Design of a Qi-compliant receiver coil

The implementation of the Wireless Power Consortium (WPC) Qi standard enables wireless charging capabilities for a variety of end applications. The size and/or power requirements of the receiver (Rx) coil may vary for each application. The Rx coil is a key component to a successful and efficient Qi standard Rx design. There are also many design methods and balancing trade-offs to consider. Therefore, designers must carefully choose their methods and proceed methodically when implementing a solution. This article will discuss in detail some of the technical issues that need to be addressed to achieve a successful Rx coil design. The article covers the Qi standard system model of the basic transformer, Rx coil measurements and system-level impact, and some methods to check whether a design can work successfully. It is assumed that the reader of this article has a basic understanding of the Qi standard inductive power system.

Transformer Qi Standard System

For many near-field wireless power systems, such as those specified by the WPC, the behavior of magnetic power transfer can be modeled using a simple transformer. Conventional transformers are typically a single physical structure with two windings wrapped around a core material that has a much higher magnetic permeability than air (Figure 1). Because conventional transformers use highly permeable materials to transfer magnetic flux, most (but not all) of the magnetic flux generated by one coil couples with the other coil. The degree of coupling can be measured by a parameter called the coupling coefficient, which is represented by k (which can range from 0 to 1).

Figure 1: Physical structure of a conventional transformer