Will this high voltage depletion mode mosfet current source work?  - Page 1
Forum section: electronics / projects

author: coppercone2
author_rank: Super Contributor
timestamp: November 11, 2023, 05:59:56 am
content: Ok I have a Keithley voltage source that does 1100V max.I thought about those 2 terminal current sources, and I came across thishttps://www.ixys.com/documents/appnotes/ixan0063.pdfSpecificallyI thought this MOSFET looked like a good candiate for the job. Its really overkill but thats why I am making the thread.https://www.digikey.com/en/products/detail/ixys/IXTH2N170D2/3586386Would this circuit work to hook it up to a 1100V rail and configure it to get 100uA or maybe 1mA from it? Then I can hook up a DMM to some safety banana jacks and have a cheap Higher Z resistance tester that I can plug into the front of the keithley and wire it to a 34401a.I am having trouble with the document though, the formula does not seem to work.. even for the first example I get -30 ohms and the answer is +40 ohms (using the values in the document for formula (2)..... Also what would be a good zener diode to use for this?Do you think this could work?Any other circuits with maybe smaller parts or better explanation I can use for this?

author: magic
author_rank: Super Contributor
timestamp: November 11, 2023, 08:04:28 am
content: The usual way of making current regulators with depletion FETs (either J or MOS) is putting a resistor in series with the source and connecting the gate to the bottom of said resistor. Current flow biases the gate negative and throttles the FET, resulting in roughly Id=Vgs(th)/R.The circuit you posted makes no sense as it applies a constant positive gate voltage to the MOSFET. That would be more suitable for enhancement FETs, highly susceptible to thermal drift (think about self heating), in addition to error caused by (variable) current bypassing the FET through the zener and R1Whatever circuit topology you use, you need to worry about power dissipation, internal junction temperature and SOA at the operating junction temperature. It may be possible to parallel current sources for lower Pd per transistor or stack cascodes on top of them for lower Vd and Pd.

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 11, 2023, 08:11:18 am
content: Don't use that circuit, use the ones in figure 3 or 5

author: coppercone2
author_rank: Super Contributor
timestamp: November 11, 2023, 08:24:47 am
content: its a 10mA source max and I want to run it at less then that usually. I can have it adjustable I guess but you need some kind of insulation for the potentiometer, keithley made like a teflon standoff shaft thingYeah I am curious because its in the APEX company thing... I did not make that circuit up or find it on some random website. i can't seem to figure out what is going on there. Try the formula they gave to.. it does not work for me? I wrote it out very carefully in a calculator program, I am sure I did not mess up any brackets or anything...........The TL431 will be OK at 1kV being around?Like I plan to turn on source, put it in 0 output, connect t he device with DUT attached, then turn on the supply to positive so it shoots up to 1000V. Its a 240A keithley. I think its a mechanical switch that disconnects and connects the 1kV.The simple zener diode circuit is very appealing for putting in a hamond box but I see what you are saying

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 11, 2023, 08:33:12 am
content: You need to understand this circuits before using them, this is not copy paste.The TL431 is at the low side, which depending on your use could be at low voltage or not, only you knows.10mA at 1100V is a lot of power for one device. This circuits are used mostly where 100uA or so are required to power something. For currents >1mA find another topology with normal enhancement mosfets

author: coppercone2
author_rank: Super Contributor
timestamp: November 11, 2023, 08:33:48 am
content: actually what chip is in fig 5, they don't sayand I specifically said 100uA - 1mA from a 10mA source in the opening is the expected current I want.....

author: coppercone2
author_rank: Super Contributor
timestamp: November 11, 2023, 09:12:09 am
content: well the reference document for that document from on-semi does not have the zener diode, but it has the other case in detail. It shows the resistor only regulator having a accuracy of 1%, and being for voltages <3V or >150V.It shows the TL321 one with a 5V rail, not mentioning the voltages.That makes me feel like there is some kinda problem with using it in higher voltages?

author: dietert1
author_rank: Super Contributor
timestamp: November 11, 2023, 09:47:29 am
content: A power mosfet that handles 10 mA * 1100 V = 11 W dissipation will have quite some parasitic capacitance. That means large voltage and current spikes when turning on the supply with a relay contact. This may be a safety issue and also a threat for the load. Using a TL431 for sensing the source resistor voltage will also inject at least 1 mA into the load.With a positive supply one should use a p-channel mosfet. It will be difficult to find a high voltage depletion mode p-channel mosfet and a TL431 only works well in the other polarity. Depending on the requirements the circuit will have more parts.If it has to be a n-channel mosfet one could use a PVI1050 to generate the gate voltage without injecting current into the load.Regards, Dieter

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 11, 2023, 11:57:37 am
content: It's incorrectly quoted.Author doesn't work for company anymore so contacting them is irrelevant. Perhaps IXYS/Littelfuse has a comment form you can file yet.The reference is given ashttps://www.aldinc.com/pdf/IntroDepletionModeMOSFET.pdfwhich shows the correct circuit, bottom of page 9 (Fig.7).Tim

author: coppercone2
author_rank: Super Contributor
timestamp: November 11, 2023, 05:36:49 pm
content: Quote from: dietert1 on November 11, 2023, 09:47:29 amHmm, can I use the Resistor only circuit to do a 1% accurate current source like the document says? Its good enough for verifying if components are grossly broken from higher voltages. I want it really to have some tool to check if say a 100k resistor is shorting or partially shorting out at high voltages, not to really determine ppm values. 5% accuracy is goodnot sure how to deal with inrush.

author: dietert1
author_rank: Super Contributor
timestamp: November 11, 2023, 06:17:05 pm
content: If you get a IXTA08N100D2 (available from Digikey), it is a 1000 V depletion mode n-channel mosfet. It's pinch-off is about 4 V and Fig. 4 shows that drain current will roughly vary a factor 4 between 100 V and 1000 V drain voltage. I'd estimate that the source resistor will get you to 30 %, but for 1 % accuracy the circuit will need additional control besides the source resistor.I would add a PVI1050 to drive the gate. Its two outputs in series generate about 10 V. The source resistor can become 3x bigger, so 30 % becomes 10 % to start with. Probably one can use the PVI1050 drive current as a fine control and for decent turn on and off. Current measurement and control could be near ground level.Regards, Dieter

author: coppercone2
author_rank: Super Contributor
timestamp: November 11, 2023, 06:32:04 pm
content: hmm well this was supposed to be a really quick project if attempted, looks like its not easy.30% is kinda dodgy even for a tester and when it needs dip chips thats turning into a big project. I only like it because it looked like 4 dead bug parts in a hamond box with a few holes. 'the function gen LCR meter attachment with oscilloscope dual channel is a good project, because you need only connectors and 1 resistor so you can measure phase shift and voltage to determine parts at any frequency. at least before I got a impedance analyzer.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 11, 2023, 06:37:36 pm
content: Quote from: coppercone2 on November 11, 2023, 05:59:56 amOverkill is not always bad, but you do need care with what else increases too.Quote from: coppercone2 on November 11, 2023, 05:59:56 amThrow that into the bin. As already mention, it is not correct for depletion mode.The link give in #8 is closer, but also has errors. The current is a sum of Iq and VREF/RF, and whilst VREF is precision, Iq is far less so.To work, this circuit needs an overhead above the VREF.The important numbers areVGS(off) VDS = 25V, ID = 250uA - 2.5 - 4.5 VSo the part might have only 2.5V at 250uA, so you needa lower VREF and ideally a much lower IQNext problem is this :Coss VGS = -10V, VDS = 25V, f = 1MHz 206 pFThat means your 1000V tester, will act as 100-200pF connected to 1000V capacitor, ready to dump into your unsuspecting circuit....A series RC, and a SPDT push button switch 'PRESS_TO_TEST' that discharges the C thru a smaller R when 'off' and charges C thru larger R (500k gives 500V at 1mA , 500mW)That safer circuit would be current regulated up to 500V and then reduce for loads > 500k, so 2M draws 400uAQuote from: coppercone2 on November 11, 2023, 05:59:56 amSometimes simplest is best. a fixed current will only high voltage test a narrow span of special resistor values.Why not just have a box with a rotary switch and some fixed high voltage test resistors ?

author: coppercone2
author_rank: Super Contributor
timestamp: November 11, 2023, 06:42:28 pm
content: do i want to jump into this rabbit hole. maybe someone else will do itsuper unrelated to what I usually like, and carpet bombing broke stuff with resistors is affordable. Because finishing the 2-10W resistor collections may be more fun then developing thisdoor strength 90% (requires 6 lockpick and 2 minutes) vs 1 tnt crate (approx the size of a stack of adafruit boxes filled with 2-10W resistors in reasonable decade).

author: coppercone2
author_rank: Super Contributor
timestamp: November 11, 2023, 06:57:52 pm
content: oohh I got a better idea thats cheaper. if I just put a ammeter on it then look to see if it follows the expected curve. Like a analog ammeter. A discontinuity should be easy to see. At least grossly broken resistors will show up, I think, like if it arcing over inside.