From Transmission Line Transformers, Fourth Edition

Sec 11.1: Introduction

In Chapter 2 it was shown that the role of a transmission line transformer core is to enhance the choking action of the coiled windings to better isolate the input from the output circuit. Even though calculations were made for only the low-frequency performance of these transformers, experimental results demonstrated that the core influenced most of the operating region (see Sec 3.4).

The analysis for toroidal cores confirms that the low-frequency response is directly related to the permeability of the core material. With higher permeabilities, fewer turns are required, allowing for higher-frequency operation. It was also shown that rods, although not yielding the high reactance of the toroids, exhibit good low-frequency response and can be used in many applications. At low frequencies, the rod was found to be relatively insensitive to the permeability of the rod material because of the high reluctance of the air path around the core.

Accurate loss measurements have shown that only a limited number of ferrite materials are useful in power applications, where high efficiency is an important consideration. This chapter describes transmission line transformers that use nickel-zinc ferrite cores with permeabilities in the moderately low range of approximately 50 to 300, to yield efficiencies in excess of 98%. No conventional transformer can approach this performance. The losses are not a function of current as in the conventional transformer, but are, in most cases, related to the impedance levels at which the transformers are operated. This suggests a dielectric loss, rather than...

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