In region ○1 (the purple shaded area) the switching frequency is higher than resonant frequency Fr1.
The converter operation is very similar to a series resonant converter. Here Lm never resonates with
resonant capacitor Cr
; it is clamped by the output voltage and acts as the load of the series resonant
tank. This is the inductive load region and the converter is always under ZVS operation regardless of
the load condition.