Abstract:
This invention relates in Synchronous Rectifier Circuits, comprises: AC input terminal, switch, driving circuit, protect opposite current circuit and a load, to improved conventional Synchronous Rectifier Circuits, can be achieve rectify function.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention related to enhancement mode power MOSFET or power JFET for synchronous rectifier, especially driving voltage connected to gate and drain of power MOSFET or power JFET replacing prior art driving voltage connected to gate and source of power MOSFET or power JFET. 
         [0003]    According to such philosophy of the present invention, the synchronous rectifier may be achieved use power MOSFET or power JFET, driving circuit, and protect opposite current circuit. Hence, functions of minimizing voltage drop between Alternating Current (AC) and Direct Current (DC) voltage output to load of the synchronous rectifier may be achieved. 
         [0004]    2. Description of Related Art 
         [0005]    As shown in  FIG. 1  is a circuit diagram of a prior art high efficiency regulate DC supply, which had been illustrated by U.S. Pat. No. 5,038,266. The circuit uses power MOSFET  22  and  23  to be the switch of rectifying and voltage regulation, transforms the DC power to the load terminal  18 . Such scheme comes with the following drawbacks: 
         [0000]    (1) To use the gate-source electrodes of the power MOSFET  22 ,  23  as input of high frequency power supply, the value of the input capacitance is large than that oft he output capacitance, which cannot downsize the power supply, therefore the power MOSFET is not suitable for the high frequency power supply.
 
(2) The maximum ratings between the gate and source of a normal power MOSFET are ±20V, the surge voltage at the secondary windings of the high frequency transformer will bum the gate-source of the power MOSFET out.
 
(3) When the voltage at the source  20  of the power MOSFET  15  is a high frequency positive voltage, input capacitance value at the gate-source is greater, the high frequency positive voltage will turn the source  20 , the gate  25 , the diode  39  and the resistor  43  into short, therefore, the power MOSFET  22  is easy to be burned out.
 
         [0006]    When the DC voltage potential at the drain  21  of the power MOSFET  22 ,  23  is greater than that at the source  20 , the high frequency power will cause great DC short current at turn-on delay time or turn-off delay time to burn the power MOSFET  22 ,  23  and the secondary winding  13 ,  14  of the power supply out. 
       SUMMARY OF THE INVENTION 
       [0007]    In order to provide DC power supply device, which may elevate the efficiency of rectification, this invention is accordance with the following objects. 
         [0008]    The first object of this invention is to provide driving voltage connected to the gate and drain of power MOSFET, can be eliminate the burnout of prior art power MOSFET and used in the high frequency power system. 
         [0009]    The second object of the present invention, is a driving voltage connected to the gate and drain of power MOSFET, can be eliminate the drawback of high power consumption of prior art rectifier utilizing diode. 
         [0010]    According to the defects of the prior art technology discussed above, a novel solution, is a driving voltage connected to the gate and drain of power MOSFET is propose in the present invention, which provides higher efficiency in rectifier. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a circuit diagram of a prior art high efficiency regulate DC supply. 
           [0012]      FIG. 2  is a circuit diagram of an N-Channel FET. 
           [0013]      FIG. 3  is a circuit diagram of first embodiment of the present invention. 
           [0014]      FIG. 4  is a circuit diagram of second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    As shown in  FIG. 2 , has an AC power source input terminal, a first terminal A and second terminal B of the input terminal, a N-Channel FET Q 1 , a first driving element R 1 , a second driving element R 2  and a load LD. 
         [0016]    A body diode BD in the N-Channel FET Q 1  shown in  FIG. 2 , a driving circuit comprises a first driving element R 1  and a second driving element R 2 ; the first terminal of first driving element R 1  connected to terminal A of AC power source and first terminal of load LD, the second terminal of first driving element R 1  connected to first terminal of the second driving element R 2  and connected together to gate of the N-Channel FET Q 1 , the second terminal of the second driving element R 2  connected to drain of the N-Channel FET Q 1  and terminal B of AC power source; source of the N-Channel FET Q 1  connected to second terminal of load LD; the driving voltage is equal to the second driving element R 2  two terminal voltage drop. 
         [0017]    As shown in  FIG. 2 , when positive of AC power source in the terminal A, terminal B is negative, the first terminal of the second driving element R 2  is positive, second terminal of the second driving element R 2  is negative, the N-Channel FET Q 1  is turned on, the driving voltage is equal to the voltage drop resistor R 2 , the path of the current flows is from terminal A of the AC power source though a load LD, source-drain of N-Channel FET Q 1 , and back to terminal B of the AC power source. 
         [0018]    As shown in  FIG. 2 , when negative of AC power source in the terminal A, terminal B is positive, the first terminal of the second driving element R 2  is negative, second terminal of the second driving element R 2  is positive, the N-Channel FET Q 1  is turned off, the rectifier is open circuit. 
         [0019]    As shown in  FIG. 3 , the operation principle of the second driving element comprises a diode D 1  and resistor R 2 , the P-junction of diode D 1  connected to gate of the N-Channel FET Q 1 , the N-junction of diode D 1  connected to first terminal of resistor R 2 , second terminal of second driving element connected to drain of the N-Channel FET Q 1  and second terminal of AC power source, the driving voltage is equal to forward voltage of diode D 1  and resistor R 2  drop voltage; the operation of the resistor R 2  the function of zener diode is same, both of the resistor R 2  can be use a zener diode replace, the driving voltage is equal to forward voltage D 1  and zener voltage of zener diode; the protect opposite current circuit comprises a voltage dividing resistor R 3 , R 4  and a transistor TR 1 , first terminal of the first voltage dividing resistor R 3  connected to terminal A of AC power source and first terminal of load LD, second terminal of first voltage dividing resistor R 3  and first terminal of second voltage dividing resistor R 4  connected together to base of transistor TR 1 , second terminal of second voltage dividing resistor R 4  connected to terminal B of AC power source and drain of the N-Channel FET Q 1 , emitter of the transistor TR 1  connected to gate of the N-Channel FET Q 1 , second terminal of first driving element R 1  and P-junction of diode D 1  connected together to emitter of transistor TR 1  and gate of the N-Channel FET Q 1 ; collector of the transistor TR 1  connected to source of the N-Channel FET Q 1  and second terminal of load LD. 
         [0020]    As shown in  FIG. 3 , when positive of AC power source in the terminal A, terminal B is negative, the first terminal of second voltage dividing resistor R 4  is positive, the second terminal of second voltage dividing resistor R 4  is negative, the transistor TR 1  is turned off, collector and emitter of the transistor TR 1  is open circuit, the path of the current flows is from terminal A of the AC power source though a load LD, source-drain of N-Channel FET Q 1  and back to terminal B of the AC power source. 
         [0021]    As shown in  FIG. 3 , when negative of AC power source in the terminal A, terminal B is positive, the first terminal of second voltage dividing resistor R 4  is negative, the second terminal of second voltage dividing resistor R 4  is positive, the transistor TR 1  is turned on, collector and emitter of the transistor TR 1  is short circuit, the rectifier is open circuit. 
         [0022]    As shown in  FIG. 4 , the operation principle of the first driving element comprises a diode D 2  and resistor R 5 , the N-junction of diode D 2  connected to gate of the P-Channel FET Q 2 , the P-junction of diode D 2  connected to second terminal of resistor R 5 , first terminal of resistor R 5  connected to drain of the P-Channel FET Q 2  and first terminal of AC power source, the driving voltage is equal to forward voltage of diode D 2  and resistor R 5  drop voltage; the operation of the resistor R 5  the function of zener diode is same, both of the resistor R 5  can be use a zener diode replace, the driving voltage is equal to forward voltage D 2  and zener voltage of zener diode; the protect opposite current circuit comprises a voltage dividing resistor R 7 , R 8  and a transistor TR 2 , first terminal of the first voltage dividing resistor R 7  connected to terminal A of AC power source and drain of the P-Channel Q 2 , second terminal of first voltage dividing resistor R 7  and first terminal of second voltage dividing resistor R 8  connected together to base of transistor TR 2 , second terminal of second voltage dividing resistor R 8  connected to terminal B of AC power source and second terminal of load LD, first terminal of second driving element R 6  and N-junction of diode D 2  and gate of the P-Channel FET Q 2  connected together to collector of transistor TR 2 , emitter of the transistor TR 2  connected to source of the P-Channel FET Q 2  and first terminal of load LD, second terminal of second driving element R 6  connected to terminal B of AC power source. 
         [0023]    As shown in  FIG. 4 , when positive of AC power source in the terminal A, terminal B is negative, the first terminal of first voltage dividing resistor R 7  is positive, the second terminal of first voltage dividing resistor R 7  is negative, the transistor TR 2  is turned off, collector and emitter of the transistor R 2  is open circuit, the path of the current flows is from terminal A of the AC power source though drain-source of P-Channel FET Q 2 , a load LD and back to terminal B of the AC power source. 
         [0024]    As shown in  FIG. 4 , when negative of AC power source in the terminal A, terminal B is positive, the first terminal of first voltage dividing resistor R 7  is negative, the second terminal of first voltage dividing resistor R 7  is positive, the transistor TR 2  is turned on, collector and emitter of the transistor R 2  is short circuit, the rectifier is open circuit.