High voltage (160V) low current limiting (0.2mA) - Page 1
Forum section: electronics / projects

author: stefan_o
author_rank: Regular Contributor
timestamp: June 11, 2017, 10:27:39 pm
content: Hey,does someone have an good idea how to achieve high-voltage current limiting? The voltage can vary up to 160 Volts, but I need a constant current limiting at 0.2mA (=200µA). Initial precision does not need to be very high. Output voltage should not (less than 500mV) change below the threshold.Any good idea how to implement that? It is to limit the current through a avalanche photo diode.Best regardsStefan

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 12, 2017, 12:03:27 am
content: Simplest unqualified solution: depletion mode MOSFET e.g. LND150, DN2540, etc.A BJT (PNP) or PMOS, current source or mirror, would also be an option, but may be less desirable for bias reasons.If high frequency response, capacitance, is an issue, find the smallest transistor you can tolerate (i.e., Rds(on) as high as possible -- presumably, Rds(on) <= (0.5V) / (0.0002A)), with Idss > 0.2mA if depletion mode. Also consider a cascode configuration, and a damped LC filter (in series with the collector/drain) to increase the impedance at high frequencies.Tim

author: stefan_o
author_rank: Regular Contributor
timestamp: June 12, 2017, 01:37:05 am
content: A depletion mode MOSFET seems to be a great idea, didn't thought of that. The LND150 also seems to be the most suitable one I've found, RDS(on) of 1kOhms is the highest I could find (using parametric search of Mouser/Digikey). For the resistor I probably need a trimmer to tune to the individual MOSFET, correct?I don't think high frequency response will be an issue.Thank you very much,Stefan

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 12, 2017, 04:07:46 am
content: If accuracy "isn't much of an issue", you might just as well leave it as-is -- otherwise, yes, a trimmer can do better. It will still be temperature sensitive, so keep that in mind.Should be easy enough to SPICE up the right resistor range and tempco required to keep it very stable, if you like.Tim

author: mariush
author_rank: Super Contributor
timestamp: June 12, 2017, 04:13:11 am
content: You could use one or two of those LR8 high voltage linear regulators from microchip :http://www.farnell.com/datasheets/2138212.pdfps. though note that datasheet says it minimum output current typically 0.3mA max 0.5mA including resistor and load current... but you could probably add a resistor in parallel with the load to eat 0.1-0.2mA

author: stefan_o
author_rank: Regular Contributor
timestamp: June 12, 2017, 02:49:46 pm
content: Problem with the LR8: The operating voltage may be different, so I cannot just add a resistor, current would be different. But the LR8 looks like a nice part for voltage regulation, much better suited than the TL783 I had in mind.Quote from: T3sl4co1l on June 12, 2017, 04:07:46 amCan you explain how? By reading thedatasheet of the LND150I assumed that I need about 4kOhm as source resistor and according to the graph on page 4 (top right) it is quite temperature stable. How would you optimize it?Best regards,Stefan

author: Marco
author_rank: Super Contributor
timestamp: June 12, 2017, 06:23:06 pm
content: Because transconductance is so low at 200 uA I'm not sure the corner is going to be sharp enough to meet that 0.5V transition between constant voltage and constant current.PS. why do you care about that though? Do you want to experiment with both constant voltage and constant current biasing?

author: PartialDischarge
author_rank: Super Contributor
timestamp: June 12, 2017, 07:10:41 pm
content: Quote from: stefan_o on June 12, 2017, 02:49:46 pmFor 200uA you need 4.3K with that depletion mosfet. The lower the current the more temp stable it is. Keep in mind that it is not a current limiter but a constant current source, ie it will try to keep that current flowing and for that it will raise the output voltage as much as it can.

author: BrianHG
author_rank: Super Contributor
timestamp: June 12, 2017, 07:12:43 pm
content: With such a low output current, can you not use a cheap 300v Rail-Rail output opamp & design a simple voltage/current regulated supply output?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 12, 2017, 07:14:32 pm
content: Ah, LND150 does indeed have quite good temp stability at low currents. Nice.Tim

author: floobydust
author_rank: Super Contributor
timestamp: June 12, 2017, 07:36:47 pm
content: Quote from: stefan_o on June 12, 2017, 02:49:46 pmI don't thinkTL783will work, it has a high min. output current spec. of 15mAI'd use discretes ala 2SC2690 CRT drivers or audio power amp VAS transistors for good linearity.

author: stefan_o
author_rank: Regular Contributor
timestamp: June 12, 2017, 08:17:49 pm
content: Quote from: MasterTech on June 12, 2017, 07:10:41 pm? There is no switching, nothing, isn't it just Ohm's law? If there is 160 V for example at the input and a current of 0.05mA output will be 160V-(RDS(on)+RSource)*I, in this case 160V-(1000Ohm+4300Ohm)*0.00005A=159.735V?Quote from: Marco on June 12, 2017, 06:23:06 pmAh, you mean RDS(on) depends on current? And with low current RDS(on) will be a lot higher? In fact, it would be prefect if a higher resistance would compensate the lower current for the voltage drop to be constant. If the voltage drop for lower currents is higher, that is bad.The easiest I think is to try it out, I already ordered a few LND150, all I need is a few resistors and see how it works.Quote from: BrianHG on June 12, 2017, 07:12:43 pmCheap 300v Opamp? I looked at Digi-Key, all are above 100 €/US$. A LND150 is less than 1 €...Quote from: floobydust on June 12, 2017, 07:36:47 pmI had the TL783 for voltage regulation in mind. For voltage I can choose the resistor low enough for it to be stable. But the LR8 has two advantages: Less power losses (resistor divider with higher values) and I can regulate down to almost 0. But this has nothing to do with the current limiting, that's the voltage one step before.

author: PartialDischarge
author_rank: Super Contributor
timestamp: June 12, 2017, 08:24:48 pm
content: Exactly, and when the current goes up it will try to restrict that increase by taking up voltage and lowering the output voltage

author: floobydust
author_rank: Super Contributor
timestamp: June 12, 2017, 08:33:37 pm
content: Quote from: stefan_o on June 12, 2017, 08:17:49 pmOk, it's just that 15mA load at 160V is a big fat resistor making 2.4W of heat and also heating up the TL783, to supply 0.2mAThis is why I say it's not going to work well here.

author: BrianHG
author_rank: Super Contributor
timestamp: June 12, 2017, 10:05:52 pm
content: Quote from: stefan_o on June 12, 2017, 08:17:49 pmI thought the 5$ LTC6090 was +/-140v, but it's actually 140v total.Digikey does have this 15$ 350v device, but, it is way out of budget and at 60ma, overkill on power output:https://www.digikey.com/product-detail/en/apex-microtechnology/PA340CC/598-1917-ND/2700701