[Amps] Power Transistor Question -- 2SC5125

[rgroh at swbell.net]

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 at g3rzp.wanadoo.co.uk>
To: Manfred Mornhinweg <mmornhin at gmx.net>; amps at 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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