High sensed, grounded load, current generator - please criticize. - Page 1
Forum section: electronics / beginners

author: uliano
author_rank: Regular Contributor
timestamp: December 14, 2022, 07:44:46 am
content: I'd like to design a current generator with the load grounded on one side (so that I can measure voltage with MCU ADC).Specifications are 250uA (but then why not something more, say 100 mA?) with a compliance of 200V or so.It all started with the will to measure the BVDSS of crap china MOSFETs as per herehttps://www.taiwansemi.com/assets/uploads/productcategoryfile/AN-1001_A1611.pdf3.1, I succeeded by serializing many lab PSU with a lot of impractical cable mess on the table and then I decided to take advantage of this to learn something and apply it to a small project.After some iterations I'm coming up with thisThe grounded load also makes a lot easier to connect gate and source together with some electronic switch (or relais) in case this small project grows toward more MOSFET measurements.the LCT1150 was already in the drawer and I ordered the INA149.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: December 14, 2022, 10:34:31 am
content: That looks like it should work.When you go to higher currents, dissipation rises also quickly 100mA @200V = 20W and an MPSA92 can not handle that. But the devil is in the details. with 200V it's easy to let the smoke escape. You have to handle all sorts of situations carefully, for example applying power in the first place, you may have to tweak it to get it stable, etc.I'd think a simpler topology would be more robust.Build some kind of adjustable stepup SMPS with a series resistor, and then just adjust the PWM input to the SMPS.Then add an uC to ramp up the voltage and do the measurements.I don't know where your 200V is coming from. But if you start with a small SMPS that can't deliver much current then the chance of releasing the magic smoke is also smaller. If your power supply can also never deliver more then about 10mA, then the thing is also relatively safe against accidental touches. You will feel the 10mA, but it won't kill you.And what is the advantage of using an EUR10 opamp? Any ol' 20ct opamp will do in this circuit. Save your fancy opamp for a circuit where it adds something to the total.

author: jonpaul
author_rank: Super Contributor
timestamp: December 14, 2022, 10:49:29 am
content: no need, PSU 0...300V>>series R>>DUT>>1..10 ohm current sense R >> returnj

author: David Hess
author_rank: Super Contributor
timestamp: December 14, 2022, 12:24:55 pm
content: That will certainly work although it seems like there should be a simpler way. I think Jim Williams showed a similar design but without the current mirror shown below. (1)The schematic should also show the reference connection for the instrumentation amplifier separately from the negative supply voltage connection since they are different pins.(1) Page 9 of theLT1102 datasheet. He did not show the reference connection of the instrumentation amplifier either.

author: uliano
author_rank: Regular Contributor
timestamp: December 14, 2022, 12:26:08 pm
content: Quote from: Doctorandus_P on December 14, 2022, 10:34:31 amencouraging, I was starting to think I had made some stupid overlook, and I'm stillQuotemy bad, it all started with 250uA and I tested up to 120V (2xdual psu in series) for the said purpose of measuring VBDSS. I withdraw (temporarily??) the requirement, let's say 1mA which should work with MPSA92 without trouble (maybe).QuoteSMPS is a new territory for me but I will look into it. Can the go straight to 0V? (my unqualified guess is no)Quotefor the moment as said is a cascade of PSU (set to deliver 10mA which is the minimum the can be set)in a next step I was thinking to wind a little transformer with multiple secondaries (2 or 4) that I could series or parallel upon requrement before an unstabilized bridge+capacitor rectifier.Quoteyou mean the INA? it is needed (at least i think so) as the absolute voltages at Rsense terminals are way too high to be handled. The other one I already had in the drawer and its low offsetINA149 has a common mode specification of +-275V

author: David Hess
author_rank: Super Contributor
timestamp: December 14, 2022, 12:31:19 pm
content: Quote from: Doctorandus_P on December 14, 2022, 10:34:31 amThe advantage to these types of circuits is that the control voltage and output current are ground referred, and the instrumentation amplifier has a specified and high common mode rejection.

author: uliano
author_rank: Regular Contributor
timestamp: December 14, 2022, 12:31:33 pm
content: Quote from: jonpaul on December 14, 2022, 10:49:29 amthat way would not be high sensed

author: uliano
author_rank: Regular Contributor
timestamp: December 14, 2022, 12:35:35 pm
content: Quote from: David Hess on December 14, 2022, 12:24:55 pmit does not have the current mirror because it stays within the opamp voltage range, if I understand correctly, do I?.Quotemy bad, i draw the sketch for myself and then I felt the urge of some scrutiny...

author: Terry Bites
author_rank: Super Contributor
timestamp: December 14, 2022, 12:47:50 pm
content: I theory this is fine, in practice you will need very tight matching of the current mirror transistors. It depends on the overall precision you're aiming for.Commercially avialble mirror based sensors are in the 2.5% range- have a look at diodes inc current monitors.The MPSA92 is fairly low gain pnp so base current errors will reduce mirror performance.What is the maximum power dissipation in these transistors? 200V is right at the limit of the MPSA92I'd go for a ready made solution- it will likely work better and cost less to build. You can use a current output monitor ic by floating it under the HV rail using a zener. Beacue the output is a current you can simply use a cascoded transistor to drop the the output to a sensible potential. See SBOA295 from TI. or Page 23 of the LT6102 datasheet. In fact you can apply this idea to many current sense ciruits.

author: uliano
author_rank: Regular Contributor
timestamp: December 14, 2022, 12:55:59 pm
content: Quote from: Terry Bites on December 14, 2022, 12:47:50 pmthe idea of feedback through INA sensing directly the load current is exactly to avoid all troubles of mirror imperfections, I amused myself breadboarding some Wildsar, Wilson (2 kinds)... and measuring their stability under load, +V, and programmed I variations. there is no way out of that road so I changed approach.Quoteyeah right, badly overlooked, let's say 1mA while 200V should be fine as theY are rated 300VQuoteI f I float something under the positive rail (I had considered it) I still have the problem of communicating with the micro which is supposedly (for future enhancement and expansion) lying close to the ground.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 14, 2022, 04:55:53 pm
content: The maximum voltage across the collector resistor is set independently of HV.I can see why this would still be worrying in a fault condition like a transistor or amplifier meltdown.You might consider adding a series this resistor and a zener or TVS. Some extra series resistance reduces Vce and thus lowers the dissipation in the transistor.This lets you choose a lower voltage transistor too.

author: uliano
author_rank: Regular Contributor
timestamp: December 14, 2022, 04:57:30 pm
content: Update:Fedex brought me the INA149 and so I immediately breadboard the sketch. It works!and it turns out that it delivers unbelievably (compared with open loop mirrors) stable current: +-1 on the fifth digit (tens of nA) varying the PSU voltage between 12 and 60 VBUT horribly unstable varying the load: -20% on current going from 10k to 100k load.Any Idea?

author: David Hess
author_rank: Super Contributor
timestamp: December 15, 2022, 01:49:58 am
content: Quote from: uliano on December 14, 2022, 12:35:35 pmAh, that is right, but high voltage compliance is limited to the input range of the instrumentation amplifier.Quote from: Terry Bites on December 14, 2022, 12:47:50 pmThe current mirror is inside the feedback loop so its accuracy is almost completely irrelevant. The only reason for the emitter degeneration is to roughly equalize the current in the two transistors.Quote from: uliano on December 14, 2022, 04:57:30 pmIt should be. The precision is largely limited by the performance of the instrumentation amplifier.QuoteCheck to see if it is oscillating. It may require some frequency compensation around the error amplifier.

author: uliano
author_rank: Regular Contributor
timestamp: December 15, 2022, 07:30:05 am
content: Quote from: David Hess on December 15, 2022, 01:49:58 amnot really, at least for my understanding.In my mind I chose INA149 (which, btw, despite the name is a unitary gain difference amplifier and not an instrumentation) for its very high V common mode +-275v thinking that till the V+ - V- difference is within the rails (provided that I grounded reference pins 1 & 5) I would be safe with whichever voltage south to the sense resistor.(see pic)Quotetonight I'll check, thanks!My fear here is that I could have overlooked some contrived common mode fact out of my actual understanding

author: David Hess
author_rank: Super Contributor
timestamp: December 15, 2022, 07:52:50 am
content: Quote from: uliano on December 15, 2022, 07:30:05 amAnother thing to check is if you are limited by the common mode rejection ratio of the instrumentation amplifier. Operating it that way is a severe strain on its precision.

author: uliano
author_rank: Regular Contributor
timestamp: December 15, 2022, 08:03:57 am
content: Here I stretched way out the suggested limits (used 10k as sense)

author: uliano
author_rank: Regular Contributor
timestamp: December 15, 2022, 08:08:44 am
content: Another approach I'm considering, once I solved the high sensing, is to trash the current mirror and use some kind of optocoupling to provide feedback to a current source that lies just under the high rail.

author: uliano
author_rank: Regular Contributor
timestamp: December 15, 2022, 06:58:48 pm
content: This is on either side of Rsense(and I'm momentarily clueless)