Analysis of the Distributed Bifilar Isolation Transformer and Current Balun

Bifilar windings have been employed in isolation transformers in order to minimize the leakage inductance and hence improve frequency and time response. A numerical analysis of the bifilar isolation transformer viewed as a 3-port balun based on odd/even mode transmission line analysis is given. The numerical predictions for the 3-port admittance parameters show that the device functions as a true current balun in the low frequency limit, but that the performance degrades as the electrical length of the bifilar winding approaches one-quarter wavelength for the odd mode. The model also shows that for an isolated, 2-terminal, load a complete null in the response occurs when the electrical length of the bifilar windings is one-half wavelength for the odd mode. A prototype bifilar isolation transformer was fabricated and characterized. The measured data is in agreement with the numerical predictions thus supporting the statements concerning performance. It is further shown that a previously proposed, 2-stage current balun consisting of a bifilar isolation transformer followed by a bifilar, common-mode choke provides superior current balance over a broad bandwidth, but is still limited to operation below the odd-mode half-wave frequency of the bifilar winding of the isolation stage.