Parasitic Cancellation for Two-Stage EMI filters
Two-stage EMI filters have more components than one-stage filters. As a result, there are more couplings inside. CPES has investigated two-stage EMI filters. It is found that the mutual inductance between two inductors and between input and output capacitors are the most important. The former one affects the filter's low frequency performance and the latter one affects the filter's high frequency performance. In order to improve both the low frequency and high frequency performance of EMI filters, the coupling between two inductors should be positive and the coupling between the two capacitors should be minimized. The parasitics are first extracted using S-parameters and Matlab and then fed to Pspice for simulation. The simulated insertion gain matches the measured one. The cancellation turn is proposed to implement to one of the capacitors in the filter to cancel the mutual inductance between two capacitors. Two inductors have the same winding direction to make their coupling positive. The experimental results show that filter's HF performance is greatly improved by 30dB at 30MHz.