Electronics > Power/Renewable Energy/EV's

Help conducting a proper ripple test for DC power supply

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Syllith:
Our company recently purchased thousands of counterfeit power supplies by accident. The client we supply them to is scared that if the DC signal it's providing isn't "clean", it will damage the batteries for their backup system. Because of this, they're wanting me to conduct a ripple test to see how much AC voltage is on the line of the counterfeit compared to a legitimate one.

To set this up, I took my oscilloscope and set it to AC coupling and set the probe to x1. I have the scope set to 200mv per division at 10uS. I also enabled bandwidth limited, although it doesn't let me choose the range and there's no indicator. Hopefully it's 20mhz? The power supply we're testing outputs 54VDC and can handle 2000w. We testing with a max load of 33 amps. We have a large load bank that can has a decent range of amperage we can pull. It has an 54 volt input, negative input, and a ground. I have attached my probe to the +54 terminal and the ground of the probe to the ground of the load bank.

After testing a few known good units, I was getting about 30mv RMS of AC. With all the counterfeit units, I was getting 130mv RMS AC.

Does it sound like I'm performing this test correctly? Does 30mv RMS of ripple sound right? How bad is 130mv in comparison? Would 130mv even be capable of destroying a huge 48 volt battery? I'm a bit new to performing this kind of test, so I'm not exactly sure what I'm looking for and google hasn't been helping much as far as specific values I should be expecting. Any help would be greatly appreciated.

tunk:
Are these any help:
https://www.youtu.be/OiAmER1OJh4
https://www.youtu.be/Edel3eduRj4
Could also be interesting to look at the peak values.

jwet:
Fundamentally you're just looking for the AC content on the output so your test is fine.  A 1x or 10x scope probe is fine, you might double check voltage ratings but good probes are usually fine to over 100v.  Ripple is usually specified with a 20 MHz bandwidth, one of the reasons scopes have a BW limit at 20 MHz.  You probably want to load the supply at different DC loads, ripple is usually worst at full load but not always- switchers can do funny things at light loads.  Sweep it over the range. Also, if you can, look at the frequency content with a spectrum analyzer or FFT mode on a scope.  100 mV vs. 30 mV on 56v DC doesn't sound like much or sound bad but it all has to do with the next guy in the chain and what his needs are- someone specified the allowable ripple input for a reason.  The acronym EMC is electromagnetic "compatibility".  The system is designed to let systems operate with one another.  I used to do military shipboard stuff- I could really appreciate the problem of putting all this electronics right next to each other on a big dirty ship- Mil Std 461 seems like a PIA but it what let's systems work.

Syllith:
Ok, so I tested 5 good units and 3 bad units. The internal components of each unit is nearly identical. Only thing I really notice is the genuine one is using some higher quality jianghai caps and the ingenuine one is using these cheaper looking ones from "RUC"? Not sure, googled for 20 minutes and couldn't find a single mention of them. Could this be the cause for the higher ripple voltage?

ejeffrey:
Definitely.  An output capacitor that is too small or too high ESL/ESR could easily increase ripple voltage.

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