Flyback converter and control circuit and control method thereof

Abstract

The invention provides a flyback converter and a control circuit and a control method thereof. The control method comprises the following steps: starting timing from conduction of a main switch tube to obtain first timing time; obtaining the sum of the primary side conduction time and the secondary side follow current time; and controlling the conduction state of the main switch tube according to the first timing time, the sum of the primary side conduction time and the secondary side follow current time and a set first duty ratio. According to the invention, the conduction state of the main switch tube is controlled according to the first timing time, the sum of the primary side conduction time and the secondary side follow current time, and the set first duty ratio, so that the working frequency of the flyback converter adapts to the change of the load, and the situation that the efficiency of the flyback converter system is low due to the too high or too low working frequency is avoided.

Description

Technical Field

[0001] This disclosure relates to the field of power electronics technology, and in particular to a flyback converter and its control circuit and control method. Background Technology

[0002] Flyback converters are characterized by simple circuit structure, electrical isolation between input and output, wide voltage regulation range, and easy multi-output, making them suitable as auxiliary switching power supplies in power electronic equipment and widely used in such applications.

[0003] Flyback converters provide power to loads through the process of storing and releasing energy. Their operating mode selection depends on the load requirements and the circuit's operating conditions. As the load changes, the flyback converter switches to the appropriate operating mode to improve system efficiency. For example, in frequency control mode, the switching frequency of the flyback converter increases as the load decreases. Existing technology optimizes the efficiency of flyback converters under light to medium loads by setting a maximum operating frequency to limit the highest switching frequency of the flyback converter.

[0004] However, the maximum power frequency described above is not suitable for all flyback converters. For example, flyback converters with small inductance require a higher switching frequency. When the maximum power frequency is set too low, the conduction loss increases, thus affecting the efficiency of the flyback converter under heavy load. Flyback converters with large inductance require a lower switching frequency. When the maximum power frequency is set too high, it will affect the output ripple of the flyback converter under medium to light load. Summary of the Invention

[0005] In view of the above problems, the purpose of this disclosure is to provide a flyback converter suitable for different inductance designs, as well as its control circuit and control method, which improves system efficiency and output ripple.

[0006] According to a first aspect of this disclosure, a control method for a flyback converter is provided, comprising:

[0007] The first timing time is obtained from the start of the main switch being turned on;

[0008] Obtain the sum of the primary edge conduction time and the secondary edge continuation time;

[0009] The conduction state of the main switch is controlled based on the first timing time, the sum of the primary side conduction time and the secondary side freewheeling time, and the set first duty cycle.

[0010] Optionally, the main switch is turned on after the first timing period is greater than the quotient of the sum of the primary side conduction time and the secondary side freewheeling time and the set first duty cycle.

[0011] Optionally, after the first timing period is greater than the quotient of the sum of the primary side conduction time and the secondary side freewheeling time and the set first duty cycle, a valley detection signal is output, and the main switch is controlled to turn on based on the valley detection signal.

[0012] The valley detection signal represents the moment when the drain voltage of the main switch reaches the valley.

[0013] Alternatively, methods for valley floor detection include:

[0014] A signal characterizing the auxiliary winding voltage is acquired. After the signal characterizing the auxiliary winding voltage drops to zero, the valley detection signal is output after a first delay.

[0015] Optionally, the absolute value of the difference between the first time and a quarter of a resonance period is less than a first threshold.

[0016] Optionally, it also includes:

[0017] The main switch is turned off based on the comparison result between the inductor current sampling signal and the set peak current signal.

[0018] Optionally, after the first timing time is greater than the quotient of the sum of the primary side conduction time and the secondary side freewheeling time and the set first duty cycle, a second time is delayed, and if the signal representing the auxiliary winding voltage is not detected to drop to zero during the second time, the main switch is controlled to turn on.

[0019] Optionally, when the first timing time is less than the quotient of the sum of the primary side conduction time and the secondary side freewheeling time and the set first duty cycle, and greater than the set third time, the main switch is controlled to conduct, and the third time is the period corresponding to the minimum switching frequency.

[0020] Optionally, if the first timing period is longer than the set fourth time and the end of the secondary side freewheeling time is still not detected, the main switch is controlled to turn on, and the fourth time is the set period in the continuous conduction mode.

[0021] Optionally, it also includes:

[0022] An error compensation signal is generated based on the output feedback signal of the flyback converter; and

[0023] The first duty cycle is set according to the error compensation signal.

[0024] Optionally, when the error compensation signal is less than the second threshold, the first duty cycle has a first minimum value;

[0025] When the error compensation signal is greater than the third threshold, the first duty cycle has a first maximum value;

[0026] When the error compensation signal is greater than the second threshold and less than the third threshold, the first duty cycle increases as the error compensation signal increases.

[0027] Optionally, the first maximum value is equal to 100%.

[0028] Optionally, it also includes:

[0029] The moment when the negative slope of the signal that detects the drain-source voltage of the main switch or the auxiliary winding voltage first reaches the slope threshold is taken as the moment when the secondary freewheeling time is detected to end, so as to obtain the sum of the primary conduction time and the secondary freewheeling time.

[0030] According to a second aspect of this disclosure, a control circuit for a flyback converter is provided, comprising:

[0031] The timing unit is used to start timing when the main switch is turned on and to obtain the first timing time.

[0032] The switch control unit acquires the sum of the primary side conduction time and the secondary side freewheeling time, and controls the conduction state of the main switch transistor based on the first timing time, the sum of the primary side conduction time and the secondary side freewheeling time, and the set first duty cycle.

[0033] Optionally, the switch control unit controls the main switch to turn on after the first timing time is greater than the quotient of the sum of the primary side conduction time and the secondary side freewheeling time and the set first duty cycle.

[0034] Optionally, after the first timing period exceeds the quotient of the sum of the primary side conduction time and the secondary side freewheeling time and the set first duty cycle, the switch control unit performs valley detection, outputs a valley detection signal, and controls the main switch to conduct according to the valley detection signal.

[0035] The valley detection signal represents the moment when the drain voltage of the main switch reaches the valley.

[0036] Optionally, the switch control unit acquires a signal characterizing the auxiliary winding voltage, and after the signal characterizing the auxiliary winding voltage drops to zero, it outputs the valley detection signal after a first delay.

[0037] Optionally, the switch control unit may also control the main switch to turn off based on the comparison result between the inductor current sampling signal and the set peak current signal.

[0038] Optionally, the switch control unit further delays the first timing time by a second time after the sum of the primary side conduction time and the secondary side freewheeling time is greater than the quotient of the set first duty cycle, and if the signal representing the auxiliary winding voltage is not detected to drop to zero during the second time, the main switch is controlled to turn on.

[0039] Optionally, the switch control unit may also control the main switch to turn on when the first timing time is less than the quotient of the sum of the primary side conduction time and the secondary side freewheeling time and the set first duty cycle, and greater than the set third time, wherein the third time is the period corresponding to the minimum switching frequency;

[0040] Alternatively, if the secondary side freewheeling time has not been detected to end when the first timing period is greater than the set fourth time, the main switch is controlled to turn on. The fourth time is the set period in the continuous conduction mode.

[0041] Optionally, it also includes:

[0042] The output feedback unit generates an error compensation signal based on the output feedback signal of the flyback converter.

[0043] The parameter setting unit sets the first duty cycle according to the error compensation signal.

[0044] According to a third aspect of this disclosure, a flyback converter is provided, including the control circuit described above.

[0045] The flyback converter and its control circuit and control method disclosed herein control the conduction state of the main switch transistor based on the first timing time, the sum of the primary side conduction time and the secondary side freewheeling time, and the set first duty cycle, so that the operating frequency of the flyback converter can adapt to different system designs and avoid the situation where the flyback converter system efficiency is low due to the operating frequency being too high or too low.

[0046] Furthermore, this disclosure also controls the main switch to turn on when the first timing time is greater than the set period in the continuous conduction mode, and before the end of the secondary side freewheeling time is detected, so as to realize the control of the flyback converter operating in CCM mode.

[0047] Furthermore, this disclosure also controls the main switch to turn on when the first timing time is less than the sum of the primary side conduction time and the secondary side freewheeling time and the set first duty cycle, and is greater than the period corresponding to the set minimum switching frequency, so as to avoid audio from entering due to the switching frequency being too low.

[0048] Furthermore, this disclosure also includes a second time delay after the first timing time is greater than the sum of the primary side conduction time and the secondary side freewheeling time and the set first duty cycle, and if no signal characterizing the auxiliary winding voltage is detected to drop to zero during the second time, the switch is controlled to turn on, so as to avoid the situation where the main switch is not turned on on time due to the small resonance amplitude.

[0049] Furthermore, this disclosure, after the first timing time is greater than the sum of the primary side conduction time and the secondary side freewheeling time and the set first duty cycle, controls the main switch to conduct at the moment when the drain voltage of the main switch resonates to the bottom, so that the main switch is turned on when the current duty cycle reaches the set corresponding first duty cycle and the first bottom is detected, thereby improving the efficiency of the flyback converter. Attached Figure Description

[0050] The above and other objects, features, and advantages of this disclosure will become clearer from the following description of embodiments of the invention with reference to the accompanying drawings, in which:

[0051] Figure 1 This diagram illustrates the structure of a flyback converter according to an embodiment of the present disclosure.

[0052] Figure 2 This diagram illustrates the structure of another flyback converter provided in an embodiment of the present disclosure.

[0053] Figure 3 This diagram illustrates the structure of yet another flyback converter provided in an embodiment of the present disclosure;

[0054] Figure 4 The diagram shows the waveforms of the peak current signal and the first duty cycle in the control circuit of the flyback converter provided in the embodiment of this disclosure;

[0055] Figure 5 The diagram shows the waveforms of the voltage signal VS and the transformer current in the control circuit of the flyback converter provided in the embodiments of this disclosure;

[0056] Figure 6 A flowchart illustrating a control method for a flyback converter provided in an embodiment of this disclosure is shown.

[0057] Figure 7 A flowchart illustrating another control method for a flyback converter provided in an embodiment of this disclosure is shown. Detailed Implementation

[0058] Various embodiments of the invention will now be described in more detail with reference to the accompanying drawings. In the various drawings, the same elements are indicated by the same or similar reference numerals. For clarity, the various parts in the drawings are not drawn to scale.

[0059] Figure 1 A schematic diagram of a flyback converter provided in an embodiment of this disclosure is shown. Figure 2 A schematic diagram of another flyback converter provided in an embodiment of this disclosure is shown. Figure 3 This diagram illustrates the structure of yet another flyback converter provided in an embodiment of the present disclosure. Figure 4 The diagram shows the waveforms of the peak current signal and the first duty cycle in the control circuit of the flyback converter provided in the embodiments of this disclosure. Figure 5 The diagram shows the waveforms of the voltage signal VS and the transformer current in the control circuit of the flyback converter provided in this embodiment.

[0060] See Figure 1 In this embodiment, the flyback converter 200 is, for example, a quasi-resonant flyback converter, including a filter 210, a rectifier bridge 220, an input capacitor Cin, a resistor C1, a capacitor C1, a diode D1, a diode D2, a resistor Rcs, a main switch Q1, a transformer T, an output capacitor Cout, and a control circuit 100.

[0061] Filter 210 receives the input voltage Vin and is used to filter out high-frequency noise and interference in the input voltage Vin. The two input terminals of rectifier bridge 220 are connected to filter 210 to convert the AC input voltage Vin into a pulsed DC voltage signal. Input capacitor Cin is connected to the two output terminals of rectifier bridge 220 to smooth the rectified pulse voltage. Resistor R1 and diode D1 are connected in series across the primary winding Np of transformer T1, capacitor C1 is connected in parallel across resistor R1, one end of the primary winding Np is connected to one end of input capacitor Cin, the other end of input capacitor Cin is grounded, and the other end of the primary winding Np is connected to the first terminal of main switch Q1. The second terminal of main switch Q1 is grounded via resistor Rcs. The control terminal of main switch Q1 is connected to control circuit 100. The secondary winding Ns of transformer T forms a loop with diode D2 and output capacitor Cout. Transformer T converts the input voltage Vin into the required output voltage Vout through magnetic coupling and provides electrical isolation. The output capacitor Cout smooths the output voltage Vout, providing a stable output voltage Vout to the load. The control circuit 100 controls the main switch Q1 to turn on or off to achieve stable regulation of the output voltage Vout.

[0062] During operation, the flyback converter 200 controls the transmission and conversion of input electrical energy by periodically turning the main switch Q1 on and off. The operation process of the flyback converter 200 includes:

[0063] Energy storage: When the main switch is turned on, the inductor current sampling signal of the primary winding rises, the transformer T stores energy, and the load is powered by the output capacitor Cout;

[0064] Energy release: When the main switch is turned off, the inductance voltage of the primary winding is reversed, the diode D2 is turned on, and the energy in the transformer T is supplied to the load through the diode D2. At the same time, the output capacitor Cout is charged to replenish the energy just lost.

[0065] In this embodiment, the control circuit 100 controls the conduction state of the main switch Q1 based on the first timing time, the sum of the primary side conduction time and the secondary side freewheeling time, and the set first duty cycle, so that the operating frequency of the flyback converter 200 can adapt to different system designs and avoid the situation where the flyback converter 200 system efficiency is low due to the operating frequency being too high or too low.

[0066] Furthermore, the control circuit 100 includes a timing unit 110 and a switch control unit 120. The switch control unit 120 includes a turn-off control unit 122, a turn-on control unit 121, and a logic unit 123.

[0067] The timing unit 110 is used to obtain the first timing time by starting the timing from zero each time the main switch Q1 is turned on.

[0068] The shutdown control unit 122 controls the main switch Q1 to turn off based on the comparison result between the inductor current sampling signal Vcs and the set corresponding peak current signal VCS. Furthermore, the shutdown control unit 122 controls the main switch Q1 to turn off when the inductor current sampling signal Vcs reaches the set corresponding peak current signal VCS.

[0069] The control unit 121 obtains the sum of the primary side conduction time and the secondary side freewheeling time ton+tons, sets the first time T1, the second time T2, the third time T3, and the fourth time T4, and controls the main switch Q1 to conduct based on the first duty cycle t set according to the sum of the primary side conduction time and the secondary side freewheeling time ton+tons.

[0070] For example, starting from the turn-on of the main switch Q1, the sum of the primary-side conduction time and the secondary-side freewheeling time, ton+tons, is obtained until the drain-source voltage Vds of the main switch Q1 first exhibits a negative voltage slope reaching a slope threshold. In an alternative embodiment, timing begins from the turn-on of the main switch Q1 until the rising edge of the drain-source voltage Vds of the main switch Q1 is detected, obtaining the primary-side conduction time ton, and timing begins from the rising edge of the drain-source voltage Vds of the main switch Q1 until the rising edge of the drain-source voltage Vds of the main switch Q1 first exhibits a negative voltage slope reaching a slope threshold, obtaining the secondary-side freewheeling time tons, and the primary-side conduction time ton and the secondary-side freewheeling time tons are added together. In an alternative embodiment, timing begins from the turn-on of the main switch Q1 until the signal characterizing the auxiliary winding voltage (exemplarily, here the voltage signal VS) first exhibits a negative voltage slope reaching a slope threshold, obtaining the sum of the primary-side conduction time and the secondary-side freewheeling time ton+tons. In an alternative embodiment, the primary-side conduction time ton is obtained from the start of the main switch Q1's conduction until the rising edge of the voltage signal VS representing the auxiliary winding voltage is detected, and the secondary-side freewheeling time tons is obtained from the start of the start of the rise of the voltage signal VS representing the auxiliary winding voltage until the voltage signal VS representing the auxiliary winding voltage first shows a negative voltage slope reaching a slope threshold. The primary-side conduction time ton and the secondary-side freewheeling time tons are then added together. In an alternative embodiment, the sum of the primary-side conduction time and the secondary-side freewheeling time ton+tons is obtained from the start of the main switch Q1's conduction until the drain voltage of the main switch Q1 first shows a negative voltage slope reaching a slope threshold. In an alternative embodiment, the primary-side conduction time ton is obtained from the start of the main switch Q1's conduction until the rising edge of the drain voltage of the main switch Q1 is detected, and the secondary-side freewheeling time tons is obtained from the start of the start of the countdown from the detection of the rising edge of the drain voltage of the main switch Q1 until the first negative voltage slope of the drain voltage of the main switch Q1 reaches a slope threshold. The primary-side conduction time ton and the secondary-side freewheeling time tons are then added together. In an alternative embodiment, the primary-side conduction time ton and the secondary-side freewheeling time tons can also be obtained by receiving a feedback control signal (a signal that drives the main switch Q1 to conduct or turn off).

[0071] It should be noted that the voltage signal VS, which represents the voltage of the auxiliary winding, can also be replaced by the current signal, which represents the voltage of the auxiliary winding.

[0072] Furthermore, the moment when the drain-source voltage Vds of the main switch Q1 or the drain voltage of the main switch Q1 or the signal characterizing the auxiliary winding voltage (e.g., voltage signal VS) first shows a negative slope reaching the slope threshold is taken as the moment when the secondary freewheeling time tons is detected to end.

[0073] Furthermore, the turn-on control unit 121 receives the drain-source voltage Vds of the main switch Q1, or the drain voltage of the main switch Q1, or the voltage signal VS representing the auxiliary winding voltage, and obtains the sum of the primary side conduction time and the secondary side freewheeling time ton+tons by detecting the drain-source voltage Vds of the main switch Q1, or the drain voltage of the main switch Q1, or the voltage signal VS representing the auxiliary winding voltage.

[0074] Furthermore, after the first timing time t1 is greater than the quotient of the sum of the primary side conduction time and the secondary side freewheeling time ton+tons and the set first duty cycle Duty, the control unit 121 controls the main switch Q1 to conduct.

[0075] Further, after the first timing time t1 exceeds the quotient of the sum of the primary-side conduction time and the secondary-side freewheeling time ton+tons and the set first duty cycle Duty, the control unit 121 performs valley detection and outputs a valley detection signal. Based on the valley detection signal, it controls the main switch Q1 to turn on. The valley detection signal represents the moment when the drain voltage of the main switch Q1 resonates to the valley. The valley detection method includes acquiring a voltage signal VS representing the auxiliary winding voltage or acquiring the drain voltage of the main switch Q1. When either the voltage signal VS or the drain voltage of the main switch Q1 drops to zero, the valley detection signal is output after a first time delay T1. The absolute value of the difference between the first time T1 and a quarter of a resonant cycle is less than a first threshold. The voltage signal VS can be the auxiliary winding voltage or a voltage division of the auxiliary winding voltage. The auxiliary winding is located on the same side as the primary winding.

[0076] Furthermore, the turn-on control unit 121 delays for a second time T2 after the first timing time t1 is greater than the sum of the primary side turn-on time and the secondary side freewheeling time ton+tons and the set first duty cycle Duty. If the voltage signal VS, which represents the auxiliary winding voltage, is not detected to drop to zero during the second time T2, the main switch Q1 is turned on.

[0077] Furthermore, when the first timing time t1 is less than the quotient of the sum of the primary side conduction time and the secondary side freewheeling time ton+tons and the set first duty cycle Duty, and greater than the set third time T3, the control unit 121 controls the main switch Q1 to conduct. The third time T3 is the period corresponding to the minimum switching frequency.

[0078] Furthermore, if the control unit 121 still does not detect the end of the secondary side freewheeling time tons when the first timing time t1 is greater than the set fourth time T4, it controls the main switch Q1 to turn on. The fourth time T4 is the set period in the continuous conduction mode.

[0079] Logic unit 123 is connected to turn-off control unit 122 and turn-on control unit 121, and is used to generate a control signal for turning the main switch Q1 on or off based on the turn-off control signal generated by turn-off control unit 122 and the turn-on control signal generated by turn-on control unit 121. For example, logic unit 123 may include an RS flip-flop. The turn-on control signal is generated by turn-on control unit 121 according to the above control logic, and the turn-off control signal is generated by turn-off control unit 122 according to the above control logic.

[0080] Furthermore, the control circuit 100 also includes an output feedback unit 140 and a parameter setting unit 150.

[0081] The parameter setting unit 150 sets multiple peak current signals according to different error compensation signals, and sets multiple first duty cycles according to different error compensation signals.

[0082] See Figure 4 The error compensation signal COMP changes according to the load. Correspondingly, the flyback converter operates in different modes under different loads. For example, based on the load change from light load to heavy load, the flyback converter can operate in Burst mode, PWM mode, PFM mode, and QR mode. Correspondingly, the Duty curve is formed by multiple first duty cycles. The multiple first duty cycles are set according to different error compensation signals COMP. The VCS curve is formed by multiple peak current signals. The multiple peak current signals are set according to different error compensation signals COMP. Curve L1 is the upper limit of the flyback converter's operating frequency, and curve L2 is the lower limit of the flyback converter's operating frequency. Further, when the error compensation signal COMP is less than a second threshold, the first duty cycle has a first minimum value; when the error compensation signal COMP is greater than a third threshold, the first duty cycle has a first maximum value; when the error compensation signal COMP is greater than the second threshold and less than the third threshold, the first duty cycle increases as the error compensation signal COMP increases. Among them, the first maximum value is equal to 100%.

[0083] The output feedback unit 140 generates an error compensation signal COMP based on the output feedback signal of the flyback converter. For example, the output feedback unit 140 can generate the error compensation signal COMP based on the output feedback signal VFB, which characterizes the output voltage. This is achieved by using two resistors (not shown in the figure) connected in series between the output voltage Vout and the secondary ground, and obtaining the output feedback signal VFB by detecting the voltage at the common terminal of the two resistors. It should be noted that this application does not limit the type or acquisition method of the output feedback signal; alternatively, the output feedback signal can also be a signal characterizing the output current or the output power.

[0084] See Figure 5 Within each switching cycle, the time it takes for the voltage signal VS, representing the auxiliary winding voltage, to reach its inflection point (from the start of the main switch's conduction until the drain-source voltage Vds of the main switch or the first negative voltage slope of the voltage signal VS representing the auxiliary winding voltage reaches a slope threshold) is the sum of the primary-side conduction time ton and the secondary-side conduction time tons. During the primary-side conduction time ton, the current Ip in the primary winding Np increases, and during the secondary-side conduction time tons, the current Is in the secondary winding Ns decreases.

[0085] See Figure 2 In this embodiment, the flyback converter 300 is, for example, an asymmetric half-bridge flyback converter, including a transformer T with a primary winding Np and a secondary winding Ns, switching transistors Q1 and Q2, a first inductor Lk and a first capacitor C1 located on the primary side of the flyback converter 300, and a first diode D1 and an output capacitor Cout located on the secondary side of the flyback converter 300.

[0086] On the primary side, the second switch Q2 and the first switch Q1 are connected in series between the input voltage Vin and the reference ground. In one possible embodiment, both the first switch Q1 and the second switch Q2 are NMOS field-effect transistors. The first inductor Lk, the primary winding Np of the transformer T, and the first capacitor C1 are connected in series between the drain and source of the second switch Q2, forming a resonant circuit when the second switch Q2 is turned on. The equivalent inductance of the primary winding of the transformer T in the resonant circuit is the magnetizing inductance. It should be noted that in low-power power supply applications, the leakage inductance of the transformer T can be used instead of the first inductor Lk.

[0087] On the secondary side, the first diode D1 is connected in series with the secondary winding Ns of the transformer T between the voltage output terminal and ground. The anode of the first diode D1 is connected to the opposite terminal of the secondary winding Ns, thereby rectifying the induced voltage, which is inversely phase to the excitation voltage of the transformer T, to provide a DC output voltage Vout. The output capacitor Cout is connected between the voltage output terminal and ground to filter the DC output voltage Vout to obtain a smooth voltage waveform.

[0088] In other embodiments, a sampling resistor is also included connected between the source of the first switching transistor Q1 and ground to obtain an inductor current sampling signal.

[0089] The control circuit 100 is connected to the control terminal of the first switching transistor Q1 to control its on or off state.

[0090] It should be noted that, for example, in this embodiment, a control signal for driving the second switch Q2 is also generated through logic unit 123.

[0091] See Figure 3In this embodiment, the flyback converter 500 is, for example, an active clamp flyback converter, including a transformer T with a primary winding Np and a secondary winding Ns, switching transistors Q1 and Q2, a first inductor Lk and a first capacitor Cclamp located on the primary side of the flyback converter 500, and a first diode D1 and an output capacitor Cout located on the secondary side of the flyback converter 500.

[0092] On the primary side, the second switch Q2 and the first switch Q1 are connected in series between the input voltage Vin and the reference ground. In one possible embodiment, both the first switch Q1 and the second switch Q2 are NMOS field-effect transistors. The first capacitor Cclamp, the first inductor Lk, and the primary winding Np of the transformer T are connected in series between the drain and source of the second switch Q2, forming a resonant circuit when the second switch Q2 is turned on. The equivalent inductance of the primary winding of the transformer T in the resonant circuit is the magnetizing inductance. It should be noted that in low-power power supply applications, the leakage inductance of the transformer T can be used instead of the first inductor Lk.

[0093] On the secondary side, the first diode D1 is connected in series with the secondary winding Ns of the transformer T between the voltage output terminal and ground. The anode of the first diode D1 is connected to the opposite terminal of the secondary winding Ns, thereby rectifying the induced voltage, which is inversely phase to the excitation voltage of the transformer T, to provide a DC output voltage Vout. The output capacitor Cout is connected between the voltage output terminal and ground to filter the DC output voltage Vout to obtain a smooth voltage waveform.

[0094] In other embodiments, a sampling resistor is also included connected between the source of the first switching transistor Q1 and ground to obtain an inductor current sampling signal.

[0095] The control circuit 100 is connected to the control terminal of the first switching transistor Q1 to control its on or off state.

[0096] It should be noted that, for example, in this embodiment, a control signal for driving the second switch Q2 is also generated through logic unit 123.

[0097] It should be noted that the flyback converter disclosed herein can also be used for other topologies such as zero-voltage turn-on topology.

[0098] Figure 6 The diagram shows a flow chart of a control method for a flyback converter provided in an embodiment of this disclosure. Figure 7 A flowchart illustrating another control method for a flyback converter provided in an embodiment of this disclosure is shown.

[0099] See Figure 6 The control method for a flyback converter provided in this disclosure includes the following steps:

[0100] Step S410: Obtain the first timing time from the start of the main switch Q1's conduction. The timer is reset and restarted each time the main switch Q1 is turned on to obtain the first timing time t.

[0101] Step S420: Control the main switch transistor to turn off based on the comparison result between the inductor current sampling signal and the set corresponding peak current signal. Control the main switch transistor Q1 to turn off when the inductor current sampling signal Vcs reaches the set corresponding peak current signal VCS. Further, multiple peak current signals VCS are set according to different output feedback. For example, the peak current signal VCS varies with the error compensation signal COMP (see...). Figure 4 The error compensation signal COMP is generated based on the output feedback signal (e.g., the output feedback signal VFB, which represents the output voltage Vout). The corresponding peak current signal VCS refers to the value corresponding to the current error compensation signal COMP.

[0102] Step S430: Obtain the sum of the primary side conduction time and the secondary side freewheeling time, and control the conduction state of the main switch transistor according to the first timing time, the sum of the primary side conduction time and the secondary side freewheeling time, and the set first duty cycle.

[0103] Further, starting from the turn-on of the main switch Q1, the sum of the primary-side conduction time and the secondary-side freewheeling time, ton+tons, is obtained until the drain-source voltage Vds of the main switch Q1 first exhibits a negative voltage slope reaching a slope threshold. In an alternative embodiment, timing begins from the turn-on of the main switch Q1 until the rising edge of the drain-source voltage Vds of the main switch Q1 is detected, yielding the primary-side conduction time ton. Timing also begins from the rising edge of the drain-source voltage Vds of the main switch Q1 until the rising edge of the drain-source voltage Vds of the main switch Q1 is detected until the rising edge of the drain-source voltage Vds of the main switch Q1 first exhibits a negative voltage slope reaching a slope threshold, yielding the secondary-side freewheeling time tons. The primary-side conduction time ton and the secondary-side freewheeling time tons are then added together. In an alternative embodiment, timing begins from the turn-on of the main switch Q1 until the signal characterizing the auxiliary winding voltage (exemplarily, here the voltage signal VS) first exhibits a negative voltage slope reaching a slope threshold, yielding the sum of the primary-side conduction time and the secondary-side freewheeling time ton+tons. In an alternative embodiment, the primary-side conduction time ton is obtained from the start of the main switch Q1's conduction until the rising edge of the voltage signal VS representing the auxiliary winding voltage is detected, and the secondary-side freewheeling time tons is obtained from the start of the start of the rise of the voltage signal VS representing the auxiliary winding voltage until the voltage signal VS representing the auxiliary winding voltage first shows a negative voltage slope reaching a slope threshold. The primary-side conduction time ton and the secondary-side freewheeling time tons are then added together. In an alternative embodiment, the sum of the primary-side conduction time and the secondary-side freewheeling time ton+tons is obtained from the start of the main switch Q1's conduction until the drain voltage of the main switch Q1 first shows a negative voltage slope reaching a slope threshold. In an alternative embodiment, the primary-side conduction time ton is obtained from the start of the main switch Q1's conduction until the rising edge of the drain voltage of the main switch Q1 is detected, and the secondary-side freewheeling time tons is obtained from the start of the start of the countdown from the detection of the rising edge of the drain voltage of the main switch Q1 until the first negative voltage slope of the drain voltage of the main switch Q1 reaches a slope threshold. The primary-side conduction time ton and the secondary-side freewheeling time tons are then added together. In an alternative embodiment, the primary-side conduction time ton and the secondary-side freewheeling time tons can also be obtained by receiving a feedback control signal (a signal that drives the main switch Q1 to conduct or turn off).

[0104] Furthermore, the first duty cycle (Duty) corresponds to the value of the current error compensation signal COMP. For example, the first duty cycle (Duty) changes with the error compensation signal COMP (see [link to documentation]). Figure 4 The first duty cycle, Duty, is generated based on the output feedback signal (e.g., the output feedback signal VFB, which characterizes the output voltage Vout).

[0105] Further, see Figure 7 Another control method for a flyback converter provided in this disclosure includes the following steps:

[0106] Step S411: Obtain the time t starting from the moment the main switch Q1 is turned on. That is, reset the timer and start counting each time the main switch Q1 is turned on to obtain the first timing time t.

[0107] Step S420: Control the main switch to turn off based on the comparison result between the inductor current sampling signal and the set corresponding peak current signal.

[0108] Step S431: Determine whether an inflection point has been detected in the signal representing the auxiliary winding voltage. For example, starting from the turn-on of the main switch Q1, it can be determined whether the drain-source voltage Vds of the main switch Q1 first exhibits a negative voltage slope reaching a slope threshold, or whether the voltage signal VS representing the auxiliary winding voltage first exhibits a negative voltage slope reaching a slope threshold, to determine whether the voltage signal VS representing the auxiliary winding voltage has reached an inflection point.

[0109] If not, proceed to step S432: determine if t > T4. If t > T4, proceed to step S438: control the main switch to turn on. Here, the fourth time T4 is the set period in continuous conduction mode. When the voltage signal VS, representing the auxiliary winding voltage, does not show an inflection point (the secondary freewheeling time tons has not yet ended), the main switch Q1 is controlled to turn on when the first timing time t reaches the set period in continuous conduction mode, thereby controlling the flyback converter to operate in CCM (Continuous Conduction Mode). Otherwise, return to step S431.

[0110] If so, proceed to step S433 to calculate ton+tons. The sum of the primary-side conduction time and the secondary-side freewheeling time, ton+tons, is the timing time from the start of the main switch's conduction until the inflection point of the voltage signal VS representing the auxiliary winding voltage is detected. Alternatively, the sum of the primary-side conduction time and the secondary-side freewheeling time, ton+tons, is the timing time from the start of the main switch's conduction until the rising edge of the voltage signal VS representing the auxiliary winding voltage is detected, and the timing time from the detection of the rising edge of the voltage signal VS representing the auxiliary winding voltage until the first negative voltage slope of the voltage signal VS representing the auxiliary winding voltage reaches the slope threshold.

[0111] Step S434: Determine Determine if the first timing time t is greater than the sum of the primary side conduction time and the secondary side freewheeling time. The quotient of the first duty cycle (Duty).

[0112] If not, proceed to step S436: determine if t > T3. Here, the third time T3 is the period corresponding to the minimum switching frequency. For example, in this embodiment, the minimum switching frequency is set to 25kHz. If t > T3, proceed to step S439: control the main switch to turn on. That is, when the voltage signal VS, representing the auxiliary winding voltage, reaches an inflection point and the current duty cycle has not reached the corresponding first duty cycle, and the first timing time t reaches the third time T3, then the main switch Q1 is turned on to prevent audio input due to an excessively low switching frequency. Otherwise, return to step S434.

[0113] If so, proceed to step S435: determine whether the signal characterizing the auxiliary winding voltage has crossed zero. For example, this is done by detecting whether the voltage signal VS characterizing the auxiliary winding voltage has crossed zero ( Figure 5 (The zero point shown).

[0114] If the voltage signal VS, which characterizes the auxiliary winding voltage, does not cross zero, proceed to step S438: After a second delay T2, proceed to step S439. That is, the first timing time t is greater than the sum of the primary side conduction time and the secondary side freewheeling time. Compared with the set first duty cycle After a second delay T2, if the voltage signal VS, which represents the auxiliary winding voltage, does not drop to zero within this second time, then the main switch Q1 is turned on. This is to avoid the main switch Q1 not turning on on time due to a small resonance amplitude causing a missed detection. The second time T2 can be set to, for example, 3 microseconds.

[0115] When the signal characterizing the auxiliary winding voltage crosses zero, step S437 is executed: a first time T1 is delayed when the signal characterizing the auxiliary winding voltage crosses zero. Then, step S439 is executed. The absolute value of the difference between the first time T1 and a quarter of a resonant cycle is less than a first threshold. That is, the first timing time t is greater than the sum of the primary side conduction time and the secondary side freewheeling time. Compared with the set first duty cycle Furthermore, the main switch Q1 is turned on approximately one-quarter of a resonant cycle after the zero-crossing delay of the voltage signal VS, which characterizes the auxiliary winding voltage. This ensures that the main switch Q1 is turned on when the current duty cycle reaches the set first duty cycle (Duty) and the first valley is detected, thereby improving the efficiency of the flyback converter.

[0116] In other embodiments, the control method for the flyback converter further includes: generating an error compensation signal based on the output feedback signal of the flyback converter. Then, a corresponding peak current signal VCS and a first duty cycle (Duty) are determined based on the current error compensation signal. Further, when the error compensation signal is less than a second threshold, the first duty cycle (Duty) has a first minimum value; when the error compensation signal is greater than a third threshold, the first duty cycle (Duty) has a first maximum value; when the error compensation signal is greater than the second threshold and less than the third threshold, the first duty cycle (Duty) increases as the error compensation signal increases. The first maximum value is equal to 100%.

[0117] As described above, these embodiments of the present invention do not exhaustively cover all details, nor do they limit the invention to the specific embodiments described. Clearly, many modifications and variations can be made based on the above description. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to effectively utilize the invention and its modifications. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A control method for a flyback converter, wherein, include: The first timing time is obtained from the start of the main switch being turned on; Obtain the sum of the primary edge conduction time and the secondary edge continuation time; The conduction state of the main switch is controlled based on the first timing time, the sum of the primary side conduction time and the secondary side freewheeling time, and the set first duty cycle.

2. The control method for the flyback converter according to claim 1, wherein, After the first timing period is greater than the quotient of the sum of the primary side conduction time and the secondary side freewheeling time and the set first duty cycle, the main switch is controlled to conduct.

3. The control method for the flyback converter according to claim 2, wherein, After the first timing period exceeds the quotient of the sum of the primary side conduction time and the secondary side freewheeling time and the set first duty cycle, a valley detection signal is output, and the main switch is controlled to turn on based on the valley detection signal. The valley detection signal represents the moment when the drain voltage of the main switch reaches the valley.

4. The control method for the flyback converter according to claim 3, wherein, Methods for valley floor detection include: A signal characterizing the auxiliary winding voltage is acquired. After the signal characterizing the auxiliary winding voltage drops to zero, the valley detection signal is output after a first delay.

5. The control method for a flyback converter according to claim 4, wherein, The absolute value of the difference between the first time and a quarter of a resonance period is less than the first threshold.

6. The control method for a flyback converter according to claim 1, wherein, Also includes: The main switch is turned off based on the comparison result between the inductor current sampling signal and the set peak current signal.

7. The control method for a flyback converter according to claim 4, wherein, After the first timing time is greater than the quotient of the sum of the primary side conduction time and the secondary side freewheeling time and the set first duty cycle, a second time is delayed, and if the signal representing the auxiliary winding voltage is not detected to drop to zero during the second time, the main switch is controlled to conduct.

8. The control method for a flyback converter according to claim 1, wherein, When the first timing time is less than the quotient of the sum of the primary side conduction time and the secondary side freewheeling time and the set first duty cycle, and greater than the set third time, the main switch is controlled to conduct, and the third time is the period corresponding to the minimum switching frequency.

9. The control method for a flyback converter according to claim 1, wherein, If the first timing period is longer than the set fourth time and the end of the secondary side freewheeling time is still not detected, the main switch is controlled to turn on. The fourth time is the set period in the continuous conduction mode.

10. The control method for a flyback converter according to any one of claims 1-9, wherein, Also includes: An error compensation signal is generated based on the output feedback signal of the flyback converter; as well as The first duty cycle is set according to the error compensation signal.

11. The control method for a flyback converter according to claim 10, wherein, When the error compensation signal is less than the second threshold, the first duty cycle has a first minimum value; When the error compensation signal is greater than the third threshold, the first duty cycle has a first maximum value; When the error compensation signal is greater than the second threshold and less than the third threshold, the first duty cycle increases as the error compensation signal increases.

12. The control method for a flyback converter according to claim 11, wherein, The first maximum value is equal to 100%.

13. The control method for a flyback converter according to claim 1, wherein, Also includes: The moment when the negative slope of the signal that detects the drain-source voltage of the main switch or the auxiliary winding voltage first reaches the slope threshold is taken as the moment when the secondary freewheeling time is detected to end, so as to obtain the sum of the primary conduction time and the secondary freewheeling time.

14. A control circuit for a flyback converter, wherein, include: The timing unit is used to start timing when the main switch is turned on and to obtain the first timing time. The switch control unit acquires the sum of the primary side conduction time and the secondary side freewheeling time, and controls the conduction state of the main switch transistor based on the first timing time, the sum of the primary side conduction time and the secondary side freewheeling time, and the set first duty cycle.

15. The control circuit for the flyback converter according to claim 14, wherein, The switch control unit controls the main switch to turn on after the first timing time is greater than the quotient of the sum of the primary side conduction time and the secondary side freewheeling time and the set first duty cycle.

16. The control circuit for the flyback converter according to claim 15, wherein, After the first timing period exceeds the quotient of the sum of the primary side conduction time and the secondary side freewheeling time and the set first duty cycle, the switch control unit performs valley detection, outputs a valley detection signal, and controls the main switch to conduct according to the valley detection signal. The valley detection signal represents the moment when the drain voltage of the main switch reaches the valley.

17. The control circuit for the flyback converter according to claim 16, wherein, The switch control unit acquires a signal characterizing the auxiliary winding voltage. When the signal characterizing the auxiliary winding voltage drops to zero, it outputs the valley detection signal after a first delay.

18. The control circuit for the flyback converter according to claim 14, wherein, The switch control unit also controls the main switch to turn off based on the comparison result between the inductor current sampling signal and the set peak current signal.

19. The control circuit for the flyback converter according to claim 17, wherein, The switch control unit further delays the first timing time by a second time after the sum of the primary side conduction time and the secondary side freewheeling time is greater than the quotient of the set first duty cycle, and if the signal representing the auxiliary winding voltage does not drop to zero during the second time, it controls the main switch tube to conduct.

20. The control circuit for the flyback converter according to claim 14, wherein, The switch control unit also controls the main switch to turn on when the first timing time is less than the quotient of the sum of the primary side conduction time and the secondary side freewheeling time and the set first duty cycle, and greater than the set third time. The third time is the period corresponding to the minimum switching frequency. Alternatively, if the secondary side freewheeling time has not been detected to end when the first timing period is greater than the set fourth time, the main switch is controlled to turn on. The fourth time is the set period in the continuous conduction mode.

21. The control circuit for the flyback converter according to any one of claims 14-20, wherein, Also includes: The output feedback unit generates an error compensation signal based on the output feedback signal of the flyback converter. The parameter setting unit sets the first duty cycle according to the error compensation signal.

22. A flyback converter, wherein, Includes the control circuit as described in any one of claims 14-21 above.

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