The figures I remember on semiconductor temperature and reliability go
something like this:
1. Typical lifetime at junction temperature of 200 degC is somewhere in
10,000 hour range
2. Lifetime doubles for each 10 degC drop in temperature
Probably could dig up something more explicit with a little searching but this
is a reasonable approximation. Other points to keep in mind:
3. Junction temperature is NOT heatsink temperature! Junction to case
thermal impedance might be in the 0.5 degC/W area then add case to heatsink of
0.5 to 1.5 degC / watt then add heatsink to ambient of (highly variable) 5
degC/W up to 15 degC/w for a total of 6 to 18 degC/W. Now pump 100W of thermal
energy through that impedance and you get a very, very high juntion
temperature. Ouch.
Interesting engineering question that has kept me involved in fun stuff over
the years.
73
Bob Groh, WA2CKY
----- Original Message ----
From: Peter Chadwick <g3rzp@g3rzp.wanadoo.co.uk>
To: Manfred Mornhinweg <mmornhin@gmx.net>; amps@contesting.com
Sent: Monday, February 19, 2007 11:02:32 AM
Subject: Re: [Amps] Power Transistor Question -- 2SC5125
Manfred said:
>First, that you allow the junction to heat up to its
absolute maximum momentary temperature, 200 degrees Celsius. Running the
transistor that hot will allow it to last only for a very short time,
maybe just hours! You should run it at no more than 150 degrees to
obtain a good (almost eternal) lifespan.<
To give some idea of how the life shortens......the company I work for used to
make transistor arrays. We sold some to go into oil well drills, guaranteed to
have a minimum life of 30 minutes (!) at a chip temperature of 260deg C. And,
naturally, we charged for them........the same chip at 125 deg C junction
temperature would exceed 1 million hours on the standard failure rate
extrapolations from burn in.
Having said all that, in my ham gear, I've had more semiconductor failures in
the last 20 years than tube failures. Possibly part of this is that I have
considerably more semiconductors than tubes, but in terms of overall equipment
reliability, it's a sobering thought.....
73
Peter G3RZP
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