Interesting any idea what causes the oscillation?
Can't you just pick a good current for the zener and let it turn on some pass element.. like this? (or a FET as others suggested?)
George H.
27KV fet?
-- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On a sunny day (Tue, 19 Aug 2014 07:13:05 -0700) it happened John Larkin wrote in :
38571 Si diodes in series
See...
HighVoltageOperationOfTL431.pdf
on the S.E.D/Schematics page of my website.
Replace TL431 with LM285-ADJ and "Toss in" a conventional MOSFET (I see no advantage using a depletion-mode FET) and perhaps a few protection diodes for safety during start-up.
Looks like a reasonable expectation would be about 1 Ohm per stage, so the higher the MOSFET voltage the better. ...Jim Thompson
--
| James E.Thompson | mens |
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| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
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| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
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I love to cook with wine. Sometimes I even put it in the food.
Not a whole lot in that toolbox there, Mr. Goodwrench.
[snip]
Oooops!
Some issues... at least according to the LM185-ADJ Spice model I have, the knee ain't so marvy... needs about 20uA cathode current to be "stiff".
Also, the reference location is between CATHODE and ADJ, rather than between ADJ and ANODE as in the TL431, so the "sex" in reversed.
...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Well, technically it is not oscillation, the ranDUMB noise is caused by sporadic micro avalanche breakdown. One can always find some (small) current range where that does not happen,BUT all zeners i have looked at (4.7V to 180V, numerous brands) exhibit the avalanche noise somewhere (or in most of) in the 20-50uA region. That configuration is not useful; it is not a 2-terminal shunt regulator.
That would be utterly silly for a 400V app.
Looking at schematic, it seems no FET is needed. But i do not calculate the 45V. Mister Ohm, NOT related to Sherlock, tells me a total voltage of
21.2V if i read it correctly. 1.20V across R1, then 8V across R2,ad 12V across R3. Corrections appreciated.
As i said, zeners are not useful: too damn noisy; the good-old avalanche spike crap which gives a negative resistance and possible oscillations.
Gotcha! Vref for a TL431 is 2.5V ;-)
Vref for the LM185-ADJ is ~1.245V ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
I think we have a little thread drift here. Somebody was talking about HV stuff for old color TVs.
-- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
It is wore than that...burn 10uA for a divider and that leaves a huge
10uA for the TL431; betcha there is a bit of rounding there...Looking again in my shunt regulator archive, once upon a time (2002), STM made the TS431 which had the exact control you mentioned. Minimum cathode current 40uA (typ). Your Spice model for the TL431 seems congruent with the STM part. I assume that your LM185-ADJ Spice model is appropriately "flipped". I found some Nat Semi LM185 models, so now i have a nice disparate set where no doubt none of them follow reality in that low current region. But at least one of them will show what to expect.
Thanks.
I've seen a bad batch of MMBT2222 have what looks like a tunnel diode effect in a particular bias region and thus appeared on the scope as a spur. Like i said, bad batch. I still have a hand full with a note on them indicating to be used as switches only.
As for zeners oscillating, I've never seen one that cause an oscillation that was bad enough to actually cause an issue. Only when they come under very low current maybe, but I think that falls true with many pn junctions?
Are you sure you didn't experience this using some glass envelope types that could be acting photovoltalic? If sure would make since when being used in very low current circuits..
Jamie
45uA (LM385-ADJ) was the lowest Iq adjustable from DigiKey or T.I.
Looks like you're up a creek. Got a paddle?
Cheers, James Arthur
What's wrong with...
In fact, with max 17uA total supply current between the two, you've got an extra 3uA to spare on the supply; that could be burned in a 5V zener. Obviously, you'll need a ~20M series dropping resistor for that, and the feedback divider must be over 133M.
With minimum gain > 10^5, the opamp output will rail over a range of less than 10uV at the input. Or for a 1/160 feedback divider, no more than
1.6mV out of 400V. Far better than the 100mV you require.The initial accuracy of the reference is not quite 0.025%, but there are tighter references out there, or you can trim it, as its stability is good.
Don't forget to use extreme precision film/foil resistors; have fun with that >= 133M part.
As you did not provide a cost or size constraint, I'm going to stamp this design exercise as "complete". Ship it! ;-)
Tim
-- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
Check; pawn to Q3.
NOPE! Look carefully at the TI datasheet: TYP min operating current Vout=Vref is 7uA for the LM385, and 8uA for the LM185/285.
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