Chris Loiacono wrote...

>OK, help me understand...
>Does this mean that a longer soft-start period will cause a higher =
inrush?

No, it just means that if the soft-start period is drawn out longer than
it needs to be for the sake of the lamp filament, it will cause
unnecessary stress on the MOSFET.

Think of what happens in the extremes:

At one extreme, we have no soft-start period at all; the MOSFET is
abruptly turned on and, if this happens to occur when the voltage is at
its peak, the lamp experiences huge inrush currents-- 7 to 10 times the
current that would normally flow through the filament.

At the other extreme, let's say we turn the MOSFET on partially, so that
it is operating as a current limiter--and then just let it twist in the
breeze indefinitely, turned partially on.  The lamp experiences no
inrush of current at all; it can't, because the MOSFET is governing how
much current passes through it.  But the MOSFET is dissipating a lot of
power, and within a minute or so it overheats and goes "pop!".

Those are the extremes, and we are attempting to find a happy medium
between them-- a soft-start period long enough that the lamp experiences
a gradual rise in current so its filament isn't shocked, yet short
enough that the MOSFET doesn't dissipate more energy than it can handle.

That's what I was trying to figure out in my simulation: how short did
the soft-start period have to be, to avoid overstressing the MOSFET.

Dave

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