Design of low-voltage, high-bandwidth radio frequency power converters

Abstract

The mass and volume required for power electronics circuitry is a dominant obstacle to the miniaturization and integration of many systems. Likewise, power electronics with greater bandwidth and efficiency are becoming vital in many applications. To realize smaller and highly responsive power electronics at low voltages, this thesis explores devices, circuits, and passives capable of operating efficiently at very high frequencies (VHF, 30-300 MHz). Operation at these frequencies enables reduction of the numerical values and physical size of the passive components that dominate power converters, and enables increased bandwidth and transient performance which is valuable in a multitude of low-voltage and low-power applications. This thesis explores the scaling of magnetic component size with frequency, and it is shown that substantial miniaturization is possible with increased frequencies even considering material and heat transfer limitations. Moreover, the impact of frequency scaling of power converters on magnetic components is investigated for different design criteria. Quantitative examples of magnetics scaling are provided that clearly demonstrate the benefits and opportunities in VHF magnetics design. It is shown to utilize the advantages of frequency scaling on passive component size that system losses and other limitations must be considered. One such area that is examined is semiconductor device requirements, where through a combination of device layout optimization for cascode structures and integrated gate drive designs on a 0.35-um CMOS process, converter performance (i.e., loss and bandwidth) can be significantly improved in the VHF regime. In this thesis a dc-dc converter topology is developed that is suitable for low-voltage power conversion and employs synchronous rectification to improve efficiency. The converter is also comprised of a high-bandwidth and high-switching-frequency inverter topology that can dynamically adjust the output power from one-quarter to full power, while maintaining good efficiency. Furthermore, with its inherent capability of gate-width switching, the inverter can further reduce gating loss by one-half resulting in substantial performance improvements at light load operation. A major contribution of this thesis is the development of a synchronous rectifier operating in the VHF regime. VHF power conversion is especially challenging at low voltages due to poor efficiency resulting from rectification loss. To overcome diode rectification loss, the benefits of synchronous rectification are discussed in the context of a 100MHz class-E resonant rectifier, which results in a 2.5 x overall converter efficiency improvement. The culmination of the developed design techniques in passives, semiconductor devices, and circuit topologies is an experimental prototype of a miniaturized 100MHz, 1W power converter utilizing synchronous rectification.