Rapid Wireless Capacitor Charging Using a Multi-Tapped Inductively-Coupled Secondary Coil

Abstract

Abstract-This paper presents an inductive coupling system designed to wirelessly charge ultra-capacitors used as energy storage elements. Although ultra-capacitors offer the native ability to rapidly charge, it is shown that standard inductive coupling circuits only deliver maximal power for a specific load impedance which depends on coil geometries and separation distances. Since a charging ultra-capacitor can be modeled as an increasing instantaneous impedance, maximum power is thus delivered to the ultra-capacitor at only a single point in the charging interval, resulting in a longer than optimal charging time. Analysis of inductive coupling theory reveals that the optimal load impedance can be modified by adjusting the secondary coil inductance and resonant tuning capacitance. A three-tap secondary coil is proposed to dynamically modify the optimal load impedance throughout the capacitor charging interval. Measurement results show that the proposed architecture can expand its operational range by up to 2.5X and charge a 2.5F ultra-capacitor to 5V upwards of 3.7X faster than a conventional architecture.

Optimal Tuning of Inductive Wireless Power Links: Limits of Performance

“…This means the load is nonlinear and will include a rectifier and filter capacitor. These are difficult to analyse and often are replaced by an "equivalent" resistor as in [4] and [9] or by more than one resistor when the rectified voltage has a relatively large range of values as in charging a capacitor [14].…”

“…Finally, for the untuned-primary, parallel-tuned secondary (UP) circuit, the load power is given by (14) obtained by taking .…”

“…Two notable performance criteria for designing wireless power systems are maximizing delivered power for a given input drive voltage [2], [11]- [14], [21], [24] and maximizing power transfer efficiency [3]- [6], [8], [15], [22], [23]. Although maximizing efficiency minimizes the required input power for a given delivered power, it can result in a higher driving voltage being required.…”

Near-Field Wireless Power Transfer

“…It is well known that using a parallel secondary tuning capacitance induces a voltage multiplication factor, making rectifier diodes easier to turn-on. 1 This makes a seriesparallel link configuration ideal for low-power (i.e., high R L ) applications, where it is difficult to generate sufficient voltage to activate non-linear rectifiers. At high powers (i.e., low R L ), the voltage multiplication factor may present voltages that go far beyond CMOS compatibility, limiting utility.…”

“…Due to the varying requirements of each standard, this chapter will not delve into implementing a system for a specific standard; rather, this chapter will introduce the fundamental concepts necessary to understand and optimize near-field wireless power systems in general. In addition, a design example of a rapid wireless ultra-capacitor charger will be discussed [1].…”