An 800-W Four-Level Supply Modulator for Efficient Envelope Tracking of RF Transmitters

Abstract

Modern RF communication and electronic warfare applications require systems capable of amplifying signals with high bandwidths and high peak to average power ratios (PAPR). One technique used to meet these demands is supply modulation also known as envelope tracking where the drain voltage of the power amplifier (PA) is dynamically changed according to the signal envelope. The limiting factor with supply modulation is typically the efficiency and switching speed of the power converter. The power converter must also be capable of producing high power levels when supplying an array of PA units or a single high power transmitter. This paper demonstrates the design and characterization of a four-level discrete supply modulator with a peak output power of 800W operating at a maximum switching frequency of 10MHz. The modulator switches between four discrete voltages from 10 to 20V. GaN on silicon HEMT power transistors are selected for the switching devices after conducting a survey of commercially available silicon MOSFET, silicon-carbide MOSFET, and GaN on silicon power transistors. Trace interconnect models are developed to achieve accurate predictions of the time domain waveforms, including ringing at the switching edges. These interconnects are first analyzed using full-wave electromagnetic simulations, and then modeled with RLC networks suitable for time domain simulations. For a test signal with equal duty cycle between the four levels, an efficiency greater than 94.0% is demonstrated for average output powers exceeding 91.6W. When tracking the envelope of an 8MHz 64-QAM signal, an average efficiency of 93.5% is achieved providing a peak power of 800W.