Mosfet Frontend Troubles

Don S · 19th July 2007, 07:44 PM

Here is my Mosfet frontend. I am having trouble in testing. Q3 and Q4 get very hot! I measured 2.0 volts across R5 which means it is passing about 10mA. What is happening? Do i need to load the drives, and or tie the Fb in? I think it is possibly oscillating.

Please help with this!

DonS

Nelson Pass · 19th July 2007, 08:56 PM

Check the voltage across R16. If each LED is 4 volts,
then you will have something around 8 volts across
R16, meaning that about 80 mA is passing through the
output devices, and at 75V rails, that's about 6 watts
per device, which is quite a bit.

Don S · 19th July 2007, 09:06 PM

Each led is 1.8v X 3 = 5.4v

Voltage across R16 is 2.02v

Current should be about 20mA

If the full rail is on the collector of Q4 max power should be 3 watts.

Voltage across R16 continues to climb as it warms up.

Don S · 19th July 2007, 09:15 PM

This may help. When R5 was 560 ohms there was no voltage across R16. I increased current in the diff pair to help turn Q6 on.

DonS

Zen Mod · 19th July 2007, 09:26 PM

Quote:

Originally posted by Don S
Here is my Mosfet frontend. I am having trouble in testing. Q3 and Q4 get very hot! I measured 2.0 volts across R5 which means it is passing about 10mA. What is happening? Do i need to load the drives, and or tie the Fb in? I think it is possibly oscillating.

Please help with this!

DonS

use just one led string for both Q3 and Q4

also - you can reference leds to +Ub (recalculated R6 or R14 to +75V, not gnd)

Don S · 19th July 2007, 09:27 PM

Sorry for these short replies. I have to allow the components to cool down between tests.

Drive + is 61 volts

Drive - is 58 volts

I hope this helps.

DonS

Don S · 19th July 2007, 09:32 PM

Zen Mod are you saying that is the problem. I kept them independant so they would have a wide range of separate options for each current source.

DonS

Don S · 19th July 2007, 10:47 PM

R14 is now open, and Q4 appears to be shorted.

I think Q6 was full on!

DonS

Zen Mod · 19th July 2007, 11:06 PM

Quote:

Originally posted by Don S
R14 is now open, and Q4 appears to be shorted.

I think Q6 was full on!

DonS

first- make Iq through Q4 fixed , for start
then (besides already mentioned idea to share led string for both CCS biases) recheck value of R1 ( you need ~ 4V Ugs for open throttle of Q6......but you don't need much more) so - make R5 from one fixed resistor and trimpot in parallel- so you can tweak " openess" of Q6.

tweak that value looking at other DC voltages in circ

DC potentials of drive points are nothing more than result of transposed DC characteristics of Q5 thingie (some called it Vbe multiplier probably,but I'm dumb for that ) .

btw- sharing bias net for two CCSes on same side is benefitial from more than one reason, but I'll mention just one- less temp-co induced differences

jupiterjune · 19th July 2007, 11:24 PM

Very cool front end - it looks a lot like the A75 front end, only SE.

JJ

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19th July 2007, 08:56 PM	#2
Nelson Pass The one and only Join Date: Mar 2001	Check the voltage across R16. If each LED is 4 volts, then you will have something around 8 volts across R16, meaning that about 80 mA is passing through the output devices, and at 75V rails, that's about 6 watts per device, which is quite a bit.

19th July 2007, 09:06 PM	#3
Don S diyAudio Member Join Date: Nov 2006	Each led is 1.8v X 3 = 5.4v Voltage across R16 is 2.02v Current should be about 20mA If the full rail is on the collector of Q4 max power should be 3 watts. Voltage across R16 continues to climb as it warms up.

19th July 2007, 09:15 PM	#4
Don S diyAudio Member Join Date: Nov 2006	This may help. When R5 was 560 ohms there was no voltage across R16. I increased current in the diff pair to help turn Q6 on. DonS

19th July 2007, 09:27 PM	#6
Don S diyAudio Member Join Date: Nov 2006	Sorry for these short replies. I have to allow the components to cool down between tests. Drive + is 61 volts Drive - is 58 volts I hope this helps. DonS

19th July 2007, 09:32 PM	#7
Don S diyAudio Member Join Date: Nov 2006	Zen Mod are you saying that is the problem. I kept them independant so they would have a wide range of separate options for each current source. DonS

19th July 2007, 10:47 PM	#8
Don S diyAudio Member Join Date: Nov 2006	R14 is now open, and Q4 appears to be shorted. I think Q6 was full on! DonS

19th July 2007, 11:24 PM	#10
jupiterjune diyAudio Member Join Date: Nov 2006	Very cool front end - it looks a lot like the A75 front end, only SE. JJ