Conducted emissions (EMI) dependence on operating temperature? - Page 1
Forum section: electronics / projects

author: TimNJ
author_rank: Super Contributor
timestamp: February 19, 2018, 03:48:48 pm
content: Hi all,I usually don't post about work on here, but I am working on reducing conducted emissions for an off-line switchmode power supply, and am struggling to see the correlation between EMI and temperature (and possibly how to improve it).This is a 60W flyback converter without power factor correction. If you take a look at the attachments, I ran a cold EMI pre-compliance test, and then a hot test, after the unit had been running for about 30 mins. Especially at the lower frequencies, some of the spikes increase 7-10dB after the unit gets hot. From a theoretical and/or practical perspective, what could be the reasons for this?Additionally, when cold the unit is dead silent, but when hot, I hear some coil whirring/whining/whistling sound. I feel that perhaps this is correlated to the reduced EMI performance and increased temperature but not sure how or why either.Anyone have any thoughts on this?Thanks!

author: Tomorokoshi
author_rank: Super Contributor
timestamp: February 19, 2018, 06:12:40 pm
content: Look into if the transformer is a bit under-rated. If the transformer is going into saturation you may be getting that sound. Peak currents may be more than you expect.Check the capacitors against the datasheet or reference design. Not sure how the circuit will behave if input or output capacitance is low at the operating frequency. Actually, because it's switching you will need it at a good 5x the switching frequency.What is the nominal switching frequency compared to the reference design? Is it that spike around 500 kHz?Are tracks between the switcher and the transformer nice and short, or is there some inadvertent large loop?

author: Wolfram
author_rank: Frequent Contributor
timestamp: February 19, 2018, 06:19:03 pm
content: Does the unit have ceramic Y capacitors in the common mode filter? Some of these can have a pretty steep temperature coefficient.

author: TimNJ
author_rank: Super Contributor
timestamp: February 19, 2018, 08:01:23 pm
content: Quote from: Tomorokoshi on February 19, 2018, 06:12:40 pmThermal tests seem to show the transformer is running decently cool, so I think it probably sized correctly. Also, it is a prototype transformer modified in house, so it may just be a little sloppy and without proper varnish etc.Bulk cap and output caps are exactly in line with the app note and switching frequency is around 85kHz under this load condition. It is a quasi-resonant flyback with frequency fold-back. I have done very little of the actual design work on this project, but I believe it to be configured and operating quite similar to app note circuit. Of course, app note is not infallible.Loop area actually looks kind of big, now that I look at it. Probably about 1" from bulk cap to transformer, and 1" from transformer to MOSFET. That said, based on the board footprint, I'm not sure how it could be made significantly better. I will bring it up to the lead designer on this.Thanks so much for all of those ideas!Quote from: Wolfram on February 19, 2018, 06:19:03 pmOn the AC input side of things, there is no CM filter with Y caps, just a single DM X cap from line to line.However, we are using 2x Y1 caps to bridge the isolation barrier between primary and secondary commons. Looking at the datasheet for that cap, I see that it is a Y5V, which is basically the worst temperature coefficient you can get at +30/-80%. That -80% number seems really troublesome. Perhaps a Y5U (+20/-55%) would be advantageous.Thank you!

author: floobydust
author_rank: Super Contributor
timestamp: February 19, 2018, 08:34:25 pm
content: If you can hear whining when the supply is hot, then I wonder if it is staying stable, the control loop. That would be the first thing I'd verify.The TL431, opto etc. may have marginal stability and loop gain drops with heat. Transformer, rectifier diodes, opto-coupler all contribute to sagging gain.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: February 19, 2018, 08:49:21 pm
content: Quote from: floobydust on February 19, 2018, 08:34:25 pmThat reminds me, check any Schottky diodes against operating temperature in the hot environment and reverse voltage.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 19, 2018, 08:56:58 pm
content: Diode recovery is noticeably worse at high temperatures. Even if you don't have PN diodes in the switching circuit (schottky diodes' AC behavior is more or less independent of temperature unless driven with high peak currents), the mains FWB could be transmitting more trash (while reverse biased) or making more of its own (due to snappier recovery even at just 60Hz).I wouldn't expect control loop to affect switching (unless it's driving into CCM when designed to be DCM), but whining is definitely a sign that something needs to be fixed there.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: February 20, 2018, 03:03:17 pm
content: Quote from: floobydust on February 19, 2018, 08:34:25 pmWas thinking the same exact thing. I checked the current sense node on the primary side (when hot) and didn't see any oscillation. I haven't checked directly on the feedback node, but usually, if there's oscillation, you can see it superimposed on the current sense signal.Thanks a lot.Quote from: T3sl4co1l on February 19, 2018, 08:56:58 pmThanks. As I understand it, at the zero crossing, the "snappy" behavior of a diode produces a spread of RF energy that inevitably finds its way back onto the line. At higher temperatures, the reverse recovery parameters also increase, resulting in greater recovery currents and more ringing/overshoot. I suppose a solution to this would be some sort of RC snubber. In this application, it is probably cost prohibitive to add. But thanks for bringing it to my (and anyone else reading's) attention.