Chris wrote:
Motorola MPS-U51
I look up the spec in the mps-u51 and see it is a to-220 like case and
can handle 1W.
The Motorola "Uniwatt" case was not really comfortable dissipating
one watt even in 25 degree free air (the book spec). I used them and
their NPN counterpart, the MPS-U01 in a number of designs in the
early '70s because they were the fastest medium-power transistors
available. We always used them with board-mounted heatsinks (unlike
the TO-220 case, which uses a lead frame and has sturdy leads, the
Uniwatt has flimsy wire leads similar to a TO-5/TO-39 package).
Any switching transistor that will handle 1-2A and has an fT in the
50 MHz range or better should work fine. Note the EBC
pinout. Today, I might replace it with a TO-237 device like the
2N6726/27/28/29/30 from Central Semi (same free-air power rating as
U51, same pinout). But note, same optimistic free-air power spec --
I'd use a clip-on heatsink if I couldn't leave my finger on it indefinitely.
<http://www.mouser.com/ProductDetail/Central-Semiconductor/2N6730/?qs=sGAEpiMZZMutXGli8Ay4kEe1J5vCvqdNDTChj11qzcA%3d>
These seem to be EOL at this time, but still
available. ZTX953/951/753/751 (Diodes, Inc.) and KSA928A (Fairchild)
are TO-92 devices that also claim high dissipation. Again, use a heatsink.
Best regards,
Charles
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