The final schematic demonstrates a valley switching flyback in which the primary switch turns on when the voltage across the switch is at a local minimum point, or valley, of the resonant ringing which occurs in discontinuous mode operation. The example here uses a C-code DLL implementation of the algorithm that determines at which of the valleys the switching will occur.

Inputs FBTH3:0 and the high/low line indicator VINTH are fed as input to the DLL. During each wake event, the DLL runs the algorithm and the output NSZX3:0 is provided to the IC U1 to specify the number of negative slope zero crossings.

The FB signal will be in one of 5 regions and the DLL “wakes” at a specified interval, reads the FBTH3:0 signal and acts accordingly. It acts as a combination of a state machine to determine the appropriate action and a register to keep track of the current number of negative slope zero crossings.

(open .sxsch) / (open .sxgpx)