Surge Protection (+ EMI Immunity) Design that will pass certification? - Page 1
Forum section: electronics / projects

author: Chendy
author_rank: Contributor
timestamp: September 06, 2019, 11:07:39 am
content: Hello,I'm working on a design for a 110 V AC / 230 V AC Board, that will need to pass all relevant certifications such that I obtain CE and FCC.The board will be used indoors in a standard commercial environment.This mains voltage AC board is functionally a breakout board:- some PCB mount Meanwell PSUs 12V & 5V- some inrush protection (simple NTC device)- some fuses and distribution of the mains to a couple of off-the-shelf battery chargers.(and to the point of my question..)- EMI filter- Surge protectionmy understanding is the EMI filter and Surge protection are likely to be needed for CE / FCC.Beyond potentially power-factor correction, is there anything else that I may need on my AC board?I'm new to certifications and compliance so please feel free to correct me..For the EMI filter, I have seen various application notes using common mode chokes and capacitors. I haven't got he final noise profile of my circuit, but I can imagine a way forwards here. Potentially using a spectrum analyser to monitor conducted emissions.What I'm especially looking for help with in this post is clamping / overvoltage protection in order to pass surge testing requirements:I have seen various designs involving GDT, MOVs, Varistors. Is there a reference design is known to pass testing? Perhaps from application note from a GDT manufacturer?Are these designs somewhat static, once you have once design within price point and passing certification testing, one can potentially re-use? As seemingly independent of the electronics downstream? - variations in over-voltage performance requirements aside.Any help appreciated

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 06, 2019, 02:40:46 pm
content: Which PSUs are you looking at? You may not need additional filtering, or much.You may not need inrush or fusing, either. Depends.The smallest and cheapest modules and bricks may not have much of anything in them (some don't even have a filter cap, e.g. CUI PBO-3-S12); average wired units have filtering and fusing but may or may not have inrush; fancier (usually ITE, medical grade) modules may have all the above (reasonable filtering, inrush protection, fusing on both lines, and an adequate surge rating).Active PFC would need filtering; you would also probably consider unfiltered DC-DC modules in that case; or just go for an integrated (dual output, PFC input) supply instead.Also, don't overlook the option of a single-output supply with a POL DC-DC for the rest. You might supply 12VDC to the main board, and have 5V, 3.3V, etc. buck converters on it. Common-ground DC-DC converters are much easier to filter.Surge is usually handled by the PS itself, in some mixture of absorption (in the NTC + CMC resistance and input cap clamping) and ride through (most flyback controllers disable themselves under high line conditions, allowing the full voltage rating for surge).If you need extended reliability, it may be wise to provide MOVs, or even suitably sized TVSs (they're big bastards, and cost ca. $100 -- advantage is they don't wear out like MOVs do).You may need a GDT + MOV to provide CM surge withstanding, without compromising ground leakage current. That's typical of IEC/UL 60950-1 for example. Otherwise, MOVs are fine.Is this all board level, or will there be e.g. an IEC cord inlet filter option?Also, don't neglect output side filtering. Sometimes a single choke or small CMC is needed. Note that PS modules are always(?) rated for noise/ripple into a 20MHz bandwidth, so it can pay to be conservative about filtering or shielding at higher frequencies. If you have grounding available (a metal enclosure, preferably), just ground the output directly to it, or bypass with caps (RF grounding) to maintain galvanic isolation.Tim

author: max_torque
author_rank: Super Contributor
timestamp: September 07, 2019, 02:11:52 pm
content: I'm designing pretty exactly this circuit at the moment for a 3ph connected product that needs CE / Certification.Because my currents are high (up to 32A) the thing that is currently worrying me is the size of the inductors necessary! Obviously, it's a trade off between inductance and physical size, as the "frequency" is fixed.So far, i've be scratching round various places buy old knackered similar products to take apart to see how they implement the surge and immunity protections......Luckily, my product is expensive, so i think we can afford to go all out on the surge protection, because the cost of the unit is significant (around £20K ) but in any case i think we'll have a few layers of protection, probably GDTs first, then the noise / surge filtering inductance (on 4 lines in my case L1,L2,L3 & N) and then a bank of MOVs. The exact layout is TBC!