TO-3 used to be a common package for power transistors, but you don't see them much anymore. It can't be just because of SMT, TO-220 is still much in use. TO-3 has a much lower thermal resistance than TO-220, so why isn't it used anymore?


1 Answer 1


TO-3 is much more labor-intensive during assembly. Also, the TO-3 package is more expensive to manufacture

It's actually cheaper overall to use several TO-220 devices or a TO-247 device.

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    \$\begingroup\$ Also, TO-3 has a fairly unfavourable geometry for placing the silicon inside the package. The amount of silicon die that fits inside is very small considering the package footprint and the material between the substrate and heatsinkable outside profile is thick. TO-220, TO-247 and especially newer designs like DirectFET allow for much better thermal performance and much more die area per footprint area - all at a much lower integration cost. \$\endgroup\$
    – user36129
    Aug 8, 2013 at 7:56

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