Surge tests - component overvoltages, overcurrents - Page 1
Forum section: electronics / projects

author: mattko
author_rank: Contributor
timestamp: October 16, 2024, 09:10:11 am
content: Hi all,during the surge tests (CWG) on DC power inputs that I regulary perform, I get some of these cases regularly:-ceramic capacitor ~50% overvoltage in duration of ~20us. Example: ceramic capacitor rated for 100 V and receives 150 V for 20 us - during a surge event.-Electrolytic capacitor ~100% overvoltage in duration of ~20us. Example: electrolytic capacitor rated for 100 V and receives 200 V for 20 us - during a surge event.-Electrolytic capacitor reverse voltage - around 40 V negative polarity for ~20us-Electrolytic capacitor high charge and discharge currents. In the order of 400 A for 20 us. ~47 uF is a typical capacitance.-shielded inductor high current - can be in the kiloamps range for ~20us.Is there some resources, articles or some experienced engineers, that can explain in more detail when are these cases acceptable and when they are not. In an ideal design there shouldn't be any of these cases, but then the design would get bigger and more costly. Datasheets rarely mention such parameters and if I ask the manufacturer they simply say not to go over the absolute maximum limits in the datasheet - easy for them to sayWith these surge tests we don't want to just pass the standard, but to be as resiliant as possible, so that the end customers are happy.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 16, 2024, 09:53:42 am
content: Capacitors are generally linear and capable until they're not (breakdown, shorting/burning?), and inductors mainly fail by thermal (melting insulation, fusing).Note that, at such currents and rates, much of the voltage might drop across a capacitor's ESR, so the element itself isn't exactly exposed to as much voltage. That helps the electrolytic.Ceramics, and cored inductors, of course saturate at high level (V, I), and you need to know by how much to properly model the transient. Characteristics don't usually extend this far, though ceramics we can at least assume they're well saturated, as they're already down significantly at rated voltage, lol.Why so high, though? Is this 8/20us, 10/1000, etc. testing? Are you not using a TVS to handle the brunt of it?Beware also, if you're probing under these conditions, use proper Kelvin connections, low inductance, isolated probes or differential sensing, and all that; you probably do, but putting it out there just in case.Tim

author: mattko
author_rank: Contributor
timestamp: October 16, 2024, 12:16:02 pm
content: Thanks Tim. This is 8/20us testing, yes. Up to 5000 V pulses at 2 Ohm output impedance. We have GDTs, varistors and TVS diodes on the input. But for example when I test at "low voltages" -> 1.5kV, the GDT doesn't conduct yet. So all of the current has to go to varistor, which has to raise the voltage a lot to start conducting current. In this case the capacitors are in the overvoltage case. when I invert the polarity it's similar. a unipolar TVS diode conducts current, but the voltage can raise to 40 V due to the high current through it. and electrolytic has negative voltage polarity on it.I'm using a high voltage differential probe and a rogowsky current probe to measure voltage and current. Many times I have to add a loop of wire in series with a inductor to be able to loop through the rogowsky current probe to measure current. But adding some inductance does not really change the circuit as we already have many tens of uH of inductance in the circuit.Quote from: T3sl4co1l on October 16, 2024, 09:53:42 amUsually there is a very fast charge or discharge of the electrolytic. the rate of voltage change on it can be 100 V in 10us, so current is around 450 A in both directions. I don't think this is very healthy for the capacitor's life expectancy.

author: thm_w
author_rank: Super Contributor
timestamp: October 16, 2024, 11:11:59 pm
content: Ceramic can usually handle large overvoltage.https://www.yuden.co.jp/ap/product/support/faq/q016.htmlYou can get a cheap tool like this if you are curious:https://www.eevblog.com/forum/reviews/completely-non-sketchy-mini-high-voltage-transistor-tester-(emeco)/I tested one 25V to over 600V here, but it was a large package 0805 50V or so.If you expect these 5kV pulses to happen regularly, thats a bit of a different story than if its just general use testing, can my device survive.In that case you may want to inspect the components before and after and look for any degradation.

author: coppercone2
author_rank: Super Contributor
timestamp: October 17, 2024, 12:12:46 am
content: I want to know about switches, I studied the normal inrush over current thing, and I see 1 cm spark shoot out of my power supply switch (20A rated) lollittle 160W supply is making sparks shoot out of a 20A switch, I love it.