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Old 17th November 2009, 10:23 PM   #1
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Default Bias stabilty tests.

I have done some research on a bias circuit in a quasi complimentary circuit with two output vertical MOSFET's.

The bias circuit works well so long as there is not too much bais current in which case thermal runaway occurrs.

With 20mA bias current it is very hard to get thermal runaway.
I tested the circuit using a household iron to heat up the heatsink to above 70 degrees C and had no problems.

However, with 90mA starting bias the bias current just kept on rising out of control.

I used a simple Vbe multiplier with no emitter degeneration glued to the heatsink.

Last edited by nigelwright7557; 17th November 2009 at 10:34 PM.
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Old 18th November 2009, 01:56 AM   #2
Bigun is offline Bigun  Canada
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sounds interesting and I probably won't have a clue as to the issue but a diagram would help me follow along and contribute if possible.

Hopefully you'll have this ironed out soon
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Old 18th November 2009, 08:35 AM   #3
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Quote:
Originally Posted by Bigun View Post
sounds interesting and I probably won't have a clue as to the issue but a diagram would help me follow along and contribute if possible.

Hopefully you'll have this ironed out soon
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Old 18th November 2009, 10:01 AM   #4
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The source resistor is in the wrong place on your lower mosfet, it should be between the output rail and point where your cfp transistor joins the mosfet. This will help thermal stability of the lower half.

What have you got mounted on your heatsink? Outputs and drivers and bias generator? Or just outputs and bias generator?
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Old 18th November 2009, 11:03 AM   #5
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The source resistor is in the wrong place on your lower mosfet, it should be between the output rail and point where your cfp transistor joins the mosfet. This will help thermal stability of the lower half.

What have you got mounted on your heatsink? Outputs and drivers and bias generator? Or just outputs and bias generator?
Just the bias and output transistors.

The driver transistors just have small bolt on heatsinks on them.
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Old 18th November 2009, 11:41 AM   #6
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OK good, that is the preferred setup as generally it stops the drivers getting as hot as the outputs.

I can only suggest trying the modification I described to see if it helps. The rest of your circuit seems well thought out.
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