Identifying Sources of Conducted Emissions by Measuring the Coherence Function

Conducted emissions (CE) is one of the electromagnetic interference (EMI) issues that pose serious compliance problems for electronic devices. For a system with several sources, estimating the contribution of each source to conducted emission at different frequencies can be a challenge. In this article, a coherence function-based signal separation method is presented and validated on two commercial power supply units (PSU). The noise generated by the PSU and measured at the line impedance stabilization network (LISN) port contains two predominantly uncorrelated signals associated with the power factor correction (PFC) and H -bridge/rectifier circuits with unknown contributions at different frequencies. Two reference signals are obtained by probing the emission signals close to the sources. By calculating the coherence between the LISN noise signal and the reference signals, the contributions of these two signals to the noise are obtained. The measurement of the signal contributions can help engineers identify the dominant sources and mitigate the emissions more efficiently over a wide range of frequencies.

I. Introduction

Conducted emissions (CE) is one of the EMI problems, which brings serious compliance issues to the electronic devices. Conducted noise generated by electronic devices can be transferred to another device through parasitic capacitance, PCB traces, power/ground planes, or cables. Noise coupled to AC cables can affect the operation of other devices and can lead to malfunctions. Therefore, it is necessary to analyze and mitigate the conduced emissions. Many researchers have addressed modeling, simulation and suppression methods for different electrical devices. In [1], a conducted emission model was constructed for a switch-mode power supply by extracting the total voltage source, characterizing the coupling path, and then combining them in a circuit model. The conducted emission mechanism and EMI suppression designs of a DC/DC semi-isolated switched mode power supply is presented in [2]. In [3], modelling and simulation methodologies of conducted emission for electronic control module are demonstrated. In [4], the effect of chassis grounding and n- filters on conducted emissions are investigated. A design of an AC voltage probe for conducted emission measurement was introduced in [5], and in [6], an in-circuit conducted emissions measurement approach without a line impedance stabilization network (LISN) is addressed by extracting the common mode and differential mode impedances.