In my experience, if you enjoy Disney's firework displays, go right ahead
and sub for the original ... !!
Seriously though, I've never had any luck going down the substitution
route
for power MOSFETs in switchers. It seems like no matter how close - or
even
apparently better - you get a match on the quoted parameters of the
original, then if it doesn't try to take the retinas out of the backs of
your eyes at first switch on, it will try to melt the supply to your house
two days later ...
Others on here might have found differently ?
Arfa
Thanks for that advice. I found the device listed on a manufacturer's
site
(
http://www.fujisemiconductor.com/html/QckSelGuides/mosfet.htm) and
it's listed with "related" devices with a small set of comparative
characteristics.
I'm (only) guessing that I(D) is maximum drain current, R(DS)on is the
maximum
drain-to-source resistance when the device is fully on, V(DSS) is the
maximum drain-to-source voltage, and Q(G) Typ would be expected
capacitance at the gate.
I was hoping that, since these specs are the only ones on this page,
and the page seems to be intended to help you choose a device for a
design,
that if I could find one of the other numbers that came in close, I'd
be OK.
But it sounds like I'm just asking for trouble. Thanks again.
Well, you might be ok. You seem to have the abreviations for the various
parameters down pat, so that's a good start. Before carrying out any repair
on it, you might want to check that the current sensing resistor in the
source has not also gone open. Check the gate coupling resistor for open.
Check the main input filter cap for poor ESR / low value. Check the main
primary side rectifier diodes for short circuit.If it uses a control chip,
check the startup resistor(s) for value, and the supply decoupling cap. If
the gate coupling resistor is blown, or even just open, suspect that the
chip is also damaged. If you can source one, replace the feedback opto as
well. Check the secondary side for short circuit rectifiers (but don't get
fooled by the fact that they are 'fast' types that read very low in one
direction anyway). Shorted secondary side reccies can place such a strain on
the primary side circuitry, that it results in a blown FET.
If you have access to a variac, you can check for chopper drive on most
supplies, by leaving out the FET initially, and 'scoping the output pin of
the control chip, using primary side rectifier "-" (main filter cap "-") as
a reference, but be careful of your 'scope grounding, and the fact that you
are very close to rectified line voltage when doing this. You can often get
the chip basically 'going' with no more than 30% line input voltage for US
110v or 15% for EU 230v, which makes the checking a lot safer.
If you can source the correct FET, and it's not too expensive, save the
postage and order two of them. You can be sure that the first one will go
bang again ...
Arfa
Arfa
