University of Tennessee, Knoxville TRACE: Tennessee Research and Creative Exchange Engineering -- Faculty Publications and Other Faculty Publications and Other Works -- EECS Works 4-2013 Design and Optimization of Printed Circuit Board Inductors for Wireless Power Transfer System Ashraf B. Islam University of Tennessee - Knoxville Syed K. Islam University of Tennessee - Knoxville Fahmida S. Tulip Follow this and additional works at: https://trace.tennessee.edu/utk_elecpubs Part of the Electrical and Computer Engineering Commons Recommended Citation Islam, Ashraf B.; Islam, Syed K.; and Tulip, Fahmida S., "Design and Optimization of Printed Circuit Board Inductors for Wireless Power Transfer System" (2013). Faculty Publications and Other Works -- EECS. https://trace.tennessee.edu/utk_elecpubs/21 This Article is brought to you for free and open access by the Engineering -- Faculty Publications and Other Works at TRACE: Tennessee Research and Creative Exchange. It has been accepted for inclusion in Faculty Publications and Other Works -- EECS by an authorized administrator of TRACE: Tennessee Research and Creative Exchange. For more information, please contact [email protected]. Circuits and Systems, 2013, 4, 237-244 http://dx.doi.org/10.4236/cs.2013.42032 Published Online April 2013 (http://www.scirp.org/journal/cs) Design and Optimization of Printed Circuit Board Inductors for Wireless Power Transfer System Ashraf B. Islam, Syed K. Islam, Fahmida S. Tulip Department of Electrical Engineering and Computer Science, University of Tennessee, Knoxville, USA Email: [email protected] Received November 20, 2012; revised December 20, 2012; accepted December 27, 2012 Copyright © 2013 Ashraf B. Islam et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ABSTRACT Wireless power transfer via inductive link is becoming a popular choice as an alternate powering scheme for biomedical sensor electronics. Spiral printed circuit board (PCB) inductors are gaining attractions for wireless power transfer ap- plications due to their various advantages over conventional inductors such as low-cost, batch fabrication, durability, manufacturability on flexible substrates, etc. In this work, design of a multi-spiral stacked solenoidal inductor for bio- medical application in 13.56 MHz band is presented. Proposed stacking method enhances the inductance density of the inductor for a given area. This paper reports an optimization technique for design and implementation of the PCB in- ductors. The proposed scheme shows higher inductance and better figure-of-merit values compared to PCB inductors reported in literature, which are desirable for wireless power transfer system. Keywords: Inductive Link; Solenoid; PCB; Wireless Power Transfer; Biomedical 1. Introduction factor (Q) [10]. But the physical dimensions of the im- planted inductors cannot be large due to various bio- With the advancement of biomedical instrumentation tech- medical constraints. Therefore, there is a need for small nologies, sensor based remote healthcare monitoring sys- sized and high inductance, high quality factor inductors tem is gaining increasing attention in the recent years. for implantable sensor applications. Inductors account for These sensors can be classified as wearable or implant- more than 90% of the size of the total implantable system able. While the wearable sensors are placed outside the and therefore the reduction of the size of the inductor body, the implantable types are placed underneath the will significantly reduce the area and the volume of the skin or inside the body cavity typically via surgical means. overall system. Implantable sensors are designed to acquire the informa- Efficiency of an inductive link in parallel resonance tion on the vital physiological phenomena by monitoring can be described as [11], blood glucose level [1-3], lactate in the bloodstream or 2 tissues [4], pH, oxygen, and minimally invasive moni- kQQ12 link 2 (1) toring of pressure in blood vessels and intracranial com- 2 12 11kQQ12 partments, etc. [5]. These sensors include associated elec- tronic circuits for signal processing and data transmission. where, Q1 and Q2 are the unloaded quality factors of the Powering these circuits is always a crucial design chal- primary and secondary coils, respectively and “k” is the lenge since the batteries are typically not allowed in an mutual coupling which has a value ranging from 0 to 1. implantable system which can come in contact with This equation confirms that the efficiency increases as 2 blood resulting in serious health risks and can be fatal in k Q1Q2 increases, and therefore the first and the foremost some cases. An alternate approach is to supply power design consideration in an inductive link design is the wirelessly for tether-less and battery-less operation of the attainment of the highest possible unloaded Q and k. circuits. Inductive coupling is the most common method These two vital parameters are functions of shape, size of wireless power transfer (WPT) to the implantable and relative positions of the coils. sensors [6-9]. For good inductive coupling, the inductors Substituting Q1 = L1/R1, Q2 = L2/R2 in the Equation should have high inductance (L) as well as high quality (1) results in, Copyright © 2013 SciRes. CS 238 A. B. ISLAM ET AL. size of the PCB inductor is chosen to be 10 mm × 10 mm 22LL12 k to comply with the size constraints of an implantable sys- RR12 link (2) tem. The structure of a commercial 4-layer PCB is util- 12 2 LL ized with FR-4 as the substrate material and copper as 22 12 11k the metal layer. RR12 The inductors are designed and simulated using Son- TM where ω is the frequency, L1 and L2 are the inductances net software [17]. The software provides precise RF and R1 and R2 are the resistances of the primary and the models (S-, Y-, Z-parameters or extracted SPICE model) secondary coils, respectively. It is evident from this equ- for planar circuits and antennas by employing a rigorous ation that increasing the inductance value of the induc- method-of-moments (MoM) electromagnetic (EM) analy- tors will increase the efficiency of the overall system. sis based on Maxwell’s equations and including all para- Therefore small sized high inductance coil has two dis- sitic, cross-coupling, enclosure and package resonance tinctive advantages—high link efficiency and smaller effects. size that increases the feasibility of placing WPT based Total inductance of a spiral can be expressed as the implantable system in various biomedical applications. algebraic sum of the self and mutual inductances present There are various types of inductors designed for vari- [18]. The positive mutual coupling is additive towards ous system applications. For example, spiral printed cir- the total inductance and the negative mutual coupling de- cuit board (PCB) planar inductors are widely used in RF creases the value of the total inductance. As such, the applications. PCBs are inexpensive, low-cost and can be inductance can be increased if the effective mutual cou- fabricated in large amounts in batches. Their geometries pling between the metal lines functions as the positive and aspect ratios can be optimized, which make them coupling. Self-inductance is also proportional to the length suitable for implantable systems placed underneath the of the conductor [19]. This concept is utilized and ex- skin or within the epidural space [12]. PCBs fabricated panded to design multi-stacked spiral solenoidal induc- on thin flexible substrates can also comply with body tors. As shown in Figure 1(a), the length of the coil in a curvatures. For example, Shah et al. proposed PCBs for single layer is increased as the spiral approaches the cen- neuroprosthetic transcranial telemetry applications [13]. ter, but the negative mutual inductance becomes promi- Micromachining techniques have also been proposed for nent and the magnetic loss also increases. This limits the fabrication of rigid hermetically sealed PCBs on silicon maximum point up to which the length of the coil can be wafers or low-temperature co-fired ceramics (LTCC) [14, increased in a single layer. Parasitic series resistance also 15]. increases with the increase in the length of the coil which In this work, design of PCB inductors for WPT in bio- affects the value of Q and increases loss in the inductor. medical applications is presented. The organization of There is a trade-off among the number of turns, induc- the paper is as follows: Section 2 discusses the design of tance, quality factor and resistive power loss in an in- three-dimensional multi-spiral inductor and mathematical ductor. In the proposed design, the inductance is further models of the inductors. Section 3 discusses the figure- increased by stacking the inductors on top of each other of-merit based optimization process. Section 4 demon- in such a way that the flow of current always follows the strates the measurement results and compares the result same direction. The flow of current in the designed in- with published works reported in literature. Section 5 ductor is inward in layer 1, it becomes outward in layer 2, draws the conclusion of the paper. and it becomes inward again in layer 3, as shown in Fig- ure 1(b). The inductance of a solenoid is proportional to 2. Design of 3-D Multi-Spiral Inductor the magnetic flux density associated with it. If the mag- 2.1. PCB Inductor netic flux density of a solenoid is increased it will result in an increase in the overall inductance value. By having In this work, PCB inductors are designed for targeted a solenoidal structure the current is allowed to flow in the biomedical applications. The targeted frequency of op- same direction and the magnetic flux inside the loop be- eration of the inductors is 13.56 MHz which fall within comes monodirectional thereby effectively increasing the the license-free industrial, scientific, and medical (ISM) overall inductance of the structure.