An ESL-Cancelling Circuit for a Shunt-Connected Film Capacitor Filter Using Vertically Stacked Coupled Square Loops

An ESL-cancelling circuit for a shunt-connected film capacitor filter using vertically stacked coupled square loops is reported in this paper. The circuit is applicable for a shunt-connected capacitor filter whose equivalent series inductance (ESL) of the shunt-path causes deterioration of filter performance at frequencies above the self-resonant frequency. Two pairs of vertically stacked magnetically coupled square loops are used in the circuit those can equivalently add negative inductance in series to the shunt-path to cancel ESL for improvement of the filter performance. The ESL-cancelling circuit for a 1-μF film capacitor was designed according to the Biot-Savart law and electromagnetic (EM)-analysis, and the prototype was fabricated with an FR4 substrate. The measured result showed 20-dB improvement of the filter performance above the self-resonant frequency as designed, satisfying Sdd21 less than -40 dB at 1 MHz to 100 MHz. This result is almost equivalent to reduce ESL of the shunt-path to less than 1 nH at 100 MHz and is also difficult to realize using any kind of a single bulky film capacitor without cancelling ESL.