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Wireless charging standards still struggling to power up

Image credit: Lxory

Wireless charging is meant to be the easy alternative to plugging in cables, but the industry is finding it tricky to put into action.

My phone failed to charge last night for a stupid reason. Despite going to all the trouble of plugging in the USB connector on the phone’s socket, what I didn’t realise is that I’d pulled the charger itself slightly out of its socket. It had sat for hours doing nothing except to drain the battery a little bit more. It’s at times like these that wireless charging looks like a great idea. However, that has its own problems.

Mark Hopgood, senior director of strategic marketing for power-chip manufacturer Dialog Semiconductor, says a common problem with the current generation of charging mats based on the Qi standard is that alignment is just as critical as checking both ends of the cable are plugged in. “The biggest complaint about Qi today is that people find they didn’t place the device quite right and it didn’t charge.”

The Qi standard, as with much older types of wireless charging used on toothbrushes, uses tight inductive coupling. The coils on the charger and the device need to be more or less the same size and arranged so that the pair act like a transformer. A loosely coupled version was proposed. “This can have smaller coils in the device or allow charging over a greater distance, but this didn’t take off because it had an adverse effect on other radios,” Hopgood explains.

To try to find an optimum approach to wireless charging, Dialog invested in the specialist Energous two years ago. That company favours using RF transmissions to recharge devices, rather than inductive coupling. It’s a very old idea. RFID tags use the energy supplied by a reader to fire up their memory and report back with the code stored inside before going back to sleep again when its capacitor runs down again. RF charging for bigger devices simply keeps the signal going longer.

At the kinds of frequencies companies like Energous propose for RF charging, the receiving antenna inside the device can be pretty small, which has advantages. “You can’t put [inductive] coils into earbuds,” Hopgood says. This means the earbuds have to be recharged inside a holder. “With this approach [RF], you can implement charging in hearing aids [directly].”

However, you will still need to be pretty close to the charger for this to work. One issue with any form of wireless charging is the energy received drops off rapidly: it’s an inverse cube-law relationship because the RF radiates out in a sphere. Dialog and Energous are focusing mainly on short distances, down to near-field charging mats or some kind of proximity charging. “Energous was first to get approval for mid-field charging from the US regulator,” Hopgood claims.

Mid-field charging works over a distance of around 15cm with an option for distances up to 90cm. In principle, RF charging could work over longer distances. “Far-field is where most of the excitement is in the industry, but it’s the least practical as well. You won’t be charging a mobile phone at several metres with watts of power,” Hopgood says.

Regulator approval is likely to be a stumbling block for RF charging. Twenty years on from its introduction, Wi-Fi continues to be plagued by concerns about health. Hopgood points to market concerns about putting Wi-Fi into devices such as bedside clock radios. “Many consumers don’t like laying next to a Wi-Fi transmitter,” he says.

Although many of the same people think nothing of leaving their mobile phone on the bedside table at night, new applications for RF - particularly those that imply the delivery of larger amounts of energy - are going to raise electrosensitivity concerns. Regulators seem likely to agree, but it’s not dead yet and it harks back to one of the early ideas for low-level energy harvesting: using the comparatively low-level emissions from Wi-Fi.

Hopgood says Dialog and Energous have explored several frequency ranges for RF charging, starting with 5.8GHz and then sub-gigahertz. Most of the action now looks to be around 2.45GHz, in the familiar licence-free zone used by Bluetooth and Wi-Fi. This has stayed licence-free because before these digital protocols came along, few wanted to use the frequency employed by microwave ovens because it’s so readily absorbed by water. As a result, the power is going to be restricted to Wi-Fi-like levels if regulators give the idea the nod – and the devices that get charged will be sensors that do not need a lot of juice, rather than cordless speakers or phones.

Industrial users get a bit more choice, Hopgood says. “We have some customers doing this. If you have proof there is nothing between the charger and the receiver, you can turn it up to something more like microwave oven levels.”

With that, you can envisage shop-floor robots protected inside a safety cage being recharged wirelessly using RF. In the home, you are going to need to get a lot closer to get things powered. However, it should be less prone to cabling errors.

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