Zero-voltage transition converter with high efficiency operating at constant switching frequency

A new type of zero-voltage-transition pulsewidth modulation (PWM) dc-dc converter is developed by adding two active switches and three diodes to the structure of a quasiresonant converter. The resonant capacitors are not discrete elements, their role is played by the parasitic capacitances of the MOSFETs. By adjusting the interval between the operation of the active switches, a duty-cycle control is obtained. Soft-switching is ensured for all switches for a wide line range and load range. One of the additional diodes is connected in the input circuit and the energy flow in the resonant stage is transferred from a path defined by the two additional transistors to a path given by the two additional diodes in the resonant tank; as a result, the resonant capacitor voltage is prevented from surpassing the input voltage; it drops to zero before the main switch is turned on. All switches are subjected to voltage and current stresses like those in their PWM classical counterparts, implying small conduction losses. The new converter exhibits a high efficiency and good regulation capability.