Re: MOSFET transient dissipation

Already covered.

Regarding the turn-on surge, I've drawn a load line on the MOSFET
characteristics curve with an assumed cold filament resistance of
0.25 ohms. It intersects the 10V Vgs curve at about 45A Id and
0.7V Vds. That's more than 30W dissipation for a brief moment
(ignoring gate drive rise time). The fall to the steady-state
dissipation of less than 0.4W will be non-linear. What I'm
concerned about is the thermal inertia during that brief period.

How about slowing down the turn-on pulse, and "warming up" the
filament (and raising its resistance) more gradually?

Run your gate drive through an RC with a reasonable time constant, and
then feed it to the gate through a gate-stopper resistor. Depending
on the time constant of the RC, and the thermal time constant of the
filament, I imagine that you can probably bring the lamp up to full
brightness in (e.g.) 100 milliseconds or so, without the current
rising to more than double its steady-state value at any point. This
might result in a better-looking power dissipation curve. You may
increase the service life of the bulbs, too.

If you do decide to bang the MOSFET all the way on as fast as
possible, it wouldn't surprise me if the MOSFET's ability to get rid
of the transient heat pulse is limited by the rate of heat conduction
through the package and tab. The current surge may be over, and the
dissipation settled down to its steady state, before much of the heat
has been conducted out to the far side of the tab. If that turns out
to be the case, adding a heatsink (for additional thermal inertia at
this point) might not buy you much.

--
Dave Platt <dplatt@xxxxxxxxxxxx> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
.

Relevant Pages

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    (sci.electronics.design)
  • Re: MOSFET transient dissipation
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    (sci.electronics.design)
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