Radio-Frequency Inverters With Transmission-Line Input Networks

A soft-switching inverter topology (the Class Phi ) is presented which draws dc source current through a transmission line or a lumped-network approximation of a distributed line. By aligning the inverter switching frequency just below the line's lambda/4-wave resonance, the Class Phi topology enforces odd-and even-harmonic content in its drain voltage and input current, respectively. The symmetrizing action of the transmission-line dynamics results in natural square-wave operation of the switch, reducing the inverter stresses (relative to a Class E) for a given power throughput. The inverter waveforms and normalized power-output capability are analyzed in simple terms, and supported by measurements of an inverter built around a length of distributed line, and an inverter incorporating a lumped L-C ladder network. The latter implementation is constructed with air-core magnetics and inter-layer capacitances that are integrated into the thickness of a printed-circuit board. A comparison with a Class E inverter of similar size and ratings demonstrates the small passive-component values and manufacturing advantages afforded by the Class Phi topology.