Abstract:
A power supply device having zero switching voltage is disclosed to include a first switch having a parasitic body diode, a first capacitor, an inductor, a transformer, a second switch having a parasitic body diode, a second capacitor, a first voltage output unit providing a first voltage output, and a second voltage output unit having a third switch and providing a second voltage output. By means of controlling the order in which the first switch, the second switch and the third switch are to be switched on, switching voltage is eliminated from the first switch and the second switch and the working efficiency of the power supply device is raised.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to power supply technology and more specifically, to a power supply device having zero switching voltage. 
     2. Description of the Related Art 
       FIG. 1  is a circuit block diagram of a conventional dual forward converter type power supply device. According to this design, the power supply device comprises a first switch  100  and a second switch  110  at its input side, and a power output unit  130  at its output side. The power output unit  130  provides a V out  voltage output. 
     According to the aforesaid dual forward converter type power supply device, when the first switch  100  and the second switch  110  are switched on, a voltage drop is produced at the parasitic body diode. Therefore, it is necessary to overcome switching loss caused by such a voltage drop during a switching operation. Overcoming this switching loss results in lowering of the power supply efficiency of the dual forward converter type power supply device. 
     Therefore, it is desirable to provide a power supply device having zero-switching voltage that eliminates the aforesaid problem. 
     SUMMARY OF THE INVENTION 
     The present invention has been accomplished under the circumstances in view. It is one object of the present invention to provide a power supply device that eliminates switching voltage, improving the working efficiency. 
     To achieve this and other objects of the present invention, a power supply device in accordance with the present invention comprises a first switch, which is a three-end element and comprises a parasitic body diode and having a first end coupled to one end of a first input voltage (V in ), a second end coupled to a first control signal, and a third end; a first capacitor, which has opposing first and second ends respectively coupled to the first and third ends of the first switch; an inductor, which has one end thereof coupled to the third end of the first switch; a transformer, which comprises a primary side, a secondary side, a first winding at the primary side, and a second winding and a third winding at the secondary side; a second switch, which is a three-end element and comprises a parasitic body diode and having a first end coupled to an opposite end of the input voltage (V in ), a second end coupled to the first control signal, and a third end; a second capacitor, which has opposing first and second ends respectively coupled to the first and third ends of the second switch; a first voltage output unit providing a first voltage output; and a second voltage output unit, which comprises a third switch and provides a second voltage output. By means of controlling the order in which the first switch, the second switch and the third switch are to be switched on, switching voltage is eliminated from the first switch and the second switch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit block diagram of a conventional dual forward converter type power supply device. 
         FIG. 2  is a circuit block diagram of a power supply device having zero switching voltage in accordance with the present invention. 
         FIG. 3  is a circuit diagram of the power supply device having zero switching voltage in accordance with the present invention. 
         FIG. 4  is a schematic drawing of the first control signal and the second control signal according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 2˜4  in which  FIG. 2  is a circuit block diagram of a power supply device having zero switching voltage in accordance with the present invention;  FIG. 3  is a circuit diagram of the power supply device having zero switching voltage in accordance with the present invention;  FIG. 4  is a schematic drawing of the first control signal and the second control signal according to the present invention. 
     As illustrated, the power supply device having zero switching voltage comprises a first switch  10 , a first capacitor  20 , an inductor  30 , a transformer  40 , a second switch  50 , a second capacitor  60 , a first voltage output unit  70 , and a second voltage output unit  80 . 
     The first switch  10  is a three-end element and comprises a parasitic body diode  11 . The parasitic body diode  11  has its first end coupled to one end of an input voltage V in , and its second end coupled to a first control signal. The first switch  10  can be, but not limited to, a power switch. The power switch can be a N-pass metal oxide semiconductor field effect transistor, N-pass junction field effect transistor, P-pass metal oxide semiconductor field effect transistor, or P-pass junction field effect transistor. The first, second and third ends of the parasitic body diode  11  can be the drain, gate and source of a metal oxide semiconductor field effect transistor respectively. According to the present preferred embodiment, the first switch  10  is a N-pass junction field effect transistor. 
     The first capacitor  20  has two opposite ends respectively coupled to the first and third ends of the first switch  10 . 
     The inductor  30  has its one end coupled to the third end of the first switch  10 . 
     The transformer  40  has a primary side, a secondary side, a first winding  41  at the primary side, and a second winding  42  and a third winding  43  at the secondary side. The first winding  41  and the third winding  43  have a predetermined turn ratio so that the first capacitor  20  has a voltage V in /2 subject to coupling through the third winding  43 . 
     The second switch  50  is a three-end element and comprises a parasitic body diode  51 . The parasitic body diode  51  has its first end coupled to the other end of the input voltage V in , and its second end coupled to the first control signal. The second switch  50  can be, but not limited to, a power switch. The power switch can be a N-pass metal oxide semiconductor field effect transistor, N-pass junction field effect transistor, P-pass metal oxide semiconductor field effect transistor, or P-pass junction field effect transistor. Further, the first control signal can be, but not limited to, a PWM (pulse wave modulation) control signal that controls the length of conduction time of the first switch  10  and the second switch  50 . Further, the first, second and third ends of the parasitic body diode  51  can be the drain, gate and source of a metal oxide semiconductor field effect transistor respectively. According to the present preferred embodiment, the second switch  50  is a N-pass junction field effect transistor. 
     The second capacitor  60  has two opposite ends respectively coupled to the first and third ends of the second switch  50 . Subject to coupling through the third winding  43 , the second capacitor  60  has a voltage V in /2. 
     The first voltage output unit  70  provides a first voltage output. As shown in  FIG. 3 , the first voltage output unit  70  comprises a first rectifier diode  71 , a second rectifier diode  72 , a first coil  73 , and a third capacitor  74 . The first rectifier diode  71  has its one end coupled to one end of the second winding  42 . The second rectifier diode  72  has its one end coupled to the other end of the second winding  42  and its other end coupled to the second end of the first rectifier diode  71 . The first coil  73  can be, but not limited to, a choke coil, having its one end coupled to the other end of the first rectifier diode  71 . The third capacitor  74  has its one end coupled to the other end of the first coil  73 , and its other end grounded. Further, the third capacitor  74  has the first voltage output at its two opposite ends. The first voltage output unit  70  is a double forward converter that is a known circuit design and therefore, no further detailed description in this regard is necessary. 
     The second voltage output unit  80  comprises a third switch  86  and a second voltage output V zv . As shown in  FIG. 3 , the second voltage output unit  80  further comprises a third rectifier diode  81 , a fourth rectifier diode  82 , a fifth rectifier diode  83 , a second coil  84 , a fourth capacitor  85  and a third switch  86 . The third rectifier diode  81  has its first end coupled to one end of the third winding  43 . The fourth rectifier diode  82  has its first end coupled to the second coil  84  and its second end coupled to one end of the third winding  43  to which the third rectifier diode  81  is coupled. The fifth rectifier diode  83  has its first end coupled to the first end of the fourth rectifier diode  82 . The second coil  84  can be, but not limited to, a chock coil having its first end coupled to the first end of the fourth rectifier diode  82  and the first end of the fifth rectifier diode  83  and its second end grounded. The fourth capacitor  85  has the second voltage output V zv  (zero voltage switching) at its two opposite ends. The third switch  86  can be a three-end element, having its first end coupled to the second end of the third rectifier diode  81 , its second end coupled to the second control signal, and its third end grounded. The third switch  86  can be a power switch that can be a N-pass metal oxide semiconductor field effect transistor, N-pass junction field effect transistor, P-pass metal oxide semiconductor field effect transistor, or P-pass junction field effect transistor. The first, second and third ends of the third switch  86  can be the drain, gate and source of a metal oxide semiconductor field effect transistor. Further, the second control signal can be, but not limited to, a PWM (pulse wave modulation) control signal. Further, there is a time delay between the conduction time of the second control signal and the conduction time of the first control signal. 
     When the first switch  10  and the second switch  50  are switched off and the third switch  86  is switched on, the fourth capacitor  85  of the second voltage output unit  80  has the second voltage output V zv  at its two ends to provide 300 W power supply temporarily; the second voltage output is coupled to the first winding  41  by the third winding  43  so that the first capacitor  20  and the second capacitor  60  have a voltage V zv /2 subject to the predetermined turn ratio; V in  at the voltage input end is added to the ends of the first capacitor  20  and the second capacitor  60  to have the first capacitor  20 , the inductor  30  and the second capacitor  60  constitute a resonant loop that forces the electric current to go through the parasitic body diodes  11  and  51  and to further short circuit the passage between the source and drain of the first switch  10  and the passage between the source and drain of the second switch  50 , avoiding voltage drop; when switching on the first switch  10  and the second switch  50  at this time, no switching loss will occur and therefore, improving the power conversion efficiency of the double forward converter. 
     By means of the application of the present invention, the power supply device having zero switching voltage eliminates switching voltage and raising the working efficiency. Therefore, the invention effectively eliminates the drawbacks of conventional dual forward converter type power supply device. 
     Although particular embodiments of the invention have been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.