U-136A Phase Shifted, Zero Voltage Transition Design Considerations

The UC3875 family of integrated circuits controls a bridge power stage by phase-shifting the switching of one half-bridge with respect to the other, allowing constant frequency pulse-width modulation with resonant, zero-voltage switching for high-efficiency performance at high frequencies. This family of circuits may be configured to provide control in either voltage- or current-mode operation, with a separate overcurrent shutdown for fast fault protection. This application note discusses the design considerations incurred in a high-frequency power supply using the phase-shifted resonant PWM control technique. It gives an overview of this switching technique, including comparisons to existing fixed-frequency, non-resonant and variable-frequency zero-voltage switching. It presents numerous design equations and associated voltage, current, and timing waveforms associated with this technique. A general-purpose phase-shifted converter design guide and procedure are introduced to assist in weighing the various design tradeoffs. An experimental 500W, 48V at 10.5A power-supply design operating from a preregulated 400V DC input is presented as an example. The details of the magnetic, power switching and control circuitry areas are discussed. A summary of comparative advantages, differences and tradeoffs to other conversion alternatives is included.