Calculating the value of the shim inductor (LS) is based on the amount of energy required to achieve zero voltage switching. This inductor needs to able to deplete the energy from the parasitic capacitance at the switch node. The following equation selects LS to achieve ZVS at 100% load down to 50% load based on the primary FET’s average total COSS at the switch node.
The actual parasitic capacitance at the switched node may differ from the estimate and LS may have to be adjusted accordingly.
Typical shim inductor DC resistance:
Estimate LS power loss (PLS) and readjust remaining power budget: