Dealing with common mode noise on SMPS - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: April 11, 2025, 10:44:24 pm
content: I'm curious if there is any consensus on how best to deal with common mode voltage on the output terminals of typical off the shelf SMPS.Discussion is based on North America residential power system where we have a split phase 120/240V with the neutral tied to Earth typically at the circuit breaker box.For example two recent projects using small Meanwell universal input power supplies:RS15-5https://www.digikey.ca/en/products/detail/mean-well-usa-inc/RS-15-5/7706168andRD35Bhttps://www.digikey.ca/en/products/detail/mean-well-usa-inc/RD-35B/7705983The RS15-5 when powered with 120V, measures about 35V WRT earth on its output terminals. When powered with 240V the common mode drops to < 1VThe RD35B is better, mV when powered with either 120 or 240V.On other generic SMPS it seems quite common to measure about 60V (half the mains voltage when powered from 120V)If we assume whatever circuit is being powered by the power supply is floating, one could argue who cares, so the ground reference just bounces up and down with little to no consequence to circuit performance. However some devices in that circuit may have degraded performance when subjected to common mode voltage beyond a certain threshold, so providing a path to Earth to get the circuit ground close to Earth potential may be beneficial in some cases.So obvious options are:1) solid Earth connection to power supply return / circuit ground2) resistor from Earth connection to power supply return / circuit ground3) capacitor from Earth connection to power supply return / circuit groundIf option 2 or 3 then a suitable resistor or capacitor value needs to be chosen. Thoughts on high R vs. low R values?

author: jonpaul
author_rank: Super Contributor
timestamp: April 11, 2025, 10:56:21 pm
content: Hello you dnt specifyut the freq or waveshape of the "35V" measured.Most SMPS have VDE Y caps L1>E L2E that produce the 1/2 mains voltages if floating test.That is NOT a CM noise .It is to REDUCE the noise as part of the EMI filter.Please include scope shots and actual data of he voltages and schematic of the connectionEnjoyj

author: mtwieg
author_rank: Frequent Contributor
timestamp: April 12, 2025, 11:39:01 am
content: Common mode signals observed on isolated power supplies are typically broken down into two components:1. Leakage currents: This created by the mains voltage. Its frequency content will be at the mains frequency and its harmonics (up to maybe 1kHz at most). Leakage currents are mainly a concern forsafetyreasons.2. Common mode noise: This is created by the switching of the SMPS. Its frequency content is usually very messy, starting at the lowest switching frequency (maybe 20kHz) and possibly extending up into the 100MHz range. Common mode noise is mainly a concern forfunctionalreasons (both for the device powered by the SMPS, and other devices powered by the same mains).Common mode noise is typically improved by adding a Y capacitor across the isolation barrier, and between the mains and earth.However those same Y capacitors will cause Leakage currents to increase proportionally.If you're trying to measure this with a DMM, your measurement is probably only seeing Leakage, not common mode noise. The datasheets for those two power supplies both describe leakage as "<2mA / 240VAC" so it's not clear why your measurements differ greatly between them. If you were to measure current instead of voltage, the measurements might make much more sense.If you don't actually need a floating output, then earthing the output of the supply is certainly one way of dealing with leakage and common mode noise. Though one should give some thought to how reliable that earth connection is, and what happens if it's broken.

author: Jester
author_rank: Frequent Contributor
timestamp: April 12, 2025, 02:18:08 pm
content: Quote from: mtwieg on April 12, 2025, 11:39:01 amThanks for your thoughtful response.Earth connection is good/solid/reliable.For this application the circuit does not need to be isolated from ground. The only advantage of keeping it isolated is that this circuit is in proximity to some fairly noisy higher power switching circuits (VFD and kW servos) and they introduce a certain amount of noise into the local Earth in the same panel. That being said the ~30V I'm measuring WRT Earth at the power supply output when not earthed is probably worse than the noise from the power electronics.Any thoughts on solid bond to Earth vs. Resistor bond to Earth advantages/disadvantages?

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: April 13, 2025, 07:00:48 am
content: There are a lot of dodgy switch mode power supplies around, mostly oddball makes from China. They usually have FCC and CE marking but are not tested to any standard and produce awful interference to any nearby (or not so near) radios.My latest finding was a lithium iron phosphate (LiFePO4) 3A charger that created enough noise to be easily picked up more than 100 foot away! That particular charger had no filtering that I could see on the print board!Sometimes you can improve matters by wrapping the input and output leads several times through a ferrite toroid, typically type 31 material, although sometimes the noise is so bad the only solution is to return the item and buy a proper product. In the case of battery chargers, the CTEK brand is completely RF quiet.Unfortunately there is just so much of this type of junk on the market, that you feel like you are sweeping water uphill...SJ

author: jonpaul
author_rank: Super Contributor
timestamp: May 14, 2025, 05:06:03 am
content: Read the many app notes and books on EMI filters, testing and design.Usually a well designed line filter will solve the issues.Jon