Ignacio Ramos

High-Efficiency Harmonically Terminated Diode and Transistor Rectifiers

This paper presents a theoretical analysis of harmonically terminated high-efficiency power rectifiers and experimental validation on a class-C single Schottky-diode rectifier and a class- F-1 GaN transistor rectifier. The theory is based on a Fourier analysis of current and voltage waveforms, which arise across the rectifying element when different harmonic terminations are presented at its terminals. An analogy to harmonically terminated power amplifier (PA) theory is discussed. From the analysis, one can obtain an optimal value for the dc load given the RF circuit design. An upper limit on rectifier efficiency is derived for each case as a function of the device on-resistance. Measured results from fundamental frequency source-pull measurement of a Schottky diode rectifier with short-circuit terminations at the second and third harmonics are presented. A maximal device rectification efficiency of 72.8% at 2.45 GHz matches the theoretical prediction. A 2.14-GHz GaN HEMT rectifier is designed based on a class-F-1 PA. The gate of the transistor is terminated in an optimal impedance for self-synchronous rectification. Measurements of conversion efficiency and output dc voltage for varying gate RF impedance, dc load, and gate bias are shown with varying input RF power at the drain. The rectifier demonstrates an efficiency of 85% for a 10-W input RF power at the transistor drain with a dc voltage of 30 V across a 98-Ω resistor.

Near-field capacitive wireless power transfer array with external field cancellation

In this paper, a near-field phased-array cancellation technique for a modular and scalable capacitive wireless transfer system is presented. The application is for kW power transfer to static electric vehicles and vehicles in motion. The goal of the distributed array approach is to increase maximum transferable power while decreasing the external electric field produced by the WPT system, accomplished by field focusing through phase control. Full-wave simulations show that high power transfer efficiency with an accompanied 40% reduction in maximum external field can be accomplished with an 8-element array with 180° alternating phase difference between adjacent elements. A comparison is made for allocated bands at 6.78, 13.56 and 27.12 MHz showing improved focusing with increasing frequency. A system prototype at a scaled power level is currently being designed, taking account tradeoffs in circuit element loss and focusing loss.

GaN Microwave DC–DC Converters

This paper presents the design and characterization of dc-dc converters operating at microwave frequencies. The converters are based on GaN transistor class-E power amplifiers (PAs) and rectifiers. Three topologies are presented, which are: 1) a PA and synchronous rectifier, requiring two RF inputs; 2) a PA and self-synchronous rectifier with a single RF input; and 3) a power oscillator with a self-synchronous rectifier with no required RF inputs. The synchronous 1.2-GHz class- E2 converter reaches a maximum efficiency of 72% at 4.6 W. By replacing the RF input at the rectifier gate with a specific termination, a self-synchronous circuit demonstrates 75% efficiency at 4.6 W, with a maximum output power of 13 W at 58% efficiency. In the third topology, the PA is replaced by a power oscillator by providing correct feedback for class-E operation, resulting in a circuit requiring no RF inputs. This oscillating self-synchronous dc-dc converter is demonstrated at 900 MHz with an efficiency of 79% at 28 V and 12.8-W output power. Self-synchronous class-E transistor rectifier operation is analyzed theoretically in the time domain and validated with harmonic-balance simulations using an improved nonlinear model for a GaN HEMT. The simplified theoretical analysis provides a useful starting point for high-efficiency self-synchronous power rectifier design, which can, in turn, be extended to high-efficiency oscillating power inverter design.

Microfabricated transmission-line transformers with DC isolation

Summary form only given. A transmission line transformer (TLT) is a device that transforms a circuit impedance and is implemented with interconnected transmission lines. While traditional TLTs, operating at UHF and low microwave frequencies, are constructed from pairs of coaxial lines wound around ferrite cores, various compact implementations at higher frequency without magnetic materials have been demonstrated with multilayer circuit boards, monolithic microwave integrated circuits (MMIC) and air-filled microcoaxial lines implemented in the PolyStrata® wafer-scale technology. Microfabricated Guanella TLTs in the microwave range typically span several GHz of bandwidth, but they are not suitable for impedance matching of power amplifiers and active devices because the input and output ports are shorted to ground and therefore lack DC isolation. A compact and DC-isolated TLT would be especially useful in small-size, low-loss and high-speed DC-to-DC converters. In this talk wideband microfabricated transformers with up to 450V of DC isolation will be described. The devices were implemented in PolyStrata(R) to achieve small size and low loss. DC isolation was obtained by use of the basic unit block shown in the figure below. It is made up of two micro-coaxial lines, where the inner conductor of the first line is connected to the outer conductor of the other. The basic unit block achieves a 1:1 impedance transformation at twice the characteristic impedance of the two coaxial lines and can be used as a part of a TLT to introduce DC isolation. If the lines have exactly the same length and for ideal interconnections, the device operating bandwidth is theoretically infinite. In practice line imbalances, non-ideal interconnections and parasitic electromagnetic modes limit the device bandwidth. A fabricated unit block operating from 1 to 12 GHz and a 1:4 DC isolated impedance transformer will be discussed. The transformers have 450 V of input/output DC isolation and are suitable for impedance matching of microwave power amplifiers and compact microwave DC to DC converters.

A compact 2.45 GHz, low power wireless energy harvester with a reflector-backed folded dipole rectenna

This paper paper describes the design procedure as well as the experimental performance of a 2.45GHz 10 μW wireless energy harvester (WEH) with a maximum total efficiency of ≈ 30% at 1 μW/cm2 incident power density. The WEH integrates a shunt high-speed rectifying diode with a folded dipole. A metal reflector increases the gain of the rectenna and a quarter wavelength differential line is used as a choke. Both a VDI WVD and a Skyworks GaAs Schottky diode are integrated with the antenna and their performance is compared.

A planar 75% efficient GaN 1.2-GHz DC-DC converter with self-synchronous rectifier

This paper presents the design and characterization of a DC-DC converter operating at 1.2GHz with a maximum efficiency of 75% at 4.6W output power, and with a maximum output power of 13W at 58% efficiency. The microwave converter consists of a GaN class-E power amplifier coupled to a GaN class-E rectifier. The circuit is planar and exhibits a power density of approximately 0.65W/cm3 at the 75% efficiency point. The rectifier can be operated with input gate drive or selfsynchronously with a single RF input.

Efficient transmitters and receivers for high-power wireless powering systems

The efficiency of a wireless powering system is maximized when the power transmitter power-added efficiency (PAE), power receiver conversion efficiency (ηC) and wireless coupling efficiency (ηW) are maximized. This paper focuses on a general approach to the design of an efficient transmitter and receiver of a high-power wireless powering system, which is valid for any type of wireless power coupling. Experimental results for high-efficiency power transmitters and receivers at various frequencies and with various power levels are discussed.

A fully monolithically integrated 4.6 GHz DC-DC converter

This paper presents the design and measurement of a proof-of-concept fully monolithically integrated DC-DC converter operating at 4.6 GHz. The converter is designed in Qorvos 0.15 μm GaN on SiC process and has a total area of 2.5mm × 3.8 mm. The distributed class-E2 converter does not use any external inductors or other lumped elements. The maximum efficiency achieved by the converter is 48% at 0.6W output power. The converter can also operate in self-synchronous mode without rectifier gate drive, albeit with a drop in performance.

RF energy harvester in the proximity of an aircraft radar altimeter

This paper presents the design and characterization of a rectenna-based energy harvester for low-power aircraft sensors. The harvester is placed on the aircraft skin in the proximity of the radar altimeter transmit antenna with power densities between 0.04 μW/cm2 to 2.2 μW/cm2 around 4.3 GHz. Several diodes were investigated via source-pull, and the chosen devices integrated with a patch antenna to directly charge a capacitor which serves as the energy storage device. The harvester is able to collect -23.2dBm of power from an incident power density of 0.65 μW/cm2, and the time it takes the harvester to reach 63% of its open-circuited voltage is characterized for several designs.

Microwave Transistor Power Rectifiers and Applications

This paper presents design, analysis and experimental results on synchronous and self- synchronous microwave transistor rectifiers implemented with GaN HEMTs at frequencies from 2 to 10GHz. The rectifier/power amplifier duality is explained and measurements confirming this mode of operation are presented. Finally, rectifier applications in wireless power transfer and high- frequency integrated dc-dc converters are discussed, including a GaN MMIC dc-dc converter switching at 4.6GHz.