Resonant converter with synchronous rectification drive circuit

[0001] This invention relates to a resonant circuit with a synchronous rectification drive circuit, and more particular to an LLC series resonant converter in a power supply.

[0002] Recently, the trend of the electronic equipments is toward the application with low voltage and high current. The power management technology is developed from the rectification of the rectification diode to the synchronous rectification of the metal-oxide-semiconductor field-effect transistor (MOSFET). The power consumed in the equipment with the rectification diode is more than that consumed in the equipment with the MOSFET; moreover, the performance of the switching power supply can be increased by the latter. The MOSFET is used for the power supply of the LCD TV with low noise and high efficiency and for the power supply of the computer and the telecommunication equipments.

[0003] Please refer to FIG. 1(a) showing the circuit of the rectification diode typed LLC series resonant converter in the prior art. The LLC series resonant converter 10 includes a switch circuit 21, a resonant circuit 22, a transformer 23 and a full-wave-rectifier circuit 24.

[0004] In FIG. 1(a), the switch circuit 21 is composed of a first switch 211 and a second switch 212, wherein the first switch 211 is composed of a main body Q₁ of a first power transistor, a first body-diode D_B1 and a first parasitic junction capacitor C_O1, and the second switch 212 is composed of a main body Q₂ of a second power transistor, a second body-diode D_B2 and a second parasitic junction capacitor C_O2. The first switch 211 is connected to the second switch 212 to form a half-bridge circuit. In addition, the switch circuit 21 can be a full-bridge circuit. The resonant circuit 22 is composed of a resonant inductor L_r, a resonant capacitor C_r and a magnetizing inductor L_m. It is known by one skilled in the art that the resonant inductor L_r can be composed of a leakage inductor of the transformer 23.

[0005] As shown in FIG. 1(a), there is a DC voltage source V_DC for the switch circuit 21 to provide an output voltage V_o on a load R_L via the resonant circuit 22, the transformer 23, the full-wave-rectifier circuit 24 and a voltage stabilizing capacitor C_L. The switch 21 is separated from the resonant circuit 22 and the full-wave-rectifier circuit 24 by the transformer 23 via a primary side winding set N_p and series connected secondary side winding sets N_S1 and N_S2. The full-wave-rectifier circuit 24 is composed of a first rectification diode D₁ and a second rectification diode D₂ connected to the voltage stabilizing C_L. The anode of the first rectification diode D₁ is connected to the positive dotted terminal of the secondary side winding set N_S1, and the anode of the second rectification diode D₂ is connected to the negative dotted terminal of the secondary side winding set N_S2. The junction of the secondary side winding sets N_S1 and N_S2 is used as the ground end of the output voltage V_o, and the cathodes of the rectification diodes D₁ and D₂ are used as the high voltage end of the output voltage V_o.

[0006] Please refer to FIG. 1(b) showing the frequency responses of the resonant converter shown in FIG. 1(a) with various circuit quality factors. As shown in FIG. 1(b), the circuit quality factor Q_K is related to the load R_L and the magnetizing current i_m, wherein K=Lm/Lr, and the LLC series resonant converter 10 is equivalent to an LC series resonant converter when K approximates to infinity.

[0007] As shown in FIG. 1(b), there are a first resonant frequency f_r1 and a second resonant frequency f_r2. The first resonant frequency f_r1 is related to the resonant inductor L_r and the resonant capacitor C_r, the second resonant frequency f_r2 is related to the resonant inductor L_r, the magnetizing inductor L_m and the resonant capacitor C_r, and the relationships thereamong are as follows.
f_r1=1/(2π√{square root over (L_r·C_r)})
f_r2=1/(2π√{square root over (L_m+L_r)·C_r)})

[0008] As shown in FIG. 1(b), there are three operation frequency regions demarcated by the first resonant frequency f_r1 and the second resonant frequency f_r2. The operation switching frequency fs corresponding to the first frequency region Region-1 has the relationship as f_s>f_r1. The operation switching frequency fs corresponding to the second frequency region Region-2 has the relationship as f_r2<f_s<f_r1. The operation switching frequency fs corresponding to the third frequency Region-3 has the relationship as f_s<f_r2. In order to achieve the zero voltage switching and the wide range of voltage stabilization, the first frequency region Region-1 and the second frequency region Region-2 are first considered to be used for the LLC series resonant converter 10; however, in order to achieve the zero voltage switching of the first switch 211 and the second switch 212 and to facilitate the design of the drive circuit, it is well known to use the second frequency region Region-2.

[0009] Please refer to FIG. 1(c) showing signals of the resonant converter shown in FIG. 1(a), which operates in the second frequency region. In FIG. 1(c), there are a first switch drive signal V_GS1, a second switch drive signal V_GS2, a resonant current i_L, a magnetizing current i_m, a power transfer current i_p and a periodic change of a resonant capacitor voltage drop V_Cr operating in the second frequency region Region-2, wherein the power transfer current i_p is zero at t₀, t₁, t₂ and t₃, there is zero voltage switching on the main body Q₁ of the first power transistor and the main body Q₂ of the second power transistor at the dead periods of t₁-t₃ and t₄-t₆, and the magnetizing current i_m at the periods is approximately a constant.

[0010] Please refer to FIG. 1(d) showing signals of the resonant converter shown in FIG. 1(a), which operates in the first frequency region. In FIG. 1(d), there are a first switch drive signal V_GS1, a second switch drive signal V_GS2, a resonant current i_L, a magnetizing current i_m, a power transfer current i_p and a periodic change of a resonant capacitor voltage drop V_Cr operating in the first frequency region Region-1, wherein the main body Q₁ of the first power transistor and the main body Q₂ of the second power transistor are respectively turned off at t₁ and t₄; thereupon the power transfer current i_p is gradually decreased, and the energy is continuously transferred to the load. Therefore, the magnetizing current i_m is continuously and linearly increased until the main body of the next power transistor is electrified, and the power transfer current i_p is zero at t₃ and t₆. The zero voltage switching on the main boy Q₁ of the first power transistor and the main body Q₂ of the second power transistor is maintained due to the magnetizing current i_m with the triangular waveform.

[0011] In FIGS. 1(c) and 1(d), the LLC series resonant converter 10 is practiced by using the rectification diode. However, under the identical conditions, the operation of the LLC series resonant converter in the second frequency region Region-2 fails if the LLC series resonant converter uses the MOSFET as the rectification switch.

[0012] Please refer to FIG. 2 showing the circuit of the synchronous rectification typed LLC series resonant converter in the prior art. FIG. 1(a) is compared with FIG. 2. The first rectification diode D₁ and the second rectification diode D₂ shown in FIG. 1(a) are replaced with the third switch 243 and the fourth switch 244, respectively, and the connection of the high voltage end of the output part and the ground end is changed so as to form the circuit shown in FIG. 2. It means that the third switch 243 is composed of a main body Q₃ of a third power transistor and a third body-diode D_B3, and the fourth switch 244 is composed of the main body Q₄ of the fourth power transistor and the fourth body-diode D_B4. The main bodies Q₃ and Q₄ of the power transistors have sources connected to the ground end of the output voltage V_o. The main body Q₃ of the third power transistor has a drain connected to the positive dotted terminal of the secondary side winding set N_S1. The main body Q₄ of the fourth power transistor has a drain connected to the negative dotted terminal of the secondary side winding set N_S2. In addition, the fourth switch rectification current i_Q4 flows into the negative dotted terminal of the secondary side winding set N_S2, and the third switch rectification current i_Q3 flows into the positive dotted terminal of the secondary side winding set N_S1.

[0013] The operation of the synchronous rectification typed LLC series resonant converter 40 in the second frequency region Region-2 is illustrated as follows. It is set that the first switch drive signal V_GS1 is identical to the fourth switch drive signal V_GS4, the second switch drive signal V_GS2 is identical to the third switch drive signal V_GS3, and the switch drive signals are identical to those in FIG. 1(c). There is the current through one of the third body-diode D_B3 and the fourth body-diode D_B4 at the dead periods of t₁-t₃ and t₄-t₆, so as to transfer the power from the secondary side to the primary side of the transformer 23, and therefore the circuit cannot operate normally and safely.

[0014] When the synchronous rectification typed LLC series resonant converter operates in the first frequency region Region-1, the converter operates normally due to the magnetizing current i_m with the triangular waveform.

[0015] If the conventional synchronous rectification typed LLC series resonant converter 40 shown in FIG. 2 operates in the second frequency region Region-2, there must be different pulse widths for the first switch drive signal V_GS1 and the fourth switch drive signal V_GS4. Similarly, there must be different pulse widths for the second switch drive signal V_GS2 and the third switch drive signal V_GS3.

[0016] In order to practice the operations in the first frequency region Region-1 and the second frequency region Region-2 for the conventional synchronous rectification drive circuit, the drive circuit is quite complicated. Therefore, the cost is high and the energy is wasted owing to ignoring to increase the conversion efficiency.

[0017] Accordingly, the drive circuit of the synchronous rectification typed LLC series resonant converter should be simplified to reduce the volume of the circuit, so as to lower the cost and to achieve high efficiency and low noise.

[0018] In order to overcome the disadvantages of the prior art described above, the present invention provides a resonant converter with a synchronous rectification drive circuit.

[0019] It is an aspect of the present invention to provide a resonant converter with a synchronous rectification drive circuit including a switch circuit, a resonant circuit, a transformer, a full-wave-rectifier circuit and a synchronous rectification drive circuit, wherein the synchronous rectification drive circuit includes four voltage-clamped drive circuits respectively coupled to the switch circuit and the full-wave-rectifier circuit, and each voltage-clamped drive circuit includes a clamp circuit and a transmission/discharge circuit for reducing the turn-off period of the switch, so as to increase the efficiency of the resonant converter when the switch circuit and the full-wave-rectifier circuit are turned off.

[0020] It is another aspect of the present invention to provide a resonant converter with a synchronous rectification drive circuit, wherein there are four voltage-clamped drive circuits divided into two groups, and the two groups are respectively driven to alternately provide identical switch drive signals, so that the resonant converter has an operation switching frequency higher than a resonant frequency thereof for the conversion performance on the wide range of the voltage regulation.

[0021] In accordance with the present invention, the resonant converter with a synchronous rectification drive circuit includes a switch circuit having at least one bridge arm having a first switch and a second switch; a resonant circuit coupled to the switch circuit and having a resonant frequency; a transformer having a primary side coupled to the resonant circuit and a secondary side; a full-wave-rectifier circuit coupled to the secondary side and having a third switch and a fourth switch; and a synchronous rectification drive circuit having a first voltage-clamped drive circuit, a second voltage-clamped drive circuit, a third voltage-clamped drive circuit and a fourth voltage-clamped drive circuit, wherein output terminals of the voltage-clamped drive circuits are coupled to the first switch, the second switch, the third switch and the fourth switch, respectively, and each of the voltage-clamped drive circuits includes a transmission/discharge circuit for reducing a turn-off period of the coupled switches during turning off the coupled switches.

[0022] In accordance with the present invention, the switch circuit is one of a half-bridge circuit and a full-bridge circuit, and each of the first switch and the second switch is a power transistor including a main body, a body-diode and a parasitic junction capacitor.

[0023] In accordance with the present invention, the resonant circuit includes a resonant capacitor, a resonant inductor and a magnetizing inductor in series.

[0024] In accordance with the present invention, each of the third switch and the fourth switch is a power transistor including a main body and a body-diode.

[0025] In accordance with the present invention, each of the voltage-clamped drive circuits includes a clamp circuit changing an AC drive signal into a DC drive signal, and includes the transmission/discharge circuit coupled to the clamp circuit and having an output terminal coupled to one of the first switch, the second switch, the third switch and the fourth switch for providing one of a first switch drive signal, a second switch drive signal, a third switch drive signal and a fourth switch drive signal.

[0026] In accordance with the present invention, the clamp circuit includes a clamp capacitor and a clamp diode.

[0027] In accordance with the present invention, the transmission/discharge circuit includes a transmission diode passing the DC drive signal therethrough, and includes a discharge transistor coupled to the clamp circuit, the transmission diode and the coupled switches for accelerating a discharge path during turning off the coupled switches.

[0028] In accordance with the present invention, the resonant converter with a synchronous rectification drive circuit includes a first drive transformer having a first primary side winding set and two first secondary side winding sets, wherein the first primary side winding set is connected to a first couple capacitor in series for receiving a first phase signal, and the first secondary side winding sets are respectively coupled to the first voltage-clamped drive circuit and the fourth voltage-clamped drive circuit, and the resonant converter with a synchronous rectification drive circuit further includes a second drive transformer having a second primary side winding set and two second secondary side winding sets, wherein the second primary winding set is connected to a second couple capacitor in series for receiving a second phase signal, and the second secondary side winding sets are respectively coupled to the second voltage-clamped drive circuit and the third voltage-clamped drive circuit.

[0029] In accordance with the present invention, the resonant converter with a synchronous rectification drive circuit includes a first isolator having a first input terminal coupled to a drive signal producing device for receiving a first phase signal and having a first output terminal coupled to the fourth voltage-clamped drive circuit for preventing the first phase signal from being interfered by a first signal from the fourth voltage-clamped drive circuit, and the resonant converter with a synchronous rectification drive circuit further includes a second isolator having a second input terminal coupled to the drive signal producing device for receiving a second phase signal and having a second output terminal coupled to the third voltage-clamped drive circuit for preventing the second phase signal from being interfered by a second signal from the third voltage-clamped drive circuit.

[0030] In accordance with the present invention, each of the first isolator and the second isolator is a drive transformer or an optical coupler.

[0031] In accordance with the present invention, the voltage-clamped drive circuits respectively provide a first, a second, a third and a fourth switch drive signals according to an operation switching frequency and the resonant frequency of the resonant converter for respectively driving the switches.

[0032] In accordance with the present invention, as the operation switch frequency is higher than the resonant frequency, the first switch drive signal is identical to the fourth switch drive signal, the second switch drive signal is identical to the third switch drive signal and, the first switch drive signal and the second switch drive signal are alternately produced.

[0033] The above aspects and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which:

[0046] The invention is described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments of this invention are presented herein for the purpose of illustration and description only; it is not intended to be exhaustive or to be limited to the precise form disclosed.

[0047] Please refer to FIG. 3 showing a resonant converter with a synchronous rectification drive circuit according to a preferred embodiment of the present invention. In the figures, similar components are denoted as similar reference numbers. It is to be noted that a switch circuit 21 of an input terminal is composed of a half-bridge circuit having a bridge arm in FIG. 3. Alternatively, the switch circuit 21 of the input terminal can be composed of a full-bridge circuit having two bridge arms. The resonant converter 60 has a synchronous rectification drive circuit 25 for implementing the synchronous rectification drive of the present invention. In addition, power transistors are used as the switches in the embodiment of the present invention; however, the term “switch” claimed in the present invention is not limited to the power transistor.

[0048] As shown in FIG. 3, the synchronous rectification drive circuit 25 includes a first voltage-clamped drive circuit 251, a second voltage-clamped drive circuit 252, a third voltage-clamped drive circuit 253 and a fourth voltage-clamped drive circuit 254. The first voltage-clamped drive circuit 251 has an input terminal for receiving a first AC drive signal v_H1, and has an output terminal coupled to the first switch 211 for providing a first switch drive signal v_GS1 to control turn on/off of the first switch 211. The second voltage-clamped drive circuit 252 has an input terminal for receiving a second AD drive signal v_H2, and has an output terminal coupled to the second switch 212 for providing a second switch drive signal v_GS2 to control turn on/off of the second switch 212. The third voltage-clamped drive circuit 253 has an input terminal for receiving a third AD drive signal v_H3, and has an output terminal coupled to the third switch 243 for providing a third switch drive signal v_GS3 to control turn on/off of the third switch 243. The fourth voltage-clamped drive circuit 254 has an input terminal for receiving a fourth AD drive signal v_H4, and has an output terminal coupled to the fourth switch 244 for providing a fourth switch drive signal v_GS4 to control turn on/off of the fourth switch 244.

[0049] As above illustrations, when the resonant converter 60 has an operation switching frequency f_s higher than a resonant frequency f_r1 thereof, i.e. the resonant converter 60 operates in a first frequency region Region-1, it is set that the first switch drive signal v_GS1 is identical to the fourth switch drive signal v_GS4, the second switch drive signal v_GS2 is identical to the third switch drive signal v_GS3, and the first switch drive signal v_GS1 and the second switch drive signal v_GS2 are alternately produced. When the resonant converter 60 operates in the second frequency region Region-2, there must be different pulse widths for the first switch drive signal v_GS1 and the fourth switch drive signal v_GS4. Similarly, there must be different pulse widths for the second switch drive signal v_GS2 and the third switch drive signal v_GS3. As shown in FIG. 3, the sequence of the first AC drive signal v_H1, the second AC drive signal v_H2, the third AC drive signal v_H3 and the fourth AC drive signal v_H4 can be regulated for various synchronous rectification drive conditions. In order to illustrate the practice of the synchronous rectification drive circuit of the present invention, the following embodiments of two synchronous rectification drive circuits 25 are provided to show the arrangement of the synchronous rectification drive circuit 25 when the resonant converter 60 operates in the first frequency region Region-1.

[0050] Please refer to FIG. 4 showing a synchronous rectification drive circuit according to the first preferred embodiment of the present invention. Each of a first voltage-clamped drive circuit 251, a second voltage-clamped drive circuit 252, a third voltage-clamped drive circuit 253 and a fourth voltage-clamped drive circuit 254 of the synchronous rectification drive circuit 25 includes a clamp circuit 301 and a transmission/discharge circuit 302. In the first voltage-clamped drive circuit 251, the clamp circuit 301 is used for raising the first AC drive signal v_H1 to a first DC drive signal v_K1, the transmission/discharge circuit 302 is coupled to the clamp circuit 301, and an output terminal is coupled to the first switch 211 for receiving a first DC drive signal v_K1 to provide the first switch drive signal v_GS1. In the second voltage-clamped drive circuit 252, the clamp circuit 301 is used for raising the second AC drive signal v_H2 to a second DC drive signal v_K2, the transmission/discharge circuit 302 is coupled to the clamp circuit 301, and an output terminal is coupled to the second switch 212 for receiving a second DC drive signal v_K2 to provide the second switch drive signal v_GS2. In the third voltage-clamped drive circuit 253, the clamp circuit 301 is used for raising the third AC drive signal v_H3 to a third DC drive signal v_K3, the transmission/discharge circuit 302 is coupled to the clamp circuit 301, and an output terminal is coupled to the third switch 213 for receiving a third DC drive signal v_K3 to provide the third switch drive signal v_GS3. In the fourth voltage-clamped drive circuit 254, the clamp circuit 301 is used for raising the fourth AC drive signal v_H4 to a fourth DC drive signal v_K4, the transmission/discharge circuit 302 is coupled to the clamp circuit 301, and an output terminal is coupled to the fourth switch 214 for receiving a fourth DC drive signal v_K4 to provide the fourth switch drive signal v_GS4.

[0051] Each clamp circuit 301 includes a clamp capacitor C_A1 and a clamp diode D_E1. The transmission/discharge circuit 302 includes a transmission diode D_E2 and a discharge transistor Q₅. The transmission diode D_E2 is used for passing corresponding DC drive signal therethrough, i.e. when the raising edge of the corresponding DC drive signal is received by the transmission diode D_E2, an input gate capacitor of the coupled switch is charged. The input gate capacitor is composed of a gate-drain capacitor C_GD and a gate-source capacitor C_GS. When the charging voltage of the gate-source capacitor C_GS is higher than the conducting voltage of the coupled switch, the coupled switch is electrically connected. The discharge transistor Q₅ is coupled to the clamp circuit 301, the transmission diode D_E2 and the coupled switch for accelerating a discharge path during turning off the coupled switch, i.e. when the falling edge of the corresponding DC drive signal is received by the transmission diode D_E2 and the discharge transistor Q₅, the discharge transistor Q₅ is electrically connected, and store charges of the input gate capacitor are discharged via the discharge transistor Q₅, so that the turn-off period of the coupled switch is reduced. Accordingly, it is advantageous to the sequential control of the synchronous rectification drive circuit 25 and raising the power efficiency.

[0052] As shown in FIG. 4, the synchronous rectification drive circuit 25 further includes a first drive transformer 255 and a second drive transformer 256. The first drive transformer 255 has a primary side winding set N_PA and two secondary side winding sets N_SA1 and N_SA2. The primary side winding set N_PA is connected to a first couple capacitor C_S1 for receiving a first phase signal v_S1. The DC part of the first phase signal v_S1 would not be transmitted to the primary side winding set N_PA of the first drive transformer 255 due to the effect of the first couple capacitor C_S1. The secondary side winding set N_SA1 is coupled to the first voltage-clamped drive circuit 251 for providing the first AC drive signal v_H1, and the secondary side winding set N_SA2 is coupled to the fourth voltage-clamped drive circuit 254 for producing the fourth AC drive signal v_H4. The second drive transformer 256 has a primary side winding set N_PB and two secondary side winding sets N_SB1 and N_SB2. The primary side winding set N_PB is connected to a second couple capacitor C_S2 for receiving a second phase signal v_S2. The DC part of the second phase signal v_S would not be transmitted to the primary side winding set N_PB of the second drive transformer 256 due to the effect of the second couple capacitor C_S2. The secondary side winding set N_SB1 is coupled to the second voltage-clamped drive circuit 252 for providing the second AC drive signal v_H2, and the secondary side winding set N_SB2 is coupled to the third voltage-clamped drive circuit 253 for producing the third AC drive signal v_H3.

[0053] Please refer to FIG. 5 showing a second preferred embodiment of the synchronous rectification drive circuit according to the present invention. As shown in FIG. 5, the synchronous rectification drive circuit 25 includes a first phase drive circuit 25a and a second phase drive circuit 25b. The first phase drive circuit 25a is coupled to a drive signal producing device 26 for receiving a first phase signal v_S1, wherein the first phase signal v_S1 is a square waveform. The first phase signal v_S1 is processed by the first phase drive circuit 25a for providing the first switch drive signal v_GS1 and the fourth switch drive signal v_GS4, so as to drive the first switch 211 and the fourth switch 244, respectively. The second phase drive circuit 25b is coupled to a drive signal producing device 26 for receiving a second phase signal v_S2, wherein the second phase signal v_S2 is another square waveform. The second phase signal v_S2 is processed by the second phase drive circuit 25b for providing the second switch drive signal v_GS2 and the third switch drive signal v_GS3, so as to drive the second switch 212 and the third switch 243, respectively.

[0054] The first phase drive circuit 25a includes the first voltage-clamped drive circuit 251, the fourth voltage-clamped drive circuit 254 and a first isolator 257. Both the first voltage-clamped drive circuit 251 and the first isolator 257 are coupled to the drive signal producing device 26 for receiving a first phase signal v_S1. The first phase signal v_S1 is converted to the first switch drive signal v_GS1 by the first voltage-clamped drive circuit 251. The first isolator 257 can be a drive transformer, an optical coupler or any other electrical equipments with isolation functions. The first isolator 257 has an output terminal coupled to the fourth voltage-clamped drive circuit 254 for preventing the first phase signal v_S1 from being interfered by signals of the fourth voltage-clamped drive circuit 254 from the load R_L. The fourth AC drive signal v_H4 is produced by the first isolator 257. The fourth voltage-clamped drive circuit 254 is coupled to the first isolator 257 for receiving the fourth AC drive signal v_H4 and converting the fourth AC drive signal v_H4 to the fourth switch drive signal v_GS4.

[0055] The first phase drive circuit 25b includes the second voltage-clamped drive circuit 252, the third voltage-clamped drive circuit 253 and a second isolator 258. Both the second voltage-clamped drive circuit 252 and the second isolator 258 are coupled to the drive signal producing device 26 for receiving a second phase signal v_S2. The second phase signal v_S2 is converted to the second switch drive signal v_GS2 by the second voltage-clamped drive circuit 252. The second isolator 258 can be a drive transformer, an optical coupler or any other electrical equipments with isolation functions. The second isolator 258 has an output terminal coupled to the third voltage-clamped drive circuit 253 for preventing the second phase signal v_S2 from being interfered by signals of the third voltage-clamped drive circuit 253 from the load R_L. The third AC drive signal v_H3 is produced by the second isolator 258. The third voltage-clamped drive circuit 253 is coupled to the second isolator 258 for receiving the third AC drive signal v_H3 and converting the third AC drive signal v_H3 to the third switch drive signal v_GS3.

[0056] The drive circuit shown in FIG. 5 can be used in other converters such as Boost typed, Buck typed and Buck-Boost typed converters.

[0057] Please refer to FIG. 6(a) showing signals of the resonant converter shown in FIG. 3 and FIG. 4 according to the present invention, wherein the periodic changes of the first switch drive signal v_GS1, the second switch drive signal v_GS2, the resonant current i_L and the resonant capacitance drop v_Cr operating in the first frequency region Region-1 are presented. Please refer to FIG. 6(b) showing signals of the resonant converter shown in FIG. 3 and FIG. 4 according to the present invention, wherein the fourth switch drive signal v_SG4, the second switch drive signal v_GS2, the fourth switch rectification current i_Q4 and the third switch rectification current i_Q3 are presented. Please refer to FIG. 6(c) showing signals presenting the zero voltage switching of the resonant converter shown in FIG. 3 and FIG. 4 according to the present invention, wherein the zero voltage switching of the first switch 211 is presented.

[0058] As shown in FIGS. 6(a)-6(c), when the resonant converter 60 has an operation switching frequency f_s higher than a resonant frequency thereof, the first switch drive signal is identical to the fourth switch drive signal, the second switch drive signal is identical to the third switch drive signal, and the first switch drive signal and the second switch drive signal are alternately produced, the first switch and the second switch have the zero voltage switching, the third switch and the fourth switch have the zero voltage switching, and the third switch rectification current and the fourth switch rectification current have the quasi-sine waveform.

[0059] Please refer to FIG. 7 showing the comparison of the power efficiency between the resonant converter shown in FIG. 1 and the resonant converter of the present invention. As shown in FIG. 7, the efficiency of the synchronous rectification resonant converter is higher than that of the rectification diode resonant converter when the output current is higher than 6 A, and the efficiency of the synchronous rectification resonant converter approximates to that of the rectification diode resonant converter when the output current is lower than 6 A.

[0060] It is characterized in the present invention that a resonant converter with a synchronous rectification drive circuit includes a switch circuit, a resonant circuit, a transformer, a full-wave-rectifier circuit and a synchronous rectification drive circuit, wherein the switch circuit at least includes a half-bridge circuit, the resonant circuit is coupled to the switch circuit and has a resonant frequency, the transformer has a primary side coupled to the resonant circuit, the full-wave-rectifier circuit is coupled to a secondary side of the transformer and includes two switches, the synchronous rectification drive circuit includes four voltage-clamped drive circuits having output terminals coupled to the switch circuit and the corresponding switch of the full-wave-rectifier circuit, and each voltage-clamped drive circuit includes a transmission/discharge circuit for reducing the turn-off period of the coupled switch during turning off the coupled switch.

[0061] While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.