Wireless charging of electronic devices is a boon to convenience, reliability, and universality. The convenience comes from not having to search for the right power cord when you need to charge your device. Reliability is enhanced because you don’t have to insert a tiny plug into a tiny jack, with the possibility of one or both failing. And, universality is enhanced because every type of device from various manufacturers may require a different type of plug, jack, or cable. And as wireless charging becomes mainstream, standards are emerging that are well on their way to coalescence and universal application.
Wireless Charging Standards
Wearable devices operating under the Power Matters Alliances (PMA) standard or the Alliance for Wireless Power (A4WP) Rezence standard will communicate with their base through Bluetooth. Devices adhering to the Wireless Power Consortium (WPC) Qi standard will rely on modulated back scatter regulation. As such, they will not have a need for any wire-based interface at all. Recharging will need to be accomplished either wirelessly or through the wholly impractical method of removing the battery from a tiny, delicate device, recharging it externally, and reinstalling it. Even if such a method were feasible, it would add cost to wearable devices, by necessitating that the battery be removable in the first place. And, judging by the early generations of wearables now coming to market, the manufacturers will have none of that.
The rechargeable toothbrush, oddly enough, is the device that can probably be credited with starting the idea of wireless charging for consumer devices. Because no metal that can hold any sort of electrical charge at all can be allowed to come in contact with the gums, power plugs were eliminated and wireless charging became essential.
How Does Wireless Charging Work?
The solution for toothbrushes is based on the workings of a transformer, where there are two coils – the primary and the secondary. An AC current is applied to the primary coil, and the magnetic field created causes an AC voltage to appear in the secondary coil. This results in inductive coupling, and the big difference is that for transformers, both coils are part of the same manufactured device. In wireless charging, one coil is in the charger and the other is in the device to be charged. They must be physically close, but they are separate.
And, because a toothbrush can sit in its charger all day except for the few minutes that it’s being used, efficiency is irrelevant. This may be good enough for a toothbrush, but not for a smartphone, which is relied upon for battery life all day long.
Resonant Inductive Coupling
Enter resonant-inductive coupling. Here, the circuits around the transmitting and receiving coils are tuned to be “resonant” with each other. This not only allows more power to be transferred more quickly, but it is possible for the devices to be physically further away from each other.
Nikola Tesla first demonstrated resonant-inductive coupling in the 1890s, and an early version of the technology was utilized for recharging implantable medical devices decades ago. Today, there are a plethora of low-power devices, such as smartphones, that need to be recharged often. With a little effort on the part of the manufacturers, a lot of crossover is possible. The three standards call for different operating frequencies for power transfer, but the PMA and A4WP organizations are merging, so their standards are likely to do so, too. Different frequencies require different antennas, so cross compatibility with the WPC’s Qi standard is less likely. However, cross compatibility is possible. One example is the Chargespot, which supports both the Qi and PMA standards.
What is Qi Wireless Charging?
Perhaps the most dominant standard for wireless charging is WPC’s Qi. Subscribers include Microsoft, Motorola Mobility, Nokia, Samsung, Sony, and Toshiba. Implementing this standard is tricky; compatibility is a problem and the charger must be closely aligned with the device under charge. A4WP’s Rezence doesn’t require such close coordination of the device under charge’s position relative to that of the charger, and unlike with Qi, more than one device can be charged simultaneously. The PMA standard, which seems headed for a merger with Rezence, has a base of support among semiconductor manufacturers. Interestingly, Apple, which has a tendency to go its own way, may be bending a bit, as reports indicate that the new, highly touted Apple watch may turn out to be chargeable via Qi chargers. However, the Apple watch hasn’t gone through Qi certification, so they may have to modify things.
The technology has matured to the level that chip manufactures such as Freescale Semiconductor, Linear Technology, Texas Instruments, and Toshiba have taken much of what’s needed to implement wireless charging, for both the receiver and transmitter, onto widely available chips. A definite trend is for most of these units to be able to accommodate more than one of these contending standards since the only changes necessary are in code as long as a compatible antenna is used.
Communication is Key
There must be constant communication between the unit under charge and the charger. An important reason for this is that both must be “tuned,” because in a consumer application, positioning between the units will vary. This is especially true in the types of applications now cropping up in coffeehouses, airports, and other public places, because precise placement of the device under charge on the charger may not be practical, due to the busy nature of the venue. Sometimes chargers integrate magnetic centers to assist in proper placement for charging.
Toshiba’s TC7761WBG is an example of a wireless power-receiver IC that manufacturers can incorporate into their devices to facilitate wireless charging. Toshiba’s chip is designed to implement the Qi standard, which is no surprise, because as mentioned earlier, Toshiba is a member of the WPC. An accompanying datasheet provides designers with complete information, and also includes a very useful block diagram (Figure 1).
The receiving unit monitors the power transmitted by the sending unit; not the amount transmitted, but, more importantly, the amount actually received. The TC7761 has established a feedback loop to ensure that no more power than the receiving unit can safely handle is absorbed.
Communication is via ASK modulation, and the Qi standard calls for communication at 2kbps. Under this regime, the transmitter only sends out power after it has recognized the receiving unit and the device has signaled that it is ready. Receiving units designed with this chip make no demand on the unit’s computing power.
While wearables are driving the most urgency to the development of wireless power, nobody really likes plugging in power cords. The present Qi standard, for example, can deliver up to five watts, which makes it a practical alternative for a plethora of low-power devices, such as smartphones and tablets. Updated Qi standards are being considered, including one that will accommodate fifteen-watt devices and a high power standard for 100 watts and more.