> I've got a cordless drill motor (~2A) that I need a high side
> driver for. At this time of day, unable to run out and buy a P-ch
> FET, I've tried some PNP transistors (eg TIP42, TIP120).
> Base drive is PWM from a 555, driving the transistor directly
> or through an NPN switcher on the PNP base. So far the
> results have not been encouraging. The waveform looks terrible
> (not the nice square wave with an N-ch FET low side I tried),
> the transistors get very hot and the motor's got no grunt

You can use an N Channel FET if (and only if) you provide a gate drive 
voltage for it that is suitably above the high side rail. This may be able 
to be done with the usual capacitor pump type circuit but it may cause 
problems here if there is no low side driver to provide a cleanish drive 
square wave. You may need to make a separate drive inverter (capacitive 
would do). For testing the easy solution is to use a 9 volt battery for the 
gate drive. With a little care you should get hundreds of hours from a  std 
9v "transistor battery" (PP3 or whatever). A slightly complexified Joule 
thief type inverter or similar one transistor self oscillating type would 
also work. The input PWM drive  may provide all you need for a capacitor 
pump or this could drive a transistor with inductor in its collector to 
"ring" to the required voltage.


        RM

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