Potential Keithley 2400 Regulation issue - Page 1
Forum section: electronics / testgear

author: zburmeister
author_rank: Newbie
timestamp: June 06, 2023, 01:43:22 am
content: Unit - Keithley 2401Setup - Sourcing 5V with 0.300A compliance in 2-wire modeTLDR - Applying a strong load transient (which is high enough to hit current compliance) to a Keithley SMU can occasionally cause a large voltage spike before regaining the control loop and regulating the proper output voltage.A brief background. I designed a board that is essentially a glorified LED driver running an ATMEGA328pb and using a handful of addressable APA102-2020 RGB LEDs aside from various other gadgets for Bluetooth and Qi. The power topology is such that the LEDs and the MCU share the same 5V rail with an appropriate amount of decoupling capacitance placed appropriately. For the sake of firmware development, I was using an external supply in the form of a Keithley 2401 SMU to supply the 5V rail. As mentioned above, it was in 2-W mode supplying 5V with a current compliance of 300mA. Everything worked great and this setup worked great for a while. Over the course of a few days, I eventually went through 3 boards in which presumably the internal protection structure of the ATMEGA was failing due to overvoltage on the 5V rail. On the failed boards, the ATMEGA gets very hot and the rail exhibits an extremely linear IV curve indicating a resistive short to ground of ~1.33Ω - hence the heat.The last board that failed, did so at the exact time I increased the current limit from 300mA to 600mA. As soon as I hit enter on the SMU the board broke. This led me to wonder if the SMU was somehow causing this since at this point I believed it was power rail related and with this last bit of information, I honed in on the SMU.To replicate my observations, I evaluated the Keithley in a few different conditions: output toggle-on with no load, output toggle-on with compliance triggering load, and lastly applying a significant load intermittently.Using a 10Ω resistor to force current compliance I added and removed the load somewhat periodically but ultimately randomly. What I saw was what I expected. The voltage applied to the load at point A, and a slow ramp to ~3V at point B. 3V was because the output compliance was set to 300mA and I was using a 10Ω resistor, therefore the voltage would go to a maximum of 0.3A*10Ω = 3V.(keithley_good.png)After a bit of tinkering I found that I could cause a relatively large spike on the output by putting the SMU into compliance (again with a 10Ω resistor), and then quickly toggling that load forcing the Keithley to auto-range between the max 1A range and something lower. It was very inconsistent and seemed to point to a pseudo-random phenomenon. Regardless of the way to trigger the issue, the resulting waveform was identical across varying loads as well as on a couple different Keithley units (also tested this on a 2401). Notice the change in V/div in the screen cap. In this case, power was applied at point A, some sort of step at B, a large ramp at C, and then ultimately it corrected and returned to the expected 3V. This point C is what I am expecting fried my MCUs.(keithley_bad.png)A close-up of the pulse(keithley_zoom.png)I think I learned a few lessons here regarding the applications of SMUs and was reminded of the importance and simplicity of a standard power supply.Regardless, my main goal here is to understand the following -- Is there something about my setup causing this? Seems unlikely as it is reproducible on multiple units with different types of load.- Is this unique to the Keithley products?- Is there an explainable cause for this? Did the control loop simply lose grip of reality?- Are there methods to reduce the impact of such transient issues? (i.e zeners, tvs, capacitance)P.s That ended up a lot longer than I expected. Thanks for reading

author: AVGresponding
author_rank: Super Contributor
timestamp: June 06, 2023, 09:15:54 am
content: This application isn't really what these SMUs are designed for. I don't understand why you aren't just using a bench PSU set to 5V with a 300mA current limit.Either way, connecting and disconnecting the DUT while the power is on is going to produce unwanted transients; as you surmise the V regulation will be put under stress, and even good PSUs with small output capacitance can deliver quite a current spike in such circumstances.You could make yourself a zener based clamp to go in between the PSU and DUT, not sure a TVS would be available in a low enough voltage, and reducing the output capacitance of your PSU/SMU is probably a shortcut to serious instability, increasing it would lead to the potential for bigger current spikes.EDIT: You mention setting "0.300A compliance", do you mean you had the SMU in current source mode? If so, the SMU would supply the voltage to the DUT necessary to make it draw 300mA... it should really be set to a 300mAlimit, in voltage source mode.

author: mawyatt
author_rank: Super Contributor
timestamp: June 06, 2023, 01:56:27 pm
content: Disclaimer, we don't have a SMU.This behavior seems dangerous whether in Voltage or Current "Compliance", wether in Constant Voltage or Constant Current Mode. With the voltage peaking you've shown this condition would destroy most 5V CMOS logic chips!!IMO a Power Supply Unit nor SMU should not behave as such, wether using a discontinuous load (connecting and disconnecting a load for example) or dynamic load, even overshoot at source turn On/Off is unacceptable (which is the 1st thing we look for in a lab type PSU, see below).In Constant Voltage Mode, the output voltage should remain stable at the specified set level until the current limit is exceeded, then either the voltage drops to keep within the current limit or the voltage collapses all together and must be reset. In Constant Current Mode, the output current should remain stable at the specified set level until the voltage limit is exceeded, then the current drops to keep within the voltage limit. Any behavior outside these limits is unacceptable IMO.Back in late 80s we had these lab grade precision power supplies (Power Designs 2010), common to most research labs, and we lost two $50K + custom PCBs with custom chips, and weeks of work to a rogue supply that did as shown by OP, except at supply turn offWhen we eventually found this "condition", we made sure that particular rogue supply would never bite anyone again (crushed it with a hammer!!). Back then we had zero tolerance for defective equipment, especially this type of defect!!Anyway, with disclaimer in mind, seems like something is wrong with this SMU (or any PSU) and should be fixed immediately before additional usage damage is incurred.Best,