Re: MOSFET transient dissipation

On Sat, 5 Dec 2009 01:47:53 +0530, "pimpom" <pimpom@xxxxxxxxxxx>
wrote:

Dave Platt wrote:
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.

A slow build-up of light is unacceptable for the reasons I gave
in the reply I just posted to Jim Thompson's post. Please see
that.

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.

That's what I'm concerned about and is really what this thread is
about. Hmmm

Here is a really crazy idea: Can you build it such that the power
transistors are thermally connected to a water mass? Like put the
power tabs into a water tank? The thermal mass of even a little bit
of water is truly impressive.
.

Relevant Pages

  • Re: MOSFET transient dissipation
    ... That's more than 30W dissipation for a brief moment ... filament (and raising its resistance) more gradually? ... on the time constant of the RC, and the thermal time constant of the ...
    (sci.electronics.design)
  • Re: MOSFET transient dissipation
    ... characteristics curve with an assumed cold filament resistance ... That's more than 30W dissipation for a brief moment ... on the time constant of the RC, ...
    (sci.electronics.design)
  • Re: MOSFET transient dissipation
    ... filament (and raising its resistance) more gradually? ... on the time constant of the RC, and the thermal time constant of the ... of the transient heat pulse is limited by the rate of heat conduction ...
    (sci.electronics.design)
  • Re: Plate Resistance
    ... which explains in detail why plate resistance Rpd is dissipative, ... characteristics of the load line is non-dissipative, ... The corresponding power lost at the plate is the product ... and therefore does not involve dissipation of any power. ...
    (rec.radio.amateur.antenna)
  • Re: a better incandescent light bulb
    ... but the overall consumption of the lamp will be ... "Small variations in resistivity along the filament cause "hot spots" to form ... increasing resistance at that point?a positive feedback ... I hypothesize that the local power consumption at the thinned area of the ...
    (rec.crafts.metalworking)