Niall Oswald

An Experimental Investigation of the Tradeoff between Switching Losses and EMI Generation With Hard-Switched All-Si, Si-SiC, and All-SiC Device Combinations

Silicon carbide (SiC) switching power devices (MOSFETs, JFETs) of 1200 V rating are now commercially available, and in conjunction with SiC diodes, they offer substantially reduced switching losses relative to silicon (Si) insulated gate bipolar transistors (IGBTs) paired with fast-recovery diodes. Low-voltage industrial variable-speed drives are a key application for 1200 V devices, and there is great interest in the replacement of the Si IGBTs and diodes that presently dominate in this application with SiC-based devices. However, much of the performance benefit of SiC-based devices is due to their increased switching speeds ( di/dt, dv/ dt), which raises the issues of increased electromagnetic interference (EMI) generation and detrimental effects on the reliability of inverter-fed electrical machines. In this paper, the tradeoff between switching losses and the high-frequency spectral amplitude of the device switching waveforms is quantified experimentally for all-Si, Si-SiC, and all-SiC device combinations. While exploiting the full switching-speed capability of SiC-based devices results in significantly increased EMI generation, the all-SiC combination provides a 70% reduction in switching losses relative to all-Si when operated at comparable dv/dt. It is also shown that the loss-EMI tradeoff obtained with the Si-SiC device combination can be significantly improved by driving the IGBT with a modified gate voltage profile.

Analysis of Shaped Pulse Transitions in Power Electronic Switching Waveforms for Reduced EMI Generation

Consideration of the higher order time derivatives of voltage and current transitions in power semiconductor devices enables the specification of “S-shaped” switching waveforms which offer an improved tradeoff between high-frequency EMI generation and switching losses. In comparison with the widely used first-order derivative trapezoidal switching waveform approximation, Fourier analysis of the proposed “S-shaped” waveform shows that it exhibits a 20 dB/dec steeper spectral gradient at high frequencies, resulting in a 20 dB greater reduction in high-frequency spectral content per decade increase in rise time. Numerical analysis of the proposed waveform shows that both peak and total RF power, employed as indicative EMI metrics, are reduced significantly with no increase in overall switching time. Experimental investigation of the effect of introducing a frequency-selective EMI transmission path shows that the overall trends in the relationships between time-domain waveform parameters and high-frequency spectral content are maintained, while the values of the waveform timing parameters which minimize the two EMI metrics are changed.

EMI Generation Characteristics of SiC and Si Diodes: Influence of Reverse-Recovery Characteristics

Silicon carbide (SiC) Schottky diodes with zero reverse-recovery current (RRC) are perceived as superior due to their reduced switching losses. The absence of reverse-recovery behavior in these devices is also expected to result in reduced electromagnetic interference (EMI), compared with the conventional silicon (Si) PIN diodes. In this letter, the influence of SiC Schottky diodes on EMI generation in hard-switched power converters is investigated. A simplified analytical model enabling the spectral envelope of the diode current waveform to be predicted is presented. Numerical simulations and experimental tests are employed to validate this model. It is found that although the reverse-recovery characteristics are very different between Si diodes and SiC Schottky diodes, the actual improvement with SiC diodes on the spectral content of the diode current waveforms is relatively small except at frequencies around 5 MHz. Factors affecting the EMI performance such as the peak amplitude and the “snappiness” of the RRC are also analyzed. Experimental measurements of the switching current waveforms for both Si diodes and SiC diodes are presented and their frequency spectra are compared.

EMI reduction with a soft-switched auxiliary commutated pole inverter

PWM-controlled power converters are increasingly used in the More Electric Aircraft (MEA). However, Electromagnetic Interference (EMI) is introduced due to the high dv/dt and di/dt slew rates of their fast-switching power devices. Though EMI is conventionally mitigated by adding filters that can be often heavy and bulky, a better solution would be to address it at source, in order to achieve higher power density systems. This paper investigates how soft-switching topologies can attenuate EMI by addressing it at source. Specifically, the auxiliary commutated pole inverter (ACPI) is employed to actively control the dv/dt of the output voltage. The effect this control has on the output voltage frequency spectrum is examined with analytical models. Subsequently, a control scheme is developed for regulating the output voltage to the desired waveform and simulation results are presented. A 1.4-kW ACPI phase-leg prototype is utilized for proof of concept. Reductions in the harmonic content of the output voltage are observed in the range of 1 to 20 MHz. It is expected that the ACPI can benefit from better EMI performance and can contribute to the overall effort of achieving high power density systems for MEA applications.

High-bandwidth, high-fidelity in-circuit measurement of power electronic switching waveforms for EMI generation analysis

The useful bandwidth of power electronic switching waveform measurements is limited by the finite resolution of measurement instrumentation and the spectral characteristics of switching waveforms, which exhibit a steep roll-off with increasing frequency. This limits the use of such measurements in EMI generation analysis, simulation and prediction. A method combining PC-based offline data processing and high-pass filtering of the waveforms prior to measurement allows the useful measurement bandwidth to be extended to 100 MHz in the case of a 1200 V, 15 A IGBT operating under realistic conditions. When utilised with commercially available current probes and passive voltage probes, this method offers high-fidelity measurements. However, it is more difficult to obtain repeatable measurements with high-voltage differential probes. The resulting spectra of IGBT collector-emitter voltage and collector current waveforms are presented; the increased bandwidth allows the high-frequency spectral gradient of −60 dB/decade to be observed across the 30–100 MHz band most critical for radiated EMI generation in IGBT-based power converters. The effect of series gate resistance variation is thus apparent, and spectral evidence is provided for the 30–100 MHz band being dominated by the turn-on transients with small gate resistances, and by the turn-off transients with large gate resistances.

IEEE Energy Conversion Congress & Exposition, Phoenix, AZ, USA

The useful bandwidth of power electronic switching waveform measurements is limited by the finite resolution of measurement instrumentation and the spectral characteristics of switching waveforms, which exhibit a steep roll-off with increasing frequency. This limits the use of such measurements in EMI generation analysis, simulation and prediction. A method combining PC-based offline data processing and high-pass filtering of the waveforms prior to measurement allows the useful measurement bandwidth to be extended to 100 MHz in the case of a 1200 V, 15 A IGBT operating under realistic conditions. When utilised with commercially available current probes and passive voltage probes, this method offers high-fidelity measurements. However, it is more difficult to obtain repeatable measurements with high-voltage differential probes. The resulting spectra of IGBT collector-emitter voltage and collector current waveforms are presented; the increased bandwidth allows the high-frequency spectral gradient of −60 dB/decade to be observed across the 30–100 MHz band most critical for radiated EMI generation in IGBT-based power converters. The effect of series gate resistance variation is thus apparent, and spectral evidence is provided for the 30–100 MHz band being dominated by the turn-on transients with small gate resistances, and by the turn-off transients with large gate resistances.

High-Bandwidth, High-Fidelity In-Circuit Measurement of Power Electronic Switching Waveforms for EMI Generation Analysis

The useful bandwidth of power electronic switching waveform measurements is limited by the finite resolution of measurement instrumentation and the spectral characteristics of switching waveforms, which exhibit a steep roll-off with increasing frequency. This limits the use of such measurements in EMI generation analysis, simulation and prediction. A method combining PC-based offline data processing and high-pass filtering of the waveforms prior to measurement allows the useful measurement bandwidth to be extended to 100 MHz in the case of a 1200 V, 15 A IGBT operating under realistic conditions. When utilised with commercially available current probes and passive voltage probes, this method offers high-fidelity measurements. However, it is more difficult to obtain repeatable measurements with high-voltage differential probes. The resulting spectra of IGBT collector-emitter voltage and collector current waveforms are presented; the increased bandwidth allows the high-frequency spectral gradient of −60 dB/decade to be observed across the 30–100 MHz band most critical for radiated EMI generation in IGBT-based power converters. The effect of series gate resistance variation is thus apparent, and spectral evidence is provided for the 30–100 MHz band being dominated by the turn-on transients with small gate resistances, and by the turn-off transients with large gate resistances.