Charging batteries without wires in a compact, efficient manner

January 13, 2014 // By Trevor Barcelo, Linear Technology
Batteries provide power to many different applications across a wide range of industries. In many of these applications, a charging connector is difficult or impossible to use. Wireless charging adds value, reliability and robustness in these and other applications.

Some products require sealed enclosures to protect sensitive electronics from harsh environments and to allow for convenient cleaning or sterilisation. Other products may simply be too small to include a connector, and in products where the battery-powered application includes movement or rotation, then charging with wires is even less feasible.

Wireless Power System Overview

As shown in Figure 1, a wireless power system is composed of two parts separated by a gap: transmit circuitry, including a transmit coil, and receive circuitry, including a receive coil. The transmit circuitry generates a high frequency alternating magnetic field around the transmit coil. This magnetic field is coupled to the receive coil and converted to electrical energy, which can be used to charge a battery or power other circuitry.

Figure 1. Wireless charging system overview

When designing a wireless power charging system, a key parameter is the amount of charging power that actually adds energy to the battery. This received power depends on many factors, including the amount of power being transmitted, the distance and alignment between the transmit coil and the receive coil, also known as the coupling between the coils, and finally, the tolerance of the transmit and receive components.

The primary goal in any wireless power design is to guarantee delivery of the required power under worst-case power transfer conditions. However, it is equally important to avoid thermal and electrical overstress in the receiver during best-case conditions. This is especially important when output power requirements are low; for example, when the battery is fully charged or nearly fully charged. In such a scenario, available power from the wireless system is high, but demanded power is low. This excess power typically leads to high rectified voltages or a need to dissipate the excess power as heat.

There are several ways to deal with excess power capacity when the demanded receiver power is low. The rectified voltage can be clamped with a power