DC-DC power conversion apparatus and method

A DC-DC power conversion apparatus and a DC-DC power conversion method are provided. The DC-DC power conversion apparatus includes a switching circuit, a main transformer circuit, a main rectifier circuit, an auxiliary transformer circuit and an auxiliary rectifier circuit. The switching circuit provides an input power to a primary winding of the main transformer circuit or a primary winding of the auxiliary transformer circuit by time-division. An AC input terminal of the main rectifier circuit is coupled to a secondary winding of the main transformer circuit. An AC input terminal of the auxiliary rectifier circuit is coupled to a secondary winding of the auxiliary transformer circuit. A power output terminal of the auxiliary rectifier circuit is coupled to a reference voltage terminal of the main rectifier circuit for lifting a voltage of the power output terminal of the main rectifier circuit.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan application serial no. 102144915, filed on Dec. 6, 2013. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

TECHNICAL FIELD

The disclosure relates to a DC-DC power conversion apparatus and method.

BACKGROUND

Presently, an isolated DC-DC power converter applied to high power generally adopts a full-bridge phase-shift circuit structure. The full-bridge phase-shift circuit can achieve zero-voltage switching (ZVS). A phase-shift pulse width modulation (PWM) control method of a fixed switching frequency can also be implemented to the full-bridge phase-shift circuit structure. However, the full-bridge phase-shift circuit structure may lose a soft-switch effect when operating under a light load (or empty load) state, and a circulating loss generated in a heavy load operation is great, which leads to decrease of efficiency.

An existing composite DC-DC power converter having both of a phase-shift circuit and a resonant circuit may achieve the soft-switch effect for full load range, and meanwhile decrease the circulating loss. However, the existing composite DC-DC power converter has to be configured with a main transformer and an auxiliary transformer. The existing composite DC-DC power converter provides power to the load through the auxiliary transformer during a freewheeling period, and during a power transmitting period (a period of providing the maximum power), the existing composite DC-DC power converter provides power to the load through the main transformer, so that the main transformer be sufficient to supply a rated power. Namely, a power level of the main transformer is required to match the rated power, so that a volume of the main transformer is larger. The structure of dual-transformer leads to larger volume of the existing composite DC-DC power converter, and causes a poor overall power density.

SUMMARY

An embodiment of the disclosure provides a DC-DC power conversion apparatus including a switching circuit, a main transformer circuit, a main rectifier circuit, an auxiliary transformer circuit and an auxiliary rectifier circuit. The switching circuit is coupled to a primary winding of the main transformer circuit and a primary winding of the auxiliary transformer circuit. The switching circuit provides an input power to the primary winding of the main transformer circuit or the primary winding of the auxiliary transformer circuit by time-division. An AC input terminal of the main rectifier circuit is coupled to a secondary winding of the main transformer circuit, where a power output terminal of the main rectifier circuit serves as a power output terminal of the DC-DC power conversion apparatus. An AC input terminal of the auxiliary rectifier circuit is coupled to a secondary winding of the auxiliary transformer circuit. A power output terminal of the auxiliary rectifier circuit is coupled to a reference voltage terminal of the main rectifier circuit for lifting a voltage of the power output terminal of the main rectifier circuit. A reference voltage terminal of the auxiliary rectifier circuit is coupled to a secondary side reference voltage.

An embodiment of the disclosure provides a DC-DC power conversion method including following steps. A main transformer circuit is configured. An auxiliary transformer circuit is configured. A switching circuit is configured, where the switching circuit provides an input power to a primary winding of the main transformer circuit or a primary winding of the auxiliary transformer circuit by time-division. An output of a secondary winding of the auxiliary transformer circuit is rectified to obtain a base voltage. An output of a secondary winding of the primary transformer circuit is rectified to obtain an adding voltage. The adding voltage is lifted by using the base voltage to obtain an output voltage.

DETAILED DESCRIPTION OF DISCLOSED EMBODIMENTS

A term “couple” used in the full text of the disclosure (including the claims) refers to any direct and indirect connections. For example, if a first device is described to be coupled to a second device, it is interpreted as that the first device is directly coupled to the second device, or the first device is indirectly coupled to the second device through other devices or connection means. Moreover, wherever possible, components/members/steps using the same referential numbers in the drawings and description refer to the same or like parts. Components/members/steps using the same referential numbers or using the same terms in different embodiments may cross-refer related descriptions.

FIG. 1is a circuit block schematic diagram of a DC-DC power conversion apparatus according to an embodiment of the disclosure. The DC-DC power conversion apparatus includes a switching circuit110, a main transformer circuit120, a main rectifier circuit130, an auxiliary transformer circuit140and an auxiliary rectifier circuit150. The switching circuit110is coupled to a primary winding of the main transformer circuit120and a primary winding of the auxiliary transformer circuit140. The switching circuit110provides an input power Vin to the primary winding of the main transformer circuit120and/or the primary winding of the auxiliary transformer circuit140by time-division.

An AC input terminal of the auxiliary rectifier circuit150is coupled to a secondary winding of the auxiliary transformer circuit140. A reference voltage terminal of the auxiliary rectifier circuit150is coupled to a secondary side reference voltage Vsr. The secondary side reference voltage Vsr can be a secondary side ground voltage or other fixed voltage. The auxiliary rectifier circuit150rectifies an output power of the secondary winding of the auxiliary transformer circuit140to provide a base voltage Vb. A reference voltage terminal of the main rectifier circuit130is coupled to a power output terminal of the auxiliary rectifier circuit150and receives the base voltage Vb to serve as a reference voltage of the main rectifier circuit130. Therefore, the main rectifier circuit130and the auxiliary rectifier circuit150can be regarded as being connected in series.

An AC input terminal of the main rectifier circuit130is coupled to a secondary winding of the main transformer circuit120. A power output terminal of the main rectifier circuit130serves as a power output terminal of the DC-DC power conversion apparatus100to provide an output voltage Vout. The main rectifier circuit130rectifies an output power of the secondary winding of the main transformer circuit120to provide an adding voltage Va. Based on the serial connection structure, the base voltage Vb can lift the adding voltage Va to obtain the output voltage Vout complied with a rated specification. Therefore, the base voltage Vb of the auxiliary rectifier circuit150can lift the output voltage Vout at the power output terminal of the main rectifier circuit130.

The power output terminal of the auxiliary rectifier circuit150can keep outputting the base voltage Vb to the reference voltage terminal of the main rectifier circuit130. The base voltage Vb can serve as a part of a rated output power of the DC-DC power conversion apparatus100. The main rectifier circuit130can compensate an insufficient part of the rated output power to provide the output power Vout (rated output power) to a load. Since the main transformer circuit120and the auxiliary transformer circuit140commonly provide power to the load during a power transmitting period, transformers with a smaller power level (transformers with a smaller volume) can be used to implement the main transformer circuit120and/or the auxiliary transformer circuit140. By using the transformers with smaller power level, a volume of the DC-DC power conversion apparatus100is decreased.

The implementation method of the DC-DC power conversion apparatus100ofFIG. 1is not limited by the disclosure. For example,FIG. 2is a circuit schematic diagram of a DC-DC power conversion apparatus200according to another embodiment of the disclosure. The DC-DC power conversion apparatus200includes the switching circuit110, the main transformer circuit120, the main rectifier circuit130, the auxiliary transformer circuit140and the auxiliary rectifier circuit150. The switching circuit110, the main transformer circuit120, the main rectifier circuit130, the auxiliary transformer circuit140and the auxiliary rectifier circuit150ofFIG. 2can be deduced with reference of related descriptions of the embodiment ofFIG. 1.

Referring toFIG. 2, the main transformer circuit120includes a transformer121. A first terminal and a second terminal of a primary winding of the transformer121are coupled to the switching circuit110. A first terminal and a second terminal of a secondary winding of the transformer121are coupled to the main rectifier circuit130. The auxiliary transformer circuit140includes a transformer141and a half-bridge resonant capacitor142. A first terminal of a primary winding of the transformer141is coupled to the switching circuit110. A first terminal of the half-bridge resonant capacitor142is coupled to a second terminal of the primary winding of the transformer141. A second terminal of the half-bridge resonant capacitor142is coupled to a primary side reference voltage Vpr. The primary side reference voltage Vpr can be a primary side ground voltage or other fixed voltages. The half-bridge resonant capacitor142may provide resonant energy. A first terminal and a second terminal of a secondary winding of the transformer141are coupled to the auxiliary rectifier circuit150.

The main rectifier circuit130includes a rectifier131, a clamping circuit132and an energy storage circuit133. An AC input terminal of the rectifier131is coupled to the secondary winding of the transformer121in the main transformer circuit120. The rectifier131can rectify an AC current of the secondary winding of the transformer121into a DC current. The clamping circuit132is coupled to a DC output terminal of the rectifier131. The clamping circuit132can clamp a voltage at the DC output terminal of the rectifier131to a predetermined voltage level to avoid generating a surge voltage. An input terminal of the energy storage circuit133is coupled to the DC output terminal of the rectifier131. An output terminal of the energy storage circuit133is coupled to a power output terminal of the main rectifier circuit130. The energy storage circuit133can filter an AC component of a voltage Vo at the DC output terminal of the rectifier131. A reference terminal of the energy storage circuit133is coupled to the reference voltage terminal of the main rectifier circuit130to receive the base voltage Vb output by the auxiliary rectifier circuit150. Based on the voltage (the base voltage Vb) at the reference voltage terminal of the main rectifier circuit130, the energy storage circuit133can provide the output voltage Vout to the power output terminal of the main rectifier circuit130.

The auxiliary rectifier circuit150includes a rectifying booster circuit151. An AC input terminal of the rectifying booster circuit151is coupled to the secondary winding of the transformer141in the auxiliary transformer circuit140. A reference voltage terminal of the rectifying booster circuit151is coupled to a secondary side reference voltage Vsr. A power output terminal of the rectifying booster circuit151is coupled to the reference voltage terminal of the auxiliary rectifier circuit150for providing the base voltage Vb. The rectifying booster circuit151can rectify an AC current of the secondary winding of the transformer141into a DC current, and increase a voltage of the secondary winding of the transformer141by a predetermined multiple.

The switching circuit110includes a first power switch SW1, a second power switch SW2, a third power switch SW3and a fourth power switch SW4. A first terminal of the first power switch SW1receives the input power Vin. A second terminal of the first power switch SW1is coupled to the first terminal of the primary winding of the transformer121in the main transformer circuit120. A control terminal of the first power switch SW1receives a control signal S1. A first terminal of the second power switch SW2is coupled to the second terminal of the first power switch SW1. A second terminal of the second power switch SW2is coupled to the primary side reference voltage Vpr. A control terminal of the second power switch SW2receives a control signal S2. A first terminal of the third power switch SW3receives the input power Vin. A second terminal of the third power switch SW3is coupled to the second terminal of the primary winding of the transformer121in the main transformer circuit120. The second terminal of the third power switch SW3is further coupled to the first terminal of the primary winding of the transformer141in the auxiliary transformer circuit140. A control terminal of the third power switch SW3receives a control signal S3. A first terminal of the fourth power switch SW4is coupled to the second terminal of the third power switch SW3. A second terminal of the fourth power switch SW4is coupled to the primary side reference voltage Vpr. A control terminal of the fourth power switch SW4receives a control signal S4.

The first power switch SW1, the second power switch SW2, the third power switch SW3and the fourth power switch SW4can be implemented by any means. For example, the first power switch SW1, the second power switch SW2, the third power switch SW3and the fourth power switch SW4can be metal oxide semiconductor (MOS) transistors, bipolar junction transistors BJT or other power transistors. For another example, the first power switch SW1and the second power switch SW2respectively include an insulated gate bipolar transistor (IGBT), and the third power switch SW3and the fourth power switch SW4respectively include a MOS transistor. The IGBT of the first power switch SW1and the second power switch SW2respectively have a body diode. The third power switch SW3and the fourth power switch SW4respectively have a body diode and a parasitic capacitance.

Operation phases of the first power switch SW1, the second power switch SW2, the third power switch SW3and the fourth power switch SW4are not limited by the disclosure. For example,FIG. 3is a timing schematic diagram of the control signals S1, S2, S3and S4ofFIG. 2according to an embodiment of the disclosure. InFIG. 3, a horizontal axis represents time t. Referring toFIG. 2andFIG. 3, during a power transmitting period T1, the first power switch SW1and the fourth power switch SW4are turned on, and the second power switch SW2and the third power switch SW3are turned off Now, the voltage of the input power Vin is exerted to the first terminal of the primary winding of the transformer121, and the primary side reference voltage Vpr is exerted to the second terminal of the primary winding of the transformer121. An inducted AC current of the secondary winding of the transformer121is rectified to a DC current by the rectifier131. The DC power output by the rectifier131can be stored in the energy storage circuit133. Therefore, the energy storage circuit133can provide the output voltage Vout to the load. Meanwhile, the primary side reference voltage Vpr is exerted to the first terminal of the primary winding of the transformer141. Charges previously stored in the half-bridge resonant capacitor142is discharged to the primary side reference voltage Vpr through the primary winding of the transformer141. An inducted AC current of the secondary winding of the transformer141is rectified to a DC current by the rectifying booster circuit151. The DC power (base voltage Vb) output by the rectifying booster circuit151is provided to the reference terminal of the energy storage circuit133through serial connection. The base voltage Vb of the rectifying booster circuit151can lift the output voltage Vout of the energy storage circuit133. Therefore, during the power transmitting period T1, the main transformer circuit120and auxiliary transformer circuit140can commonly provide electric power to the load.

During a first freewheeling period T2, the first power switch SW1and the third power switch SW3are turned on, and the second power switch SW2and the fourth power switch SW4are turned off. Now, parasitic capacitances Cp3and Cp4of the third power switch SW3and the fourth power switch SW4are respectively discharged and charged by the energy of a magnetizing inductor Lm2of the primary winding of the transformer141. The body diode Db3of the third power switch SW3is forward biased, such that the third power switch SW3is turned on under a zero voltage. The clamping circuit132can reset a primary side current of the transformer121during the first freewheeling period T2, such that a circulating current of the transformer121is zero, and a conduction loss generated by the circulating current is decreased. The first power switch SW1can be turned off under the zero current.

During a commutation power transmitting period T3, the second power switch SW2and the third power switch SW3are turned on, and the first power switch SW1and the fourth power switch SW4are turned off. Now, the voltage of the input power Vin is exerted to the second terminal of the primary winding of the transformer121, and the primary side reference voltage Vpr is exerted to the first terminal of the primary winding of the transformer121. An inducted AC current of the secondary winding of the transformer121is rectified to a DC current by the rectifier131. The DC power output by the rectifier131can be stored in the energy storage circuit133. Therefore, the energy storage circuit133can provide the output voltage Vout to the load. Meanwhile, the voltage of the input power Vin is exerted to the first terminal of the primary winding of the transformer141. Charges previously stored in the half-bridge resonant capacitor142are discharged to the primary side reference voltage Vpr through the primary winding of the transformer141. An inducted AC current of the secondary winding of the transformer141is rectified to a DC current by the rectifying booster circuit151. The DC power (base voltage Vb) output by the rectifying booster circuit151is provided to the reference terminal of the energy storage circuit133through serial connection. The base voltage Vb of the rectifying booster circuit151can lift the output voltage Vout of the energy storage circuit133. Therefore, during the commutation power transmitting period T3, the main transformer circuit120and auxiliary transformer circuit140can commonly provide electric power to the load.

During a second freewheeling period T4, the second power switch SW2and the fourth power switch SW4are turned on, and the first power switch SW1and the third power switch SW3are turned off. Now, parasitic capacitances Cp3and Cp4of the third power switch SW3and the fourth power switch SW4are respectively charged and discharged by the energy of the magnetizing inductor Lm2of the primary winding of the transformer141. The body diode Db4of the fourth power switch SW4is forward biased, such that the fourth power switch SW4is turned on under a zero voltage. The clamping circuit132can reset the primary side current of the transformer121during the second freewheeling period T4, such that the circulating current of the transformer121is zero, and a conduction loss generated by the circulating current is decreased. The second power switch SW2can be turned off under the zero current.

After the energy storage circuit133filters the AC component of the voltage Vo at the DC output terminal of the rectifier131, the energy storage circuit133can provide the output voltage Vout to the load. The electric power shown by a slash shading part ofFIG. 3is provided by the auxiliary transformer circuit140. According toFIG. 3, it is known that the auxiliary transformer circuit140can keep outputting the base voltage Vb to provide a part of the rated output power through the auxiliary rectifier circuit150during the power transmitting period T1, the first freewheeling period T2, the commutation power transmitting period T3and the second freewheeling period T4. The main transformer circuit120can provide the adding voltage Va during the power transmitting period T1and the commutation power transmitting period T3to compensate the insufficient part of the rated output power. Therefore, transformers with a smaller power level (transformers with a smaller volume) can be used to implement the main transformer circuit120and/or the auxiliary transformer circuit140. By using the transformers with smaller power level, a volume of the DC-DC power conversion apparatus100is decreased.

FIG. 4is a circuit schematic diagram of a DC-DC power conversion apparatus according to still another embodiment of the disclosure. The DC-DC power conversion apparatus400includes the switching circuit110, the main transformer circuit120, the main rectifier circuit130, the auxiliary transformer circuit140and the auxiliary rectifier circuit150. The switching circuit110, the main transformer circuit120, the main rectifier circuit130, the auxiliary transformer circuit140and the auxiliary rectifier circuit150ofFIG. 4can be deduced with reference of related descriptions of the embodiment ofFIG. 1. The switching circuit110ofFIG. 4includes the first power switch SW1, the second power switch SW2, the third power switch SW3and the fourth power switch SW4. The first power switch SW1, the second power switch SW2, the third power switch SW3and the fourth power switch SW4shown inFIG. 4can be deduced with reference of related descriptions of the embodiments ofFIG. 2andFIG. 3. The main rectifier circuit130ofFIG. 4includes the rectifier131, the clamping circuit132and the energy storage circuit133. The auxiliary rectifier circuit150ofFIG. 4includes the rectifying booster circuit151. The rectifier131, the clamping circuit132, the energy storage circuit133and the rectifying booster circuit151can be deduced with reference of related descriptions of the embodiment ofFIG. 2.

Referring toFIG. 4, the rectifying booster circuit151includes a diode451, a diode452, a capacitor453and a capacitor454. An anode of the diode451is coupled to the first terminal of the secondary winding of the transformer141in the auxiliary transformer circuit140. A cathode of the diode451is coupled to the power output terminal of the rectifying booster circuit151. A cathode of the diode452is coupled to the anode of the diode451. An anode of the diode452is coupled to the secondary side reference voltage Vsr. A first terminal of the capacitor453is coupled to the power output terminal of the rectifying booster circuit151for providing the base voltage Vb. A second terminal of the capacitor453is coupled to the second terminal of the secondary winding of the transformer141in the auxiliary transformer circuit140. A first terminal of the capacitor454is coupled to the second terminal of the capacitor453. A second terminal of the capacitor454is coupled to the secondary side reference voltage Vsr. When the current flows to the anode of the diode451from the first terminal of the secondary winding of the transformer141, the current can charge the capacitor453. When the current flows to the second terminal of the capacitor453from the second terminal of the secondary winding of the transformer141, the capacitor453has a charge pump function to boost the base voltage Vb. Therefore, the rectifying booster circuit151simultaneously have a half-bridge rectifying function and a voltage doubling function (the charge pump function).

The energy storage circuit133includes an inductor431and an output capacitor432. A first terminal and a second terminal of the inductor431are respectively coupled to the input terminal of the energy storage circuit133and the output terminal of the energy storage circuit133. A first terminal of the output capacitor432is coupled to the output terminal of the energy storage circuit133for providing the output voltage Vout. A second terminal of the output capacitor432is coupled to the reference terminal of the energy storage circuit133for receiving the base voltage Vb provided by the rectifying booster circuit151.

The rectifier131includes a diode411, a diode412, a diode413and a diode414. An anode of the diode411is coupled to the first terminal of the secondary winding of the transformer121in the main transformer circuit120. A cathode of the diode411is coupled to the power output terminal of the rectifier131for providing the rectified power to the input terminal of the energy storage circuit133. A cathode of the diode412is coupled to the anode of the diode411. An anode of the diode412is coupled to the reference terminal of the rectifier131for receiving the base voltage Vb provided by the rectifying booster circuit151. A cathode of the diode413is coupled to the cathode of the diode411. An anode of the diode413is coupled to the second terminal of the secondary winding of the transformer121in the main transformer circuit120. A cathode of the diode414is coupled to the anode of the diode413. An anode of the diode414is coupled to the reference terminal of the rectifier131for receiving the base voltage Vb provided by the rectifying booster circuit151.

The clamping circuit132includes a clamping capacitor421, a diode422and a diode423. A first terminal of the clamping capacitor421is coupled to the DC output terminal of the rectifier131. A cathode of the diode422is coupled to a second terminal of the clamping capacitor421. An anode of the diode422is coupled to the reference terminal of the clamping circuit132for receiving the output (the base voltage Vb) of the auxiliary rectifier circuit150. An anode of the diode423is coupled to the second terminal of the clamping capacitor421. A cathode of the diode423is coupled to the first terminal of the output capacitor432.

The DC-DC power conversion apparatus400can convert the input power Vin into the DC voltage and current required by the load. The DC voltage and current required by the load can be implemented by controlling the timing of the power switches SW1-SW4in the switching circuit110according to a phase-shift pulse width modulation (PWM) method. The DC-DC power conversion apparatus400combines features of full-bright phase-shift PWM and LLC half-bridge resonant converter of a fixed frequency to achieve a low switching loss of zero-voltage switching (ZVS) of the right leg switches SW3and SW4and zero-current switching (ZCS) of the left leg switches SW1and SW2. Operation timing of the power switches SW1-SW4ofFIG. 4may refer to related description ofFIG. 5.

FIG. 5is a timing schematic diagram of the signals shown inFIG. 4according to an embodiment of the disclosure. InFIG. 5, a horizontal axis represents time t. Vpril and Ipril inFIG. 5respectively represent a primary side voltage Vpril and a primary side current Ipril of the transformer121ofFIG. 4. The control signals S1, S2, S3and S4shown inFIG. 5may refer to related descriptions ofFIG. 3. Referring toFIG. 4andFIG. 5, during the power transmitting period T1, the first power switch SW1and the fourth power switch SW4are turned on, and the second power switch SW2and the third power switch SW3are turned off. Now, the voltage of the input power Vin is exerted to the first terminal of the primary winding of the transformer121, and the primary side reference voltage Vpr is exerted to the second terminal of the primary winding of the transformer121. The inducted AC current of the secondary winding of the transformer121turns on the diode411and the diode414, and the power output through the secondary winding of the transformer121can be stored in the inductor431of the energy storage circuit133for providing power to the load. Meanwhile, the power output by the secondary winding of the transformer121can charge the output capacitor432and the load through the clamping capacitor421and the diode423in the clamping circuit132, so as to improve system efficiency. The base voltage Vb output by the rectifying booster circuit151is provided to the reference terminal of the rectifier131, the reference terminal of the clamping circuit132and the reference terminal of the energy storage circuit133through serial connection. The base voltage Vb of the rectifying booster circuit151can lift the output voltage Vout of the energy storage circuit133. Therefore, during the power transmitting period T1, the main transformer circuit120and the auxiliary transformer circuit140can commonly provide electric power to the load.

During the first freewheeling period T2, the first power switch SW1and the third power switch SW3are turned on, and the second power switch SW2and the fourth power switch SW4are turned off. Now, the parasitic capacitances Cp3and Cp4of the third power switch SW3and the fourth power switch SW4are respectively discharged and charged by the energy of a magnetizing inductor Lm2of the primary winding of the transformer141. The body diode Db3of the third power switch SW3is forward biased, such that the third power switch SW3is turned on under the zero voltage. The clamping circuit132can reset the primary side current Ipril of the transformer121during the first freewheeling period T2, such that a circulating current of the transformer121is zero, and a conduction loss generated by the circulating current is decreased. The first power switch SW1can be turned off under the zero current.

During the commutation power transmitting period T3, the second power switch SW2and the third power switch SW3are turned on, and the first power switch SW1and the fourth power switch SW4are turned off Now, the voltage of the input power Vin is exerted to the second terminal of the primary winding of the transformer121, and the primary side reference voltage Vpr is exerted to the first terminal of the primary winding of the transformer121. The inducted AC current of the secondary winding of the transformer121turns on the diode412and the diode413, and the power output through the secondary winding of the transformer121can be stored in the inductor431of the energy storage circuit133for providing power to the load. Meanwhile, the power output by the secondary winding of the transformer121can charge the output capacitor432and the load through the clamping capacitor421and the diode423in the clamping circuit132, so as to improve system efficiency. The base voltage Vb output by the rectifying booster circuit151is provided to the reference terminal of the rectifier131, the reference terminal of the clamping circuit132and the reference terminal of the energy storage circuit133through serial connection. The base voltage Vb of the rectifying booster circuit151can lift the output voltage Vout of the energy storage circuit133. Therefore, during the commutation power transmitting period T3, the main transformer circuit120and the auxiliary transformer circuit140can commonly provide electric power to the load.

During the second freewheeling period T4, the second power switch SW2and the fourth power switch SW4are turned on, and the first power switch SW1and the third power switch SW3are turned off. Now, parasitic capacitances Cp3and Cp4of the third power switch SW3and the fourth power switch SW4are respectively charged and discharged by the energy of the magnetizing inductor Lm2of the primary winding of the transformer141. The body diode Db4of the fourth power switch SW4is forward biased, such that the fourth power switch SW4is turned on under a zero voltage. The clamping circuit132can reset the primary side current Ipril of the transformer121during the second freewheeling period T4, such that the circulating current of the transformer121is zero, and a conduction loss generated by the circulating current is decreased. The second power switch SW2can be turned off under the zero current.

In the embodiment ofFIG. 4, the half-bridge LLC resonant converter is combined to the full-bridge phase-shift converter structure according to a method of sharing a lagging switch leg, such that the full load range can all achieve the soft-switch effect, and the circulating loss is also decreased. The induced voltages at the secondary windings of the transformer121and the transformer141are added through the serial connection and transmitted to the output terminal of the DC-DC power conversion apparatus400. The transformer141can keep providing the voltage Vb to the main rectifier circuit130during a whole switch switching period. The transformer121is required to compensate the insufficient part of the output voltage Vout. Therefore, the power level and magnetic component specification of the transformer121can be greatly decreased to achieve effects of volume reduction and improvement of converter power density. According to the circuit of the present embodiment, 20% of the volume of the main transformer can be saved without influencing the global switch soft-switch effect of the primary side circuit. Moreover, the transformer121and the transformer141can commonly share the total output power, and utilization of the transformer141is accordingly improved. Besides that the volume is decreased to improve the whole converter power density, reduction of the magnetic component specification can also save the cost.

A DC-DC power conversion method is described below. The DC-DC power conversion method includes following steps. The main transformer circuit120, the auxiliary transformer circuit140and the switching circuit110are configured, where the switching circuit110provides the input power Vin to the primary winding of the main transformer circuit120or the primary winding of the auxiliary transformer circuit140by time-division. An output of the secondary winding of the auxiliary transformer circuit140is rectified to obtain the base voltage Vb. An output of the secondary winding of the primary transformer circuit120is rectified to obtain the adding voltage Va. The adding voltage Va is lifted by using the base voltage Vb to obtain the output voltage Vout.

In some embodiment, the step of obtaining the base voltage Vb includes keeping outputting the base voltage Vb to provide a part of the rated output power, where the adding voltage Va can compensate the insufficient part of the rated output power. Therefore, the power level of and the magnetic component specification of the main transformer circuit120and the auxiliary transformer circuit140can be greatly decreased to achieve the effects of volume reduction and improvement of the converter power density.