On Mon, Dec 15, 2003 at 06:59:23AM -0800, Declan Moriarty enlightened us thusly
> On Mon, Dec 15, 2003 at 05:44:19AM -0800, Brad O'Dell enlightened us thusly
> > Hello everybody,
> > I was repairing a socket 370 motherboard and needed to replace a noticeably
> > damaged Harris HUF76121D3S (part is branded 76121D, also Fairchild has this part).
> > All I have easy access to is a RFP30N06LE. Will this be a suitable replacement
> > besides the package style (TO-252AA versus TO-220AB)?
> >
> > starter data:
> > both are N-channel enhancement mode logic level MOSFETs
> > replacement has higher voltage rating (Vdss and Vdgr 60V vs. 30V)
> > replacement has higher current rating (Id 30A vs. 20A)
> > replacement seems to be faster
> > however, drain-to-source ON resistance is more with replacement (Rds(ON) max.
> > .047ohm vs. .030ohm). Would this significantly affect circuit operation? Also
> > the HUF76121 Rds(on) was rated for the part maximum current of 20A, whereas the
> > RFP30N06 Rds(on) was rated for that part's maximum current of 30A. How would I
> > recalculate the RFP device's Rds(on) with the lower current of 20A that will be
> > used?
> >
> The rule of thumb is to replace with better, and replace all together in
> a current sharing arrangement.
>
> I am expert at breaking this rule ;-). I would not worry. Swap away. I
> can't imagine any problems resulting from extra heat, as you have a
> bigger package, right? Not many designers make use of propogation time
> in switching circuits. Don't recalculate the RFP device's specs
> whatever you do, as you will only worry yourself :-).
>
> The question I am asking is - why did the first one die? what killed it?
>
>
This reads very poorly (as usual). Before everyone corrects me, let me
try again.
> Not many designers make use of propogation time in switching circuits.
should be more like
Designers are not inclined to rely on having a fet they have turned
on in the off state. It does happen for a nanosecond or three, but you
don't use it. So the faster switching is not an issue.
--
With best Regards,
Declan Moriarty.
--
Author: Declan Moriarty
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