Mains on PCB - Page 1
Forum section: electronics / projects

author: tszaboo
author_rank: Super Contributor
timestamp: September 15, 2015, 11:10:53 am
content: I would like to talk about a topic which doesn't happen often on the EEVBLOG Forum. I'm a design engineer with dozens of (low voltage) designs, so usually I know what I'm doing. This time, please treat me like a beginner. I would like to boldly go into a new territory (for me at least):The mains input power.Does anyone know a good guide how to deal with it? Checking the required isolation distances is easy. Filtering is not that straightforward, what kind of mains filter to use when there is a say 30W SMPS onboard? Is there any EMC guide that I can use? How are you making the boards development friendly, when you only have bare boards and bodge wires on the boards? Are there any good practices that I can follow?I'm thinking first I would try with PCB mounted IEC inputs, and board mounted encapsulated power supplies.Please share your experiences and any documentation that you follow when doing these designs!

author: con-f-use
author_rank: Unknown
timestamp: September 15, 2015, 01:20:19 pm
content: (Can't contribute more, just very interested in this.)

author: Throy
author_rank: Regular Contributor
timestamp: September 16, 2015, 06:56:20 am
content: I would suggest using a line filter. Würth Elektronik has an application note that might be of some use to you.http://www.we-online.com/web/en/electronic_components/produkte_pb/application_notes/netzfilter___die_letzte_huerde_im_schaltnetzteil.phpAs far as EMC, it all depends on what standard you're designing to. In there, it will state what requirements you need to meet.https://en.wikipedia.org/wiki/List_of_EMC_directives

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: September 16, 2015, 08:54:20 am
content: While most board i designed are main powered, i can't say am an expert on this. What i usealy do is to check what SMPS controller chip manufacturer offer in their application note. some of those manufacturer like TI and Power int. do EMC test for their development board or design example. So i just use what they use. 30W is not much and for sure you can find something close to that in those AN.Quote from: NANDBlog on September 15, 2015, 11:10:53 amyap i have one ... don't do it that way.bare board and bodge wire on main are not good practice, any thing can go wrong and it's life threatening thing to do. i was electrocuted 100 time for doing this. use an insulation transformer , if the HV part is only an SMPS then power it up to test if supply work and giving the right voltage. then power the LV with a lab ps with current limiting to match your on board ps capability. So if your lab ps is over loaded by your design you will notice that.i fix bodge wire with glue so they don't move any more, so i don't have a risk that one of them touch somehow the HV or any other part of the circuit and ruin my day.

author: tszaboo
author_rank: Super Contributor
timestamp: September 16, 2015, 12:05:00 pm
content: Quote from: Throy on September 16, 2015, 06:56:20 amThank you! Exacly the type of information I'm looking for.Quote from: hamdi.tn on September 16, 2015, 08:54:20 amI'm not going to design SMPS working from the mains, that is not cost effective for the numbers being made.For example: I want to make our supporting equipment more compact. Now our calibrator system used by our service engineers (flying around the globe) has a eurostyle power input, bring your own 24V power supply. Now this is often times an issue. By putting a AC/DC from Tracopower (for example) to the board, they only need an IEC cable, or a Mickey mouse cable to make it work.Also, If I let the other departments do this, we end up with bulky power supplies, internal cabling, mounting, and so on...This is just one example.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 16, 2015, 12:29:20 pm
content: I could sell you a couple of these boards and bobbins and cores perhaps,Works pretty well up to 100W or so. UC3842 based flyback, with assembly options for different snubbing, EMC filtering, input/output voltages, etc. Off the shelf parts.Experience so far has been, low voltages are easier than high voltages. The capacitance between primary and secondary, times the voltages on the windings, is magnified for high voltages. I've got a 300V unit that throws out a mess in the 50-80MHz range, with harmonics beyond 100MHz. The 6.3V model is quiet as can be with a few strategically placed Y-type caps.Tim

author: tszaboo
author_rank: Super Contributor
timestamp: September 16, 2015, 12:50:14 pm
content: Quote from: T3sl4co1l on September 16, 2015, 12:29:20 pmI could order any range of TDK, Traco, Meanwell or pre-made power supply, but thank you for the offer.On the other hand, I would very much like to hear about the practices you used when designing this.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 16, 2015, 01:22:18 pm
content: Ah, but those power supplies are already complete -- you don't learn the design and construction. Studying existing designs is informative, but it doesn't tell you about the space in which those choices were made, how much tolerance some parts have or what was a late-revision hack job..Testing EMC is kind of tricky with e.g. just a scope though. You can see spikes and ringing, and you can build some of your own equipment without too much trouble (a ground plane setup, LISN, near field probes), but you won't really get a fully quantitative idea of the frequency spikes and harmonics unless you have a proper spectrum analyzer (ideally one with the appropriate average and quasi-peak detectors).As for guides, check around. There are appnotes (heh, mediocre though they usually are), guides, books... Sadly I haven't checked out any personally, but there are some classics others can recommend. The best advice I can give is, play with RLC filters and get experience with how they interact, and build a mental model of how fields work, transmission lines and impedance and stuff.Tim

author: tszaboo
author_rank: Super Contributor
timestamp: September 22, 2015, 12:10:42 pm
content: Quote from: T3sl4co1l on September 16, 2015, 01:22:18 pmI'm not really interested of the design of a SMPS. It is not cost effective to build it yourself unless you make a lot of it, or you have very special requirements. I mean there is nothing wrong with using a pre-made power supply module, there is no need to reinvent it every time. I would need t ocreate all the components in the ERP system, Altium, test it much more thoroughly then a module.