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Old 19th July 2007, 07:44 PM   #1
Don S is offline Don S  United States
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Default Mosfet Frontend Troubles

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
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Old 19th July 2007, 08:56 PM   #2
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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.

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Old 19th July 2007, 09:06 PM   #3
Don S is offline Don S  United States
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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.
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Old 19th July 2007, 09:15 PM   #4
Don S is offline Don S  United States
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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
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Old 19th July 2007, 09:26 PM   #5
Zen Mod is offline Zen Mod  Serbia
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Default Re: Mosfet Frontend Troubles

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)
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Old 19th July 2007, 09:27 PM   #6
Don S is offline Don S  United States
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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
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Old 19th July 2007, 09:32 PM   #7
Don S is offline Don S  United States
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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
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Old 19th July 2007, 10:47 PM   #8
Don S is offline Don S  United States
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R14 is now open, and Q4 appears to be shorted.

I think Q6 was full on!

DonS
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Old 19th July 2007, 11:06 PM   #9
Zen Mod is offline Zen Mod  Serbia
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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
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Old 19th July 2007, 11:24 PM   #10
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Very cool front end - it looks a lot like the A75 front end, only SE.

JJ
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