Charge-Based Soft-Switching Analysis and Joint Selection of Commutating Inductance and Dead Time in Phase-Shifted Full-Bridge Converters

Zero-voltage switching (ZVS) of the lagging leg in phase-shifted full-bridge (PSFB) converters is usually assessed using an energy criterion that assumes voltage-independent capacitance. For superjunction MOSFETs, whose output capacitance falls nearly three orders of magnitude with voltage, this misjudges threshold and losses: 86% of the studied device’s output charge lies below 22 V, the charge- and energy-related equivalent capacitances differ 10.3-fold, and the threshold work is underestimated 8-fold (38µJ against 309 µJ). This paper presents a charge-based approach in which the threshold work, the switching-node trajectory, and the residual voltage are integrals over the datasheet output-capacitance Coss (u) characteristic. The commutating current is derived for a diode bridge and a held-on synchronous rectifier, and below the threshold the transition has a turning point that makes the dead-time condition an equality. A design procedure selects the commutating inductance, dead time, and number of paralleled primary devices jointly: for a 3.3 kW reference converter, the feasible interval is 3.9–17.8 µH (held-on rectifier) or 10.6–16.4 µH (diode bridge), the optimum 8 µH/310 ns, and more than two devices no longer pay. PLECS Spice simulation with vendor models reproduces the commutation current within 10% and confirms the predicted incomplete transition at light load.