Flyback conversion circuit and control method and control chip thereof

Abstract

The embodiment of the invention provides a flyback conversion circuit, a control method thereof and a control chip. The flyback conversion circuit comprises a primary side unit, a secondary side unit and an auxiliary unit, the primary side unit comprises a main switch tube and a primary side winding, the first end of the primary side winding is electrically connected with a power supply, the second end of the primary side winding is electrically connected with the first end of the main switch tube, and the second end of the main switch tube is electrically connected with reference ground; the secondary side unit comprises a secondary side winding; the auxiliary unit comprises an auxiliary switch tube, a first capacitor and an auxiliary winding; the first end of the auxiliary winding is electrically connected with the first end of the first capacitor, the second end of the first capacitor is electrically connected with the first end of the auxiliary switch tube, and the second end of the auxiliary switch tube is electrically connected with the second end of the auxiliary winding; the second end of the auxiliary winding is electrically connected with the reference ground; the second end of the primary winding, the first end of the auxiliary winding and the first end of the secondary winding are dotted terminals. The flyback conversion circuit can realize zero-voltage switching in different working modes, so that the working efficiency is improved.

Description

Technical Field

[0001] This invention relates to the field of switching circuit control, and more particularly to a flyback converter circuit and its control method and control chip. Background Technology

[0002] Quasi-resonant flyback converters (QR flyback) enable valley-level turn-on of the primary-side power switches, significantly reducing switching losses and becoming the mainstream topology for low-power switching power supplies. However, with the trend towards higher frequencies, turn-on losses increase significantly under high-voltage input conditions, severely impacting system efficiency. To address this issue, the industry has proposed several novel topologies, including active clamp flyback, asymmetrical half-bridge flyback, zero-voltage flyback (ZVS flyback), and flyback circuits that achieve ZVS (Zero Voltage Switching) through secondary-side synchronous rectification. However, the drive circuits and control methods corresponding to these four flyback circuits for achieving zero-voltage switching are relatively complex. More importantly, all four circuits achieve zero-voltage switching by turning the current flowing through the magnetizing inductor into a negative direction, thereby discharging the junction capacitance of the main switch transistor. Therefore, this method can only be implemented in critical mode or discontinuous mode. When entering continuous mode, the current in the magnetizing inductor cannot turn into a negative direction, and zero-voltage switching will not be possible. Therefore, achieving zero-voltage switching by turning the current in the magnetizing inductor into a negative direction is not a universal solution. Summary of the Invention

[0003] In view of this, embodiments of the present invention provide a flyback converter circuit and its control method and control chip. The flyback converter circuit can achieve zero-voltage switching in different operating modes, thereby improving the circuit's operating efficiency.

[0004] To solve the above-mentioned technical problems, this application adopts the following technical solution: A flyback converter circuit includes a primary-side unit, a secondary-side unit, and an auxiliary unit; The primary-side unit includes a main switch and a primary-side winding. The first end of the primary-side winding is electrically connected to the power supply, the second end of the primary-side winding is electrically connected to the first end of the main switch, and the second end of the main switch is electrically connected to a reference ground. The secondary-side unit includes a secondary-side winding, and the first end of the secondary-side winding can be electrically connected to the high-potential end of the load. The auxiliary unit is used to provide conditions for zero-voltage switching of the main switching transistor; the auxiliary unit includes an auxiliary switching transistor, a first capacitor, and an auxiliary winding; a first end of the auxiliary winding is electrically connected to a first end of the first capacitor, a second end of the first capacitor is electrically connected to a first end of the auxiliary switching transistor, a second end of the auxiliary switching transistor is electrically connected to a second end of the auxiliary winding; the second end of the auxiliary winding is electrically connected to the reference ground; The second end of the primary winding, the first end of the auxiliary winding, and the first end of the secondary winding are all the same name.

[0005] The flyback converter circuit includes an auxiliary unit, which includes an auxiliary switch, a first capacitor, and an auxiliary winding. By controlling the auxiliary switch, the main switch achieves ZVS, thereby improving the circuit's operating efficiency.

[0006] A control method applicable to the flyback converter circuit described above; the control method includes at least the following steps: Before the main switch of the flyback converter circuit is turned on, the auxiliary switch of the flyback converter circuit is turned on, so that the junction capacitance voltage of the main switch is reduced. When the junction capacitance voltage of the main switch decreases, the main switch is then turned on. Once the main switch is turned on, the auxiliary switch is then turned off.

[0007] This control method reduces the junction capacitance voltage of the main switch by controlling the auxiliary switch to turn on before the main switch is turned on, thus creating conditions for zero-voltage switching of the main switch. This facilitates the realization of zero-voltage switching of the main switch and improves working efficiency.

[0008] A control chip, suitable for the aforementioned flyback converter circuit, the control chip includes a CS terminal, an FB terminal, a ZCD terminal, a GD1 terminal, and a GD2 terminal. The CS terminal is a current sampling pin used to control the peak current. The FB terminal is a negative feedback pin, used to receive feedback voltage and adjust the output power of the system; The ZCD terminal is a trough detection pin, used to achieve trough locking and output voltage detection; The GD1 terminal is the drive output pin of the main switch in the flyback circuit; the GD2 terminal is the drive output pin of the auxiliary switch in the auxiliary unit. The control chip is also used to execute the control method described above.

[0009] This control chip can control the flyback converter circuit mentioned above, enabling the main switch to achieve zero-voltage conduction and improving working efficiency. Attached Figure Description

[0010] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort, but they should fall within the protection scope of this application.

[0011] Figure 1 This is a schematic diagram of an active clamp flyback circuit in the prior art; Figure 2 A schematic diagram of a flyback converter circuit provided in an embodiment of this application; Figure 3 for Figure 2 The equivalent circuit diagram of the flyback converter circuit when the main switch is turned on is shown. Figure 4 for Figure 2 The equivalent circuit diagram of the flyback converter circuit when the main switch is turned off is shown. Figure 5 for Figure 2 The equivalent circuit diagram of the flyback converter circuit is shown when the main switch is off and the auxiliary switch is on. Figure 6 A schematic diagram of a flyback converter circuit for a multiplexing auxiliary unit provided in one embodiment of this application; Figure 7 This is a schematic diagram of another flyback converter circuit provided in another embodiment of this application; Figure 8 This is a schematic diagram of another auxiliary unit circuit provided in another embodiment of this application; Figure 9 This is a schematic diagram of another flyback converter circuit provided in another embodiment of this application; Figure 10 This is a schematic diagram of another flyback converter circuit provided in another embodiment of this application; Figure 11 A flowchart of the flyback converter circuit control method provided in the embodiments of this application; Figure 12 A schematic diagram of the control waveforms of the main switch and the auxiliary switch; Figure 13 In continuous mode Figure 10 The diagram shows the main waveforms of the flyback converter circuit. Figure 14 In intermittent mode Figure 10 The diagram shows the main waveforms of the flyback converter circuit. Detailed Implementation

[0012] To better understand the technical solution of the present invention, the embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0013] It should be understood that the described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0014] The terminology used in the embodiments of this invention is for the purpose of describing particular embodiments only and is not intended to limit the invention. The singular forms “a,” “the,” and “the” as used in the embodiments of this invention and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. Electrical connection primarily refers to the transmission of signals, including direct and indirect electrical connections.

[0015] It should be understood that the term "and / or" used in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, the character " / " in this article generally indicates that the preceding and following related objects have an "or" relationship.

[0016] Common flyback circuits with ZVS functionality include active clamp flyback, asymmetrical half-bridge flyback, zero-voltage flyback (ZVS flyback), and flyback circuits that achieve ZVS (Zero Voltage Switching) through secondary-side synchronous rectification. These circuits all achieve ZVS by turning the current flowing through the magnetizing inductor into a negative direction and are not suitable for continuous mode. Figure 1 Taking the active clamp flyback circuit shown as an example, its principle for achieving ZVS is as follows: its transformer leakage inductance L m Energy first passes through the clamping capacitor C clamp Stored, and then controlled by switching transistor Q. 02 Releasing the stored energy in the capacitor causes the magnetizing inductor current to become negative. When a sufficient negative current is reached, Q is controlled. 02 When the switch is turned off, the negative magnetizing inductor current discharges through the junction capacitance of the switch Q1, enabling the switch Q1 to achieve ZVS (Zero-Voltage Switching), thereby improving efficiency.

[0017] Based on this, this application provides a flyback converter circuit that achieves ZVS without needing to reverse the current in the magnetizing inductor, offering versatility applicable in continuous, critical, and discontinuous modes. Specifically, as... Figure 2As shown, the flyback converter circuit includes a primary-side unit 10 and a secondary-side unit 20. The primary-side unit 10 includes a main switch Q1 and a primary winding Np. The first terminal of the primary winding Np is electrically connected to the power supply Vin, and the second terminal of the primary winding Np is electrically connected to the first terminal of the main switch Q1. The second terminal of the main switch Q1 is electrically connected to the reference ground GND. The secondary-side unit 20 includes a secondary winding Ns, the first terminal of which can be electrically connected to the high-potential terminal of the load. The working principle of the primary and secondary units is the conventional flyback converter principle. Simply put, by controlling the main switch Q1 of the primary-side unit 10 to conduct, the transformer is energized, storing energy in the transformer T. Then, by controlling the main switch Q1 to turn off, the energy in the transformer T is transferred to the secondary-side unit. To achieve the ZVS function of the main switch, the flyback converter circuit provided in this embodiment also includes an auxiliary unit 30.

[0018] Specifically, the auxiliary unit 30 includes an auxiliary switch Q2, a first capacitor C1, and an auxiliary winding Na; the first end of the auxiliary winding Na is electrically connected to the first end of the first capacitor C1, the second end of the first capacitor C1 is electrically connected to the first end of the auxiliary switch Q2, the second end of the auxiliary switch Q2 is electrically connected to the second end of the auxiliary winding Na; and the second end of the auxiliary winding Na is electrically connected to the reference ground GND. The second end of the primary winding Ns, the first end of the auxiliary winding Na, and the first end of the secondary winding Ns are all terminals with the same name.

[0019] This application improves the converter's efficiency by adding an auxiliary unit 30 to a traditional flyback converter circuit to achieve ZVS (Zero Switching) for the main switch Q1, and is applicable to continuous mode. In continuous mode, if the system remains in a hard-on state, it will cause significant switching losses and greatly reduce efficiency. This application achieves ZVS in continuous mode by adding the auxiliary unit 30, which requires no external power supply. Switching devices, such as MOSFETs, typically have a body diode and a junction capacitance. For ease of analysis, as shown... Figure 2 As shown, the body diode and junction capacitance of the main switch are denoted as D1 and Coss, respectively, and the body diode and junction capacitance of the auxiliary switch are denoted as D2 and C2, respectively. The specific working principle is as follows: When the main switch Q1 is turned on, its equivalent circuit is as follows: Figure 3 As shown, Lk is the leakage inductance of the transformer, Lm is the magnetizing inductance, and the input voltage of the power supply Vin is denoted as V. in The red arrow indicates the direction of the magnetizing current. At this time, Lk and Lm are jointly magnetized, and the secondary diode Do is in the off state due to reverse bias (Do is blurred in the diagram to indicate it is in the off state). At this time, the body diode of the auxiliary switch is turned on, so the first capacitor C1 is in a state of positive at the bottom and negative at the top. Ignoring the effect of leakage inductance, its voltage is V. in / Np *N a , where N p N represents the number of turns in the primary winding. a The number of turns for the auxiliary winding.

[0020] like Figure 4 As shown, when the main switch Q1 is off, the secondary diode conducts because the magnetizing current in the magnetizing inductor Lm is not zero, and the transformer transfers energy to the secondary unit. At this time, the auxiliary diode Q2 is in the off state (Q2 is represented as being in the off state by being virtualized), therefore the voltage across the first capacitor C1 remains unchanged. With the main switch Q1 in the off state, its voltage is the sum of the input voltage and the reflected voltage of the secondary winding, i.e., V0. in +V o / N s *N p V o N is the output voltage of the secondary unit. p N represents the number of turns in the primary winding. s This represents the number of turns in the secondary winding. The direction of the red arrow indicates the direction of the demagnetizing current.

[0021] When the transformer energy transfer cycle ends, that is, when the main switch Q1 turns on, the auxiliary switch Q2 is turned on. At this time, the secondary diode Do is in the off state. Figure 5 As shown, at this time, the voltage across the magnetizing inductor Lm becomes positive at the top and negative at the bottom, while the voltage across the leakage inductance Lk becomes positive on the right and negative on the left. Considering the magnetizing inductance is very large, its influence is ignored. At this time, a high-frequency resonance will occur between the junction capacitance Coss of the main switch and Lk. The junction capacitance of Q1 discharges. When the junction capacitance voltage of Q1 becomes sufficiently low, the main switch Q1 is turned on, thus achieving zero-voltage turn-on, i.e., realizing the ZVS function of the main switch. After the main switch Q1 is effectively turned on, the auxiliary switch Q2 is turned off. The main switch Q1 and the auxiliary switch Q2 will have a period of simultaneous conduction. The voltage relationship expression in the circuit at this time is as follows:

[0022] Among them, V C1 V is the voltage across the first capacitor. Lk V is the voltage across the leakage inductance Lk. ds The voltages at the first and second terminals of the main switch are given. Using the above voltage relationship, the voltage across the leakage inductance can be calculated, allowing for the calculation of the discharge current of the main switch's junction capacitance Coss. This provides a theoretical basis for the debugging and optimization of the ZVS function.

[0023] In summary, this application achieves ZVS for the main switch Q1 by adding an auxiliary unit and specifically controlling the main and auxiliary switches to make the junction capacitance and leakage inductance of Q1 resonate, consuming the charge on the junction capacitance of Q1 and reducing the voltage of the junction capacitance. This does not require the current in the magnetizing inductor Lm to become negative. Regardless of the direction of the current in Lm, the resonance between the junction capacitance Coss of Q1 and Lk can discharge the voltage at the terminal of Coss, thus creating conditions for Q1 to achieve ZVS conduction. This method is applicable not only to critical mode and discontinuous mode, but also to ZVS implementation in continuous mode.

[0024] It should be noted that the leakage inductance Lk and the magnetizing inductance Lm are only schematic diagrams in the circuit schematic and do not necessarily mean that the device is actually included. They can be implemented by designing a transformer, or of course, by using an inductor. This application does not limit this.

[0025] When Q1 is turned on, due to the configuration of the primary and auxiliary windings with the same terminal, the body diode D2 of the auxiliary switch is turned on, and the first capacitor C1 stores energy, providing the resonant voltage when the subsequent junction capacitance Coss resonates with Lk. In one embodiment, a diode connected in reverse parallel can also be provided for the auxiliary switch to provide a path for the energy stored in the first capacitor C1.

[0026] Furthermore, in one embodiment, such as Figure 10 As shown, the auxiliary unit 30 also includes a first resistor R1 and a second resistor R2. The first resistor R1 and the second resistor R2 are connected in series and then in parallel with the auxiliary winding Na. R1 and R2 are used to obtain the system trough, that is, Figure 10 In the embodiment shown, the auxiliary winding Na is divided by a first resistor R1 and a second resistor R2 to obtain the divided voltage V. zcd And based on this voltage divider V zcd Obtaining trough information. In this embodiment, the excitation current information V is obtained by sampling using the sampling resistor Rcs. cs This is used to control the turn-off signal of the main switch Q1; and to obtain the system's trough information and output voltage information by using the first resistor R1 and the second resistor R2 for voltage division; based on the negative feedback signal V generated by the feedback network... fb By continuously adjusting the peak current of the flyback converter circuit and the frequency of the switching transistors, drive signals G for Q1 and Q2 are generated. Q1 and G Q2 This enables power regulation and achieves ZVS for Q1.

[0027] More specifically, in one embodiment, both the main switch and the auxiliary switch can be MOSFETs. When the main switch is a MOSFET, its first terminal is the drain and its second terminal is the source. When the auxiliary switch is a MOSFET, its first terminal is the drain and its second terminal is the source. In this case, the reference ground of the auxiliary winding Na is connected to the source of the MOSFET Q2. Unlike the conventional zero-voltage flyback (ZVS) circuit connection, this allows for ZVS in continuous mode.

[0028] Furthermore, in one embodiment, to reuse hardware, the auxiliary unit can also be reused as an auxiliary power supply to provide auxiliary voltage. The circuit design contains many active devices, such as control chips, operational amplifiers, and controllable switching transistors. This auxiliary voltage can provide operating voltage for these active devices. In this way, the auxiliary winding and auxiliary power supply winding of the main switching transistor ZVS are combined into one, avoiding the need for additional windings and increased costs. A detailed circuit diagram of this embodiment is shown below. Figure 6 As shown, the auxiliary unit 30 also includes a fifth diode D5 and a fifth capacitor C5; the anode of the fifth diode D5 is electrically connected to the second terminal of the first capacitor C1, the first terminal of the fifth capacitor C5 is electrically connected to the cathode of the fifth diode D5, and the second terminal of the fifth capacitor C5 is electrically connected to the reference ground GND; the cathode of the fifth diode D5 is the auxiliary voltage output terminal, which is electrically connected to the active device in the circuit design to provide the working voltage.

[0029] Furthermore, in one embodiment, to further improve efficiency, an auxiliary circuit is added for implementing ZVS of the main switch transistor, specifically, as follows: Figure 7 As shown, the auxiliary unit also includes a third switch Q3 (the body diode and junction capacitance of Q3 are denoted as D3 and C3, respectively), a sixth capacitor C6, and a second auxiliary winding Na2. The first terminal of the second auxiliary winding Na2 is electrically connected to the first terminal of the sixth capacitor C6, the second terminal of the sixth capacitor C6 is electrically connected to the first terminal of the third switch Q3, and the second terminal of the third switch Q3 is electrically connected to the second terminal of the second auxiliary winding Na2. The second terminal of the second auxiliary winding Na2 is electrically connected to reference ground GND. The first terminals of the second auxiliary winding Na2 and the first terminal of the auxiliary winding Na are identical. The number of turns in the auxiliary winding Na is greater than the number of turns in the second auxiliary winding Na2. The control method for the third switch is the same as that for the auxiliary switch, and the circuit principle is also the same. The difference lies in the different operating periods of the third and auxiliary switches. The auxiliary circuit containing the auxiliary switch operates during periods of lower power supply voltage, while the auxiliary circuit containing the third switch operates during periods of higher power supply voltage. That is, the power supply voltage is lower when the auxiliary switch is operating than when the third switch is operating.

[0030] In this embodiment, the auxiliary switch Q2 or the third switch Q3 is controlled to operate according to the power supply voltage of the flyback converter circuit. Since the conduction of Q2 or Q3 is not lossless, to further reduce the conduction loss caused by the auxiliary switch, it is assumed that the number of turns in the auxiliary winding Na is greater than the number of turns in the second auxiliary winding Na2. When the power supply input is high voltage, the voltage of the first capacitor C1 is greater than the voltage of the sixth capacitor C6. At this time, the loss of turning on Q2 is slightly greater than that of Q3. Therefore, to achieve ZVS of the main switch, the auxiliary circuit containing Q3 is controlled to operate to reduce losses and improve efficiency. When the power supply input is low voltage, because the switching loss of Q2 is relatively small, to avoid the influence of line damping and parasitic resistance, which would prevent the junction capacitance Coss of the main switch from fully resonating and discharging, the ZVS effect is weakened. At this time, the auxiliary circuit containing Q2 is controlled to operate to ensure the ZVS effect of the main switch.

[0031] Furthermore, the auxiliary winding and the second auxiliary winding can be implemented using a push-pull winding configuration, specifically, as shown below. Figure 8 As shown, the auxiliary winding is equipped with a center tap, which divides the auxiliary winding into a third winding Na3 and a fourth winding Na4. The auxiliary unit 30 also includes a seventh capacitor C7 and a fourth switch Q4. The first terminal of the seventh capacitor C7 is electrically connected to the center tap, the second terminal of the seventh capacitor C7 is electrically connected to the first terminal of the fourth switch Q4, and the second terminal of the fourth switch Q4 is electrically connected to the reference ground GND. In this embodiment, because the number of turns of the winding between the first terminal of the auxiliary winding and the reference ground is greater than the number of turns of the coil between the center tap and the reference ground, when the power supply input is high voltage, the voltage of the first capacitor C1 is greater than the voltage of the seventh capacitor C7. At this time, the loss of turning on Q2 is slightly greater than that of Q4. Therefore, in order to achieve ZVS of the main switch, the auxiliary circuit where Q4 is located is controlled to work to reduce losses and improve efficiency. When the power supply input is low voltage, because the switching loss of Q2 is relatively small, in order to avoid the influence of line damping and parasitic resistance, the junction capacitance Coss of the main switch cannot fully resonate and discharge, and the ZVS effect is weakened. At this time, the auxiliary circuit where Q2 is located is controlled to work to ensure the ZVS effect of the main switch. Therefore, furthermore, the power supply voltage when the auxiliary switch Q2 is operating is set to be lower than the power supply voltage when the fourth switch Q4 is operating. The control method for the fourth switch is the same as that for the auxiliary switch, and the circuit principle is also the same. The difference lies in the different operating periods of the fourth switch and the auxiliary switch. When the power supply input is low voltage, the auxiliary circuit containing Q2 operates; when the power supply input is high voltage, the auxiliary circuit containing Q4 operates. This improves efficiency and enhances the ZVS effect of the main switch.

[0032] Based on the aforementioned flyback converter circuit, this application provides a control method for the auxiliary switch and the main switch to achieve the ZVS function of the main switch, such as... Figure 11 As shown, the auxiliary conduction steps include the following: Step S11: Control the auxiliary switch to turn on: Specifically, before controlling the main switch of the flyback converter circuit to turn on, control the auxiliary switch of the flyback converter circuit to turn on, so that the junction capacitance voltage of the main switch is reduced. Step S12: Control the main switch to turn on: Specifically, when the junction capacitance voltage of the main switch decreases, the main switch is then controlled to turn on, thereby achieving zero-voltage turn-on of the main switch, reducing the switching loss of the main switch, and improving circuit efficiency. Step S13: Control the auxiliary switch to turn off: Specifically, after the main switch is turned on, control the auxiliary switch to turn off.

[0033] The on-time of the main switch can be controlled based on the peak current. When the main switch is turned on, the magnetizing inductor enters a freewheeling state, transferring energy to the secondary unit. By introducing an auxiliary unit and the above control method, the auxiliary switch is controlled to turn on before the main switch is triggered, achieving resonance between the junction capacitance and leakage inductance of the main switch, causing a drop in the voltage at the junction capacitance of the main switch; then the main switch is turned on. Compared to directly controlling the main switch to turn on, the control method provided in this application reduces the junction capacitance voltage of the main switch, thus reducing its conduction loss and facilitating the realization of ZVS for the main switch. Furthermore, due to factors such as the switch drive delay, the auxiliary switch is not immediately turned off when the main switch is turned on, but is delayed for a period of time before being turned off, preventing the ZVS function from failing due to the delay in the main switch's turn-on. Specifically, it prevents the leakage inductance / magnetizing inductance and the main switch junction capacitance from continuing to resonate, causing the ZVS function to fail, if the auxiliary switch is turned off but the main switch is not turned on in time.

[0034] It should be noted that in conventional control methods, the condition for triggering the main switch to turn on can be that the number of system troughs reaches a preset value, or that the switching frequency of the main switch is limited. The control method provided in this application improves upon conventional control methods by first controlling the auxiliary switch to turn on before triggering the main switch, and then controlling the main switch to turn on, thereby achieving ZVS (Zero-Voltage Switching) of the main switch and improving efficiency. Therefore, the triggering condition for the auxiliary switch to turn on can also be the triggering condition for the main switch in conventional control methods, i.e., the number of system troughs reaches a preset value or the switching frequency of the main switch is limited, while the triggering condition for the main switch occurs after the auxiliary switch turns on, causing a decrease in the junction capacitance of the main switch.

[0035] Specifically, step S12: controlling the main switch to turn on again after the junction capacitance voltage of the main switch decreases includes the following steps: After the auxiliary switch is turned on, there is a first time delay t. d1 Then, control the main switch to turn on; wherein, during the first time period t d1During the corresponding time period, the junction capacitance of the main switch and the leakage inductance of the transformer in the flyback converter circuit resonate, causing the junction capacitance voltage of the main switch to decrease, thereby providing conditions for the main switch to achieve ZVS.

[0036] Combination Figure 12 The control waveforms of the main switch and auxiliary switch shown are illustrated. Time t1 is the trigger condition for the auxiliary switch. Specifically, this can be either the number of system troughs reaching a preset value or the switching frequency of the main switch Q1 reaching a limited value, at which point the auxiliary switch Q2 is triggered to conduct. After the auxiliary switch Q2 conducts, there is a first delay t. d1 Then control the main switch Q1 to turn on.

[0037] Among them, the first duration t d1 It can be a preset value, and with Positive correlation, further can be directly applied to the first duration t d1 Preset Among them, L k C is the leakage inductance value of the transformer. oss This refers to the junction capacitance value of the main switch transistor. Of course, in practical engineering applications, due to L... k and C oss It is nonlinear and its precise value is difficult to obtain. Therefore, it can also be adjusted according to the ZVS state of the main switch.

[0038] Furthermore, in one embodiment, step S13: after the main switch is turned on, controlling the auxiliary switch to turn off includes the step of: after the main switch is turned on, delaying for a second time period before controlling the auxiliary switch to turn off.

[0039] In this embodiment, as Figure 12 As shown, the conduction time t1 of the auxiliary switch Q2 is delayed by a first time t. d1 After reaching time t2, the main control switch is turned on, and then a second delay of t is made based on t2. d2 The time t3 is reached when the auxiliary switch Q2 is turned off. Second duration t d2 This setting prevents Q2 from turning off too early, which would cause the main switch junction capacitor to be charged by the current from the magnetizing inductor. This would result in a continuous increase in the voltage across the main switch junction capacitor, leading to ZVS failure. Furthermore, the second duration t... d2 The setting can also reduce the impact of the main switch turn-on delay. In this case, the second duration t d2 It is generally positively correlated with the turn-on delay of the main switch, and can be set to be the same, typically 50ns. The turn-on delay includes the turn-on delay of the switch itself, the delay of the drive circuit, etc.

[0040] Of course, the above control method can also be implemented through hardware circuits, such as... Figure 9As shown, in one embodiment, the flyback converter circuit further includes a driving unit 40; the driving unit includes at least a first driving circuit 41 and a second driving circuit 42; the driving unit 40 is used to receive the turn-on control signal of the auxiliary switch and transmit it to the control terminal of the auxiliary switch; the turn-on control signal is delayed by the first driving circuit to generate the turn-on control signal of the main switch and input to the control terminal of the main switch; the turn-on control signal of the main switch is passed through the second driving circuit to generate the turn-off control signal of the auxiliary switch, and the second driving circuit is at least used to delay the turn-on control signal of the main switch to obtain the turn-off control signal of the auxiliary switch.

[0041] Furthermore, in one embodiment, the delay is achieved through an RC delay circuit. In this embodiment, as... Figure 9 As shown, the first driving circuit 41 includes a current-limiting resistor Rx, a first delay resistor Rd1, and a first delay capacitor Cd1; the second driving circuit 42 includes a second delay resistor Rd2, a second delay capacitor Cd2, and a driving switch K; the delay circuit formed by the first delay resistor Rd1 and the first delay capacitor Cd1 generates a first duration t. d1 Adjust the turn-on delay of the main switch Q1. A second duration t is generated using a delay circuit consisting of the second delay resistor Rd2 and the second delay capacitor Cd2. d2 The turn-off delay of the auxiliary switch Q2 is adjusted. Specifically, one end of the first delay resistor Rd1 is used to receive the drive signal G of the auxiliary switch. Q2 Drive signal G Q2 This includes the turn-on control signal for the auxiliary switch; one end is electrically connected to the first terminal of the first delay capacitor Cd1, and the second terminal of the first delay capacitor Cd1 is electrically connected to the reference ground GND; the first terminal of the second delay resistor Rd2 is connected to the first terminal of the first delay capacitor Cd1, and the second terminal of the second delay resistor Rd2 is electrically connected to the first terminal of the second delay capacitor Cd2, and the second terminal of the second delay capacitor Cd2 is electrically connected to the reference ground GND; the control terminal of the drive switch K is electrically connected to the second terminal of the second delay resistor Rd2, and the first terminal of the drive switch K receives the drive signal G of the auxiliary switch Q2 through the current limiting resistor Rx. Q2 It is electrically connected to the control terminal of the auxiliary switch Q2, and the second terminal of the drive switch K is electrically connected to the reference ground GND.

[0042] Combination Figure 12 right Figure 9 The principle of the drive unit 40 is explained: drive signal G Q2 This includes the turn-on control signal for the auxiliary switch (assuming it's a high-level signal), which is generated at time t1. At time t1, the auxiliary switch Q2 is turned on, and after a delay of a first duration t by the first drive circuit 41... d1After reaching time t2, the high level drives the main switch Q1 to turn on, and then the second drive circuit 42 delays for a second duration t. d2 Then, at time t3, this high level turns on the driving switch K, which in turn turns the control terminal of the auxiliary switch Q2 low, turning Q2 off. The driving switch K can be a controllable switch such as a transistor or a field-effect transistor.

[0043] Of course, delay can also be achieved through other delay circuits or chips, and this application does not limit this.

[0044] Furthermore, when the flyback converter circuit adopts such as Figure 7 The circuit shown, that is, when the auxiliary unit of the flyback converter circuit includes a third switch, a sixth capacitor, and a second auxiliary winding, in order to improve efficiency and enhance the ZVS effect of the main switch, in one embodiment, the control method further includes the following steps: Determine whether the power supply voltage of the flyback converter circuit is greater than the preset value; If the power supply voltage is less than the preset value, then the above-mentioned auxiliary conduction steps are executed, namely steps S11-S13. If the power supply voltage is greater than the preset value, then the following steps are performed: Before the main switch of the flyback converter circuit is turned on, the third switch of the flyback converter circuit is turned on, so that the junction capacitance voltage of the main switch is reduced. When the junction capacitance voltage of the main switch decreases, the main switch is then turned on. Once the main switch is turned on, the third switch is then turned off.

[0045] In this embodiment, the operating time periods of the third switch and the auxiliary switch are distinguished by judging the power supply voltage level, so as to improve efficiency and enhance the ZVS effect of the main switch.

[0046] Furthermore, taking a power supply voltage range of 85V-265V as an example, the preset value can be set to 160V. In this embodiment, a power supply voltage greater than 160V is considered a high power input, and the third or fourth switching transistor works, achieving ZVS of the main switching transistor while having higher efficiency. A power supply voltage less than 160V is considered a low power input, and the auxiliary switching transistor works to ensure the ZVS effect of the main switching transistor.

[0047] Similarly, when the auxiliary unit of the flyback converter circuit adopts such as Figure 8 The circuit shown, that is, when the auxiliary unit includes a seventh capacitor and a fourth switching transistor and the auxiliary winding has a center tap, the control method further includes the following steps: Determine whether the power supply voltage of the flyback converter circuit is greater than the preset value; If the power supply voltage is less than the preset value, then the above-mentioned auxiliary conduction steps are executed, namely steps S11-S13. If the power supply voltage is greater than the preset value, then the following steps are performed: Before the main switch of the flyback converter circuit is turned on, the fourth switch of the flyback converter circuit is turned on, so that the junction capacitance voltage of the main switch is reduced. When the junction capacitance voltage of the main switch decreases, the main switch is then turned on. Once the main switch is turned on, the fourth switch is then turned off.

[0048] In this embodiment, the operating time periods of the fourth switch and the auxiliary switch are distinguished by judging the power supply voltage level, so as to improve efficiency and enhance the ZVS effect of the main switch.

[0049] To implement the above control method and control the main switching transistor and auxiliary switching transistor, embodiments of this application also provide a control chip, such as... Figure 10 As shown in the diagram. U1 is a control chip based on the aforementioned control method. This control chip includes at least the CS, FB, ZCD, GD1, and GD2 terminals. The CS terminal is a current sampling pin for peak current control; the FB terminal is a negative feedback pin used to receive feedback voltage and adjust the system's output power; the ZCD terminal is a trough detection pin used for trough locking and output voltage detection; the GD1 terminal is the drive output pin of the main switch in the flyback circuit; and the GD2 terminal is the drive output pin of the auxiliary switch in the auxiliary unit.

[0050] Furthermore, it is understandable that when the flyback converter circuit is like... Figure 7 The diagram also includes a third switch Q3 or, as shown, ... Figure 8 When the fourth switch Q4 is also included, the control chip U1 is correspondingly configured with the GD3 terminal, which serves as the drive output pin for either the third or fourth switch. Of course, since the auxiliary switch and the third / fourth switch operate in a time-sharing manner, the GD2 terminal can also be reused as the drive output pin for either the third or fourth switch.

[0051] The flyback converter circuit and control method provided in this application can achieve zero-voltage switching (i.e., ZVS) of the main switch in both continuous and discontinuous modes, thereby improving system efficiency. Figure 13 and Figure 14 As shown, G Q1 G represents the drive signal for the main switching transistor. Q2 I represents the drive signal for the auxiliary switching transistor. LmV represents the excitation current; cs This represents the sampling current of the main switch transistor; V coss This represents the junction capacitance voltage of the main switch transistor. By adding auxiliary units and corresponding control methods to the flyback converter circuit, it is possible to achieve... Figure 13 , Figure 14 The core of the signal waveform is to create a high-frequency resonance between t1 and t2 between the leakage inductance and the junction capacitance of the main switch, thereby reducing the junction capacitance voltage V of the main switch. coss This enables the ZVS function of the main switch transistor.

[0052] The above-disclosed embodiments are merely preferred embodiments of this application and should not be construed as limiting the scope of this application. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this specification shall still fall within the scope of this application.

Claims

1. A flyback converter circuit, characterized in that, Includes primary edge elements, secondary edge elements, and auxiliary elements; The primary-side unit includes a main switch and a primary-side winding. The first end of the primary-side winding is electrically connected to the power supply, the second end of the primary-side winding is electrically connected to the first end of the main switch, and the second end of the main switch is electrically connected to a reference ground. The secondary-side unit includes a secondary-side winding, and the first end of the secondary-side winding can be electrically connected to the high-potential end of the load. The auxiliary unit is used to provide conditions for zero-voltage switching of the main switching transistor; the auxiliary unit includes an auxiliary switching transistor, a first capacitor, and an auxiliary winding; a first end of the auxiliary winding is electrically connected to a first end of the first capacitor, a second end of the first capacitor is electrically connected to a first end of the auxiliary switching transistor, a second end of the auxiliary switching transistor is electrically connected to a second end of the auxiliary winding; the second end of the auxiliary winding is electrically connected to the reference ground; The second end of the primary winding, the first end of the auxiliary winding, and the first end of the secondary winding are all the same name.

2. The flyback converter circuit according to claim 1, characterized in that, The auxiliary unit is also used to provide an auxiliary voltage; the auxiliary unit further includes a fifth diode and a fifth capacitor; the anode of the fifth diode is electrically connected to the second terminal of the first capacitor, the first terminal of the fifth capacitor is electrically connected to the cathode of the fifth diode, and the second terminal of the fifth capacitor is electrically connected to the reference ground; the cathode of the fifth diode is the auxiliary voltage output terminal.

3. The flyback converter circuit according to claim 1, characterized in that, The auxiliary unit further includes a third switch, a sixth capacitor, and a second auxiliary winding; the first end of the second auxiliary winding is electrically connected to the first end of the sixth capacitor, the second end of the sixth capacitor is electrically connected to the first end of the third switch, and the second end of the third switch is electrically connected to the second end of the second auxiliary winding; the second end of the second auxiliary winding is electrically connected to the reference ground; the first end of the second auxiliary winding and the first end of the auxiliary winding are of the same name; the number of turns of the auxiliary winding is greater than the number of turns of the second auxiliary winding; when the auxiliary switch is working, the voltage of the power supply is less than the voltage of the power supply when the third switch is working; or, The auxiliary winding has a center tap; the auxiliary unit further includes a seventh capacitor and a fourth switch; the first terminal of the seventh capacitor is electrically connected to the center tap, the second terminal of the seventh capacitor is electrically connected to the first terminal of the fourth switch, and the second terminal of the fourth switch is electrically connected to the reference ground; when the auxiliary switch is working, the voltage of the power supply is less than the voltage of the power supply when the fourth switch is working.

4. The flyback converter circuit according to claim 1, characterized in that, It also includes a driving unit; the driving unit includes at least a first driving circuit and a second driving circuit; the driving unit is used to receive the turn-on control signal of the auxiliary switch and transmit it to the control terminal of the auxiliary switch; the turn-on control signal is delayed by the first driving circuit to generate the turn-on control signal of the main switch and input to the control terminal of the main switch; the turn-on control signal of the main switch is passed through the second driving circuit to generate the turn-off control signal of the auxiliary switch, and the second driving circuit is at least used to delay the turn-on control signal of the main switch to obtain the turn-off control signal of the auxiliary switch.

5. The flyback converter circuit according to claim 4, characterized in that, The first driving circuit includes a current-limiting resistor, a first delay resistor, and a first delay capacitor; the second driving circuit includes a second delay resistor, a second delay capacitor, and a driving switch; one end of the first delay resistor is used to receive the driving signal of the auxiliary switch, and the other end is electrically connected to the first end of the first delay capacitor, and the second end of the first delay capacitor is electrically connected to the reference ground; the first end of the second delay resistor is connected to the first end of the first delay capacitor, the second end of the second delay resistor is electrically connected to the first end of the second delay capacitor, and the second end of the second delay capacitor is electrically connected to the reference ground; the control terminal of the driving switch is electrically connected to the second end of the second delay resistor, the first end of the driving switch receives the driving signal of the auxiliary switch at least through the current-limiting resistor and is electrically connected to the control terminal of the auxiliary switch, and the second end of the driving switch is electrically connected to the reference ground.

6. A control method, characterized in that, Applicable to the flyback converter circuit as described in claim 1; the control method includes at least the following auxiliary conduction steps: Before the main switch of the flyback converter circuit is turned on, the auxiliary switch of the flyback converter circuit is turned on, so that the junction capacitance voltage of the main switch is reduced. When the junction capacitance voltage of the main switch decreases, the main switch is then turned on. Once the main switch is turned on, the auxiliary switch is then turned off.

7. The control method according to claim 6, characterized in that, The step of controlling the main switch to turn on after the junction capacitance voltage of the main switch decreases includes the following steps: After the auxiliary switch is turned on, there is a first time delay before the main switch is turned on; wherein, during the time period corresponding to the first time, the junction capacitance of the main switch and the leakage inductance of the transformer of the flyback converter circuit resonate to reduce the junction capacitance voltage of the main switch. The step of controlling the auxiliary switch to turn off after the main switch is turned on includes the following steps: After the main switch is turned on, the auxiliary switch is turned off after a second delay.

8. The control method according to claim 7, characterized in that, The first duration is a preset value, and is related to Positive correlation; Among them, L k C is the leakage inductance value of the transformer. oss The junction capacitance value of the main switch transistor; Alternatively, the second duration is positively correlated with the turn-on delay duration of the main switch.

9. The control method according to claim 6, characterized in that, When the auxiliary unit of the flyback converter circuit includes a third switch, a sixth capacitor, and a second auxiliary winding, or when the auxiliary unit includes a seventh capacitor and a fourth switch and the auxiliary winding has a center tap, the control method further includes the following steps: Determine whether the power supply voltage of the flyback converter circuit is greater than a preset value; If the power supply voltage is less than the preset value, then the auxiliary conduction step is executed; If the power supply voltage is greater than the preset value, then the following steps are performed: Before the main switch of the flyback converter circuit is turned on, the third or fourth switch of the flyback converter circuit is turned on, so that the junction capacitance voltage of the main switch is reduced. When the junction capacitance voltage of the main switch decreases, the main switch is then turned on. Once the main switch is turned on, the third or fourth switch is then turned off.

10. The control method according to any one of claims 6-9, characterized in that, The conduction time of the main switch is controlled based on the peak current; the conduction time of the auxiliary switch is determined by whether the number of system troughs reaches a preset value or by the switching frequency limit of the main switch.

11. A control chip, characterized in that, The control chip is applicable to the flyback converter circuit as described in any one of claims 1-5, and includes a CS terminal, an FB terminal, a ZCD terminal, a GD1 terminal, and a GD2 terminal. The CS terminal is a current sampling pin used to implement current peak control. The FB terminal is a negative feedback pin, used to receive feedback voltage and adjust the output power of the system; The ZCD terminal is a trough detection pin, used to achieve trough locking and output voltage detection; The GD1 terminal is the drive output pin of the main switch in the flyback circuit; the GD2 terminal is the drive output pin of the auxiliary switch in the auxiliary unit. The control chip is used to execute the control method as described in any one of claims 6-10.

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