Sir, I am making DC regulated variable power supplies various ratings. wherein I use Transistor as series pass . Depending upon the current I add number of transister in parellel. Mostly I use 2N 3055, 2N 3773 and BU 508 for the unit. Since the transistor are not reliable during short circuit test. Can u sujest circuit using MOSFET instead of Transistors. My units are short circuit protected with variable Current limit setting. I use dual op amp MC 1458 for current and voltage controls, with TL 431 as 2.5V internal reference. A line in reply will be highly appreciated. Regds. Devaraj. Real Power Vision, Bangalore

 

1) Please do not "hijack" other peoples threads.

2) BJT's in parallel don't work because they have draw more and more current as they get warmer. They get warmer as they draw more and more current. Whichever BJT happens to draw the most current first burns out first. MOSFETS draw less and less current as they get warmer.

MOSFETS don't do very well in the linear region, though. You might look into SMPS motifs.

 

do you have emitter resistors to equalise the current through each transistor and what sort of voltages and currents are we talking about ?

 

It should be possible to run lots of transistors in parallel. I have seen a laser supply with 32 2N3055's in parallel.

 

It should be possible to run lots of transistors in parallel. I have seen a laser supply with 32 2N3055's in parallel.

Indeed, if you arrange a way of balancing the current through each one, since 2N3055 differ slightly from each other.

 

You can use resistors to limit the current for transistors in parallel to balance them - and you can use a length of wire, as well. It might be less expensive and lighter in weight to use a foot or two of 18 gauge (or suitable) wire to act as a resistor. You'll have to model it.

Take a look at National Semiconductor's datasheet for the LM117/LM317 voltage regulator; there are examples of paralleling transistors to increase the regulator's output. You could also add a comparator or op amp to monitor the voltage across the output resistors. The higher the voltage across the resistors, the more current is being output. You could have the output of the comparator/op amp charge an LC circuit which would turn the pass transistors off, giving you a "crowbar" (overcurrent protection)