Optimizing the magnetics and precise power technology are crucial for designing effective inductive wireless power transfer systems. Depending upon the technique, the amount of energy transferred can differ by a thousand times. Lower-power WPT systems are used in consumer electronics devices such as cell phone charging pads, rechargeable toothbrushes and implantable medical devices while high-power systems are increasingly being used in wireless charging of electric vehicles. The two types of application share fundamental concepts, but there are critical differences in the magnetic aspects such as coupling, as well as other elements, for product designers to navigate.
We like the pair of presentations by power supply manufacturer RECOM Power in Austria that deal with both applications while also explaining the interactions with power system design. Some highlights are given here. The complete presentations with much greater detail are available on the company’s website. Another helpful source is the RECOM AC/DC Book of Knowledge.
The principal components in an inductive WPT design are a transmitter coil and a receiver coil. Power is transferred wirelessly using the magnetic field between the transmitter to receiver. On the transmit side the DC input power source typically feeds a half-bridge or full-bridge topology to power a DC-to-AC power inverter. The inverter creates an alternating magnetic field using an LC series resonance tank to transmit power to the receiver. On the receiving side, series resonance components convert the incoming magnetic field to current, and a high-power rectifier converts the AC current into a DC voltage. An output regulator is used to provide a stable DC voltage to the load.
Coil configurations can vary widely depending on the application and the type of power coupling. Both resonant and non-resonant inductive coupling are commonly used.
Consumer and industrial applications
Consumer WPT technology conforms to the relevant industry standard for inductive charging. The Qi (pronounced “chee”) system from the Wireless Power Consortium is highly efficient but requires positional alignment of the transmitter and receiver coils. The competing AirFuel Alliance standard uses magnetic resonance power transfer with tuned coils that are less sensitive to misalignment and support longer distances up to a few meters.
There are other significant differences between the Qi and AirFuel systems. For example, the Qi wireless charging standard has a frequency range of 110–205kHz for low power applications (5W) and 80–300kHz for medium-power applications (up to 120W). The AirFuel specification uses a higher frequency of 6.78MHz.
The transmitter and receiver coil design, distance, and alignment play a key role in the efficiency of the WPT design. In the simplest low-power WPT applications such as an electric toothbrush the relative locations of the transmitter and receiver coils are tightly controlled, but there is greater variation in higher-power systems.
High-power wireless charging for electric vehicles
In a basic EV wireless charging concept, the vehicle would be simply parked over a charging coil and power would be transferred by inductive wireless power transfer to recharge its batteries. Wireless communication would ensure that power was only transferred when it is safe to do so, much as a modern mobile phone communicates with a Qi-enabled charger pad to ensure that no foreign objects are present in the charging field before power is applied.
The main difference between a mobile phone and EV wireless power charger is the power level used. For a high-power wireless charger, efficiency must be maximized, whereas phone chargers are typically only 70% efficient. This figure is acceptable for a low-cost commodity product, but would be wasteful for an EV wireless charger, where a system efficiency figure of closer to 85% is required (AC power to DC power).
There are three ways in which the power transfer efficiency can be improved: more tightly coupled magnetic circuits, higher frequency operation and better matching. About a thousand times more energy can be transferred in a coupled magnetic field than a capacitively coupled field when practical voltage and current limitations are considered. Therefore, inductive and magnetic resonance coupling lend themselves to the highest power transfer.
Resonant inductive coupling using intermediary resonators. The coupling coefficient can be enhanced by inserting intermediary coils which act as ’magnetic lenses‘ to focus the magnetic flux. Higher power resonant inductive coupling systems may use three or more of these coils. These intermediary coils are resonant tank circuits with a capacitor in parallel with the winding which resonates at the frequency of the alternating magnetic field.
The resonators boost the effective magnetic field strength from the transmitting coil and concentrate the effective received field into the receiving coil, increasing the coupling efficiency significantly. Additionally, even if only part of the projected magnetic flux is intercepted by the intermediary circuits, they will still resonate, so separation distance and alignment are not so critical as with two simple flat coils.
Intermediary resonators do not have to be placed symmetrically. If the limiting factor for power transfer is sufficient magnetic flux, then paired resonators placed close to the transmitter coil will magnify the local magnetic field through the coupling factors. Such intermediary coils are essential for WPT applications where the distance and alignment between the transmitting and receiving coils is not fixed, for example in an electric road that recharges a moving vehicle driving over it.
Power supply needs to be stable and precise
The power supply for WPT must provide a stable and precise output voltage or current to ensure consistent and efficient power transfer. Fluctuations can impact efficiency and reliability. WPT systems may experience varying loads, especially in scenarios where multiple devices are wirelessly charged simultaneously. The power supply should be capable of adapting to these changes in load to maintain optimal performance.
WPT systems often require isolation between the power supply and the wireless power transfer components for safety reasons. Isolated power supplies prevent electrical hazards and ensure compliance with safety standards. A metal object, such as a coin, a key, or a nail, may be inadvertently located near the magnetic fields generated by the WPT system, creating a potential safety hazard and affecting the electrical characteristics of the WPT system.
High magnetic fields may also produce adverse symptoms in living objects located between the transmitter and receiver. Identifying such unintended objects and avoiding transmitting power to them is known as foreign object detection (FOD). FOD is typically accomplished by calculating the power received at the receiver and communicating it back to the transmitter. A mismatch in the two values after correction factors have been applied is assumed to be due to a foreign object absorbing power and triggers a shutdown.
Power supplies should incorporate temperature monitoring mechanisms and overtemperature protection (OTP), especially in wireless charging pads or modules. Standard protection features should also include short circuit protection (SCP), and overvoltage protection (OVP). Power supply frequency, communication interfaces and electromatic compatibility are other key factors.
Headquartered in Gmunden, RECOM manufactures a full range of standard and customized DC/
DC and AC/DC converters in every power class from sub-1W to tens of kW, along with switching regulators and LED drivers in a wide selection of formats. For more information and to see the full presentations on WPT, visit www.recom-power.com.
