Flyback converter and control circuit and control method thereof

Abstract

The application provides a flyback converter and a control circuit and a control method thereof. The flyback converter is subjected to working condition detection. When it is detected that the flyback converter is in a non-driving state for a predetermined time length, the main switch tube in the flyback converter is subjected to soft driving in at least one continuous switching period after starting driving, so that the voltage at the coupling node of the main switch tube and the transformer has a very slow change rate, and the stress of the secondary side rectifier tube under special working conditions is effectively inhibited. When it is detected that the flyback converter works in a steady state, the main switch tube is subjected to soft driving or zero voltage turn-on driving according to the load size, so that the stress of the secondary side rectifier tube can be reduced under a wide output voltage range and in all working conditions, and the complexity of system control is reduced.

Description

Technical Field

[0001] This application relates to the field of switching converter technology, specifically to a flyback converter and its control circuit and control method. Background Technology

[0002] The asymmetrical half-bridge (AHB) flyback converter is a novel type of soft-switching converter. As a type of switching power supply, it is typically used in isolated DC-DC conversion applications. In practical applications, by adjusting the on-time of the high-side and low-side switches in the half-bridge structure of the AHB flyback converter, not only can the output voltage be adjusted, but the main and auxiliary switches can also achieve soft-switching (i.e., zero-voltage switching, ZVS) functionality, thereby improving the power conversion efficiency of the AHB flyback converter.

[0003] Compared to conventional flyback converters, AHB flyback converters are more efficient. Under heavy loads, they typically operate in a complementary state between the two switches, such as BCM (Continuous On) mode. Under light loads, they generally reduce the switching frequency to allow the flyback converter to enter DCM (Discontinuous On) mode.

[0004] In wide output range applications, the transformer in the AHB flyback converter is relatively small, leading to increased stress on the secondary rectifier diodes. When the AHB flyback converter operates in BCM mode, its main switch is ZVS turned on, resulting in very low stress on the secondary rectifier diodes. However, when the AHB flyback converter operates in DCM mode, existing solutions require controlling the ZVS transistor (which can be any of the following: auxiliary switch, a switch connected in parallel with the auxiliary switch, a switch connected to the secondary winding, or a switch connected to the auxiliary winding) to be turned on for an additional period before the main power transistor is turned on. This generates a negative current in the magnetizing inductor, which discharges the junction capacitance of the main switch before the main switch is turned on, achieving zero-voltage turn-on of the main switch. This reduces the stress on the secondary rectifier diodes even in DCM mode.

[0005] However, the above solutions are all based on the premise that the magnetizing inductor has established a stable negative current. Under some special operating conditions, such as during system cold start (e.g., power-on), load step, or the first few cycles after restarting from hibernation in Burst mode, since an effective magnetizing voltage has not yet been formed at both ends of the transformer, even if the ZVS transistor of the system turns on on time, it cannot generate enough negative current to complete the junction capacitance discharge. This causes the main switching transistor to still turn on at a higher voltage, failing to achieve the ZVS effect. Consequently, the secondary rectifier unit will still have significant stress, making device selection difficult. Summary of the Invention

[0006] In view of the above-mentioned technical problems, the purpose of this application is to provide a flyback converter and its control circuit and control method. The flyback converter is driven and controlled by a combination of soft drive mode and zero voltage turn-on drive mode. This allows the system to have low secondary rectifier stress in the first few switching cycles after restarting, even in special operating conditions such as power-on, dynamic operation, or sudden hibernation and restart. This ensures low secondary stress in the system over a wide output voltage range and under all operating conditions. The control method is simple.

[0007] According to a first aspect of this application, a control method for a flyback converter is provided, the flyback converter including a transformer and a main switching transistor.

[0008] The control method includes:

[0009] The flyback converter is subjected to operating condition monitoring;

[0010] When it is detected that the flyback converter is in an undriven state for a predetermined period of time, the main switch is soft-driven for at least one consecutive switching cycle after the drive is started, so as to reduce the rate of change of voltage at the coupling node between the main switch and the transformer.

[0011] When the flyback converter is detected to be operating in a steady state, the main switch is either soft-driven or zero-voltage turn-on driven according to the load size.

[0012] Optionally, the soft drive can be implemented by reducing the turn-on speed of the main switch in at least one consecutive switching cycle after the start-up drive, so as to reduce the rate of voltage change at the coupling node between the main switch and the transformer.

[0013] Optionally, when the flyback converter is detected to meet any of the following operating conditions, it can be determined that the flyback converter is in a non-driven state for a predetermined period of time:

[0014] The flyback converter was detected to be in power-on mode;

[0015] The flyback converter was detected to be in a dynamic operating condition with a sudden load change;

[0016] The flyback converter was detected to be operating in hiccup mode;

[0017] The flyback converter was detected to be operating in sleep mode.

[0018] Optionally, during soft driving, the turn-on speed of the main switch in at least one consecutive switching cycle after the drive is started is reduced by extending the time for the drive signal of the main switch to change from an invalid level to an effective level.

[0019] Optionally, in DCM mode, when the flyback converter is detected to be operating in steady state, the main switch is driven to turn on at zero voltage in each switching cycle under steady state.

[0020] Optionally, in DCM mode, when the load is less than a preset load threshold, the main switch is selected to be soft-driven, and when the load is greater than the preset load threshold, the main switch is selected to be zero-voltage turn-on driven.

[0021] Optionally, when the switching frequency of the flyback converter is detected to be greater than a preset frequency threshold, it is determined that the load is greater than a preset load threshold.

[0022] When the switching frequency of the flyback converter is detected to be less than a preset frequency threshold, it is determined that the load is less than a preset load threshold.

[0023] Optionally, when it is detected that the turn-off time of the main switch in each switching cycle is less than a preset time threshold, it is determined that the switching frequency of the flyback converter is greater than a preset frequency threshold.

[0024] When the off-time of the main switch in each switching cycle is detected to be greater than a preset time threshold, it is determined that the switching frequency of the flyback converter is less than a preset frequency threshold.

[0025] Optionally, when the flyback converter operates in a hiccup mode with multiple consecutive hiccup cycles, the main switch is selected to be soft-driven in the first M switching cycles of each hiccup cycle after the first hiccup cycle. Each hiccup cycle includes N switching cycles, where M and N are both positive integers and 1 ≤ M < N.

[0026] Optionally, the flyback converter further includes a zero-voltage switch;

[0027] The zero-voltage turn-on drive of the main switch is achieved by controlling the zero-voltage switch to turn on additionally for a first time before the main switch is turned on.

[0028] Optionally, the zero-voltage switch includes at least one of the following:

[0029] An auxiliary switching transistor is connected in series with the main switching transistor between the input voltage terminal and the reference ground;

[0030] A synchronous rectifier tube connected to the secondary winding of the transformer in the flyback converter;

[0031] The first switching transistor is connected to the auxiliary winding of the transformer in the flyback converter;

[0032] A second switching transistor connected in parallel with the auxiliary switching transistor.

[0033] According to a second aspect of this application, a control circuit for a flyback converter is provided, the flyback converter including a transformer and a main switching transistor.

[0034] The control circuit includes:

[0035] The operating condition detection unit performs operating condition detection on the flyback converter, and outputs a first trigger signal when it detects that the flyback converter is in a no-drive state for a predetermined period of time, and outputs one of the first trigger signal and the second trigger signal according to the load size when it detects that the flyback converter is operating in a steady state.

[0036] The soft drive control unit, upon receiving the first trigger signal, performs soft drive on the main switch transistor to reduce the rate of voltage change at the coupling node between the main switch transistor and the transformer.

[0037] The zero-voltage turn-on control unit drives the main switch transistor to turn on at zero voltage when it receives the second trigger signal.

[0038] According to a third aspect of this application, a flyback converter is provided, comprising: a control circuit as disclosed in any embodiment of this application.

[0039] The beneficial effects of this application include at least the following:

[0040] The flyback converter and its control circuit and method provided in this application employ a combination of soft-drive mode and zero-voltage turn-on drive mode to drive and control the flyback converter. This includes soft-drive of the main switch transistor during at least one consecutive switching cycle (e.g., the previous one or several switching cycles) when the flyback converter is detected to have restarted drive under a special operating condition (i.e., when the flyback converter has been in a no-drive state for a predetermined period). By reducing the rate of voltage change at the coupling node between the main switch transistor and the transformer, the secondary rectifier unit also changes at a slower rate during these switching cycles, thereby reducing the secondary rectifier voltage. The application addresses the stress on the secondary rectifier diodes by selecting either soft-drive or zero-voltage turn-on drive for the main switch transistor based on the load size when the flyback converter is detected to be operating in steady state. Compared to traditional solutions, this application effectively reduces the stress on the secondary rectifier diodes even under special operating conditions (such as restarting after a long period of inactivity). This results in lower stress on the secondary rectifier diodes across a wide output voltage range and all operating conditions. Furthermore, during restarting under special operating conditions, no additional measures are needed to ensure sufficient initial voltage on the resonant circuit; simply switching to soft-drive mode is sufficient to effectively reduce the stress on the secondary rectifier diodes, simplifying the control method. After the flyback converter reaches steady state, under light load conditions, soft-drive (with a lower turn-on speed) can be selected to turn on the main switch transistor, resulting in minimal turn-on losses. Under heavy load conditions, zero-voltage turn-on drive (with a faster turn-on speed) can be selected to turn on the main switch transistor, offering faster turn-on speed and lower turn-on losses.

[0041] It should be noted that the above general description and the following detailed description are merely exemplary and explanatory, and do not limit this application. Attached Figure Description

[0042] Figure 1 A schematic diagram of a flyback converter provided according to an embodiment of this application is shown;

[0043] Figure 2 Show Figure 1 Block diagram of the control signal generation unit;

[0044] Figure 3 Show Figure 1 A schematic diagram of the timing waveforms of some signals from the flyback converter in the first operating condition;

[0045] Figure 4 Show Figure 1 A schematic diagram of the timing waveforms of a portion of the flyback converter under the second operating condition;

[0046] Figure 5 Show Figure 1 A schematic diagram of the timing waveforms of a portion of the flyback converter under the third operating condition;

[0047] Figure 6 A flowchart illustrating the control method for an asymmetric half-bridge flyback converter according to an embodiment of this application is shown. Detailed Implementation

[0048] The preferred embodiments of this disclosure are described in detail below with reference to the accompanying drawings, but this disclosure is not limited to these embodiments. This disclosure covers any alternatives, modifications, equivalent methods, and solutions made within the spirit and scope of this disclosure.

[0049] In order to provide the public with a thorough understanding of this disclosure, specific details are described in detail in the following preferred embodiments of this disclosure, but those skilled in the art can fully understand this disclosure without these details.

[0050] The present disclosure is described in more detail below by way of example with reference to the accompanying drawings. It should be noted that the drawings are in a simplified form and use non-precise scales, and are only used to facilitate and clarify the illustration of the embodiments of the present disclosure.

[0051] To facilitate understanding of this application, a more complete description will be provided below with reference to the accompanying drawings. Preferred embodiments of this application are shown in the drawings. However, this application may be implemented in various forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of this application.

[0052] References to "one embodiment" or "some embodiments" as described in this specification mean that one or more embodiments of this application include a specific feature, structure, or characteristic described in connection with that embodiment. Therefore, the phrases "in one embodiment," "in some embodiments," "in other embodiments," "in still other embodiments," etc., appearing in different parts of this specification do not necessarily refer to the same embodiment, but rather mean "one or more, but not all, embodiments," unless otherwise specifically emphasized. The terms "comprising," "including," "having," and variations thereof mean "including but not limited to," unless otherwise specifically emphasized.

[0053] In the description of this application, words such as "exemplary" or "for example" are used to indicate that they are examples, illustrations, or descriptions. Any embodiment described as "exemplary" or "for example" in this application should not be construed as being more preferred or advantageous than other embodiments. The term "and / or" in this document describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, and B alone. "Multiple" refers to two or more. Furthermore, to facilitate a clear description of the technical solutions of the embodiments of this application, the terms "first," "second," etc., are used to distinguish identical or similar items with substantially the same function and effect. Those skilled in the art will understand that the terms "first," "second," etc., do not limit the quantity or execution order, and that "first," "second," etc., do not necessarily imply differences.

[0054] In addition, the same reference numerals in the figures indicate the same or similar structures, so repeated descriptions of them will be omitted. That is, the various parts in this specification are described in a combination of parallel and progressive manner. Each part focuses on the differences from other parts, and the same or similar parts between the various parts can be referred to each other.

[0055] Figure 1 A schematic diagram of a flyback converter provided in an embodiment of this application is shown. It should be noted that... Figure 1 The technical solution of this application is only illustrated by taking an asymmetric half-bridge flyback converter as an example. However, the technical solution disclosed in this application can also be applied to other types of flyback converters, such as active clamp flyback converters.

[0056] In this application, the flyback converter 100 includes: a transformer TR containing a primary winding Np and a secondary winding Ns (in some other embodiments, the transformer TR also includes an auxiliary winding Na), a main switch Q1, an auxiliary switch Q2, a resonant capacitor Cr, a rectifier unit D1, an output capacitor Co, and a control circuit 110.

[0057] Figure 1In this configuration, the main switch Q1 and the auxiliary switch Q2 are connected in series between the input voltage Vin and the reference ground. The drain of the main switch Q1 is connected to the input voltage Vin, and its gate is connected to the control circuit 110. The drain of the auxiliary switch Q2 is connected to the source of the main switch Q1, and its source is connected to the reference ground through the sampling resistor Rcs. Its gate is connected to the control circuit 110. Capacitors C1 and C2 are the junction capacitances of the main switch Q1 and the auxiliary switch Q2, respectively. During the same switching cycle, the main switch Q1 and the auxiliary switch Q2 are turned on in a time-division manner to transfer the input voltage Vin from the primary side to the secondary side of the flyback converter. Figure 1 In this configuration, both the main switch Q1 and the auxiliary switch Q2 are NMOS field-effect transistors. Of course, in other possible embodiments, the main switch Q1 and the auxiliary switch Q2 can also be PMOS field-effect transistors or GA devices.

[0058] The magnetizing inductance and leakage inductance of the primary winding Np are equivalent to inductances Lm and Lk, respectively, and together with the resonant capacitor Cr and the auxiliary switch Q2, they form a resonant circuit.

[0059] Figure 1 The illustration shows an example where the main switch Q1 is the upper switch (also called the high-side switch) and the auxiliary switch Q2 is the lower switch (also called the low-side switch). It can be understood that in other embodiments of this application, the auxiliary switch Q2 can also be configured as the upper switch (or high-side switch), while the main switch Q1 can be configured as the lower switch (or low-side switch). Furthermore, the resonant capacitor Cr can also be replaced by connecting it between the drain of the auxiliary switch Q2 and the inductor Lk.

[0060] Figure 1 An example of a rectifier unit D1 being a rectifier diode is shown, wherein the anode of the rectifier diode is connected to the secondary winding Ns, and the cathode of the rectifier diode is connected to the output terminal of the flyback converter. The positive terminal of the output capacitor Co is connected to the output terminal of the flyback converter, and the negative terminal of the output capacitor Co is connected to the reference ground. Simultaneously, the corresponding terminal of the secondary winding Ns is also connected to the reference ground. Furthermore, the output terminal of the flyback converter is connected to a load that receives the electrical energy (e.g., voltage and current) converted by the flyback converter. Of course, in other possible embodiments, the rectifier unit D1 may also be a synchronous rectifier diode, and the rectifier unit D1 may be configured to be coupled to the reference ground.

[0061] The control circuit 110 generates drive signals Vgs1 and Vgs2 to drive the main switch Q1 and the auxiliary switch Q2, respectively. The control circuit 110 includes a control signal generation unit 120, a driver 130, and a driver 140. The control signal generation unit 120 generates control signals, which are then converted into drive signals Vgs1 and Vgs2 after being driven by the drivers 130 and 140, respectively.

[0062] refer to Figure 2 , Figure 2 It shows Figure 1 The structural block diagram of the control signal generation unit is shown below. In this embodiment, the control signal generation unit 120 includes: a working condition detection unit 210, a soft drive control unit 220, and a zero-voltage turn-on control unit 230.

[0063] The operating condition detection unit 210 is used to detect the operating condition of the flyback converter 100. When it detects that the flyback converter 100 is in a non-driven state for a predetermined period, it outputs a first trigger signal. When it detects that the flyback converter 100 is operating in a steady state, it outputs one of the first trigger signal and a second trigger signal according to the load size. The soft drive control unit 220 is connected to the operating condition detection unit 210 and is used to soft drive the main switch Q1 upon receiving the first trigger signal, reducing the rate of change of the voltage Vsw at the coupling node (i.e., the switching node SW) between the main switch Q1 and the transformer TR. The zero-voltage turn-on control unit 230 is connected to the operating condition detection unit 210 and is used to drive the main switch Q1 to zero-voltage turn-on upon receiving the second trigger signal.

[0064] The operating condition detection unit 210 determines that the flyback converter 100 has been in a non-driven state for a predetermined period of time when it detects that the flyback converter 100 meets any of the following operating conditions: the flyback converter 100 is in a power-on state; the flyback converter 100 is in a dynamic state with a sudden load change; the flyback converter 100 is operating in a burst mode; or the flyback converter 100 is operating in a sleep mode. Generally, the time the flyback converter 100 operates in the burst mode is usually shorter than the time the flyback converter 100 operates in the sleep mode. Of course, there are special cases, and this application does not strictly limit this.

[0065] For example, when it is necessary to determine whether the flyback converter 100 is in the power-on state, it can be determined by detecting the input voltage Vin of the flyback converter 100 and / or the power-on start signal. For example, when the input voltage Vin of the flyback converter 100 is detected to rise from zero to a preset voltage threshold, or when the power-on start signal indicating system power-on is detected to change from an invalid state to an effective state, it can be determined that the flyback converter is in the power-on state.

[0066] For example, when it is necessary to determine whether the flyback converter 100 is in a dynamic operating condition of sudden load change, it can be determined by detecting the error compensation signal COMP, load current, and other current / voltage signals that can indicate the load size of the flyback converter 100. For example, when a large change is detected in the error compensation signal COMP, load current, and other current / voltage signals that can indicate the load size, it can be determined that the flyback converter is in a dynamic operating condition of sudden load change.

[0067] For example, when it is necessary to determine whether the flyback converter 100 is in hiccup mode, it can be determined by detecting the error compensation signal COMP, load current, or other current / voltage signals that can indicate the load size of the flyback converter 100, or by detecting the mode selection signal indication state of the flyback converter 100. For example, when the error compensation signal COMP, load current, or other current / voltage signals that can indicate the load size are detected to be within a predetermined value range, or when the mode selection signal of the flyback converter 100 is detected to be in a specific indication state, it can be determined that the flyback converter 100 is in hiccup mode.

[0068] For example, when it is necessary to determine whether the flyback converter 100 is working in sleep mode, the flyback converter 100 can be determined by detecting the sleep start signal of the flyback converter 100. For example, when the sleep start signal is detected to change from an invalid state to an effective state, it can be determined that the flyback converter is working in sleep mode.

[0069] Furthermore, the number of rising (or falling) edges of the drive signals of the main switch Q1 and / or auxiliary switch Q2 in the flyback converter 100 can be counted. When the corresponding count value is detected not to increase within a predetermined time period, it can also be determined that the flyback converter 100 is in a no-drive state within the predetermined time period. Of course, other conventional operating condition detection methods can also be used in other embodiments of this application to detect the operating condition of the flyback converter 100.

[0070] In this embodiment, when soft-driving of the main switch Q1 is required, the soft-drive control unit 220 achieves soft-drive by reducing the turn-on speed of the main switch Q1 in at least one consecutive switching cycle after the flyback converter 100 starts driving, thereby reducing the rate of change of the voltage Vsw at the coupling node SW between the main switch Q1 and the transformer TR in the flyback converter 100. For example, during soft-drive, a smaller drive current can be given to turn on the main switch Q1 at a slower speed.

[0071] In some embodiments, the drive signal Vgs1 of the main switch Q1 can be adjusted. By reducing the slope (e.g., rising slope) of the drive signal Vgs1 when it changes from an invalid level to an effective level during soft drive, or by extending the time it takes for the drive signal Vgs1 to change from an invalid level to an effective level, the turn-on speed of the main switch Q1 in at least one consecutive switching cycle after the start of drive can be reduced. Specifically, the time for the drive signal Vgs1 of the main switch Q1 to change from an invalid level to an effective level during soft drive is set to be greater than the time it takes during normal drive. This results in a slower turn-on speed for the main switch Q1 during soft drive compared to normal drive, thereby reducing the rate of change of the voltage Vsw at node SW, and consequently reducing the rate of change of the voltage across the secondary rectifier unit D1, thus reducing the stress on the secondary rectifier tube during soft drive. When the drive signal Vgs1 is at an effective level, it can control the main switch Q1 to complete normal turn-on.

[0072] When it is necessary to drive the main switch Q1 to turn on at zero voltage, the zero voltage turn-on control unit 230 can be configured to control the zero voltage switch to turn on for an additional first time before the main switch Q1 is turned on, thereby generating a negative current in the magnetizing inductor Lm during the first time. After the zero voltage switch is turned off, the negative current in the magnetizing inductor Lm is used to discharge the junction capacitance C1 of the main switch Q1. When the drain-source voltage of the main switch Q1 is zero, the main switch Q1 is turned on, realizing the zero voltage turn-on of the main switch Q1. Since the drain-source voltage of the main switch Q1 is zero, the turn-on loss is low, and the voltage stress across the secondary rectifier is low.

[0073] In flyback converters operating under conditions of prolonged inactivity such as before startup, sudden load changes, dynamic disturbances, hiccups, or hibernation, this invention employs soft-drive to turn on the main switch Q1 upon restarting. This reduces stress on the secondary rectifier diodes of the flyback converter. Since soft-drive turn-on results in higher turn-on losses, this invention only applies soft-drive to the main switch Q1 during the first few switching cycles after restarting. After the flyback converter reaches steady-state operation, under light load conditions, soft-drive can still be applied to the main switch Q1 due to the small load, resulting in relatively low turn-on losses and stress on the secondary rectifier diodes. Under heavy load conditions, zero-voltage turn-on is applied to the main switch Q1, meaning it is turned on when the drain-source voltage of the main switch Q1 is zero. This results in low turn-on losses and low stress on the secondary side of the main switch Q1. This invention combines soft drive and zero-voltage drive to drive the main switch Q1, which can ensure low switching loss, high working efficiency and low stress on the secondary rectifier tubes (including diodes, MOSFETs and other transistors) under various operating conditions, and facilitates device selection.

[0074] Optionally, the zero-voltage switch can be at least one of the following: auxiliary switch Q2, synchronous rectifier (not shown) connected to the secondary winding Ns, switch (not shown) connected to the auxiliary winding in transformer TR, or switch (not shown) connected in parallel with auxiliary switch Q2.

[0075] refer to Figure 3 , Figure 3 It shows Figure 1 A schematic diagram of the timing waveforms of a portion of the flyback converter under the first operating condition.

[0076] like Figure 3 As shown, the operating condition detection unit 210 detects that the flyback converter 100 is in an undriven state during the time period t1-t2. At this time, the operating condition detection unit 210 outputs a first trigger signal to trigger the flyback converter 100 to enter the soft drive mode. That is, the soft drive control unit 220 is triggered to perform soft drive on the main switch Q1 in the first two (or other numbers) switching cycles after the flyback converter 100 restarts drive. For example, by adjusting the rising slope of the drive signal Vgs1, the turn-on speed of the main switch Q1 is reduced, so that the voltage across the secondary rectifier unit D1 can change at a slower rate, thereby effectively reducing the stress on the secondary rectifier tube. After the soft drive is completed, the operating condition detection unit 210 outputs a second trigger signal to trigger the flyback converter 100 to enter the zero-voltage turn-on drive mode. That is, the zero-voltage turn-on control unit 230 is triggered to generate a zero-voltage turn-on drive signal V_ZVS before the main switch Q1 is turned on in the subsequent switching cycle, and control the zero-voltage switch to turn on for an additional time. The zero-voltage turn-on of the main switch Q1 is used to reduce the stress of the secondary rectifier in steady state.

[0077] Furthermore, in some embodiments, under light load, the operating condition detection unit 210 is configured to continuously output a second trigger signal when it detects that the flyback converter 100 is operating in DCM mode and has reached a steady state, so as to control the flyback converter 100 to always or as close as possible to zero voltage turn-on drive mode under steady state, so as to reduce losses.

[0078] In other embodiments, under light load, the operating condition detection unit 210 is configured to output a first trigger signal when the load is detected to be less than a preset load threshold in DCM mode, triggering the flyback converter 100 to enter soft drive mode and select soft drive for the main switch Q1; while when the load is detected to be greater than the preset load threshold, it outputs a second trigger signal to trigger the flyback converter 100 to enter zero voltage turn-on drive mode and select zero voltage turn-on drive for the main switch Q1, thereby reducing control complexity.

[0079] For example, the load can be determined to be greater than or less than the preset load threshold by comparing the switching frequency of the flyback converter 100 with the preset frequency threshold. For instance, when the switching frequency of the flyback converter 100 is detected to be greater than the preset frequency threshold, the load is determined to be greater than the preset load threshold; when the switching frequency of the flyback converter 100 is detected to be less than the preset frequency threshold, the load is determined to be less than the preset load threshold.

[0080] In some embodiments, the switching frequency of the flyback converter 100 can be determined by comparing the turn-off time (denoted as toff) of the main switch Q1 in each switching cycle with a preset time threshold. For example, when the turn-off time toff of the main switch Q1 in each switching cycle is detected to be less than the preset time threshold, it can be determined that the switching frequency of the flyback converter 100 is greater than the preset frequency threshold. In this case, the main switch Q1 is soft-driven in the next switching cycle without zero-voltage turn-on control. When the turn-off time toff of the main switch Q1 in each switching cycle is detected to be greater than the preset time threshold, it can be determined that the switching frequency of the flyback converter 100 is less than the preset frequency threshold. In this case, the main switch Q1 is driven to turn on in the next switching cycle. In this way, the stress on the secondary rectifier under light load can be controlled, and the system's requirements for power consumption and control complexity under light load can be better balanced.

[0081] refer to Figure 4 , Figure 4 It shows Figure 1 A schematic diagram of the timing waveforms of a portion of the flyback converter under the second operating condition.

[0082] like Figure 4As shown, if the operating condition detection unit 210 detects a change in the load of the flyback converter 100 (such as load reduction) during the time period t1-t2, the operating condition detection unit 210 will output a first trigger signal during the dynamic process of load change, triggering the flyback converter 100 to enter the soft drive mode. That is, the soft drive control unit 220 will perform soft drive on the main switch Q1 for at least one switching cycle after the flyback converter 100 starts driving. For example, by adjusting the rising slope of the drive signal Vgs1, the turn-on speed of the main switch Q1 can be reduced, so that the voltage across the secondary rectifier unit D1 can change at a slower rate, thereby effectively reducing the stress on the secondary rectifier tube. After detecting that the flyback converter 100 has returned to steady state, the switching frequency of the flyback converter 100 is detected to be lower than a predetermined frequency threshold by detecting whether the turn-off time of the main switch Q1 or the auxiliary switch Q2 is greater than a predetermined threshold in each switching cycle under steady state. If the turn-off time tooff of the main switch Q1 is detected to be greater than the predetermined threshold in a certain switching cycle, the operating condition detection unit 210 continues to output the first trigger signal, and the soft drive control unit 220 performs soft drive on the main switch Q1 in the next switching cycle to reduce the turn-on speed of the main switch Q1, so that the voltage across the secondary rectifier unit D1 can change at a slower rate, thereby effectively reducing the stress on the secondary rectifier tube. If the off-time tooff of the main switch Q1 in a certain switching cycle is not greater than a predetermined threshold, the operating condition detection unit 210 outputs a second trigger signal. The zero-voltage turn-on control unit 230 generates a zero-voltage turn-on drive signal V_ZVS in the next switching cycle before the main switch Q1 is turned on, and controls the zero-voltage switch to turn on for an additional first time. The zero-voltage turn-on of the main switch Q1 is used to reduce the stress on the secondary rectifier in steady state.

[0083] Under light load, the flyback converter 100 may also operate in hiccup mode. Furthermore, the operating condition detection unit 210 can also be configured to, when detecting that the flyback converter 100 is operating in hiccup mode with multiple consecutive hiccup cycles, select the main switch Q1 for soft drive in the first M switching cycles of each hiccup cycle after the first hiccup cycle. Each hiccup cycle includes N switching cycles, where M and N are both positive integers, and 1 ≤ M < N.

[0084] refer to Figure 5 , Figure 5 It shows Figure 1 A schematic diagram of the timing waveforms of a portion of the flyback converter under the third operating condition.

[0085] like Figure 5As shown, the operating condition detection unit 210 detects, for example, that the flyback converter 100 is operating in hiccup mode at time t1 or during the time period t1-t2, where the time period t1-t2 corresponds to the first hiccup cycle. At this time, the operating condition detection unit 210 outputs a first trigger signal, triggering the drive control unit 220 to start from the second hiccup cycle (e.g., starting from time t2), and in at least one subsequent hiccup cycle, during the first M switching cycles of each hiccup cycle ( Figure 5 The diagram shows the case where M=1, but it's understood that M can also be set to other values. Soft-driving the main switch Q1 reduces its turn-on speed, allowing the voltage across the secondary rectifier unit D1 to change at a slower rate, thus reducing stress on the secondary rectifier. In other switching cycles of the hiccup mode, the main switch Q1 is driven normally.

[0086] Combination Figure 3-5 It is understood that this application, by introducing a control scheme combining soft drive and zero-voltage turn-on drive into the flyback converter 100, can ensure that the drain-source voltage V of the main switch Q1 is turned on under various operating conditions of the flyback converter 100. DS_Q1 The voltage can change at a slower rate, allowing the voltage on the secondary rectifier unit to rise at a slower rate as well, thereby effectively reducing the stress on the secondary rectifier tube under all operating conditions.

[0087] Furthermore, this application also provides a control method for a flyback converter, which can be applied to the flyback converter 100 and its control circuit 110 disclosed in any of the foregoing embodiments. In specific implementation, as follows... Figure 6 As shown, the control method includes performing the following steps:

[0088] 610. Perform operating condition testing on the flyback converter.

[0089] 620. When it is detected that the flyback converter is in an undriven state for a predetermined period of time, the main switch is soft-driven for at least one consecutive switching cycle after the drive is started, so as to reduce the rate of change of voltage at the coupling node between the main switch and the transformer.

[0090] 630. When the flyback converter is detected to be operating in steady state, select either soft drive or zero-voltage turn-on drive for the main switch transistor according to the load size.

[0091] For example, when the flyback converter is detected to meet any of the following operating conditions, it can be determined that the flyback converter is in a non-driven state for a predetermined period of time: the flyback converter is detected to be in a power-on state; the flyback converter is detected to be in a dynamic state of sudden load change; the flyback converter is detected to be operating in a hiccup mode; the flyback converter is detected to be operating in a sleep mode.

[0092] Optionally, the zero-voltage switch in step 630 may include at least one of the following: an auxiliary switch connected in series with the main switch between the input voltage terminal and the reference ground; a synchronous rectifier connected to the secondary winding of the transformer in the flyback converter; a first switch connected to the auxiliary winding of the transformer in the flyback converter; and a second switch connected in parallel with the auxiliary switch.

[0093] In steps 620 and 630, soft driving can be achieved by reducing the turn-on speed of the main switch in at least one consecutive switching cycle after the drive is started, thereby reducing the rate of voltage change at the coupling node between the main switch and the transformer. In some embodiments, during soft driving, the turn-on speed of the main switch in at least one consecutive switching cycle after the drive is started is reduced by extending the time for the drive signal of the main switch to change from an invalid level to an effective level.

[0094] In step 630, the zero-voltage turn-on drive of the main switch can be achieved by controlling the zero-voltage switch to turn on additionally for the first time before the main switch is turned on.

[0095] In DCM mode, in some embodiments, step 630 further includes: when the flyback converter is detected to be operating in steady state, performing zero-voltage turn-on drive on the main switch in each switching cycle under steady state.

[0096] In DCM mode, in some embodiments, step 630 further includes: selecting soft drive for the main switch when the load is less than a preset load threshold, and selecting zero-voltage turn-on drive for the main switch when the load is greater than the preset load threshold. For example, when the switching frequency of the flyback converter is detected to be greater than a preset frequency threshold, it is determined that the load is greater than the preset load threshold; when the switching frequency of the flyback converter is detected to be less than the preset frequency threshold, it is determined that the load is less than the preset load threshold.

[0097] Furthermore, when the off-time of the main switch in each switching cycle is detected to be less than a preset time threshold, the switching frequency of the flyback converter is determined to be greater than a preset frequency threshold; when the off-time of the main switch in each switching cycle is detected to be greater than a preset time threshold, the switching frequency of the flyback converter is determined to be less than a preset frequency threshold.

[0098] When the flyback converter operates in a hiccup mode with multiple consecutive hiccup cycles, step 630 further includes: selecting the main switch to be soft-driven in the first M switching cycles of each hiccup cycle after the first hiccup cycle, where each hiccup cycle includes N switching cycles, M and N are both positive integers, and 1≤M<N.

[0099] In summary, the flyback converter control scheme disclosed in this application adopts a combination of soft drive mode and zero-voltage turn-on drive mode to drive and control the flyback converter. This effectively reduces the stress on the secondary rectifier diodes even under special operating conditions (such as when the drive is stopped for a long time and then restarted), ensuring that the system has low stress on the secondary rectifier diodes over a wide output voltage range and under all operating conditions. The control method is simple.

[0100] Furthermore, after the flyback converter operates in steady state, under light load conditions, a soft drive (with a lower turn-on speed) can be selected to turn on the main switch, and the turn-on loss will not be significant due to the light load. Under heavy load conditions, a zero-voltage turn-on drive (with a faster turn-on speed) can be selected to turn on the main switch, resulting in a faster turn-on speed and lower turn-on loss.

[0101] Finally, it should be noted that the above embodiments are merely examples for clearly illustrating this application and are not intended to limit the implementation. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this application.

Claims

1. A control method for a flyback converter, the flyback converter comprising a transformer and a main switching transistor, The control method includes: The flyback converter is subjected to operating condition monitoring; When it is detected that the flyback converter is in an undriven state for a predetermined period of time, the main switch is soft-driven for at least one consecutive switching cycle after the drive is started, so as to reduce the rate of change of voltage at the coupling node between the main switch and the transformer. When the flyback converter is detected to be operating in a steady state, the main switch is either soft-driven or zero-voltage turn-on driven according to the load size.

2. The control method according to claim 1, wherein, The soft drive is achieved by reducing the turn-on speed of the main switch in at least one consecutive switching cycle after the start-up drive, so as to reduce the rate of voltage change at the coupling node between the main switch and the transformer.

3. The control method according to claim 1, wherein, When the flyback converter is detected to meet any of the following operating conditions, it can be determined that the flyback converter is in a non-driven state for a predetermined period of time: The flyback converter was detected to be in power-on mode; The flyback converter was detected to be in a dynamic operating condition with a sudden load change; The flyback converter was detected to be operating in hiccup mode; The flyback converter was detected to be operating in sleep mode.

4. The control method according to claim 2, wherein, In soft-drive mode, by extending the time it takes for the drive signal of the main switch to change from an invalid level to an effective level, the turn-on speed of the main switch in at least one consecutive switching cycle after the drive is started is reduced.

5. The control method according to claim 1, wherein, In DCM mode, when the flyback converter is detected to be operating in steady state, the main switch is driven to turn on at zero voltage in each switching cycle under steady state.

6. The control method according to claim 1, wherein, In DCM mode, when the load is less than a preset load threshold, the main switch is selected to be soft-driven, and when the load is greater than the preset load threshold, the main switch is selected to be zero-voltage turn-on driven.

7. The control method according to claim 6, wherein, When the switching frequency of the flyback converter is detected to be greater than a preset frequency threshold, it is determined that the load is greater than a preset load threshold. When the switching frequency of the flyback converter is detected to be less than a preset frequency threshold, it is determined that the load is less than a preset load threshold.

8. The control method according to claim 7, wherein, When the off time of the main switch in each switching cycle is less than a preset time threshold, it is determined that the switching frequency of the flyback converter is greater than a preset frequency threshold. When the off-time of the main switch in each switching cycle is detected to be greater than a preset time threshold, it is determined that the switching frequency of the flyback converter is less than a preset frequency threshold.

9. The control method according to claim 1, wherein, When the flyback converter operates in a hiccup mode with multiple consecutive hiccup cycles, the main switch is selected to be soft-driven in the first M switching cycles of each hiccup cycle after the first hiccup cycle. Each hiccup cycle includes N switching cycles, where M and N are both positive integers and 1≤M<N.

10. The control method according to any one of claims 1-9, wherein, The flyback converter also includes a zero-voltage switch; The zero-voltage turn-on drive of the main switch is achieved by controlling the zero-voltage switch to turn on additionally for a first time before the main switch is turned on.

11. The control method according to claim 10, wherein, The zero-voltage switching transistor includes at least one of the following: An auxiliary switching transistor is connected in series with the main switching transistor between the input voltage terminal and the reference ground; A synchronous rectifier tube connected to the secondary winding of the transformer in the flyback converter; The first switching transistor is connected to the auxiliary winding of the transformer in the flyback converter; A second switching transistor connected in parallel with the auxiliary switching transistor.

12. A control circuit for a flyback converter, the flyback converter comprising a transformer and a main switching transistor. The control circuit includes: The operating condition detection unit performs operating condition detection on the flyback converter, and outputs a first trigger signal when it detects that the flyback converter is in a no-drive state for a predetermined period of time, and outputs one of the first trigger signal and the second trigger signal according to the load size when it detects that the flyback converter is operating in a steady state. The soft drive control unit, upon receiving the first trigger signal, performs soft drive on the main switch transistor to reduce the rate of voltage change at the coupling node between the main switch transistor and the transformer. The zero-voltage turn-on control unit drives the main switch transistor to turn on at zero voltage when it receives the second trigger signal.

13. A flyback converter, comprising: The control circuit as described in claim 12 of this application.

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