Ringing/Oscillation in amplified zener sim - diyAudio
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Old 9th March 2012, 06:48 PM   #1
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Default Ringing/Oscillation in amplified zener sim

So I'm using an amplified zener on the low side of a full bridge rectifier to drop ~200V off a HV supply.

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The output voltage looks good, and is coming right where I want it.

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The issue is (or maybe isn't) ringing at the output of the bridge caused by the amplified zener. Here is what I'm seeing at the output of the bridge:

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Zoom -

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Zoom to single cycle -

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Thoughts?

Is the ringing real, or a just a simulation/model artifact?

If it's real - thoughts on getting rid of it?
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Old 9th March 2012, 07:22 PM   #2
Elvee is offline Elvee  Belgium
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First thing to do is to test your pseudo zener in isolation: when it's hooked up in a circuit with AC sources, rectifier bridges and various components like inductors, it's difficult to know exactly what's going on.
With simulation, it's the easiest thing of the world to do, so you have no excuse.

Then my prime suspect would be L1: I have a suspicion it isn't damped, and in sim that would mean a very high Q.
Even in reality, such an arrangement could create funny effects.

First make an AC analysis on your zener, using the actual static conditions: this will show any possible quirk in the impedance curve
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Old 10th March 2012, 06:12 AM   #3
RJM1 is offline RJM1  United States
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If you add a 1µF capacitor from D3, D4 cathode to ground, this should stop your ringing.

You could also add a resistor in series with the 1µF capacitor of somewhere about 33 to 100 ohms to reduce the turn on surge and still eliminate the ringing.

You can simulate your circuit without your amplified zener and get the same ringing.

Last edited by RJM1; 10th March 2012 at 06:27 AM.
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Old 10th March 2012, 09:24 AM   #4
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Quote:
Originally Posted by Elvee View Post
First thing to do is to test your pseudo zener in isolation: when it's hooked up in a circuit with AC sources, rectifier bridges and various components like inductors, it's difficult to know exactly what's going on.
With simulation, it's the easiest thing of the world to do, so you have no excuse.

Then my prime suspect would be L1: I have a suspicion it isn't damped, and in sim that would mean a very high Q.
Even in reality, such an arrangement could create funny effects.

First make an AC analysis on your zener, using the actual static conditions: this will show any possible quirk in the impedance curve
I agree. There's an undamped L-C that would ring.
Why are R1, R2, R3 so large? What is this circuit for?

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Old 10th March 2012, 10:52 AM   #5
RJM1 is offline RJM1  United States
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Tube plate supply my guess with the 6H inductor
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Old 10th March 2012, 01:40 PM   #6
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Originally Posted by janneman View Post
I agree. There's an undamped L-C that would ring.
Yep.

Putting a filter cap ahead of the inductor totally damps the flyback oscillation. I'm actually not sure WHY I didn't have one originally.

Quote:
Originally Posted by janneman View Post
Why are R1, R2, R3 so large? What is this circuit for?
B+ supply in a tube preamp. R1/R2 are filter/dropping resistors and R3 is the load.

Basically B+ is only going to the plates of 2x6SL7 (4 sections total) and at DC it is only drawing ~3mA of total current.

So... 500VCT transformer and I need ~410VDC @ 3mA.
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Old 11th March 2012, 03:00 AM   #7
gootee is offline gootee  United States
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Capacitors don't do damping. Resistors do damping.
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Old 11th March 2012, 03:37 AM   #8
gootee is offline gootee  United States
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Practical snubber design procedure is at post 292, here:

paralleling film caps with electrolytic caps
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