6 Synchronous Rectifier Modes of Operation

Synchronous rectifier operation modes in a PSFB converter can be different from other isolated topologies such as an LLC-SRC. A synchronous rectifier in an LLC-SRC works like an ideal diode. Detecting the synchronous rectifier FET voltage or current reveals whether current is conducting through the body diode of the FET, so that you know whether to turn on the synchronous rectifier FET. When synchronous rectifier current is lower than a certain level, turn off the synchronous rectifier FET.

As PSFB rectifier stage (shown in Figure 33) operates like a buck converter. For example, when V_SEC is positive, L_O is energized through the leg 4 high-side switch (controlled by the Out4H signal) and current returning to the transformer through the leg 3 low-side switch (controlled by the Out3L signal). When the transformer winding voltage is zero, I_LO current continuously flows through the leg 4 low-side switch (controlled by the Out4L signal) and current returns to L_O through the leg 3 high-side switch (controlled by Out3H). This is the freewheeling period, during which the inductor current will continue to flow through the body diodes of the synchronous rectifier FETs, even you don’t turn them on. When the current during the freewheeling period is high, you must turn on all four of the synchronous rectifier FETs in the full-bridge rectifier to allow channel conduction instead of body diode conduction to lower the conduction losses. In general, there are three modes of operations of the synchronous rectifier in a PSFB, as shown in Figure 34 through Figure 36. When the load current is very low, it is acceptable to keep all synchronous rectifier FETs turned off (mode 0) to lower the switching and driving losses. When the load current is a little bit higher while the PSFB is still in discontinuous conduction mode, you can start to turn synchronous rectifier FETs on during the L_O energizing period (mode 1). When the load current goes even higher, the PSFB operates in continuous conduction mode, turning all synchronous rectifier FETs on during the L_O de-energizing period (mode 2) to help lower the conduction losses. The ability to turn all four synchronous rectifier FETs of the full-bridge rectifier on during the freewheeling period is unique to a PSFB.