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Power Electronics Electromagnetic Compatibility
The electromagnetic compatibility issues in power electronic systems are essentially the high levels of conducted electromagnetic interference (EMI) noise because of the fast switching actions of the power semiconductor devices. The advent of high-frequency, high-power switching devices resulted in the widespread application of power electronic converters for human productions and livings. The high-power rating and the high-switching frequency of the actions might result in severe conducted EMI. Particularly, with the international and national EMC regulations have become more strictly, modeling and prediction of EMI issues has been an important research topic.
By evaluating different methodologies of conducted EMI modeling and prediction for power converter systems includes the following two primary limitations: 1) Due to different applications, some of the existing EMI modeling methods are only valid for specific applications, which results in inadequate generality. 2) Since most EMI studies are based on the qualitative and simplified quantitative models, modeling accuracy of both magnitude and frequency cannot meet the requirement of the full-span EMI quantification studies, which results in worse accuracy. Supported by National Natural Science Foundation of China under Grant 50421703, this dissertation aims to achieve an accurate prediction and a general methodology. Several works including the EMI mechanisms and the EMI quantification computations are developed for power electronic systems. The main contents and originalities in this research can be summarized as follows.
I. Investigations on General Circuit Models and EMI Coupling Modes
In order to efficiently analyze and design EMI filter, the conducted EMI noise is traditional decoupled to common-mode (CM) and differential-mode (DM) components. This decoupling is based on the assumption that EMI propagation paths have perfectly balanced and time-invariant circuit structures. In a practical case, power converters usually present inevitable unsymmetrical or time-variant characteristics due to the existence of semiconductor switches. So DM and CM components can not be totally decoupled and they can transform to each other. Therefore, the mode transformation led to another new mode of EMI: mixed-mode EMI. In order to understand fundamental mechanisms by which the mixed-mode EMI noise is excited and coupled, this dissertation proposes the general concept of lumped circuit model for representing the EMI noise mechanism for power electronic converters. The effects of unbalanced noise source impedances on EMI mode transformation are analyzed. The mode transformations between CM and DM components are modeled. The fundamental mechanism of the on-intrinsic EMI is first investigated for a switched mode power supply converter. In discontinuous
conduction mode, the DM noise is highly dependent on CM noise because of the unbalanced diode-bridge conduction. It is shown that with the suitable and justified model, many practical filters pertinent to mixed-mode EMI are investigated, and the noise attenuation can also be derived theoretically. These investigations can provide a guideline for full understanding of the EMI mechanism and accuracy modeling in power electronic converters. (Publications: A new technique for modeling and analysis of mixed-mode conducted EMI noise, IEEE Transactions on Power Electronics, 2004; Study of differential-mode EMI of switching power supplies with rectifier front-end, Transactions of China Electrotechnical Society, 2006)
II. Identification of Essential Coupling Path Models for Conducted EMI Prediction
Conducted EMI prediction problem is essentially the problem of EMI noise source modeling and EMI noise propagation path modeling. These modeling methods can be classified into two approaches, mathematics-based method and measurement-based method. The mathematics method is very time-consuming because the circuit models are very complicated. The measurement method is only valid for specific circuit that is conveniently to be measured, and is lack of generality and impracticability. This dissertation proposes a novel modeling concept, called essential coupling path models, derived from a circuit theoretical viewpoint, means that the simplest models contain the dominant noise sources and the dominant noise coupling paths, which can provide a full feature of the EMI generations. Applying the new idea, this work investigates the conducted EMI coupling in an AC/DC half-bridge converter. Three modes of conducted EMI noise are identified by time domain measurements. The lumped circuit models are derived to describe the essential coupling paths based on the identification of the EMI coupling modes. Meanwhile, this study illustrates the extraction of the parameters in the afore-described models by measurements, and demonstrates the significance of each coupling path in producing conducted EMI. It is shown that the proposed method is very effective and accurate in identifying and capturing EMI features. The equivalent models of EMI noise are sorted out by just a few simple measurements. Under these approaches, EMI performance can be predicted together with the filtering strategies. (Publications: Identification of essential coupling path models for conducted EMI prediction in switching power converters, IEEE Transactions on Power Electronics, 2006; Noise source lumped circuit modeling and identification for power converters, IEEE Transactions on Industrial Electronics, 2006)
III. High Frequency Conducted EMI Source Modeling
The conventional method of EMI prediction is to model the current or voltage source as a periodic trapezoidal pulse train. However, the single slope approximation for rise and fall transitions can not characterize the real switching transitions involved in high frequency resonances. In most common noise source models simple trapezoidal waveforms are used where the high frequency information of the EMI spectrum is lost. Those models made several important assumptions which greatly impair accuracy in the high frequency range of conducted noise. To achieve reasonable accuracy for EMI modeling at higher frequencies, the relationship between the switching transitions modeling and the EMI spectrum is studied. An important criterion is deduced to give the reasonable modeling frequency range for the traditional simple approximation method. For the first time, an improved and simplified EMI source modeling method based on multiple slope approximation of device switching transitions is presented. To confirm the proposed method, a buck circuit prototype using an IGBT module is implemented. Compared with the superimposed envelops of the measured spectra, it can be seen that the effective modeling frequency is extended to more than 10 MHz, which verifies that the proposed multiple slopes switching waveform approximation method can be applied for full-span EMI noise quantification studies. (Publications: Multiple slope switching waveform approximation to improve conducted EMI spectral analysis of power converters, IEEE Transactions on Electromagnetic Compatibility, 2006; Power converter EMI analysis including IGBT nonlinear switching transient model, IEEE Transactions on Industrial Electronics, 2006)
IV. Loop Coupling EMI Modeling in Power Electronic Systems
Practical examples of power electronic systems that have various electrical, electromechanical and electronics apparatus emit electromagnetic energy in the course of their normal operations. In order to predict the EMI noise in a system level, it is significant to model the EMI propagation characteristics through electromagnetic coupling between two apparatus circuit within a power electronic system. The PEEC modeling technique which was first introduced in 1970s has recently becomes a popular choice in relation to the electromagnetic analysis and EMI coupling. In previous studies, the integral equation based method was mostly applied in the electrical modeling and analysis of the interconnect structure in very large scale integration systems, only at the electronic chip and package level. By introducing the partial inductance theory of PEEC modeling technique, this work investigates the EMI loop coupling issues in power electronic circuits. The work models the magnetic flux coupling due to EMI current on one conductor and another by mutual inductance. To model the EMI coupling between the grounding circuits, this study divides the ground impedance into two parts: one is the internal impedance and the other is the external inductance. The external inductance due to the fields external to the rectangular grounding loop and flat conductor is modeled. To verify the mathematical models, the steel plane grounding test configurations are constructed and the DM and CM EMI coupling generation and modeling technique are experimentally studied. The comparison between the measured and calculated EMI noise voltage validates the proposed analysis and models. These investigations and results can provide a powerful engineering application of analyzing and solving the coupling EMI issues in power electronic circuits and systems. (This part of work is one of the main contributions of the awarded project of Military Science and Technology Award in 2006, where the author is No. 4 position. Publication: Loop coupled EMI analysis based on partial inductance models, Proceedings of the Chinese Society of Electrical Engineering, 2007)
V. Conducted EMI Prediction for PWM Conversion Units
PWM-based power conversion units are the main EMI noise sources in power systems. Due to the various PWM strategies and the large number of switches, a common analytical approach for the PWM-based switched converter systems has not been dated. Determination of the frequency spectrum of a PWM converter is quite complex and is often done by using an FFT analysis of a simulated time-varying switched waveform. This approach requires considerable computing capacity and always leaves the uncertainty as to whether a subtle simulation round-off or error may have slightly tarnished the results obtained. By introducing the principle of the double Fourier integral, this work presents a general method for modeling the conduced EMI sources of PWM conversion units by identifying double integral Fourier form to suit each PWM modulation. Appling the proposed method, three PWM strategies have been discussed. The effects of different modulation schemes on EMI spectrum are evaluated. The EMI modeling and prediction efforts from an industrial application system are studied comprehensively. Comparison between the measured and the predicted spectrum confirms the validity of the EMI modeling and prediction method. This method breaks through the limitations of time-consuming and considerable accumulated error by traditional time-domain simulations. A standard without relying on simulation but a common analytical approach has been obtained. Clearly, it can be regarded as a common analytical approach that would be useful to be able to model and predict the exact EMI performance of the PWM-based power electronic systems. (Publications: DM and CM EMI Sources Modeling for Inverters Considering the PWM Strategies, Transactions of China Electrotechnical Society, 2007. High Frequency Model of Conducted EMI for PWM Variable-speed Drive Systems, Proceedings of the Chinese Society of Electrical Engineering, 2008)
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