As we have not seen a circuit diagram (AFAIK) it's not 100% certain what is
being done, BUT - the holding arrangement SHOULD be the equivalent of a
buck converter with  current circulating in the inductor when the FET is
off and the FET being PWM'd to maintain current. Properly done the FET is a
true switch and dissipation is low.
The recirculating diode should be of suitable speed to suit the switching
frequency - but faster does not usually hurt too much. (A sharp diode
transition can produce substantial RF noise).

   R
--=20
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