Feature: Power electronics
an insertion gain rather than an insertion loss. This behaviour is caused by resonance within the filter circuit itself. Under the 50Ω/50Ω condition, this insertion gain is not
particularly obvious. However, under other source and load impedance conditions, the resonance shiſts to below 100kHz and the magnitude of the insertion gain becomes significantly worse. The following case study illustrates this problem: A variable speed drive manufacturer recently encountered
an issue when using a third-party EMI filter between the drive and the load. Its team observed significantly high common- mode current circulating between the unit and protective earth. Given that this was a megawatt-level converter, excessive low-frequency common-mode current was a serious concern. Further investigation showed that the dominant common-
mode noise frequency was exactly the third harmonics of the IGBT switching frequency, around 48kHz. This was unexpected, as the EMI filter appeared to worsen the problem rather than improve it. After reviewing the filter manufacturer’s datasheet, a clear
conclusion emerged. At 48kHz, the filter exhibits an insertion gain rather than an insertion loss under realistic operating conditions. Because the datasheet only presents results under a 50Ω/50Ω condition, this behaviour is not obvious. Under that test condition, the common-mode insertion gain at 48kHz appears to be close to 0dB. However, when the filter is evaluated under different
source and load impedances, the resonance shifts to a lower frequency and moves clearly into the insertion-gain region. This explains why in the real system the filter amplified common-mode noise instead of attenuating it. The root cause, therefore, lies in the impedance mismatch
between the real system and the filter characterisation conditions used in the datasheet.
Installation and layout Lastly, filter performance – particularly in common mode – depends heavily on the quality of the earthing connections. A short, direct and low-impedance connection to earth or chassis is essential to achieve the intended filter performance.
Figure 5: Different filter attenuations
Figure 6: Correct way of installing an off-the-shelf filter
www.electronicsworld.co.uk March 2026 23
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