Common-mode Current and EMI Analysis for Wide Bandgap Device Based Power Converters

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August 2022

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Wide bandgap devices such as Silicon Carbide (SiC) and Gallium Nitride (GaN) are facilitating higher switching frequencies for power electronic converters. Their faster switching speed combined with low on-state resistance helps in increasing the efficiency and power density of the converters. However, high voltage and current slew rates (dv/dt and di/dt) associated with wide bandgap devices also increase Electromagnetic Interference (EMI) which may cause malfunctioning or failure of the converter itself or other sensitive circuits onboard. EMI issues are further worsened when high switching speed and high frequency operation is used for high power applications. Apart from hard switched wide bandgap power converters, dv/dt rates can also be high in wide bandgap converters which offer Zero Voltage Switching (ZVS) resulting into high Common-Mode (CM) currents through parasitic capacitances. High CM current leads to EMI. Dual Active Bridge (DAB) converter is a DC-DC converter offering ZVS along with bi-directional power transfer, isolation, and voltage matching. It finds applications in Electric Vehicles (EVs), battery charging stations, Solid State Transformers (SSTs) etc. High dv/dt, high switching frequency combined with multiple parasitic elements present in an isolated converter like DAB, make EMI mitigation difficult. This dissertation presents CM current and EMI analysis for single phase DAB converter. Conventional and Neutral Point Clamp (NPC) based architectures are studied with regard to their suitability to reduce CM current. Design and control variables in Active NPC (ANPC) - DAB converter are used to reduce CM conducted as well as radiated EMI. While doing so, it is insured that all possible modulation schemes used in DAB converters remain unaffected. To further reduce CM current than what is possible by design and/or control, an Input Parallel Output Parallel (IPOP) ANPC-DAB architecture is proposed for high power DAB converters. This architecture uses power sharing and dv/dt cancellation to reduce CM current injection. Factors causing nonideal dv/dt cancellations are discussed with their possible solutions. Finally in a contrasting work, EMI is investigated as a noninvasive system monitoring tool. A low cost radiated EMI sensing circuit is developed to detect dynamic current unbalancing in parallel connected wide bandgap devices. Apart from its hardware validation for paralleled SiC MOSFETs, possibilities to use it for EMI precompliance testing and online junction temperature monitoring are also discussed.

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Engineering, Electronics and Electrical

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