Wireless Device Charging

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Wireless charging of portable devices such as smartphones, tablets, and laptops seems like a good idea that prevents many problems. It allows for a totally sealed product enclosure, a tangible advantage in difficult environments such as military, industrial, and medical applications.

It eliminates the need for a connector, a plus in extremely compact designs such as smart watches. It negates the issue of not having the right charging cable. It prevents accidental or deliberate damage to a charging connector or even malicious electrical damage to the charger itself (by injecting high currents and voltages). It appears to be so convenient: just place your device on the charging pad, wait a few hours, and the unit is charged. It clearly makes sense at public charging stations, where the charging pad can be built into a tabletop with no need for cords or user handling a physical cable, Figure 1.

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Figure 1: Using a wireless charging configuration, a user would place their smartphone (or other device) on their nightstand, without the need to plug the device in.

The principle of wireless charging — which is really a manifestation of the wireless transfer of power — is not new; it has been explored since the understanding of the transformer and inductive coupling was developed and used in the mid-1800s. The transformer is still used extensively for AC-voltage step up/step down, galvanic isolation, and many other power and signal applications, almost always with an iron core. However, in wireless charging, the primary and secondary windings do not have a common core, but instead are separated by air over distance of about 25 mm (1 inch) or more, depending on configuration.

While the principles of transformers are simple and well-understood, using it for a wireless charger is not easy, for many reasons. Both the charging power source and the device to be charged have wire coils, which are place closed to each other and with their axes aligned. An alternating current in the source induces an alternating current in the receiver, which can then be rectified and used to charge a battery, Figure 2.

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Figure 2: The wireless charger architecture is not a simple pair of transformer coils in proximity; among other functions, it requires a reverse communication from the device being charged to the power source (Source: Texas Instruments).

(Note that wireless charging is already used in some ultralow-power implanted medical devices such as pacemakers, but its use has been limited by unavoidable impediments such as the centimeters of body tissue between the power source and the receiver, physical misalignment and orientation issues, and other factors. Thus, despite its attractiveness at first glance compared to having surgery to replace a battery, the latter is still a better solution in many cases.)

For wireless charging to work well, with small size and good efficiency (power transfer ratio), the AC-drive frequency must match the resonant frequency of the coils used at each end. Metal objects between the two coils (such as a paper clip) can detune the coils and so decrease efficiency and cause faults. Keep in mind that wired charging has a 100% power-transfer efficiency (minus a small IR loss) while wireless charging has typical efficiency between 50 and 80%, so more power must be used on the source side, or a longer charging time must be acceptable. Finally, the receive coil and associated circuitry (capacitors, receiver, and charger-related ICs) adds to BOM complexity, component cost, and size, so a device fitted for wireless charging will be a little bulkier than one using wired charging.

Wireless charging gains traction

Despite the electrical, physical, and cost realities associated with wireless charging, the potential benefits are significant enough that consumer-product vendors are starting to incorporate it into their products. To make wireless charging a viable approach, standards are needed to ensure compatibility between the charging source (power transmitter) and the device being charged (power receiver).

At present, three standards are vying to become the dominant one. These are from the Wireless Power Consortium, or WPC with the Qi standard (not to be confused with inductor parameter "Q"); the Power Matters Alliance (PMA); and the Alliance for Wireless Power or A4WP (with their Rezence standard). In January 2015, the Alliance for Wireless Power and the Power Matters Alliance announced plans to merge, and in September the merged entity on a new name, the AirFuel Alliance.

Each standards group is lining up major end-product vendors who will commit to adopting that standard, since it is not practical to put one than one receiver type into a device. However, some vendors of transmitter components do offer universal multi-standard circuits which will adapt to any of the standards; this is technically feasible and manageable with respect to form factor and packaging, although there are additional costs.

Each wireless power approach offers tradeoffs in performance and capacity; the table is an overview of the key attributes. In brief, PMA’s approach uses on magnetic induction, which requires devices to be placed on a charging surface for power transfer to happen. In contrast, the A4WP standard uses on resonance charging, which delivers power out over a greater distance, so devices can be a foot or further  away to receive power (but with reduced efficiency).

Although they use the same underlying principles, each uses a different combination of frequency, coupling constant K, and inductor Q (quality) factor. WPC and PMA needs high K values and can use lower-Q, lower-cost inductors, and thus have lower positional flexibility; the transmitter and receiver must be no more than about 10 to 20 mm apart. The A4WP approach is a loosely coupled system, which results in a large amount of positional freedom, with a wider charger surface.

An important consideration is the deliverable charging-power level, in order to have a one-to-two hour maximum charging time for the device; a laptop battery needs more energy for a full charge than a smartphone does. The first iteration of the WPC Qi standard targeted smartphones, and was designed to provide 5 W of output power corresponding to a charging rate of approximately 1 A. As the potential for wireless charging has expanded, vendors have developed ICs and power components for both higher and lower power levels, such as 2 W and 10 W.

The transit and receive coils, although "merely" passive components, are very critical to the design, size, and efficiency of a wireless charging system. While lower-power levels ease the electrical and thermal design, and thus less costly, it doesn't necessarily shrink the size of the coil. In general, the coil sizes of the transmitter and receiver must be roughly matched for effective coupling.

Coil vendors now offer standard catalog components which are specifically targeted for wireless-charging applications, with suitable diameters, made as thin as possible, and the required electrical specifications such as inductance, DC resistance, and Q. Typical 5-W coils have a diameter of about 40 to 50 mm, and the receiver coil is about 35 mm across. If the coil is smaller, such as below 25 mm for a wearable device, power-transfer efficiency suffers; however, most wearable-device batteries have fairly low capacity and so don’t need much energy to be pumped across to them.

Wireless charging is not just a matter of sending power to a receiving coil as an open-loop event. The two sides must communicate to set up the link, establish charge rate, and also know when to terminate charging. For example, the Qi standard uses digital feedback with packets sent back over the magnetic coupling path, and the receiver communicates at a 2-kHz rate using load modulation. The standard's identification-and-configuration command packets assure that power is being transferred to the only correct device, if there is more than one in the sweet spot. Charge-complete and end-power-transfer packets stop power transfer when the battery is charged, there is a foreign object in the path, or other conditions require that power transfer be terminated.

ICs key to wireless charging adoption

Vendors of ICs and other components see wireless charging as an opportunity to deploy their power-related expertise and leverage the versatility of application-specific and optimized components.

A chip set and reference design for low-power Qi charging, such as for low-power wearable devices, is available from Texas Instruments and is based on their bq500211 Qi-compliant wireless-power transmitter and bq51003 Qi wireless receiver ICs. The simplified block diagram, Figure 3, gives an overview of the wireless charging system which delivers receiver output of 5 V and up to 1 A.

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Figure 3: The high-level block diagram of a 5-W wireless Qi charging system using a pair of power-specific ICs from Texas Instruments only begins to show the complexity of a complete design (Source: Texas Instruments).

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BQ500211ARGZT

Texas Instruments Wireless Charging Devices View

 

This overview does not reveal the full complexity of the design, of course, but the full schematic shows there is still a significant amount of circuitry needed, Figure 4 and Figure 5. As with all wireless charging systems, the coil details (physical size, form factor, placement, and electrical specifications) are critical to an effective and compact design; there are many coil-related tradeoffs to consider.

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Figure 4:  The transmit side of the complete wireless-charging circuit using the Texas Instruments bq500211 Qi-compliant wireless-power transmitter IC requires a few external passives plus MOSFETs and their drivers (Source: Texas Instruments).

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Figure 5: The receive side of the Qi charging circuit is more complicated than the transmit side; here, it uses the Texas Instruments bq51003 Qi wireless receiver, supplemented with the bq24232 battery charger IC (Source: Texas Instruments)

Conclusion

Wireless charging for portable devices is the latest role for wireless power transfer, a concept that has been known and used for over 100 years. Whether it becomes a mainstream technique for wearables, smartphones, laptops, and other devices is still unclear, although many top-tier consumer vendors offer it in some of their products. Despite its obvious virtues in many situations, it does bring additional cost and bulk to the design, as well as possible incompatibilities between standards. There are also questions of whether consumers will un-learn their well-established and reassuring habit of carrying a small charger and cable, and if the new Type-C USB connector for power and high-speed data will be a more attractive alternative.




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