The importance of EMI model of power electronics in a system and how to build it

With the increasing applications of power electronics in systems such as automotive sectors and smart cities, there are many electromagnetic interference (EMI) issues from these power converters which should be paid more attention. Without appropriate assessment, the conducted emissions (CEs) generated by these devices may highly affect the function of the whole system.

For instance, the CEs could interfere Power Line Communications (PLC) applications which raises the coexistence of power and data lines. Besides the coexistence issue, the aggregated CE from several converters in the system may induce severe problems to other devices, even those converters can pass the standards separately.

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Fig 1. Aggregated CE on DC bus with multiple converters.

Therefore, with the aim to solve the EMI issues (such as the coexistence issues and aggregated CE as mentioned) in a system due to power converters, the EMI characteristics of each power converter should be implemented in the CE analysis of the whole system. Thus we use the EMI model to represent the EMI characteristics of power converter in time-domain or frequency-time to assist the analysis of EMI issues of the system.

It is obvious that the accuracy of EMI model plays an essential role in the solution of EMI issues of a system. Without appropriate and accuracy EMI model in the analysis of a system, the derived solution can even make the issues worse. Therefore, it is quite important to develop an accurate EMI model for power converters.

There are basically two types of EMI model of power electronics: circuit model and behavioral model. The circuit model can represent not only the functional behavioral of the power converter, but also contains sufficient parasitic components to represent the EMI behavioral especially in high frequency range. This kind of model is simulated in time-domain and sometimes it can be really time-consuming and quite difficult to find every parasitic components. However, since this model has representations for each component in the circuit, it is quite useful to analyze the EMI issues due to each components.

On the contrary, the behavioral model is also called black-box model, which is used to represent the frequency-domain behavior of converters. The approach regards the power converter under modelling as a black box, and a sets of measurement are carried out at the output ports of converter for model parameter identification.  

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Fig 2. Two modelling approaches of power converters.

The choice of modelling approach depends on the requirement of the EMI analysis and the final application. For instance, if the EMI analysis is focus on system level, the black-box model is more suitable due to its simplified procedures to build. In other cases, if the prototype of a simple DC/DC converter needs to be improved for EMC standards, it is necessary to adjust the circuit components in the model, therefore, in this situation, the circuit model is more suitable for this purpose.

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