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.