ZERO-VOLTAGE TRANSITION CONVERTER WITH HIGH-EFFICIENCY OPERATING AT CONSTANT SWITCHING FREQUENCY

A new type of zero-voltage-transition pulsewidth modulation (PWM) de-d e converter is developed by adding two active switches and three diode s to the structure of a quasiresonant converter. The resonant capacito rs are not discrete elements, their role is played by the parasitic ca pacitances of the MOSFET's. By adjusting the interval between the oper ation of the active switches, a duty-cycle control is obtained, Soft-s witching is ensured for all switches for a wide line range and load ra nge. One of the additional diodes is connected in the input circuit an d the energy flow in the resonant stage is transferred from a path def ined by the two additional transistors to a path given by the two addi tional diodes in the resonant tank; as a result, the resonant capacito r voltage is prevented from surpassing the input voltage; it drops to zero before the main switch is turned on. All switches are subjected t o voltage and current stresses as those in their PWM classical counter parts, implying small conduction losses. The new converter exhibits a high efficiency and good regulation capability.