Design of High-Density Filter Building Blocks for SiC-based Three-Phase Power Converters

The advent of wide-bandgap (WBG) devices like silicon carbide (SiC) MOSFETs has resulted in a paradigm shift toward high-density and high-efficiency integration of power electronics systems. This being the result of relatively high switching frequencies (>10 kHz) compared to conventional Si IGBT counterparts, which reportedly can minimize the size of passive components such as DC-link capacitors and line harmonic filters. Unfortunately, with faster switching speeds and high slew rates, the common-mode (CM) and differential-mode (DM) conducted emissions interference (EMI) noise is worsened. The effects are manifested at the utility interface with grid-tied applications (three-phase rectifiers or back-to-back converters) in the form of high CM and DM emissions, total harmonic distortion (THD) and current harmonics. While at the motor end, long cable and bearing/leakage current effects are prevalent. As such, typically bulky passive filters are recommended to comply with industry regulations and allow safe and reliable system operation, which can be detrimental on the overall system power density. Hence, it is imperative to minimize the filter volume/weight contribution to fully utilize the benefits of WBG power converters. As an added feature, modular filter building block (FBB) configurations inspired by the building block nature of power electronics converters are needed to address scalability to higher power levels (through interleaving or paralleling) without the need for significant filter redesign.

As such, for grid-tied applications (AC-DC converters), the interleaving of parallel converters adopted to achieve superior harmonic attenuation for grid-side currents at the expense of low harmonic filter volume. Therefore, interleaved converters are explored in Chapters 2 and 3. However, to block inter-channel circulation, additional use of coupled inductors (CI) can outweigh the benefits of interleaving. Therefore, modular FBB architectures with unique methods to handle circulating currents are proposed. At the same time, the FBB is designed to meet power quality and EMI limits for any given number of channels, up to the maximum number of channels, N, allowed at the point of common coupling (PCC). Consequently, a qualitative and quantitative comparison of FBB candidates is performed, and the indirectly coupled FBB using a secondary loop interconnection is proposed as a viable modular FBB candidate.

Correspondingly, for DC-AC inverters, modular filters can be realized using a masked impedance and decoupling approach. The test case being a DC-fed motor drive for aircraft propulsion systems. Techniques, such as optimized parallel RC dampers to reduce the peak bearing current and CM/DM magnetic integration of a DC side filter with an embedded DC current sensor and embedded decoupling path with gate driver for high frequency commutation, are implemented to reduce the overall weight of the system. The challenges with low temperature rise margin due to high ambient temperature and low peak Partial Discharge Inception Voltage (PDIV) are addressed. In addition, a novel pulse with modulation (PWM) scheme is proposed to further enhance the bandwidth of the proposed AC filter, specifically targeted to reduce the peak bearing current and improve the specific power and motor lifetime.

A negative consequence of high-density filter integration is the impact of self and mutual parasitic couplings of filter sub-components on filter attenuation, which is studied on a back-to-back converter system (AC-AC). Simplified lumped models that are representative of the high frequency filter behavior are developed to desensitize the impact of individual filter sub-components. Thereafter, unique winding and placement techniques are proposed to compensate for the impact of self and mutual parasitic couplings on the noise spectrum.

Overall, this work presents potential FBB topologies for varying modes of power conversion (AC-DC, DC-AC, and AC-AC), ultimately aimed at reducing the volume/weight of the system. Methods to minimize the passive component volume/weight from the point of view of topology, magnetic integration, and PWM techniques are discussed, while the implications of a high-density integration at high frequency is presented. Generalized practical design guidelines are formulated to aid in accurate high-density filter design for WBG converters.