cheap near field probe and EMC analyser for radiated emissions checking of SMPS? - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: February 04, 2021, 09:51:39 pm
content: Supposing i change the value of a Y capacitor in a offline flyback SMPS.I then want to see if the official EN55032 radiated emissions scan would bet better or worse..but dont (yet) want the expense of a radiated emissions scan.Can i do the comparitive scan with a near field probe and eg a Laplace instruments SA1002A EMC analyser? (We have one of these in house and so its cheaper)SA1002A EMC analyserhttps://www.laplace.co.uk/wp-content/uploads/2019/10/SA_1002-1.pdf

author: Andreas
author_rank: Super Contributor
timestamp: February 05, 2021, 10:17:36 pm
content: My opinion:A near field probe is only a tool to localize the maximum emission.(At a SMPS you should know the root cause).For radiated emission a comparison is very difficult because you never meet the same distance.(and since the distance has a r-square or 3rd order influence in near field).So I would rather invest into a (wideband) LISN and a limiter for the spectrum analyser.with best regardsAndreas

author: ocset
author_rank: Super Contributor
timestamp: February 06, 2021, 10:15:18 am
content: QuoteThanks, you mean do conducted emissions instead?.......at the problem frequencies of 50MHz to 200MHz?..kind of see your point because ultimately, the radiated emissions come from the input wires of the SMPS's inside the system?We appreciate we can't see if it passes or fails with our near field probe, but surely we can tell that if we do a modification to the components (eg Y capacitor values) , that the radiated emissions get better or worse?

author: Andreas
author_rank: Super Contributor
timestamp: February 06, 2021, 10:35:12 am
content: Quote from: treez on February 06, 2021, 10:15:18 amThats new information.But on the other side:For optimum radiation (resonance) you need a "antenna geometry" of at least Lambda/4 which you usually do not have within your SMPS but shurely at your power chord which acts as a antenna.So usually most of the energy leaves the cirquit as common mode conducted emission before it gets radiated from a antenna.And playing with Y capacitors is the typical usecase to reduce conducted emissions if you are doing it right.(shorting the conducted emissions to the metal housing/heat sink of the SMPS to reduce the current loop area of parasitic capacitances).And there are some LISN devices up to at least 80-230 MHz for the automotive and aerospace use.https://www.eevblog.com/forum/projects/5uh-aerospace-lisn-how-dumb-would-i-be-to-throw-one-together/msg3154038/#msg3154038with best regardsAndreas

author: ocset
author_rank: Super Contributor
timestamp: February 07, 2021, 01:32:41 am
content: Thanks, thats great for doing a conducted scan at up to 200MHz.In the meantime , could anyone assist with getting us through Radiated EMC in this charity system?Please help our kiddies post-lockdown pager system game kit pass radiated EMC? I am doing this with a company in South Africa for charity. Its as attached.It is an earthed metal enclosure A, which contains a 20W offline SMPS and a 12V battery. This enclosure feeds a 12V cable to enclosure B which contains a 10W , 12V to 5V SMPS. This 5V is then fed to each of four removable pagers, (so they get charged up) which the kiddies can remove and talk to each other. (they will be in a “hospitals and doctors” game)It fails EMC at the moment. (Radiated emissions failure at frequencies from 80MHz to 190MHz) We wish to sell it into the EU, where they need some post-Covid relief as much as everyone else.Should the cable between A and B be earth screened and the earth screen connected to both enclosure A and B?The enclosure B is open at the front, so is it therefore a waste of time to earth enclosure B? As such, is it also a waste of time to take earthed 12V cable from A to B?The wires running from the 12v_to_5v SMPS to each pager, should they be loomed as close as possible to the walls of enclosure B?The enclosure B has to be open at the front so that kiddies can grab the pagers.We are thinking we may need a common mode choke at the point in enclosure A where the 12V cable leaves A to go to B? Also, we could put a common mode choke at the input and output of the 12v_to_5v SMPS. (NiZn torroid based common mode chokes with just a few turns) We could also put a flux band round the offline flyback transformer in A. We could also increase the Y capacitor across the flyback transformer in A. At the moment its only 100pF. We don’t want PCB respins if poss as the customers don’t want to pay much.

author: ocset
author_rank: Super Contributor
timestamp: February 10, 2021, 07:01:25 am
content: QuoteThanks, these look really good, but unfortunately we cant get our system inside one of these, as it is too big.(its the 'kiddy's panel' in the post above this one)

author: fcb
author_rank: Super Contributor
timestamp: February 10, 2021, 10:52:23 am
content: How much does it fail by? Can you post the sweeps.Is the noise from the mains SMPS, the 12-5 or the pagers? Have you done tests to isolate the source?Have you "hit the ferrites" yet? Is the test house in the UK, SA or somewhere else?For noise in region you are mentioning, I doubt tweaking the X/Y caps at the front will solve it. Perhaps ringing around the snubber across the flyback primary?

author: nctnico
author_rank: Super Contributor
timestamp: February 10, 2021, 11:54:27 am
content: Radiated emissions in that frequency range can point to the rectifier diode. A diode with a softer recovery may help as well, as slowing down the switching MOSFET or putting a ferrite bead in series with the diode. Using a H field probe (Google RF explorer for an affordable set) you can pinpoint the component or trace through which current flows with the frequency content. Once you found the component, you know where to make a change in the circuit.Testing can be done by using a HF current transformer on the wires and a spectrum analyser. You can DIY HF current transformers from a clamp-on ferrite bead:https://interferencetechnology.com/the-hf-current-probe-theory-and-application/That way you can check whether the emissions come from the mains cable or the 12V cable. And then you'll need to figure out which one of the 2 boxes is causing the problem.You'll have to make relative measurements though. Take a baseline measurement without changes and then what effect a modification has.

author: Andreas
author_rank: Super Contributor
timestamp: February 10, 2021, 09:23:38 pm
content: Hello,sorry that I have to say this:You have chosen the most bad combination of choices.- Power supplies for toys: there are special requirements/regulations according to earth currents and isolation.so I doubt that you can do much with Y-capacitors since the 100 pF from above might be already at the limit.(but I am no expert at this).- 2-3 possible sources of EMI in 2 different housings is a bad/costly concept EMI-wise.But I don't know wether you did this for isolation purposes.Pin-pointing is essential in this case.Is it the mains SMPS or the 12 to 5V converter?Does the mains line radiate or the low voltage side?If it is on the low voltage connection:Wether a shielding helps or not is in the most cases a question of correct connection of the shield to the two housings.A "pigtail" connection will not help at these frequencies. A correct connection will need special connectors which support shielded cables.But be aware that earth currents from the Y-capacitors are now also on the low voltage housing if the earth connection is broken.So 2 common mode chokes (one on each side) could be cheaper than a shielded connection.It should be possible to place some "Y" capacitors (not necessarily Y class) on the low voltage line to the isolated housing without increasing earth currents.On the SMPS housing side I do not know wether it spoils the isolation.If the open front side should be a issue (what I doubt, except if the low voltage line is very short) you probably could place a metal shield connected to the housing between the DC/DC and the charging connector for the pagers.with best regardsAndreas

author: jonpaul
author_rank: Super Contributor
timestamp: February 11, 2021, 12:13:10 pm
content: You need to distinguish between conducted and radiated interference.The X and Y caps affect the conducted not radiated. A CM choke may be in order.The EMC near field probe will pickup radiated, not conducted.use a LISN for conducted.Beware that the line to ground leakage current is proportional to the Y capacitance and that the 4700 pF caps are the highest value to meet the normal leakage current specs at 120V.Changing the Y caps incurs a safety aspect.Here is a note on conducted EMI capshttp://www.minuszerodegrees.net/line_supression/evoxrifa_rfi_fact.pdfBon chanceJon

author: ocset
author_rank: Super Contributor
timestamp: February 14, 2021, 06:36:52 pm
content: It is well known that passing radiated emissions to EU/USA standard with an SMPS that is not surrounded in a metal enclosure is pretty well impossible. Even if it is in a metal enclosure, it will still fail radiated emissions if a cable leaves the enclosure to go somewhere else. (as in the attached). In that case, a common mode choke and Y caps will be needed in order to stop the cable from radiating. Would you agree?In the attached diagram, both SMPS's in each enclosure will need common mode choke and Y caps at their input and outputs. You agree?