Flyback converter and method for the same

Abstract

This disclosure provides a flyback switching circuit, including a primary-side power switch, a secondary-side rectifier switch, and a control circuit. When the flyback switching circuit operates normally and has not entered a surge mode, after the primary-side power switch is turned off, the control circuit controls the secondary-side rectifier switch to turn on until the de-energizing condition of the secondary-side rectifier switch is met. Before the primary-side power switch is turned on in the next switching cycle, the control circuit controls the secondary-side rectifier switch to remain on for an additional period of time. When the flyback switching circuit enters and then exits a surge mode, before the primary-side power switch is turned on in the next switching cycle, the control circuit controls the secondary-side rectifier switch to no longer remain on for an additional period of time.

Description

Technical Field

[0001] This disclosure relates to an electronic circuit, and more specifically, particularly to a flyback switching circuit. Prior Technology

[0002] A flyback converter is a typical converter in AC-to-DC conversion applications. A typical circuit of a flyback converter is shown in Figure 1, which includes: a transformer T with a primary-side coil T0 and a secondary-side coil T1, a primary-side power switch Q0, a secondary-side rectifier switch Q1, and a voltage-off buffer RCD.

[0003] In flyback switching applications, zero-voltage turn-on (ZVTO) technology is typically used to reduce switching losses on the primary-side power switch. ZVTO refers to a technique where, before the primary-side power switch Q0 turns on, the secondary-side power switch Q1 conducts for an additional period after its freewheeling phase, or an auxiliary power switch Q2 connected in series with the auxiliary coil T2 (as shown by the dotted line in Figure 1) conducts for a period, thus achieving zero-voltage turn-on for the primary-side power switch Q0. After this additional conduction time, a negative magnetizing current is generated on the primary side. This negative magnetizing current amplifies the amplitude of the discontinuous oscillation of the primary-side power switch Q0, resulting in a relatively low turn-on voltage for Q0.

[0004] However, when the circuit load decreases, causing the converter to enter burst mode, and then exits burst mode as the load increases, the voltage across the capacitor in the voltage-off buffer RCD will drop very low. If the secondary rectifier switch Q1 is turned on for an additional period of time before the primary power switch Q0 is turned on, the current flowing through the secondary rectifier switch during this additional on-time will be very large, resulting in a large voltage ripple in the output voltage. Summary of the Invention

[0005] According to one embodiment of this disclosure, a flyback switching circuit is proposed, including a primary-side power switch, a secondary-side rectifier switch, and a control circuit. The primary-side power switch is coupled to the primary winding of a transformer. The secondary-side rectifier switch is coupled to the secondary winding of the transformer. The control circuit provides primary-side control signals and secondary-side control signals to control the primary-side power switch and the secondary-side rectifier switch, respectively. Under the control of the control circuit, the primary-side power switch and the secondary-side rectifier switch are periodically turned on and off, converting the input voltage into an output voltage. When the flyback switching circuit operates normally and does not enter a surge mode, after the primary-side power switch is turned off, the control circuit controls the secondary-side rectifier switch to turn on until the de-energizing condition of the secondary-side rectifier switch is met. Before the primary-side power switch is turned on in the next switching cycle, the control circuit controls the secondary-side rectifier switch to turn on for an additional period of time. When the flyback converter enters and exits the rush mode, the control circuit controls the secondary rectifier switch not to conduct for an additional period of time before the primary power switch is turned on in the next switching cycle.

[0006] According to another embodiment of this disclosure, a method for a flyback converter circuit is proposed. The flyback converter circuit has a primary-side power switch and a secondary-side rectifier switch that are periodically controlled to switch between each other to convert an input voltage into an output voltage. The method includes: when the flyback converter circuit is operating normally and has not entered a surge mode, after the primary-side power switch is turned off, controlling the secondary-side rectifier switch to turn on until the de-energizing condition of the secondary-side rectifier switch is met, and controlling the secondary-side rectifier switch to turn on for an additional period of time before the primary-side power switch is turned on in the next switching cycle; and when the flyback converter circuit enters a surge mode and then exits the surge mode, controlling the secondary-side rectifier switch to no longer turn on for an additional period of time before the primary-side power switch is turned on in the next switching cycle. Simple Explanation of the Diagram

[0007] To better understand the embodiments of this disclosure, the disclosure will be described in detail with reference to the following drawings. Like elements are represented by the same reference numerals. The following drawings are for illustrative purposes only and may only show a portion of the device, and are not necessarily drawn to scale. [Figure 1] A schematic diagram of the circuit structure of a flyback converter. [Figure 2] is a schematic diagram of the circuit structure of a flyback converter according to an embodiment of the present disclosure. [Figure 3] illustrates a circuit structure diagram of a flyback switching circuit according to an embodiment of the present disclosure. [Figure 4] is a schematic diagram of the circuit structure of the first controller according to an embodiment of the present disclosure. [Figure 5] is a flowchart of a method for a flyback switching circuit according to an embodiment of the present disclosure. Implementation

[0008] Specific embodiments of this disclosure will now be described in detail. It should be noted that the embodiments described herein are for illustrative purposes only and are not intended to limit the scope of this disclosure. In the following description, numerous specific details are set forth in order to provide a thorough understanding of this disclosure. However, it will be apparent to those skilled in the art that these specific details are not necessary to implement this disclosure. In other instances, known circuits, materials, or methods have not been specifically described to avoid obscuring this disclosure.

[0009] Throughout this specification, references to "an embodiment," "an embodiment," "an example," or "an example" mean that a particular feature, structure, or characteristic described in connection with that embodiment or example is included in at least one embodiment disclosed herein. Therefore, the phrases "in an embodiment," "in an embodiment," "an example," or "an example" appearing in various places throughout this specification do not necessarily refer to the same embodiment or example. Furthermore, particular features, structures, or characteristics can be combined in one or more embodiments or examples in any suitable combination and / or sub-combination. Moreover, those skilled in the art will understand that the accompanying drawings provided herein are for illustrative purposes and are not necessarily drawn to scale. It should be understood that when an element is referred to as "coupled to" or "connected to" another element, it can be directly coupled to or coupled to the other element, or there may be intermediate elements. Conversely, when an element is referred to as "directly coupled to" or "directly connected to" another element, there are no intermediate elements. The same reference numerals indicate the same elements. The term "and / or" as used herein includes any and all combinations of one or more of the related listed items.

[0010] Figure 2 is a schematic diagram of the circuit structure of a flyback converter circuit 200 according to an embodiment of the present disclosure. In the embodiment shown in Figure 2, the flyback converter circuit 200 includes: a primary-side power switch Q0 coupled to the primary-side coil T0 of a transformer T; a secondary-side rectifier switch Q1 coupled to the secondary-side coil T1 of a transformer T; and a control circuit 110 providing a primary-side control signal GPS and a secondary-side control signal GSR to control the primary-side power switch Q0 and the secondary-side rectifier switch Q1, respectively. The primary-side power switch Q0 and the secondary-side rectifier switch Q1 are periodically turned on and off under the control of the control circuit 110, converting the input voltage Vin into an output voltage VO. Specifically, when the flyback converter circuit is operating normally and has not entered the surge mode, after the primary power switch Q0 is turned off, the control circuit 110 controls the secondary rectifier switch Q1 to turn on until the condition for the secondary rectifier switch Q1 to turn off is met. Before the primary power switch Q0 is turned on in the next switching cycle, the control circuit 110 controls the secondary rectifier switch Q1 to turn on for an additional period of time. When the flyback converter circuit 200 enters the surge mode and then exits the surge mode, before the primary power switch Q0 is turned on in the next switching cycle, the control circuit 110 controls the secondary rectifier switch Q1 to no longer turn on for an additional period of time.

[0011] In one embodiment of this disclosure, the disconnection conditions of the secondary rectifier switch Q1 include: the current flowing through the secondary rectifier switch Q1 drops to a reference value (such as a reference value near zero), such as the voltage across the secondary rectifier switch Q1 representing the current flowing through the secondary rectifier switch Q1 reaching a disconnection threshold.

[0012] In one embodiment of this disclosure, after the flyback conversion circuit 200 enters the surge mode, the control circuit 110 controls the primary-side power switch Q0 and the secondary-side rectifier switch Q1 not to perform switching operations.

[0013] In one embodiment of this disclosure, when the flyback switching circuit 200 enters and exits the rush mode, in the first switching cycle immediately after exiting the rush mode, the control circuit 110 controls the secondary-side rectifier switch Q1 not to conduct for an additional period of time before the primary-side power switch Q0 is turned on in the next switching cycle; and after exiting the rush mode and after one switching cycle, the control circuit 110 resumes controlling the secondary-side rectifier switch Q1 to conduct for an additional period of time before the primary-side power switch Q0 is turned on in the next switching cycle. In other embodiments disclosed herein, when the flyback switching circuit 200 enters and exits the rush mode, during the initial n switching cycles immediately after exiting the rush mode, the control circuit 110 controls the secondary-side rectifier switch Q1 to not conduct for an additional period of time before the primary-side power switch Q0 is turned on in the next switching cycle, where n is an integer greater than 1; and after exiting the rush mode and after the n switching cycles have passed, the control circuit 110 resumes controlling the secondary-side rectifier switch Q1 to conduct for an additional period of time before the primary-side power switch Q0 is turned on in the next switching cycle.

[0014] In one embodiment of this disclosure, the duration for which the secondary-side rectifier switch Q1 is additionally turned on is determined by the input voltage and the output voltage. In another embodiment of this disclosure, the additional conduction of the secondary-side rectifier switch Q1 is terminated when the current flowing through the secondary-side rectifier switch Q1 reaches a current threshold.

[0015] In one embodiment disclosed herein, the primary-side power switch Q0 and the secondary-side rectifier switch Q1 comprise metal-oxide-semiconductor field-effect transistors (MOSFETs). However, those skilled in the art will understand that the primary-side power switch Q0 and the secondary-side rectifier switch Q1 may comprise other controllable switching devices, such as bipolar junction transistors (BJTs), insulated-gate bipolar transistors (IGBTs), gallium nitride field-effect transistors (GaN FETs), silicon carbide field-effect transistors (SiC FETs), or common-source-common-gate stacked circuits (such as a stacked circuit of a junction field-effect transistor (JFET) and a MOSFET), etc.

[0016] Figure 3 illustrates a schematic diagram of the circuit structure of the flyback conversion circuit 300 according to an embodiment of the present disclosure. The embodiment shown in Figure 3 specifically illustrates a schematic diagram of the circuit structure of the control circuit 110. Specifically, in the embodiment shown in Figure 3, the control circuit 110 includes: an error amplifier 101, which amplifies and integrates the difference between the feedback voltage VFB (representing the output voltage VO) and the reference voltage Vref to generate a compensation signal CMP; and a hysteresis comparator 102, which compares the compensation signal CMP with the threshold voltage VTH to generate a judgment signal BRT, indicating whether the flyback conversion circuit 300 has entered or exited the rush mode. The control circuit 110 further includes: a first controller 103, which generates a secondary-side signal syn0 and a secondary-side control signal GSR based on a signal representing the current flowing through the secondary-side rectifier switch Q1 (such as the voltage VDSR across the secondary-side rectifier switch Q1), a compensation signal CMP, and a judgment signal BRT; a transmission module 104, which converts the secondary-side signal syn0 into a primary-side signal syn1; and a second controller 105, which generates a primary-side control signal GPS based on the primary-side signal syn1.

[0017] In one embodiment of this disclosure, the transmission module 104 includes an isolation capacitor. In other embodiments of this disclosure, the transmission module 104 may also include an isolation feedback element such as an optocoupler.

[0018] In one embodiment of this disclosure, if the compensation signal CMP is less than the threshold voltage VTH, it indicates that the output voltage VO is low (i.e., the load is light), and the flyback conversion circuit 300 enters the rush mode; if the load increases, the compensation signal CMP increases, and when the compensation signal CMP increases to the sum of the threshold voltage VTH and the hysteresis value of the hysteresis comparator 102, it indicates that the flyback conversion circuit 300 exits the rush mode.

[0019] In one embodiment of this disclosure, the first controller 103 generates a secondary-side control signal GSR in response to a signal representing the current flowing through the secondary-side rectifier switch Q1 (such as the voltage VDSR across the secondary-side rectifier switch Q1). If the voltage across the secondary-side rectifier switch Q1 is less than the turn-on threshold, the first controller 103 controls the secondary-side rectifier switch Q1 to turn on; if the voltage across the secondary-side rectifier switch Q1 is greater than the turn-off threshold, the first controller 103 controls the secondary-side rectifier switch Q1 to turn off. Subsequently, if it is determined that the signal BRT indicator circuit 300 has not entered the surge mode, the first controller 103, in response to the compensation signal CMP, turns on the secondary-side rectifier switch Q1 for an additional period of time and sends the secondary-side signal syn0 to the primary side through the transmission module 104, causing the primary-side power switch Q0 to turn on, and the flyback conversion circuit 300 enters a new switching cycle. If the signal BRT indicates that the flyback converter circuit 300 has entered the surge mode, the first controller 103 controls the secondary-side rectifier switch Q1 to not perform a switching action; at the same time, it outputs the corresponding secondary-side signal syn0 to the primary side, so that the primary-side power switch Q0 also does not perform a switching action. For example, at this time, the first controller 103 outputs a low-level secondary-side signal syn0, which is equivalent to the first controller 103 not sending a signal to indicate that the primary-side power switch Q0 is turned on. If the signal BRT indicates that the signal has exited the surge mode, the first controller 103, in response to the compensation signal CMP, directly sends the secondary-side signal syn0 to the primary side through the transmission module 104, so that the second controller 105 controls the primary-side power switch Q0 to turn on.

[0020] Specifically, if the flyback converter circuit does not enter the surge mode, when the compensation signal CMP indicates that the primary-side power switch Q0 is about to be turned on, the first controller 103 controls the secondary-side rectifier switch Q1 to conduct for an additional period of time to achieve zero-voltage turn-on function. At this time, the secondary-side signal syn0 (e.g., high potential) is sent to the primary side through the transmission module 104, causing the second controller 105 to control the primary-side power switch Q0 to turn on. However, when the flyback converter circuit enters and exits the surge mode, when the compensation signal CMP indicates that the primary-side power switch Q0 is about to be turned on, the first controller 103 does not control the secondary-side rectifier switch Q1 to conduct for an additional period of time. At this time, the conduction of the secondary-side rectifier switch Q1 is blocked, that is, the zero-voltage turn-on function is blocked. The secondary-side signal syn0 is directly sent to the primary side through the transmission module 104, causing the second controller 105 to control the primary-side power switch Q0 to turn on directly.

[0021] In one embodiment of this disclosure, if the signal BRT indicates that the flyback switching circuit 300 has exited the burst mode, then in the initial n switching cycles after exiting (e.g., n cycles can be counted using a counter), the first controller 103, in response to the compensation signal CMP, directly sends the secondary side signal syn0 (e.g., high potential) to the primary side through the transmission module 104, so that the second controller 105 controls the primary side power switch Q0 to turn on.

[0022] At the second controller 105, in response to the primary-side signal syn1, the second controller 105 controls the primary-side power switch Q0 to either turn on or not perform a switching action. After the primary-side power switch Q0 is turned on, the second controller 105 can turn off the primary-side power switch Q0 according to the current flowing through the primary-side power switch Q0 (when the current flowing through the primary-side power switch Q0 reaches the current limit), such as peak current control well known to those skilled in the art; or the second controller 105 can adopt a constant on-time method, turning off the primary-side power switch Q0 when the on-time of the primary-side power switch Q0 reaches a certain duration. How to control the off-time of the primary-side power switch Q0 is a technical method well known to those skilled in the art and is not the technical inventive point disclosed herein, and will not be described in detail here for the sake of brevity.

[0023] Figure 4 is a schematic diagram of the circuit structure of the first controller 103 according to an embodiment of the present disclosure. In the embodiment shown in Figure 4, the first controller 103 includes: a first signal generator 31, which generates a first control signal GSR1 based on a signal VDSR representing the current flowing through the secondary-side rectifier switch Q1, for controlling the freewheeling current of the secondary-side rectifier switch Q1; a second signal generator 32, which generates a second control signal GSR2 based on a compensation signal CMP and a judgment signal BRT, for controlling the additional conduction of the secondary-side rectifier switch Q1; and a selector 33, which selects either the second control signal GSR2 or the compensation signal CMP as the secondary-side signal syn0 based on the judgment signal BRT.

[0024] When the judgment signal BRT indicates that the flyback conversion circuit has not entered the surge mode, the second signal generator 32 generates the second control signal GSR2 in response to the compensation signal CMP, so as to control the secondary side rectifier switch Q1 to conduct for an additional period of time before the primary side power switch Q0 is turned on in the next switching cycle.

[0025] In the embodiment shown in Figure 4, the first controller 103 further includes a logic OR circuit 34, which performs a logic OR operation on the first control signal GSR1 and the second control signal GSR2 to obtain a secondary-side control signal GSR, so as to control the secondary-side rectifier switch Q1.

[0026] Figure 5 is a flowchart 500 of a method for a flyback converter circuit according to an embodiment of the present disclosure. The flyback converter circuit has a primary-side power switch and a secondary-side rectifier switch that are periodically controlled to switch to convert an input voltage into an output voltage. The method includes steps 501 and 502.

[0027] Step 501: When the flyback switching circuit is operating normally and has not entered the surge mode, after the primary-side power switch is turned off, the secondary-side rectifier switch is controlled to turn on until the secondary-side rectifier switch's off condition is met. Before the primary-side power switch is turned on in the next switching cycle, the secondary-side rectifier switch is controlled to turn on for an additional period of time. In one embodiment disclosed herein, the off condition for the secondary-side rectifier switch includes: the current flowing through the secondary-side rectifier switch decreasing to a reference value.

[0028] Step 502: When the flyback conversion circuit enters the surge mode and then exits the surge mode, the secondary rectifier switch is controlled to not be turned on for an additional period of time before the primary power switch is turned on in the next switching cycle.

[0029] In one embodiment of this disclosure, the method further includes: when the flyback converter circuit enters the surge mode, controlling the primary-side power switch and the secondary-side rectifier switch not to perform switching operations.

[0030] In one embodiment of this disclosure, when the flyback switching circuit enters and exits the rush mode, in the first switching cycle after exiting the rush mode, the secondary rectifier switch is controlled not to conduct for an additional period of time before the primary power switch is turned on in the next switching cycle; and after exiting the rush mode and after one switching cycle, the secondary rectifier switch is controlled to conduct for an additional period of time before the primary power switch is turned on in the next switching cycle.

[0031] In one embodiment of this disclosure, when the flyback switching circuit enters and exits the rush mode, during the initial n switching cycles immediately after exiting the rush mode, the secondary rectifier switch is controlled to not conduct for an additional period of time in the next switching cycle of the primary power switch, where n is an integer greater than 1; and after exiting the rush mode and after the n switching cycles have passed, the secondary rectifier switch is controlled to conduct for an additional period of time before being turned on in the next switching cycle of the primary power switch.

[0032] In one embodiment of this disclosure, the method further includes: amplifying and integrating the difference between the feedback voltage representing the output voltage and the reference voltage to generate a compensation signal; comparing the magnitude of the compensation signal and the threshold voltage to generate a judgment signal to indicate whether the flyback conversion circuit has entered or exited the surge mode.

[0033] The aforementioned flyback switching circuit and method according to various embodiments of this disclosure, when exiting the system after entering the rush mode, does not turn on the secondary-side rectifier switch for an additional period of time at the start of the switching cycle, but directly turns on the primary-side power switch. This avoids the large output voltage ripple caused by excessive secondary-side current due to the voltage drop across the RCD buffer capacitor caused by the rush mode. Therefore, the circuit performance is improved.

[0034] Although this disclosure has been described with reference to several exemplary embodiments, it should be understood that the terminology used is illustrative and exemplary, and not restrictive. Because this disclosure can be embodied in many forms without departing from its spirit or substance, it should be understood that the above embodiments are not limited to any of the foregoing details, but should be interpreted broadly within the spirit and scope defined by the appended claims. Therefore, all variations falling within the scope of the claims or their equivalents should be covered by the appended claims.

[0035] 200: Flyback Conversion Circuit Vin: Input voltage T0: Primary side coil T1: Secondary coil T: Transformer Q0: Primary-side power switch Q1: Secondary side rectifier switch VO: Output voltage GPS: Primary side control signal GSR: Secondary control signal 110: Control Circuit 103: First Controller 104: Transfer Module 105: Second Controller syn0: Secondary side signal syn1: Primary side signal RCD: Voltage-off type buffer T2: Auxiliary coil Q2: Auxiliary power switch 300: Flyback Conversion Circuit VDSR: Voltage VFB: Feedback Voltage Vref: Reference voltage 101: Error Amplifier CMP: Compensation Signal 102: Hysteresis Comparator VTH: Threshold voltage BRT: Determine the signal VDSR: Voltage GSR1: First control signal GSR2: Second Control Signal 31: First signal generator 32: Second signal generator 33: Selector 34: Logic OR circuit 500: Method Flowchart 501, 502: Steps

Claims

1. A flyback switching circuit, comprising: A primary-side power switch is coupled to a primary-side coil of a transformer; a secondary-side rectifier switch is coupled to the primary-side coil of the transformer. A control circuit is provided to provide a primary-side control signal and a secondary-side control signal to control the primary-side power switch and the secondary-side rectifier switch, respectively. The primary-side power switch and the secondary-side rectifier switch are periodically turned on and off under the control of the control circuit to convert an input voltage into an output voltage. Specifically: when the flyback converter is operating normally and has not entered a surge mode, after the primary-side power switch is turned off, the control circuit controls the secondary-side rectifier switch to turn on until a disconnection condition for the secondary-side rectifier switch is met. Before the primary-side power switch is turned on in the next switching cycle, the control circuit controls the secondary-side rectifier switch to be turned on for an additional period of time. When the flyback converter enters and then exits the surge mode, before the primary-side power switch is turned on in the next switching cycle, the control circuit controls the secondary-side rectifier switch to no longer be turned on for an additional period of time.

2. The flyback switching circuit as described in claim 1, wherein: After the flyback conversion circuit enters the surge mode, the control circuit controls the primary-side power switch and the secondary-side rectifier switch not to perform switching operations.

3. The flyback switching circuit as described in claim 1, wherein: When the flyback switching circuit enters and exits the burst mode, in the first switching cycle immediately after exiting the burst mode, the control circuit controls the secondary-side rectifier switch to not conduct for an additional period of time before the primary-side power switch is turned on in the next switching cycle. After exiting the burst mode and after one switching cycle, the control circuit resumes controlling the secondary-side rectifier switch to conduct for an additional period of time before the primary-side power switch is turned on in the next switching cycle.

4. The flyback switching circuit as described in claim 1, wherein: When the flyback switching circuit enters and exits the burst mode, during the initial n switching cycles after exiting the burst mode, the control circuit controls the secondary rectifier switch to not conduct for an additional period of time before the primary power switch is turned on in the next switching cycle, where n is an integer greater than 1. After exiting the burst mode and after the n switching cycles, the control circuit resumes controlling the secondary rectifier switch to conduct for an additional period of time before the primary power switch is turned on in the next switching cycle.

5. The flyback switching circuit as claimed in claim 1, wherein the control circuit comprises: An error amplifier is used to amplify and integrate the difference between a feedback voltage representing the output voltage and a reference voltage to generate a compensation signal. A hysteresis comparator is used to compare the compensation signal with a threshold voltage to generate a judgment signal indicating whether the flyback switching circuit has entered or exited the surge mode; a first controller is used to generate a primary-side signal and a secondary-side control signal based on a signal representing a current flowing through the secondary-side rectifier switch, the compensation signal, and the judgment signal; a transmission module is used to convert the secondary-side signal into a primary-side signal; and a second controller is used to generate the primary-side control signal based on the primary-side signal.

6. The flyback switching circuit as claimed in claim 5, wherein the first controller comprises: A first signal generator for generating a first control signal based on the signal representing the current flowing through the secondary rectifier switch to control a freewheeling current in the secondary rectifier switch; a second signal generator for generating a second control signal based on the compensation signal and the judgment signal to control additional conduction of the secondary rectifier switch; and a selector for selecting either the second control signal or the compensation signal as the secondary signal based on the judgment signal.

7. A method for a flyback converter circuit having a primary-side power switch and a secondary-side rectifier switch that are periodically controlled to switch to convert an input voltage into an output voltage, the method comprising: When the flyback switching circuit is operating normally and has not entered a surge mode, after the primary power switch is turned off, the secondary rectifier switch is controlled to turn on until a disconnection condition for the secondary rectifier switch is met. Before the primary power switch is turned on in the next switching cycle, the secondary rectifier switch is controlled to turn on for an additional period of time. When the flyback switching circuit enters the surge mode and then exits the surge mode, the secondary rectifier switch is controlled not to turn on for an additional period of time before the primary power switch is turned on in the next switching cycle.

8. The method as described in claim 7, further comprising: When the flyback converter circuit enters the surge mode, it controls the primary-side power switch and the secondary-side rectifier switch not to perform switching operations.

9. The method as described in request item 7, wherein: When the flyback switching circuit enters and exits the burst mode, in the first switching cycle after exiting the burst mode, the secondary rectifier switch is controlled not to conduct for an additional period of time before the primary power switch is turned on in the next switching cycle. After exiting the burst mode and after one switching cycle, the secondary rectifier switch is controlled to conduct for an additional period of time before the primary power switch is turned on in the next switching cycle.

10. The method as described in request item 7, wherein: When the flyback switching circuit enters and exits the burst mode, during the initial n switching cycles immediately after exiting the burst mode, the secondary rectifier switch is controlled to not conduct for an additional period of time in the next switching cycle of the primary power switch, where n is an integer greater than 1; after exiting the burst mode and after the n switching cycles have elapsed, the secondary rectifier switch is controlled to conduct for an additional period of time before being turned on in the next switching cycle of the primary power switch.

Images