Silent switching: EMI suppression without conventional EMI filters

Zhang, Zhe (2019) Silent switching: EMI suppression without conventional EMI filters. EngD thesis, University of Nottingham.

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Electromagnetic Interference (EMI) has been a major concern when utilising Wide Band Gap (WBG) devices such as SiC (Silicon Carbide) and GaN (Gallium Nitride) for power conversion system. Electromagnetic interference arises from the rapid voltage and current transitions occurring during operation of the switching cell. Suppression of electromagnetic interference is typically achieved by a combination of filtering including differential and common mode filters, to attenuate conducted emissions and shielding to attenuate radiated emissions. The purpose of adding filters is to block the noise propagation (high impedance) and to bypass the noise (low impedance at high frequency for capacitors). On the top of understanding the source of EMI and impedance of noise propagation path, the attenuation from passive filters can be controlled to suppress EMI noise to acceptable levels that meet with various EMC (Electromagnetic compatibility) standards. Typically such features are added at converter and system levels but not integrated into the switching cell. At frequencies below several MHz, passive filters typically provide enough noise attenuation, however, at higher frequencies, filter performance is compromised by parasitic inductance and capacitance within the filter system and by mutual electromagnetic coupling between filter stages. In addition the bypass route for high frequency noise back to its noise source might not be effective due to the parasitic impedance in the noise current path. Therefore the realisation of wide-band noise filters becomes challenging with conventional filter configurations.

As the conventional approach of tackling EMI, passive EMI filters are used to attenuate noise outside the switching cell. The passive EMI filters normally include a combination of common mode and differential mode filters which are used at both input and output to attenuate conducted noise and shunt high frequency noise back to its noise source. Such filters may consist of two inductors are wound on a common core as a common mode whose magnetic leakage is used as differential mode choke and several noise shunt capacitors across each line and the ground to return high frequency noise. Normally the EMI filter would occupy about 10% - 20% of the overall system size[1][2]. As the switching speed becomes faster, much larger EMI filter would be necessary for higher level of EMI noise attenuation [3].

In this thesis, in contrast to this conventional method, we propose an alternative approach to solve EMI problems within the switching cell. By applying partial or complete conductive partitions or by embedding an electrically conductive screen layer, internally generated switching noise is returned to the source via an internal screen layer. Common mode noise which would otherwise be capacitively coupled to ground is returned direct to the screen. Internally generated magnetic fields are attenuated in screen layer or in additional shielding layers whose thickness and extent are chosen to produce the desired degree of attenuation. On the top of screening and shielding techniques, differential mode noise is attenuated by low-pass filters that are applied in each of the electrical ports and whose high-freqeuncy return currents are directed back to the screen and hence to the noise source. This thesis also focuses on the achievement of high attenuation, across a wide bandwidth, for differential mode noise at the electrical ports. A new concept of suppressing electromagnetic interference by using multistage integrated filters within switching cells has been proposed to contain EMI noise. Furthermore, the techniques of partitioning each filter stage to improve noise attenuation at very high frequencies and feedthrough connection to decouple EMI field coupling have been proved to achieve attenuation levels of over -120dB up to 1GHz. In the end, a GaN power converter with 1MHz switching frequency is used as a test bed to confirm the effectiveness of EMI noise attenuation with each intervention. A silent switching environment has been confirmed on this GaN power converter when all interventions applied.

Item Type: Thesis (University of Nottingham only) (EngD)
Supervisors: Johnson, Mark
Keywords: EMI; CM current; Screening Shielding integration; Feedthrough connection; EMI confinement containment
Subjects: T Technology > TK Electrical engineering. Electronics Nuclear engineering > TK7800 Electronics
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 55961
Depositing User: Zhang, Zhe
Date Deposited: 26 Apr 2022 09:14
Last Modified: 26 Apr 2022 09:15

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