We have been presented with an engineering problem that we are not sure how to solve: we need to be able to measure high-frequency ripple current (up to several MHz) on a DC power rail that can be anywhere from zero to 500 volts, and get that in a form that can be fed into a logging device. The DC component of the current can be up to 100 amperes. Any engineers here who can give some advice?
@Felthry Any specific logging device or are you rolling the entire solution?
@zetasyanthis probably going to use an oscilloscope and possibly also a some sort of dSPACE unit or equivalent
but it's been pointed out to us that we're kind of getting ahead of ourselves here and we need to do the basics before worrying about metrology
@zetasyanthis our target is 30MHz, and it's going to be on top of a DC bias of 100A--that's going to saturate most cores
@Felthry Ah, I hadn't thought about the saturation issue. Hmm.
@zetasyanthis We considered a Rogowski coil, which is essentially just an air-core current transformer, but, well. Air-core.
@Felthry Hmm. What kind of resolution do you need?
@zetasyanthis Our lab (unfortunately) uses Tektronix scopes mostly (we don't like tektronix as much as agilent (no, keysight is a bad name, I'm calling it agilent)), so let me see if they make one of those...
@Felthry I don't see why tek vs. agilent is an issue there? It just outputs a BNC. And there's a whole series, looks like. https://www.keysight.com/en/pc-1659326/oscilloscope-current-probes?cc=US&lc=eng
@Felthry Also, fyi, most oscilloscopes are 8 bit
@zetasyanthis Yeah, I know--I think we have some really expensive sixteen-bit one somewhere though
but also we're interested in lower frequency stuff, which means you can increase the ENOB somewhat with averaging...or something? signal processing stuff is complicated
@zetasyanthis current probes almost always use some proprietary connector on top of the BNC though? I know the current probes we have already can let you know when the jaw is open for example
@Felthry "Compatible with any Keysight oscilloscope with a 1 MΩ input BNC" (Which means any. The picture shows a normal BNC cable.)
@Felthry If you click on the link, you'll see. :P 600A, 30Mhz.
@zetasyanthis Ah, didn't realize I had to enable javascript to see the iamge, that's weird
@Felthry Oh, that kinda is. Either way, I think that does what you want? There's a 300A model too. Not sure on the resolution. You'll have to check.
@zetasyanthis Possibly! The price is a bit ehhhh though--we'll focus on designing the buck converter first and then get around to metrology. Will leave a spot for current measurements.
Fair. Keysight's stuff is pricey, but it's good at least.
I'm also a little curious what this is for. I've designed buck regulators before (and was actually working on a few just the other day), but 100A is quite a bit.
@zetasyanthis you managed to lose the @!
Research on three-level buck converters, specifically circulating currents when using multiple in parallel, as well as how to mitigate problems with neutral point drift.
This is based around DC power transmission systems, as used on some ships. The idea is that this small(!) device will have a 10kW max power output
@zetasyanthis The lab is also heavily focused towards EMC (i.e. compliance with UL, FCC, etc standards), and conducted emissions are legally defined as undesired harmonic content up to 30MHz either on the output or back up the input (or through grounding straps, or...). That's why we're looking at 30MHz bandwidth specifically.
@Felthry AHA, that makes perfect sense!
@Felthry Oh, oops! And okay, that's pretty neat! I've dealt with 12V DC distribution in computer racks before (and am dealing with it now a bit), but nothing on the scale of a ship. (I think these power shelves are about 15kVA each. Unsurprisingly, Delta makes them.)
@zetasyanthis The research is being done on a grant from the office of naval research so of course it'd be about shipboard stuff (not a huge fan of working on a thing for the navy, but none of this is classified research and all of it will apply equally to freight ships so....)
@zetasyanthis Actually, looking into it, it seems like Tek sells the _exact_ same current probes, just with a tektronix logo instead of a keysight one on them
I assume they both buy from some third-party manufacturer and put their own logos on them.
@Felthry I'm not that surprised. Even Lecroy rebrands some of their probes.
@zetasyanthis that rogowski probe you recommended has a minimum detectable current of 250mA, which suggests it's probably got a pretty high noise floor unfortunately. Not sure how good you can get with rogowski coils, though.
@Felthry Ah, dang. They have a few probes with different sensitivities. Might be another can help.
@zetasyanthis I think poor noise performance is kind of baked into the concept of a rogowski coil unfortunately. not having a magnetic core means they don't concentrate flux into the coil, which is what makes cored devices work so well
we need better paramagnetic materials!
@Felthry I mean, there are current transformers too. (I checked digikey, but those go up to about 1 kHz max at the current you're wanting.)
@zetasyanthis Yeah, that's the problem! I'm wondering how hard it would be to roll our own current transformer, at this point, but I don't know the first thing about high frequency magnetics
Some kind of current shunt resistor might be the only option
@Felthry Actually, yeah, that could work. You could A/C couple off the current sense resistor before driving an op-amp with it, potentially. Definitely want some limiting resistors on those lines, too.
@zetasyanthis Not sure. Fairly high; this is for a research application and we want the best metrology we can manage. Twelve or sixteen bits, perhaps? At sixteen, you'll definitely be losing a bit or two to noise (I won't pretend we know the first thing about designing a PCB to mitigate noise)
@Felthry Also re: this, can current transformers not give you high enough bandwidth?