Class-E rectifiers and power converters: the operation of the class-E topology as a power amplifier and a rectifier with very high conversion efficiencies

In the late 70’s, the interest in reducing the value and size of reactive components moved power supply specialists to operate dc-to-dc converters at hundreds of kHz or even MHz frequencies. Passive energy storage (mainly magnetics) dominates the size of power electronics, limiting also its cost, reliability and dynamic response. Motivated by miniaturization and improved control bandwidth, they had to face the frequency-dependent turn-on and turn-off losses associated with the use of rectangular waveforms in the hard-switched topologies of that time. Similar to approaches for RF/microwave power amplifiers (PAs), the introduction of resonant circuits allowed shaping either a sinusoidal voltage or current, with parasitic reactive elements absorbed by the topology in the neighborhood of the switching frequency. The resulting resonant power converters, obtained by cascading a dc-to-ac resonant inverter with a high-frequency ac-to-dc rectifier, first transform the dc input power into controlled ac power, and then convert it back into the desired dc output [1]. This paper provides some historic notes on the operation of the class-E topology, introduced worldwide to the RF/microwave community by Nathan O. Sokal [2], as a power inverter and as a rectifier, with very high conversion efficiencies up to microwave frequencies. Recent research advances and implementations of class-E rectifiers and dc-to-dc converters at UHF and beyond are included. Offering competitive performance in terms of efficiency for RF power recovery, together with a wide bandwidth for low-loss power conversion, their potential for some modern applications is highlighted.The authors would like to acknowledge support in part by the Spanish Ministry of Economy, Industry and Competitiveness (MINECO) through TEC2014-58341-C4-1-R and TEC2017-83343-C4-1-R projects, co-funded with FEDER, and in part by Lockheed Martin Endowed Chair at the University of Colorado

Device characterization and modeling for the design of UHF Class-E inverters and synchronous rectifiers

In this paper, the advantages derived from an appropriate characterization and modeling of active and passive devices, leading to the optimized design of Ultra-High Frequency (UHF) Class-E inverters and synchronous rectifiers, are highlighted. While the combination of a couple of low-frequency and RF measurement techniques is shown to be valid for the extraction of a simplified model as a switch, a more complex approach may be required if also addressing the design of the continuous wave (CW) driving network or if interested in taking fully advantage of other transistor characteristics. Design examples, based on GaN HEMTs and a GaAs E-pHEMT, are presented, in which the parasitics of the employed coils and capacitors are also taken into consideration. Wireless transmitting and powering applications have been addressed.This work was supported by the Spanish Ministry MINECO through projects TEC2011-29126-C03-01, co-funded with FEDER, and Consolider CSD2008-00068

GaN HEMT class-E rectifier for DC+AC power recovery

A 915 MHz GaN HEMT-based class-E rectifier is proposed in this paper to be used for DC+AC wireless power recovery. Taking advantage of the time reversal (TR) duality principle, the rectifier was derived from a class-E inverter, whose output network was designed for high-efficiency operation over a wide range of resistive loads. The addition of an appropriate gateside termination allows the device to be turned-on without an additional RF source for gate driving. The rectifier reduced sensitivity to load variation, as well as its capability for efficiently and linearly recovering the envelope of an AM RF excitation, were then characterized. An average efficiency of 82% has been measured for the combined RF-to-DC and RF-to-AC power conversion of a 1.6 W modulated carrier. Frequency multiplexing and frequency modulation alternatives for high-level DC+AC wireless power transmission are finally presented.This work was supported by the Ministry of Economy and Competitiveness (MINECO) through TEC2014-58341-C4-1-R and TEC2014-58341-C4-2-R projects, co-funded with FEDER. The authors wish to thank Prof. Christian Brañas, University of Cantabria, by his assistance with the transformer characterization. David Vegas and José R. Pérez-Cisneros also thank the support provided by the pre-doctoral BES-2015-072203 and the pre-doctoral mobility EEBB-I-15-10447 grants, respectively

Class-E rectifiers and power converters

This paper reviews the use of the class-E topology for RF-to-DC and DC-to-DC power conversion. After covering its early history, the class-E rectifier is introduced in the context of the time-reversal duality principle, to be then integrated with an inverter in a class-E2 DC/DC converter. Recent examples and applications at UHF and microwave bands are finally presented. A review of RF rectifiers based on Schottky diodes or FET transistors, is followed by a discussion of synchronous and self-synchronous implementations of the double class-E DC/DC converter, using advanced GaN HEMT transistors.This work was supported in part by the Spanish Ministry of Economy and Competitiveness (MINECO) under project TEC2014-58341-C4-1-R, co-funded with FEDER, and in part by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DEAR0000216 and the DARPA MPC program, ONR award N00014-11-1-0931

UHF power conversion with GaN HEMT class-E2 topologies

This paper reviews the use of UHF double class-E (class-E2) topologies for dc/dc power conversion. After introducing this attractive resonant converter in the context of the time-reversal duality principle, two different lumped-element networks are described for appropriately terminating the drain of the switching devices. Recent implementation examples, taking advantage of GaN HEMT processes, are then presented. The potential for a fast dynamic response is validated (with a slew rate over 2 V/nS), while also the feasibility for an appropriate operation without requiring external RF gate driving signals. A solution for approximating a load-insensitive operation is finally exposed

Design of UHF class-E inverters and synchronous rectifiers for efficient transmission topologies

In this paper, the design of efficient transmitting architectures is addressed. Taking advantage of the modeling and characterization of novel active devices as GaN HEMT and E-pHEMT, several topologies have been approached. An outphasing transmitter at 770 MHz have been implemented from two class-E RFPA designed over package GaN HEMTs. In addition, for multiband applications, a dual band outphasing transmitter (able of operating either at 770 MHz or 960 MHz) has been also realized. A Chireix reactive combiner allows positioning the drain impedance loci to produce high efficiency and good dynamic range profiles, for both designs. Average drain efficiency figure over 61% has been measured for a 8.4 dB PAPR WCDMA signal, in the dual-band implementation. Besides, in the first approach, an efficient value of 57.5% was estimated when reproducing a LTE with a PAPR of 9.6 dB. On the other hand, wireless powering applications have been into account in the design of self-biased and self-synchronous class-E rectifier, based on an E-pHEMT. Two implementations at 900 MHz and 2.45 GHz, with efficient values of 76% and 64% at power levels of -4 dBm and -1 dBm, respectively, and peak figures of 88% and 77%, have been measured.This work was supported by MINECO through projects TEC2011-29126-C03-01, co-funded with FEDER, and Consolider CSD2008-00068

UHF Class-E power amplifier design for wide range variable resistance operation

This paper presents a simple switch model for a GaN HEMT device, extracted to estimate the output power and efficiency load-pull contours when the transistor is used in a UHF class-E power amplifier (PA). The impact of model parameters on the theoretical achievable efficiency versus output power backoff (PBO) profile is considered, to then be used in a load-insensitive class-E design methodology. A simple lumped element terminating network, derived from [1], was selected to approximate the desired zero voltage switching (ZVS) operation along a wide range of resistive loads. A 700 MHz outphasing transmitter, amenable to be transformed into a resonant dc/dc power converter, has been implemented for validation. Drain efficiency peaks of 82.2% and 78% have been measured for each application case, with values above 70% and 60% at 10 dB of backoff and 30% of nominal dc output power, respectively.This work was supported by the Ministry of Economy and Competitiveness (MINECO) through TEC2014-58341-C4-1-R and TEC2017-83343-C4-1-R projects, co-funded with FEDER. D. Vegas and M. Pampín also thank the support provided by the pre-doctoral BES-2015-072203 and BES-2012-059599 grants, respectively

Supply modulated GaN HEMT power amplifiers - From transistor to system

Power amplifiers (PAs) for mobile communication applications are required to fulfil stringent requirements concerning linearity while keeping a high efficiency over a wide power range and bandwidth. To achieve this, a number of advanced PA topologies have been developed, mostly based on either load modulation, such as Doherty PAs or load modulation balanced PAs, or on supply modulation such as envelope tracking or envelope elimination and restoration. Supply modulation has an advantage over other topologies as the power range of high efficiency can be realised over arbitrary bandwidths, only limited by the bandwidth of the PA. This does, however, come at the cost of a significantly more complicated voltage supply. Instead of a static supply voltage, the PA needs to be provided with one which is rapidly changing, requiring a supply modulator capable of powering the PA while modulating its supply voltage. This thesis investigates a number of challenges in supply modulated power amplifiers, ranging from the transistor itself to circuit design and system level considerations and focusses on power levels up to 10 W and frequencies between 1 GHz and 4 GHz. Transistors, as the centre-piece of a PA, determine how well the PA reacts to a changing supply voltage. In this work, the traits that make GaN HEMTs suitable for supply modulated PAs were investigated, and gain variation with changing supply voltage was established as an important parameter. This gain variation is described in detail and its impacts on PA performance are discussed. By comparing transistors in literature, gain variation has been demonstrated to be a prevalent characteristic in transistors with GaN HEMTs showing a very wide range of gain variation. Using a small-signal model based on measurements, the voltage dependent behaviour of the feedback capacitance CGD is, for the first time, identified as the origin of small-signal gain variation. This is traced down to the gate field plate which is commonly used to combat surface trapping effects in GaN HEMTs. With this in mind, two different ways of changing the transistor geometry to reduce the impact of gain variation and thus optimise the transistor for operation in supply modulated PAs are discussed and demonstrated using a 250 nm GaN HEMT. As a result of the non-linearity of the feedback and gate-source capacitances, the input impedance of GaN HEMTs changes with supply voltage and drive power. This prevents the transistor from being matched at all supply voltages and input powers and introduces phase distortion. Using simulation and measurement, the impact of input impedance on linearity and efficiency of supply modulated power amplifiers is demonstrated on a 2.9 GHz 10 W PA. Careful selection of the input impedance allows improvement of AM/PM distortion of a supply modulated PA with little cost in terms of AM/AM and PAE. I Supply modulators have a significant impact on efficiency and linearity of the ET system. One supply modulator topology with the potential to generate a supply voltage with a high modulation bandwidth is the RF modulator in which the input DC voltage is turned into an RF signal and rectified, resulting in an output voltage which depends on the excitation of the PA. While PAs are well understood in every detail, there are gaps in the understanding of RF rectifiers. Using active load-pull/source-pull measurements, intrinsic gate and drain waveforms of a GaN HEMT operated as a rectifier are demonstrated for the first time. This allows in-detail evaluation of the impact of the gate termination in self-synchronous rectifiers. It also allows detailed analysis of the loss mechanisms in rectifiers and formulation of the required impedances to realise efficient self-synchronous operation, resulting in efficiencies exceeding 90% over wide power ranges. Using waveform engineering, a new type of RF modulator, with potentially very high bandwidths, based on even harmonic generation/injection is proposed. The necessary operating conditions of the rectifier part of the modulator are emulated using an active load-pull/source-pull system to successfully demonstrate that the rectifier behaves as predicted. Using a simple demonstrator, preliminary measurements were conducted and the RF modulator was shown to work, reaching efficiencies up to 78%. As PA and supply modulator are combined, they present impedances to each other. These impedances have a significant impact on the behaviour of both sub-systems. A simple way to characterise both the impedance presented to the PA by the modulator and the impedance presented to the modulator by the PA is described. Using a state-of-the-art modulator, these impedances are measured, the modulator impedance is demonstrated to be close to the simulated value. These measurements also demonstrate that the impedances change significantly with the operating conditions

An E-pHEMT self-biased and self-synchronous class E rectifier

In this paper, the design of a self-biased and self-synchronous class E rectifier, based on an Enhancement-mode Pseudomorphic High Electron Mobility Transistor (E-pHEMT), is proposed. Characterized by a small value of the switch-mode time-constant (the on-state resistance times the output capacitance), high power efficiency figures may be obtained when forcing zero-voltage and zero-voltage-derivative switching conditions (ZVS and ZVDS). The self-synchronous operation, made possible by the device gate-to-drain coupling capacitance, leads to a compact design, while the gate-to-source Schottky junction allows self-biasing the gate terminal in order to improve the efficiency versus input power profile. Simulations, based on an extracted simplified non-linear model, are combined with measured results for implementations at 900 MHz and 2.45 GHz. Efficiency values as high as 76% and 64% have been estimated at power levels of -4 dBm and -1 dBm, respectively, with peak figures of 88% and 77%.This work was supported by MINECO through projects TEC2011-29126-C03-01, co-funded with FEDER, and Consolider CSD2008-0006