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Old 24th February 2005, 05:22 PM   #1
maylar is offline maylar  United States
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Default metal vs plastic transistors

Is there any inherent advantage to using TO-3 packages vs their flat pack counterparts?

The only thing I can think of is a possible thermal advantage, with better case to heatsink or case to ambient cooling. Maybe junction to case as well, if the die bonding is better in a metal herm sealed package. But I can't find any numbers in the spec sheets to know for sure.

Any thoughts on this?
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Old 24th February 2005, 05:38 PM   #2
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Actually the manufacturers spec a better thermal figure
for the plastics, but I'm not sure that it's simply them
being less conservative.

In any case, the TO-3's are becoming more expensive and
less available, and I have only a few years worth left in stock.

Sonically, I don't hear a difference, the reliability and
measurements look the same, and the customers seem to care
less as time goes on.
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Old 24th February 2005, 06:08 PM   #3
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The plastic ones are easier to mount, but other than that, I like the TO-3's better except for the fact you have to have more holes in the heatsink.

The TO-3's heat up slower, and I like that as well. IMO you don't need as big of a heatsink with the metal TO-3s as with the flat plastic ones.
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Old 25th February 2005, 12:58 AM   #4
anatech is offline anatech  Canada
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Huh?, the metal packages heat up slower?? In my experience they heat up the same.

The plastic packages save production costs, so that's what industry wants. They are just as reliable as long as they are clamped to the heatsink. I'm not convinced the single screw has as good contact (just being old?)
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Old 25th February 2005, 06:37 AM   #5
AndrewT is offline AndrewT  Scotland
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Generally the plastic version has a lower Tjmax than TO3.
Calculating back to Rjc shows that most plastic have a lower thermal resistance than TO3. Does the manufacturer recognise that the olastic part is potentially more vulnerable at elevated temps?
This confirms earlier suspicion that a bigger heatsink is recommended for plastic to keep them cooler,
If you intend operating at elevated temp then 200 deg C versions have a better SOA (I believe that 200 degC is only available in TO3).
regards Andrew T.
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Old 25th February 2005, 07:02 AM   #6
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Look at IR's Super TO 220 package - 200 - 300 W of Ptot is normal by this devices. Sure, they must be fitted by clips or by belt by two screws - all problems with plastic case is caused by one screw fitting .
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Old 25th February 2005, 09:39 AM   #7
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TO3's have a much higher torque rating than plastic.
(i broke a couple of the square dude casings)

Because of the eliptic shape of the TO3 housing, and two holes for torqueing them, pressure is distributed more evenly, contact surface is much better.
Plastic cases have elastic behavior when torqueing, not really a point with the metal TO3, also a reason for bad connecting surface.
Resistance case to heatsink is 0.30 compared to 0.50 for a TO247, at the best.
With class A amplifiers that is a major disadvantage when using plasticos, my view.
The difference of 0.20 is for every output device.
That means the heatsink will have to surrender the 0.20, 0.20 is quite a difference in heatsink.
To make it worse, the 0.20 has to be divided by the number of devices mounted on it.

As Mr Pass said: fat chance the TO3's will be there for very much longer.
Not just for output devices, also for other active parts.
The mil. spec. often came in TO3 cases.
I am not keen on spending leasure time marching, but mil spec devices always had the same effect as a dirty mag.

I am about to raid NP's stash !
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Old 25th February 2005, 09:51 AM   #8
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Originally posted by jacco vermeulen
TO3's have a much higher torque rating than plastic.
(i broke a couple of the square dude casings)
... but why tighten to screws so hard?... The only thing you want is to get rid of all air bubbles in the "sweat paste".
/Per-Anders (my first name) or P-A as my friends call me
PA03 LM4780 amplifier group buy, SIGN UP HERE If the interest is big enough there will be a group buy
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Old 25th February 2005, 09:59 AM   #9
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That was accidental, i can be clumsy, Per.

I noticed a few times that bolts loosened on plastic cases, a professional audio friend already adviced me to torque output devices well, especially on class A stages.

My thought was that plastic and metal do not mix well, with large temperature differences.
Thermal expansion for plastic is much different from the one for metal.
Even combining alloy and metal causes expansion problems.
A major problem with ships, btw : cruise ships have alloy superstructures and steel hulls.
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Old 25th February 2005, 11:37 AM   #10
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Use hi-temp thread lock on the screws. It stands up to brake rotors heat cycling, it will stand up to transistor heat cycling.
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