Parallel Transistors?

[wjsmarine]

Hi George, members.

Dusting off the cobwebs I seem to recall the emitter resistors oppose hfe as emitter current increases due the higher volt drop taking place - this is their purpose and allow each parallel device to balance.

Given we are talking .6v turn on you can see that not much voltage is required to throttle back the transistor drive, so scale your emitter resistors accordingly e.g. if 10A flowing then only 0.06 ohm will give you 0.6v but still dissipate 6W of heat.

Also don't forget those transistors will be cooking too with Vce*Ice Watts each and are subject to thermal runaway if not controlled. Mosfets have a positive temperature coefficient and therefore don't runaway, instead shut down as they get hotter. I hope this helps.

Regards,
Bill

[wjsmarine]

Oops! Wrong information.
Like all semiconductors Mosfets also have a negative temperature coefficient NOT positive like I mentioned previously. But they still tend to shut down as temperature rises and therefore current share better than bipolar transistors.
Cheers,
Bill

[George]

But still - are FETs as easy to control in a current regulation situation? Also - are there many that can dissipate over 100W of power? I've never used a FET before to limit current and am not really sure how easy it would be to get a good steady current regulation from one.

I was thinking - what if I use a length of copper wire say 0.1 Ohm from each of the emmiters, the copper would naturally increase it's resistance as it gets hot and let less power through the transistor - so then it would have a semi dynamic leveling system rather than just a static resistor?

[wjsmarine]

Hi George, members.

Mosfets are capable of big power, no problem, obviously heat sinking is a factor. I would be careful using pcb tracks as a resistor where you are relying on heat as an aid in resistance changing - there is a fine line (no pun intended) between getting hot enough and a fuse, but if you decide to do this see Roman Black's site
http://www.romanblack.com/pcbtemp.htm
for a utility to allow calculating track sizes.

I have had good success with an arrangement that controls Source current by robbing Gate drive. Use a Source resistor (N channel device) to ground and place the b-e junction of an NPN bipolar across it. The collector goes to Gate with a 1k. This will rapidly switch so the average Source current is controlled by the Source resistor - value determined by 0.65/desired current e.g. 0.065 ohms will give 10amps average.

If this meets your needs I would start small and scale up accordingly.

I hope this helps.

Regards,
Bill