Common Mode Voltage Evaluation for choosing Quiet MCU and Optimizing PCB Design: Electromagnetic Emissions Measurement for Integrated Circuits

Abstract

Similar to the fact that the differential signaling interfaces radiate less amount than the typical single ended interfaces, radiation from wire harness connected to electronic control units using single chip micro computers can be reduced by adjusting the common mode voltage fluctuation of supply lines. The radiation from wire harness can be minimized when the Vcc and the Vss fluctuate with same magnitude in opposite phase. As estimating these fluctuations of real products using lots of components on complex PCB layout by simulation technique is quite difficult at this moment, alternative solution to investigate the common mode voltage fluctuation has been investigated. By the Workbench Faraday cage method and advanced usage of it with 180deg four ports hybrid balun, common mode voltage fluctuations of Vcc and Vss with phase information can be monitored. Examples of this measured data are discussed and a way of reducing common mode radiation from wire harness is explained.

Investigation of the Workbench Faraday Cage Method, IEC 61967-5

“…Although it is an official IEC standard, there are only a few scientific publications with theoretical-or measurement validation of the method. In [4] the common mode current caused by Vcc/Vss voltage fluctuations from a microcomputer was investigated by the WBFC method. At some frequencies there was a good correlation between WBFC measurements and 3 m SAC measurements while other frequencies showed large differences.…”