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TL431 shortcircuit and AC unity gain capacitor

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geotFM

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Hello.I am built this shunt PSU but i shortcircuited the output and TL431 failed.I think the failure has to do with the AC unity gain capacitor C1=220uF.

Is a 220uF a normal value or shoud i use something lower around 22uF?

Does this capacitor discharges through reference pin of TL431 in case of an output shortcircuit and destroying TL431 ?
If yes ,how can i protect TL431 from being destroyed?
 

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The problem may be occurring due to the fact that in short-circuiting the output, the voltage supplying the reference of the shunt regulator accross resitor divider, is also changed.
 

Hi,

Have you looked at the capacitance stability chart in the datasheet or the supplementary app note SLVA482 Understanding Stability Boundary Conditions Charts in TL431, TL432 Data Sheet?

Provisionally, there's no reason why a 220uF would be an issue, it's in the "safe area" (outside oscillatory areas) but it does seem rather high to me.

Would it be better to have the large value capacitor going from Q2 collector to ground/before the TL431 feedback and use a smaller cap across the divider top resistor?
 

Right, the problem is that by shorting the output the 220uF on the reference drives the reference negative in a big way.

No, 220u isn't normal. Generally I avoid anything above 1u unless it's an actual power application and this issue is one reason it's nice to do that.


First, in general when you're making RC's you can chose the R to keep the C in a 'normal' range. With 10k/1k you have room to go up in R before having problems (>100k or 1Meg typically)

But second I think your AC feedback frequency choice is also off the charts here. I'd change to 220n, or 22n and I expect it will still work.
 
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    d123

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Yes, it's likely the large negative voltage generated at the TL431 control input zapped the device when the output was shorted.
I see no good reason for C1 so just eliminate it.
Unity gain is actually worse for stability in the feedback loop than a higher gain.
You already have C2 to handle any output transient current changes.

If you still want a cap, you could connect a Schottky diode from the control input to ground (anode to ground) to prevent the input from going significantly negative.
 
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    d123

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So adding the protection diode as a discharging path solves the issue...Why you suggest a shottky diode instead of a rectifying one like 1n4004?Will a 1N5819 shottky diode do the job ?
 

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10 uF is still an unrealistic large compensation capacitor. Why not design first reasonable compensation first and then the protection? I expect that lag-lead compensation (capacitor with series resistance) will give good phase margin and also protect TL431.
 

I have seen some applications on the net that use such a cap to get even lower noise results.Some also use a 10uF cap with a 1K series resistor as you mention.Maybe i wont use this cap at all.
 

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