Analysis and experimentation with passive cancellation of common-mode noise in buck converter

A primary source of common-mode (CM) electromagnetic interference (EMI) in high frequency switching converters is the parasitic capacitance between switching element and its heat sink. A passive method for cancellation of CM-EMI is analyzed in detail in this paper. Compared to with active cancellation techniques, it is much simpler and requires no additional switches or semiconductor components. The principle of operation using steady-state idealized waveforms is described. A small-signal analytical model is derived to predict CM noise with and without the passive cancellation circuit. Effect of circuit parasites is addressed and design procedure is described. The analytical findings are confirmed using PSpice simulation. The passive CM-EMI cancellation circuit is applied to a laboratory prototype 500 W, 100 kHz buck converter. Experimental results are presented validating the CM-EMI reduction.