Three-Phase High Frequency Isolated DC?DC Converter with PWM Generator

Basic power circuit of an isolated three -phase DCDC converter consists of three stages inverter stage in this stage the power switches (S1 to S6) practically realized by IGBT or MOSFET with body diode and parallel connected RC snubber circuits. Second one is high frequency isolated transformer. this transformer have three units of single phase high frequency transformer with star-connected primary and mesh connected secondary and thread output rectifier stage. Keeping in view of low voltage high current applications, different types of rectifiers such as forward type, center-tap, bridge, current doublers and Tripler are considered. On the basis of secondary side load sharing, transformer design and thermal heat dissipation, Three-phase, full diode bridge with three inductors are selected for low voltage high power and frequency. To control the output voltage against input source voltage and load variations, width of gate signals of power switches of three phase full bridge inverter are varied by the PWM control method. On the bases of duty cycle of switches, two control methods namely symmetric and asymmetric control are proposed The duty ratio of the upper group power switches (S1, S3, S5) in inverter legs is same as that of lower group of power switches (S2, S4, S6) in symmetrical control on the other hand in asymmetrical control duty ratio differs. To understand the steady-state analysis of proposed converter, power switching devices and passive components are considered to be lossless. The output voltage ripple is small and neglected and output inductors are large enough to ensure continuous of operation and ripple-free current. Switches of each inverter-leg are symmetrically and asymmetrically controlled and 120 degree phase shifted between each inverter-leg. To regulate the output voltage against line and load variation, duty cycle of power switch vs. of three phases, full bridge inverter is varied. Furthermore, duty cycle of switches also depends on the turns ratio of transformer.