Minimum Current Sink for High Voltage Regulator - Page 1
Forum section: electronics / projects

author: proschuno
author_rank: Contributor
timestamp: August 22, 2025, 10:42:51 am
content: Greetings, I am currently working on this style of regulator in which an LM317 is used to regulate voltages higher than it's ratings by being cascoded with a high voltage mosfet. I have an input of 550V and I wish to use it from about 530 Volts (it's highest considering dropout) down to 40V. I have a 20K resistor replacing R4 and a 500K pot paralleled with a 560K resistor to make a ~250K pot in place of the pot in the schematic.My problem is that the regulator needs about 5mA to function, just using a resistor means burning a lot of amps at high voltages, or having no regulation at lower voltages by using a higher value resistor . Beardboarding any active current sink involved it often blowing up my whole regulator when I switched on the high voltage (there's a switch to turn on the transformer, and another to engage the HV). Is there a slow turn on design that anyone can suggest that I can add on to the output to help it to regulate?

author: Ian.M
author_rank: Super Contributor
timestamp: August 22, 2025, 11:33:45 am
content: *DON'T* use a LM317! The high minimum load current for regulation (worst case 10mA*) means you have to burn off too much energy in the cascode MOSFET.A regulator with a much lower min. load current will solve most of your problems *IF* the cascode MOSFET is biassed correctly. Unfortunately even biassing the MOSFET gate can be challenging with over 500V in, unless you use a depletion mode MOSFET.Also, we can't see your schematic . . .* Min. load current of 5mA is only for the long obsolete LM117.

author: David Hess
author_rank: Super Contributor
timestamp: August 22, 2025, 11:48:59 am
content: The alternatives are to use either a lower quiescent current floating regulator in place of the 317, or a discrete design for a lower quiescent current floating regulator with an operational amplifier, reference, and pass element.I thought Texas Instruments had a lower quiescent version of the 317, but I do not see it.

author: Zero999
author_rank: Super Contributor
timestamp: August 22, 2025, 11:54:43 am
content: Quote from: David Hess on August 22, 2025, 11:48:59 amThe LM317L used to have a lower minimum load current, but the scumbags changed the die to one with a higher minimum load requirement, without bothering to issue a new part number.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 22, 2025, 01:18:18 pm
content: There are schemes where you can float an opamp up near the supply voltage to drive a pass transistor. The pass transistor can be lower voltage than the supply in some cases. A string of zeners is usually involved. I can probably find a schematic for that, but IMHO all these schemes were lower voltage parts are used will tend to fail if shorted or abused in any way. They work best in a controlled situation where the load is constant and there are no user disconnects.

author: David Hess
author_rank: Super Contributor
timestamp: August 22, 2025, 07:40:50 pm
content: Quote from: Conrad Hoffman on August 22, 2025, 01:18:18 pmIt is not difficult, and the high voltage pass transistor bootstraps the regulator control circuits with a zener from its output to the supply. Everything is much easier if some low power high voltage transistors are included to properly bias the regulator from the high voltage supply.Check the 723 datasheets for examples of floating regulators to get an idea of how it is done. Break out some microcurrent parts and you can get the quiescent current to below 100 microamps, and below 10 microamps with some effort. Shown below are examples from the LM10 application note but the design would need modern parts and changes to get lower quiescent current.Also, frequency compensation of a low quiescent current regulator can be challenging.

author: Picuino
author_rank: Super Contributor
timestamp: August 22, 2025, 08:03:33 pm
content: What about a switching power supply design?They can be made with low noise.AN-118: High Voltage, Low Noise, DC/DC Converters A Kilovolt with 100 Microvolts of Noisehttps://www.analog.com/en/resources/app-notes/an-118fb.htmlhttps://www.analog.com/media/en/technical-documentation/app-notes/an-118.pdf

author: Zero999
author_rank: Super Contributor
timestamp: August 22, 2025, 09:39:38 pm
content: Quote from: Ian.M on August 22, 2025, 11:33:45 amThe LM217 will work with a minimum load of 5mA and is still available.https://www.st.com/resource/en/datasheet/lm217.pdfhttps://uk.rs-online.com/web/p/voltage-regulators/1891015https://www.digikey.com/en/products/detail/stmicroelectronics/LM217T/591645Heck, you'll probably get away with 3.5mA, if the voltage differential isn't extreme and poorer regulation is acceptable.

author: David Hess
author_rank: Super Contributor
timestamp: August 22, 2025, 10:23:51 pm
content: Quote from: Picuino on August 22, 2025, 08:03:33 pmMost are designed for lower operating voltages, and it would not be any more efficient unless it is very complex.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 22, 2025, 10:25:32 pm
content: Quote from: proschuno on August 22, 2025, 10:42:51 amAs you have found, a LM317 is a terrible fit here, but you can buy 'better LM317m with higher Vmax and lower Iq ' - look at the MICROCHIP LR8K4 with 10uA typical ground current, 10mA Adjustable 1.2V~438V DPAK

author: proschuno
author_rank: Contributor
timestamp: August 22, 2025, 11:57:23 pm
content: I'm using this in a variable voltage supply that I'm retrofitting, as shown in the pics. Homebuilt by some amateur in the 60's, it originally had tubes as a regulator but I gutted it and retrofitted it with semiconductor parts. It has banana jacks with binding posts so I could screw in an appropriate sized resistor to set the operating current but I'd be a hell a lot more convenient and safer to have something contained inside.I've tried a basic zener diode - enhancement mosfet set up and while it worked well, it would sometimes blow up when turning on the HV and shorting the regulator to blowing it up as well. I might try an appropriately rated depletion mosfet that would be biased with just a resistor and no zeners to blow.However I might just revert to the resistor on the output that I can scale to whatever voltage I'm using; dumb simple and no moving parts. The circuit I'm using has proven reliable and quite powerful, much more than I can get out of using tubes. The transformer is huge and I have the humongous capacitors lying around (5 left after this build), so I wanted a circuit that would maximize the energy delivered. The pass transistor is a 1000V 200W Enhancement mosfet screwed onto a huge heatsink with a fan constantly on.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 23, 2025, 05:35:26 am
content: Quote from: proschuno on August 22, 2025, 11:57:23 pmFor high voltages, you could just whack a pot into a FET Gate.I find Bourns have 2W 1Meg cermet pots with plastic shafts, and the FET source follower impedance is low for 2-100mA, the FET curve change is less than any current limiting sense resistor.A short on a 400V supply is always going to be tough. The rise times can be sub-10ns and the FET capacitances determines how the voltage and currents split.5-10A ballpark peaks of short duration are possible, so parasitic inductances start to matter.A depletion mosfet can current regulate with just a resistor.An enhancement FET needs added parts like NPN or dual NPN for current limiter, but they are well behaved, and enhancement FETs are alotcheaper.With either you still need decent clamp diodes, carefully mounted with low inductance, and maybe some safety series Rout.

author: MariuszD
author_rank: Regular Contributor
timestamp: August 23, 2025, 06:52:12 am
content: Quote from: proschuno on August 22, 2025, 11:57:23 pmWhat transistor did you use? Vertical MOSFETs are unreliable for linear operation. At high voltages, the SOA is very limited. Often, the manufacturer doesn't provide the SOA for the DC or gives incorrect SOA characteristics because they forgot about thermal instability.Show us the schematic.In a cascode circuit using the LM317, it's difficult to guaranty that the voltage across the LM317 will remain within a safe range during transient states.

author: Zero999
author_rank: Super Contributor
timestamp: August 23, 2025, 10:30:59 am
content: Quote from: PCB.Wiz on August 22, 2025, 10:25:32 pmI don't trust TI data sheets.Look at their LM317L data sheet.https://www.ti.com/lit/ds/symlink/lm317l.pdfThe legacy chip has a lower minimum load current than the new chip.I suspect they've got legacy and new chips confused on the LM317M data sheet.EDIT:The LM317L-N has a minimum load current requirement of 2.5mA, so long as 3V ≤ (VIN− VOUT) ≤ 15V. It's only rated to 100mA though.https://www.ti.com/lit/ds/symlink/lm317l-n.pdfQuote from: proschuno on August 22, 2025, 11:57:23 pmWhy the heck did you do that?Thermionic valves are one of the best things to use for linear high voltage applications; much more rugged than semiconductors. I suggest redesigning it to use a semiconductor for the reference and keep the glassware on the output stage.

author: mtwieg
author_rank: Frequent Contributor
timestamp: August 23, 2025, 12:45:18 pm
content: Quote from: Zero999 on August 22, 2025, 11:54:43 amI looked at the datasheet out of curiosity and yikes. Very different specifications, with no way to order one or the other. And no change to device markings either. If you order a reel though, they say the reel packaging indicates which chip is usedQuote from: Zero999 on August 23, 2025, 10:30:59 amNot sure what you're trying to say here. The decision to change chips/specs without issuing a new part number is scummy, but I don't see anything wrong with the datasheet itself. Don't shoot the messenger!