2 A PSFB converter with an active clamp

As shown in Figure 2-1, inserting an active clamp leg formed by a capacitor (C_CL) and a MOSFET (Q_CL) before the output inductor enables active clamp leg current conduction within the effective duty-cycle (D_eff) period, thus clamping the secondary winding voltage (V_SEC) and rectifier voltage stress to the C_CL voltage – V_CL. In order to have low voltage stress on the output rectifier, you must select a large-enough C_CL for low capacitor voltage ripple. A rule of thumb is to select the inductor-capacitor (LC) resonant period formed by L_r and C_CL to be much longer than the switching period (T_s) [3] expressed by Equation 1:

The rectifier voltage stress will clamp to around V_INx N_S/N_P with the active snubber, which is about half of the voltage stress without any clamp circuit.

Unlike a passive snubber, an active snubber doesn’t dissipate the ringing energy on the power resistor. Instead, it circulates the energy in the LC resonant tank as a lossless snubber. When the output winding voltage becomes nonzero, energy will transfer from the primary winding to the secondary winding to energize the output inductor and conduct current through the Q_CL body diode, even if Q_CL isn’t turned on. Turning on Q_CL after its body has already conducted current will ensure zero voltage switching (ZVS) on Q_CL. Therefore, you can expect higher converter efficiency on a PSFB converter with an active snubber over a PSFB converter with a passive snubber in an identical specification.