I have a question regarding mounting THT power transistors (TO220, TO247...). I am building a output full bridge for a welder and I thought about not using PCB at all for the power terminals (D,S). It's pretty normal to use the tab of a transistor as a contact that is clamped to the heatsink whick function as a "high current" conductor. The tab is usually the drain..but what about the source? I am thinking about clamping the mosfet legs between a busbar and a clamping "washer".

I don't see any problem with that but I haven't seen it done anywhere so I thought about asking what you think.

  • \$\begingroup\$ TO-220 legs are not designed to be clamped like that and any pressure out-of-plane could crack the case. Furthermore, they are relatively close together and all in the same plane. How much current are we talking about here? \$\endgroup\$
    – vir
    Sep 25 at 20:17
  • \$\begingroup\$ A soldered joint is best. Clamping or bolting could be used (note, not on TO-247, there's no tab, only a back side) but more importantly, what are you going to do about the supporting components (capacitors etc.)? Loop inductance can be quite a lot lower in a planar (PCB) design. \$\endgroup\$ Sep 25 at 20:21
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    \$\begingroup\$ SOT-227B case is made for such mount situations. \$\endgroup\$
    – Jens
    Sep 25 at 20:21
  • \$\begingroup\$ You are indeed correct. I know it is a weird application and there are much better packages to use so don't crucify me please. Technicaly I can snip off the drain pin (since drain is connected to the tab) and I would bend the gate pin upwards to interface with PCB containing the drivers. Then I am left with only a source pin. I think I can take care of the out-of-plane clamping issue so cracking shouldn't be a much of a problem. We are talking about paralleling multiple fets to get to about 2-300A? In my mind the clamp could be "better" than soldering because of the surface contact area. \$\endgroup\$
    – tomstr21
    Sep 25 at 20:25
  • \$\begingroup\$ For 300A there are devices in packages with screw terminals and are designed to be bolted to a heatsink. Paralleling transistors for 300A might be challenging in regards to ensuring each device stays within it’s SOA. \$\endgroup\$
    – Kartman
    Sep 27 at 13:16


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