TL431 High Voltage Regulator - How to get it thermally stable? - diyAudio
 TL431 High Voltage Regulator - How to get it thermally stable?
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 14th March 2014, 02:49 AM #1 diyAudio Member   Join Date: Jun 2011 TL431 High Voltage Regulator - How to get it thermally stable? Hi, The idea of this circuit is to design a power regulator that outputs adjustable regulated voltage between 40 and 50 volt. However I seem to unable to get the output thermally stable. The voltage drifts from 42volt to 38 volt in about 10 minutes. Can anybody help me to improve this design? Thanks in advance.
 14th March 2014, 03:34 AM #2 diyAudio Member     Join Date: Feb 2009 Location: Melbourne, Oz Jazz, your image may need reloading or ?
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Quote:
Thanks trobbins. Somehow the image does not show up. I thought it was waiting for audit before publish

TL431+Mosfet-40v.JPG

 14th March 2014, 04:27 AM #4 diyAudio Member   Join Date: Oct 2010 Location: SF Bay Area There is no source for the Q1 base current. You need to supply a positive voltage to the TL431 cathode/ 2n5551 base via a resistor from a positive voltage source.
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Quote:
 Originally Posted by Art M There is no source for the Q1 base current. You need to supply a positive voltage to the TL431 cathode/ 2n5551 base via a resistor from a positive voltage source.

Hi Art,

Spice simulation shows there is a 7.8uA current through the Q1 base. If I take the 10K R5 out, the current(about 9.2uA) from TL431 is completely fed into Q1 base. Is this enough to drive the Q1 into amplification?

How to calculate the value for the resistor you mentioned?

 14th March 2014, 06:54 AM #6 In the hills diyAudio Member   Join Date: Jan 2002 Location: Northern California As Art said the top of the TL431 is missing a connection. Just tie it to the ungrounded lead to set for 2.5 V. The data sheet has temp comp schemes but get it regulating first. __________________ wizards assistant
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Join Date: Jun 2011
Quote:
 Originally Posted by wayne As Art said the top of the TL431 is missing a connection. Just tie it to the ungrounded lead to set for 2.5 V. The data sheet has temp comp schemes but get it regulating first.
Hi Wayne,

But the Q1 base to ground is 0.7v when it is conducting. How to make the TL431 regulating?

 14th March 2014, 07:36 AM #8 diyAudio Member   Join Date: Dec 2010 Q1 and R5 are useless and incorrect. Both, Q5 and U1 are inverting amplifying devices. The result will be an oscillator. Connect TL431 cathode directly to the gate of IRFP240 or insert a thermally stable Zener diode between them (level shifte, for higher output voltage). Last edited by franzm; 14th March 2014 at 07:46 AM.
 14th March 2014, 07:40 AM #9 diyAudio Member   Join Date: Jun 2008 Looking only at the 431, the minimum current for regulation on the 431 itself is 0.4mA. If you have the capability it's good to give it some margin plus Q1's base current. If you can spare the current then run something like 5 - 10mA through it. In your sim, try a ~7K resistor from the cathode of the 431 to the output voltage rail. Take out the 10K resistor. Your 33K/1.5K divider on the 431 ref pin is now setting the output V+ at 57.5V which is higher than the input voltage so you'll either have to raise the supply voltage or lower the value of the resistor in the 33K position to get a lower output voltage.
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Join Date: Jun 2011
Quote:
 Originally Posted by Hearinspace Looking only at the 431, the minimum current for regulation on the 431 itself is 0.4mA. If you have the capability it's good to give it some margin plus Q1's base current. If you can spare the current then run something like 5 - 10mA through it. In your sim, try a ~7K resistor from the cathode of the 431 to the output voltage rail. Take out the 10K resistor. Your 33K/1.5K divider on the 431 ref pin is now setting the output V+ at 57.5V which is higher than the input voltage so you'll either have to raise the supply voltage or lower the value of the resistor in the 33K position to get a lower output voltage.
Do you mean something like this ?TL431+Mosfet.JPG

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