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current sharing on psu pass transistors
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Kleinstein:
A little over 3 A give quite a lot of power per transistor, especially if at worst case voltage (e.g. 35 V). The 0.2 Ohms are about sufficient for current sharing, but there are still tolerances so that one of the transistors may see some 4 A.

It may still be acceptable if good transformer tap switching is used. Otherwise I would suggest reducing the current per transistor to some 2 A.

Compared to 0.2 Ohms the wiring can also add some resistance and thus effect the current sharing if in a poor way.
duak:
Heatsinks tend to have a temperature gradient from the bottom to the top where the temperature is highest.  Are the devices arranged horizontally or vertically?

Are you using a nylon shoulder washer to insulate the mounting screw from the collector tab?  If so, it should be replaced with a glass-filled nylon shoulder washer or better.  Simple Nylon is not up to the task because it creeps at temperature and you lose clamping force leading to thermal runaway.  A better way to clamp the devices is with a springy bracket pressing down on the case over the die.

Please provide a picture of the heatsink.  If you can't, then a description will help.

If all else fails, It should be possible to reduce the temperature of an individiual device by putting some resistance in series with its base connection.  This is easier than fiddling with the value of its emitter resistor.
m3vuv:
well the transistor that was getting hot went sc e/c,ive removed it,the voltage  accross the balast resisters are readings are  tr1/173mv tr2/175.9mv tr3/178.mv tr4/158mv tr5/180.6mv,the heatsink is black anodised aluminium and measures 150mm x 150mm x 30 deep,all transistors are mounted on silpads with dow corning 340 thermal compound and bur free etc.i assume it was a faulty transistor.
dom0:

--- Quote from: Kleinstein on September 28, 2019, 04:04:06 pm ---Compared to 0.2 Ohms the wiring can also add some resistance and thus effect the current sharing if in a poor way.

--- End quote ---

That's why for many transistors in parallel the C and E connections should be mirrored, i.e.

Kleinstein:

--- Quote from: dom0 on September 29, 2019, 12:23:34 pm ---
--- Quote from: Kleinstein on September 28, 2019, 04:04:06 pm ---Compared to 0.2 Ohms the wiring can also add some resistance and thus effect the current sharing if in a poor way.

--- End quote ---

That's why for many transistors in parallel the C and E connections should be mirrored, i.e.

(Attachment Link)

--- End quote ---
That type of connection is good for switching, but not for analog control. In analog control the collector resistance has very little effect, but emitter side wiring drop would add up.  It should be more like the emitter resistors meeting at one point.
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