12-10-2024 02:27 PM
Hello, apologize in advance if this has been asked, I didn't see it.
This is a pre-sales question for an automated system intended to replace bench tests. The products being tested have a relatively large input capacitance (10mF...30mF) on the power input and relatively low steady-state current draw. There is little no current limiting series resistance, mainly limited by power supply foldback. In some cases power can be hard-switched on (e.g. via electromechanical relays / EMR or field-effect transistor / FET). In other words a delta T of perhaps 200us (200 microseconds) to 1ms (1,000 microseconds). The power being switched in may be 5V, 12V, or even as high as 30V. Using a basic I = C*dV/dt equation it's clear the theoretical inrush current blows up to an unrealistic number (because of course the source and path will have some resistance, the capacitors will have some ESR, etc.)
I = C * dV / dt
= 0.03 F * 30 V / 200E-6 s
= 4,500 Apk
Realistically, in some cases we have measured 20Apk to even 70Apk, which gives one an idea of how much foldback / equivalent series resistance is present in the system (I'll spare you that math 🙂 ).
The existing equipment we're attempting to replace for this (and other) measurements is a Tek scope with a hall effect current probe (model numbers aren't really important). We've found some clever ways to get around overloading the equipment, but essentially we need two different measurements:
Going forward with automated equipment, I think it's reasonable we don't need to switch as hard (test equipment can reasonably limit inrush), but I need to have a comparative uncertainty analysis to be sure we can still test and reasonably detect an off-nominal input capacitance condition.
For example, knowing an inrush current is 10Apk for the system, should be fine. Let's say capacitor limits are +/-1%, we have 100x better basic accuracy within those limits using a 16-bit DAQ sampling at 1MS/s across a sense resistor. (No, we are not going to use an LCR meter for this application, I'll spare you the details.)
I am planning to have NI switches in the circuit to switch among multiple units under test, probably an EMR for this high inrush test, but I'd rather put the work on the supply or our own PMOS FET rather than wear out the EMR switch by hot-switching. As such, I'm not satisfied with the detail in the 4150 and 4139 specifications and I'm asking for additional info. For more background,
PXIe-4150 questions:
Thanks,
Andrew
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12-12-2024 12:03 PM
Hi, Andrew. You have a lot of helpful detail in your question, but I’m confused about the big picture.
What exactly do you need your automated test system to do? On the one hand it sounds like you want it to power your DUTs and measure their current draw, but on the other you say you’re replacing a scope plus current probe, which obviously can’t supply any power to the DUT. Do you want the system to supply power, or just measure current from a separate power supply?
Assuming you need the system to supply power, how much power do you need to supply? You mention measuring inrush with “low Amps/div” and long-term current with “high Amps/div” which sounds backwards to me. What is the continuous current draw of your DUT? If the power requirement is low enough, you should be able to use a PXIe-4139.
I’m also unclear on if you actually want to measure a high inrush or if you just want to be able to infer the bulk capacitance from the inrush you see, in which case having the SMU limit the inrush shouldn’t be a problem.
Then you throw in measuring the current with a DAQ board and a shunt when you’re already sourcing with an SMU, which can directly measure the current it’s supplying.
So you can see I’m a bit confused. I will say that extended range pulsing on the 4139 probably doesn’t work like you think it does. Exceeding the standard power limit is done by generating a pulse in the driver, which checks the requested V and I setpoint and limits vs. the duration of the requested pulse. So it is driver-limited.
I don’t know that much about the PXIe-4150, but I do know it requires an external power supply of some sort.
Anyway, if you can clear up my confusion a bit, I may be able to be a bit more helpful.
12-17-2024 06:46 PM - edited 12-17-2024 06:47 PM
"What exactly do you need your automated test system to do?"
Both supply power and provide measurements.
The current system and procedure correlates 3-4 independent benchtop instruments (voltage, current probes, scope, and power supply) that I believe can all be done with one SMU.
"Then you throw in measuring the current with a DAQ board and a shunt when you’re already sourcing with an SMU, which can directly measure the current it’s supplying."
Assuming the SMU can't measure the inrush current in the way I expect, maybe I'll need to add a DAQ and a sense resistor, but I doubt that will be the case (more details below.)
"Assuming you need the system to supply power, how much power do you need to supply?"
Depends on the time frame, obviously, and I'm suggesting we need to soft-switch for production test. To do so, I'm suggesting we break apart the tests into two tests - 1 for inrush and 2 for during steady-state - using an SMU such as the PXIe-4139.
Test 1 - set current pulse value and duration, measure dV/dt, calculate capacitance. Because of the 10Apk limitation of the PXIe-4139, this doesn't turn on the DUT, it only characterizes the input capacitance. I argue this is OK and meets the intention of what we were already doing.
Test 2 - set output voltage and wait for DUT to fully turn on, then measure steady-state current.
Assuming I lose this argument (break apart the existing test into two separate tests) I think I'd need to use something like the PXIe-4150 supply instead of the PXie-4139 SMU.
"You mention measuring inrush with “low Amps/div” and long-term current with “high Amps/div” which sounds backwards to me."
Yikes, I said that?! Sorry about that! You're correct.
"What is the continuous current draw of your DUT?"
<100mA
"Exceeding the standard power limit is done by generating a pulse in the driver, which checks the requested V and I setpoint and limits vs. the duration of the requested pulse. So it is driver-limited"
I think this answers my question #4, which I pretty much expected, so I'm more preferring the additional information I requested in queestion #5 (specifically, I'm looking for better resolution on the number of ms allowable for each of the 10Apk pulses I mentioned.)
"I don’t know that much about the PXIe-4150, but I do know it requires an external power supply of some sort."
Hmm... ok, that's disappointing. Both because the external supply pretty much doubles the cost, and also because it's not stated in the specifications very clearly (e.g. the 4150 graphs.)
So I'm trying to keep focused on the high-level - does this make more sense to align with my previous questions?
12-23-2024 06:05 PM - edited 12-23-2024 06:15 PM
First, I'd like to make sure you realize there are two versions of the 4139: the original 20W version and a newer 40W version. Their pulsing specs are different, and the pulsing graphs you pulled are for the 40W version. The 20W version didn't merit a graph, I guess, since its specs are pretty straightforward. Let's start with that one.
That 0.2 J energy spec is the one you need. It tells you how much energy you can get from a single pulse. So taking your 20V example, 20V x 10A = 200W. 0.2J / 200W = 1 msec. Similar math for all the other voltages.
The "maximum cycle average power" spec is stating that if you're pulsing 200W, the downtime after your pulse has to be long enough to keep the average output power to 10 W. If the output power during the off time is 0, that means you'll have to wait 19 times as long after the pulse as the pulse was on. e.g. 1 msec on, 19 msec off. The "maximum duty cycle" spec means that 5% is the maximum allowed duty cycle for an extended-range pulse even when the power levels are lower.
If you're not familiar with how pulsing is specified in NI-DCPower, I suggest you take a look at https://www.ni.com/docs/en-US/bundle/ni-dcpower/page/pulsing.html, which might clarify some terminology. One thing that might not be obvious is that an SMU can do pulsing without using the "pulsing" API as long as the requested levels and limits fit within the DC power capabilities of the device. The pulsing API is primarily to get access to pulsing beyond the DC limits.
The specs also contain the information you requested for the 40W 4139. Look for the piece of the specs that starts like this:
I won't try to work through the equations here, but I think you should be able to figure them out.
One thing you might consider trying is to create a simulated 4139 and see what the software allows you to do. I'm no expert on how to set that up, but it should be pretty straightforward. It used to be done in MAX, but now I think there's a newer tool to do it.
I now understand better what you're trying to do, but I'm still a bit confused, partly because of comments like this: "Because of the 10Apk limitation of the PXIe-4139, this doesn't tu