Improved transmission-line attenuators for integrated power filters in the RF band

Differential mode electromagnetic interference (EMI) at radio frequencies (RF) in compact integrated power electronic converter modules-as due in part to semiconductor switching and structural electromagnetic coupling-can be suppressed by the use of lossy transmission line lowpass filters. A previously reported lowpass filter design for such a system employed a Cu-Al2O3-Ni-BaTiO3-Ni-Al2O3-CU lossy uniform width planar transmission line. where the low-permittivity Al2O3 ceramic wafer merely separates the Cu and Ni conductors. The higher-conductivity Cu conductors served to handle kW-level power frequencies. Above power frequencies, skin-depth restrictions cause current diversion from the Cu line into the inner segment of Ni-BaTiO3-Ni that provides subsequent attenuation of milliwatt-level spurious RF interference. Velocity reduction due to the high-permittivity barium titanate (BaTiO3) ceramic (epsilon(r) similar to 12,000) enabled compact integrated package size construction (e.g., 10 mm x 1.5 mm x 130 mm). With the aim of increasing the lowpass cut-off response slope, the current paper examines a prototype non-uniform (in width) planar Ni-BaTiO3-Ni constituent attenuator segment. For ease of one-dimensional ABCD matrix analysis, a four-section Ni-BaTiO3-Ni planar attenuator-comprised of four cascaded uniform sections of decreasing step width-was fabricated as a first approximation to an exponential taper. While good agreement was obtained between the predicted and experimental responses of the reverse-connected (narrow-to-wide) attenuator up to about 30 MHz, the discrepancy between the experimental and theoretical forward-connected (wide-to-narrow) frequency responses is attributed here to three-dimensional current constrictions at step interfaces. It is postulated that this can be overcome by the alternate use of a linearly- or an exponentially-tapered line.