A control method and device for a flyback converter and a flyback converter

Abstract

The present invention discloses a control method, device, and flyback converter for a flyback converter. One embodiment of the control method comprises the following steps: during each operating cycle of the flyback converter, the main power switch is turned on for a first duration, allowing the primary winding to store energy; the main power switch is turned off for a first pause time, during which time the energy in the primary winding is released via the secondary winding when the rectifier diode is turned on, and the leakage inductance energy of the primary winding is transferred to the clamping capacitor when the body diode of the clamping tube is turned on; the clamping tube is turned on for a second duration, allowing the leakage inductance energy stored in the clamping capacitor to be transferred to the secondary circuit via the transformer for output; and the clamping tube is turned off for the second pause time. The present invention has simple control, low circuit cost, and can achieve both low cost and high efficiency in high-frequency applications. It is suitable for various flyback operating modes.

Description

Technical Field

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

[0002] Flyback converters are widely used in small and medium-power switching power supplies due to their simple topology, high reliability, input-output electrical isolation, and low cost. Currently, one of the main reasons for the low efficiency of flyback converters is the presence of transformer leakage inductance, which typically accounts for 2%-5% of the magnetizing inductance. The energy in this leakage inductance needs to be processed; otherwise, it is directly dissipated as heat, significantly impacting efficiency. Furthermore, when the flyback converter's main power transistor is turned off, a large voltage spike is generated, increasing the stress on the power transistor. Furthermore, the resonance between the leakage inductance and the parasitic junction capacitance can lead to more severe EMI issues.

[0003] Therefore, it's necessary to add a clamping circuit to handle the flyback converter's leakage inductance energy, suppress the main switch's turn-off voltage spike, and feed the leakage inductance energy back to the main power circuit to improve efficiency and reliability. Currently, existing clamping circuits include passive RCD clamping, passive LCD clamping, and active clamping. Passive RCD absorption only clamps to reduce the voltage stress on the main switch, but cannot feed back leakage inductance energy, thus failing to improve efficiency. Passive LCD clamping, on the other hand, is relatively complex and difficult to implement.

[0004] As shown in FIG1(a), which is a schematic diagram of the first active clamp flyback circuit in the prior art, and FIG1(b), which is an operating waveform diagram of the first active clamp flyback circuit in the prior art, the addition of the clamping tube Q2 can not only suppress the voltage stress of the switch tube, but also recover the leakage inductance energy. However, after research, the inventors of the present application found that in this solution, since the clamping capacitor Cr needs to store sufficient energy to achieve zero-voltage turn-on of the main power switch tube Q1, a large capacitance is required. On the one hand, this increases the volume and cost, and on the other hand, the current flowing through the clamping tube increases, so it is necessary to select a better clamping tube Q2. This is generally used in situations where the main switch tube needs to achieve ZVS. In addition, if an adaptive ZVS effect is to be achieved, a more complex control strategy is required. Therefore, this solution is more suitable for medium and high power situations.

[0005] FIG2(a) is a schematic diagram of a second type of active-clamped flyback converter in the prior art, and FIG2(b) is an operating waveform diagram of the second type of active-clamped flyback converter in the prior art. By controlling the clamping tube Q2 to conduct during the demagnetization period of the transformer, with the conduction time being proportional to the demagnetization time, leakage inductance energy can be processed. However, the inventors of the present application have discovered through research that this solution also has defects. On the one hand, the clamping tube Q2 in this solution has a long conduction time, and the effective current of the clamping circuit is large, which is not conducive to improving efficiency. On the other hand, the demagnetization time needs to be detected, and the control strategy is more complicated. Summary of the Invention

[0006] In view of this, the present invention and the technical problem to be solved are to provide a control method, device and flyback converter of a flyback converter, which can not only reduce the voltage stress of the main switch tube, but also recover the leakage inductance energy and improve the efficiency, and have a simple structure and low cost.

[0007] The present invention addresses the aforementioned technical issues by providing a flyback transformer leakage inductance energy clamping circuit comprising a clamping diode and a clamping capacitor. The clamping diode and the clamping capacitor are connected in series, and their positions can be interchanged. After the series connection, they are then connected in parallel with the primary winding of the transformer. The clamping diode provides a channel for absorbing and releasing leakage inductance energy, while the clamping capacitor serves as a medium for temporarily storing leakage inductance energy. The non-complementary conduction control of the clamping diode and the main power switch makes the clamping circuit applicable to flyback circuits operating in continuous, discontinuous, and critical modes. This addresses the complex and costly control issues of existing flyback transformer clamping circuits, achieving both low cost and high efficiency in high-frequency applications, thereby enhancing its versatility and practicality. Specifically, the clamping diode is only turned on for a short, fixed time after the main power switch is turned off, which is much shorter than the main power switch's off time. During this time, the leakage inductor is demagnetized and its energy is released into the clamping capacitor. After the demagnetization is complete, the clamping capacitor re-magnetizes the leakage inductor. During this period, the energy stored in the clamping capacitor is fed back to the output capacitor via the transformer.

[0008] As a first aspect of the present invention, an embodiment of a control method is provided as follows:

[0009] 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 clamping transistor, a clamping capacitor, and a primary winding of the transformer; the secondary-side circuit comprising a rectifier diode and a secondary winding of the transformer; the control device comprising a feedback circuit, a primary-side controller, and a driver, the control device being configured to execute the control method to control the turning on and off of the main power switch and the clamping transistor; the main power switch and the clamping transistor being controlled to be turned off at the initial state of each operating cycle of the flyback converter; the control method comprising the following steps in each operating cycle of the flyback converter:

[0010] The feedback circuit obtains a first signal representing the magnitude of the output voltage of the flyback converter and outputs the signal to the primary-side controller; the primary-side controller obtains a second signal representing the magnitude of the current flowing through the source of the main power switch tube;

[0011] The primary-side controller generates a first pulse signal based on the first signal and the second signal, turns on the main power switch for a first duration, so that the primary winding stores energy; the primary-side controller also generates a second pulse signal based on a falling edge signal of the first pulse signal, and outputs the second pulse signal to the driver;

[0012] After the first duration, turning off the main power switch tube for a first pause time, so that during the first pause time, energy in the primary winding is released through the secondary winding when the rectifier diode is turned on, and leakage inductance energy of the primary winding is transferred to the clamping capacitor when the body diode of the clamping tube is turned on;

[0013] After the first pause time, the driver turns on the clamping transistor for a second duration according to the second pulse signal, so that the leakage inductance energy stored in the clamping capacitor is transferred to the secondary side circuit through the transformer and then output;

[0014] After the second duration, the clamp tube is opened for a second pause time.

[0015] Furthermore, the second duration is a fixed time.

[0016] Furthermore, the second duration is hundreds of nanoseconds.

[0017] As a first aspect of the present invention, another embodiment of the control method provided is as follows:

[0018] 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, a clamp capacitor, and a primary winding of the transformer; the secondary-side circuit comprising a synchronous rectifier and a secondary winding of the transformer; the control device comprising a feedback circuit, a primary-side controller, a secondary-side controller, an isolation circuit, and a driver; the control device being configured to execute the control method to control the turning on and off of the main power switch, the clamp, and the synchronous rectifier; the main power switch, the clamp, and the synchronous rectifier being controlled to be turned off at the initial state of each operating cycle of the flyback converter; the control method comprising the following steps in each operating cycle of the flyback converter:

[0019] The feedback circuit obtains a third signal representing the output voltage of the flyback converter and outputs it to the primary-side controller; the primary-side controller obtains a fourth signal representing the source current of the main power switch tube; and the secondary-side controller obtains a fifth signal representing the drain-source voltage of the synchronous rectifier tube.

[0020] The primary-side controller generates a third pulse signal according to the third signal and the fourth signal, turns on the main power switch for a third duration, so that the primary winding stores energy;

[0021] After the third duration, turning off the main power switch tube for a third pause time;

[0022] After the third pause time, the secondary-side controller generates a fourth pulse signal according to the fifth signal, turns on the synchronous rectifier for a fourth duration, so that energy in the primary winding is released through the secondary winding when the synchronous rectifier is turned on, and during the fourth duration, leakage inductance energy of the primary winding is transferred to the clamping capacitor when the body diode of the clamping tube is turned on; the secondary-side controller also generates a fifth pulse signal based on a falling edge signal of the fourth pulse signal, and transmits the fifth pulse signal to the driver via the isolation circuit;

[0023] After the fourth duration, the driver turns on the clamping transistor for a fifth duration according to the fifth pulse signal, so that the leakage inductance energy stored in the clamping capacitor is transferred to the secondary side circuit through the transformer and then output;

[0024] After the fifth duration, the clamp tube is disconnected for a fourth pause time.

[0025] Furthermore, the secondary side controller obtains an enable signal to enable the secondary side controller to obtain a fifth signal representing the drain-source voltage of the synchronous rectifier tube, and enables the secondary side controller to generate a fifth pulse signal based on the falling edge signal of the fourth pulse signal.

[0026] Furthermore, the enable signal is generated some time before the rising edge of the third pulse signal.

[0027] Furthermore, when the fifth signal reaches the first threshold, the fourth pulse signal is generated, the synchronous rectifier tube is turned on, and the fourth duration begins; when the fifth signal reaches the second threshold, the fourth pulse signal is stopped from being generated, the synchronous rectifier tube is turned off, and the fourth duration ends; the first threshold and the second threshold are both negative thresholds, and the absolute value of the first threshold is greater than the absolute value of the second threshold.

[0028] Furthermore, the fifth duration is a fixed time.

[0029] Furthermore, the fifth duration is hundreds of nanoseconds.

[0030] As a second aspect of the present invention, an embodiment of the control device provided is as follows:

[0031] A control device, applied to a flyback converter, includes a primary-side circuit, a secondary-side circuit, a transformer, and the control device. The primary-side circuit includes a main power switch, a clamp, a clamp capacitor, and a primary winding of the transformer. The secondary-side circuit includes a rectifier diode and a secondary winding of the transformer. The control device includes a feedback circuit, a primary-side controller, and a driver. The control device is configured to control the on / off states of the main power switch and the clamp. The main power switch and the clamp are controlled to be off at the initial state of each operating cycle of the flyback converter. The control device performs the following actions during each operating cycle of the flyback converter:

[0032] The feedback circuit obtains a first signal representing the magnitude of the output voltage of the flyback converter and outputs the signal to the primary-side controller; the primary-side controller obtains a second signal representing the magnitude of the current flowing through the source of the main power switch tube;

[0033] The primary-side controller generates a first pulse signal based on the first signal and the second signal, turns on the main power switch for a first duration, so that the primary winding stores energy; the primary-side controller also generates a second pulse signal based on a falling edge signal of the first pulse signal, and outputs the second pulse signal to the driver;

[0034] After the first duration, turning off the main power switch tube for a first pause time, so that during the first pause time, energy in the primary winding is released through the secondary winding when the rectifier diode is turned on, and leakage inductance energy of the primary winding is transferred to the clamping capacitor when the body diode of the clamping tube is turned on;

[0035] After the first pause time, the driver turns on the clamping transistor for a second duration according to the second pulse signal, so that the leakage inductance energy stored in the clamping capacitor is transferred to the secondary side circuit through the transformer and then output;

[0036] After the second duration, the clamp tube is opened for a second pause time.

[0037] Furthermore, the second duration is a fixed time.

[0038] Furthermore, the second duration is hundreds of nanoseconds.

[0039] As a second aspect of the present invention, another embodiment of the control device provided is as follows:

[0040] A control device, applied to a flyback converter, includes a primary-side circuit, a secondary-side circuit, a transformer, and the control device. The primary-side circuit includes a main power switch, a clamp, a clamp capacitor, and a primary winding of the transformer. The secondary-side circuit includes a synchronous rectifier and a secondary winding of the transformer. The control device includes a feedback circuit, a primary-side controller, a secondary-side controller, an isolation circuit, and a driver. The control device is configured to control the on / off states of the main power switch and the clamp. The main power switch and the clamp are controlled to be off at the initial state of each operating cycle of the flyback converter. The control device performs the following actions during each operating cycle of the flyback converter:

[0041] The feedback circuit obtains a third signal representing the output voltage of the flyback converter and outputs it to the primary-side controller; the primary-side controller obtains a fourth signal representing the source current of the main power switch tube; and the secondary-side controller obtains a fifth signal representing the drain-source voltage of the synchronous rectifier tube.

[0042] The primary-side controller generates a third pulse signal according to the third signal and the fourth signal, turns on the main power switch for a third duration, so that the primary winding stores energy;

[0043] After the third duration, turning off the main power switch tube for a third pause time;

[0044] After the third pause time, the secondary-side controller generates a fourth pulse signal according to the fifth signal, turns on the synchronous rectifier for a fourth duration, so that energy in the primary winding is released through the secondary winding when the synchronous rectifier is turned on, and during the fourth duration, leakage inductance energy of the primary winding is transferred to the clamping capacitor when the body diode of the clamping tube is turned on; the secondary-side controller also generates a fifth pulse signal based on a falling edge signal of the fourth pulse signal, and transmits the fifth pulse signal to the driver via the isolation circuit;

[0045] After the fourth duration, the driver turns on the clamping transistor for a fifth duration according to the fifth pulse signal, so that the leakage inductance energy stored in the clamping capacitor is transferred to the secondary side circuit through the transformer and then output;

[0046] After the fifth duration, the clamp tube is disconnected for a fourth pause time.

[0047] Furthermore, the secondary side controller obtains an enable signal to enable the secondary side controller to obtain a fifth signal representing the drain-source voltage of the synchronous rectifier tube, and enables the secondary side controller to generate a fifth pulse signal based on the falling edge signal of the fourth pulse signal.

[0048] Furthermore, the enable signal is generated some time before the rising edge of the third pulse signal.

[0049] Furthermore, when the fifth signal reaches the first threshold, the fourth pulse signal is generated, the synchronous rectifier tube is turned on, and the fourth duration begins; when the fifth signal reaches the second threshold, the fourth pulse signal is stopped from being generated, the synchronous rectifier tube is turned off, and the fourth duration ends; the first threshold and the second threshold are both negative thresholds, and the absolute value of the first threshold is greater than the absolute value of the second threshold.

[0050] Furthermore, the fifth duration is a fixed time.

[0051] Furthermore, the fifth duration is hundreds of nanoseconds.

[0052] As a third aspect of the present invention, an embodiment of a flyback converter is provided as follows:

[0053] A flyback converter includes: a primary-side circuit, a secondary-side circuit, a transformer, and a control device; the primary-side circuit includes a main power switch tube, a clamp tube, a clamp capacitor, and the primary winding of the transformer; the secondary-side circuit includes a rectifier diode or a synchronous rectifier tube and the secondary winding of the transformer; when the secondary-side circuit includes a rectifier diode and the secondary winding of the transformer, the control device is any specific implementation scheme in one embodiment of the above-mentioned control device; when the secondary-side circuit includes a synchronous rectifier tube and the secondary winding of the transformer, the control device is any specific implementation scheme in another embodiment of the above-mentioned control device. In the control method and control device of the present invention, the clamp tube feeds leakage inductance energy back to the main power circuit after being turned on for a period of time after the main circuit is turned off. The clamp tube turns off at the time when the demagnetization of the excitation inductance is completed or before the main circuit is turned on again. Therefore, the control method and device of the flyback converter provided by the present invention are applicable to flyback circuits of various working modes. Specifically, the present invention has at least the following technical advantages over the prior art:

[0054] 1. The clamping circuit is simple and highly reliable. At the same time, the clamping capacitance is small, and the clamping tube can be selected from a switching tube with small current specifications, which enables the flyback converter to achieve a smaller size and lower cost.

[0055] 2. The clamping circuit suppresses the voltage spike when the switch is turned off, while recovering some leakage inductance energy, thereby increasing the efficiency of the flyback converter and improving EMI performance.

[0056] 3. Since the demagnetization time of the leakage inductance is longer when the load is larger, the pulse width of the clamping tube can be adjusted based on load compensation to better achieve the leakage inductance energy recovery effect.

[0057] 4. The conduction time of the clamping tube is short and does not affect the working state of the main power. The control method and control device of the present invention are applicable to flyback circuits of various working modes and have greater versatility and practicality.

[0058] Other features and advantages of the present invention will be described in the following description, and in part will become apparent from the description, or will be understood by practicing the present invention. The purposes and other advantages of the present invention can be realized and obtained through the structures particularly pointed out in the description, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] FIG1( a ) is a schematic diagram of a first active clamp flyback circuit in the prior art;

[0060] FIG1( b ) is a working waveform diagram of a first active clamp flyback circuit in the prior art;

[0061] FIG1( c ) is a working waveform diagram of a second active clamp flyback converter in the prior art;

[0062] FIG2( a ) is a circuit diagram of a flyback converter used in the control method according to the first embodiment of the present invention;

[0063] FIG2( b ) is a working waveform diagram of the flyback converter used in the control method according to the first embodiment of the present invention;

[0064] FIG3( a ) is a circuit diagram of a flyback converter used in a control method according to a second embodiment of the present invention;

[0065] FIG3( b ) is a working waveform diagram of the flyback converter to which the control method according to the second embodiment of the present invention is applied. DETAILED DESCRIPTION

[0066] 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.

[0067] 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.

[0068] 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 here. 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.

[0069] 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.

[0070] 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.

[0071] First embodiment

[0072] This embodiment provides a first control method for a flyback converter. FIG2( a ) is a circuit schematic diagram of a flyback converter to which the control method of the first embodiment of the present invention is applied. The flyback converter of FIG2( a ) 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 Q1, a clamp Q2, a clamp capacitor Cr and a primary winding of the transformer T1; the secondary-side circuit includes a rectifier diode D1 and a secondary winding of the transformer T1; the control device includes a feedback circuit 242, a primary-side controller 240 and a driver 241. The control device is configured to execute the control method of this embodiment to control the turning on and off of the main power switch Q1 and the clamp Q2; the main power switch Q1 and the clamp Q2 are controlled to be turned off in the initial state of each working cycle of the flyback converter.

[0073] The control method of this embodiment includes the following steps in each working cycle of the flyback converter:

[0074] The feedback circuit 242 obtains a first signal representing the magnitude of the flyback converter output voltage Vo and outputs it to the primary-side controller 240 ; the primary-side controller 240 obtains a second signal representing the magnitude of the current flowing through the source of the main power switch Q1 ;

[0075] The primary-side controller 240 generates a first pulse signal SW1 based on the first signal and the second signal, turning on the main power switch Q1 for a first duration, thereby storing energy in the primary winding. The primary-side controller 240 also generates a second pulse signal SW2 based on the falling edge of the first pulse signal SW1 and outputs it to the driver 241.

[0076] After the first duration, the main power switch Q1 is turned off for a first pause time, so that during the first pause time, the energy in the primary winding is released through the secondary winding when the rectifier diode D1 is turned on, and the leakage inductance energy of the primary winding is transferred to the clamping capacitor when the body diode of the clamping tube Q2 is turned on;

[0077] After the first pause time, the driver 241 turns on the clamp transistor Q2 for a second duration according to the second pulse signal SW2, so that the leakage inductance energy stored in the clamp capacitor Cr is transferred to the secondary side circuit through the transformer T1 and then output;

[0078] After the second duration, the clamp transistor Q2 is turned off for a second pause time.

[0079] The innovative process of this embodiment is described below:

[0080] Through research, the inventors of the present application have discovered that in a flyback converter with an active clamping function on the primary side, due to the presence of a clamping capacitor, when the drain-source voltage of the main power switch tube reaches N*Vout+Vin (where N is the turns ratio of the primary winding and the secondary winding 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 will be turned on, and the energy of the leakage inductance will be transferred to the clamping capacitor. Ultimately, the drain-source voltage of the main power switch tube reaches N*Vout+Vin+△Vc (△Vc is the voltage increase of the clamping capacitor after the leakage inductance energy is transferred to the clamping capacitor in each cycle). Since the clamping capacitor stores energy in each cycle, in order to avoid the risk of breakdown of the drain-source voltage of the main power switch tube due to accumulation of leakage inductance energy from cycle to cycle, it is necessary to release the energy in each cycle.

[0081] In the prior art control method shown in Figure 1(b), during each operating cycle of the flyback converter, the clamping diode performs two tasks when it turns on: first, it absorbs the drain-source voltage spike of the main power switch and recycles the leakage inductance energy to the output; second, it achieves zero-voltage turn-on of the main power switch. To this end, the clamping diode's turn-on time is designed to occur after the secondary winding is demagnetized, allowing the voltage on the clamping capacitor to reversely excite the primary winding, thereby achieving zero-voltage turn-on of the main power switch. Due to the small leakage inductance energy, when the clamping diode turns on, the clamping capacitor voltage drops rapidly, making it impossible to continuously reverse excite the primary winding at a stable voltage. Therefore, the prior art control method requires a larger clamping capacitor to store more energy to maintain a stable clamping capacitor voltage, thereby stabilizing the negative current slope of the primary winding. Furthermore, the clamping capacitor's charging current, i = C*dV / dt, is very high when the clamping capacitor capacitance C is large, necessitating a clamping switch with a higher current rating.

[0082] In the control method of the prior art shown in FIG1(c), during each operating cycle of the flyback converter, the clamp switch tube is turned on to absorb the drain-source voltage spike of the main power switch tube and recover the leakage inductance energy to the output end. The conduction time is proportional to the demagnetization time. Therefore, the conduction time is long, the effective current flowing through the clamp switch tube is large, the clamping loop loss is large, the leakage inductance energy recovery effect is poor, and the efficiency cannot be effectively improved.

[0083] Considering that in low-power applications, the value of achieving zero-voltage conduction of the switch tube is not great, and low-power applications pursue lower costs, a simpler control method is needed that can not only reduce the voltage stress of the main power switch tube, but also recover leakage inductance energy and improve efficiency.

[0084] To this end, the control method of this embodiment is proposed. The clamping tube is activated after the energy in the primary winding is released through the secondary winding and the leakage inductance energy of the primary winding is transferred to the clamping capacitor. That is, the clamping tube is activated before the secondary winding is demagnetized, releasing the leakage inductance energy stored in the clamping capacitor to the output end of the secondary circuit. In this case, the leakage inductance energy does not need to be used to achieve ZVS of the main power switch tube. Therefore, the value of the clamping capacitor only needs to meet the requirement of absorbing the drain-source voltage spike of the main power switch tube. Therefore, there is no need to use a large-capacity clamping capacitor. In addition, the charge and discharge current i=C*dV / dt of a small clamping capacitor is small, so the clamping tube can be selected with a small current specification, which helps reduce the cost and size of the flyback converter.

[0085] It can be seen that although this embodiment cannot achieve zero-voltage turn-on of the main power switch tube, it has great advantages in terms of cost and volume. In low-power applications, this embodiment is more competitive than the existing technology.

[0086] The output power range of the low-power flyback converter of the present invention is generally below 150W. It should be noted that this power range does not constitute a limitation on the scope of protection of the present invention. Those skilled in the art can choose whether to adopt the solutions of this embodiment and other embodiments of the present application according to actual needs.

[0087] FIG2( b ) is a working waveform diagram of the flyback converter applied to the control method of the first embodiment of the present invention. The working process of the flyback converter of FIG2( a ) is described below in conjunction with FIG2( b ):

[0088] Phase 1 (t0-t1): The primary-side controller 240 provides a first pulse signal SW1 to turn on the main power switch Q1, causing the primary inductor current IL_p to flow in the primary winding in a forward direction. After the first phase, the first pulse signal SW1 is stopped, turning off the main power switch Q1, and the flyback converter enters the second phase.

[0089] Phase 2 (t1-t2): The main power switch Q1 is turned off, and the primary inductor current IL_p rapidly charges the drain-source capacitance Cds of the main power switch Q1, causing the drain-source voltage to rise. When the voltage reaches Vin + N*Vout, the secondary rectifier diode D1 turns on, and the transformer T1 begins to demagnetize. Due to the presence of leakage inductance Lk, the leakage inductance energy causes the drain-source voltage of the main power switch Q1 to continue to rise until it reaches N*Vout + Vin + △Vc (△Vc is the voltage increase of the clamping capacitor after the leakage inductance energy is transferred to the clamping capacitor in each cycle). The body diode of the clamping transistor Q2 turns on, providing a low-impedance path for the leakage inductance energy, which is then stored in the clamping capacitor Cr.

[0090] Phase 3 (t2-t3): During this period, the leakage inductance energy is still being transferred to the clamping capacitor. At this time, the driver 241 provides a second pulse signal SW3 to turn on the clamping transistor Q2, providing a lower impedance loop for the leakage inductance energy, so that the remaining leakage inductance energy is stored in the clamping capacitor. At this time, the leakage inductance energy is completely transferred, that is, the leakage inductance is completely demagnetized. Then, the clamping capacitor Cr resonates with the leakage inductance, reversely exciting the leakage inductor Lk. The excitation voltage is Vc-N*Vo. At the same time, the leakage inductance energy stored in the clamping capacitor is fed back to the output end through the transformer, realizing leakage inductance energy recovery. The clamping transistor Q2 is turned on for a fixed time T1.

[0091] The clamping tube Q2 is preferably an enhanced N-channel MOS tube, and a switch tube with a smaller current specification and package can be selected compared to the prior art.

[0092] The clamping capacitor Cr has a smaller capacitance than that of the prior art, generally between 1-20nF.

[0093] Preferably, the second duration of this embodiment (i.e., the on-time T1 of the clamp transistor Q2) can be a fixed pulse width, typically several hundred nanoseconds, so that the leakage inductance energy stored in the clamp capacitor can be transferred to the output end within several resonant cycles of the leakage inductance Lk and the clamp capacitor Cr, thereby minimizing the current in the resonant circuit and the loss during the leakage inductance energy transfer process.

[0094] Preferably, the second duration (i.e., the on-time T1 of the clamping transistor Q2) of this embodiment can be a pulse width that varies with the load. Since the leakage inductance demagnetization time increases with increasing load, the on-time of the clamping transistor Q2 can be pulse-width compensated based on the load to better achieve leakage inductance energy recovery.

[0095] Second embodiment

[0096] This embodiment provides a second control method for a flyback converter. FIG3( a ) is a circuit schematic diagram of a flyback converter to which the control method of the second embodiment of the present invention is applied. Referring to FIG3( a ), the 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 Q1 , a clamp Q2 , a clamp capacitor Cr, and a primary winding of the transformer T1 . The secondary-side circuit includes a synchronous rectifier Q3 and a secondary winding of the transformer T1 . The control device includes a feedback circuit 242 , a primary-side controller 240 , a secondary-side controller 243 , an isolation circuit 244 , and a driver 241 . The control device is configured to execute the control method of this embodiment to control the turning on and off of the main power switch Q1 , the clamp Q2 , and the synchronous rectifier Q3 . The main power switch Q1 , the clamp Q2 , and the synchronous rectifier Q3 are all controlled to be turned off at the initial state of each working cycle of the flyback converter.

[0097] The control method of this embodiment includes the following steps in each working cycle of the flyback converter:

[0098] The feedback circuit 242 obtains a third signal representing the magnitude of the flyback converter output voltage Vo and outputs it to the primary-side controller 240. The primary-side controller 240 obtains a fourth signal representing the magnitude of the current flowing through the source of the main power switch Q1. The secondary-side controller 243 obtains a fifth signal representing the drain-source voltage of the synchronous rectifier Q3.

[0099] The primary-side controller 240 generates a third pulse signal SW1 according to the third signal and the fourth signal, and turns on the main power switch Q1 for a third duration, so that the primary winding stores energy.

[0100] After the third duration, the main power switch Q1 is turned off for a third pause time;

[0101] After the third pause time, the secondary-side controller 243 generates a fourth pulse signal SR based on the fifth signal, turns on the synchronous rectifier Q3 for a fourth duration, so that the energy in the primary winding is released through the secondary winding when the synchronous rectifier Q3 is turned on, and during the fourth duration, the leakage inductance energy of the primary winding is transferred to the clamp capacitor Cr when the body diode of the clamp transistor Q2 is turned on. The secondary-side controller 243 also generates a fifth pulse signal SW2 based on the falling edge signal of the fourth pulse signal SR, and transmits the fifth pulse signal SW2 to the driver 241 via the isolation circuit 244.

[0102] After the fourth duration, the driver 241 turns on the clamp transistor Q2 for a fifth duration according to the fifth pulse signal, so that the leakage inductance energy stored in the clamp capacitor Cr is transferred to the secondary side circuit through the transformer T1 and then output;

[0103] After the fifth duration, the clamping transistor Q2 is turned off for a fourth pause time.

[0104] The control method inventive concept of this embodiment is the same as that of the first embodiment. The difference is that the secondary side circuit of the flyback converter used in this embodiment adopts a synchronous rectification scheme, which can further improve the efficiency of the flyback converter. However, the synchronous rectifier tube also needs to be controlled. Therefore, the control method is more complicated than that of the first embodiment. Those skilled in the art can choose whether the secondary side circuit adopts diode rectification or synchronous rectifier tube rectification according to actual conditions.

[0105] FIG3( b ) is a working waveform diagram of the flyback converter applied to the control method of the second embodiment of the present invention. The working process of the flyback converter of FIG3( a ) is described below in conjunction with FIG3( b ):

[0106] Phase 1 (t0-t1): The primary-side controller 240 provides a first pulse signal SW1 to turn on the main power switch Q1, causing the primary inductor current IL_p to flow in the primary winding in a forward direction. After the first phase, the first pulse signal SW1 is stopped, turning off the main power switch Q1, and the flyback converter enters the second phase.

[0107] Phase 2 (t1-t2): The main power switch Q1 is turned off, and the primary inductor current IL_p rapidly charges the drain-source capacitor Cds of the main power switch Q1, causing the drain-source voltage to rise. When the drain-source voltage reaches Vin + N*Vout, the secondary-side controller 243 generates a third pulse signal SR to turn on the synchronous rectifier Q3, and the transformer T1 begins to demagnetize. Due to the leakage inductance Lk, the leakage inductance energy causes the drain-source voltage of the main power switch Q1 to continue to rise until it reaches N*Vout + Vin + ΔVc (ΔVc is the voltage increase of the clamping capacitor after the leakage inductance energy is transferred to the clamping capacitor in each cycle). At this point, the body diode of the clamping transistor Q2 turns on, providing a low-impedance path for the leakage inductance energy, allowing most of the leakage inductance energy to be stored in the clamping capacitor Cr.

[0108] The third stage (t2-t3): At this time, the leakage inductance has been demagnetized and the energy is stored in the clamping capacitor Cr. During this stage, the transformer T1 is in the demagnetization stage until time t3. When the transformer T1 is about to be demagnetized, the secondary-side controller 243 stops providing the third pulse signal SR, and the synchronous rectifier tube Q3 is turned off. When the third pulse signal SR is stopped, a falling edge signal is generated. After being processed by the secondary-side controller 243 and transmitted to the driver 241 through the isolation circuit, a second pulse signal SW2 is generated, which controls the clamping tube Q2 to be turned on for a period of time T1. During this period, the leakage inductance energy stored in the clamping capacitor Cr is transmitted to the secondary output end.

[0109] Furthermore, in this embodiment, the secondary side controller 243 obtains the enable signal, enables the secondary side controller 243 to obtain the fifth signal representing the drain-source voltage of the synchronous rectifier tube Q3, and enables the secondary side controller 243 to generate the fifth pulse signal SW2 based on the falling edge signal of the fourth pulse signal SR.

[0110] Furthermore, the enable signal is generated some time before the rising edge of the third pulse signal SW1, thereby ensuring that the fifth pulse signal SW2 can be forcibly turned off when it continues to the next enable signal in the current continuous mode, avoiding the simultaneous existence of the third pulse signal SW1, which causes damage to the main power switch tube and the synchronous rectifier tube.

[0111] Furthermore, when the fifth signal reaches the first threshold, the fourth pulse signal SR is generated, the synchronous rectifier Q3 is turned on, and the fourth duration begins; when the fifth signal reaches the second threshold, the fourth pulse signal SR is stopped from being generated, the synchronous rectifier Q3 is turned off, and the fourth duration ends; the first threshold and the second threshold are both negative thresholds, and the absolute value of the first threshold is greater than the absolute value of the second threshold, so that the synchronous rectifier is turned off before the demagnetization is completed, creating conditions for the generation of the fifth pulse signal SW2.

[0112] Likewise, in this embodiment, the clamping transistor Q2 is preferably an enhancement-mode N-channel MOS transistor, and a switch transistor with a smaller current specification and package size can be selected compared to the prior art.

[0113] Likewise, the clamping capacitor Cr in this embodiment has a smaller capacitance than that in the prior art, typically between 1-20 nF.

[0114] Preferably, the isolation circuit may be capacitive isolation or other isolation methods.

[0115] Preferably, the end time of the fifth duration (i.e., the turn-off time of the clamping tube Q2) is near the time when the secondary inductor current IL_s drops to 0, thereby avoiding the loss problem caused by the resonance of the excitation inductance and the clamping capacitor after demagnetization.

[0116] Preferably, the fifth duration (i.e., the on-time T1 of the clamp transistor Q2) of this embodiment can be a fixed pulse width, typically several hundred nanoseconds, so that the leakage inductance energy stored in the clamp capacitor can be transferred to the output end within several resonant cycles of the leakage inductance Lk and the clamp capacitor Cr, thereby minimizing the effective current of the resonant circuit and reducing losses during the leakage inductance energy transfer process.

[0117] Third embodiment

[0118] This embodiment provides a first control device, which is applied to the flyback converter of Figure 2(a). The control device includes a feedback circuit 242, a primary side controller 240 and a driver 241. The control device of this embodiment is configured to execute any specific implementation scheme of the control method in the first embodiment in each working cycle of the flyback converter of Figure 2(a).

[0119] The beneficial effects of each specific implementation method of this embodiment are consistent with the beneficial effects of the corresponding implementation method of the first embodiment, and will not be repeated here.

[0120] Fourth embodiment

[0121] This embodiment provides a second control device, which is applied to the flyback converter of Figure 3(a). The control device includes a feedback circuit 242, a primary side controller 240, a secondary side controller 243, an isolation circuit 244 and a driver 241. The control device of this embodiment is configured to execute any specific implementation scheme of the control method in the second embodiment in each working cycle of the flyback converter of Figure 3(a).

[0122] The beneficial effects of each specific implementation method of this embodiment are consistent with the beneficial effects of the corresponding implementation methods of the second embodiment, and are not repeated here.

[0123] Fifth embodiment

[0124] This embodiment provides a flyback converter, including: a primary side circuit, a secondary side circuit, a transformer and a control device; the primary side circuit includes a main power switch tube, a clamp tube, a clamp capacitor and the primary winding of the transformer; the secondary side circuit includes a rectifier diode or a synchronous rectifier tube and the secondary winding of the transformer; when the secondary side circuit includes a rectifier diode and the secondary winding of the transformer, the control device is any specific implementation scheme of the third control device; when the primary side circuit includes a synchronous rectifier tube and the secondary winding of the transformer, the control device is any specific implementation scheme of the control device of the fourth embodiment.

[0125] The beneficial effects of the specific implementations of this embodiment are indirectly consistent with the beneficial effects of the corresponding implementations of the first and second embodiments, and are not repeated here.

[0126] According to the above contents of the present invention, by utilizing common technical knowledge and customary means in this field, without departing from the above inventive concept of the present invention, the present invention may also make other various forms of modification, replacement or change, all of which fall within the scope of protection of the present invention.

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 tube, a clamp tube, a clamp capacitor, and a primary winding of the transformer; the secondary-side circuit comprising a rectifier diode and a secondary winding of the transformer; the control device comprising a feedback circuit, a primary-side controller, and a driver, the control device being configured to execute the control method to control the turning on and off of the main power switch tube and the clamp tube; the main power switch tube and the clamp tube being controlled to be turned off at the initial state of each working cycle of the flyback converter; characterized in that The control method comprises the following steps in each working cycle of the flyback converter: The feedback circuit obtains a first signal representing the magnitude of the output voltage of the flyback converter and outputs the signal to the primary-side controller; the primary-side controller obtains a second signal representing the magnitude of the current flowing through the source of the main power switch tube; The primary-side controller generates a first pulse signal based on the first signal and the second signal, turns on the main power switch for a first duration, so that the primary winding stores energy; the primary-side controller also generates a second pulse signal based on a falling edge signal of the first pulse signal, and outputs the second pulse signal to the driver; After the first duration, turning off the main power switch tube for a first pause time, so that during the first pause time, energy in the primary winding is released through the secondary winding when the rectifier diode is turned on, and leakage inductance energy of the primary winding is transferred to the clamping capacitor when the body diode of the clamping tube is turned on; After the first pause time, the driver turns on the clamping transistor for a second duration according to the second pulse signal, so that the leakage inductance energy stored in the clamping capacitor is transferred to the secondary side circuit through the transformer and then output; After the second duration, the clamp tube is opened for a second pause time.

2. The control method according to claim 1, characterized in that: The second duration is a fixed time.

3. The control method according to claim 2, characterized in that: The second duration is hundreds of nanoseconds.

4. 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, a clamp capacitor, and a primary winding of the transformer; the secondary-side circuit comprising a synchronous rectifier and a secondary winding of the transformer; the control device comprising a feedback circuit, a primary-side controller, a secondary-side controller, an isolation circuit, and a driver, the control device being configured to execute the control method to control the turning on and off of the main power switch, the clamp, and the synchronous rectifier; the main power switch, the clamp, and the synchronous rectifier being controlled to be turned off at the initial state of each working cycle of the flyback converter; and characterized in that: The control method comprises the following steps in each working cycle of the flyback converter: The feedback circuit obtains a third signal representing the output voltage of the flyback converter and outputs it to the primary-side controller; the primary-side controller obtains a fourth signal representing the source current of the main power switch tube; and the secondary-side controller obtains a fifth signal representing the drain-source voltage of the synchronous rectifier tube. The primary-side controller generates a third pulse signal according to the third signal and the fourth signal, turns on the main power switch for a third duration, so that the primary winding stores energy; After the third duration, turning off the main power switch tube for a third pause time; After the third pause time, the secondary-side controller generates a fourth pulse signal according to the fifth signal, turns on the synchronous rectifier for a fourth duration, so that energy in the primary winding is released through the secondary winding when the synchronous rectifier is turned on, and during the fourth duration, leakage inductance energy of the primary winding is transferred to the clamp capacitor when the body diode of the clamp is turned on; The secondary-side controller further generates a fifth pulse signal based on the falling edge signal of the fourth pulse signal, and transmits the fifth pulse signal to the driver via the isolation circuit; After the fourth duration, the driver turns on the clamping transistor for a fifth duration according to the fifth pulse signal, so that the leakage inductance energy stored in the clamping capacitor is transferred to the secondary side circuit through the transformer and then output; After the fifth duration, the clamp tube is disconnected for a fourth pause time.

5. The control method according to claim 4, characterized in that: The secondary side controller obtains an enable signal to enable the secondary side controller to obtain a fifth signal representing the drain-source voltage of the synchronous rectifier tube, and enables the secondary side controller to generate a fifth pulse signal based on the falling edge signal of the fourth pulse signal.

6. The control method according to claim 5, characterized in that: The enable signal is generated some time before the rising edge of the third pulse signal.

7. The control method according to claim 4, characterized in that: When the fifth signal reaches the first threshold, the fourth pulse signal is generated, the synchronous rectifier tube is turned on, and the fourth duration begins; when the fifth signal reaches the second threshold, the fourth pulse signal is stopped from being generated, the synchronous rectifier tube is turned off, and the fourth duration ends; the first threshold and the second threshold are both negative thresholds, and the absolute value of the first threshold is greater than the absolute value of the second threshold.

8. The control method according to claim 4, characterized in that: The fifth duration is a fixed time.

9. The control method according to claim 4, characterized in that: The fifth duration is hundreds of nanoseconds.

10. 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 tube, a clamp capacitor, and a primary winding of the transformer; the secondary-side circuit comprising a rectifier diode and a secondary winding of the transformer; the control device comprising a feedback circuit, a primary-side controller, and a driver, the control device being configured to control the on and off of the main power switch tube and the clamp tube; the main power switch tube and the clamp tube being controlled to be off at the initial state of each working cycle of the flyback converter; characterized in that The control device performs the following actions in each working cycle of the flyback converter: The feedback circuit obtains a first signal representing the magnitude of the output voltage of the flyback converter and outputs the signal to the primary-side controller; the primary-side controller obtains a second signal representing the magnitude of the current flowing through the source of the main power switch tube; The primary-side controller generates a first pulse signal based on the first signal and the second signal, turns on the main power switch for a first duration, so that the primary winding stores energy; the primary-side controller also generates a second pulse signal based on a falling edge signal of the first pulse signal, and outputs the second pulse signal to the driver; After the first duration, turning off the main power switch tube for a first pause time, so that during the first pause time, energy in the primary winding is released through the secondary winding when the rectifier diode is turned on, and leakage inductance energy of the primary winding is transferred to the clamping capacitor when the body diode of the clamping tube is turned on; After the first pause time, the driver turns on the clamping transistor for a second duration according to the second pulse signal, so that the leakage inductance energy stored in the clamping capacitor is transferred to the secondary side circuit through the transformer and then output; After the second duration, the clamp tube is opened for a second pause time.

11. The control device according to claim 10, characterized in that: The second duration is a fixed time.

12. The control device according to claim 10, characterized in that: The second duration is hundreds of nanoseconds.

13. 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 tube, a clamp capacitor, and a primary winding of the transformer; the secondary-side circuit comprising a synchronous rectifier tube and a secondary winding of the transformer; the control device comprising a feedback circuit, a primary-side controller, a secondary-side controller, an isolation circuit, and a driver, the control device being configured to control the on and off states of the main power switch tube and the clamp tube; the main power switch tube and the clamp tube being controlled to be off at the initial state of each operating cycle of the flyback converter; characterized in that: The control device performs the following actions in each working cycle of the flyback converter: The feedback circuit obtains a third signal representing the output voltage of the flyback converter and outputs it to the primary-side controller; the primary-side controller obtains a fourth signal representing the source current of the main power switch tube; and the secondary-side controller obtains a fifth signal representing the drain-source voltage of the synchronous rectifier tube. The primary-side controller generates a third pulse signal according to the third signal and the fourth signal, turns on the main power switch for a third duration, so that the primary winding stores energy; After the third duration, turning off the main power switch tube for a third pause time; After the third pause time, the secondary-side controller generates a fourth pulse signal according to the fifth signal, turns on the synchronous rectifier for a fourth duration, so that energy in the primary winding is released through the secondary winding when the synchronous rectifier is turned on, and during the fourth duration, leakage inductance energy of the primary winding is transferred to the clamp capacitor when the body diode of the clamp is turned on; The secondary-side controller further generates a fifth pulse signal based on the falling edge signal of the fourth pulse signal, and transmits the fifth pulse signal to the driver via the isolation circuit; After the fourth duration, the driver turns on the clamping transistor for a fifth duration according to the fifth pulse signal, so that the leakage inductance energy stored in the clamping capacitor is transferred to the secondary side circuit through the transformer and then output; After the fifth duration, the clamp tube is disconnected for a fourth pause time.

14. The control device according to claim 13, characterized in that: The secondary side controller obtains an enable signal to enable the secondary side controller to obtain a fifth signal representing the drain-source voltage of the synchronous rectifier tube, and enables the secondary side controller to generate a fifth pulse signal based on the falling edge signal of the fourth pulse signal.

15. The control device according to claim 14, characterized in that: The enable signal is generated some time before the rising edge of the third pulse signal.

16. The control device according to claim 13, characterized in that: When the fifth signal reaches the first threshold, the fourth pulse signal is generated, the synchronous rectifier tube is turned on, and the fourth duration begins; when the fifth signal reaches the second threshold, the fourth pulse signal is stopped from being generated, the synchronous rectifier tube is turned off, and the fourth duration ends; the first threshold and the second threshold are both negative thresholds, and the absolute value of the first threshold is greater than the absolute value of the second threshold.

17. The control device according to claim 13, characterized in that: The fifth duration is a fixed time.

18. The control device according to claim 13, characterized in that: The fifth duration is hundreds of nanoseconds.

19. A flyback converter, characterized in that: include: A primary side circuit, a secondary side circuit, a transformer and a control device; the primary side circuit includes a main power switch tube, a clamp tube, a clamp capacitor and the primary winding of the transformer; the secondary side circuit includes a rectifier diode or a synchronous rectifier tube and the secondary winding of the transformer; when the secondary side circuit includes a rectifier diode and the secondary winding of the transformer, the control device is the control device described in any one of claims 10 to 12; when the secondary side circuit includes a synchronous rectifier tube and the secondary winding of the transformer, the control device is the control device described in any one of claims 13 to 18.

Images