Synchronous rectifying drive circuit with energy feedback for voltage doubler rectifer

This application claims priority of Chinese Patent Application No. 200910099060.2 filed Jun. 8, 2009, which is incorporated herein by reference.

Exemplary embodiments of the present invention relate to a drive circuit of a synchronous rectifier, and more specifically, to energy feedback in a current-driven synchronous rectifier (SR) designed for a voltage doubler rectifier.

With the development of semiconductor technology, the on-resistance of low and middle voltage MOSFET has decreased. In order to reduce conduction loss, a synchronous rectifier (SR) is used in low voltage high current switching power applications. In some low and middle voltage DC-DC and AC-DC applications, the output voltage could be 48 volts (V) and/or higher. If a half-wave rectifier or a center-tapped rectifier is used in such high voltage applications, the voltage stress of the rectifier is too high to use low voltage rectifiers to reduce the conduction loss. When the voltage of a MOSFET is above 200V, both the cost of the MOSFET and the on-resistance increases, and thus is not suitable for synchronous rectification applications. Therefore, for a conventional rectifying circuit, when the output voltage of a rectification circuit is above 60V, normal diode rectification is usually adopted. However, there could be certain oscillation between a transformer leakage inductance and an intrinsic capacitance in circuit. The oscillation may cause a voltage spike and as a result, the voltage stress of the rectifying components may be much higher than that in the ideal conditions.

A voltage doubler rectifier is able to eliminate a voltage spike across the rectifying components and reduce the voltage stress, by clamping the withstanding voltage of the rectifier at the same level of the output voltage. Therefore, a low voltage MOSFET can be used for synchronous rectification to reduce the conduction loss. However the conventional method of driving a MOSFET by sensing current signal requires a current transducer (CT) with large turns in its windings. In particular, when the output current is very high, there will be too much energy transferred to the drive circuit. If this part of redundant energy could not be fed back to the source of the output voltage, there would be additional loss. In light of the particularity of the voltage doubler rectifying circuit, one of the SR drive circuits needs to be ground-floated. So when using traditional current-driven rectifiers, it's impossible to have both SR's driving energy fed back to the output.

A current-driven synchronous rectifying drive circuit designed for a T-type voltage doubler rectifier with an energy feedback circuit including a clamp and energy feedback circuit, a high frequency transformer, a current transducer, an energy storage capacitor, an output capacitor, a first and a second synchronous rectifier, and a first drive circuit connected to the first synchronous rectifier and a second drive circuit connected to the second synchronous rectifier.

A more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, exemplary embodiments of the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

FIG. 1 is a schematic diagram of a T-type voltage doubler diode rectifier;

FIG. 2 is a schematic diagram of a T-type voltage doubler synchronous rectifier with two current transducers;

FIG. 3 is a schematic diagram of a T-type voltage doubler synchronous rectifier with an energy feedback circuit and two current transducers;

FIG. 4 is a schematic diagram of a T-type voltage doubler synchronous rectifier with an energy feedback circuit and one current transducer;

FIG. 5 is a schematic diagram of a common voltage doubler synchronous rectifier with a MOSFET energy feedback circuit and one current transducer;

FIG. 6 is a schematic diagram of a T-type voltage doubler synchronous rectifier with a MOSFET energy feedback circuit and one current transducer;

FIG. 7 is a schematic diagram of another T-type voltage doubler synchronous rectifier with a MOSFET energy feedback circuit and one current transducer;

FIG. 8 is a schematic diagram of the third T-type voltage doubler synchronous rectifier with a MOSFET energy feedback circuit and one current transducer;

FIG. 9 is a schematic diagram of the fourth T-type voltage doubler synchronous rectifier with a MOSFET energy feedback circuit and one current transducer;

FIG. 10 is a schematic diagram of a common voltage doubler synchronous rectifier with a MOSFET energy feedback circuit and two current transducers;

FIG. 11 is a schematic diagram of a T-type voltage doubler synchronous rectifier with a MOSFET energy feedback circuit and two current transducers;

FIG. 12 is a schematic diagram of another T-type voltage doubler synchronous rectifier with a MOSFET energy feedback circuit and two current transducers;

FIG. 13 is a schematic diagram of the third T-type voltage doubler synchronous rectifier with a MOSFET energy feedback circuit and two current transducers; and

FIG. 14 is a schematic diagram of the fourth T-type voltage doubler synchronous rectifier with a MOSFET energy feedback circuit and two current transducers.

Reference will be made below in detail to exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numerals used throughout the drawings refer to the same or like parts.

In order to feed the redundant energy (received by the SRs from the CT) back to the output, the present invention proposes a new solution. By using a T-type voltage doubler rectifier, the driving voltage of the upper SR of the rectifier can be clamped to the voltage of the energy storage capacitor, and the redundant energy in the lower SR is fed back to the same energy storage capacitor through a diode. The solution is simple to implement and it improves the driving efficiency as explained in further detail below.

FIG. 1 is a schematic diagram of a T-type voltage doubler diode rectifier. When the secondary voltage of the high frequency transformer is negative on top and positive on bottom, the diode D 1 turns on, and the voltage of the capacitor C 1 is charged to Vo. When the secondary voltage of the high frequency transformer is positive on top and negative on bottom, the diode D 2 turns on, and the voltage of the capacitor C 2 is charged to 2 Vo.

FIG. 2 is a schematic diagram of a T-type voltage doubler synchronous rectifier with two current transducers. The whole circuit consists of two drive circuits, two current transducers and a power circuit. The power circuit consists of a high frequency transformer T 1 , two synchronous rectifiers SR 1 and SR 2 , two capacitors C 1 and C 2 . The non-dotted terminal of the high frequency transformer T 1 is connected to the drain of SR 1 and to the positive terminal of the output capacitor C 2 . The dotted terminal of the high frequency transformer T 1 is connected to the positive terminal of the energy storage capacitor C 1 . The source of SR 1 is connected to the non-dotted terminal of the primary winding N 1 of the current transducer CT 1 . The gate of SR 1 is connected to the output port of its drive circuit. The drain of SR 2 is connected to the dotted terminal of the primary winding N 3 of the other current transducer CT 2 . The source of SR 2 is connected to the negative terminal of the output capacitor. The gate of SR 2 is connected to the output port of its drive circuit. The negative terminal of the energy storage capacitor C 1 is connected to the dotted terminal of the primary winding N 1 of the current transducer CT 1 as well as the non-dotted terminal of the primary winding N 3 of the other current transducer CT 2 .

The current transducer CT 1 consists of a magnetic core, a primary winding N 1 and a secondary winding N 2 . One terminal of the winding N 1 is connected to one terminal of the energy storage capacitor C 1 . The other terminal of the winding N 1 is connected to one terminal of SR 1 . The winding N 2 is connected to the drive circuit of SR 1 . The current transducer CT 2 consists of a magnetic core, a primary winding N 3 and a secondary winding N 4 . One terminal of the winding N 3 is connected to one terminal of the energy storage capacitor C 1 . The other terminal of the winding N 3 is connected to one terminal of SR 2 . The winding N 4 is connected to the drive circuit of SR 2 .

The signal shaping and reset circuit consists of a diode D 1 , a diode D 2 , a diode D 3 , a resistor R 1 and a NPN transistor Q 1 . The cathode of the diode D 1 is connected to the dotted terminal of the secondary winding N 2 of the current transducer CT 1 and to the collector of the transistor Q 1 . The anode of the diode D 1 is connected to one node of the resistor R 1 , the base of the transistor Q 1 and the anode of the diode D 2 . The other node of the resistor R 1 is connected to the cathodes of the diode D 2 and D 3 , and the non-dotted terminal of the secondary winding N 2 of the current transducer CT 1 . The anode of diode D 3 is connected to the emitter of the transistor Q 1 .

The push-pull driver consists of a NPN transistor Q 2 , a PNP transistor Q 3 and a resistor R 2 . The base of the transistor Q 2 is connected to the base of the transistor Q 3 as well as one node of the resistor R 2 . The other node of the resistor R 2 is connected to the dotted terminal of the secondary winding N 2 of the current transducer CT 1 . The emitter of the transistor Q 2 is connected to the emitter of the transistor Q 3 as well as the gate of SR 1 . The collector of the transistor is connected to the anode of the diode D 3 .

The self-bias circuit consists of a diode D 4 and an energy storage capacitor C 3 . The anode of the diode D 4 is connected to the collector of the transistor Q 1 . The cathode of the diode D 4 is connected to the positive terminal of the energy storage capacitor C 3 as well as the collector of the transistor Q 2 . The negative terminal of the energy storage capacitor C 3 is connected to the collector of the transistor Q 3 .

FIG. 3 is a schematic diagram of a T-type voltage doubler synchronous rectifier with an energy feedback circuit and two current transducers. Compare with FIG. 2 , the main difference is that a clamp and energy feedback circuit is added in FIG. 3 . The clamp and energy feedback circuit consists of a diode D 11 and D 12 . The anode of the diode D 11 is connected to the positive terminal of the energy storage capacitor C 3 . The cathode of the diode D 11 is connected to the positive terminal of the energy storage capacitor C 1 . The anode of the diode D 12 is connected to the positive terminal of the energy storage capacitor C 4 . The cathode of the diode D 12 is connected to the positive terminal of the energy storage capacitor C 1 .

FIG. 4 is a schematic diagram of a T-type voltage doubler synchronous rectifier with an energy recycling circuit and one current transducer. Compared with FIG. 3 , the main differences are that only one current transducer CT 1 is used and a protecting circuit is added in FIG. 4 to prevent false drive signal. The power circuit consists of a high frequency transformer T 1 , two synchronous rectifiers SR 1 and SR 2 , and two capacitors C 1 and C 2 . The non-dotted terminal of the high frequency transformer T 1 is connected to the non-dotted terminal of the primary winding N 1 of the current transducer CT 1 . The dotted terminal of the high frequency transformer T 1 is connected to the positive terminal of the energy storage capacitor C 1 . The source of the synchronous rectifier SR 1 is connected to the negative terminal of the energy storage capacitor C 1 as well as the drain of the synchronous rectifier SR 2 . The drain of SR 1 is connected to the positive terminal of the output capacitor C 2 and the dotted terminal of the primary winding N 1 of the current transducer CT 1 . The gate of SR 1 is connected to the output port of its drive circuit. The negative terminal of the output capacitor C 2 is connected to the drain of SR 2 . The gate of SR 2 is connected to the output port of its drive circuit.

The current transducer CT 1 consists of a magnetic core, a primary winding N 1 , two secondary windings N 2 and N 3 . One terminal of the winding N 1 is connected to one terminal of the high frequency transformer T 1 . The other terminal of the winding N 1 is connected to one terminal of SR 1 . The winding N 2 is connected to the drive circuit of SR 1 . The winding N 3 is connected to the drive circuit of SR 2 .

The protecting circuit consists of a resistor R 3 , a diode D 9 and a transistor Q 7 . One node of the resistor R 3 is connected to the cathode of the diode D 9 as well as the drain of SR 1 . The other node of the resistor R 3 is connected to the anode of the diode D 9 and the base of the transistor Q 7 . The collector of the transistor Q 7 is connected to the drive circuit. The emitter of the transistor Q 7 is connected to the source of SR 1 .

FIG. 5 is a schematic diagram of a common voltage doubler synchronous rectifier with a MOSFET energy feedback circuit and one current transducer. Comparing with FIG. 4 , the main differences are that a MOSFET energy feedback circuit is used to replace the diode D 11 , and the cathode of the diode D 12 is connected to the positive terminal of the output capacitor C 2 and the main circuit is a common voltage doubler rectifier.

The MOSFET energy feedback circuit consists a N-channel MOSFET S 1 , a resistor R 7 , a resistor R 8 , a resistor R 9 , a resistor R 10 , a capacitor C 5 , a diode D 13 and a transistor Q 9 . The source of the N-channel MOSFET S 1 is connected to one node of the resistor R 10 . The drain of the N-channel MOSFET S 1 is connected to the positive terminal of the energy storage capacitor C 3 . The gate of the N-channel MOSFET S 1 is connected to the collector of the transistor Q 9 . The other node of the resistor R 10 is connected to the positive terminal of the energy storage capacitor C 4 . The emitter of the transistor Q 9 is connected to the source of SR 2 . The base of the transistor Q 9 is connected to the anode of the diode D 13 . The cathode of the diode D 13 is connected to the source of SR 1 . One node of the capacitor C 5 and one node of the resistor R 8 are connected to the anode of the diode D 13 . The other nodes of C 5 & R 8 are connected to the cathode of the diode D 13 . One node of the resistor R 9 is connected to the source of N-channel MOSFET S 1 . The other node of the resistor R 9 is connected to the gate of N-channel MOSFET S 1 . One node of the resistor R 7 is connected to the drain of SR 1 . The other node of the resistor R 7 is connected to the gate of N-channel MOSFET S 1 .

The common voltage doubler rectifier consists of a high frequency transformer T 1 , two synchronous rectifiers SR 1 and SR 2 , two capacitors C 1 and C 2 . The non-dotted terminal of the high frequency transformer T 1 is connected to the dotted terminal of the primary winding N 1 of the current transducer CT 1 . The dotted terminal of the high frequency transformer T 1 is connected to the positive terminal of the output capacitor C 2 . The source of SR 1 is connected to the non-dotted terminal of the primary winding N 1 of the current transducer CT 1 as well as the drain of SR 2 . The drain of SR 1 is connected to the positive terminal of the output capacitor C 1 . The gate of SR 1 is connected to the output port of its drive circuit. The negative terminal of the output capacitor C 2 is connected to the source of SR 2 . The gate of SR 2 is connected to the output port of its drive circuit. The negative terminal of the output capacitor C 1 is connected to the positive terminal of the output capacitor C 2 .

FIG. 6 is a schematic diagram of a T-type voltage doubler synchronous rectifier with a MOSFET energy feedback circuit and one current transducer. Compared with FIG. 5 , the main differences are that the cathode of the diode D 12 is connected to the positive terminal of the capacitor C 1 , and the main circuit is a T-type voltage doubler rectifier. The T-type voltage doubler rectifier consists of a high frequency transformer T 1 , two synchronous rectifiers SR 1 and SR 2 , two capacitors C 1 and C 2 . The non-dotted terminal of the high frequency transformer T 1 is connected to the non-dotted terminal of the primary winding N 1 of the current transducer CT 1 . The dotted terminal of the high frequency transformer T 1 is connected to the positive terminal of the energy storage capacitor C 1 . The source of the synchronous rectifier SR 1 is connected to the negative terminal of the energy storage capacitor C 1 as well as the drain of the synchronous rectifier SR 2 . The drain of SR 1 is connected to the positive terminal of the output capacitor C 2 and the dotted terminal of the primary winding N 1 of the current transducer CT 1 . The gate of SR 1 is connected to the output port of its drive circuit. The negative terminal of the output capacitor C 2 is connected to the drain of SR 2 . The gate of SR 2 is connected to the output port of its drive circuit.

FIG. 7 is a schematic diagram of another T-type voltage doubler synchronous rectifier with a MOSFET energy feedback circuit and one current transducer. Compared with FIG. 6 , the main differences are that the cathode of the diode D 12 is connected to the positive terminal of the output capacitor C 2 , and the power circuit is another T-type voltage doubler rectifier. The T-type voltage doubler rectifier in FIG. 7 consists of a high frequency transformer T 1 , two synchronous rectifiers SR 1 and SR 2 , two capacitors C 1 and C 2 . The non-dotted terminal of the high frequency transformer T 1 is connected to the dotted terminal of the primary winding N 1 of the current transducer CT 1 . The dotted terminal of the high frequency transformer T 1 is connected to the source of SR 2 . The negative terminal of the capacitor C 1 is connected to the non-dotted terminal of the primary winding N 1 of the current transducer CT 1 . The positive terminal of the capacitor C 1 is connected to the source of SR 1 . The drain of SR 1 is connected to the positive terminal of the output capacitor C 2 . The gate of SR 1 is connected to the output port of its drive circuit. The drain of SR 2 is connected to the source of SR 1 . The gate of SR 2 is connected to the output port of its drive circuit. The negative terminal of the output capacitor C 2 is connected to the source of SR 2 .

FIG. 8 is a schematic diagram of a third T-type voltage doubler synchronous rectifier with a MOSFET energy feedback circuit and one current transducer. Compared with FIG. 6 , the main differences are that the cathode of the diode D 12 is connected to the positive terminal of the output capacitor C 2 , and the main circuit is another T-type voltage doubler rectifier. The T-type voltage doubler rectifier in FIG. 8 consists of a high frequency transformer T 1 , two synchronous rectifiers SR 1 and SR 2 , two capacitors C 1 and C 2 . The non-dotted terminal of the high frequency transformer T 1 is connected to the non-dotted terminal of the primary winding N 1 of the current transducer CT 1 . The dotted terminal of the high frequency transformer T 1 is connected to the drain of SR 2 . The negative terminal of the capacitor C 1 is connected to the dotted terminal of the primary winding N 1 of the current transducer CT 1 . The positive terminal of the capacitor C 1 is connected to the drain of SR 1 . The source of SR 1 is connected to the drain of SR 2 . The gate of SR 1 is connected to the output port of its drive circuit. The source of SR 2 is connected to the negative terminal of the output capacitor C 2 . The gate of SR 2 is connected to the output port of its drive circuit. The positive terminal of the output capacitor C 2 is connected to the drain of SR 1 .

FIG. 9 is a schematic diagram of a fourth T-type voltage doubler synchronous rectifier with a MOSFET energy feedback circuit and one current transducer. Compare with FIG. 6 , the main difference is that the main circuit is another T-type voltage doubler rectifier. The T-type voltage doubler rectifier in FIG. 9 consists of a high frequency transformer T 1 , two synchronous rectifiers SR 1 and SR 2 , two capacitors C 1 and C 2 . The non-dotted terminal of the high frequency transformer T 1 is connected to the dotted terminal of the primary winding N 1 of the current transducer CT 1 . The dotted terminal of the high frequency transformer T 1 is connected to the positive terminal of the capacitor C 1 . The negative terminal of the capacitor C 1 is connected to the source of SR 2 . The drain of SR 2 is connected to the source of SR 1 . The gate of SR 2 is connected to the output port of its drive circuit. The source of SR 1 is connected to the non-dotted terminal of the primary winding N 1 of the current transducer CT 1 . The drain of SR 1 is connected to the positive terminal of the output capacitor C 2 . The gate of SR 1 is connected to the output port of its drive circuit. The negative terminal of the output capacitor C 2 is connected to the source of SR 2 .

FIG. 10 is a schematic diagram of a common voltage doubler synchronous rectifier with a MOSFET energy feedback circuit and two current transducers. Compare with FIG. 5 , the main differences are that two current transducers are used, and the protecting circuits are removed. The current transducer CT 1 in FIG. 10 consists of a magnetic core, a primary winding N 1 and a secondary winding N 2 . One terminal of the winding N 1 is connected to one terminal of the energy storage capacitor C 1 . The other terminal of the winding N 1 is connected to one terminal of SR 1 . The winding N 2 is connected to the drive circuit of SR 1 . The current transducer CT 2 consists of a magnetic core, a primary winding N 3 and a secondary winding N 4 . One terminal of the winding N 3 is connected to one terminal of the energy storage capacitor C 2 . The other terminal of the winding N 3 is connected to one terminal of SR 2 . The winding N 4 is connected to the drive circuit of SR 2 .

FIG. 11 is a schematic diagram of a T-type voltage doubler synchronous rectifier with a MOSFET energy feedback circuit and two current transducers. Compare with FIG. 6 , the main differences are that two current transducers are used, and the protecting circuits are removed. The current transducer CT 1 in FIG. 11 consists of a magnetic core, a primary winding N 1 and a secondary winding N 2 . One terminal of the winding N 1 is connected to one terminal of the energy storage capacitor C 1 . The other terminal of the winding N 1 is connected to one terminal of SR 1 . The winding N 2 is connected to the drive circuit of SR 1 . The current transducer CT 2 consists of a magnetic core, a primary winding N 3 and a secondary winding N 4 . One terminal of the winding N 3 is connected to one terminal of the energy storage capacitor C 2 . The other terminal of the winding N 3 is connected to one terminal of SR 2 . The winding N 4 is connected to the drive circuit of SR 2 .

FIG. 12 is a schematic diagram of another T-type voltage doubler synchronous rectifier with a MOSFET energy feedback circuit and two current transducers. Comparing with FIG. 7 , the main differences are that two current transducers are used, and the protecting circuits are removed. The current transducers CT 1 and CT 2 are the same with those in FIG. 11 .

FIG. 13 is a schematic diagram of a third T-type voltage doubler synchronous rectifier with a MOSFET energy feedback circuit and two current transducers. Compared with FIG. 8 , the main differences are that two current transducers are used, and the protecting circuits are removed. The current transducers CT 1 and CT 2 are the same with those in FIG. 11 .

FIG. 14 is a schematic diagram of a fourth T-type voltage doubler synchronous rectifier with a MOSFET energy feedback circuit and two current transducers. Compare with FIG. 9 , the main differences are that two current transducers are used, and the protecting circuits are removed. The current transducers CT 1 and CT 2 are the same with those in FIG. 11 .

As is evident from the exemplary embodiments explained herein, an exemplary element of the present invention includes a current-driven synchronous rectifying drive circuit which has an energy feedback circuit that could feed the driving energy in both SRs back to the output. The whole circuit includes a clamp circuit, an energy feedback circuit, a high frequency transformer T 1 , a current transducer CT 1 , an energy storage capacitor C 1 , an output capacitor C 2 , two synchronous rectifiers SR 1 and SR 2 , and two drive circuits for SR 1 and SR 2 respectively. The driving voltage of SR 1 of the T-type voltage doubler rectifier is clamped to the voltage of the energy storage capacitor, and the energy of SR 2 is fed back to the same energy storage capacitor. Each drive circuit may have a signal shaping and reset circuit, a self-bias circuit, and a push-pull driver.

The signal shaping and reset circuit is used to convert the synchronous rectifier's current signal (which is detected by the secondary winding of the current transducer CT 1 ) into voltage signal and shape it into a pulse signal. It also enables the current transducer CT 1 to reset when the current of the synchronous rectifier turns zero. The input port of the signal shaping and reset circuit is connected to one of the secondary windings of the current transducer CT 1 and the output port is connected to the input port of the push-pull driver.

The self-bias circuit is used to store the energy from the current transducer CT 1 in to capacitor C 1 and to generate a voltage source which can regulate with the load and power consumption of the drive circuits in the synchronous rectifiers and power the whole drive circuit. The input port of the self-bias circuit is connected to the secondary winding of the current transducer CT 1 while the output port is connected to the push-pull driver.

The push-pull driver is used to amplify the driving signal generated by the signal shaping and reset circuit to drive the synchronous rectifiers. The input port of the push-pull driver is connected to the output port of the signal shaping and reset circuit while the output is connected to the gate of the synchronous rectifier.

The clamp and energy feedback circuit is used to clamp the voltage of the self-bias circuit to the energy storage capacitor C 1 , and feed the redundant energy transferred to the current transducer CT 1 back to the same energy storage capacitor C 1 . The input port of the clamp and energy circuit is connected to the self-bias circuit while the output port is connected to the storage capacitor C 1 .

Compared with the prior art technology discussed above, exemplary embodiments of the present invention has made improvements by reducing energy fed back thus improving efficiency, reducing a number of turns of the secondary winding of the current transducer thus reducing the cost of the CT, and providing a circuit that is now available in miscellaneous application, e.g., drive circuit with one CT or two.

The drive circuit and the power circuit are connected by the non-dotted terminal of the high frequency transformer T 1 is connected to the non-dotted terminal of the primary winding N 1 of the current transducer CT 1 . The dotted terminal of the high frequency transformer T 1 is connected to the positive terminal of the energy storage capacitor C 1 . The source of the synchronous rectifier SR 1 is connected to the negative terminal of the energy storage capacitor C 1 as well as the drain of the synchronous rectifier SR 2 . The drain of SR 1 is connected to the positive terminal of the output capacitor C 2 as well as the dotted terminal of the primary winding N 1 of the current transducer CT 1 . The gate of SR 1 is connected to the output port of its drive circuit. The negative terminal of the output capacitor C 2 is connected to the drain of SR 2 . The gate of SR 2 is connected to the output port of its drive circuit.

When two current transducers are used, the drive circuit and the main circuit are connected wherein the non-dotted terminal of the high frequency transformer T 1 is connected to the drain of SR 1 as well as the positive terminal of the output capacitor C 2 . The dotted terminal of the high frequency transformer T 1 is connected to the positive terminal of the energy storage capacitor C 1 . The source of SR 1 is connected to the non-dotted terminal of the primary winding N 1 of the current transducer CT 1 . The gate of SR 1 is connected to the output port of its drive circuit. The drain of SR 2 is connected to the dotted terminal of the primary winding N 3 of the other current transducer CT 2 . The source of SR 2 is connected to the negative terminal of the output capacitor C 2 . The gate of SR 2 is connected to the output port of its drive circuit. The negative terminal of the energy storage capacitor C 1 is connected to the dotted terminal of the primary winding N 1 of the current transducer CT 1 as well as the non-dotted terminal of the primary winding N 3 of the other current transducer CT 2 .

Thus, in an exemplary embodiment, the signal shaping and reset circuit consists of a diode D 1 , a diode D 2 , a diode D 3 , a resistor R 1 and a NPN transistor Q 1 . The cathode of the diode D 1 is connected to the dotted terminal of the secondary winding N 2 of the current transducer CT 1 as well as the collector of the transistor Q 1 . The anode of the diode D 1 is connected to one node of the resistor R 1 , the base of the transistor Q 1 and the anode of the diode D 2 . The other node of the resistor R 1 is connected to the cathodes of the diode D 2 & D 3 , and the non-dotted terminal of the secondary winding N 2 of the current transducer CT 1 . The anode of diode D 3 is connected to the emitter of the transistor Q 1 .

The push-pull driver consists of a NPN transistor Q 2 , a PNP transistor Q 3 and a resistor R 2 . The base of the transistor Q 2 is connected to the base of the transistor Q 3 as well as one node of the resistor R 1 . The other node of the resistor R 2 is connected to the dotted terminal of the secondary winding N 2 of the current transducer CT 1 . The emitter of the transistor Q 2 is connected to the emitter of the transistor Q 3 as well as the gate of SR 1 . The collector of the transistor Q 3 is connected to the anode of the diode D 3 .

The self-bias circuit consists of a diode D 4 and an energy storage capacitor C 3 . The anode of the diode D 4 is connected to the collector of the transistor Q 1 . The cathode of the diode D 4 is connected to the positive terminal of the energy storage capacitor C 3 as well as the collector of the transistor Q 2 . The negative terminal of the energy storage capacitor C 3 is connected to the collector of the transistor Q 3 .

The clamp and energy feedback circuit consists of a diode D 11 . The anode of the diode D 11 is connected to the positive terminal of the energy storage capacitor C 3 . The cathode of the diode D 11 is connected to the positive terminal of the energy storage capacitor C 1 .

In another exemplary embodiment of the present invention, a MOSFET energy feedback circuit is used to replace the clamp diode D 11 in the drive circuit of synchronous rectifier SR 1 . Such an energy feedback circuit can be applied in voltage doubler rectifiers. It has the features that the drive voltages of the output voltage doubler rectifier can be clamped to a certain level, and the redundant energy can be fed back. The MOSFET energy feedback circuit consists a N-channel MOSFET S 1 , a resistor R 7 , a resistor R 8 , a resistor R 9 , a resistor R 10 , a capacitor C 5 , a diode D 13 and a transistor Q 9 . The source of the N-channel MOSFET S 1 is connected to one node of the resistor R 10 . The drain of the N-channel MOSFET S 1 is connected to the positive terminal of the energy storage capacitor C 3 . The gate of the N-channel MOSFET S 1 is connected to the collector of the transistor Q 9 . The other node of the resistor R 10 is connected to the positive terminal of the energy storage capacitor C 4 . The emitter of the transistor Q 9 is connected to the source of SR 2 . The base of the transistor Q 9 is connected to the anode of the diode D 13 . The cathode of the diode D 13 is connected to the source of SR 1 . One node of the capacitor C 5 and one node of the resistor R 8 are connected to the anode of the diode D 13 . The other nodes of C 5 & R 8 are connected to the cathode of the diode D 13 . One node of the resistor R 9 is connected to the source of N-channel MOSFET S 1 . The other node of the resistor R 9 is connected to the gate of N-channel MOSFET S 1 . One node of the resistor R 7 is connected to the drain of synchronous rectifier SR 1 . The other node of the resistor R 7 is connected to the gate of N-channel MOSFET S 1 .

In another exemplary embodiment of the present invention, when one current transducer is used, a protecting circuit is placed between the drain and the source of each SR. This protecting circuit consists of a resistor R 3 , a diode D 9 and a transistor Q 7 . One node of the resistor R 3 is connected to the cathode of the diode D 9 as well as the drain of SR 1 . The other node of the resistor R 3 is connected to the anode of the diode D 9 and the base of the transistor Q 7 . The collector of the transistor Q 7 is connected to the drive circuit. The emitter of the transistor Q 7 is connected to the source of SR 1 .

While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes, omissions and/or additions may be made and equivalents may be substituted for elements thereof without departing from the spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, unless specifically stated any use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another.