Offline Flyback..shorting out the ferrite beads gives what EMC standard fail? - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: March 28, 2021, 08:05:03 pm
content: Hi,The attached (PDF schem and LTspice sim) is a basic version of an Offline 120W flyback schematic that has been prototype designed by a consultancy (VAC input = 190-270VAC).You may assume it has passed EMC radiated and conducted.The three Ferrite beads shown have gone obselete.Supposing they were all replaced by shorting wire links, then what standard, at what frequency, would you expect to see an EMC failure?Ferrite bead now obselete BL02RN2R1M2B...https://www.murata.com/en-eu/products/productdetail?partno=BL02RN2R1M2%23

author: srb1954
author_rank: Super Contributor
timestamp: March 29, 2021, 07:15:47 am
content: Quote from: treez on March 28, 2021, 08:05:03 pmNot really enough information to determine the effect of the change.Did you get a test report from an accredited EMC test lab?Knowing how close the design was to the limits would give a better chance of predicting whether it fails without the beads.

author: jonpaul
author_rank: Super Contributor
timestamp: March 29, 2021, 12:33:43 pm
content: Hello: If the consultant was competent then there was good reason for the beads. In general beads are low cost and most effective for DM noise in bands 5-300 MHz.Necessary for conducted EMI at output, CM and DM noise, due to stray capacitances, Series L of shunt C, shield effectiveness, etc.Still Impossible to respond further to your note without:1/ Compliance requirement and market area of product eg EU/CISPR/IEC regs, USA FCC etc.2/ reason to remove beads instead of finding a substitue for the Ferrite bead now obsolete BL02RN2R1M2B...Suggest to Ask the original designer, or a Linear Tech forum or app note author for the IC.Bon Chance\Jon

author: ocset
author_rank: Super Contributor
timestamp: March 30, 2021, 04:51:16 am
content: Thanks, the skin depth at 5MHz is 9um, so generally 5MHz sees really high resistance in tracks and wires. As such, in region 5-300MHz, would you not say that disturbances are virtually exclusively common mode?As you know, common mode basically comprises the wave nature of disturbances, as noise radiates out of the circuit.

author: jonpaul
author_rank: Super Contributor
timestamp: March 30, 2021, 10:03:49 am
content: Rebonjour Treez:...the skin depth at 5MHz is 9um, ... 5MHz sees really high resistance in tracks and wires.Please mention switch frequency, is it 5 MHz?.. in region 5-300MHz, ... disturbances are virtually exclusively common mode....basically comprises the wave nature of disturbances, as noise radiates out of the circuit.Sorry I do not understand your notes. The skin depth has little relationship to the emission of conducted noise CM vs DM.Any SMPS inherently generates both CM and DM noise due to the flow of capacitive displacement currents which impinge on the various conductors.The balance of CM and DM is dependent on many factors but both are usually present in intensity that surpression and design measures are needed to achieve compliance.Also, unbalance in impedances will cause CM noise to DM noise conversion.I suggest to check some of the excellent textbooks and papers on CM and DM noise generation mechanisms and calculations.We always had to consider both in out designs, and place BOTH CM and DM chokes and beads where necessary.Normally, filters are required at both input and output ports of the finished SMPS.Notice that a CM choke can be designed with substantial DM inductance, so to serve both functions.I hope this will be useful.Bon Chance!Jon