Magnetics Business & Technology - Winter 2015 - (Page 8)

FEATURE ARTICLE An Introduction to Resonant Inductive Power Transfer By Gianpaolo Lisi, Systems Manager | Kilby Laboratories Kalyan Siddabattula, Systems Manager, Wireless and Low Power Charging | Texas Instruments With the growing number of personal electronic devices that the average person is constantly relying on, wireless power transfer is becoming an essential mechanism to keep batteries charged at all times. Moreover, thanks to the intrinsic isolation that this technology provides, today's manufacturers can design more robust, fully enclosed and even water-proof electronic products. Magnetic resonance can be used to engineer wireless power transfer systems with some attractive features. In this article we present the key properties of series resonant inductive wireless power transfer and their impact on systems with a low coupling coefficient. In applications where it is desirable to transfer power to a receiver in different positions and orientations, the coupling coefficient can be quite small and can vary substantially. This is due to the non-uniformity of the magnetic field generated by a generic inductive structure and to the varying distance and orientation between transmitter and receiver coil. The color plot in Figure 2 shows the coupling coefficient between the two coils as coil 2 is positioned in different coordinates with respect to coil 1. Note that maximum coupling is not necessarily achieved when the two coils are perfectly aligned. Specific coil designs can be used to minimize or leverage this effect based on the application of interest. Introduction Wireless power transfer can be achieved with a number of different techniques, all with their pros and cons. Magnetic induction is one that has found application in many electronic systems. Some of the reasons behind the wide acceptance of magnetic induction as a wireless power transfer mechanism are the capability of transferring power through many commonly used materials (such as wood and plastic), good tolerance to objects misalignment, safety for living beings and the possibility of using extremely thin structures (such as printed laminate coils) as transmitter and receiver antennas. Coils, Coupling Coefficient, Mutual Inductance An example of inductive wireless power transfer system is a transmitter copper coil excited by an alternating (AC) voltage or current source and a receiver copper coil connected to a load. When the transmitter and the receiver coils are brought in proximity of each other, they behave like a double dipole or coupled inductors (Figure 1). a b Figure 1. Inductively-coupled copper coils (a); and schematic symbol of coupled inductors (b). Figure 2. Example of coupled coils and their coupling coefficient (percentage) as a function of position of coil 2 with respect to coil 1 (coils are parallel to each-other). Impedance Matching, Maximum Power Transfer, Maximum Efficiency Equations (1 - 2) can be represented graphically by the equivalent circuit of Figure 3. Assuming that coil 1 is excited by a voltage source VS(t) = VS sin(ω*t) and that coil 2 is connected to a resistive load RLOAD (see Figure 4), the circuit of Figure 3 can be solved in the phasors domain. The electrical properties of the coupled inductors in Figure 1 are described by equations (1 - 2): V1 (t) = L1 (diL1) / dt - Lm (diL2) / dt (1) V2 (t) = Lm (diL1) / dt - L2 (diL2) / dt (2) Where L1 and L2 are the self-inductances of coils 1 and 2, respectively, and LM = k √(L1 L2) is their mutual inductance. The coefficient k describes the mutual magnetic coupling between the two coils and is known as coupling coefficient. It is an adimentional number which can assume values between zero and one (or zero and one hundred when expressed in percentage). The coupling coefficient between two coils is determined by their geometric properties and their relative position in space. 8 Magnetics Business & Technology * Winter 2015 Figure 3. Equivalent circuit of the system in Figure 1, R1 and R2 represent the equivalent series resistances of coils 1 and 2, respectively.

Table of Contents for the Digital Edition of Magnetics Business & Technology - Winter 2015

Editor's Choice
What the Heck Happened to the Magnet Industry?
An Introduction to Resonant Inductive Power Transfer
Research & Development
Magnetics 2016: Preview
2016 Resource Guide
Industry News
Marketplace / Advertising Index
Spontaneous Thoughts: Small Ball

Magnetics Business & Technology - Winter 2015