Flying Capacitor Four-Level Supply Modulator with Active Balancing for RF Power Amplifier Applications

This paper presents a four-level flying capacitor multi-level (FCML) converter operating as a drain supply modulator (DSM) for radio frequency power amplifiers (RFPAs). The proposed FCML-based DSM excludes the standard output LC filter and employs a direct multi-level DSM approach, changing the output according to an external RFPA drain-supply level demand. A simple state-machine-based controller ensures that the flying capacitor voltages remain balanced for arbitrary sequences of requested output voltage levels. The functionality of the state-machine controlled converter is verified with a GaN-based DSM prototype with output voltage levels of 0V, 6.67V, 13.33 V, and 20 V generated from an input dc voltage of 20 V, and with the state-machine controller sampling period of 5$\mu$s. Experimental results show the converter is able to maintain capacitor charge balance across the flying capacitors while outputting the requested RFPA level demand. The maximum output current of the converter is 0.8 A, while efficiencies measured at each output level range between 99.0-99.8%.