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

Question

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?

Answer 1

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.