Forward-flyback converter with active-clamp circuit

Forward-flyback converter with active clamping circuit

The present invention discloses a forward-flyback converter with an active clamping circuit, in particular to a converter that can be used in a wide load variation range and can take into account the conversion efficiency of heavy load and light load output at the same time, and has both cost-effectiveness. Isolated flexible switching DC-DC converter.

Due to the enhanced functions of electronic products on the market, power supplies are required to provide greater output energy to respond. At the same time, small size and light weight of products are desired. These trends have resulted in two major demands for power converters: increasing power density And improve conversion efficiency. Based on these two requirements, the active clamp forward-flyback converter and its derivative circuits, which can continuously transfer energy to the output terminal and have flexible switching functions, have begun to be widely used in various power products.

The traditional active clamp forward-flyback converter is shown in the first figure, which has a DC power supply<i>V</i>_<i>in</i>, Used to supply DC input voltage, a main switch<i>S</i>₁, An auxiliary switch<i>S</i>₂, A resonant inductor<i>L</i>_<i>r</i>(Mostly transformer leakage inductance), a resonant capacitor<i>C</i>_<i>r</i>(For switch<i>S</i>₁and<i>S</i>₂The sum of the capacitance between the poles), a clamp capacitor<i>C</i>_<i>c</i>, One has a primary winding<i>N</i>₁, A first secondary winding<i>N</i>₂With a second secondary winding<i>N</i>₃Transformer<i>T</i>_<i>r</i>(<i>N</i>₁，<i>N</i>2 and<i>N</i>₃The turns ratio is<i>n</i>₁：<i>n</i>₂：<i>n</i>₃), a first diode<i>D</i>₁, A second diode<i>D</i>₂, An output inductor<i>L</i>_<i>o</i>, An output capacitor<i>C</i>_<i>o</i>, And a load<i>R</i>_L. The active clamp forward-flyback converter can output a stable DC voltage<i>V</i>_<i>o</i>. Due to transformer<i>T</i>_<i>r</i>The secondary side adopts a center-tapped design, so the input energy is in the main switch<i>S</i>₁Both the on and off periods can be transferred to the output terminal, so that the circuit has a larger energy processing capability. At the same time, in a switching cycle, the current can be continuously output so that the output capacitance<i>C</i>_<i>o</i>The current ripple is small, so the required output filter capacitance is also small. In addition, the Zero Voltage Switching (ZVS) design with active clamping can effectively reduce the switching loss of the circuit. Therefore, a higher switching frequency can be selected to reduce the volume and weight of passive components. Active clamp design utilizes auxiliary switch<i>S</i>₂With clamp capacitor<i>C</i>_<i>c</i>Clamping circuit composed in series to achieve the main switch<i>S</i>₁When the switch is off, the purpose of voltage clamping and magnetic reset of the transformer. At the same time, this circuit can achieve two switches through complementary driving.<i>S</i>₁and<i>S</i>₂The zero voltage switching. Since the active clamp circuit can achieve flexible switching, switch span voltage clamping, and transformer magnetic reset by using a streamlined architecture, it has the advantage of low cost.

Active clamp forward-the main switch of the flyback converter<i>S</i>₁Zero voltage switching occurs at the auxiliary switch<i>S</i>₂After cut-off and main switch<i>S</i>₁A short period of preset dead time before turning on, in this interval, it flows through the main switch<i>S</i>₁Current<i>i</i>_<i>S</i>1Oscillate to a negative value to discharge the capacitance between switch poles to 0, and then force the main switch<i>S</i>₁The anti-parallel diode conducts flow, and the main switch is switched during the conduction period of the anti-parallel diode<i>S</i>₁Turn on to achieve the main switch<i>S</i>₁The zero voltage conduction. However, the actual circuit operation cannot achieve the main switch smoothly as described above.<i>S</i>₁The reason for zero voltage switching is explained as follows. The second figure is a schematic diagram of the discharge process of the capacitance between the switches of the active clamp forward-flyback converter. The second secondary winding at this time<i>N</i>₃Current<i>i</i>_<i>N</i>3Gradually drop, the first secondary winding<i>N</i>₂Current<i>i</i>_<i>N</i>2Gradually rise, according to Amperes law, with<i>i</i>_<i>N</i>2and<i>i</i>_<i>N</i>3The gradual change, the primary winding<i>N</i>₁Current<i>i</i>_<i>N</i>1It will switch from outgoing dot to incoming dot. when<i>i</i>_<i>N</i>1After being converted into dot, the current<i>i</i>_<i>S</i>1Will quickly become smaller, switch the capacitance between poles<i>C</i>_<i>r</i>The discharge speed is thus drastically reduced. Because of this feature,<i>v</i>_<i>Cr</i>Unable to switch on the main<i>S</i>₁It drops to 0 before turning on, so the main switch cannot be effectively reached<i>S</i>₁The zero-voltage switching during turn-on causes the conversion efficiency to decrease. This shortcoming is especially serious when the output is heavy. In order to solve this deficiency, the conventional technology in the art has adopted the first secondary winding<i>N</i>₂With the first diode<i>D</i>₁Add an additional inductance or a Saturable Reactor (Saturable Reactor) to suppress the first secondary winding<i>N</i>₂Current<i>i</i>_<i>N</i>2Capacitance between switch poles<i>C</i>_<i>r</i>The rate of rise during discharge. The addition of additional inductive components does help the main switch<i>S</i>₁Achieve zero-voltage switching, and effectively solve the problem of switch overheating, but the additional inductive components will cause a lot of iron loss under the design of high-frequency operation, so there is still room for improvement in the solution of this problem.

Different from the low conversion efficiency of the active-clamped forward-flyback converter when the output is under heavy load, the active-clamped flyback converter does not have the first secondary winding to transfer energy in the forward direction.<i>N</i>₂, So the capacitance between the switch poles<i>C</i>_<i>r</i>When discharging, the resonance current<i>i</i>_<i>Lr</i>Will not rapidly decrease due to the generation of secondary side current and affect the main switch<i>S</i>₁The function of zero voltage switching. In an active clamp flyback converter, its resonant current<i>i</i>_<i>Lr</i>The peaks and valleys are determined by the load<i>R</i>_<i>L</i>The size is determined. When the load<i>R</i>_<i>L</i>When larger, the resonance current<i>i</i>_<i>Lr</i>There will be larger peaks and valleys, so it is beneficial to the main switch when the output is heavy<i>S</i>₁The zero voltage switching.

In order to effectively improve the zero-voltage switching mechanism of the active-clamp forward-flyback converter under heavy load output, a feasible strategy is formed by using the zero-voltage switching design of the active-clamp flyback converter. In addition, in recent years, due to the wide load fluctuation requirements of power supply equipment, the efficiency performance during light-load operation has gradually been paid attention to. When the active-clamp forward-flyback converter is operating at light load, current flows through both branches of the secondary side, resulting in unnecessary conduction losses. In Intels latest Voltage Regulator Module (VRM) specification (VRM/EVRD 11.1), it specifies the requirements for improving efficiency under light load, although the current use of isolated power converters for communication and network The system or other industrial fields have not yet determined this specification, but it is very likely that this requirement will be proposed in the near future.

For this reason, in view of the lack of conventional technology, the inventor thought and improved the idea of invention, and finally invented the "forward-flyback converter with active clamping circuit" of this case.

The main purpose of this case is to provide an active clamp forward-flyback converter that can simultaneously take into account the conversion efficiency of heavy load and light load output under a wide load variation range. Under heavy load output, it uses an active clamp flyback converter zero-voltage switching mechanism to improve the shortcomings of insufficient resonant current of the traditional active clamp forward-flyback converter; under light load output, its utilization is extended The time for the resonant current to change from negative to positive to ensure that the switch can achieve zero-voltage switching, and the design makes the diode of the flyback sub-circuit not have enough forward bias voltage to make the sub-circuit unable to continue to work, so as to reduce unnecessary The component power loss.

The purpose of this case is that the flyback sub-circuit on the secondary side is designed in Continuous Conduction Mode (CCM), and the cathode of the diode is connected to the positive terminal of the output capacitor, so when the main switch is turned off , The magnetic energy of the transformer reset (Reset) can be directly transferred to the output capacitor and the load terminal via the secondary winding of the flyback sub-circuit, which is conducive to energy conversion.

Another purpose of this case is to add a flywheel diode (Flywheeling Diode) to the forward sub-circuit of the secondary side, so that the discharge current slope of the output inductor is not affected by the voltage of the clamping capacitor, and the output inductor is designed In Discontinuous Conduction Mode (DCM), the duty cycle ratio of the main switch is<i>D</i>It will automatically decrease as the load becomes lighter, and the voltage of the clamp capacitor will also decrease as the duty cycle ratio decreases. When the load continues to become lighter, the voltage of the clamping capacitor will be so low that the voltage across the secondary winding coupled to the flyback sub-circuit is not enough to bias the flyback diode forward. The flyback sub-circuit will no longer work, so that unnecessary component power loss can be reduced, so that the efficiency of the circuit of the present invention under light load can be improved. At the same time, the starting point of this mechanism can be determined by designing the turns ratio of the primary and secondary windings. The starting point is generally selected at a load operating point with too low efficiency.

Another purpose of this case is to continue working with the forward sub-circuit with only DCM operation under light load. Therefore, the circuit of the present invention is equivalent to an actively clamped DCM forward converter, because the resonant current of this structure is negative The process of turning positive is no longer affected by the flyback sub-circuit, so the slope of the current change is a relatively flat line segment. When the load becomes lighter, the time for the resonant current to be negative will be prolonged. This feature helps to achieve zero-voltage switching of the switch under light load.

Another purpose of this case is to use the innovative connection relationship on the secondary side to make the reverse bias of the diode lower than that of the traditional active clamp forward-flyback converter, so low cost and low withstand voltage can be used. element. And because the output inductor operates in DCM, the required inductance will be much smaller than the inductance of the traditional active clamp forward-flyback conversion circuit, so the volume and cost of the inductance can be reduced, so the present invention is cost-effective .

In order to make the above-mentioned objects, features, and advantages of the present invention more obvious and understandable, the following is a detailed description of preferred embodiments in conjunction with the accompanying drawings:

The third figure is a schematic circuit diagram of a buck-boost active-clamp forward-flyback converter according to the first preferred embodiment of the present invention, which includes a buck-boost active-clamp forward-backward converter A chi-type DC-DC converter and a control circuit can achieve the output voltage regulation of the DC-DC converter circuit and the fast response function under dynamic load changes by means of the control circuit. The polarity of each voltage and the direction of the current in the active clamp forward-flyback converter have been defined in the third figure. The key to the non-operation of the flyback sub-circuit when the input energy is continuously transferred to the output terminal through the secondary winding of the transformer and can perform light load is a transformer<i>T</i>_<i>r</i>, The transformer has a primary winding<i>N</i>₁, A first secondary winding<i>N</i>₂And a second secondary winding<i>N</i>₃, The winding<i>N</i>₂With this winding<i>N</i>₃It is wound on the secondary side and designed in series, so center-tapped windings or separate windings can be used. The windings<i>N</i>₁, Winding<i>N</i>₂And winding<i>N</i>₃The turns ratio is<i>n</i>₁：<i>n</i>₂：<i>n</i>₃. In the present invention, the implementation of the forward-flyback DC-DC converter with active clamping is described as follows.

As shown in the third figure, the transformer<i>T</i>_<i>r</i>The primary side includes a DC power supply<i>V</i>_<i>in</i>, Winding<i>N</i>₁, Connect the input DC power supply<i>V</i>_<i>in</i>With winding<i>N</i>₁Resonant inductance<i>L</i>_<i>r</i>, Main switch<i>S</i>₁With auxiliary switch<i>S</i>2. Parallel to the main switch<i>S</i>₁Resonant capacitor<i>C</i>_<i>r</i>, Where the resonant inductance<i>L</i>_<i>r</i>Available transformer<i>T</i>_<i>r</i>Leakage inductance or an external inductance or the sum of the two, the main switch<i>S</i>₁With auxiliary switch<i>S</i>₂All of them are MOSFETs with built-in parallel diodes, resonant capacitors<i>C</i>_<i>r</i>Is the main switch<i>S</i>₁With auxiliary switch<i>S</i>₂The sum of capacitance between electrodes. Main switch<i>S</i>₁Connect to the winding in its drain direction<i>N</i>₁, The source (Source) direction is connected to the DC power supply<i>V</i>_<i>in</i>The negative terminal. The implementation of the clamp circuit can use a common buck-boost design, as shown in the third figure, that is, the auxiliary switch<i>S</i>₂Connect to the winding in the source direction<i>N</i>₁And main switch<i>S</i>1 contact point, a clamping capacitor<i>C</i>_<i>c</i>, One end of which is connected to the resonant inductor<i>L</i>_<i>r</i>With input DC power<i>V</i>_<i>in</i>, The other end is connected to the auxiliary switch<i>S</i>2 Drain; or using a boost design, the circuit diagram of a boost type active clamp forward-flyback converter according to the second preferred embodiment of the present invention is shown in Figure 4; or The step-down design is adopted. The circuit diagram of a step-down active-clamp forward-flyback converter according to the third preferred embodiment of the present invention is shown in Figure 5. Figures 3, 4 and 5, the transformer<i>T</i>_<i>r</i>The secondary side includes a set of forward sub-circuits, which are composed of windings<i>N</i>₂, The first diode<i>D</i>₁, The second diode<i>D</i>₂And output inductor<i>L</i>_<i>o</i>Composed of the output inductor<i>L</i>_<i>o</i>Work in discontinuous conduction mode. It also includes a set of flyback sub-circuits working in continuous conduction mode, which is composed of the third winding<i>N</i>₃With the third diode<i>D</i>₃Composed of the third diode<i>D</i>₃The cathode is connected to the output capacitor<i>C</i>_<i>o</i>The positive extreme. If the present invention is applied to the output of low voltage and high current, the diode on the secondary side<i>D</i>₁、<i>D</i>₂and<i>D</i>₃It can be replaced by a synchronous rectifier switch.

The sixth figure is a schematic circuit diagram of the controller of the active clamp forward-flyback converter according to the first to third preferred embodiments of the present invention. The controller includes a voltage feedback and optical coupling circuit, a pulse width modulation (PWM) control and frequency response compensation circuit, an inverter (Inverter Gate), a main switch dead time adjustment circuit, and an auxiliary switch dead time adjustment circuit. Zone time adjustment circuit and a high and low switch gate drive circuit.

According to the state of switch conduction and diode conduction (Conduct), in the normal operation mode with large load current, each switching cycle can be divided into eight working stages. Figure 7 (a) to Figure 7 (h) respectively show an action schematic diagram of the equivalent circuit of the buck-boost active clamp forward-flyback converter according to the first preferred embodiment of the present invention . In order to facilitate the explanation of the principle, the operation schematic diagram of the equivalent circuit in the seventh figure (a) to the seventh figure (h), considering the magnetizing inductance of the transformer<i>L</i>_<i>m</i>, Treat it as parallel to the first winding of the transformer<i>N</i>₁On both ends, the action of the control circuit is not shown in the figure. The waveform of the present invention under heavy load and light load output is shown in Figure 8.<i>v</i>_<i>GS</i>1and<i>v</i>_<i>GS</i>2Respectively switch<i>S</i>₁and<i>S</i>₂The gate drive signal,<i>v</i>_<i>N</i>1Is the primary winding<i>N</i>₁Voltage signal,<i>v</i>_<i>Cr</i>Is the resonant capacitor<i>C</i>_<i>r</i>Voltage signal,<i>i</i>_<i>Lr</i>Is flowing through the resonant inductor<i>L</i>_<i>r</i>The current signal,<i>i</i>_<i>Lm</i>Is the magnetizing inductance flowing through the transformer<i>L</i>_<i>m</i>The current signal,<i>i</i>_<i>D</i>1、<i>i</i>_<i>D</i>2and<i>i</i>_<i>D</i>3Respectively flow through the diode<i>D</i>₁、<i>D</i>₂and<i>D</i>₃ofcurrent signal. According to the action schematic diagram of the equivalent circuit in Fig. 7 and the steady-state working waveform diagram in Fig. 8, the operation principle of each working stage of the present invention in the normal operation mode is explained as follows: 1. The first stage: (<i>t</i>₀≦<i>t</i>&lt;<i>t</i>₁) As shown in Figure 7 (a), the main switch at this time<i>S</i>₁On, auxiliary switch<i>S</i>₂Cut off, input DC voltage<i>V</i>_<i>in</i>Equal to resonant inductance<i>L</i>_<i>r</i>With primary winding<i>N</i>₁The sum of the cross voltages, due to the first secondary winding<i>N</i>₂A positive voltage is induced on it, so the first diode<i>D</i>₁Load that turns on and transfers input energy to the output<i>R</i>_<i>L</i>, The second secondary winding<i>N</i>₃A negative voltage is induced on it, so the third diode<i>D</i>₃It is cut off due to reverse bias, and this stage is like a normal forward conversion circuit.

2. The second stage: (<i>t</i>₁≦<i>t</i>&lt;<i>t</i>₂) As shown in Figure 7(b), the switching element at this time<i>S</i>₁、<i>S</i>₂Cut off, transformer primary current<i>i</i>_<i>Lr</i>Resonant capacitor<i>C</i>_<i>r</i>Charge, because<i>C</i>_<i>r</i>The capacity is very small, so the resonant capacitor<i>C</i>_<i>r</i>Cross pressure<i>v</i>_<i>Cr</i>Quickly rise from 0 to<i>V</i>_<i>in</i>。

3. The third stage: (<i>t</i>₂≦<i>t</i>&lt;<i>t</i>₃) As shown in Figure 7 (c),<i>v</i>_<i>Cr</i>Rise to a high enough value to make the primary side of the transformer cross-voltage<i>v</i>_<i>N</i>1Equal to 0, at this time, the first diode<i>D</i>₁And the second diode<i>D</i>₂Simultaneously diversion and output inductance<i>L</i>_<i>o</i>Transfer the stored energy to the load<i>R</i>_<i>L</i>, And the output inductor<i>L</i>_<i>o</i>Current<i>i</i>_<i>Lo</i>It starts to decrease linearly. The resonance tank circuit on the primary side at this stage is caused by<i>L</i>_<i>r</i>and<i>C</i>_<i>r</i>composition.

4. The fourth stage: (<i>t</i>₃≦<i>t</i>&lt;<i>t</i>₄) As shown in Figure 7 (d),<i>v</i>_<i>Cr</i>Raised to make the auxiliary switch<i>S</i>₂The anti-parallel diode diversion, gradually rising<i>v</i>_<i>Cr</i>Will make<i>i</i>_<i>Lr</i>Gradually decreases, so the winding current induced on the secondary side of the transformer, that is, flows through the first diode<i>D</i>₁Current<i>i</i>_<i>D</i>1, Will gradually decrease until<i>i</i>_<i>D</i>1=0, and flow through the second diode<i>D</i>₂Current<i>i</i>_<i>D</i>2Will gradually rise until<i>i</i>_<i>D</i>2＝<i>i</i>_<i>Lo</i>. exist<i>i</i>_<i>D</i>1Before it drops to 0, the voltage across the primary side of the transformer<i>v</i>_<i>N</i>1All remain at 0.

S. The fifth stage: (<i>t</i>₄≦<i>t</i>&lt;<i>t</i>₅) As shown in Figure 7(e), at this stage,<i>i</i>_<i>Lr</i>Drop below<i>i</i>_<i>Lm</i>, So the primary winding current<i>i</i>_<i>N</i>1Will be turned into a negative direction and flow out dot, at this time the second secondary winding<i>N</i>₃Will induce current<i>i</i>_<i>N</i>3Inflow dot. Because the third diode<i>D</i>₃Diversion makes the primary winding<i>N</i>₁Voltage<i>v</i>_<i>N</i>1By voltage-<i>V</i>_<i>o</i>．<i>n</i>₁/<i>n</i>₃Clamped, at the same time resonant inductance<i>L</i>_<i>r</i>With clamp capacitor<i>C</i>_<i>c</i>Form a resonant circuit. To make the auxiliary switch<i>S</i>₂To achieve zero voltage switching, it must be<i>i</i>_<i>Lr</i>Flow through the auxiliary switch<i>S</i>₂When the reverse parallel connection of the diode and the auxiliary switch is not reversed<i>S</i>₂Conduction.

6. The sixth stage: (<i>t</i>₅≦<i>t</i>&lt;<i>t</i>₆) As shown in Figure 7 (f), the output inductor current at this stage<i>i</i>_<i>Lo</i>Has been reduced to 0, the rest of the circuit working principle is the same as the fifth stage.

7. The seventh stage: (<i>t</i>₆≦<i>t</i>&lt;<i>t</i>₇) As shown in the seventh figure (g), the auxiliary switch<i>S</i>₂Cut off, connect the clamp capacitor<i>C</i>_<i>c</i>The path is cut off, at this time, the resonant inductance<i>L</i>_<i>r</i>With resonant capacitor<i>C</i>_<i>r</i>A new high-speed resonant circuit is formed. With this new resonant circuit, the reverse flow<i>i</i>_<i>Lr</i>Makes the resonant capacitance<i>C</i>_<i>r</i>Start to discharge, and its cross voltage<i>v</i>_<i>Cr</i>exist<i>t</i>₇The point in time dropped to zero.

8. The eighth stage: (<i>t</i>₇≦<i>t</i>&lt;<i>t</i>₈) As shown in Figure 7 (h), the resonant capacitor<i>C</i>_<i>r</i>After fully discharged to 0, the main switch<i>S</i>₁The anti-parallel diode starts to divert, in order to make the main switch<i>S</i>₁To achieve zero voltage switching, the current must flow through the main switch<i>S</i>₁The main switch when the diode is connected in reverse and not reversed<i>S</i>₁Conduction. At this stage, the output voltage is still output due to the voltage across the transformer winding<i>V</i>_<i>o</i>Restrained, therefore<i>i</i>_<i>Lr</i>Linear increase, its rising rate of change can be determined by (<i>V</i>_<i>in</i>＋<i>V</i>_<i>o</i>．<i>n</i>₁/<i>n</i>₃)/<i>L</i>_<i>r</i>Decision; at the same time, flowing through the second secondary winding<i>N</i>₃Current<i>i</i>_<i>N</i>3Decrease linearly and merge in<i>t</i>₈The point in time dropped to zero.

As shown in the eighth figure, in the case of light load output, the flyback sub-circuit in the present invention will stop working (<i>i</i>_<i>N</i>3=0), similar to the multi-phase synchronous rectification type VRM channel removal (Phase Shedding) function can reduce the power loss of components and wires. At the same time, because the flyback sub-circuit loses its function, the present invention will be equivalent to an active clamp forward converter working in DCM, due to the main switching current of this architecture<i>i</i>_<i>S</i>1The slope of the current change from negative to positive is a relatively smooth line segment, so the time for the main switch current to be negative will be extended. This feature helps to achieve zero voltage switching under light load. The above functions are all conducive to the improvement of conversion efficiency.

In addition, compared with the traditional active clamp forward-flyback conversion circuit, the present invention adds a diode, but all the diodes have lower reverse bias voltages than the former, so low-cost options can be used. Low-voltage components. See the first figure, the diode reverse bias (cathode-anode cross voltage) of the traditional active clamp forward-flyback converter is:<maths><img file="TW201014140A_D0001.tif" /></maths>

See the first preferred embodiment of the present invention in the third figure. The reverse bias voltages of the diodes used in the present invention are:<maths><img file="TW201014140A_D0002.tif" /></maths><maths><img file="TW201014140A_D0003.tif" /></maths>

The present invention can also be derived into a circuit with multiple sets of outputs. As shown in Figure 9, it is a buck-boost active clamp forward-reverse with multiple sets of output voltages according to the fourth preferred embodiment of the present invention. The schematic diagram of the circuit of the free-wheeling converter; the forward sub-circuit is electrically connected in parallel with a first output capacitor<i>C</i>_<i>o</i>1With a first load<i>R</i>_<i>L</i>1, Responsible for the main output<i>V</i>_<i>o</i>1(The first DC output voltage); a flyback sub-circuit, which is electrically connected to a second output capacitor in parallel<i>C</i>_<i>o</i>2With a second load<i>R</i>_<i>L</i>2, Responsible for secondary output<i>V</i>_<i>o</i>2(Second DC output voltage), and the number of secondary outputs can be increased as needed. This design can be applied to power supplies that require multiple outputs. When the load current of the main output becomes smaller, the duty cycle is<i>D</i>It will also be automatically reduced so that the secondary output stops supplying power. The starting point of this mechanism can be set when the system enters the standby mode, and the power saving function can be achieved by stopping power supply to the power supply of unnecessary circuits.

In summary, the present invention discloses an active clamp forward-flyback converter that is used in a wide range of load fluctuations, which is beneficial to improve conversion efficiency, and because the diode reverse bias used is more traditional The active clamp forward-flyback converter is low, so low-cost low withstand voltage components can be selected, and it is cost-effective, so it does have its progress and novelty.

Therefore, even though this case has been described in detail by the above-mentioned embodiments and can be modified in many ways by those familiar with the art, it does not deviate from the protection of the scope of the attached patent application.

The first figure: it shows the circuit diagram of a traditional active clamp forward-flyback converter; the second figure: it shows the discharge of the inter-electrode capacitance of a traditional active clamp forward-flyback converter Schematic diagram of the operation; the third diagram: it shows a schematic circuit diagram of a buck-boost active-clamp forward-flyback converter according to the first preferred embodiment of the present invention; the fourth diagram: it shows a The circuit diagram of the boost type active clamp forward-flyback converter according to the second preferred embodiment of the present invention; Figure 5: It shows a third preferred embodiment of the present invention The circuit diagram of the step-down active clamp forward-flyback converter; Figure 6: It shows an active clamp forward-flyback converter according to the first to third preferred embodiments of the present invention The functional block diagram of the controller of the converter; Figure 7 (a) to Figure 7 (h): They respectively show a forward buck-boost active clamp according to the first preferred embodiment of the present invention -Schematic diagram of the action of the equivalent circuit of the flyback converter; Figure 8: It shows a buck-boost active clamp forward-flyback converter according to the first preferred embodiment of the present invention Working waveform diagrams under heavy load and light load; and the ninth figure: it shows a buck-boost active clamp forward-backlash with multiple sets of output voltages according to the fourth preferred embodiment of the present invention Schematic diagram of the circuit of the type converter.