Zeners for shunt regulation and ground offset - diyAudio
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Old 24th April 2011, 06:19 AM   #1
NiklasJ is offline NiklasJ  Sweden
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Default Zeners for shunt regulation and ground offset

I'm trying to get something like this to work. My thought was to get both the needed input stage voltages (about +510 VDC and -35VDC for ccs'ed LTP tail) from one secondary winding by use of zener string shunt regulation and ground/earth offset.
(In this case the other hv secondary has other usage planned for it so thats the reason I went with the voltage doubler circuit and "230V" secondarys)

What do you guys think? What are the problems (there always are it seems...) and how can I improve on this circuit?
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Last edited by NiklasJ; 24th April 2011 at 06:29 AM. Reason: sspelling...
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Old 24th April 2011, 07:35 AM   #2
NiklasJ is offline NiklasJ  Sweden
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What I'm really interested in knowing is what are the potential problems/weaknesses in using zeners to offset a power supply reference with respect to ground/earth? And, can you accomplish the same result some other better, easy/clean design sort of way?
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Old 24th April 2011, 07:17 PM   #3
Elvee is offline Elvee  Belgium
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Honestly, I do not like very much the look of this supply, but as long as you have done your homework (current in any of the zeners for any combination of output currents for any mains voltage, etc), it should work as intended.

You are talking about +510 and -35 supplies, but the way it is drawn is a 545V supply with a 35V tap.
This is OK, as you draw the 60mA current from the extremes, but if you really use it as +510 and -35 sources, there might be problems with zener currents.
If you simply need an auxiliary -35V, I think adding a capacitive supply to the 0V side of the main supply would give more independence, see this example:
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Old 24th April 2011, 08:36 PM   #4
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I'm not a fan of long zener strings. Depending on voltages and currents, if one fails (shorted is typical) more tend to fail, then the whole string goes. Also, each zener may have a decent shaped knee, but all strung together the performance may not be that great. OTOH, I used to work for a place that sold high voltage power supplies built that way, and they mostly worked OK.
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Old 24th April 2011, 09:44 PM   #5
NiklasJ is offline NiklasJ  Sweden
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Elvee, Conrad,thanks very much for your time and efforts to help.
Yes I plan to use voltage from the extremes, driving 2 series coupled long tailed pairs. The gnd offset only makes it possible to loose an input cap really...And as you pointed out Conrad, that long zener string feels like an accident waiting to happen, but thats what that iteration needed to cope with the different current requirements I had planned for that layout (~5-65mA) using 5w zeners. Have already realized that was a bad idea anyway.
So perhaps a capacitive b- supply and some other variant of regulation for the hv b+ would be next for me to investigate. Perhaps something like this to take the load off the zeners: (attachement) but takes the fun/beauty of simplicity away... Well, hope I learn some more as I go along with this.
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Old 25th April 2011, 06:56 AM   #6
Elvee is offline Elvee  Belgium
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You can retain the shunt regulator, but in a streamlined form.

Here is an example: the MOS cascodes the output of the TL431, and a series resistor R1, bears the brunt of the dissipation.
In series with the drain of M1, the effect on the quality of the regulation is negligible.
Performances will be better than a plain zener, and the reliabilitywill be improved too.
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Old 28th April 2011, 07:37 AM   #7
NiklasJ is offline NiklasJ  Sweden
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After some thinking, here is what I think I will try first.
After psu filter, a current source that is shunt regulated at the output. Low ripple according to ltspice, ~300uV, but who knows, perhaps it sucks the life out of music... (that is with the max designed output ~65mA@~530V, much much lower at more modest output )
What I dont like is that tl431 circuit, too much ic for my taste...but it has good reputation. ?
The irfpg40 shunt mosfet is a tough beast and perhaps severe overkill for the application, but it has high power rating and doesnt cost a fortune.
The ccs set potentiometer could (should) be changed to a 250 or 500 Ohm variant for this use as the resistors for gnd divider, shunt voltage and capacitor equalizer wants about 4mA even without any output load so not much use to be able to set the ccs under 5-6mA perhaps and a lower R pot would increase setting resolution.
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Last edited by NiklasJ; 28th April 2011 at 08:07 AM. Reason: addition to text
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Old 28th April 2011, 08:24 AM   #8
NiklasJ is offline NiklasJ  Sweden
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I'm not totally set in using that divider gnd reference scheme, I do think it makes sense to use one psu for the circuits using it. In this case what it does is make it possible to use a solid state ccs at the input stage long tailed pair tail and NOT having a input capacitor.
Another option to the voltage divider would be similar to my initial scetch with zeners but using volage regulator tubes instead, 3x serial VR150 to get +450V to gnd and a VR75 below that to make -75V. Then I could ditch that semi/ic shunt also but I would have a veritable forest of tubes on the chassis...

Also I'm contemplating using a similar scheme for a cathode follower type output tube driver, where you would set the output tube bias by offsetting the whole cf driver circuit/psu with respect to gnd. I think it could be a good idea but I haven't yet simulated what would happen if the driver circuit draws grid current from the output stage (very likely scenario in my application =otl circlotron/parallell single ended)

Perhaps I have done total overkill here, I don't know.
Please comment or advice.
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Old 28th April 2011, 12:03 PM   #9
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by NiklasJ View Post
What I dont like is that tl431 circuit, too much ic for my taste...but it has good reputation. ? .
OK, designing purely discrete shunt regulators isn't difficult either: here are one or two examples, good, better, ... etc
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Old 28th April 2011, 12:17 PM   #10
AndrewT is offline AndrewT  Scotland
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A very nice example of what a very few extra components placed with knowledge can achieve.
V2 to V3 is relocating R6 to R12.
V3 to V4 is adding a damped capacitor across R13.

Even the very basic V1 is only 140mVpp in 510Vdc ! This is pretty good.
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