Date of Award

Spring 1-1-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical, Computer & Energy Engineering

First Advisor

Dragan Maksimovic

Second Advisor

Zoya Popovic

Third Advisor

Robert Erickson

Fourth Advisor

Khurram Afridi

Fifth Advisor

Miguel Rodriguez

Abstract

Supply modulation, or envelope tracking (ET), is a system efficiency improvement technique in radio-frequency (RF) transmitters where a power converter operating as drain supply modulator (DSM) provides an RF power amplifier (RFPA) with time-varying envelope waveforms. DSMs need to have high bandwidth, low tracking error and high efficiency.

This thesis focuses on the design of the DSM using GaN-on-SiC technology for a 20 V ET system with X-band RF transmitter. The signals considered are 20 MHz OFDM and 20 MHz QPSK with 10 W peak output power. The topology chosen is a multi-phase pulse-width modulated (PWM) buck converter. Zero-voltage switching (ZVS) technique is used for high efficiency operation at high switching frequencies. A fourth order output filter design method that supports ZVS operation is proposed to accommodate both high bandwidth and high efficiency.

Monolithic gate driver and half-bridge power-stage integration is a key step in addressing challenges in 50-100 MHz buck converter design. An analytical loss model is developed and used to optimize the monolithic GaN chips, which are then used to construct high-efficiency, wide-bandwidth synchronous buck converter prototypes implemented as DSMs. The converter prototypes are tested at various duty cycles and output power levels. The loss model and the design optimization results are verified by experimental results. The tradeoffs in multi-phase converter design is also discussed based on a total efficiency optimization method. Several multi-phase converter prototypes are tested, analyzed and compared.

The resulting design shows record 97% efficiency at 10 MHz and 91% efficiency at 100~MHz. Accurate tracking of a 20 MHz LTE (100RB) envelope signal is demonstrated with 89.7% power stage efficiency and 83.3% total efficiency. Successful operation of the complete ET transmitter system is demonstrated.

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