Wireless transmission of power has numerous advantages. For example, it makes fault-prone plugs redundant and allows devices to be built into moisture-proof enclosures. Users also don’t have to put up with the hassle of plugging in cables, and most wireless power transfer applications exist in the field of portable device battery charging.
There are several established standards in this field. However, there are many applications that do not require any standards. Thus, individually optimized power transfer can be used. Figure 1 shows an inductive power transfer concept where two coils are brought close together, alternating current is generated in the primary coil, and alternating current is induced in the secondary coil through the magnetic field generated, as in a transformer.
Figure 1: Inductive power transfer concept with primary side control and receiver
In principle, the primary transmitter can be built with a simple oscillator and few discrete components, which is very efficient for transmission at low power levels. For higher power, an integrated transmitter circuit such as the LTC4125 should be used. The transmitter tunes very accurately to a given resonant frequency. This enables very high power transfer with specific components.
The LTC4125 can also detect foreign objects on the primary coil. For example, if a piece of metal is placed against a coil, eddy currents will form in the metal. They heat up the metal (especially at high power) and can cause injury. At low power levels, foreign objects only cause very weak heating and do not pose a major risk. The LTC4125 can detect a metal object and then reduce power or interrupt power transfer. To save power, the LTC4125 can adjust the transmit power according to the power requirements of the secondary side.
Figure 2 shows an example of a demonstration circuit with specific components. The diagram shows what happens when there is a specific amount of offset or separation between the two coils. In transformers, the coupling factor is usually between 0.95 and 1. In wireless power transfer systems, coupling coefficients of 0.8 to 0.05 are common. In Figure 2, the coil offset (unit: mm) is shown on the x-axis. On the y-axis the separation between the two coils (also in millimeters) is displayed.
Therefore, for a battery charging power of 1W, if the two coils are perfectly vertically aligned (ie, the coil offset is zero), the separation distance between the two coils can be as large as 12mm. The higher the power, the closer and more accurately the two coils must be aligned. The transmittable power can be adjusted by the selection of circuit elements. However, the relationship between coil offset and coil spacing will be similar to that shown in the example.
Figure 2: Effects of offset and spacing between two coils
For longer distance wireless power transfer, RF power transfer can be used. There are test setups that work in the ISM band. However, their transmittable power and transfer efficiency are much lower than the inductive coupling methods described here.