# TO-3 Transistor replacement needed that does not protrude through the chassis

I have a linear power supply that uses two 2N3771G. They are TO-3 style, and are exposed outside the chassis, on the heatsink.

I need to move this power supply into a new chassis, for commercial reasons.

I'm hoping to find a replacement transistor that can mount inside the new chassis, but not stick out through it. The new chassis will have heatsinks exposed on the outside.

Here are the specs of the 2N3771G:

$V_{CEO}$ = 40V max
$I_C$ = 30A max
$h_{FE}$ = 15 min
$h_{FE}$ = 60 max

How can I find a substitute?

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How much power are these transistors burning? –  The Photon May 30 '12 at 4:59
@ThePhoton - Wood is burned, not power. Power is consumed. –  stevenvh May 30 '12 at 5:47
@stevenvh, in engineering, "burn" also has the meaning of "turn to heat." Some words have multiple meanings. –  markrages May 30 '12 at 5:53
@markrages - steam train engineering? :-) –  stevenvh May 30 '12 at 5:53
@markrages - Oh, I know! They're the origin of the word engineer. And they burned a lot of coal. –  stevenvh May 30 '12 at 6:37

You could look for a straight replacement in one of the plastic cases like TO-3P or TO-247, such as STW3040 (which is "not recommended for new design") or BUF420AW (surprisingly expensive.)

A better choice might be a pair of TO-220 transistors, but you need to look at the surrounding circuit first. Bipolar transistors can't be simply paralleled, but need some emitter resistors to ensure they share current properly and don't run away thermally.

For linear operation, most of the important design considerations will be expressed by the safe operating area graph (assuming you use the same size of heat sink, so thermal considerations are OK):

You are considering DC operation, which is the lowest line on the curve. You need to make sure the operation stays within that lower line. To just do a part substitution without circuit analysis, you can compare the SOA curve of the replacement with the original part, and make sure the original curve fits under the replacement curve.

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+1 for mentioning TO-3P and TO-247. This info, combined with a manufacturer's online parametric search tool, will yield good results. –  zebonaut May 30 '12 at 8:14
digikey.com/product-detail/en/BUW48/BUW48-ND/1037781 Can someone tell me if this is the right one? (I'm not an electrical engineer, and am a little over my head) –  Kevin Welsh May 31 '12 at 17:30
You need to understand a fair amount of electronics to do this substitution successfully. Maybe you would do better to mount the TO-3 on a smaller heatsink internally, then connect the heatsink to the large external heatsink. Then no circuit changes are required and you can continue to use the inexpensive TO-3 part. –  markrages May 31 '12 at 20:16