At 02:55 PM 05/01/2012, you wrote:
>  At work I repair some ballasts that use 2 * TIP41's, these die on a
>regular basis often burning a nice hole in the PCB at the same time as
>disintegrating.
>
>After trying catch diodes (didn't work), I experimented with replacing
>TIP41's with BUJ 302a from NXP, these have a breakdown voltage of around
>1kv.
>
>The other day one of these repaired ones died, but one BUJ died O/C betwee=
n
>all three pins, and the other O/C between CE and S/C between BE. I've not
>seen a transistor used in transformer multivibrator circuit fail O/C
>before. Is it possible NXP have some form of internal resistor or perhaps
>this is a result of some inherent manufacturing technique of high breakdow=
n
>transistors?
>
>Obviously failing open circuit is much much safer than short circuit.
>
>Colin

Most typically semiconductors fail short circuit. If the
connections to them fail (either for mechanical reasons such as
thermal fatigue or because there is enough current to vaporize
the bonding wires) they may appear to fail open when measured
at the package leads.

In your case, it sounds like you may effectively have two fuses
in series (perhaps one is the PCB, and the other is the bonding wires)
and the more flimsy one will be the one that fails first. The TIP41
is rated for something like 50% more continuous collector current than
the BUJ. So, I suggest it's a difference in construction, but a chance one.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.co=
m
Embedded software/hardware/analog  Info for designers:  http://www.speff.co=
m



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