ABSTRACT
Pulse-width-modulation (PWM) DC-DC converters have been the most common switching DC-DC converters. However, their switching frequency is limited due to large switching losses. To reduce the weight and size of a DC-DC converter, its switching frequency should be raised. To reduce switching losses and raise switching frequency, many soft-switching techniques have been adopted for DC-DC converters [1, 2]. Phase-shift full-bridge converters are widely used for step-down applications due to their advantages such as low voltage stresses of the switching devices, a fixed switching frequency, and ZVS operation of power switches. However, large circulating current causes large conduction losses [3, 4]. The asymmetrical full-bridge converter was proposed in [5]. It has various advantages such as zero switching loss, no conduction loss penalty, and fixed switching frequency. However, voltage stresses across power switches are fixed to input voltage even when steep conversion ratio is required. In order to overcome these problems, an isolated ZVS buck converter based on full-bridge topology is proposed in this paper. The proposed converter witch is shown in Fig. 1 uses active clamp techniques. Instead of DC-blocking capacitor in the conventional asymmetric full-bridge buck converter, a clamping capacitor is connected across the 2nd bridge to clamp the 2nd bridge switch voltages and provide ZVS operation. The proposed converter features clamped switch voltages, fixed switching frequency, soft-switching operations of all power switches, and low voltage stresses in small duty cycle operation. Therefore, the proposed converter shows high efficiency and it is suitable to a steep conversion application.
