[quote author=Chris Magagna link=topic=76363.msg577124#msg577124 date=1319331164]
Can you explain this a bit? A source current on the base of 20mA and 1.28A is a gain of 64.
If I search on Digikey for NPN transistors where Ic >= 1.5A and gain is at least 70, I get pages of results (for example http://search.digikey.com/us/en/products/MPS651RLRAG/MPS651RLRAGOSCT-ND/1139913 ). Why wouldn't something like this work?
I'm still trying to figure transistors out so any advice is greatly appreciated.
When you're using a transistor as a switch and you want to get a low voltage drop across the transistor when it is turned on, you need to drive it with somewhat more base current than its hfe would indicate. The voltage drop of a transistor when it is driven hard is called its Vce(sat). For that particular transistor, Vce(sat) is quoted as 0.5V at 2A collector current and 200mA base current (i.e. 10:1 not 75:1). When you drive it with 20mA base current and 1.28A collector current, the transistor will not be fully saturated and its voltage drop will be somewhat higher. Not only might it be too high for your application, but the power dissipated by the transistor in this condition is Vce * Ic plus a smaller contribution from the base current. The maximum power dissipation for that transistor is 625mW, so you need to keep Vce down to about 0.45v when it is passing 1.28A to avoid overheating the transistor.
As a rule of thumb, I use single transistors to switch up to 500mA and mosfets above that.