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Old 10th October 2007, 07:00 AM   #1
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Default Problem with a solid state CCS

Hi all,

I'm building a SE amp; the input stage is made with a 6N6P valve, loaded with a SS CCS at 20mA.
The CCS schematic is THIS ; it uses a TL431 shunt regulator to regulate the current.
It works fine, during the test on the bench the current was stable and perfectly regulated; once connected to the high voltage power supply (400V) it begin to oscillate, I can see a sine wave of about 1V superimposed to the output voltage at the mosfet source (measured with respect to the comm level).
The sine has a frequency of about 13 Mhz!
It seems the TL431 oscillate when I increase the power supply over, let's say, 30V, trying to keep the current stable.
How can I solve the problem?
Any idea?
Thanks in advance.


Ciao,
Giovanni
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Old 10th October 2007, 12:31 PM   #2
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You could try a 'Gate-Stopper' of around 100 K in series with the gate of the Mosfet....

Mount as close to the Mosfet as possible....

Worth a try....
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Old 10th October 2007, 01:02 PM   #3
ilimzn is offline ilimzn  Croatia
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100 K?? You sure it's not supposed to be 100 ohm?
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Old 10th October 2007, 01:38 PM   #4
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Already tried.
A 10K "gate stopper" resistor installed there create a 5V, 310Khz perfect sine output wave...
I think I will change CCS topology.
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Old 10th October 2007, 03:14 PM   #5
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Quote:
Originally posted by ilimzn
100 K?? You sure it's not supposed to be 100 ohm?
Absolutely!--

Its what Ive always used to good effect!

croccodillo--

Just try a 100K before re-design...Might save you the trouble....
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Old 10th October 2007, 03:41 PM   #6
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Just a shot in the dark here without the circuit to bend test, but you might try some additional feedback in parallel with the 10k resistor as well. Off the top of my head, you might try a 2.2k resistor in series with 270pF across that R11 (10k). There are a few other things i can think of to stabilize it, but one at a time i suppose.
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