Conducted EMI

The purpose of an input conducted EMI filter is to keep the high-frequency conducted noise inside the case. The main noise source is the switching power supply. Filtering on any of the input/output (I/O) lines is also important to keep noise from any internal circuit, like microprocessors, inside the case. 

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Figure 2-6 Harmonics of a Current Pedestal (with the CISPR22 Conducted EMI Compliance Region of 150kHz to 30MHz Highlighted)...

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Finally, we observe that standard conducted EMI emission limits are typically only up to 30 MHz. We can ask — why weren t the limits set even higher The reason is that by 30 MHz, any conducted noise is expected to automatically suffer severe attenuation in the mains wiring, and therefore won t really be able to travel far enough to cause interference much down the road. However, since the cables can certainly still radiate (locally), typical EMI radiation limits cover the range from 30 MHz to 1 GHz. 

Note This technique does nothing to cancel the noise injected through the interface capacitance (i.e. between the primary to secondary windings). But despite that limitation, a 10 dBpV reduction in conducted EMI is still possible (at various points in the EMI spectrum). So this could certainly be worth trying out, if there is a last-minute problem and a major redesign of the board needs to be avoided. 

Note A quick diagnostic test for understanding a particular high-frequency conducted EMI problem (measured by a LISN at the inputs of the power supply) is to twist the output cables of the power supply tightly together (along with their respective return wires). This induces field cancellation (also called flux containment), thus reducing the radiation from the output cables (if present). 

Basso, C. Conducted EMI Filter Design for the NCP1200 , On Semiconductor Application Note AND8032/D... 

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EMI signal transduction pathways can be divided in accordance with the common mode and differential mode signals. Noise conduction pathways three-level inverter system are shown in Figure 5. There are three kinds of conducted EMI signal sources in the speed control system that can affect the control circuit, just as follows. 

Guo Pan-feng, Tan Guo-jun, Zhao Yan-ping, Zhang Xiao, Discussing Conductive EMI Suppression Technology for Switching Power Supply, Journal of Microwaves, 2010. 

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