Aim-TTi SMU4201 oscillating when used as conventional power supply - Page 1
Forum section: electronics / testgear

author: tom66
author_rank: Super Contributor
timestamp: April 04, 2025, 05:49:10 pm
content: We ordered an R&S NGM202 for another project, but due to shipping delays, we received this unit as a loaner to get started. We're using it as a precision power supply (~0.1% accuracy) with external control and 4-wire sensing at the board to test a battery metering model.The board includes capacitors, ICs, etc. - nothing unusual for an embedded system - but it's causing this SMU to oscillate for reasons I can't comprehend.The oscillation occurs regardless of using 4-wire mode, though it's worse in 4-wire. The only control loop control the SMU has is a 'high reactance' setting, which seems to change the oscillation frequency, but it doesn't prevent it. The oscillation varies in frequency and amplitude with output voltage. It is not mains frequency oscillation.Is this just something SMUs do when loads aren't as simple as semiconductors, and I'll need to get a precision power supply instead? Or is it an issue with -this- SMU?https://drive.google.com/file/d/1db_cU51IPt7YeUabq1OLxVE5sOe2ah89/view?usp=sharing

author: Kleinstein
author_rank: Super Contributor
timestamp: April 04, 2025, 06:21:32 pm
content: SMUs are a bit prone to oscillation with difficult loads. The large range of possible current / voltage ranges and loads complete testing is difficult.To get reasonable fast response there is usually relatively little capacitance at the output.The difficult loads are low loss capacitive ones, possibly combined with some constant current load.To help with such loads added capacitance with a bit more series resistance can help - ideally this would be available in the SMU, but that large range makes this tricky.Another tricky load are SMPS circuits - these can have negative input resistance in some frequency ranges. So even a good supply can oscillate, especially going between CC and CV mode.

author: the Chris
author_rank: Regular Contributor
timestamp: April 04, 2025, 07:19:15 pm
content: Quote from: Kleinstein on April 04, 2025, 06:21:32 pmThis has been my experience as well. I was feeding an SX1308 based DC-DC boost converter with some LEDs attached the other day using a Keithley 236 SMU and you could easily see it oscilating between CC and CV mode when passing the set output voltage at which point the converter basically puts through V_in to V_out. It was oscillating quicker than the display was able to show the current change so I first was irritated about what was going on until I saw the compliance led flashing erratically.Also, trying to use a cheap electronic load to calibrate an E3632A (with two DMM to adjust current and voltage independently from what the load says) led to an oscillation between CC and CV.By personal experience, even relatively new SMU like the Keysights B29xx that are specified for pulses as quick as 1ms can take over half a millisecond to achieve target current if the DUT shows non-linear behaviour, which in turn can cause oscillation with a fast responding DUT.

author: tom66
author_rank: Super Contributor
timestamp: April 05, 2025, 10:01:36 am
content: Hmm, the interesting thing is I'm not getting anywhere near the current limit - the board normally draws around 500-600mA, with maybe the odd peak up to 700mA, but the current limit is set to 3.15A (the supply's maximum). So it can't be CC/CV mode oscillation. I do imagine the load does have some nonlinear behaviour drawing less current as voltage increases but it's surprising to see it have so much of an effect on stability.I'll try adding a large output capacitor across the 'main' terminals and if that fails, I'll go for a series resistor too.

author: tom66
author_rank: Super Contributor
timestamp: April 07, 2025, 10:52:16 am
content: Tried all sorts:- 0.5-4.7ohm series resistance with 1000uF (0.5/1/2/4.7 ohm total combinations)- 1 ohm with 2000uF capacitance at the load- 1000uF directly connected at the load, no resistanceAll seem to cause the SMU to oscillate. Oscillation does go away when using 2-wire mode, but that kinda defeats the point of a precision PSU...More than 4.7 ohm causes the sense voltage trip which is totally understandable.Cap was an electrolytic. Oscillation frequency is in the range 30-100Hz. High reactance mode does stop oscillation in most cases but makes the supply extremely unstable with any load changes, introducing its own problems.Any other suggestions would be appreciated!

author: Kleinstein
author_rank: Super Contributor
timestamp: April 07, 2025, 11:36:50 am
content: The wires for sense and drive should be close togehter. Often they suggesting twisting, which can help. This would be expecially a thing with lower impedance loads.Oscillation at only 30-100 Hz is odd. Normally oscillation is more like at the BW limit of the regulator and thus more in the 50 KHz to low MHz range.Things may get slow with low current, like in the µA range. Knowing the rought load current could give a feeling on how relevant the capacitor size is.For the low frequency I would not expect the 4 wire sense wires a bit off and thus having not compensated inductance should be the issue.Some 1000 µF is already a lot of capacitance. The more sensible dampening part would be with less capacitance, more like 1-10 µF and some 1-10 ohms.It the SMU at least stable with a well behaved simple resistive load - if not this could indicate some defect or similar problem.

author: tom66
author_rank: Super Contributor
timestamp: April 07, 2025, 01:17:20 pm
content: Normal load for my board is 200-500mA, depending on state. Operating voltage is in the range 4.5-7V.I tried a few different configurations. A resistive load (1 ohm or 4.7 ohm) on its own is fine. The supply is perfectly stable there. A capacitive load (4.7 ohm + 1000uF in parallel) causes some bizarre low frequency oscillation where the supply switches from positive to negative polarity, despite output voltage being commanded as only positive. That is truly crazy to me. It switches at a low frequency, around 2-3Hz, and outputs a higher voltage than commanded, AND then lies to me about the current -and- voltage, and doesn't obey the polarity request either!!The unit passes its basic built in self calibration but I've no access to the main calibration mode nor the equipment to do any such calibration. I'm wondering if I have a dud here but it seems strange for a unit to fault out in this way.Video of this behaviour:https://drive.google.com/file/d/1FeH6DyAW5-G8dPIDhpaHwMFxhORhY_6W/view?usp=drivesdk

author: Kleinstein
author_rank: Super Contributor
timestamp: April 07, 2025, 03:31:28 pm
content: It is a good question if the is defective unit or a general problem of the design. The load with 1000 µF can already be tricky for a not well designed lab power supply if the capacitor is a low ESR type. The large current range and 4 wire sense adds a bit more to the challange.If it would be a defect than something like a cold solder joint or chacked open capacitor in the compensation, maybe a bad (e.g. wrong type) OP-amp that causes extra phase shift or too large a dead zone from a trimmer for the standing current going open.It looks like the regulator goes from hard one way to hard the other way and over shoots all the way to saturation (without enabling the high voltage). Such "large singnal" instability can happen with regulator wind up. I have seen similar problems (though faster) with a lab supply desing in a simulation. It is a rather nasty case to handle and adding anti windup seems to be non standard for power supplies. It is at least tricky with analog regulator and may compromise accuracy.

author: voltsandjolts
author_rank: Unknown
timestamp: April 07, 2025, 03:38:05 pm
content: I've found TTi technical support to be very good. Describe the simplest test setup you have that shows the problem and I'm sure they would try to replicate and advise, given a few days.

author: thm_w
author_rank: Super Contributor
timestamp: April 07, 2025, 11:01:18 pm
content: You have slew rate on "Max" in the second video which seems wrong.You can see some other equipment here that only support up to 100uF on the output:https://www.tek.com/en/documents/application-note/making-stable-low-current-measurements-high-test-connection-capacitancehttps://www.keysight.com/blogs/en/tech/bench/2023/10/18/what-are-the-different-smu-operation-modes-and-when-do-you-use-each-one-updatedQuoteIn the spec if you have "High Reactance Load Stability" on, then it can do up to 200uF.https://resources.aimtti.com/datasheets/AIM-SMU4000_Series_Data_Sheet_issue_1.pdf

author: tom66
author_rank: Super Contributor
timestamp: April 08, 2025, 09:07:04 am
content: I did repeat the test and with 100uF on the output in HR mode ON it does not misbehave - so likely operator error there as I had not realised the capacitance limit was so low. Though I still can't understand the bizarre readings from the instrument, with incorrect voltage and current specified. And I'd have thought oscillation between +/- would at least trip some kind of fault indication when the supply is set for positive output only; that could damage sensitive loads.I did contact TTI support, and interestingly enough, they were -not- able to replicate the issue I observed with 1000uF. They suggested the unit may indeed be defective.I'm still not sure why it misbehaves powering my board, but it could just be because it doesn't like the characteristics of the SMPSUs - the board doesn't have a huge amount of capacitance on the battery input, certainly within the limit for the HR mode.