At no load the current is not continuous (in a sense) in the inductor in the output direction (it is AC, with zero crossings, OK it might be considered continuous, but AC), thus there are subtle differences if you use the SS TF for the continuous case.
The TF now has a heavy dependence on the ON time of the lower FET, which may not appear at all in your TF.
Increasing the ON time ever so slightly of the lower fet (at no load) will cause net reverse power flow and a lowering of Vout, it would be easy for a system to be some what oscillatory here... as there is no load and very little damping on the o/p...
There would be appreciable AC current in the o/p caps also - not so desire-able...
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Ideally at no load, both devices should be running a duty cycle of zero, with a little bit of pulse skipping (burst mode) every 0.5 sec to keep the output up, top device only, 0.5% on time, burst length designed to get o/p to Vout + 5%, then off...until Vout falls to Vout x 0.95...
This action is taken from a slow error amp...
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or simply disable switching the lower fet for currents less than 5% of Imax, (app dependent)