CN112713778B

CN112713778B - Switching control circuit and method for controlling flyback power supply circuit

Info

Publication number: CN112713778B
Application number: CN202010161114.XA
Authority: CN
Inventors: 张炜旭; 杨大勇; 陈裕昌; 陈昭锜; 李俊庆; 罗立狄; 钟豪文
Original assignee: Richtek Technology Corp
Current assignee: Richtek Technology Corp
Priority date: 2019-10-24
Filing date: 2020-03-10
Publication date: 2022-03-29
Anticipated expiration: 2040-03-10
Also published as: CN112713778A; TWI721802B; TW202118213A

Abstract

A switching control circuit and method for controlling a flyback power supply circuit. The switching control circuit includes: the power transformer, the primary side control circuit and the secondary side control circuit. The power transformer is coupled between the input voltage and the output voltage in an electrically insulated manner. The primary side control circuit is used for controlling a primary side switch in the flyback power supply circuit. The secondary side control circuit is used for generating a synchronous rectification control signal to control a synchronous rectification switch in the flyback power supply circuit. The synchronous rectification control signal is provided with a synchronous rectification pulse and a soft switching pulse, the synchronous rectification pulse is used for controlling the synchronous rectification switch to conduct the synchronous rectification time interval so as to realize secondary side synchronous rectification, and the soft switching pulse is used for controlling the synchronous rectification switch to conduct the soft switching time interval, so that the primary side switch realizes soft switching.

Description

Switching control circuit and method for controlling flyback power supply circuit

Technical Field

The present invention relates to a switching control circuit, and more particularly to a switching control circuit for controlling a flyback power supply circuit. The invention also relates to a method for controlling the flyback power supply circuit.

Background

The prior art related to the present invention is: "K. -H.Chen, T. -J.Liang., Design of queue-responsive flight control IC with DCM and CCM operation,2014International Power Electronics Conference, IEEE", "US 8917068B2, S.Chen, J.jin, queue-responsive control and driving Circuit and Method for a flight controller, 2014", "US 2011/0305048A 1, T. -Y.Yang, Y. -C.Suft, C.Lin, Active-Clamp Circuit for queue-responsive flight Power Converter, 2011", "A.A.Saliva, Design for flexibility, Design for QR Converter, Design for switch, Design for synchronization 2013, switch 20101, switch for discovery, switch, I.W. for discovery, and discovery.

Referring to fig. 1A and fig. 1B, fig. 1A shows a flyback power supply circuit (flyback power supply circuit 1) in the prior art, and fig. 1B shows an operation waveform diagram of the flyback power supply circuit corresponding to the prior art. The primary control circuit 80 controls the primary switch S1 to switch the power transformer 10 to generate the output voltage Vo, and the secondary control circuit 90 generates the synchronous rectification control signal S2C to control the synchronous rectification switch S2 to perform the synchronous rectification on the secondary side.

The prior art shown in fig. 1A and 1B has the disadvantage that the synchronous rectification switch S2 cannot be synchronized with the primary-side switch S1 on the primary side accurately in real time, and the primary-side switch S1 has poor power conversion efficiency without Soft Switching (Soft Switching).

Compared with the prior art shown in fig. 1A and 1B, the synchronous rectification switch S2 can be precisely synchronized with the primary switch S1, and the soft switching of the primary switch S1 can be realized by the soft switching pulse of the synchronous rectification switch S2, so as to effectively improve the power conversion efficiency. In addition, the switching control circuit of the present invention can adaptively determine the optimal conduction period and delay period of the soft switching pulse according to different operation modes.

Disclosure of Invention

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 comprising: a power transformer coupled between the input voltage and the output voltage in an electrically isolated manner; a primary side control circuit for generating a switching signal to control a primary side switch in the flyback power supply circuit to switch a primary side winding of the power transformer, wherein the primary side winding is coupled to the input voltage; and a secondary side control circuit for generating a Synchronous rectification control signal to control a Synchronous rectification switch in the flyback power supply circuit and switch a secondary side winding of the power transformer to generate the output voltage, wherein the Synchronous rectification control signal has a Synchronous Rectification (SR) pulse and a Soft Switching (SS) pulse, the Synchronous rectification pulse is used to control the Synchronous rectification switch to conduct a Synchronous rectification period to realize secondary side Synchronous rectification, and the Soft Switching pulse is used to control the Synchronous rectification switch to conduct a Soft Switching period, thereby enabling the primary side switch to realize Soft Switching; wherein the power transformer is magnetically sensitive when the primary side switch is turned on, and transfers energy obtained by the magnetically sensitive when the primary side switch is turned off to the output voltage; when the flyback power supply circuit operates in a boundary conduction mode, the synchronous rectification switch is conducted to demagnetize through the synchronous rectification pulse by the power transformer, and after the secondary side winding of the power transformer is demagnetized, the secondary side control circuit continuously conducts the synchronous rectification switch through the soft switching pulse, so that soft switching is realized when the primary side is conducted next time, and the soft switching pulse has a first conduction time interval; or when the flyback power supply circuit operates in a discontinuous conduction mode, the synchronous rectification switch is conducted by the power transformer through the synchronous rectification pulse to be demagnetized, after the demagnetization of the secondary winding of the power transformer is completed, the secondary side control circuit controls the synchronous rectification switch to be non-conductive, then the synchronous rectification switch is conducted again by the secondary side control circuit through the soft switching pulse, so that the soft switching is realized when the primary side is conducted next time, and the soft switching pulse has a second conduction time interval.

In a preferred embodiment, the soft switching pulse draws a negative current from the output voltage by turning on the secondary winding, thereby causing the primary side to switch soft on the next turn on.

In a preferred embodiment, the secondary side control circuit detects the voltage associated with the synchronous rectification switch to detect completion of demagnetization of the secondary side winding of the power transformer.

In a preferred embodiment, the switching control circuit further comprises a signal shaping circuit for shaping the voltage associated with the synchronous rectification switch and providing the shaped voltage to the secondary side control circuit to detect completion of demagnetization of the secondary side winding of the power transformer.

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

In a preferred embodiment, the switching control circuit further comprises a signal shaping circuit for shaping a voltage related to the power transformer and providing the shaped voltage to the primary side control circuit to detect completion of demagnetization of the secondary side winding of the power transformer.

In a preferred embodiment, the primary side control circuit generates a clock signal for determining a highest switching frequency of the switching signal, wherein when the clock signal is generated before 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 for a first delay period, and when the clock signal is generated after 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 for a second delay period; wherein the primary-side switch is inhibited from conducting during the first delay period and the second delay period; wherein 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 demagnetization of the secondary side winding of the power transformer is completed according to the current flowing through the synchronous rectification switch and the current threshold, wherein 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 comprises 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, in order to change an input voltage and produce an output voltage, wherein a power transformer of the flyback power supply circuit, couple to the input voltage and the output voltage in an electrically insulated way; the method comprises the following steps: generating a switching signal at a primary side of the flyback power supply circuit to control a primary side switch of the flyback power supply circuit to switch a primary side winding of the power transformer, wherein the primary side winding is coupled to the input voltage; generating a Synchronous rectification control signal on the secondary side of the flyback power supply circuit to control a Synchronous rectification switch in the flyback power supply circuit and switch a secondary side winding of the power transformer to generate the output voltage, wherein the Synchronous rectification control signal has a Synchronous Rectification (SR) pulse and a Soft Switching (SS) pulse, the Synchronous rectification pulse is used to control the Synchronous rectification switch to conduct a Synchronous rectification period to realize secondary side Synchronous rectification, and the Soft Switching pulse is used to control the Synchronous rectification switch to conduct a Soft Switching period, so that the primary side switch realizes Soft Switching; wherein the power transformer is magnetically sensitive when the primary side switch is turned on, and transfers energy obtained by the magnetically sensitive when the primary side switch is turned off to the output voltage; when the flyback power supply circuit operates in a boundary conduction mode, the synchronous rectification switch is conducted to demagnetize through the synchronous rectification pulse by the power transformer, and after the secondary side winding of the power transformer is demagnetized, the synchronous rectification switch is continuously conducted through the soft switching pulse, so that soft switching is realized when the primary side is conducted next time, and the soft switching pulse has a first conduction time interval; or when the flyback power supply circuit operates in a discontinuous conduction mode, the synchronous rectification switch is conducted to demagnetize through the synchronous rectification pulse by the power transformer, the synchronous rectification switch is controlled to be not conducted after the secondary side winding of the power transformer is demagnetized, then the synchronous rectification switch is conducted again through the soft switching pulse by the secondary side control circuit, so that soft switching is realized when the primary side is conducted next time, and the soft switching pulse has a second conduction time period.

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

In a preferred embodiment, the step of detecting completion of demagnetization of the secondary winding of the power transformer comprises: detecting a voltage associated with the power transformer through an auxiliary winding of the power transformer on the primary side to detect completion of demagnetization of the secondary winding of the power transformer.

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

In a preferred embodiment, the method further comprises: generating a frequency signal at the primary side for determining a highest switching frequency of the switching signal, wherein when the frequency signal is generated before demagnetization of the secondary winding of the power transformer is completed, the primary side switch is controlled to be turned on after the frequency signal is delayed for a first delay period, and when the frequency signal is generated after demagnetization of the secondary winding of the power transformer is completed, the primary side switch is controlled to be turned on after the frequency signal is delayed for a second delay period; wherein the primary-side switch is inhibited from conducting during the first delay period and the second delay period; wherein the first delay period is longer than the second delay period.

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

In a preferred embodiment, the method further comprises: and a signal transformer for transmitting the frequency signal from the primary side of the flyback power supply circuit to the secondary side of the flyback power supply circuit.

The purpose, technical content, features and effects of the invention will be more easily understood through the following detailed description of specific embodiments.

Drawings

Fig. 1A shows a prior art flyback power supply circuit.

Fig. 1B is a waveform diagram illustrating the operation of the flyback power supply circuit corresponding to the prior art in fig. 1A.

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

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

Fig. 3 shows a waveform diagram of an embodiment of a switching control circuit according to the present invention.

Fig. 4 shows a waveform diagram of an embodiment of a flyback power supply circuit according to the present invention.

Fig. 5 shows a detailed waveform diagram corresponding to fig. 3.

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

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

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

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

FIG. 10 is a schematic diagram of a switching control circuit according to an embodiment of the present invention.

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

Description of the symbols in the drawings

1 flyback power supply circuit

10, 10' power transformer

100,107,110,111 switching control circuit

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 frequency signal

Co output capacitor

Cp parasitic capacitance

Ith _ ZC current threshold

Ip primary side current

Secondary side current of ISR

n number of turns ratio

PSR synchronous rectification pulse

PSS, PSS' soft switching pulses

S1 Primary side switch

S1C switching signal

S2 synchronous rectification switch

S2C synchronous rectification control signal

SRZC signal

time point t0-t5, t2

Td1, Td2 delay period

TSR synchronous rectification period

TSS, TSS' on period

Voltage of V3

VDS1, VDS2 Voltage

Vin input voltage

Vo output voltage

Vth _ knee threshold

Vth _ vly trough threshold

Vth _ ZC current threshold

W1 primary winding

W2 Secondary winding

W3 auxiliary winding

Detailed Description

The drawings in the present disclosure are schematic and are intended to show the coupling relationship between circuits and the relationship between signal waveforms, and the circuits, signal waveforms and frequencies are not drawn to scale.

Referring to fig. 2A, fig. 2A shows an embodiment of a switching control circuit (switching control circuit 100) in the present invention, as shown in the figure, the switching control circuit 100 is used to control a flyback power supply circuit 1000 to convert an input voltage Vin to generate an output voltage Vo, so as to provide power to a load circuit (not shown, well known to those skilled in the art and not described herein). 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 isolated from the input voltage Vin and the output voltage Vo, and the primary-side switch S1 is coupled to the primary-side winding W1 of the power transformer 10, wherein the primary-side winding W1 is coupled to the input voltage Vin. The synchronous rectification switch S2 and the secondary winding W2 of the power transformer 10 are connected in series between the output voltage Vo and the secondary side ground node. In the present embodiment, the synchronous rectification switch S2 is coupled between the secondary winding W2 of the power transformer 10 and the secondary side ground node. The synchronous rectifier switch S2 may also be coupled between the secondary winding W2 of the power transformer 10 and the output voltage Vo, as illustrated in the embodiment shown in fig. 2B. For simplicity, the embodiment shown in fig. 2A in which the synchronous rectifier switch S2 is coupled between the secondary winding W2 of the power transformer 10 and the secondary side ground node will be described, but the same spirit can be applied to the other form shown in fig. 2B.

The primary-side control circuit 20 is configured to generate a switching signal S1C, and the switching signal S1C is configured to control the primary-side switch S1 to switch the primary-side winding W1 of the power transformer 10, wherein the primary-side winding W1 is coupled to the input voltage Vin. The secondary control circuit 30 is configured to generate a synchronous rectification control signal S2C to control the synchronous rectification switch S2 to turn on and off, so as to switch the secondary winding W2 of the power transformer 10 to generate the output voltage Vo. Wherein the VDS1 is a voltage of the drain of the primary-side switch S1, and the VDS2 is a voltage of the first terminal of the synchronous rectification switch S2. In this embodiment, the first terminal of the synchronous rectification switch S2 is a drain (current flowing terminal), and the second terminal of the synchronous rectification switch S2 is a source (current flowing terminal). It should be noted that, in the embodiment where the synchronous rectification 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 first terminal of the synchronous rectification switch S2 is a source (current inflow terminal), and the second terminal of the synchronous rectification switch S2 is a drain (current outflow terminal).

Referring to fig. 3, fig. 3 is a waveform diagram of a switching control circuit according to an embodiment of the invention. In this embodiment, the flyback power supply circuit of the present invention operates in a Discontinuous Conduction Mode (DCM). According to the present invention, the synchronous rectification control signal S2C has a synchronous rectification pulse PSR and a Soft Switching (SS) pulse PSS, and when the primary switch S1 is turned on and then turned off again (as shown in t3 of fig. 3), the synchronous rectification pulse PSR is used to control the synchronous rectification switch S2 to turn on a synchronous rectification period TSR to achieve secondary-side synchronous rectification, wherein the synchronous rectification period TSR is substantially synchronous with the on-time of the induced current of the secondary winding W2, in other words, the synchronous rectification period TSR starts at a time point (t3) when the secondary winding W2 transfers energy from the primary winding W1 to generate the secondary current ISR, and the synchronous rectification period TSR ends at a time point (t4) when the secondary current ISR of the secondary winding W2 drops to 0, so as to improve the power conversion efficiency. Wherein n is the ratio of the number of turns of the primary winding to the secondary winding.

Referring to fig. 3, on the other hand, the soft switching pulse PSS is used to realize the soft switching of the primary switch S1. In detail, in the embodiment, when the load of the flyback power supply circuit 1000 is a relatively light load, i.e. the load is not greater than a predetermined load threshold, and the flyback power supply circuit 1000 operates in the discontinuous conduction mode, the power transformer 10 senses magnetism (t 2-t3, fig. 3) when the primary switch S1 is turned on, and transfers energy obtained during the sensing magnetism to the output voltage Vo when the primary switch S1 is turned off; when the synchronous rectification pulse PSR controls the synchronous rectification switch S2 to be turned on to demagnetize the secondary winding W2 of the power transformer 10 (demanderized, t4, fig. 3), the synchronous rectification switch S2 is first controlled to be non-conductive (t4-t5, fig. 3), and when the synchronous rectification switch S2 is turned on again according to the soft switching pulse PSS (e.g. t0 or t5 of fig. 3), the power transformer 10 induces a negative secondary current ISR in the secondary winding W2, i.e. in fig. 3, during the on period TSS (e.g. t0-t1), the secondary current ISR is negative, the secondary current ISR draws a negative current from the output voltage Vo (i.e. the negative secondary current ISR) by turning on the secondary winding W2, transfers energy to the secondary winding W2, when the synchronous rectification switch S2 is turned off again at the end of the soft switching pulse PSS (e. t1), as shown in fig. 3, the power transformer 10 induces a negative primary current Ip in the primary winding W1, during which time (e.g., t1-t2), the negative primary current Ip discharges 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 charges the charge back to the input capacitance Ci through the primary winding W1, and when the primary switch S1 is subsequently turned on, the primary switch S1 is Soft-switched (SS-Soft Switching). In a preferred embodiment, the negative primary-side current Ip discharges the parasitic capacitance Cp of the primary-side switch S1 to substantially 0V, which enables Zero-Voltage Switching (ZVS-Zero Voltage Switching) of the primary-side switch S1.

The "load" refers to power consumed by the load circuit, that is, power supplied to the load circuit by the output voltage Vo and power consumed by the load circuit. The so-called "light load" (relatively small load) and "heavy load" (relatively large load) are separated by the predetermined load threshold, which needs to be different according to the design parameters of the respective flyback power supply circuit, such as the input voltage, the output voltage, the inductance of the transformer, etc. For example, when the output power is greater than a predetermined load threshold, the load is heavy, and the boundary conduction mode is operated, and the output power is not greater than another or the same predetermined load threshold, the load is light, and the discontinuous conduction mode is well known to those skilled in the art and will not be described herein. Of course, the so-called "light load" and "heavy load" can also be separated by the output current related to the output power in practical circuit applications, and will not be described herein.

In addition, the aforementioned "soft switching" means that, before the transistor (e.g., corresponding to the primary-side switch S1) is turned on, the residual voltage of the parasitic capacitance of the transistor is discharged to a lower voltage through the discharge-disabled path (e.g., corresponding to the primary-side winding W1) by the discharge current, and charges the charge back to the device (e.g., the input capacitance Ci) that is not damaged, so that when the transistor is turned on, the drain-source voltage of the transistor is reduced to a lower voltage, and the charge stored in the parasitic capacitance is not discharged by the on-resistance of the transistor in the process, thereby improving the power conversion efficiency.

Further, it should be noted that: since the parasitic effect of the circuit components or the matching between the components is not necessarily perfect, although the parasitic capacitance Cp is discharged to 0V, it may not be discharged to 0V exactly, but only close to 0V, that is, according to the present invention, it is acceptable that there is a certain degree of error between the voltage of the discharged parasitic capacitance Cp and 0V due to the non-ideal circuit, that is, the aforementioned discharge to "substantially" is 0V, and the other points mentioned in the present document are also "substantially".

Referring to fig. 4, fig. 4 is a waveform diagram of a flyback power supply circuit according to an embodiment of the present invention. In this embodiment, the flyback power supply circuit of the present invention operates in a Boundary Conduction Mode (BCM-Boundary Conduction Mode). The present embodiment is similar to the embodiment of fig. 3, and is different in that, as shown in fig. 4, when the synchronous rectification pulse PSR ends (when the synchronous rectification period TSR ends), that is, when the secondary side current ISR drops to 0 (as t 4in fig. 4), the synchronous rectification control signal S2C is simultaneously followed by the soft switching pulse PSS '(as t4 to t5 in fig. 4), in other words, in the present embodiment, the synchronous rectification pulse PSR of the synchronous rectification control signal S2C is connected to the soft switching pulse PSS', so that the appearance of the conduction period of the synchronous rectification control signal S2C during the period of non-conduction of the primary side switch appears to have only one pulse, wherein the soft switching pulse PSS 'has a conduction period TSS'. It should be noted that, in a preferred embodiment, the secondary side current ISR is positive (in this embodiment, the output current is positive) during the synchronous rectification pulse PSR, and at least a portion of the secondary side current ISR is negative (i.e., negative) during the soft switching pulse PSS'.

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

The switching control circuit of the present invention can detect the completion of the demagnetization of the secondary winding W2 of the power transformer 10 in several different ways. In one embodiment, the secondary-side control circuit 30 detects the voltage associated with the synchronous rectifier switch S2 to detect that the demagnetization of the secondary winding W2 of the power transformer 10 is completed, i.e., when the secondary-side current ISR achieves 0 current. Referring to fig. 5, fig. 5 is a schematic diagram showing a detailed waveform corresponding to fig. 3. Specifically, taking fig. 5 as an example, the secondary control circuit 30 detects the drain voltage VDS2 of the synchronous rectifier switch S2, when the secondary current ISR is positive and the synchronous rectifier switch S2 is turned on according to the synchronous rectifier pulse PSR, the drain voltage VDS2 of the synchronous rectifier switch S2 is negative (e.g., t3-t 4in fig. 5), and when the demagnetization of the secondary winding W2 of the power transformer 10 is completed, i.e., when the secondary current ISR is decreased from positive to 0, the secondary control circuit 30 may increase from negative to 0 according to the drain voltage VDS2 of the synchronous rectifier switch S2, so as to detect the completion of the demagnetization of the secondary winding W2 of the power transformer 10. It should be noted that "detecting the completion of the demagnetization of the secondary winding of the power transformer" means determining the ending time point of the demagnetization procedure according to the relevant parameters, that is, when the secondary side current ISR reaches 0 current, for the sake of simplicity of description, the "detecting the completion of the demagnetization of the secondary winding of the power transformer" is used as an indication, and the following is the same.

Referring to fig. 5, fig. 6A and fig. 6B show two embodiments of the secondary side control circuit (secondary

side control circuits

30A and 30B) in the switching control circuit of the present invention.

As shown in fig. 6A, in an embodiment, the secondary-side control circuit 30A includes a current comparator 31A for comparing the secondary-side current ISR with a current threshold Ith _ ZC to generate a signal SRZC indicating that the secondary-side current ISR is 0 current, which can indicate that the demagnetization of the secondary-side winding W2 of the power transformer 10 is completed, i.e., when the secondary-side current ISR achieves 0 current. In one embodiment, the current threshold Ith _ ZC is a settable value. Specifically, the current threshold value Ith _ ZC may be set to a threshold value close to 0, and in one embodiment, the current threshold value Ith _ ZC may be set to a threshold value close to 0 but greater than 0.

As shown in fig. 6B, in an embodiment, the secondary-side control circuit 30B includes a voltage comparator 31B for comparing a signal related to the secondary-side current ISR, such as but not limited to the drain voltage VDS2 of the synchronous rectifier switch S2, with a current threshold Vth _ ZC to generate a signal SRZC indicating that the secondary-side current ISR is 0 current, which may be used to indicate that the secondary-side winding W2 of the power transformer 10 is demagnetized. In one embodiment, the current threshold Vth _ ZC is a settable value. Specifically, the current threshold Vth _ ZC may be set to a threshold value close to 0, and in one embodiment, the current threshold Vth _ ZC may be set to a threshold value close to 0 but less than 0.

In other embodiments, the demagnetization of the secondary winding W2 of the power transformer 10 can be detected by the primary side control circuit. Referring to fig. 7, fig. 7 shows an embodiment of the switching control circuit (the switching control circuit 107) in the present invention, in which the primary side control circuit 20 'detects the voltage associated with the power transformer 10' through the auxiliary winding W3 of the power transformer 10 'to detect that the demagnetization of the secondary winding W2 of the power transformer 10' is completed, i.e. when the secondary side current ISR achieves 0 current. In another embodiment, the primary-side control circuit 20 ' may detect the drain voltage VDS1 of the primary-side switch S1 to detect the voltage associated with the power transformer 10 ' to detect that the demagnetization of the secondary winding W2 of the power transformer 10 ' is completed, i.e., when the secondary current ISR achieves 0 current.

Referring to fig. 3 and 4, in an embodiment of the switching control circuit of the invention, the primary side control circuit 20 includes a clock signal CLK for determining the highest switching frequency of the switching signal S1C, as shown in fig. 4, when the clock signal CLK is generated before the demagnetization of the secondary winding W2 of the power transformer 10 is completed, i.e. the secondary current ISR achieves 0 current, the primary side control circuit 20 controls the primary side switch S1 to be turned on after a delay period Td1 from the clock signal CLK, specifically, in the embodiment, the load is larger, therefore, the clock signal CLK is generated before the demagnetization of the secondary winding W2 of the power transformer 10 is completed, i.e. the secondary current ISR achieves 0 current, according to the invention, in an embodiment, the soft switching pulse PSS' (e.g. t4-t5 in fig. 4) is triggered by the clock signal CLK to continue the synchronous rectification pulse PSR, at the same time, the clock signal CLK also triggers the delay period Td1, and since a part of the delay period Td1 overlaps the soft switching pulse PSS', the triggering of the switching signal S1C is prohibited during the delay period Td1, i.e., the primary switch S1 is prohibited from being turned on, so as to prevent the primary switch S1 and the synchronous rectification switch S2 from being turned on at the same time.

Referring to fig. 3, in another embodiment, when the clock signal CLK is pulsed after the demagnetization of the secondary winding W2 of the power transformer 10 is completed, i.e., the secondary current ISR achieves 0 current, the primary-side control circuit 20 controls the primary-side switch S1 to be turned on after a delay time Td2 from the generation of the pulse of the clock signal CLK, and specifically, in this embodiment, the load is light, so that the clock signal CLK is generated before the demagnetization of the secondary winding W2 of the power transformer 10 is completed, i.e., the secondary current ISR achieves 0 current, according to the present invention, in one embodiment, the soft switching pulse PSS (e.g., t0-t1 in fig. 3) is triggered by the clock signal CLK, and at the same time, the delay time Td2 is triggered by the clock signal CLK, and the triggering of the switching signal S1C is prohibited during the delay time Td2, i.e., the primary-side switch S1 is prohibited from being turned on, to prevent the primary-side switch S1 and the synchronous rectification switch S2 from turning on simultaneously.

In one embodiment, the delay period Td1 is longer than the delay period Td 2.

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

Referring to fig. 8 and 9, fig. 8 shows an embodiment of a primary side control circuit (the primary side control circuit 20) in the switching control circuit of the present invention, and fig. 9 shows a waveform diagram corresponding to the embodiment of the switching control circuit of the present invention. In one embodiment, the primary-side 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-side switch S1 falls below a knee point threshold Vth _ knee, that is, the time point (the end time point of the demagnetization process) when the secondary current ISR achieves 0 current, specifically, as shown in fig. 8 and 9, in one embodiment, the comparator 21 is configured to compare the drain voltage VDS1 of the primary-side switch S1 with the knee point threshold Vth _ knee to generate the knee point signal V1_ knee for indicating whether the drain voltage VDS1 of the primary-side switch S1 is below the knee point. In another embodiment, the primary-side control circuit 20 determines the time point when the demagnetization of the secondary winding W2 of the power transformer 10 is completed, i.e. the secondary current ISR achieves 0 current, according to whether the voltage V3 of the auxiliary winding W3 falls below a knee point threshold Vth _ knee, and specifically, as shown in fig. 8 and 9, in one embodiment, the comparator 22 is configured to compare the voltage V3 of the auxiliary winding W3 with the knee point threshold Vth _ knee to generate the knee point signal V1_ knee, which indicates whether the voltage V3 of the auxiliary winding W3 is lower than the knee point.

Referring to fig. 8 and 9, in an embodiment, the primary-side control circuit 20 determines whether the drain voltage VDS1 of the primary-side switch S1 is at a valley according to whether the drain voltage VDS1 of the primary-side switch S1 drops below a valley threshold Vth _ vly, and specifically, as shown in fig. 8 and 9, in an embodiment, the comparator 22 is configured to compare the drain voltage VDS1 of the primary-side switch S1 with the valley threshold Vth _ vly to generate a valley signal V1_ vly indicating whether the drain voltage VDS1 of the primary-side switch S1 is at a valley. In another embodiment, the primary-side control circuit 20 determines whether the voltage V3 of the auxiliary winding W3 is at a valley according to whether the voltage V3 of the auxiliary winding W3 falls below a valley threshold Vth _ vly, and specifically, as shown in fig. 8 and 9, in one embodiment, the comparator 22 is configured to compare the voltage V3 of the auxiliary winding W3 with the valley threshold Vth _ vly to generate a valley signal V1_ vly indicating whether the voltage V3 of the auxiliary winding W3 is at a valley. In one embodiment, after the delay periods (Td1, Td2), the timing of turning on the primary-side switch S1 is determined according to whether the drain voltage VDS1 of the primary-side switch S1 is at the trough. In a preferred embodiment, the timing of turning on the primary-side switch S1 is determined according to whether the drain voltage VDS1 of the primary-side switch S1 is close to 0 after the delay periods (Td1, Td 2).

Referring to fig. 10, fig. 10 shows a schematic diagram (a switching control circuit 110) of a switching control circuit according to an embodiment of the present invention, which is similar to the aforementioned embodiment, in which the switching control circuit 110 further includes a signal transformer 40 for transmitting a clock signal CLK from the primary side control circuit 20 ″ to the secondary side, for example, for synchronously triggering the aforementioned soft switching pulses PSS, PSS'.

Referring to fig. 11, fig. 11 shows a schematic diagram of a switching control circuit and a signal shaping circuit therein (the switching control circuit 111, the

signal shaping circuit

50A or 50B) according to an embodiment of the invention. As shown in fig. 11, in this embodiment, similar to the previous embodiments, the switching control circuit 111 further includes a signal shaping circuit (e.g., the

signal shaping circuit

50A or 50B) in this embodiment, and in one embodiment, the signal shaping circuit 50A is configured to perform signal processing on the drain voltage VDS1 of the primary-side switch S1 before being transmitted to the primary-side control circuit 20. In one embodiment, the signal shaping circuit 50B is configured to perform signal processing on the drain voltage VDS2 of the synchronous rectification switch S2 before transmitting the processed signal to the secondary-side control circuit 30.

As shown in fig. 11, in an embodiment, the signal shaping circuit (corresponding to the

signal shaping circuit

50A or 50B) includes a voltage dividing circuit (e.g., a voltage dividing resistor) and a filtering circuit (e.g., a filtering capacitor) for filtering noise.

The present invention has been described with respect to the preferred embodiments, but the above description is only for the purpose of making the content of the present invention easy to understand for those skilled in the art, and is not intended to limit the scope of the present invention. The embodiments described are not limited to single use, but may be used in combination, for example, two or more embodiments may be combined, and some components in one embodiment may be substituted for corresponding components in another embodiment. Further, equivalent variations and combinations are contemplated by those skilled in the art within the spirit of the present invention, and the term "processing or computing or generating an output result based on a signal" is not limited to the signal itself, and includes, if necessary, performing voltage-to-current conversion, current-to-voltage conversion, and/or scaling on the signal, and then processing or computing the converted signal to generate an output result. It is understood that equivalent variations and combinations, not necessarily all illustrated, will occur to those of skill in the art, which combinations are not necessarily intended to be limiting. Accordingly, the scope of the present invention should be determined to encompass all such equivalent variations as described above.

Claims

1. 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 comprising:

a power transformer coupled between the input voltage and the output voltage in an electrically isolated manner;

a primary side control circuit for generating a switching signal to control a primary side switch in the flyback power supply circuit to switch a primary side winding of the power transformer, wherein the primary side winding is coupled to the input voltage; and

a secondary side control circuit for generating a synchronous rectification control signal to control a synchronous rectification switch in the flyback power supply circuit and switching a secondary side winding of the power transformer to generate the output voltage, wherein the synchronous rectification control signal has a synchronous rectification pulse for controlling the synchronous rectification switch to conduct a synchronous rectification period to realize secondary side synchronous rectification and a soft switching pulse for controlling the synchronous rectification switch to conduct a soft switching period, thereby enabling the primary side switch to realize soft switching;

wherein the power transformer is magnetically sensitive when the primary side switch is turned on, and transfers energy obtained by the magnetically sensitive when the primary side switch is turned off to the output voltage;

when the flyback power supply circuit operates in a boundary conduction mode, the synchronous rectification switch is conducted to demagnetize through the synchronous rectification pulse by the power transformer, and after the secondary side winding of the power transformer is demagnetized, the secondary side control circuit continuously conducts the synchronous rectification switch through the soft switching pulse, so that soft switching is realized when the primary side is conducted next time, and the soft switching pulse has a first conduction time interval; or

When the flyback power supply circuit operates in a discontinuous conduction mode, the synchronous rectification switch is conducted to demagnetize through the synchronous rectification pulse by the power transformer, after the demagnetization of the secondary side winding of the power transformer is completed, the secondary side control circuit controls the synchronous rectification switch to be not conducted, then the synchronous rectification switch is conducted again through the soft switching pulse by the secondary side control circuit, so that the soft switching is realized when the primary side is conducted next time, and the soft switching pulse has a second conduction time period;

the primary side control circuit generates a frequency signal for determining a highest switching frequency of the switching signal, wherein when the frequency signal is generated before demagnetization of the secondary winding of the power transformer is completed, the primary side control circuit controls the conduction of the primary side switch after the frequency signal delays for a first delay time period, and when the frequency signal is generated after demagnetization of the secondary winding of the power transformer is completed, the primary side control circuit controls the conduction of the primary side switch after the frequency signal delays for a second delay time period;

wherein the primary-side switch is inhibited from conducting during the first delay period and the second delay period;

wherein the first delay period is longer than the second delay period.

2. The switching control circuit of claim 1 wherein the soft switching pulse draws a negative current from the output voltage by turning on the secondary winding, thereby causing the primary side to switch soft on the next turn on.

3. The switching control circuit of claim 1, wherein the secondary-side control circuit detects a voltage associated with the synchronous rectifier switch to detect completion of demagnetization of the secondary-side winding of the power transformer.

4. The switching control circuit of claim 3, further comprising a signal shaping circuit for shaping the voltage associated with the synchronous rectifier switch and providing the shaped voltage to the secondary control circuit to detect completion of demagnetization of the secondary winding of the power transformer.

5. The switching control circuit of claim 1, wherein the primary-side control circuit detects a voltage associated with the power transformer through an auxiliary winding of the power transformer to detect completion of demagnetization of the secondary winding of the power transformer.

6. The switching control circuit of claim 5, further comprising a signal shaping circuit for shaping a voltage associated with the power transformer and providing the shaped voltage to the primary control circuit to detect completion of demagnetization of the secondary winding of the power transformer.

7. The switching control circuit of claim 1, wherein the secondary-side control circuit has a current threshold, and the secondary-side control circuit determines whether the secondary-side winding of the power transformer is demagnetized according to the current flowing through the synchronous rectifier switch and the current threshold, wherein the current threshold is a settable value.

8. The switching control circuit of claim 1, wherein the first conduction period is longer than the second conduction period.

9. The switching control circuit of claim 1, further comprising a signal transformer for transmitting the clock signal from the primary-side control circuit to the secondary-side control circuit.

10. A method for controlling a flyback power supply circuit, in order to change an input voltage and produce an output voltage, wherein a power transformer of the flyback power supply circuit, couple to the input voltage and the output voltage in an electrically insulated way; the method comprises the following steps:

generating a switching signal at a primary side of the flyback power supply circuit to control a primary side switch of the flyback power supply circuit to switch a primary side winding of the power transformer, wherein the primary side winding is coupled to the input voltage; and

generating a synchronous rectification control signal at the secondary side of the flyback power supply circuit to control a synchronous rectification switch in the flyback power supply circuit and switch a secondary side winding of the power transformer to generate the output voltage, wherein the synchronous rectification control signal has a synchronous rectification pulse and a soft switching pulse, the synchronous rectification pulse is used for controlling the synchronous rectification switch to conduct a synchronous rectification time interval to realize secondary side synchronous rectification, and the soft switching pulse is used for controlling the synchronous rectification switch to conduct a soft switching time interval to realize soft switching of the primary side switch;

when the flyback power supply circuit operates in a boundary conduction mode, the synchronous rectification switch is conducted to demagnetize through the synchronous rectification pulse by the power transformer, and after the secondary side winding of the power transformer is demagnetized, the synchronous rectification switch is continuously conducted through the soft switching pulse, so that soft switching is realized when the primary side is conducted next time, and the soft switching pulse has a first conduction time interval; or

When the flyback power supply circuit operates in a discontinuous conduction mode, the synchronous rectification switch is conducted to demagnetize through the synchronous rectification pulse by the power transformer, after the demagnetization of the secondary side winding of the power transformer is completed, the synchronous rectification switch is controlled to be not conducted, then the synchronous rectification switch is conducted again through the soft switching pulse, so that the soft switching is realized when the primary side is conducted next time, and the soft switching pulse has a second conduction time period;

wherein the step of generating the switching signal comprises: generating a frequency signal at the primary side for determining a highest switching frequency of the switching signal, wherein when the frequency signal is generated before demagnetization of the secondary winding of the power transformer is completed, the primary side switch is controlled to be turned on after the frequency signal is delayed for a first delay period, and when the frequency signal is generated after demagnetization of the secondary winding of the power transformer is completed, the primary side switch is controlled to be turned on after the frequency signal is delayed for a second delay period, wherein the primary side switch is prohibited to be turned on during the first delay period and the second delay period, and the first delay period is longer than the second delay period.

11. The method of claim 10 wherein said soft switching pulse draws a negative current from said output voltage by turning on said secondary winding, thereby causing said primary side to switch soft on the next turn on.

12. The method of claim 10, wherein a voltage associated with the synchronous rectifier switch is detected on the secondary side to detect completion of demagnetization of the secondary winding of the power transformer.

13. The method of claim 12, further comprising: shaping the voltage associated with the synchronous rectifier switch to detect completion of demagnetization of the secondary winding of the power transformer.

14. The method of claim 10, wherein detecting completion of demagnetization of the secondary winding of the power transformer comprises: detecting a voltage associated with the power transformer through an auxiliary winding of the power transformer on the primary side to detect completion of demagnetization of the secondary winding of the power transformer.

15. The method of claim 14, further comprising: shaping a voltage associated with the power transformer to detect completion of demagnetization of the secondary winding of the power transformer.

16. The method of claim 10, wherein determining whether demagnetization of the secondary winding of the power transformer is complete comprises: and determining whether the secondary side winding of the power transformer is demagnetized according to the current flowing through the synchronous rectification switch and a current threshold value, wherein the current threshold value is a settable value.

17. The method of claim 10, wherein the first conduction period is longer than the second conduction period.

18. The method of claim 10, further comprising: and a signal transformer for transmitting the frequency signal from the primary side of the flyback power supply circuit to the secondary side of the flyback power supply circuit.