complete, that is, when the secondary side current ISR reaches zero current.

Please continue to refer to Figures 3 and 4. In one embodiment, in the switching control circuit of the present invention, the primary side control circuit 20 includes a clock signal CLK to determine the highest switching frequency of the switching signal S1C, as shown in Figure 4. As shown in the figure, the clock signal CLK is completed after the demagnetization of the secondary winding W2 of the power transformer 10, that is, when the secondary side current ISR reaches zero current, the primary side control circuit 20 starts from the clock signal CLK and passes a delay period After Td1, the primary side switch S1 is controlled to be turned on. Specifically, in this embodiment, the load is relatively large. Therefore, the clock signal CLK is demagnetized in the secondary winding W2 of the power transformer 10, that is, the secondary side current ISR reaches 0. The current is generated before the time. According to the present invention, in one embodiment, the flexible switching pulse PSS' (such as t4-t5 in Figure 4) is triggered by the clock signal CLK to continue the aforementioned synchronous rectification pulse PSR. At the same time, the clock signal CLK also triggers the delay period Td1. Since part of the delay period Td1 overlaps the flexible switching pulse PSS', during the delay period Td1, the triggering of the switching signal S1C is disabled, that is, The primary-side switch S1 is prohibited from being turned on, so as to prevent the primary-side switch S1 and the synchronous rectifier switch S2 from being turned on at the same time.

Please continue to refer to Figure 3. In another embodiment, the time pulse signal CLK is demagnetized in the secondary winding W2 of the power transformer 10, that is, when the secondary side current ISR reaches zero current and the pulse wave is generated, the primary side The control circuit 20 starts when the clock signal CLK generates a pulse, and after a delay period Td2, controls the primary side switch S1 to be turned on. Specifically, in this embodiment, the load is lighter. Therefore, the clock signal CLK is applied to the secondary of the power transformer 10 The demagnetization of the side winding W2 is completed, that is, it is generated before the secondary side current ISR reaches zero current. According to the present invention, in one embodiment, the flexible switching pulse PSS is triggered by the clock signal CLK (such as t0 in Figure 3). -t1). At the same time, the clock signal CLK also triggers the delay period Td2. During the delay period Td2, the triggering of the switching signal S1C is disabled, that is, the primary side switch S1 is prohibited from conducting to avoid the primary side switch S1 and the synchronous rectifier switch S2 are turned on at the same time.

In an embodiment, the aforementioned delay period Td1 is longer than the delay period Td2.

In one embodiment, the clock signal CLK is generated by the primary side control circuit 20.

Please refer to Figures 8 and 9 at the same time. Figure 8 shows an embodiment of the primary side control circuit (primary side control circuit 20) in the switching control circuit of the present invention. Figure 9 shows the switching control corresponding to the present invention. Schematic diagram of the waveform of an embodiment of the circuit. In one embodiment, the primary control circuit 20 determines that the demagnetization of the secondary winding W2 of the power transformer 10 is completed according to whether the drain voltage VDS1 of the primary switch S1 drops below a knee threshold Vth_knee, that is, the secondary current The time point when the ISR reaches zero current (the end time point of the demagnetization procedure), specifically, as shown in Figs. 8 and 9, in one embodiment, the comparator 21 is used to compare the drain of the primary side switch S1 The voltage VDS1 and the knee threshold Vth_knee generate a knee signal V1_knee to indicate whether the drain voltage VDS1 of the primary switch S1 is lower than the knee point. In another embodiment, the primary control circuit 20 determines that the demagnetization of the secondary winding W2 of the power transformer 10 is completed according to whether the voltage V3 of the auxiliary winding W3 drops below a knee threshold Vth_knee, that is, the secondary current ISR is reached The time point of zero current, specifically, as shown in Figs. 8 and 9, in one embodiment, the comparator 22 is used to compare the voltage V3 of the auxiliary winding W3 with the knee threshold Vth_knee to generate the knee signal V1_knee, It is used to indicate whether the voltage V3 of the auxiliary winding W3 is lower than the knee point.

Please continue to refer to FIGS. 8 and 9. In one embodiment, the primary-side control circuit 20 determines the drain voltage of the primary-side switch S1 according to whether the drain voltage VDS1 of the primary-side switch S1 drops below a trough threshold Vth_vly Whether VDS1 is in a trough, specifically, as shown in FIGS. 8 and 9, in one embodiment, the comparator 22 is used to compare the drain voltage VDS1 of the primary side switch S1 with the trough threshold Vth_vly to generate the trough signal V1_vly , To indicate whether the drain voltage VDS1 of the primary side switch S1 is in the trough. In another embodiment, the primary side control circuit 20 determines whether the voltage V3 of the auxiliary winding W3 is at a trough according to whether the voltage V3 of the auxiliary winding W3 drops below a trough threshold Vth_vly, specifically, as shown in Figs. 8 and 9 As shown in the figure, in one embodiment, the comparator 22 is used to compare the voltage V3 of the auxiliary winding W3 with the valley threshold Vth_vly to generate a valley signal V1_vly to indicate whether the voltage V3 of the auxiliary winding W3 is at a valley. In one embodiment, after the aforementioned delay period (Td1, Td2), the time point when the primary side switch S1 is turned on is determined according to whether the drain voltage VDS1 of the primary side switch S1 is at a trough. In a preferred embodiment, after the aforementioned delay period (Td1, Td2), the time point when the primary side switch S1 is turned on is determined according to whether the drain voltage VDS1 of the primary side switch S1 is close to 0.

Please refer to Figure 10. Figure 10 shows a schematic diagram of an embodiment of the switching control circuit of the present invention (switching control circuit 110). This embodiment is similar to the previous embodiment. In this embodiment, the switching control circuit 110 further includes a signal transformer. 40. Used to transmit the clock signal CLK from the primary side control circuit 20" to the secondary side, for example, to synchronously trigger the aforementioned flexible switching pulses PSS and PSS'.

Please refer to FIG. 11. FIG. 11 shows a schematic diagram of an embodiment of the switching control circuit of the present invention and the signal shaping circuit (switching control circuit 111, signal shaping circuit 50A or 50B). As shown in FIG. 11, this embodiment is similar to the previous embodiments. In this embodiment, the switching control circuit 111 further includes a signal shaping circuit (such as a signal shaping circuit 50A or 50B). In one embodiment, the signal shaping circuit 50A It is used to transmit the drain voltage VDS1 of the primary side switch S1 to the primary side control circuit 20 after signal processing. In one embodiment, the signal shaping circuit 50B is used to process the drain voltage VDS2 of the synchronous rectifier switch S2 before sending it to the secondary side control circuit 30.

As shown in Figure 11, in one embodiment, the signal shaping circuit (corresponding to the aforementioned signal shaping circuit 50A or 50B) includes a voltage divider circuit (such as a voltage divider resistor) and a filter circuit (such as a filter capacitor) ) To filter out noise.

The present invention has been described above with respect to preferred embodiments, but the above descriptions are only for making it easier for those skilled in the art to understand the content of the present invention, and are not intended to limit the scope of rights of the present invention. The illustrated embodiments are not limited to individual applications, but can also be combined. For example, two or more embodiments can be used in combination, and part of the composition in one embodiment can also be used to replace another embodiment. Corresponding component parts. In addition, under the same spirit of the present invention, those skilled in the art can think of various equivalent changes and various combinations. For example, the “processing or calculation based on a certain signal or generating a certain output result” in the present invention is not limited to According to the signal itself, it also includes performing voltage-current conversion, current-voltage conversion, and/or ratio conversion on the signal when necessary, and then process or calculate an output result according to the converted signal. It can be seen from this that under the same spirit of the present invention, those skilled in the art can think of various equivalent changes and various combinations, and there are many combinations of them, which are not listed here. Therefore, the scope of the present invention should cover all the above and other equivalent changes.

1: Flyback power supply circuit

10, 10’: Power transformer

100, 107, 110, 111: switching control circuit

20, 20’, 20”: Primary side control circuit

30, 30A, 30B: Secondary side control circuit

31A: current comparator

31B: Voltage comparator

40: signal transformer

50A, 50B: signal shaping circuit

80: Primary side control circuit

90: Secondary side control circuit

Ci: Input capacitance

CLK: Clock signal

Co: output capacitance

Cp: parasitic capacitance

Ith_ZC: current threshold

Ip: primary side current

ISR: Secondary side current

n

PSR: Synchronous rectification pulse

PSS, PSS’: Flexible switching pulse

S1: Primary side switch

S1C: Switch signal

S2: Synchronous rectifier switch

S2C: Synchronous rectification control signal

SRZC: Signal

t0-t5, t2’: time point

Td1, Td2: Delay period

TSR: Synchronous rectification period

TSS, TSS’: On period

V3: Voltage

VDS1, VDS2: Voltage

Vin: input voltage

Vo: output voltage

Vth_knee: knee point threshold

Vth_vly: trough threshold

Vth_ZC: current threshold

W1: Primary winding

W2: Secondary winding

W3: auxiliary winding

Figure 1A shows a flyback power supply circuit of the prior art.

FIG. 1B shows a schematic diagram of the operation waveforms of the flyback power supply circuit of the prior art corresponding to FIG. 1A.

Figure 2A shows an embodiment of the switching control circuit in the present invention.

Figure 2B shows an embodiment of the switching control circuit in the present invention.

Figure 3 shows a schematic diagram of waveforms corresponding to an embodiment of the switching control circuit of the present invention.

FIG. 4 shows a schematic diagram of waveforms corresponding to an embodiment of the flyback power supply circuit of the present invention.

Figure 5 shows the detailed waveform diagram corresponding to Figure 3.

Figures 6A and 6B show two embodiments of the secondary side control circuit in the switching control circuit of the present invention.

Figure 7 shows an embodiment of the switching control circuit in the present invention.

Figure 8 shows an embodiment of the primary side control circuit in the switching control circuit of the present invention.

Fig. 9 shows a waveform diagram corresponding to an embodiment of the switching control circuit of the present invention.

Figure 10 shows a schematic diagram of an embodiment of the switching control circuit of the present invention.

FIG. 11 shows a schematic diagram of an embodiment of the switching control circuit of the present invention and the signal shaping circuit therein.

CLK: Clock signal

Ip: primary side current

ISR: Secondary side current

n: turns ratio

PSR: Synchronous rectification pulse

PSS: Flexible switching pulse

S1C: Switch signal

S2C: Synchronous rectification control signal

t0-t5,t2’: time point

Td2: Delay period

TSR: Synchronous rectification period

TSS: On period

VDS1, VDS2: Voltage

Vin: input voltage

Vo: output voltage

Claims (18)

A switching control circuit for controlling a flyback power supply circuit to convert an input voltage to generate an output voltage. The switching control circuit includes: a power transformer coupled to the input voltage and Between the output voltage; a primary side control circuit for generating a switching signal to control a primary side switch of the flyback power supply circuit, and switch a primary side winding of the power transformer, wherein The primary winding is coupled to the input voltage; and a secondary control circuit for generating a synchronous rectification control signal to control a synchronous rectification switch in the flyback power supply circuit to switch the power transformer A secondary winding generates the output voltage, wherein the synchronous rectifying control signal has a synchronous rectifying (Synchronous Rectifying, SR) pulse wave and a soft switching (Soft Switching, SS) pulse wave, and the synchronous rectifying pulse wave is used to control The synchronous rectification switch is turned on for a synchronous rectification period to achieve secondary-side synchronous rectification, and the flexible switching pulse is used to control the synchronous rectification switch to conduct a flexible switching period, thereby enabling the primary-side switch to achieve flexible switching; wherein the power transformer When the primary-side switch is turned on, the magnetic is induced, and when the primary-side switch is turned non-conductive, the energy obtained during the magnetic induction is transferred to the output voltage; wherein when the flyback power supply circuit is operated in the boundary conduction mode , After the power transformer turns on the synchronous rectifier switch to demagnetize by the synchronous rectification pulse wave, after the demagnetization of the secondary winding of the power transformer is completed, the secondary control circuit then uses the flexible switching The synchronous rectification switch is continuously turned on by a pulse wave, so that the primary-side switch achieves flexible switching when the primary-side switch is turned on next time, and the flexible switching pulse wave has a first conduction period; or When the flyback power supply circuit operates in discontinuous conduction mode, the power transformer turns on the synchronous rectifier switch by the synchronous rectification pulse to demagnetize, and the demagnetization of the secondary winding of the power transformer is completed After that, the secondary-side control circuit controls the synchronous rectifier switch to not conduct, and then, the secondary-side control circuit turns on the synchronous rectifier switch again by the flexible switching pulse, so that the primary-side switch is turned on next time Flexible switching, the flexible switching pulse has a second conduction period; wherein the secondary-side control circuit has a current threshold, and the secondary-side control circuit determines the power transformer according to the current flowing through the synchronous rectifier switch and the current threshold Whether the demagnetization of the secondary winding is completed, wherein the current threshold is a settable value. The switching control circuit as described in claim 1, wherein the flexible switching pulse draws a negative current from the output voltage by turning on the secondary side winding, thereby making the primary side switch turn on next time Time to achieve flexible switching. The switching control circuit described in the first item of the scope of patent application, wherein the secondary side control circuit detects the voltage related to the synchronous rectifier switch to detect the completion of the demagnetization of the secondary side winding of the power transformer. The switching control circuit described in item 3 of the scope of patent application further includes a signal shaping circuit for shaping the voltage related to the synchronous rectifier switch and providing it to the secondary side control circuit to detect the power transformer The demagnetization of the secondary winding is completed. The switching control circuit described in the first item of the scope of patent application, wherein the primary side control circuit detects the voltage related to the power transformer through an auxiliary winding of the power transformer to detect the secondary side of the power transformer The demagnetization of the side winding is completed. For example, the switching control circuit described in item 5 of the scope of patent application further includes a signal shaping circuit for shaping the voltage related to the power transformer and providing it to the primary side control circuit to detect the two parts of the power transformer. The demagnetization of the secondary winding is completed. As for the switching control circuit described in claim 1, wherein the first conduction period is longer than the second conduction period. The switching control circuit described in item 1 of the scope of patent application further includes a signal transformer for transmitting the clock signal from the primary side control circuit to the secondary side. A switching control circuit for controlling a flyback power supply circuit to convert an input voltage to generate an output voltage. The switching control circuit includes: a power transformer coupled to the input voltage and Between the output voltage; a primary side control circuit for generating a switching signal to control a primary side switch of the flyback power supply circuit, and switch a primary side winding of the power transformer, wherein The primary winding is coupled to the input voltage; and a secondary control circuit for generating a synchronous rectification control signal to control a synchronous rectification switch in the flyback power supply circuit to switch the power transformer A secondary winding generates the output voltage, wherein the synchronous rectifying control signal has a synchronous rectifying (Synchronous Rectifying, SR) pulse wave and a soft switching (Soft Switching, SS) pulse wave, and the synchronous rectifying pulse wave is used to control The synchronous rectification switch is turned on for a synchronous rectification period to achieve secondary-side synchronous rectification, and the flexible switching pulse is used to control the synchronous rectification switch to conduct a flexible switching period, thereby enabling the primary-side switch to achieve flexible switching; wherein the power transformer When the primary-side switch is turned on, the magnetic is induced, and when the primary-side switch is turned non-conductive, the energy obtained during the magnetic induction is transferred to the output voltage; wherein when the flyback power supply circuit is operated in the boundary conduction mode , After the power transformer turns on the synchronous rectifier switch to demagnetize by the synchronous rectification pulse wave, after the demagnetization of the secondary winding of the power transformer is completed, the secondary control circuit then uses the flexible switching pulse Wave and continuously turn on the synchronous rectifier switch, so that the primary side switch achieves flexible switching when it is turned on next time, and the flexible switching pulse has a first conduction period; or when the flyback power supply circuit is operated in discontinuous conduction mode When the power transformer turns on the synchronous rectifier switch to demagnetize by the synchronous rectification pulse wave, after the demagnetization of the secondary winding of the power transformer is completed, the secondary side control circuit controls the synchronous rectifier switch to demagnetize Turn on, and then the secondary-side control circuit turns on the synchronous rectifier switch again by the flexible switching pulse, so that the primary-side switch achieves flexible switching when it is turned on next time, and the flexible switching pulse has a second conduction period; The primary-side control circuit generates a clock signal to determine a highest switching frequency of the switching signal. When the clock signal is generated before the demagnetization of the secondary winding of the power transformer is completed, the primary-side control The circuit controls the primary side switch to turn on after the clock signal is delayed by a first delay period, and when the clock signal is generated after the demagnetization of the secondary winding of the power transformer is completed, the primary control circuit is connected to the The clock signal is delayed by a second delay period to control the primary side switch to turn on; wherein during the first delay period and the second delay period, the primary side switch is prohibited from turning on; wherein the first delay period is longer than The second delay period. A method for controlling a flyback power supply circuit to convert an input voltage to generate an output voltage, wherein a power transformer of the flyback power supply circuit is electrically insulated coupled to the input voltage And the output voltage; the method includes: generating a switching signal on the primary side of the flyback power supply circuit to control a primary side switch of the flyback power supply circuit to switch the power transformer A primary winding, wherein the primary winding is coupled to the input voltage; and Generate a synchronous rectification control signal on the secondary side of the flyback power supply circuit to control a synchronous rectifier switch in the flyback power supply circuit, and switch a secondary winding of the power transformer to generate the output The synchronous rectifying control signal has a synchronous rectifying (Synchronous Rectifying, SR) pulse and a soft switching (Soft Switching, SS) pulse, the synchronous rectifying pulse is used to control the synchronous rectifying switch to conduct a synchronous rectifying period In order to achieve secondary-side synchronous rectification, the flexible switching pulse is used to control the synchronous rectification switch to conduct a flexible switching period, thereby enabling the primary-side switch to achieve flexible switching; wherein the power transformer induces magnetism when the primary-side switch is turned on , And when the primary side switch turns non-conducting, the energy obtained during magnetic induction is transferred to the output voltage; wherein when the flyback power supply circuit is operated in the boundary conduction mode, the power transformer uses the synchronous Rectify the pulse to turn on the synchronous rectifier switch for demagnetization. After the demagnetization of the secondary winding of the power transformer is completed, the flexible switching pulse is then used to continuously turn on the synchronous rectifier switch so that the primary side When the switch is turned on next time, the flexible switching is achieved, and the flexible switching pulse has the first conduction period; or when the flyback power supply circuit is operated in discontinuous conduction mode, the power transformer is generated by the synchronous rectification pulse The synchronous rectifier switch is turned on to demagnetize, and after the demagnetization of the secondary winding of the power transformer is completed, the synchronous rectifier switch is controlled to be non-conductive, and then the secondary-side control circuit is turned on again by the flexible switching pulse The synchronous rectifier switch enables the primary-side switch to achieve flexible switching when it is turned on next time, and the flexible switching pulse has a second conduction period; wherein the step of determining whether the demagnetization of the secondary winding of the power transformer is completed includes: The current flowing through the synchronous rectifier switch and a current threshold determine whether the demagnetization of the secondary winding of the power transformer is completed, wherein the current threshold is a settable value. The method described in claim 10, wherein the flexible switching pulse draws a negative current from the output voltage by turning on the secondary winding, so that the primary side switch is turned on next time Flexible switching. The method described in claim 10, wherein the voltage related to the synchronous rectifier switch is detected on the secondary side to detect the completion of the demagnetization of the secondary side winding of the power transformer. The method described in item 12 of the scope of the patent application further includes: shaping the voltage related to the synchronous rectifier switch to detect the completion of the demagnetization of the secondary winding of the power transformer. For the method described in claim 10, the step of detecting the completion of the demagnetization of the secondary winding of the power transformer includes: detecting the demagnetization related to the power transformer by an auxiliary winding on the primary side of the power transformer The voltage of the power transformer is used to detect the completion of the demagnetization of the secondary winding of the power transformer. The method described in item 14 of the scope of the patent application further includes: shaping the voltage related to the power transformer to detect the completion of the demagnetization of the secondary winding of the power transformer. The method described in claim 10, wherein the first conduction period is longer than the second conduction period. The method described in item 10 of the scope of patent application further includes: using a signal transformer to transmit the clock signal from the primary side of the flyback power supply circuit to the secondary side of the flyback power supply circuit . A method for controlling a flyback power supply circuit to convert an input voltage to generate an output voltage, wherein a power transformer of the flyback power supply circuit is electrically insulated and coupled to the input voltage And the output voltage; the method includes: A switching signal is generated on the primary side of the flyback power supply circuit to control a primary side switch of the flyback power supply circuit to switch a primary winding of the power transformer, wherein the primary winding Is coupled to the input voltage; and generates a synchronous rectification control signal on the secondary side of the flyback power supply circuit to control a synchronous rectifier switch in the flyback power supply circuit to switch one of the power transformers The output voltage is generated by the secondary winding, wherein the synchronous rectifying control signal has a synchronous rectifying (Synchronous Rectifying, SR) pulse wave and a soft switching (Soft Switching, SS) pulse wave, and the synchronous rectifying pulse wave is used to control the The synchronous rectification switch is turned on for a synchronous rectification period to achieve secondary side synchronous rectification, and the flexible switching pulse is used to control the synchronous rectification switch to conduct a flexible switching period, thereby enabling the primary side switch to achieve flexible switching; wherein the power transformer is When the primary side switch is turned on, it senses magnetism, and when the primary side switch turns to non-conduction, the energy obtained when the magnetism is sensed is transferred to the output voltage; wherein when the flyback power supply circuit is operated in the boundary conduction mode, After the power transformer turns on the synchronous rectifier switch to demagnetize by the synchronous rectification pulse, after the demagnetization of the secondary winding of the power transformer is completed, the flexible switching pulse continuously turns on the Synchronous rectifier switch, so that the primary side switch achieves flexible switching when it is turned on next time, and the flexible switching pulse has a first conduction period; or when the flyback power supply circuit is operated in discontinuous conduction mode, the power transformer The synchronous rectification switch is turned on by the synchronous rectification pulse to demagnetize. After the demagnetization of the secondary winding of the power transformer is completed, the synchronous rectification switch is controlled to be non-conducting, and then the secondary control circuit The flexible switching pulse turns on the synchronous rectifier switch again, so that the primary side switch achieves flexible switching when it is turned on next time, and the flexible switching pulse has a second conduction period; wherein the method further includes: generating a moment on the primary side The pulse signal is used to determine a highest switching frequency of the switching signal, wherein when the clock signal is in the secondary winding of the power transformer When it is generated before the demagnetization is completed, the primary side switch is controlled to be turned on after the clock signal is delayed by a first delay period, and when the clock signal is generated after the demagnetization of the secondary winding of the power transformer is completed, The clock signal is delayed by a second delay period to control the primary side switch to turn on; wherein during the first delay period and the second delay period, the primary side switch is prohibited from turning on; wherein the first delay period Longer than the second delay period.

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