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Investigation into the EMC emission from Switched Mode Power Supplies (SMPSs) & similar Switched Electronic Load Controllers (SELCs) under different load conditions

A report for OFCOM by York EMC Services Ltd, University of York

Author: Dr D Bozec


A study was undertaken to consider the EMC emission from Switched Mode Power Supplies (SMPSs) and similar Switched Electronic Load Controller (SELC) devices tested under different load conditions.

Various products based on the SMPS/SELC technology were purchased for this investigation. Specific loads were designed for each of the equipment under tests (EUTs) to enable the EMC testing to be performed for the 25%, 50% and 100% load condition. EMC testing was performed on three items (two based on SMPS technology and one on SELC technology) for harmonic emission, conducted emission and radiated magnetic and electric field emissions.

Results of the testing showed that substantial levels of emissions were measured in the harmonic and conducted emission tests. Few emissions were measured for the radiated type emission (magnetic and electric). In the case of SMPS based devices, the 100% load condition was found to be the most appropriate load to measure the maximum emission, whereas in the case of the SELC based devices, lower load values (25% and 50%) were found to lead to higher emission levels.

Numerical modelling was performed on the three items to determine the feasibility of using SPICE models to predict harmonic emission and conducted interference between 9kHz and 30MHz. Comparison with harmonic test results showed that it is possible to accurately model the amount of harmonic emission in the case of both SMPS and SELC based devices.

The modelling of the conducted emission was found to be more difficult however, overall, the general emission trend was predicted but only with poor accuracy

A Code of Practice was written as part of this investigation for the design SMPSs and SELCs giving guidance on how to design such devices to ensure good EMC characteristics. The Code of Practice is aimed at engineers who have experience of general electronic design but need to gain knowledge of EMI problems and solutions peculiar to SMPS/SELC.


Format Document
PDF Main report (2,234 kB)
PDF Appendix A - Interim Report (1,020 kB)
PDF Appendix B - Test Plan (474 kB)
PDF Appendix C - Load Guidelines Booklet (251 kB)
PDF Appendix D - Circuit Diagrams (516 kB)
PDF Appendix E - Test Report - Printer SMPS (1,189 kB)
PDF Appendix F - Test Report - Plug In SMPS (1,412 kB)
PDF Appendix G - Test Report - One Way Rotary Dimmer (SELC) (943 kB)
PDF Appendix H - Modelling Report (4,829 kB)
PDF Appendix I - Code of Practice (468 kB)

Last updated: 2004-May-19