This paper presents a unified framework for the modeling, analysis, and design of load-independent Class E and Class EF inverters and rectifiers. These circuits are able to maintain zero-voltage switching and, hence, high efficiency for a wide load range without requiring tuning or use of a feedback loop, and to simultaneously achieve a constant amplitude ac voltage or current in inversion and a constant dc output voltage or current in rectification. As switching frequencies are gradually stepping into the megahertz (MHz) region with the use of wide-bandgap (WBG) devices such as GaN and SiC, switching loss, implementing fast control loops, and current sensing become a challenge, which loadindependent operation is able to address, thus allowing exploitation of the high-frequency capability of WBG devices. The traditional Class E and EF topologies are first presented, and the conditions for load-independent operation are derived mathematically; then, a thorough analytical characterization of the circuit performance is carried out in terms of voltage and current stresses and the power-output capability. From this, design contours and tables are presented to enable the rapid implementation of these converters given particular power and load requirements. Three different design examples are used to showcase the capability of these converters in typical MHz power conversion applications using the design equations and methods presented in this paper. The design examples are chosen toward enabling efficient and high-power-density MHz converters for wireless power transfer (WPT) applications and dc/dc conversion. Specifically, a 150-W 13.56-MHz Class EF inverter for WPT, a 150-W 10-MHz miniature Class E boost converter, and a lightweight wirelessly powered drone using a 20-W 13.56-MHz Class E synchronous rectifier have been designed and are presented here. Index Terms-DC-AC power converters, resonant inverters, wireless power transmission, zero voltage switching. I. INTRODUCTION T HE full exploitation of wide-bandgap (WBG) devices is driving the design of ever higher frequency converters
Class EF and Class E/F inverters are hybrid inverters that combine the improved switch voltage and current waveforms of Class F and Class F -1 inverters with the efficient switching of Class E inverters. As a result, their efficiency, output power and power output capability can be higher in some cases than the Class E inverter. Little is known about these inverters and no attempt has been made to provide an in depth analysis on their performance. The design equations that have been previously derived are limited and are only applicable under certain assumptions. This paper is the first to provide a comprehensive set of analytical analysis of Class EF and Class E/F inverters. The Class EF2 inverter is then studied in further detail and three special operation cases are defined that allow it to either operate at maximum power-output capability, maximum switching frequency or maximum output power. Final design equations are provided to allow for rapid design and development. Experimental results are provided to confirm the accuracy of the performed analysis based on a 23 W Class EF2 inverter at 6.78 MHz and 8.60 MHz switching frequencies. The results also show that the Class EF2 inverter achieved an efficiency of 91 % compared to a 88 % efficiency when operated as a Class E inverter.
This paper investigates the performance of Class E inverters used in wireless power transfer applications based on resonant inductive coupling. The variations in the load and the distance between the coils cause Class E inverters to operate in non-optimal switching conditions, which result in inefficient operation and can lead to permanent damage to its switching transistor. Therefore, a novel approach to tune Class E inverters electronically is proposed. The tuning method relies on saturable reactors to ensure that the inverter operates at optimal switching conditions regardless of variations in the load and the distance between the coils. In addition, a more accurate model of inductive links is presented in order to provide a better understanding of the major power losses in resonant inductive links. Experimental results are presented to confirm the improved accuracy of the inductive link model and the validity of the tuning method.
Abstract-This paper will present the modelling, analysis and design of a load-independent Class EF inverter. This inverter is able to maintain zero-voltage switching (ZVS) operation and produce a constant output current for any load value without the need for tuning or replacement of components. The loadindependent feature of this inverter is beneficial when used as the primary coil driver in multi-megahertz high power inductive wireless power transfer (WPT) applications where the distance between the coils and the load are variable. The work here begins with the traditional load-dependent Class EF topology for inversion and then specifies the criteria that are required to be met in order achieve load-independence. The design and development of a 240 W load-independent Class EF inverter to drive the primary coil of a 6.78 MHz WPT system will be discussed and experimental results will be presented to show the load-independence feature when the distance between the coils of the WPT system changes. A. IntroductionResonant soft-switching converters, such as Class E and Class EF 2 inverters, are commonly used in high power inductive WPT systems that operate at multi-megahertz frequencies due to their efficient operation and simple construction [1], [2]. However, they are only designed to operate at optimum switching conditions for a fixed load and therefore are highly dependent on the load value. They are less tolerant to load variations compared to other inverters, such as Class D inverters, which causes them to become less efficient as the load deviates from its optimum value. Consequently, this limits the WPT system to function efficiently only at a fixed coil separation distance and for a narrow load range.There have been attempts to tune the Class E inverter while in operation to compensate for any variations in the load by using saturable reactors and varactors [3]. While these methods can increase the load range that Class E inverter can tolerate, they require either the user to tune them manually by observing the MOSFET's drain waveform or require a control loop to be implemented. Additionally, the tuning elements can limit the Class E inverter to operate at higher power levels.It was shown in [4], [5] that the Class E and Class φ 2 inverters when used with a finite-DC inductor instead of the usual choke, can be designed such that the they achieve zerovoltage switching (ZVS) and produce a constant output voltage as the load resistance varies. These designs extends the load range at which the Class E and Class φ 2 inverters can operate efficiently from infinite load resistance (open circuit) to a certain minimum load resistance. Although these designs can be Fig. 1. The Class EF inverter applied to several applications, such as high frequency DC/DC converters, they cannot be applied efficiently in inductive WPT application where the distance between the coils changes. This is because the load will range from zero resistance (short circuit) when the coils are completely separated from each other to a certain ma...
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