Abstract:
A DC-DC power converter in which the voltage across the main switch due to leakage inductance of the transformer is clamped and leakage energy of the transformer is recycled instead of being dissipated so as to improve operating efficiency.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The invention relates to a DC-DC power converter. More specifically, the invention relates to a converter in which the voltage across the main switch due to the leakage inductance of the transformer is clamped and the leakage energy of the transformer is recycled instead of being is dissipated by the circuit so as to improve the circuit efficiency. 
     2. Description of Related Art 
     A well-known conventional DC/DC flyback converter is shown in FIG. 1, where Lk  10  is the leakage inductance of the transformer T  12 . The typical switching waveforms of FIG. 1 are shown in FIG.  2 . When switch S  14  is turned off at t 2 , the leakage current charges the parasitic output capacitance of switch S  14  (output capacitance of S is not shown in FIG.  1 ), which causes a high voltage spike across switch S  14 . After the leakage energy is completely released, the voltage across switch S  14  reaches its steady-state value. As a result, a high voltage rating for switch S  14  is required. 
     To eliminate this voltage spike, a number of circuit topologies have been reported in the literature. Among them, the R-C-D snubber, shown in FIG. 3 is one of the most popular ways to minimize the voltage spike as shown in FIG.  2 . The snubber circuit consists of diode D 1   20 , capacitor Cs  22  and resistor Rs  24 . When switch S  14  is turned off, the leakage current flows through diode D 1   20  and charges capacitance Cs  22 . If capacitance Cs  22  is relatively large, the voltage across Cs  22  does not change so as to clamp the voltage. In this case, the leakage energy of the transformer is first charged to Cs  22  and then is dissipated by the resistor Rs  24 . As a result, the voltage clamp is achieved at the expense of low conversion efficiency. 
     SUMMARY OF THE INVENTION 
     The invention is a DC-DC converter in which the voltage across the main switch due to the leakage inductance of the transformer is clamped and the leakage energy of the transformer is recycled instead of being dissipated by the circuit so as to improve the circuit efficiency. The DC-DC converter has a voltage source which is connected to a diode. A first transformer primary winding is in series with a first capacitor. This winding and capacitor are connected across the voltage source and diode. A second transformer primary winding is in series with a second capacitor. They are also connected across the voltage source and diode. The first and second transformer primary windings have first and second leakage inductances respectively. 
     A switch has one terminal connected to terminals of the first transformer primary winding and the first capacitor. The switch also has a second terminal connected to the terminals of the second transformer primary winding and the second capacitor. The transformer first and second primary windings and the transformer secondary winding are included in the transformer. The transformer has a magnetizing inductance providing a delivered output to the transformer secondary winding. A parallel load capacitor and load resistor are connected across the transformer secondary winding and diode. 
     The advantage of the inventive DC-DC converter is that the voltage across the main switch due to the leakage inductance of the transformer is clamped. In addition, the leakage energy of the transformer is recovered by charging the first and second capacitors and the delivered output by the magnetizing inductance instead of being dissipated by the circuit so as to improve the circuit efficiency. Another objective of the invention is to use as few components as possible and use only one active switch to reduce the cost. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a schematic drawing of a conventional DC-DC flyback converter (prior art). 
     FIG. 2 shows the switching waveforms of FIG.  1 . 
     FIG. 3 shows a detailed schematic drawing of a DC-DC flyback converter with R-C-D Snubber (prior art). 
     FIG. 4 shows a detailed schematic drawing of the invented DC-DC converter with leakage energy recovery of the transformer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The inventive circuit is shown in FIG.  4 . The transformer T  40  has two primary windings (i.e. first and second) N 1   30  and N 2   32 , respectively, and one (i.e. first) secondary winding N 3   34 . The windings N 1   30  and N 2   32  have the same number of turns. LK 1   36  and LK 2   38  (LK 1 =LK 2 ) are the first and second leakage inductances, respectively, of two primary windings N 1   30  and N 2   32  in transformer T  40 . First and second capacitors C 1   42  and C 2   44 , respectively, (C 1 =C 2 ) are the clamp capacitors to clamp the voltage across switch S  48  during the switch off period. Switch S  48  is a power semiconductor switch which for example could be a MOSFET or an insulated gate bipolar transistor (IGBT). First diode D 1   50  is in series with power source  52  to block the reverse energy to the source when C 1   42  and C 2   44  release the leakage energy to the load through the transformer  40 . Second diode Do  54  is the output rectifier and capacitor Co  56  is the filter capacitor to reduce the output voltage ripple. Resistor Ro  58  with voltage Vo across it represents the load on the converter. 
     Before the switch S  48  is on, C 1   42  and C 2   44  are charged to a high voltage value Vcmax by the magnetizing current. 
     As the switch S  48  turns on, capacitors C 1   42  and C 2   44  are in series, and two primary windings N 1   30  and N 2   32  are in series through switch S  48 . The voltage Va  60  is higher than the input voltage source Vin  52 , and diode D 1   50  is off. The voltages across capacitors C 1   42  and C 2   44  are applied to the windings N 2   32  and N 1   30  respectively. The energy stored in capacitors C 1   42  and C 2   44  is delivered to magnetizing inductance Lm  64 . As a result, the magnetizing current increases and the voltages across C 1   42  and C 2   44  decrease in a resonant form, until the voltage Va  60  is equal to the input voltage  52 , and D 1   50  is conducting when the voltage Va  60  is clamping to the input voltage  52 . The capacitors C 1   42  and C 2   44  provide the energy to the magnetizing inductance Lm  64  during these time intervals. The transformer is modeled as a magnetizing inductance Lm  64  with an ideal transformer with coupled windings N 1 , N 2  and N 3  in this figure. The magnetizing inductance Lm  64  is shown in parallel with primary winding N 1   30 . The magnetizing inductance Lm  64  could be reflected to winding N 2   32  with the same value if N 1 =N 2  because they are coupled. The power is delivered to the output through the magnetizing inductance to the secondary side because the magnetizing inductance Lm  64  can also be reflected to the secondary winding (i.e. N 3   34 ) which is connected to the load  58  through the output diode  54 . 
     The inout voltage Vin  52  is applied to the windings N 1   30  and N 2   32  through switch S  48 . The current in the magnetizing inductor Lm  64  increases linearly, and the voltages across C 1  and C 2  are clamping to half of Vin. Therefore, the input power source provides the energy to the magnetizing inductor Lm  64  during this period. 
     When the switch S  48  turns off, the transformer&#39;s magnetizing current is first to charge capacitors C 1   42  and C 2   44 . After the voltages across C 1   42  and C 2   44  are higher than Vx (where Vx=Vin+N 1 /N 3 ×Vo), Do begins to conduct. The magnetizing energy stored in the transformer is then transferred to the output and the magnetizing current linearly decreases. Meanwhile, the energy stored in leakage inductance Lk 1   36  and Lk 2   38  is transferred to capacitors C 1   42  and C 2   44  instead of being dissipated by the circuit in prior arts. As the currents in Lk 1   36  and Lk 2   38  decrease to zero, the voltages on C 1   42  and C 2   44  reach the maximum value, Vcmax, where          V     c                 max       =       V   in     +       N1   N3     ·     V   0       +       I   kp              L   k1       /     C   1                                  
     where I kp  is the peak current in the leakage inductor Lk 1   36  or Lk 2   38  when switch S  48  turns off. 
     The maximum voltage across switch S  48  is:          V     ds      max       =       V   in     +         N1   +   N2     N3     ·     V   0       +       I   kp              L   k1       /     C   1                                  
     It is shown that the leakage energy is full recovered and directly transferred to the load, instead of being dissipated by the circuit compared with the circuits in the prior art. As a result the invented circuit has potential high power conversion efficiency and low cost. 
     Another advantage is that the voltage across the main switch due to the leakage inductance of the transformer is clamped. Still another advantage is that the circuit uses only one active switch and only a few components. 
     While the preferred embodiments of the invention have been shown and described; numerous variations and alternative embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.