3.1 Phase-Shifted Full-Bridge with Current Doubler Rectifier

The Phase-Shifted Full-Bridge topology consists of a full bridge at the primary producing bipolar drive voltage for the SMPS transformer. The main function of the transformer is to provide required galvanic isolation and provide a suitable step-down conversion ratio. On the secondary, the current doubler rectifier stage is implemented by using two low-side rectifiers and two output inductors. Each synchronous rectification leg runs interleaved depending on transformer pulse polarity.

Figure 3-1. PSFB-CDR Topology with Output Short-Circuit Protection Stage

The phase-shifted full-bridge topology works by having both primary side legs switching at 50% duty cycle with dead time (160 ns for this design). There is no direct duty cycle variation, but rather a phase shift is introduced which produces bipolar voltage pulses for the Main Switch mode transformer. The phase-shift control range for this design is Ø = 0⁰ to 171⁰, resulting in 0 to 95% duty control. Additionally, an external shim inductance may be added to enforce ZVS at a given operating point.

Note: A DC blocking capacitor is used to block DC currents going into the transformer. This enforces a volt-second balance at primary and avoids transformer saturation during transient events. Firmware implementation can also help prevent uneven volt-second at the transformer (i.e., phase-shift update done every other cycle).