ThB4-3 : Radio-frequency Inverters with Transmission-line Input Networks

A soft-switching inverter topology (the Class Φ) 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 λ/4-wave resonance, the Class Φ topology enforces oddand even-harmonic content in its drain voltage and input current, respectively. The symmetrizing action of the transmissionline 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 Φ topology.