3.9 Load Transients and Reverse Current Protection in Current Doubler Stage

Load transients may cause high change in phase shift and abruptly decrease inductor current demand. This pushes the output inductors in DCM. Note that at this point, the duty demand is very small, and SR MOSFETs are switched ON most of the time.

This causes reverse currents from charged output capacitors to go into the SR MOSFETs. During this time, the inductor also stores energy in the opposite direction. When the SR MOSFETs are switched OFF in every switching cycle, this stored energy in the oppositive direction causes repeated high inductive spikes on the SR MOSFET. Such an event causes SR MOSFETs to repeatedly avalanche, resulting in MOSFET damage on the secondary.

To avoid this problem, the SR MOSFETs must be switched OFF below a certain inductor current threshold and work in Asynchronous mode. This protection is implemented in firmware by separately enabling Synchronous Rectification outside of the control loop.