keith@ux.acs.umn.edu (Keith MaloneyHuss) (03/17/90)
I have a problem with a power supply that I recently built. It drives a laser (5 mW) and is a high voltage (2-10 kV) DC supply. Basically it uses a 555 that drives a power transistor switching 12 VDC into the primary of a transformer that steps this up to about 300 VAC which through a diode/capacitor ladder gived the required high voltage DC. My problem is that I'm blowing out transistors. I have a diode across the transistor to save it from the transformer's backlash (for lack of a better word). Is it better to put the diode across the transformer primary, or across the transistor emitter/collector junction? Or does it matter? The problem seems to be overheating. I am using an NTE248 PNP transistor that I believe is rated at about 100 watts or so. It is in a TO-3 can and is well heat sinked to a chassis with heat transfer goo, but it still overheats. If you haven't guessed already, I'm a beginner at this. The laser tube draws 6.5 mA at 2000 VDC if that is relevant. Would a power MOSFET be a better choice for this application? Or do I need a more robust Darlington? I would appreciate any advice. Thanks. --keith