Excell to the rescue!
My basic basic protection circuit is shown above. It is not particularly sexy, and many such circuits, or variations of it, are used in power amps. A nice article by Michael Kiwanuka describing the tradeoffs appeared in Electronics World in 2002.
If you remember the SOA on the previous page, you know there are a few
kinks in the curves. Our protection circuit should follow that curve
because we want to wring as much power out of the device as possible
without destroying it. Therefore, we will have similar kinks in the
protection circuit.
Stop Press 1:
I have changed the protection curve slightly to have a bit more power in complex loads. Referring to Part 3 in the May Elektor issue, the following component values are now recommended:
R6,R13: 10k
R7, R11: 2.2k
R8, R10: 1.5k
D3, D4: zener 15V 0.5W
The diagram at the left shows the top half of a power amp output
stage.
Qout are the output transistors, Q1 is the protection transistor. Ic
causes a voltage to develop across the power transistor emitter
resistors Re. If that voltage becomes high enough (high enough
current), Q1 will start to conduct and activate the protection.
Through R6, R5, R14 etc, there is an additional input from Vce, so the actual protection point becomes a function of both Ic and Vce, as was explained before. That Vce-link has two kinks introduced by zener diodes D2 and D3: above a certain Vce they start to conduct, and further rising of Vce above the kinks flatten the Vce influence on the protection point.
The complete (involved) detailed calculation is given in the Elektor article. However, there is an MS XL spreadsheet that makes it much easier to 'play' with values. But note that although I did quite some testing, I'm not guaranteeing that is is faultless! Anyway, feedback appreciated!
Stop Press 2: Since the paX protection system was published in the May 2008, Elektor issue, some persons reported that the protection system may fire prematurely. René van Heijningen has done some tests for me and the cause was leakage on the detector side of the opto-coupler U1, an MCT6.
The following change has been tested (thank you René!) and takes care of the issue:
Change resistor R1 from 10k to 18k;
Add a resistor R1a of 3.3k from ground to pin 5 or 8 (this is point ProtN in fig 2) of the optocoupler. You can put this resistor on the bottom of the circuit board.
