Solved: Ringing/Radiated Emissions in SMPS, huristic for how much is acceptable? - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: March 10, 2025, 12:24:59 pm
content: I've got a SMPS inverter design that outputs ~24W / 24V at ~100kHz and uses a toroidal inductor.edit: This design is a constant current supply and operates a certain portion of the time in discontinuous mode.The inductor rings at around 700kHz at low output powers. (frequency and amplitude of ringing depends on the amount of snubbing applied)I'd like the design to run as cool as possible (use as little snubbing as possible, and use the fastest switching speeds possible), to use a passive RC snubber network, and to operate at low output power levels.Short of sending it to a test lab, are there any good heuristics for determining how much ringing is acceptable?Something like "set up a SDR / spectrum analyzer and make sure it's emissions at X feet are less than Y times as strong as a local AM radio station"It's a 4 layer PCB with ground plane under all power traces. I've got a roughly 1.5" toroid and a 1.5" square copper pour that flip from +24V to -24V at the switching/ringing frequency that might emit some noise. I heavily filter the power input and output in an attempt to avoid radiating.My thinking is that If a low-current 2V signal (100Mbit ethernet phy, +/-1.0V) can fail at 100MHz due to radiated emissions, then perhaps a high-current 48V signal (+/-24V SMPS) can fail at a few megahertz. (Although, it "feels" unlikely due to the electrical size of the PCB / etc. being much smaller, lack of cabling being intentionally driven at HF)

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 10, 2025, 04:25:42 pm
content: Quote from: incf on March 10, 2025, 12:24:59 pmWhat specifically do you mean by "ring"? You mention it's an issue at low power levels, sounds like you might be describing the ringing seen on DC-DC converters when operating in discontinuous conduction mode (which is very distinct from other types of "ringing").QuoteIf you want realistic estimates of actual emissions levels then at the absolute minimum you need proper antennas (for whatever band(s) you need to cover) with accurate characterization data.If you don't have at least that, there's really no way to avoid a trip to a proper test lab. They will give you the test data (make sure you ask them for the raw spectra, not just the worst case peaks or quasipeak data).Then you take your own setup and measure emissions yourself, using the test lab data as a baseline. Keeping that setup the same (antenna distance, orientation, etc), you have to modify the EUT to reduce those emissions by whatever margin is required by the test lab results. If you do so then you should pass next time at the lab.QuoteEMC issues are usually system-level, and can't be inferred from some general specifications like baud rate or watts or rise/fall times. Subtle changes in how the EUT is grounded or shielded can make huge differences. That's why realistic testing is necessary.Also you should investigate what load/line conditions you will be required to test emissions under. This will depend on the standard being applied, as well as your own risk management.

author: incf
author_rank: Frequent Contributor
timestamp: March 10, 2025, 05:09:25 pm
content: You are right, I was vague.Yes, I'd like to support discontinuous mode operation for my application. See attached*what I call "constant current output mode" with a low output current limit results in discontinuous mode operation---I feel as though if the ringing occurs at ~700kHz, and three local AM radio stations that emits at a known power levels are operating at 600-800kHz, then I might be be able to use the local AM radio stations as a calibration standard for emissions. I only care about order of magnitude type measurements. This approach might permit one to use any random antenna provided that the strength of the calibration source is known and the frequency is "close".FCC publishes "contour maps" for AM radio stations and the 0.5mV/m contour for one suitable AM station passes right overtop of me.

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 10, 2025, 07:14:11 pm
content: Glad I asked then.I've never seen a case where DCM ringing contributed significantly to EMI. Just do an FFT to that waveform and watch how it changes as load current increases. The component at the ringing frequency should be lower than the harmonic components of the switching frequency (so long as there's only one or two ringing cycles per switching period). In theory if the ringing had many cycles, it could dominate the spectrum, but that's a fairly exceptional situation. There's generally no advantage to trying to damp/snub this ringing.

author: incf
author_rank: Frequent Contributor
timestamp: March 10, 2025, 07:42:31 pm
content: That makes sense, the DCM ringing is a single (low) frequency whereas the switching is a smear of higher harmonics. Any of which might be problematic.