A control method, a control device and a flyback converter

Abstract

The present invention discloses a control method, a control device, and a flyback converter. The control method includes, in each operating cycle, the following steps: turning on the main power switch for a first duration to store energy in the primary winding; turning on the rectifier for a second duration to release the energy stored in the primary winding via the secondary winding, while simultaneously transferring the leakage inductance energy of the primary winding to the clamping capacitor; turning on the clamping switch for a third duration to transfer the leakage inductance energy stored in the clamping capacitor to the secondary circuit via the transformer for output; and turning on the rectifier again for a fourth duration to turn on the main power switch before the negative current reaches zero. Time intervals exist between the first and second durations, between the fourth and third durations, and between the fourth and first durations of the next operating cycle. The present invention can reduce the capacitance of the clamping capacitor, thereby reducing the cost of the clamping switch.

Description

Technical Field

[0001] The present invention relates to the field of switching converters, and in particular to a control method, a control device and a flyback converter. Background Art

[0002] In the field of small and medium-power switching power supplies, the flyback converter is the most popular circuit topology. However, as switching power supplies develop towards high frequency and small size, the losses of the flyback converter are receiving more and more attention, especially the switching losses of the primary-side power switch tube. In addition, due to the existence of leakage inductance, the drain-source of the primary-side power switch tube needs to withstand voltage spikes. Therefore, a corresponding clamping circuit needs to be added to the flyback converter to limit the voltage spike of the primary-side power switch tube. The usual clamping circuit is the RCD clamping circuit. However, clamping circuits such as the RCD clamp and the LCD clamp are lossy absorption circuits, which further leads to reduced performance of the flyback converter and hinders the high-frequency development of the flyback converter.

[0003] In order to further increase the operating frequency of the flyback converter and reduce the switching loss, the industry has proposed a secondary active clamp flyback converter. Figure 1a The schematic diagram of the existing secondary active clamp flyback converter circuit is shown in FIG. Figure 1b for Figure 1a Timing diagram of a secondary-side active-clamp flyback converter. The secondary-side active-clamp flyback converter generates reverse current on the secondary side by extending the on-time of the secondary-side synchronous rectifier after the transformer is demagnetized. When the secondary-side synchronous rectifier is turned off, negative current also flows on the primary side, thereby achieving zero voltage switching (ZVS) of the primary-side power switch.

[0004] Although the secondary active clamp flyback converter can achieve zero-voltage turn-on of the primary-side power switch tube, the primary-side power switch tube still requires an RCD clamping circuit to limit the voltage spike at its drain-source terminal. In addition, the control method is complex and requires variable frequency control to achieve zero-voltage turn-on of the primary-side power switch tube under a wide input voltage range and load range. A higher frequency is required at high voltage and light load, which in turn increases the transformer iron loss and RCD clamping loss.

[0005] If a primary active clamp flyback converter appears, Figure 2a The schematic diagram of the existing primary active clamp flyback converter circuit is shown in FIG. Figure 2b for Figure 2aTiming diagram of a primary-side active-clamp flyback converter. A primary-side active-clamp flyback converter achieves zero-voltage turn-on of the main power switch by turning on the clamp switch for a period of time before the main power switch turns on. The clamp capacitor limits the drain-source voltage spike of the main power switch, eliminating the need for an RCD clamp. However, to store sufficient energy to achieve zero-voltage turn-on of the main power switch, the clamp capacitor requires a large capacitance. This large capacitance increases current, necessitating the selection of a better clamp switch. Therefore, this method is not suitable for low-cost, low- and medium-power applications. Summary of the Invention

[0006] In view of this, the technical problem to be solved by the present invention is to provide a control method, a control device and a flyback converter, which can not only achieve zero-voltage turn-on of the main tube but also recover leakage inductance energy. In addition, the present invention can reduce the capacitance of the clamping capacitor, thereby reducing the cost of the clamping switch tube.

[0007] As a first aspect of the present invention, an embodiment of the control method provided is as follows:

[0008] A control method is applied to a flyback converter, the flyback converter comprising a primary-side circuit, a secondary-side circuit, a transformer, and a control device; the primary-side circuit comprising a main power switch, a clamp switch, a clamp capacitor, and a primary winding of the transformer; the secondary-side circuit comprising a rectifier and a secondary winding of the transformer; the control device comprising a primary-side controller, a secondary-side controller, an isolation circuit, and control logic; the primary-side controller is configured to control the on / off state of the main power switch and the clamp switch, the secondary-side controller is configured to control the on / off state of the rectifier, the isolation circuit is configured to bidirectionally transmit synchronization signals between the primary-side controller and the secondary-side controller, and the control logic is configured to execute the control method; the main power switch, the clamp switch, and the rectifier are initially off in each operating cycle of the flyback converter; the control method, in each operating cycle of the flyback converter, sequentially comprises:

[0009] Turning on the main power switch tube for a first duration so that the primary winding stores energy;

[0010] Turning on the rectifier tube for a second duration so that the energy stored in the primary winding is released via the secondary winding, and at the same time, the leakage inductance energy of the primary winding is transferred to the clamping capacitor;

[0011] Turning on the clamp switch for a third duration so that leakage inductance energy stored in the clamp capacitor is transferred to the secondary-side circuit through the transformer and then output;

[0012] and turning on the rectifier tube again for a fourth duration, so that the main power switch tube is turned on before the negative current reaches zero;

[0013] There is a first time interval between the first duration and the second duration, a second time interval between the fourth duration and the third duration, and a third time interval between the fourth duration and the first duration of the next working cycle.

[0014] Furthermore, the third duration is proportional to the output power of the flyback converter.

[0015] Furthermore, the fourth duration begins when the junction capacitance voltage of the main power switch tube resonates to a peak, and at this time, the drain-source voltage of the rectifier tube resonates to a corresponding valley.

[0016] Furthermore, the control method further includes, in each working cycle: obtaining an input voltage signal representing the current input voltage of the flyback converter, comparing the input voltage signal with a first threshold, and determining whether to turn on the rectifier tube again for a fourth duration based on the comparison result, specifically:

[0017] When the input voltage signal is less than or equal to the first threshold, the rectifier tube is not turned on again for a fourth duration;

[0018] When the input voltage signal is greater than the first threshold, the rectifier tube is turned on again for a fourth duration.

[0019] Furthermore, the determining, based on the comparison result, whether to turn on the rectifier tube again for the fourth duration is achieved by determining whether the primary-side controller transmits a synchronization signal to the secondary-side controller. Specifically:

[0020] When the input voltage signal is less than or equal to the first threshold, the primary-side controller transmits a synchronization signal to the secondary-side controller, so that the secondary-side controller does not turn on the rectifier tube again for a fourth duration;

[0021] When the input voltage signal is greater than the first threshold, the primary-side controller does not transmit a synchronization signal to the secondary-side controller, so that the secondary-side controller turns on the rectifier tube again for a fourth duration.

[0022] As a second aspect of the present invention, an embodiment of the control device provided is as follows:

[0023] A control device is applied to a flyback converter, the flyback converter comprising a primary-side circuit, a secondary-side circuit, a transformer, and the control device; the primary-side circuit comprising a main power switch tube, a clamp switch tube, a clamp capacitor, and a primary winding of the transformer; the secondary-side circuit comprising a rectifier tube and a secondary winding of the transformer; the main power switch tube, the clamp switch tube, and the rectifier tube are initially off during each operating cycle of the flyback converter; the control device comprises:

[0024] A primary-side controller, configured to control the on and off of the main power switch tube and the clamp switch tube;

[0025] A secondary-side controller configured to control the on and off of the rectifier tube;

[0026] an isolation circuit configured to bidirectionally transmit synchronization signals between the primary-side controller and the secondary-side controller;

[0027] and a control logic configured to sequentially perform the following control actions in each operating cycle of the flyback converter:

[0028] Turning on the main power switch tube for a first duration so that the primary winding stores energy;

[0029] Turning on the rectifier tube for a second duration so that the energy stored in the primary winding is released via the secondary winding, and at the same time, the leakage inductance energy of the primary winding is transferred to the clamping capacitor;

[0030] Turning on the clamp switch for a third duration so that leakage inductance energy stored in the clamp capacitor is transferred to the secondary-side circuit through the transformer and then output;

[0031] and turning on the rectifier tube again for a fourth duration, so that the main power switch tube is turned on before the negative current reaches zero;

[0032] There is a first time interval between the first duration and the second duration, a second time interval between the fourth duration and the third duration, and a third time interval between the fourth duration and the first duration of the next working cycle.

[0033] Further, the control logic is configured such that the third duration is proportional to the output power of the flyback converter.

[0034] Furthermore, the control logic is configured such that the fourth duration begins when the junction capacitance voltage of the main power switch tube resonates to a peak, and at this time, the drain-source voltage of the rectifier tube resonates to a corresponding valley.

[0035] Furthermore, the control logic is configured such that the control method further comprises, in each working cycle: obtaining an input voltage signal representing a current input voltage of the flyback converter, comparing the input voltage signal with a first threshold, and determining whether to turn on the rectifier tube again for a fourth duration based on the comparison result, specifically:

[0036] When the input voltage signal is less than or equal to the first threshold, the rectifier tube is not turned on again for a fourth duration;

[0037] When the input voltage signal is greater than the first threshold, the rectifier tube is turned on again for a fourth duration.

[0038] Further, the control logic is configured such that the determination of whether to turn on the rectifier tube again for the fourth duration according to the comparison result is achieved by whether the primary-side controller transmits a synchronization signal to the secondary-side controller, specifically:

[0039] When the input voltage signal is less than or equal to the first threshold, the primary-side controller transmits a synchronization signal to the secondary-side controller, so that the secondary-side controller does not turn on the rectifier tube again for a fourth duration;

[0040] When the input voltage signal is greater than the first threshold, the primary-side controller does not transmit a synchronization signal to the secondary-side controller, so that the secondary-side controller turns on the rectifier tube again for a fourth duration.

[0041] As a third aspect of the present invention, an embodiment of a flyback converter is provided as follows:

[0042] A flyback converter comprises: a primary-side circuit, a secondary-side circuit, a transformer, and a control device as described in any one of the above items; the primary-side circuit comprises a main power switch tube, a clamp switch tube, a clamp capacitor, and the primary winding of the transformer; the secondary-side circuit comprises a rectifier tube and the secondary winding of the transformer; the control device is used to control the conduction and shutdown of the main power switch tube, the clamp switch tube, and the rectifier tube; the main power switch tube, the clamp switch tube, and the rectifier tube are initially in the off state during each working cycle of the flyback converter.

[0043] Compared with the prior art, the control method and control device of the present invention have the following beneficial effects: the function of absorbing the drain-source voltage spike of the main power switch tube and the function of achieving zero-voltage turn-on of the main power switch tube are completely decoupled, which not only solves the drain-source voltage spike problem of the main power switch tube in the flyback converter and avoids the impact of the RCD clamping circuit loss on the flyback converter efficiency, but also the clamping capacitor in the active clamping circuit has a smaller capacitance than the clamping capacitor in the primary active clamping flyback converter, which facilitates the selection of a lower-cost clamping switch tube. At the same time, zero-voltage turn-on of the main power switch tube can be achieved under the full range of input voltages, further improving the efficiency of the flyback converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1a The schematic diagram of the existing secondary active clamp flyback converter circuit is as follows;

[0045] Figure 1b for Figure 1a Timing diagram of the secondary active clamp flyback converter;

[0046] Figure 2a The schematic diagram of the existing primary active clamp flyback converter circuit is as follows;

[0047] Figure 2b for Figure 2a Timing diagram of the primary active clamp flyback converter;

[0048] Figure 3 A circuit diagram of a flyback converter to which the control method of the present invention is applicable;

[0049] Figure 4 is a timing diagram of the first embodiment of the control method of the present invention;

[0050] Figure 5 is a flow chart of a second embodiment of the control method of the present invention;

[0051] Figure 6 for Figure 5 Timing diagram when Vin is less than or equal to the first threshold. DETAILED DESCRIPTION

[0052] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments in this application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0053] In order to enable those skilled in the art to better understand the present invention, the following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of this application.

[0054] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application and the above-mentioned drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequential order. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present application described herein. In addition, the terms "including" and "having" and any of their variations are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0055] It should be understood that, in the specification, claims and drawings, when a step is described as being connected to another step, the step may be directly connected to the other step, or be connected to the other step through a third step; when an element / unit is described as being "connected" to another element / unit, the element / unit may be "directly connected" to the other element / unit, or be "connected" to the other element / unit through a third element / unit.

[0056] In addition, the figures of this disclosure are merely schematic diagrams of the present disclosure and are not necessarily drawn to scale. Identical reference numbers in the figures denote identical or similar parts, and therefore repeated descriptions thereof will be omitted. Some of the blocks shown in the figures are functional entities that do not necessarily correspond to physically or logically separate entities. These functional entities may be implemented using software, in one or more hardware modules or integrated circuits, or in different networks and / or processor devices and / or microcontrollers.

[0057] Figure 3 For a circuit diagram of a flyback converter to which the control method of the present invention is applicable, see Figure 3The flyback converter includes a primary-side circuit, a secondary-side circuit, a transformer T1, and a control device; the primary-side circuit includes a main power switch tube S1, a clamp switch tube S3, a clamp capacitor C1, and a primary winding of the transformer; the secondary-side circuit includes a rectifier tube S2 and a secondary winding of the transformer; the control device includes a primary-side controller 220, a secondary-side controller 211, an isolation circuit 221, and control logic; the primary-side controller 220 is used to control the on / off of the main power switch tube S1 and the clamp switch tube S3, the secondary-side controller 211 is used to control the on / off of the rectifier tube, the isolation circuit 221 is used to bidirectionally transmit a synchronization signal SYNC between the primary-side controller 220 and the secondary-side controller 211, and the control logic ( Figure 3 The main power switch tube S1, the clamp switch tube S3 and the rectifier tube S2 are initially turned off in each working cycle of the flyback converter.

[0058] It should be noted that Figure 3 The figure also shows the feedback circuit 223, the output capacitor Co and the secondary side detection circuit inside the secondary side controller 211 related to the present invention. The functions of these circuits are common knowledge to those skilled in the art and are therefore not described in detail.

[0059] Among them, the primary side controller 220 is configured to control the conduction and shutdown of the main power switch tube S1 and the active clamp switch tube S3 according to the input voltage signal Vin, the feedback signal FB, the drain-source voltage signal Vds-MP of the flyback converter, and the synchronization signal SYNC; the secondary side controller 220 is configured to control the conduction and shutdown of the rectifier tube S2 according to the drain-source voltage signal Vds-SR of the rectifier tube. The secondary side controller 220 can also be configured to control the conduction and shutdown of the rectifier tube S2 according to the synchronization signal SYNC according to actual conditions.

[0060] First embodiment

[0061] This is a timing diagram of the first embodiment of the control method of the present invention, see Figure 4 The control method of this embodiment includes the following steps in each working cycle of the flyback converter:

[0062] Turning on the main power switch S1 for a first duration to allow the primary winding to store energy;

[0063] Turning on the rectifier tube S2 for the second duration allows the energy stored in the primary winding to be released through the secondary winding. At the same time, the leakage inductance energy of the primary winding will be transferred to the clamping capacitor C1.

[0064] The clamp switch S3 is turned on for a third duration, so that the leakage inductance energy stored in the clamp capacitor C1 is transferred to the secondary side circuit through the transformer T1 and then output;

[0065] and turning on the rectifier tube S2 again for a fourth duration, so that the main power switch tube S1 is turned on before the negative current reaches zero;

[0066] There is a first time interval between the first duration and the second duration, a second time interval between the fourth duration and the third duration, and a third time interval between the fourth duration and the first duration of the next working cycle.

[0067] In this embodiment, a first time interval needs to be set between the first duration and the second duration, a second time interval needs to be set between the fourth duration and the third duration, and a third time interval needs to be set between the fourth duration and the first duration of the next working cycle. The purpose is to prevent the main power switch tube S1, the clamp switch tube S3 and the rectifier tube S2 from being turned on at the same time.

[0068] In this embodiment, the primary-side controller 220 is configured to control the on / off of the main power switch S1 and the active clamp switch S3 according to the input voltage signal Vin, the feedback signal FB, the drain-source voltage signal Vds-MP of the flyback converter, and the synchronization signal SYNC; the secondary-side controller 220 is configured to control the on / off of the rectifier S2 only according to the drain-source voltage signal Vds-SR of the rectifier.

[0069] This embodiment is Figure 2b The existing primary active clamp flyback converter control method can reduce the clamp capacitor value and lower the cost of the clamp switch tube. The detailed analysis is as follows:

[0070] Due to the presence of the clamping capacitor, when the drain-source voltage of the main power switch tube S1 reaches N*Vout+Vin (where N is the turns ratio of the primary and secondary windings of the transformer, Vout is the output voltage of the flyback converter, and Vin is the input voltage of the flyback converter), the body diode of the clamping switch tube S3 will turn on, and the leakage inductance energy will be transferred to the clamping capacitor to prevent the drain-source voltage of the main power switch tube S1 from exceeding N*Vout+Vin. Since the clamping capacitor stores energy in each cycle, it needs to be released in each cycle; otherwise, the clamping capacitor will not be able to store the leakage inductance energy for the next cycle.

[0071] In the control method of the prior art, the clamping switch is turned on once during each operating cycle. This conduction of the clamping switch is required to absorb the drain-source voltage spike of the main power switch and achieve zero-voltage turn-on of the main power switch. The on-time of the clamping switch is designed to occur after the secondary winding is demagnetized, so that the voltage on the clamping capacitor reversely excites the primary winding, thereby achieving the second function, zero-voltage turn-on of the main power switch. Due to the small leakage inductance energy, when the clamping switch is turned on, the clamping capacitor voltage drops rapidly, making it impossible to continuously reverse excite the primary winding at a stable voltage. Therefore, the control method of the prior art requires the use of a larger clamping capacitor to store more energy in order to maintain a stable voltage across the clamping capacitor, thereby stabilizing the negative current slope of the primary winding. Furthermore, since the charging current of the clamping capacitor, i, = C*dV / dt, is large when the clamping capacitor capacitance C is large, the charging current will be very large, requiring a clamping switch with a higher current rating.

[0072] In the control method of this embodiment, the method for absorbing the drain-source voltage spike of the main power switch tube is as follows: before the demagnetization of the secondary winding is completed, the clamp switch tube is turned on for a third duration to release the leakage inductance energy stored in the clamp capacitor to the output end of the secondary side circuit; the method for achieving zero voltage turn-on of the main power switch tube S1 is as follows: when the rectifier tube S2 is turned on again, the output voltage reverses the secondary winding, and after the fourth duration, the rectifier tube S2 is turned off. After the turn-off, the polarity of the secondary winding is reversed, and the primary winding generates a negative current. This negative current can completely drain the charge stored in the junction capacitance between the drain and source of the main power switch tube S1, thereby achieving zero voltage turn-on of the main power switch tube S1. It can be seen that in this embodiment, the function of absorbing the drain-source voltage spike of the main power switch tube and the function of achieving zero-voltage turn-on of the main power switch tube are completely decoupled. Not only can the drain-source voltage spike problem of the main power switch tube in the flyback converter be solved and the influence of the RCD clamping circuit loss on the efficiency of the flyback converter be avoided, but the capacitance of the clamping capacitor in the active clamping circuit is smaller than that of the clamping capacitor in the prior art, thereby facilitating the selection of a lower-cost clamping switch tube. At the same time, zero-voltage turn-on of the main power switch tube can be achieved under the full range of input voltages, further improving the efficiency of the flyback converter.

[0073] During the second duration, the leakage inductance current flowing through the primary winding charges the clamping capacitor C1 via the body diode of the clamping switch tube S3, thereby transferring the leakage inductance energy of the primary winding to the clamping capacitor C1. During the primary winding leakage inductance energy transfer process, the voltage of the clamping capacitor C1 reaches a maximum when the current of the primary winding reaches zero. The maximum capacitance of the clamping capacitor can be designed based on this. Specifically, the maximum capacitance of the clamping capacitor can be determined based on the maximum input voltage and the maximum output voltage of the flyback converter.

[0074] Preferably, the third duration is proportional to the output power of the flyback converter, that is, the greater the output power, the longer the third duration. The reason is that the greater the output power, the greater the current of the primary side winding, and the corresponding leakage inductance energy will also be greater. Therefore, the more leakage inductance energy stored in the clamping capacitor, and therefore the longer the third duration, so that the leakage inductance energy of the clamping capacitor can be completely released.

[0075] Preferably, during the third duration, the voltage of the clamping capacitor C1 gradually decreases. At the end of the third duration, the voltage of the clamping capacitor C1 is still greater than the reflected voltage. This is because when the rectifier tube S2 is turned on again, the drain-source voltage of the main power switch tube S1 is equal to N*Vout+Vin, while the voltage of the clamping capacitor C1 is still greater than the reflected voltage. That is, the voltage across the clamping capacitor C1 is greater than N*Vout+Vin, and the body diode of the active clamping switch tube S2 is cut off, thereby preventing energy from the secondary-side output end from being transferred to the clamping capacitor C1. The reflected voltage is the product of the primary-to-secondary turns ratio of the transformer and the output voltage of the flyback converter.

[0076] Preferably, the second duration ends when the secondary winding current decreases to the set forward current value, so that the third duration is still within the secondary winding demagnetization time, avoiding the clamping capacitor and the leakage inductance of the primary side from resonating to the negative half cycle, causing the leakage inductance energy to return to the clamping capacitor.

[0077] Preferably, the fourth duration begins when the junction capacitance voltage of the main power switch tube S1 resonates to a peak, at which time the drain-source voltage of the rectifier tube S2 resonates to a corresponding valley. The purpose is to turn on the rectifier tube S2 again when the junction capacitance voltage of the main power switch tube S1 resonates to a peak, thereby enabling valley-turning on of the rectifier tube S2 and reducing turn-on losses of the rectifier tube S2. Specifically, turning on the rectifier tube S2 again when the junction capacitance voltage of the main power switch tube S1 resonates to a peak can cause the reflected voltage to reversely excite the excitation inductance of the primary winding. When the secondary winding current reaches a set negative current, the rectifier tube S2 is turned off. At this time, the primary winding generates a negative current, which turns on the main power switch tube S1 before the negative current reaches zero, thereby achieving zero-voltage turn-on of the main power switch tube S1.

[0078] The following combination Figure 3 The flyback converter shown in Figure 4 A detailed analysis of the timing diagram of the first embodiment of the control method of the present invention shows that the following six stages are repeated in each working cycle of the flyback converter:

[0079] The first stage (t0-t1): that is, during the first duration, the primary-side controller 220 generates a high-level drive signal LSGD to control the main power switch tube S1 to turn on, the input voltage Vin excites the primary winding, the current IL-P in the primary-side circuit flows through the transformer T1 and increases linearly, and the transformer continuously stores energy. When the current IL-P in the primary-side circuit reaches a certain value, the primary-side controller 220 generates a low-level drive signal LSGD to control the main power switch tube S1 to turn off.

[0080] The second stage (t1-t2): that is, during the first time interval, the control device keeps the main power switch tube S1, the rectifier tube S2, and the clamp switch tube S3 all turned off. The current IL-P in the primary side circuit charges the junction capacitance Cds of the main power switch tube S1. The drain-source voltage Vds-MP of the main power switch tube S1 continues to rise. When it rises to Vin+N*Vout, this stage ends, where Vin is the input voltage of the flyback converter, Vout is the output voltage of the flyback converter, and N is the turns ratio of the primary winding to the secondary winding of the transformer T1.

[0081] Phase 3 (t2-t3): When the drain-source voltage of the main power switch S1 rises to Vin+N*Vout, the drain-source voltage Vds-SR of the rectifier S2 drops to zero volts. The first time interval ends and the second time interval begins. The secondary-side controller 211 generates a high level SRGD to control the rectifier S2 to turn on. The energy stored in the transformer T1 begins to be transferred to the output end of the flyback converter, that is, the transformer T1 begins to be demagnetized. At this time, the current IL-P in the primary-side circuit will rapidly decrease linearly. The current IL-P will charge the clamping capacitor C1 through the body diode of the clamping switch S3. When the current IL-P is zero, the voltage across the clamping capacitor C1 reaches its maximum.

[0082] During the fourth phase (t3-t4), when the secondary winding current IL-S decreases to a positive current value, the secondary-side controller 211 generates a low-level signal SRGD, turning off the rectifier S2, and the second duration ends. When the rectifier S2 turns off, the secondary-side controller 211 transmits a synchronization signal SYNC to the primary-side controller 220. Upon receiving the synchronization signal SYNC, the primary-side controller 220 generates a high-level signal HSGD, turning on the clamp switch S3, and the third duration begins. The leakage inductance energy stored in the clamp capacitor C1 is transferred to the secondary output terminal via the transformer T1, causing the voltage on the clamp capacitor C1 to gradually decrease. The maximum on-time of the clamp switch S3 will not exceed half of the resonant period of the resonant connection between the clamp capacitor C1 and the leakage inductor Lk, and the on-time is linearly related to the feedback signal FB obtained by the feedback circuit 223. When the output power is high, the feedback signal FB is also higher, and the primary-side controller 220 will also appropriately extend the on-time of the clamp switch S3 according to the feedback signal FB. When the on-time of the clamp switch S3 reaches the set on-time, the primary-side controller 220 generates a low-level HSGD signal to control the clamp switch S3 to turn off. When the clamp switch S3 is turned off, the drain-source voltage of the main power switch S1 is Vin+N*Vout, that is, the voltage across the clamp capacitor C1 is not less than N*Vout.

[0083] In the fifth stage t4-t5: after the clamp switch tube S3 is turned off, it enters the second time interval, and the junction capacitance Cds of the main power switch tube S1 and the excitation inductance Lm of the primary winding resonate.

[0084] In the sixth phase t5-t6: when the junction capacitance voltage of the main power switch tube S1 resonates to the Nth peak, the drain-source voltage of the rectifier tube S2 resonates to the Nth valley. The secondary-side controller 211 generates a high-level SRGD signal to control the rectifier tube S2 to turn on, entering the fourth duration. The reflected voltage reversely excites the excitation inductance Lm of the primary winding. When the secondary winding current IL-S reaches the set negative current, the secondary-side controller 211 generates a low-level SRGD signal to control the rectifier tube S2 to turn off, entering the third time interval. At this time, the primary winding current IL-P generates a negative current. Before the negative current reaches zero, the primary-side controller 220 generates a high-level LSGD signal to control the main power switch tube S1 to turn on, thereby achieving zero-voltage turn-on of the main power switch tube S1.

[0085] In the third stage, the voltage of the clamp capacitor C1 reaches a maximum, which exceeds N*Vout. Therefore, in the fourth stage, when the clamp switch S3 is turned on, the drain-source voltage of the main power switch S1 will suddenly rise.

[0086] The longest on-time of the clamp switch tube S3 will not exceed half of the resonant cycle time of the clamp capacitor C1 and the leakage inductor Lk.

[0087] Second embodiment

[0088] With respect to the control method of the first embodiment described above, the inventors have observed that when the input voltage is lower than N*Vout, the drain-source voltage of the primary-side main power switch tube S1 can naturally drop to 0V, thereby achieving zero voltage switching. Therefore, there is no need to turn on the rectifier tube S2 again for the fourth duration.

[0089] Figure 5 This is a timing diagram of the second embodiment of the control method of the present invention, see Figure 5 The control method of this embodiment differs from that of the first embodiment in that, in each working cycle, it further includes: obtaining an input voltage signal representing the current input voltage of the flyback converter, comparing the input voltage signal Vin with a first threshold value Vth, and determining whether to turn on the rectifier tube S2 again for a fourth duration based on the comparison result. Specifically:

[0090] When the input voltage signal Vin is less than or equal to the first threshold value Vth, the rectifier tube S2 is not turned on again for the fourth duration;

[0091] When the input voltage signal Vin is greater than the first threshold Vth, the rectifier tube S2 is turned on again for a fourth duration.

[0092] Wherein, determining whether to turn on the rectifier tube again for the fourth duration according to the comparison result is achieved by whether the primary side controller 220 transmits the synchronization signal SYNC to the secondary side controller 211, specifically;

[0093] When the input voltage signal Vin is less than or equal to the first threshold value Vth, the primary-side controller 220 transmits the synchronization signal SYNC to the secondary-side controller 211, so that the secondary-side controller 211 does not turn on the rectifier tube S3 again for the fourth duration;

[0094] When the input voltage signal Vin is greater than the first threshold Vth, the primary-side controller 220 does not transmit the synchronization signal SYNC to the secondary-side controller 211 , so that the secondary-side controller 211 turns on the rectifier tube S3 again for the fourth duration.

[0095] Preferably, the input voltage signal Vin is obtained by directly dividing the voltage through a sampling resistor.

[0096] When the input voltage signal Vin is greater than the first threshold value Vth, the working sequence of the control method of this embodiment is the same as that of the first embodiment, see Figure 4 .

[0097] When the input voltage signal Vin is less than or equal to the first threshold value Vth, the working timing of the control method of this embodiment is as follows: Figure 6 In this case, the primary-side controller 220 is configured to control the on / off of the main power switch S1 and the active clamp switch S3 based on the flyback converter's input voltage signal Vin, the feedback signal FB, the main power switch drain-source voltage signal Vds-MP, and the synchronization signal SYNC. The secondary-side controller 220 is configured to control the on / off of the rectifier S2 based on the rectifier drain-source voltage signal Vds-SR and the synchronization signal SYNC.

[0098] It should be noted that when the input voltage signal Vin is less than or equal to the first threshold Vth, when the first threshold is set close to N*Vout, the drain-source voltage Vds-MP of the main power switch tube S1 can also be turned on at close to zero voltage during the valley. The calculation formula Vds-MP=Vin-N*Vout can also reduce the switching loss of the main power switch tube S1.

[0099] The following combination Figure 3 The flyback converter shown in Figure 6 The timing diagram shown in the figure is analyzed in detail. Under this control timing, the following six stages are repeated in each working cycle of the flyback converter:

[0100] The first stage (t0-t1): is the same as the first embodiment and will not be described again.

[0101] In the second stage (t1-t2): the process is the same as that in the first embodiment and will not be described again.

[0102] In the third stage (t2-t3): the process is the same as that in the first embodiment and will not be described again.

[0103] In the fourth stage (t3-t4): the difference from the first embodiment is that when the input voltage is less than or equal to the first threshold value Vth, the primary side controller 220 transmits the synchronization signal SYNC to the secondary side controller 211, so that the secondary side controller 211 will not turn on the rectifier tube S3 again for the fourth duration.

[0104] In the fifth stage (t4-t5): the process is the same as that in the first embodiment and will not be described again.

[0105] In the sixth phase (t5-t6): when the primary-side controller 220 detects that the drain-source voltage Vds-MP of the main power switch tube S1 resonates to the Nth valley, the primary-side controller 220 generates a high-level LSGD signal to control the main power switch tube S1 to turn on (the next working cycle has begun at this time), and the rectifier tube S2 will not be turned on again when the secondary-side controller 211 detects the synchronization signal SYNC.

[0106] The drain-source voltage of the main power switch S1 at the valley is Vds-MP=Vin-N*Vout. Since Vin is less than or equal to the first threshold Vth, the main power switch S1 can also be turned on at zero voltage.

[0107] Third embodiment

[0108] This embodiment provides a control device for Figure 3 The flyback converter shown in FIG. 1 includes a primary-side circuit, a secondary-side circuit, a transformer T1, and a control device of this embodiment. The primary-side circuit includes a main power switch S1, a clamp switch S3, a clamp capacitor C1, and a primary winding of the transformer T1. The secondary-side circuit includes a rectifier S2 and a secondary winding of the transformer T1. The main power switch S1, the clamp switch S3, and the rectifier S2 are initially turned off during each operating cycle of the flyback converter. The control device of this embodiment includes:

[0109] The primary-side controller 220 is configured to control the on and off of the main power switch S1 and the clamp switch S3;

[0110] The secondary side controller 211 is configured to control the on and off of the rectifier tube S2;

[0111] The isolation circuit 221 is configured to bidirectionally transmit a synchronization signal SYNC between the primary-side controller 220 and the secondary-side controller 211 ;

[0112] and a control logic configured to execute, in each working cycle, a control action of any one of the specific implementation modes of the control method in the first embodiment and the specific implementation modes of the control method in the second embodiment.

[0113] The beneficial effects of the control device of this embodiment correspond to the specific implementation methods in the first embodiment / the second embodiment and are not described again.

[0114] Fourth embodiment

[0115] This embodiment provides a flyback converter. Figure 3 As shown, it includes: a primary-side circuit, a secondary-side circuit, a transformer T1, and any one of the specific implementation methods of the control device in the third embodiment; the primary-side circuit includes a main power switch tube S1, a clamp switch tube S3, a clamp capacitor C1, and the primary winding of the transformer T1; the secondary-side circuit includes a rectifier tube S2 and the secondary winding of the transformer T1; the control device is used to control the conduction and shutdown of the main power switch tube S1, the clamp switch tube S3, and the rectifier tube S2; the main power switch tube S1, the clamp switch tube S3, and the rectifier tube S2 are initially turned off in each working cycle of the flyback converter.

[0116] The beneficial effects of the control device of this embodiment are indirectly consistent with the specific implementation methods in the first embodiment / the second embodiment, and are not described again.

[0117] The above are merely preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be construed as limiting the present invention. Persons skilled in the art will appreciate that equivalent power supplies, improvements, and modifications may be made without departing from the spirit and scope of the present invention. These equivalent power supplies, improvements, and modifications should also be considered within the scope of protection of the present invention. Examples will not be used here for further elaboration. The scope of protection of the present invention shall be determined by the scope defined in the claims.

Claims

1. A control method, applied to a flyback converter, the flyback converter comprising a primary-side circuit, a secondary-side circuit, a transformer, and a control device; the primary-side circuit comprising a main power switch, a clamp switch, a clamp capacitor, and a primary winding of the transformer; the secondary-side circuit comprising a rectifier and a secondary winding of the transformer; the control device comprising a primary-side controller, a secondary-side controller, an isolation circuit, and control logic; the primary-side controller being configured to control the on / off states of the main power switch and the clamp switch, the secondary-side controller being configured to control the on / off states of the rectifier, the isolation circuit being configured to bidirectionally transmit synchronization signals between the primary-side controller and the secondary-side controller, and the control logic being configured to execute the control method; the main power switch, the clamp switch, and the rectifier being in an off state at the beginning of each operating cycle of the flyback converter; and characterized in that: The control method includes, in each working cycle of the flyback converter, sequentially: Turning on the main power switch tube for a first duration so that the primary winding stores energy; Turning on the rectifier tube for a second duration so that the energy stored in the primary winding is released via the secondary winding, and at the same time, the leakage inductance energy of the primary winding is transferred to the clamping capacitor; Turning on the clamp switch for a third duration so that leakage inductance energy stored in the clamp capacitor is transferred to the secondary-side circuit through the transformer and then output; and turning on the rectifier tube again for a fourth duration, so that the main power switch tube is turned on before the negative current reaches zero; There is a first time interval between the first duration and the second duration, a second time interval between the fourth duration and the third duration, and a third time interval between the fourth duration and the first duration of the next working cycle.

2. The control method according to claim 1, characterized in that: The third duration is proportional to the output power of the flyback converter.

3. The control method according to claim 1, characterized in that: The fourth duration begins when the junction capacitance voltage of the main power switch tube resonates to a peak, and at this time, the drain-source voltage of the rectifier tube resonates to a corresponding valley.

4. The control method according to any one of claims 1 to 3, characterized in that: The control method further comprises, in each working cycle, obtaining an input voltage signal representing a current input voltage of the flyback converter, comparing the input voltage signal with a first threshold, and determining whether to turn on the rectifier tube again for a fourth duration based on the comparison result, specifically: When the input voltage signal is less than or equal to the first threshold, the rectifier tube is not turned on again for a fourth duration; When the input voltage signal is greater than the first threshold, the rectifier tube is turned on again for a fourth duration.

5. The control method according to claim 4, characterized in that: The determination of whether to turn on the rectifier tube again for the fourth duration according to the comparison result is achieved by determining whether the primary-side controller transmits a synchronization signal to the secondary-side controller. Specifically: When the input voltage signal is less than or equal to the first threshold, the primary-side controller transmits a synchronization signal to the secondary-side controller, so that the secondary-side controller does not turn on the rectifier tube again for a fourth duration; When the input voltage signal is greater than the first threshold, the primary-side controller does not transmit a synchronization signal to the secondary-side controller, so that the secondary-side controller turns on the rectifier tube again for a fourth duration.

6. A control device, applied to a flyback converter, the flyback converter comprising a primary-side circuit, a secondary-side circuit, a transformer, and the control device; the primary-side circuit comprising a main power switch tube, a clamp switch tube, a clamp capacitor, and a primary winding of the transformer; the secondary-side circuit comprising a rectifier tube and a secondary winding of the transformer; the main power switch tube, the clamp switch tube, and the rectifier tube are initially in an off state during each operating cycle of the flyback converter; characterized in that: The control device comprises: A primary-side controller, configured to control the on and off of the main power switch tube and the clamp switch tube; A secondary-side controller configured to control the on and off of the rectifier tube; an isolation circuit configured to bidirectionally transmit synchronization signals between the primary-side controller and the secondary-side controller; and a control logic configured to sequentially perform the following control actions in each operating cycle of the flyback converter: Turning on the main power switch tube for a first duration so that the primary winding stores energy; Turning on the rectifier tube for a second duration so that the energy stored in the primary winding is released via the secondary winding, and at the same time, the leakage inductance energy of the primary winding is transferred to the clamping capacitor; Turning on the clamp switch for a third duration so that leakage inductance energy stored in the clamp capacitor is transferred to the secondary-side circuit through the transformer and then output; and turning on the rectifier tube again for a fourth duration, so that the main power switch tube is turned on before the negative current reaches zero; There is a first time interval between the first duration and the second duration, a second time interval between the fourth duration and the third duration, and a third time interval between the fourth duration and the first duration of the next working cycle.

7. The control device according to claim 6, characterized in that: The control logic is configured such that the third duration is proportional to an output power of the flyback converter.

8. The control device according to claim 6, characterized in that: The control logic is configured such that the fourth duration begins when the junction capacitance voltage of the main power switch tube resonates to a peak, and at this time, the drain-source voltage of the rectifier tube resonates to a corresponding valley.

9. The control device according to any one of claims 6 to 8, characterized in that: The control logic is configured to further include, in each working cycle: obtaining an input voltage signal representing a current input voltage of the flyback converter, comparing the input voltage signal with a first threshold, and determining whether to turn on the rectifier tube again for a fourth duration based on the comparison result, specifically: When the input voltage signal is less than or equal to the first threshold, the rectifier tube is not turned on again for a fourth duration; When the input voltage signal is greater than the first threshold, the rectifier tube is turned on again for a fourth duration.

10. The control device according to claim 9, characterized in that: The control logic is configured such that determining whether to turn on the rectifier tube again for the fourth duration according to the comparison result is implemented by determining whether the primary-side controller transmits a synchronization signal to the secondary-side controller, specifically: When the input voltage signal is less than or equal to the first threshold, the primary-side controller transmits a synchronization signal to the secondary-side controller, so that the secondary-side controller does not turn on the rectifier tube again for a fourth duration; When the input voltage signal is greater than the first threshold, the primary-side controller does not transmit a synchronization signal to the secondary-side controller, so that the secondary-side controller turns on the rectifier tube again for a fourth duration.

11. A flyback converter, characterized in that: include: A primary-side circuit, a secondary-side circuit, a transformer, and a control device according to any one of claims 6 to 10; the primary-side circuit includes a main power switch tube, a clamp switch tube, a clamp capacitor, and the primary winding of the transformer; the secondary-side circuit includes a rectifier tube and the secondary winding of the transformer; the control device is used to control the on and off of the main power switch tube, the clamp switch tube, and the rectifier tube; the main power switch tube, the clamp switch tube, and the rectifier tube are in the off state at the initial stage of each working cycle of the flyback converter.

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