Wireless power has become an increasingly popular means of charging a wide variety of products ranging from smartphones and tablets to toothbrushes, hearing aids and, soon, hybrid electric (HEV) and fully electric (EV) vehicles. Next month vendors will be showing off a slew of wireless charging products at the 2014 International CES so it seems a good time to offer an update on developments in wireless power technology and applications, which I first investigated in this space two years ago (focusing then on EVs).
By way of review, the principle of wireless power transfer is simple: it’s an open core transformer consisting of primary transmitter and secondary receiver coils and associated electronics. In basic Magnetic Induction (MI) schemes an electromagnetic field of a given frequency is generated by alternating current in the transmitter, inducing a voltage in the receiver coil. Current then flows and is rectified and power can be transferred to a load. In a wireless inductive charging system the primary coil resides in the charging device and the secondary coil is located in the portable device being charged.
Perhaps the most famous historical example of wireless energy transfer is Nikola Tesla’s demonstration in 1893 at the Chicago World’s Fair, where light bulbs were lit wirelessly with high frequency ambient electric fields. Even back in the days of Tesla it was well known that resonance could be employed to improve wireless power transmission. When resonant magnetic coupling is used, the transmitter and receiver inductors are tuned to the same natural frequency. Transmitting and receiving coils are usually single layer solenoids or flat spirals with parallel capacitors, which, in combination, allow the receiving element to be tuned to the transmitter frequency.
One of the big limitations of wireless power continues to be the lack of standardization. There are three major standards organizations currently pushing different approaches to wireless charging. For MI technology there are two main prevailing standards groups: the Wireless Power Consortium (WPC) and Power Matters Alliance (PMA), both with specific requirements for the positioning and alignment of the receiver coil on the transmitter to address efficiency, etc. In WPC- and PMA-based MI technologies power can be transferred over a wide range of frequencies, with the resonant frequency at which the power is transferred based on the load impedance. In the case of Magnetic Resonance (MR) technologies, power is transferred only at a particular resonant frequency. The Alliance for Wireless Power (A4WP) offers the first standard based on MR, for which, generally speaking, freedom of positioning and or multi-device charging capability for enhanced user convenience are key features.
Despite the three standards-making organizations there is no question that a multi-mode solution able to seamlessly recognize both coupled MI- or MR-based devices and transfer power in either case effectively and efficiently remains the ideal—but not yet available—long term solution.
Compatibility products for the Qi, (pronounced “Chee”) global wireless charging standard, developed and licensed by WPC, includes accessories for the Google Nexus 7 tablet and the Blackberry Z3, Samsung Galaxy S4, and Nokia Lumia 1020 smartphones, among other handheld devices. When the mobile device rests on the charging pad, the receiver communicates to the transmitter, requesting the appropriate amount of power. The transmitter transfers power to the receiver via coupled inductors, with the primary coil in the transmitter, and the secondary coil in the receiver.
Since the Rx coil is a key component in a successful and efficient design of a Qi-compliant system there are many design options and trade-offs to consider. For example, a Qi-compliant receiver coil (Rx) circuit for wireless power includes a series resonant capacitor, (C1), and a parallel resonant capacitor, (C2) which must be sized correctly per the WPC specification. In addition, accurately monitoring voltage and current on both transmit and receive sides is necessary in order to maximize efficiency in magnetic induction technologies.
Currently, the Qi standard allows for devices to be charged with up to 5 watts of power; forthcoming version of the Qi standard promise to pump 10 to 15 watts of energy to enable rapid charging of tablets and notebook computers. Operating frequency is 105 – 205kHz and Qi provides for electrical power transfer at distances of up to 4 cm (1.6 inches).
Another wireless charging standard is the work of the Alliance for Wireless Power (A4WP), founded by Samsung and Qualcomm and using near field resonant magnetic wireless charging at the 6.78MHz ISM band to generate its magnetic field. It is capable of enabling the simultaneous charging of multiple devices and the flexible positioning of devices on a charging pad. Devices can charge from up to 1.5 inches away. The A4WP does not support the close induction coupling mechanism of the WPC.
Power Matters Alliance (PMA) is a global, not-for-profit industry organization working to create a power paradigm for battery-equipped devices using wireless charging technology. The PMA’s activities are currently organized according to working groups covering inductive power and resonance power (to provide multi-mode solutions), as well as certification, regulatory and marketing. PMA is an IEEE Standards Association (IEEE-SA) Industry Connections member and IEEE also is developing the P2100.1 Standard Specifications for Wireless Power and Charging Systems, the first in a series of anticipated standards addressing parallel wireless power and charging technology specifications for both transmitter and receiver devices.
Industry analysts are predicting that we may see wireless charging capability integrated into automotive systems in the next couple of years via charging “stations” that can sit above ground, be embedded in garage floor parking surfaces or perhaps even placed outdoors in roadways, resembling manhole covers.
Wireless charging technology for automobiles utilizes electromagnetic resonance, making a magnetic connection between the charging infrastructure (transmitter) and a vehicle equipped with a wireless receiving coil. Among the fundamental issues at hand that need to be addressed are the inherently lower efficiency and wasted energy of wireless charging compared to, say, recharging EVs or HEVs via plugged connection to the electrical grid. The reduced efficiency of wireless power compared to wired power is relatively acceptable for charging low-power mobile gadgets but much less so for automotive use. The low efficiency is due to losses when transferring power over the gap between transmitters and receivers. Lost power also is released as heat, and this can become a safety issue in higher power applications.
Consequently, every part of the wireless charging system for automotive apps must be optimized for high efficiency. For example, very low loss capacitors must be selected for resonant coupling to avoid spoiling the quality factor (Q) of the circuit and reducing efficiency (the general definition of quality factor is based on the ratio of apparent power to the power losses in a device). Capacitor losses also will generate heat in the charging system and further contribute to a reduction in the qualify factor of the resonant circuit. What’s more, the ability to match the capacitor to the coil at the desired resonant frequency is needed to minimize adjustment and tuning, which can be difficult at best in stationary/off-vehicle components as well as in moving vehicle circuits.