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Shunt regulator PSRR dropout?
Shunt regulator PSRR dropout?
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Old 3rd January 2019, 11:41 PM   #1
JeffYoung is offline JeffYoung  Ireland
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Shunt regulator PSRR dropout?
Default Shunt regulator PSRR dropout?

I'm looking at a 49V shunt regulator that I've adapted from a design by Pete Millet:

beefcake shunt.jpg


When I run an AC analysis on it, I get an ugly spike at 280kHz where I lose almost 20dB of attenuation:

PSRR dropout.jpg


I can mostly smooth this out by using an ultra-low-impedance electrolytic capacitor for C8. Here's what it looks like with a 0.038 ohm ESR cap:

PSRR attenuated.jpg

But that's an awfully specialized cap and it doesn't seem like this circuit should be that finicky. Is there a better way to handle this?

Thanks,
Jeff.
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Old 3rd January 2019, 11:56 PM   #2
SemperFi is offline SemperFi  Wake Island
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At that frequency an electrolytic isnt much good. >100kHz ltspice cant sim resonances correctly unless u put in all parasitics. Which u cannot know before build. So u cant take it too seriously.
More than 50dB atteuation at that frequency is pretty good.
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Old 4th January 2019, 10:56 AM   #3
DF96 is offline DF96  England
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Yes. Regulators use feedback loops. Feedback loops have potential HF instabilities, the details of which depend on details of component and layout parasitic capacitances.
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Old 4th January 2019, 11:04 AM   #4
jan.didden is offline jan.didden  Europe
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Play with gate stopper value, and put small zobels across the collector Rs.
As DF said, you need to shape the loop gain.
A C across R8/R16 will increase LF PSRR but you may not like that curve visually ;-)

Jan
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Last edited by jan.didden; 4th January 2019 at 11:06 AM.
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Old 4th January 2019, 08:13 PM   #5
JeffYoung is offline JeffYoung  Ireland
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Shunt regulator PSRR dropout?
I can damp the -52dB spike with either a really big stopper (12K) or a Zobel over the collector resistor. Neither helps the -60dB hump following the spike. Both move the spike to a lower frequency.

Increasing the current through the TL431 moves the spike to a higher frequency.

Dropping the TL431 current to 2.5mA (TI's minimum spec is 1mA) and using a 680ohm gate stopper moves the spike low enough that a more "normal" electrolytic can knock it to -64dB.

Is this worth doing, or is this more likely just SPICE playing with my head?

Thanks,
Jeff.
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Old 4th January 2019, 11:51 PM   #6
JeffYoung is offline JeffYoung  Ireland
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Shunt regulator PSRR dropout?
Here we have:

blue: original
red: stonkin' big 12K gate stopper & higher TL431 current
cyan: original with ultra-low impedance electrolytic (Nichicon UHW)
green: stonkin' big 12K gate stopper with ultra-low impedance electrolytic

Shunt regulator PSRR dropout?-regplot-jpg

Are there any issues with such a big gate stopper? 'Cause the green curve sure looks nice....
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File Type: jpg regplot.jpg (50.5 KB, 187 views)
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Old 5th January 2019, 12:53 AM   #7
JeffYoung is offline JeffYoung  Ireland
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Shunt regulator PSRR dropout?
I can get basically the same curve by replacing the gate stoppers with Zobels between the gate and drain of the MOSFETs.

Shunt regulator PSRR dropout?-beefcake-shunt-zobel-jpg
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File Type: jpg beefcake shunt zobel.jpg (30.1 KB, 177 views)
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Old 5th January 2019, 11:20 AM   #8
DF96 is offline DF96  England
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Spice can help you see the effect of changing or adding components. What it won't do, unless you tell it, is model the effects of the parasitics of the actual real circuit which you may one day build. At present what you are mainly seeing is the component parasitics included in the Spice models, which will vary a bit from sample to sample anyway.
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Old 5th January 2019, 11:34 AM   #9
JeffYoung is offline JeffYoung  Ireland
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Shunt regulator PSRR dropout?
@DF96, so will the Zobel on average give better results, or are we so far into the weeds that I should just stick with a "normal" gate stopper?
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Old 5th January 2019, 11:57 AM   #10
jan.didden is offline jan.didden  Europe
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Assuming the parasitics of the devices are reasonably modelled, one parasitic to look out for is the C across R27. Try with say 3pF across R27, see what that does to the original, then try again the other things.

The zobel from gate to drain should have exactly the same effect as the same zobel across the collector R.

Jan
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