Diamond Output buffer eats trannies (thermal)

[blu_line]

Hello all,

My diamond output buffer, see PDF, eats output transistors, being Q1,Q4,Q27 and Q30.

I know it has to do with thermal issues. So i tried the following :
Place Q1,Q4,Q27 and Q30 together on the heatsink.
And couple Q6 with Q2 and Q28 with a piece of aluminium.
And the same with Q8, Q3 and Q29.
According to the logical rules this should be a perfect temeparture compensation , cause when Q2 and Q28 heat (and also Q3 and Q29) the current through Q6 and Q8 reduces and so the BE voltage over Q2,Q28, Q3 and Q29 decreases. Fine you should say. Well, when powering-up this immediately pops the power trannies.

So i also tried ; place Q1,Q4,Q27, Q30, Q2, Q28, Q3 and Q29 together on the heatsink. This seems to work.
But i see the quiescent current rise slowly as the heatsink warms-up. So that is not correct either.

And next to that.....
I used series resistors (13 ohm) in the powerline to protect for over-current.
When i remove these i get fireworks also. (i either set-up)

Somebody got some ideas ?

grtz

Simon

[lumanauw]

Diamond buffer intended to have no gain, output=input.
Is the CFP (which usually prefers gain) suitable for this?

[blu_line]

Is the CFP (which usually prefers gain) suitable for this?

You mean the Skizlai pair ?
It should be highly usable, see Douglas Self's Book !

grtz

Simon

[peranders]

I think your main problem is the relationship 10/0.1 ohms in the emitter resistors, it's too much and you have a difficult way of temperature stabilization.

[blu_line]

@peranders
So i have to reduce it ?
say; none and 0.1 or ?

And which transistors to put together on the heatsink ?


grtz

Simon

[sajti]

Originally posted by blu_line
@peranders
So i have to reduce it ?
say; none and 0.1 or ?

And which transistors to put together on the heatsink ?


grtz

Simon

My recommendation:

Use 0.33ohm instead of 0.1ohm. I think, that it will solve Your problem, without any changes of the heatsink combination.
I think that Your current mirrors are over complicated, and the current depended by the supply voltage. I believe, that very simple current regulator do the job as well!
I think that You have to put all the BD139/140 on the same heatsink. This will give some thermal compensation. Or put all the power transistors to one big heatsink. Maybe it will work even with the 0.1ohm.


sajti

[ilimzn]

1) Q2, Q28, Q6, Q8, Q3, Q29 must be on the same heatsink not in contact with the main heatsink.
2) R2, 29, 48, 34 need to be higher, you only have <17.5mV under quiescent conditions which will not adequatly compensate 2mv/K from the transistors, and will not adequatly current share, as well as potentially be thermally unstable.
3) R9 and R10 set the quiescent current to ~~175mA per output pair (which seems high!) while the current sources only source 1.7mA, this is not enough for your output configuration.
4) The buffer has VERY close to Av=1, which means any feedback from output to input is an invitation to oscillation - this may be the probelm with your transistors instantly popping - given what you have written about thermal coupling of transistors that you tried, one would expect thermal runaway to be much slower. Id the input grounded when you try powering it up?
5) I see the reason for the current mirror current sources, as these can operate very close to Vcc, but you would probably do just fine with the common two transistor CCS. You may also want to divide R27 and R26 into two series resistors and put a LED or zener to the corresponding Vcc as otherwise you get bias current dependant on power supply.

[mtlin12]

Hi Simon,

I estimate the current from the Wilson current source is about
3.5mA, 3.5mA*10/0.1=350mA is not very large even to BD139.

And I think the first ( couple Q6 with Q2 and Q28 with a piece of
aluminium) is more reasonable than the second one, but if the
temperature of Q2.Q3 Q28.Q29 rise too quickly, the Vbe will
lower than the Q6.Q8, the quiescent current of Q1.Q4...will add
about 200mA per 10 Centigrade each. Because Q1's Vbe will lower
2mV per 1 Centigrade .

I guess this is the major reason why power transistor will blowout
quickly then the second one (all on the same "large"? heatsink) .

M.T.

p.s. u can see my sziklai-DB power stage PDF,
I use another method to provide the quiescent current =Vbe/2*Re .

[chalky]

There are a couple of things I don't understand about the schematic, but maybe I've missed something. Why are the resistor values different for the top and bottom sections of the circuit? Do your current sources give around 4mA? If this is so how do the output transistors get biased on?

[chalky]

Ignore daft question about output transistor biasing - I'm too used to seeing the standard diamond buffer arrangement.