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: one inrush measurement at fast sampling rates and low Amps/div setting on the scope, and one long-term measurement at slow sampling rates and high Amps/div setting on the scope. 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, In the 4150 configuration an example of the system is something like PX6771950, and In the 4139 configuration an example of the system is something like PX6772373 (see PXI Advisor.) PXIe-4150 questions: Is an external supply (AUX POWER IN) really required to achieve performance in Figure 1 of the specs? My initial reading made me think "no" because I saw no such note in the specs or datasheet, however... In the PXI Advisor it specifically says so (see image below) If required, the external supply is crazy expensive (almost as much, or as much) as the 4150 itself... just an observation. (I guess that was passive-aggressive for any Sales people reading.) I'm tempted to just plug in a $200 benchtop supply instead. More driver question than anything - see table 11. I would like to just get a 30V step with maximum output current capability for >=5ms. What is the current range (is it the max 10A) if I configure a 30V voltage-mode step? If so, is there any idea how much foldback (excess current for a brief <=5ms period) there would be? (10Apk for 10A range, 20Apk for 10A range, etc.) My planned method is to use a sense resistor and Multifunction DAQ to determine current at turn-on. Option 1 Voltage mode: As long as I know that output current capability max of the 4150 is 10A or so, I may be able to detect deviations in peak current and reliably assume the deviations is due to the product, not the test equipment. Option 2 Current mode: Alternatively, if I can't get enough current (10A or so) in voltage mode, perhaps I need to configure current mode and do some math to estimate how it will affect voltage on each individual product. Seems riskier... Are there any max load capacitance needs of the 4150? I only see a note about 1uF on a noise specification.PXIe-4139 questions Figure 1 of the specifications gives a fairly clear picture of what this instrument can do at 10A. It shows 10Apk for 1ms...10ms at 30V. Is there any way to exceed (in the driver) 10Apk for shorter (lets say 1ms) pulses? More a driver question than anything - see Figure 1 - is the 1ms...10ms driver limited somehow (e.g. will throw an error if exceeded) or is it just a hardware limitation that shows up without driver error? I'm pretty sure it's the later, because if it's the former, I don't know a method to avoid errors at 10Apk, various supply voltages, other than "guess and check" each pulse width. If hardware as I suspect, could I get any characterization / finer granularity on pulse widths (e.g. short to ground) for these cases? See also image below, mark-up of Figure 1 10Apk 3.3V output 10Apk 5.0V output 10Apk 12V output 10Apk 20V output 10Apk 30V output Same as 4150 question - are there any max load capacitance spec of the 4139? I see no notes. Per note 11, I don't see the article "Examples of Determining Extended Range Pulse Parameters and Optimizing Slew Rate using NI SourceAdapt" when I search for it on ni.com or on another search engine. Maybe this is now SourceAdapt Custom Transient Response Ranges ? In any case, this post PXIe-4139 Output Stage Shunt Resistance from Jared R. in 2016 does a great job of identifying the real resistance involved in these kinds of power questions. I simply want to raise awareness I can't find the article shown in the specs. Thanks, Andrew

i. Regarding graphs with more detail, sorry. The graphs that are there are just to give a rough idea of what's possible, with the intention that a little math is required if you want more detail. ii. "Pulse On-Time vs Pulse Current and Pulse Voltage" is in fact the title of the very graph that you wish had more detail. It's not an article. It's supposed to give a quick idea of the range of values to expect, and which equation in the spec to focus on first. iii. Regarding simulation, I thought maybe you already had NI software installed. If not, it may not be worth it. You'd need to install our actual driver to test what its limits were. My point is that you can do that without having the hardware. iv. The drawing helps a lot, as does knowing your DUT doesn't turn on till at least 20V. What I still don't get is what the advantage is of breaking things into two tests. What if you just set your 4139 to source 30V with a 1A limit? It would charge your 10-30 mF in a second or less, and you could determine the actual C from the dV/dt. You could then run all the other tests once the SMU leaves current limit, meaning the C is charged. Is the issue that you want the test to go faster than that? Anyway, on to the 7 questions: 4150: 1. Answered. 99% sure an external supply is required. 2. I suspect the 4150 behaves like our SMUs in that it will current limit at whatever limit you specify, which has to be within the selected range. It may be possible to go a few % above max range, but not beyond that. You will definitely not hit the specified <5msec voltage settling time to 30 V with a 20mF load. ... And I'm just now recalling that you may in fact be planning to switch among multiple DUTs, presumably with the power source already set at the desired voltage, e.g. 30V. In that case, you will get some extra current, but any overrange runs the risk of the power supply or SMU self-protecting. I'll address this use-case after I get through the rest of the questions. 3. There are no capacitive load limits. 4139: 4. 10A is the maximum current the device can source and/or measure. 5. The driver will limit your extended range pulse according to the equations in the specs. 6. There are no capacitive load limits. 7. Clicking on the "Examples of Determining Extended Range Pulse Parameters and Optimizing Slew Rate using NI SourceAdapt" link in the specs just takes me to a section further down in the spec sheet that gives examples of how to use the graphs and equations we've already discussed. Back to the DUT-switching issue. An SMU (or power supply) will limit its current as it changes from one voltage to another. So starting at 0V and moving to 30V, the SMU will never exceed its programmed current limit, e.g. 1A. That may mean that it takes a while for the output to get to its setpoint, if the load is capacitive. Or it may mean it never gets there, if the load is sufficiently heavy, e.g. a short circuit. If you want to switch between DUTs, the safest thing to do would be to return the SMU voltage to zero between DUTs. If you leave the SMU at 30V and then switch to another DUT that is sitting at 0V, there will be a current spike that will likely exceed the SMU current range. This should be fine, as the SMU should immediately lower its voltage to limit the current, but that does take time, and so there's a big current spike. As you stated, this would be hard on any relays involved, and it means there's some period of time where you don't know exactly how much charge/energy you've dumped into your DUT, since you exceeded the SMU measurement range. There's also a small risk that if for some reason the SMU is not able to bring its current under control quickly enough, it will trip an error to protect itself. So switching at 0V is just an easy way to reduce the likelihood of hot-switching any relays or tripping any errors. If you really want to charge up your 20mF DUTs as quickly as possible, you could stick a 100mF cap on the output of the 4139. That would give your DUTs lots of inrush current, destroying any electromechanical relays and forcing you to your FET solution. The 4139 would be isolated from that inrush current, although it would have to recharge the 100mF, which would have been discharged by 20% or so. That recovery would take just as long as charging the 20mF would originally (same amount of charge to deliver), but maybe other tests could be happening during that time.

Digital Multimeters (DMMs) and Precision DC Sources

PXIe-4150 vs. PXIe-4139 Pre-Sales Question, Testing Inrush Current of High Capacitance (10mF...30mF) Loads.

Solved!

Author

Good point, thank you!

Message 11 of 11

(142 Views)

Digital Multimeters (DMMs) and Precision DC Sources

PXIe-4150 vs. PXIe-4139 Pre-Sales Question, Testing Inrush Current of High Capacitance (10mF...30mF) Loads.

Re: PXIe-4150 vs. PXIe-4139 Pre-Sales Question, Testing Inrush Current of High Capacitance (10mF...30mF) Loads.