Operation of the Bifilar Common-mode Voltage Suppressor

The common-mode (CM) voltage suppressor (CMVS) is essentially a tightly-coupled, center-grounded inductor. While the CM choke ideally presents an open circuit impedance to CM current, the CM voltage suppressor presents a short circuit to CM voltage. The efficacy of the CM voltage suppression depends on the magnetic coupling coefficient between the two symmetrical halves of the inductor. Bifilar winding is typically necessary in order to effect sufficiently tight magnetic coupling. However, bifilar winding causes the device to behave in a distributed sense. This, in turn, hard limits the bandwidth of the device–when the winding is one-half of the odd-mode wavelength (in the bifilar transmission line) long, it acts as a CM open circuit and a differential mode (DM) short circuit. It is further shown that the effective upper operating frequency limit is about one octave below the half wavelength frequency. These assertions are supported by an analytical model, finite element numerical simulations, and experimental data. The author believes the model and the predictions of bandwidth limitations in the bifilar CM voltage suppressor have not previously been published.