TL431 High Voltage Regulator - How to get it thermally stable? - diyAudio
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Old 14th March 2014, 02:49 AM   #1
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Default 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.

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Old 14th March 2014, 03:34 AM   #2
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Jazz, your image may need reloading or ?
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Old 14th March 2014, 04:07 AM   #3
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Default image reloaded

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Originally Posted by trobbins View Post
Jazz, your image may need reloading or ?
Thanks trobbins. Somehow the image does not show up. I thought it was waiting for audit before publish

Here is the reload.

TL431+Mosfet-40v.JPG
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Old 14th March 2014, 04:27 AM   #4
Art M is offline Art M  United States
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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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Old 14th March 2014, 06:35 AM   #5
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Quote:
Originally Posted by Art M View Post
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,

Thanks a lot for your reply.

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?
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Old 14th March 2014, 06:54 AM   #6
wayne is offline wayne  United States
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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.
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Old 14th March 2014, 07:34 AM   #7
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Quote:
Originally Posted by wayne View Post
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?
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Old 14th March 2014, 07:36 AM   #8
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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.
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Old 14th March 2014, 07:40 AM   #9
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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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Old 14th March 2014, 07:57 AM   #10
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Quote:
Originally Posted by Hearinspace View Post
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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