# Transistor Balance Issue



## TigerNut (Dec 18, 2009)

http://www.ineltron.de/english/powerex-data/ks621k30.pdf

I found this datasheet scan. The transistor you used is a three-stage Darlington bipolar transistor, not a Fet or a Fetlington. Bipolar transistors have a thermal runaway issue where the current through a given device goes up as the temperature goes up. So whichever of your five transistors was dissipating the most power, would conduct more and more current, until it fried itself. You need to make sure that your drive circuit has feedback to control the current and that it also has a thermal feedback feature to prevent the runaway condition. The datasheet has several graphs that give the 'safe operating area' for the transistor, and you need to be sure that your individual transistors cannot under any condition go outside the SOA. 
When you have multiple transistors connected in parallel it's more difficult to guarantee this, because the individual tolerances of the feedback resistors will mean that the transistors don't all operate exactly the same. 5% tolerance parts are far too variable to guarantee an equal power share between the transistors.

A Darlington transistor is not especially suitable for high current applications because the transistor that's taking most of the current is kept in the active region - you can't drive it into saturation using the base connection alone. On the transistor you showed, they have the extra BX connection to the main transistor's base, which you could use to drive the main transistor into saturation and thereby reduce it's Vce drop and associated power dissipation, probably by a factor of 2.


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## Snakub (Sep 8, 2008)

Could you recommend a temperature sensor setup? Could I somehow measure current off of the shunt? I am using an arduino micro controller.


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## ngrimm (Oct 19, 2007)

Seems like you could start by sticking a thermistor on each of the transistor cases and compare the temperature of each with your ohmmeter while driving. Or maybe a hand held infrared temperature gun like they have at Harbor freight. Even if it isn't exact, it should work to make comparisons. Then you could select the gate drive resistors that keep the temperature the same. Maybe I am over simplifying though.


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## TigerNut (Dec 18, 2009)

The critical parameter that has to be measured is the junction temperature. You can estimate that from the temperature at the heatsink, and knowing the thermal transfer coefficients, but your temperature sensor has to be thermally coupled as close as possible to the actual junction. 

Doing digital feedback on a thermal control loop is pretty tricky (other than just doing a high-temperature shutdown), and people have been designing analog feedback systems for transistor drives for decades, so I'd say you would be best off researching that angle before trying to do a digital loop on the arduino. Check into high power audio drivers... they have to deal with complex-impedance loads as well as power sharing between drivers. There's a lot of design information using bipolars here:
http://sound.westhost.com/projects.htm


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## ga2500ev (Apr 20, 2008)

Would the OP simply be better off with a better power section? As I pointed out in that thread, Darlingtons are not a good choice. They draw too much power at load and have runaway thermal effects as you guys have been discussing.

Snakub, go take a read of M. Paul Holmes open source controller thread here:

http://ecomodder.com/forum/showthread.php/paul-sabrinas-cheap-diy-144v-motor-controller-6404.html

Even if you just build the power section with the MOSFETs, diodes, and caps, you'll be in a better position to get a working controller. Also be sure to check any jackbauer posts in there about using IGBTs as the power element. In either case, you should get a better result.

ga2500ev


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