I'm out of town for a few days, so can't run any tests.  But in the=20
meanwhile...

The turn-off should have minimal effect on the MOSFET until the current=20
starts dropping off through the flyback diode (and sense resistor).  And=20
that seems to be the effect -- with a fast enough flyback diode the=20
MOSFET runs quite cool.  It's just figuring out how to minimize that=20
diode heat so that it can be contained inside the enclosure.

Technically, you're right that the duty-cycle should not make a=20
difference to the flyback diode heat... with everything perfect. But I=20
wonder how much energy is being dissipated in the flyback diode during=20
the ripple of the 1A hold phase.  I can double the frequency of that=20
ripple, to see if it makes any noticeable difference... though I doubt=20
it would be much.




On 11/26/2015 4:57 PM, IVP wrote:

> I would guess that the duty cycle might be irrelevant. It's the quality o=
f
> the transitions that matters. If it's anything but an 'instant' and singl=
e
> L-H
> or H-L then there's going to be heat because time is being spent in the
> FET's linear region. All these extra components around the FET are an
> attempt to minimise undesired transitions without slowing down (ie keep
> the slopes as vertical as possible) the required ones
>
> Joe
>

--=20
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