SLYT835 March   2023


  1. 1Introduction
  2. 2A PSFB converter with an active clamp
  3. 3Active clamp leg design considerations
  4. 4Summary
  5. 5References


Phase-shifted full-bridge (PSFB) converters (see Figure 1-1) are widely applied to high-power applications, mainly because a PSFB converter can achieve soft switching on its input switches and thus facilitate high converter efficiency [1]. Although soft switching greatly reduces switching losses, the output rectifier parasitic capacitance resonates with the transformer leakage inductors – modeled as Lr in Figure 1-1 – resulting in voltage ringing with high voltage stress [2].

The voltage stress of the output rectifier could be as high as 2 x VIN x NS/NP, where NP and NS are the transformer’s primary and secondary windings, respectively. Traditionally, applying a passive snubber [2] (such as the resistor-capacitor-diode [RCD] snubber in Figure 1-1) at the output rectifier prevents the rectifier voltage from going too high and enables the use of a lower-voltage-rated component with a better figure of merit for lower power dissipation.

When applying metal-oxide semiconductor field-effect transistors (MOSFETs) as a synchronous rectifier (SR), you can expect lower Coss and RDS(on) on lower-voltage-rated MOSFETs at the same cost level compared to higher-voltage-rated MOSFETs. However, using a passive snubber means that part of the energy that caused the voltage ringing will dissipate in the passive snubber and result in an efficiency reduction.

This article introduces an active (rather than a passive) snubber and its related control, which minimizes rectifier voltage stress to achieve higher converter efficiency while also greatly reducing energy dissipation in the snubber circuit and without sacrificing operational range.

Figure 1-1 A PSFB power stage with a passive clamp and key waveforms.