Control circuit and control method of half-bridge flyback converter
By controlling the circuit and method in the continuous hiccup mode of the half-bridge flyback converter, zero-voltage turn-on of the first switch and timely turn-off of the second switch are achieved, solving the switching loss problem of the asymmetrical half-bridge flyback converter under light load, and improving transmission efficiency and system reliability.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- JOULWATT TECH INC LTD
- Filing Date
- 2020-09-10
- Publication Date
- 2026-07-17
AI Technical Summary
In the intermittent conduction mode, the hard turn-on of the main switch in the asymmetric half-bridge flyback converter results in large turn-on losses, and the efficiency is low, especially under light load.
By employing control circuits and methods, the half-bridge flyback converter controls the second switching transistor to turn off in the first N-1 switching cycles and then turn on the first switching transistor with zero voltage in the Nth switching cycle. The first switching transistor is then turned off when the magnetizing inductor current is zero. Combined with zero-crossing detection, soft switching is achieved, reducing switching losses.
Reduce switching losses and improve transmission efficiency under light and medium loads, achieve soft-switching startup, and reduce overall system power loss.
Smart Images
Figure CN117118238B_ABST
Abstract
Description
Technical Field
[0001] This disclosure relates to the field of power electronics technology, and more specifically, to a control circuit and control method for a half-bridge flyback converter. Background Technology
[0002] A flyback converter includes a transformer that transfers electrical energy to the secondary winding of the transformer while the primary winding is disconnected from the input power supply. Flyback converters have a simple circuit structure, low cost, and a wide input voltage range, and are therefore widely used in various electronic devices.
[0003] In some applications, flyback converters employ an asymmetric half-bridge topology, or may also include an active clamping circuit. In an asymmetric half-bridge flyback converter, on the primary side of the transformer, the flyback converter includes a main switch and an auxiliary switch connected between the input terminal and ground. One end of the transformer's primary winding is connected to the intermediate node between the main switch and the auxiliary switch. In a flyback converter employing an active clamping circuit, on the primary side of the transformer, the flyback converter includes not only the main switch connected between the transformer's primary winding and ground, but also an auxiliary switch and a capacitor connected in parallel with the transformer's primary winding. Both types of flyback converters also include a first inductor and a first capacitor that, together with the auxiliary switch, form a resonant circuit. The first inductor can be the leakage inductance of the transformer.
[0004] Asymmetric half-bridge flyback converters are more efficient than conventional flyback converters. Under heavy load, they generally operate in a complementary state between the two switches, while under light load, the switching frequency is reduced to put the flyback converter into DCM (Discontinuous Conduction Mode). However, in discontinuous conduction mode, the hard turn-on of the main switch will bring about a large turn-on loss, and the higher the switching frequency, the greater the loss.
[0005] Therefore, it is necessary to provide improved technical solutions to overcome the above-mentioned technical problems existing in the prior art. Summary of the Invention
[0006] In view of this, the purpose of this disclosure is to provide a control circuit and control method for a half-bridge flyback converter to solve the problems in the prior art.
[0007] According to a first aspect of this disclosure, a control circuit for a half-bridge flyback converter is provided. The half-bridge flyback converter includes a transformer, a first switch and a second switch connected in series, and a first capacitor and a first inductor connected in series across the second switch. The control circuit includes:
[0008] The first switch control module controls the on and off of the first switch transistor according to the first switch control signal;
[0009] The second switch control module controls the on and off of the second switch transistor according to the second switch control signal.
[0010] The half-bridge flyback converter operates in a hiccup mode with multiple consecutive hiccup cycles. Each hiccup cycle includes N switching cycles. The second switching control module controls the second switching transistor to turn off in the first N-1 switching cycles, and then controls the first switching control module to turn on the first switching transistor with zero voltage. The second switching transistor is also controlled to turn off when the magnetizing inductor current is zero in the Nth switching cycle, where N is an integer greater than 1.
[0011] Optionally, the first switch control module and the second switch control module control the first switch and the second switch to be complementaryly turned on during the first N-1 switching cycles, and to remain off after being turned off in the Nth switching cycle until the next hiccup cycle.
[0012] Optionally, the first switch control module generates the first switch control signal based on the comparison result of the first sampling parameter and the preset first parameter curve, and the second switch control module generates the second switch control signal based on the comparison result of the second sampling parameter and the preset second parameter curve.
[0013] Optionally, the second switch control module controls the second switch to continue conducting for a period of time when the magnetizing inductor current is zero during the first N-1 switching cycles, thereby generating a negative magnetizing inductor current and releasing the junction capacitance charge of the first switch.
[0014] Optionally, the control circuit further includes a period control module, the period control module comprising:
[0015] The error amplification unit generates an error amplification signal based on the output voltage.
[0016] A first curve setting unit, connected to the error amplification unit, generates the preset first parameter curve based on the error amplification signal; and
[0017] The second curve setting unit is connected to the error amplification unit and generates the preset second parameter curve according to the error amplification signal.
[0018] Optionally, the preset first parameter curve is a current curve, and the first sampling parameter is the current flowing through the first switching transistor; the preset second parameter curve is a frequency curve, and the second sampling parameter is the operating frequency.
[0019] Optionally, the preset first parameter curve and the preset second parameter curve are adjusted so that the first switch control module and the second switch control module respectively adjust the turn-on time and turn-off time of the first switch and the second switch, thereby adjusting the number N of the switching cycles included in the hiccup cycle, and the output power of the half-bridge flyback converter is proportional to the value of N.
[0020] Optionally, the preset first parameter curve and the preset second parameter curve are adjusted so that the first switch control module and the second switch control module respectively adjust the turn-on time and turn-off time of the first switch and the second switch to adjust the frequency of the hiccup cycle, and the output power of the half-bridge flyback converter is proportional to the frequency of the hiccup cycle.
[0021] Optionally, the preset first parameter curve and the preset second parameter curve are adjusted so that the first switch control module and the second switch control module respectively adjust the turn-on time and turn-off time of the first switch and the second switch to adjust the peak value of the magnetizing inductor current, and the output power of the half-bridge flyback converter is proportional to the peak value of the magnetizing inductor current.
[0022] Optionally, the half-bridge flyback converter includes an auxiliary winding, and the control circuit further includes:
[0023] The zero-crossing detection unit is connected to the first switch control module and is used to detect the moment when the source-drain voltage across the first switch tube is zero based on the voltage of the auxiliary winding, and to control the first switch control module to turn on the first switch tube when the source-drain voltage is zero.
[0024] Optionally, the second switch control module further controls the second switch to conduct for a period of time before the first switch is turned on during the first switching cycle, generating a negative magnetizing inductance current to release the junction capacitance charge of the first switch.
[0025] Optionally, the first and second switching transistors are connected in series between the power input terminal and the reference ground.
[0026] According to a second aspect of the present invention, a control method for a half-bridge flyback converter is provided, the half-bridge flyback converter comprising a transformer, a first switch and a second switch connected in series, and a first capacitor and a first inductor connected in series across the second switch, the control method comprising:
[0027] The second switch is turned on after the first switch is turned off.
[0028] The second switch is turned off during the first N-1 switching cycles, and then the first switch is turned on with zero voltage.
[0029] The second switch is turned off when the magnetizing inductor current is zero in the Nth switching cycle, thus entering the resonant cycle.
[0030] The half-bridge flyback converter operates in a hiccup mode with multiple consecutive hiccup cycles according to the control method. Each hiccup cycle includes N switching cycles, where N is an integer greater than 1.
[0031] Optionally, the control method further includes: controlling the second switch to continue conducting for a period of time when the magnetizing inductor current is zero during the first N-1 switching cycles, thereby generating a negative magnetizing inductor current.
[0032] Optionally, the control method further includes:
[0033] The second switch is controlled to be turned on for a period of time before the first switch is turned on during the first switching cycle;
[0034] When the source-drain voltage across the first switch is detected to be zero, the first switch is turned on.
[0035] According to the control circuit and control method of the half-bridge flyback converter in the embodiments of this disclosure, the half-bridge flyback converter is controlled to operate in hiccup mode. Each hiccup cycle includes N switching cycles. By controlling the second switch to be turned on for a period of time in each of the first N-1 switching cycles, the first switch can achieve ZVS turn-on after it is turned off. In the last switching cycle, the second switch is turned off when the excitation current is close to zero to avoid generating a large negative excitation current. This can reduce the turn-on loss, reduce the energy loss of the resonant cycle after the two switches are turned off, and achieve soft-switching start of the first switch, thereby improving the transmission efficiency of the flyback converter under light load.
[0036] Furthermore, the transmission efficiency of the flyback converter can be improved by increasing the value of N, increasing the frequency of the hiccup cycle, or increasing the peak value of the magnetizing inductor current.
[0037] Furthermore, by turning on the second switch before the first switch is turned on during the first switching cycle, and turning off the second switch when the source-drain voltage across the first switch is detected to be zero, and then turning on the first switch, the first switch can achieve ZVS turn-on during the first switching cycle. In this way, the first switch can achieve soft switching during the entire hiccup cycle, further reducing turn-on losses and resulting in low overall system power loss. Attached Figure Description
[0038] Figure 1 The circuit block diagram of an asymmetric half-bridge topology for a conventional flyback converter is shown.
[0039] Figure 2 The circuit block diagram of another asymmetric half-bridge topology of a conventional flyback converter is shown.
[0040] Figure 3 Show Figure 1 and Figure 2 The waveform diagram of the asymmetric half-bridge flyback converter is shown below.
[0041] Figure 4 A circuit block diagram showing the topology and control circuit of a half-bridge flyback converter according to a first embodiment of the present disclosure is provided.
[0042] Figure 5 Show Figure 4 The waveform diagram of the half-bridge flyback converter is shown below.
[0043] Figures 6a-6c Show each Figure 4 The waveform diagrams shown are of the half-bridge flyback converter under three different control modes.
[0044] Figure 7 A circuit block diagram showing the topology and control circuit of a half-bridge flyback converter according to a second embodiment of the present disclosure is provided.
[0045] Figure 8 Show Figure 7 The waveform diagram of the half-bridge flyback converter is shown. Detailed Implementation
[0046] The preferred embodiments of this disclosure are described in detail below with reference to the accompanying drawings, but this disclosure is not limited to these embodiments. This disclosure covers any alternatives, modifications, equivalent methods, and solutions made within the spirit and scope of this disclosure.
[0047] In order to provide the public with a thorough understanding of this disclosure, specific details are described in detail in the following preferred embodiments of this disclosure, but those skilled in the art can fully understand this disclosure without these details.
[0048] The present disclosure is described in more detail below by way of example with reference to the accompanying drawings. It should be noted that the drawings are in a simplified form and use non-precise scales, and are only used to facilitate and clarify the illustration of the embodiments of the present disclosure.
[0049] Figure 1 The diagram shows a circuit block diagram of an asymmetric half-bridge topology for a conventional flyback converter. For clarity, only the main circuitry of the flyback converter is shown in the diagram, and the switch control module for providing switching control signals to the switching transistors is not shown.
[0050] exist Figure 1In the asymmetric half-bridge topology shown, the flyback converter 100 includes 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 Cr located on the primary side of the flyback converter, and a first diode D1 and an output capacitor Co located on the secondary side of the transformer T.
[0051] On the primary side of transformer T, a first switching transistor Q1 and a second switching transistor Q2 are connected in series between the voltage input terminal and the primary side reference ground. In one possible embodiment, both the first switching transistor Q1 and the second switching transistor Q2 are NMOS field-effect transistors. The primary winding Np of transformer T, the first inductor Lk, and the first capacitor Cr are connected in series between the source and drain of the second switching transistor Q2, forming a resonant circuit when the second switching transistor Q2 is turned on. The equivalent inductance of the primary winding of transformer T in the resonant circuit is the magnetizing inductance Lm. Preferably, in low-power power supply applications, the leakage inductance of transformer T can be used instead of the first inductor Lk.
[0052] On the secondary side of transformer T, a first diode D1 is connected in series with the secondary winding Ns of transformer T between the voltage output terminal and the secondary reference 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 transformer T, to provide a DC output voltage Vo. An output capacitor Co is connected between the voltage output terminal and the secondary reference ground to filter the DC output voltage Vo to obtain a smooth voltage waveform. In an alternative embodiment, a synchronous rectifier switch can be used instead of the first diode D1.
[0053] Preferably, the flyback converter further includes a sampling resistor Rcs connected between the source of the second switch Q2 and a reference ground, for obtaining the current flowing through the first switch Q1 during the period when the first switch Q1 is turned on and the second switch Q2 is turned off.
[0054] Figure 2 A circuit block diagram of another asymmetric half-bridge topology for a conventional flyback converter is shown. For clarity, only the main circuitry of the flyback converter is shown in the figure, and the switching control module for providing switching control signals to the switching transistors is not shown.
[0055] exist Figure 2 In the asymmetric half-bridge topology shown, the flyback converter 200 includes 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 Cr located on the primary side of the flyback converter, and a first diode D1 and an output capacitor Co located on the secondary side of the transformer T.
[0056] On the primary side of transformer T, the second switch Q2 and the first switch Q1 are connected in series between the voltage input terminal and the primary reference ground. In one possible embodiment, both the first switch Q1 and the second switch Q2 are NMOS field-effect transistors. The primary winding Np of transformer T, the first inductor Lk, and the first capacitor Cr are connected in series between the source and drain 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 transformer T in the resonant circuit is the magnetizing inductance Lm. Preferably, in low-power power supply applications, the leakage inductance of transformer T can be used instead of the first inductor Lk.
[0057] On the secondary side of transformer T, a first diode D1 is connected in series with the secondary winding Ns of transformer T between the voltage output terminal and the secondary reference 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 transformer T, to provide a DC output voltage Vo. The output capacitor Co is connected between the voltage output terminal and the secondary reference ground to filter the DC output voltage Vo to obtain a smooth voltage waveform.
[0058] Preferably, the asymmetric half-bridge flyback converter further includes a sampling resistor Rcs connected between the source of the second switch Q2 and the reference ground, for obtaining the current flowing through the first switch Q1 during the period when the first switch Q1 is turned on and the second switch Q2 is turned off.
[0059] Figure 3 Show Figure 1 and Figure 2 The diagram shows the operating waveforms of the asymmetric half-bridge flyback converter. In the figure, Vg1 and Vg2 represent the switching control signals of the first switch Q1 and the second switch Q2, respectively; Im represents the magnetizing inductor current flowing through the magnetizing inductor Lm; and Vsw represents the source-drain voltage across the first switch Q1 or the voltage at node SW.
[0060] like Figure 3As shown, the asymmetric half-bridge flyback converter operates in normal state and in DCM state. At time t1, the first switch control signal Vg1 controls the first switch Q1 to turn on, and the input voltage Vin charges the first inductor Lk, the magnetizing inductor Lm, and the first capacitor Cr. During the time interval t1-t2, the magnetizing inductor current Im increases. At time t2, the first switch control signal Vg1 controls the first switch Q1 to turn off. During the dead time interval t2-t3, the source-drain voltage Vsw increases. At time t3, the second switch control signal Vg2 controls the second switch Q2 to turn on. During the time interval t3-t4, the source-drain voltage Vsw approaches Vin, the first capacitor Cr and the first inductor Lk resonate, and electrical energy is transferred from the primary side of the transformer to the secondary side, and the magnetizing inductor current Im decreases. At time t4, the magnetizing inductor current Im drops to zero, the second switch Q2 turns off, and the resonant period begins during the time interval t4-t5. At time t5, the first switch control signal Vg1 controls the first switch Q1 to turn on again and enter the next cycle. At this time, the source-drain voltage Vsw across the first switch Q1 has not dropped to zero voltage, and the first switch Q1 is hard turned on. At this time, the switching loss is large. Therefore, the asymmetric half-bridge flyback converter in DCM mode has large switching loss under light or medium load.
[0061] This invention optimizes the control circuit of the flyback converter with the above-mentioned asymmetric half-bridge topology, so that the first switch Q1 can be turned on when the source-drain voltage is close to zero, thereby realizing the ZVS turn-on of the asymmetric half-bridge flyback converter, reducing switching losses and improving transmission efficiency.
[0062] Figure 4 A circuit block diagram showing the topology and control circuitry of a half-bridge flyback converter according to a first embodiment of this disclosure is provided. Figure 4 In the illustrated half-bridge topology, the half-bridge flyback converter 400 includes a main circuit 410 and a control circuit 420. The control circuit 420 includes a first switch control module 440, a second switch control module 450, and a cycle control module 430. The control circuit 420 controls the half-bridge flyback converter 400 to operate in a hiccup mode. The hiccup mode is a mode with multiple consecutive hiccup cycles (BUR), where each hiccup cycle (BUR) includes N switching cycles, and N is an integer greater than 1.
[0063] The first switch control module 440 and the second switch control module 450 are respectively used to control the turn-on and turn-off of the first switch transistor Q1 and the second switch transistor Q2 according to the first switch control signal Vg1 and the second switch control signal Vg2. The period control module 430 is connected to the first switch control module 440 and the second switch control module 450, and is used to generate an error amplification signal Vota according to the output voltage Vo, and then generate a preset first parameter curve and a preset second parameter curve according to the error amplification signal Vota. The first switch control module 440 generates the first switch control signal Vg1 according to the comparison result of the first sampling parameter and the preset first parameter curve, and the second switch control module 450 generates the second switch control signal Vg2 according to the comparison result of the second sampling parameter and the preset second parameter curve. Furthermore, the second switch control module 450 controls the second switch transistor Q2 to turn off in the first N-1 switching cycles according to the comparison result, and then controls the first switch control module 440 to turn on the first switch transistor Q1 when the source-drain voltage is zero, and controls the second switch transistor Q2 to turn off when the excitation inductor current in the Nth switching cycle is zero. This allows the first switch Q1 to be turned on when the source-drain voltage is close to zero during at least N-1 switching cycles of the hiccup cycle, achieving soft turn-on, reducing switching losses, and improving transmission efficiency. Furthermore, the second switch can be turned off in time during the last switching cycle to avoid generating a large negative magnetizing inductor current that would cause resonant energy loss.
[0064] Specifically, such as Figure 4 As shown, the main circuit 410 of the half-bridge flyback converter 400 includes 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 Cr located on the primary side of the transformer T, and a first diode D1 and an output capacitor Co located on the secondary side of the transformer T.
[0065] On the primary side of transformer T, the second switch Q2 and the first switch Q1 are connected in series between the voltage input terminal and the primary reference ground. In one possible embodiment, both the first switch Q1 and the second switch Q2 are NMOS field-effect transistors. The primary winding Np of transformer T, the first inductor Lk, and the first capacitor Cr are connected in series between the source and drain 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 transformer T in the resonant circuit is the magnetizing inductance Lm. Preferably, in low-power power supply applications, the leakage inductance of transformer T can be used instead of the first inductor Lk.
[0066] On the secondary side of transformer T, a first diode D1 is connected in series with the secondary winding Ns of transformer T between the voltage output terminal and the secondary reference 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 opposite to the excitation voltage of transformer T, to provide a DC output voltage Vo. An output capacitor Co is connected between the voltage output terminal and the secondary reference ground to filter the DC output voltage Vo to obtain a smooth voltage waveform. In an alternative embodiment, a synchronous rectifier switch can be used instead of the first diode D1.
[0067] Preferably, the half-bridge flyback converter further includes a sampling resistor Rcs connected between the drain of the first switch Q1 and a reference ground, for obtaining the current flowing through the first switch Q1 during the period when the first switch Q1 is turned on and the second switch Q2 is turned off.
[0068] It is understood that the main circuit of the half-bridge flyback converter in this embodiment can also be Figure 1 The topology shown.
[0069] Furthermore, the half-bridge flyback converter 400 also includes an auxiliary line 470, and the control circuit 200 further includes a zero-crossing detection unit 460. The auxiliary line 470 includes an auxiliary winding Na and resistors R1 and R2 connected in series with the auxiliary winding Na. The auxiliary winding Na is coupled to the primary winding Np or the secondary winding Ns. In this embodiment, the corresponding terminal of the auxiliary winding Na is grounded. The zero-crossing detection unit 460 is connected at the node of resistors R1 and R2 and is also connected to the first switch control module 440. The zero-crossing detection unit 460 is used to detect the moment when the source-drain voltage across the first switch transistor Q1 is zero based on the voltage of the auxiliary winding Na, and at the moment when the source-drain voltage is zero, the first switch control module 440 controls the first switch transistor Q1 to turn on.
[0070] Furthermore, under the control of switch control signals Vg1 and Vg2, the first switch Q1 and the second switch Q2 are turned on and off according to a predetermined switching cycle. In this embodiment, the first switch control module 440 and the second switch control module 450 control the first switch Q1 and the second switch Q2 to be complementaryly turned on during the first N-1 switching cycles of each hiccup cycle, and to remain off after being turned off in the Nth switching cycle until the next hiccup cycle. When the first switch Q1 is turned on and the second switch Q2 is turned off, the first capacitor charges, causing the voltage VC1 across the first capacitor Cr to increase. When the first switch Q1 is turned off and the second switch Q2 is turned on, the resonant circuit operates, and the first capacitor Cr discharges by providing a resonant current, transferring electrical energy from the primary side to the secondary side of the transformer. The DC output voltage Vo is regulated by adjusting the duty cycle of the switch control signals.
[0071] Specifically, the periodic control module 430 includes: an error amplification unit 431, a first curve setting unit 432, and a second curve setting unit 433. The error amplification unit 431 compares the output voltage Vo (after voltage division) with a reference voltage to generate an error amplification signal Vota. The first curve setting unit 432 is connected to the error amplification unit 431 and generates a preset first parameter curve based on the error amplification signal Vota. The second curve setting unit 433 is connected to the error amplification unit 431 and generates a preset second parameter curve based on the error amplification signal Vota. The preset first parameter curve and the preset second parameter curve are curves corresponding to changes in physical quantities such as current or voltage.
[0072] The first switch control module 440 includes a first comparison unit 441 and a first drive unit 442. The first comparison unit 441 receives a first sampling parameter and compares it with a preset first parameter curve to generate a comparison result. The first drive unit 442 generates a first switch control signal Vg1 based on the comparison result. The second switch control module 450 includes a second comparison unit 451 and a second drive unit 452. The second comparison unit 451 receives a second sampling parameter and compares it with a preset second parameter curve to generate a comparison result. The second drive unit 452 generates a second switch control signal Vg2 based on the comparison result. The preset first parameter curve is, for example, a current curve, and the first sampling parameter corresponds to the current flowing through the first switch transistor Q1. The preset second parameter curve is, for example, a frequency curve, and the second sampling parameter corresponds to the operating frequency. The first sampling parameter, the second sampling parameter, the preset first parameter curve, and the preset second parameter curve can also be set to other physical quantity parameters.
[0073] In this embodiment, the first switch control module 440 controls the turn-on and turn-off of the first switch Q1, for example, based on the comparison result of the first sampling parameter and the preset first parameter curve. The second switch control module 450 controls the turn-on and turn-off of the second switch Q2, for example, based on the comparison result of the second sampling parameter and the preset second parameter curve. Furthermore, the second switch control module 450 controls the second switch Q2 to continue conducting for a period of time during the first N-1 switching cycles when the magnetizing inductor current is zero, generating a negative magnetizing inductor current. Since the first capacitor Cr and the first inductor Lk resonate during the conduction period of the second switch Q2, when it is turned off, the first inductor Lk and the magnetizing inductor Lm continue to flow, releasing the charge on the junction capacitance of the first switch Q1. Therefore, in the next switching cycle, the first switch Q1 conducts with a source-drain voltage close to zero. The zero-crossing detection unit 460 detects the voltage across the auxiliary winding to determine the moment when the source-drain voltage across the first switching transistor Q1 is zero. When the source-drain voltage is zero, the first switching control module 440 turns on the first switching transistor Q1. Furthermore, the second switching control module 450 controls the second switching transistor Q2 to turn off when the magnetizing inductor current is zero in the last (Nth) switching cycle, thus avoiding the generation of a negative magnetizing current and preventing the loss of resonant energy.
[0074] According to the control circuit of the half-bridge flyback converter in the embodiment of this disclosure, the flyback converter enters a hiccup mode under light load conditions. The hiccup mode consists of multiple consecutive hiccup cycles, each of which includes N-1 critical on-off switching cycles and one intermittent on-off switching cycle. By controlling the on-time of the second switch, the first switch achieves ZVS turn-on within at least N-1 switching cycles, and the second switch is turned off in a timely manner during the intermittent on-off switching cycle to avoid generating a large negative magnetizing inductor current that causes energy loss. This reduces the turn-on loss in each switching cycle and improves the output power under light load conditions.
[0075] Figure 5 Show Figure 4 The diagram shows the operating waveforms of the half-bridge flyback converter. Figure 5 In the diagram, Vg1 and Vg2 represent the switching control signals of the first switch Q1 and the second switch Q2, respectively; Im represents the magnetizing inductance current flowing through the magnetizing inductor Lm; and Vsw represents the source-drain voltage across the first switch Q1.
[0076] like Figure 5As shown, the half-bridge flyback converter operates in normal state and hiccup mode. Under the control of switch control signals Vg1 and Vg2, the first switch Q1 and the second switch Q2 are turned on and off according to a predetermined switching cycle. Specifically, at time t1, the first switch control signal Vg1 controls the first switch Q1 to turn on, and the second switch signal Vg2 controls the second switch Q2 to turn off. The input voltage Vin charges the first inductor Lk, the magnetizing inductor Lm, and the first capacitor Cr. Then, during the time period t1-t2, the magnetizing inductor current Im and the voltage across the first capacitor Cr both increase, the source-drain voltage Vsw across the first switch Q1 drops to near zero, and the direction of the magnetizing inductor current Im is set to positive at this time. At time t2, the first switch control module 440 outputs the first switch control signal Vg1 according to the comparison result to control the first switch transistor to turn off, and the source-drain voltage Vsw gradually rises to the highest point. After a dead time, the second switch control module 450 outputs the second switch control signal Vg2 according to the comparison result to control the second switch transistor Q2 to turn on. During the time period t2-t4, the source-drain voltage Vsw remains at the highest point, for example, close to Vin. The second switch transistor Q2, the first capacitor Cr, and the first inductor Lk form a resonant circuit, and electrical energy is transferred from the primary side of the transformer to the secondary side, and the magnetizing inductor current Im decreases. At time t3, the magnetizing inductor current Im drops to zero. The second switch Q2 continues to conduct for a period of time until time t4. During the time interval t3-t4, the conduction of the second switch Q2 generates a negative magnetizing inductor current, causing the charge on the junction capacitance of the first switch Q1 to be released. The source-drain voltage across the first switch Q1 gradually drops to zero. Thus, after a dead time following the turn-off of the second switch Q2, the first switch Q1 is turned on at zero voltage at time t4. The time interval t1-t4 is one switching cycle, specifically one critical conduction switching cycle. In this embodiment, N is 3. One hiccup cycle includes two critical conduction switching cycles and one intermittent conduction switching cycle. Similarly, during the second critical conduction switching cycle, at time t5 when the magnetizing inductor current drops to zero, the second switch Q2 continues to conduct for a period of time, enabling the first switch Q1 to achieve zvs turn-on in the next switching cycle. However, at time t6, which is when the magnetizing inductor current drops to zero in the last switching cycle, the second switch Q2 is turned off, entering the resonant cycle. The period from t6 to t7 is the resonant time. Furthermore, at time t7, the first switch Q1 turns on, initiating the next hiccup cycle, with the frequency corresponding to the hiccup cycle being fBUR. Those skilled in the art will know that there is a certain dead time during the switching process of the first and second switches; the dead time will not be described in detail here.
[0077] Through the above embodiments, the control circuit of the half-bridge flyback converter of this application enables the flyback converter to achieve ZVS turn-on of the first switch in at least N-1 switching cycles of each hiccup cycle, and to turn off the second switch in time in the last switching cycle, thereby reducing the switching loss of the flyback converter under light or medium load and improving the output power, thus improving the reliability of the half-bridge flyback converter.
[0078] Figures 6a-6c Show each Figure 4 The waveform diagrams shown are of the half-bridge flyback converter under three different control modes.
[0079] To further improve output power, the cycle control module 430 generates different preset first parameter curves and preset second parameter curves to change the on and off times of the first and second switching transistors, thereby increasing output power. For example, changing the on-time of the first and second switching transistors changes the value of N, or changing the frequency of the hiccup cycle or the peak value of the magnetizing inductor current to change the output power, respectively corresponding to... Figures 6a-6c The waveform diagram.
[0080] Specifically, the cycle control module 430 controls the first and second switching transistors to turn on and off in different sequences according to different preset first parameter curves and preset second parameter curves. The cycle control module 430 adjusts the preset first parameter curves and preset second parameter curves, causing the first switch control module 440 and the second switch control module 450 to respectively adjust the on and off times of the first switching transistor Q1 and the second switching transistor Q2, thereby adjusting the number N of switching cycles included in the hiccup cycle. The output power of the half-bridge flyback converter is proportional to the value of N. For example... Figure 6a As shown, by extending the switching cycle of the first and second switching transistors by one more switching cycle, the resonant period is shortened. In the next hiccup cycle, the number of switching cycles is increased (from N to N+1), while the frequency and the peak value of the magnetizing inductor current remain unchanged, thereby increasing the output power. The changes in each signal during each switching cycle are shown in the diagram. Figure 5 .
[0081] In another embodiment, the cycle control module 430 controls the first and second switching transistors to turn on and off in different sequences according to different preset first parameter curves and preset second parameter curves. The cycle control module 430 adjusts the preset first parameter curves and preset second parameter curves, causing the first switch control module 440 and the second switch control module 450 to respectively adjust the on and off times of the first switching transistor Q1 and the second switching transistor Q2, thereby adjusting the frequency of the hiccup cycle. The output power of the half-bridge flyback converter is proportional to the frequency of the hiccup cycle. Figure 6bAs shown, by shortening the resonant period, the frequency of the hiccup cycle is increased from the original fBUR to fBUR2, while the number of switching cycles and the peak value of the magnetizing inductor current remain unchanged, thereby increasing the output power. The transformation of each signal in each switching cycle is referenced. Figure 5 .
[0082] In another embodiment, the period control module 430 controls the first and second switching transistors to turn on and off in different sequences according to different first and second parameter curves. The period control module 430 adjusts the preset first and second parameter curves, causing the first switch control module 440 and the second switch control module 450 to respectively adjust the turn-on and turn-off times of the first switching transistor Q1 and the second switching transistor Q2, thereby adjusting the peak value of the magnetizing inductor current. Furthermore, the output power of the half-bridge flyback converter is proportional to the peak value of the magnetizing inductor current. Figure 6c As shown, by extending the conduction time of the first switch in each switching cycle, the peak value of the magnetizing inductor current is increased from Ipek1 to Ipek2, while the number of switching cycles N and the frequency remain unchanged, thus increasing the output power. The signal transformations in each switching cycle can be referred to... Figure 5 .
[0083] Figure 7 A circuit block diagram showing the topology and control circuitry of a half-bridge flyback converter according to a second embodiment of the present disclosure is provided. Figure 7 In the half-bridge topology shown, the half-bridge flyback converter 500 includes a main circuit 510, an auxiliary circuit 570, and a control circuit 520. The main circuit 510, auxiliary circuit 570, and control circuit 520, including the first switch control module 540, the second switch control module 550, the period control module 530, and the zero-crossing detection unit 560, are all consistent with the first embodiment and will not be described again here.
[0084] In this embodiment, the second switch control module 550 is also used to control the second switch Q2 to conduct for a period of time in the first switching cycle of each hiccup cycle and before the first switch Q1 is turned on, to generate a negative magnetizing inductance current, so that the junction capacitance charge of the first switch Q1 is released, so that the first switch Q1 can achieve ZVS turn-on in the first switching cycle, and thus the first switch Q1 can achieve ZVS turn-on in the entire hiccup cycle.
[0085] Specifically, when the flyback converter enters the resonant cycle and is about to enter the next hiccup cycle, the second switching control module 550 compares the operating frequency with the frequency curve and controls the second switching transistor Q2 to conduct for a period of time before the first switching transistor Q1 conducts in the first switching cycle of the next hiccup cycle, thereby generating a negative magnetizing inductance current. This negative magnetizing inductance current causes the junction capacitance charge of the first switching transistor Q1 to be released, and the source-drain voltage of the first switching transistor Q1 approaches zero. Furthermore, in the first switching cycle, when the zero-crossing detection unit 560 detects that the source-drain voltage across the first switching transistor Q1 is zero, the first switching control module 540 controls the first switching transistor Q1 to conduct at zero voltage. Thus, the first switching transistor can achieve ZVS turn-on throughout the entire hiccup cycle, reducing turn-on losses. Further, the zero-crossing detection unit 560 can also be connected to the second switching control module 550 to control the second switching transistor Q2 to turn off when the source-drain voltage across the first switching transistor Q1 is detected to be zero.
[0086] Furthermore, the control circuit 520 may also include a start-up generation unit 580, which sends a turn-on control signal to the second switch control module 550 during the first switching cycle of each hiccup cycle and before the first switch Q1 is turned on, so that the second switch control module 550 controls the second switch Q2 to be turned on for a period of time to generate a negative magnetizing inductor current.
[0087] Furthermore, the first curve setting unit 532 is used to generate a preset first parameter curve l1, the second curve setting unit 533 is used to generate a preset second parameter curve l2 and a third parameter curve l3, and the start generation unit 580 sends a conduction control signal to the second switch control module 550 according to the operating frequency fcs and the third parameter curve.
[0088] It is understood that the main circuit of the half-bridge flyback converter in this embodiment can also be Figure 1 The topology shown.
[0089] Figure 8 Show Figure 7 The waveform diagram of the half-bridge flyback converter is shown.
[0090] like Figure 8As shown, at time t0, the second switch Q2 is turned on for a period of time, generating a negative magnetizing inductor current Im. It is turned off before time t1. During the time interval t0-t1, the zero-crossing detection unit 560 detects that the source-drain voltage across the first switch has dropped to zero, turning off the second switch Q2. After a dead time, at time t1, the first switch Q1 is turned on. During the time interval t1-t2, the first switch Q1 is turned on, the second switch Q2 is turned off, and the magnetizing inductor current Im rises. During the time interval t2-t4, the first switch Q1 is turned off, and the second switch Q2 is turned on, preparing for the soft turn-on of the first switch in the next switching cycle. At time t4, the first switch Q1 is turned on. At time t5, the second switch Q2 continues to be turned on for a period of time, enabling the first switch Q1 to achieve ZVS turn-on in the next switching cycle. At time t6, the second switch Q2 is turned off when the magnetizing inductor current is zero, and the resonant cycle begins during the time interval t6-t7. At time t7, the second switch Q2 generates a negative magnetizing inductor current, initiating the next cycle and enabling the first switch Q1 to achieve ZVS turn-on at time t8. Those skilled in the art will know that there is a certain dead time during the turn-on and turn-off switching processes of both the first and second switches; therefore, the dead time will not be described in detail here.
[0091] The control circuit of the half-bridge flyback converter in this embodiment turns on the second switch for a period of time before the first switch is turned on in the first switching cycle, generating a negative magnetizing inductor current. This enables the first switch to achieve soft start throughout the entire hiccup cycle, thereby further reducing turn-on losses and increasing output power.
[0092] This invention also provides a control method for a half-bridge flyback converter. The half-bridge flyback converter includes a transformer, a first switch and a second switch connected in series, and a first capacitor and a first inductor forming a resonant circuit when the second switch is on. The control method includes: controlling the first switch to turn off and then turning on the second switch; controlling the second switch to continue conducting for a period of time during the first N-1 switching cycles when the magnetizing inductance current is zero, generating a negative magnetizing inductance current; controlling the second switch to turn off during the first N-1 switching cycles and then turning on the first switch with zero voltage; controlling the second switch to turn off when the magnetizing inductance current is zero in the Nth switching cycle, entering the resonant cycle. The half-bridge flyback converter operates in a hiccup mode with multiple consecutive hiccup cycles according to the control method, each hiccup cycle including N switching cycles, where N is an integer greater than 1. This half-bridge flyback converter can be... Figures 4-8 Any half-bridge flyback converter in the embodiments.
[0093] Furthermore, the control method also includes: controlling the second switch to turn on before the first switch to turn on during the first switching cycle; and controlling the first switch to turn on when the source-drain voltage across the first switch is detected to be zero.
[0094] In summary, according to the control circuit and control method of the half-bridge flyback converter in the embodiments of this disclosure, the half-bridge flyback converter is controlled to operate in hiccup mode. Each hiccup cycle includes N switching cycles. By controlling the second switch to be turned on for a period of time in each of the first N-1 switching cycles, the first switch can achieve ZVS turn-on after it is turned off. In the last switching cycle, the second switch is turned off when the excitation current is close to zero to avoid generating a large negative excitation current. This can reduce the turn-on loss, reduce the energy loss of the resonant cycle after the two switches are turned off, and achieve soft-switching start of the first switch, thereby improving the transmission efficiency of the flyback converter under light load.
[0095] Furthermore, the transmission efficiency of the flyback converter can be improved by increasing the value of N, increasing the frequency of the hiccup cycle, or increasing the peak value of the magnetizing inductor current.
[0096] Furthermore, by turning on the second switch for a period of time before the first switch is turned on during the first switching cycle, and turning on the first switch when the source-drain voltage across the first switch is detected to be zero, the first switch is turned on by ZVS during the first switching cycle. Thus, the first switch can be soft-switched on throughout the entire hiccup cycle, further reducing turn-on losses and resulting in low overall system power loss.
[0097] The half-bridge flyback converters in the above embodiments are all described using an asymmetric half-bridge flyback converter as an example. However, it is understood that this disclosure is not limited thereto. Based on similar working principles, the solutions in the above embodiments can also be applied to half-bridge flyback converters with other structures, such as half-bridge flyback converters with active clamps.
[0098] The embodiments described above do not constitute a limitation on the scope of protection of this technical solution. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the above embodiments should be included within the scope of protection of this technical solution.
Claims
1. A control circuit for a half-bridge flyback converter, the half-bridge flyback converter comprising a transformer, a first switch and a second switch connected in series, and a first capacitor and a first inductor connected in series across the second switch, the control circuit comprising: The first switch control module controls the on and off of the first switch transistor according to the first switch control signal; The second switch control module controls the on and off of the second switch transistor according to the second switch control signal. The half-bridge flyback converter operates in a hiccup mode with multiple consecutive hiccup cycles. Each hiccup cycle includes N switching cycles. The second switching control module controls the second switching transistor to turn off in the first N-1 switching cycles, and then controls the first switching control module to turn on the first switching transistor with zero voltage. The second switching transistor is also controlled to turn off when the magnetizing inductor current is zero in the Nth switching cycle, where N is an integer greater than 1.
2. The control circuit according to claim 1, wherein, The first switch control module and the second switch control module control the first switch transistor and the second switch transistor to conduct complementaryly during the first N-1 switching cycles, and maintain the off state after being turned off in the Nth switching cycle until the next hiccup cycle.
3. The control circuit according to claim 1, wherein, The first switch control module generates the first switch control signal based on the comparison result of the first sampling parameter and the preset first parameter curve, and the second switch control module generates the second switch control signal based on the comparison result of the second sampling parameter and the preset second parameter curve.
4. The control circuit according to claim 3, wherein, The second switching control module controls the second switching transistor to continue conducting for a period of time when the magnetizing inductor current is zero during the first N-1 switching cycles, thereby generating a negative magnetizing inductor current and releasing the junction capacitance charge of the first switching transistor.
5. The control circuit according to claim 3, wherein, It also includes a periodic control module, which comprises: The error amplification unit generates an error amplification signal based on the output voltage. A first curve setting unit, connected to the error amplification unit, generates the preset first parameter curve based on the error amplification signal; and The second curve setting unit is connected to the error amplification unit and generates the preset second parameter curve according to the error amplification signal.
6. The control circuit according to claim 3, wherein, The preset first parameter curve is a current curve, and the first sampling parameter is the current flowing through the first switching transistor; the preset second parameter curve is a frequency curve, and the second sampling parameter is the operating frequency.
7. The control circuit according to claim 3, wherein, The preset first parameter curve and the preset second parameter curve are adjusted so that the first switch control module and the second switch control module respectively adjust the turn-on time and turn-off time of the first switch and the second switch, thereby adjusting the number N of the switching cycles included in the hiccup cycle, and the output power of the half-bridge flyback converter is proportional to the value of N.
8. The control circuit according to claim 3, wherein, The preset first parameter curve and the preset second parameter curve are adjusted so that the first switch control module and the second switch control module respectively adjust the turn-on time and turn-off time of the first switch and the second switch, thereby adjusting the frequency of the hiccup cycle, and the output power of the half-bridge flyback converter is proportional to the frequency of the hiccup cycle.
9. The control circuit according to claim 3, wherein, The preset first parameter curve and the preset second parameter curve are adjusted so that the first switch control module and the second switch control module respectively adjust the turn-on time and turn-off time of the first switch and the second switch, thereby adjusting the peak value of the magnetizing inductor current, and the output power of the half-bridge flyback converter is proportional to the peak value of the magnetizing inductor current.
10. The control circuit according to claim 4, wherein, The half-bridge flyback converter includes an auxiliary winding, and the control circuit further includes: The zero-crossing detection unit is connected to the first switch control module and is used to detect the moment when the source-drain voltage across the first switch tube is zero based on the voltage of the auxiliary winding, and to control the first switch control module to turn on the first switch tube when the source-drain voltage is zero.
11. The control circuit according to claim 4, wherein, The second switch control module also controls the second switch to conduct for a period of time before the first switch is turned on during the first switching cycle, generating a negative magnetizing inductance current to release the junction capacitance charge of the first switch.
12. The control circuit according to claim 1, wherein, The first and second switching transistors are connected in series between the power input terminal and the reference ground.
13. A control method for a half-bridge flyback converter, the half-bridge flyback converter comprising a transformer, a first switch and a second switch connected in series, and a first capacitor and a first inductor connected in series across the second switch, the control method comprising: The second switch is turned on after the first switch is turned off. The second switch is turned off during the first N-1 switching cycles, and then the first switch is turned on with zero voltage. The second switch is turned off when the magnetizing inductor current is zero in the Nth switching cycle, thus entering the resonant cycle. The half-bridge flyback converter operates in a hiccup mode with multiple consecutive hiccup cycles according to the control method. Each hiccup cycle includes N switching cycles, where N is an integer greater than 1.
14. The control method according to claim 13, wherein, It also includes: controlling the second switch to continue conducting for a period of time when the magnetizing inductor current is zero during the first N-1 switching cycles, thereby generating a negative magnetizing inductor current.
15. The control method according to claim 13, wherein, Also includes: The second switch is controlled to be turned on for a period of time before the first switch is turned on during the first switching cycle; When the source-drain voltage across the first switch is detected to be zero, the first switch is turned on.