Modeling and Analysis of a Current-Fed ZCS Full-Bridge DC/DC Converter with Adaptive Soft-Switching Energy

A full dc and ac analysis of a previous proposed boost-type-input soft-switching full-bridge converter is presented. The primary-side switches turn-on/off under a zero-current condition (ZCS). The soft-switching is realized by using a simple snubber, formed by two uni-directional switches and a resonant capacitor, in the primary side. The snubber´s switches commute under a zero-voltage condition. The resonant energy used for getting ZCS is self-adaptable, depending only on the value of the input current. Except when the input current is at its maximum value, less resonant energy is used, keeping the conduction losses at a low value. A dc analysis allows for the calculation of the duty-cycle. The duty-cycle loss, the maximum voltage across the snubber capacitor, and the duration of the soft-switching assisted-intervals (the charging and discharging intervals of the snubber capacitor) are represented for different values of the resonant capacitor and the leakage inductance of the converter transformer. These graphics allow for an optimized design, by trading-off the voltage stress on the resonant capacitor (i.e. on the primary-side switches) with the loss of duty-cycle (i.e. with the ZCS regulation range). The range of the input voltage and load variation for which both output voltage regulation and soft-switching are assured is determined. Soft-switching is obtained for a wide line and load range. A current-controlled feedback circuit, using a digital signal processor (DSP) with a soft-start scheme was implemented. A small-signal ac analysis of the power stage allowed for the design of the digital controllers.