Welcome to the Planet Analog blog on wireless power. This is the first in a series of articles that will address the quickly-evolving art and science of transmitting useful amounts of power without wires. Yes, we are all aware and still in awe of Nikola Tesla’s experiments ca. 1890. But this dream is finally becoming a reality, and the topic is more relevant than ever.
Tesla, aged 40, 1896
Tesla clearly knew back then that he was on to something -- and we are now figuring it out in ways that are safe, useful, and cost-effective. There are now more than 50 million mobile phones that use inductive charging. Electric buses in Turin and Genoa, Italy, induction stovetops, and a plethora of other applications exist today as examples of how the technique is proliferating. So what’s changed to enable this relatively sudden growth in the application of wireless power?
The factors driving this change are what we will discuss in this blog. It’s a fascinating mix of materials development, IC design and process capability, consumer demand, corporate visions of world-dominance, and individual innovation. Over the course of this series, we will explore these topics, dive into the nuts and bolts of various techniques, delve into safety concerns, and explore the ever-present political interests that always accompany the creation of new standards.
Let’s start with the demand-side of the equation. The most powerful forces driving wireless power innovation and adoption are associated with the limitations of mobile-phone battery life and present-day charging techniques. The mobile phone industry even has a name for it: Battery Anxiety.
According to a recent Veloxity study, 84% of mobile phone users in NYC experience the symptoms of battery anxiety. Battery anxiety can be described as that queasy feeling one gets as the phone fuel gauge drops. It results in modified cellphone usage patterns, including decreased web surfing, picture taking, or other data-intensive activities. It also results in decreased revenue for service providers and decreased phone utility.
No wonder battery life is a key selling feature and batteries keep getting bigger. Philips produced a comedic depiction of the disorder here. Typical remedies include anxious searches for a recharge throughout the day, hunts for AC outlets, portable battery packs, and cases with integrated batteries.
Now imagine a world where cellphone charging spots are embedded into your environment wherever you need them to be. A typical day might include moving your phone from your bedside charger to your car charger to your office charger to a train or plane charger to a hotel or restaurant charger... well, you get the idea. In this scenario, phones are not going from full to empty to full again as fast as possible; they are always being topped up. More like energy sipping than gulping. Add to that image the ability of the phone to adapt to the location in which it is charging: A phone placed in a car charging spot will, via NFC, respond to the user’s program and start streaming music, GPS, contact data, and whatever else the user wants available in the car.
Access to these applications and data would be made through safe user interface systems like the in-dash touchscreen, steering wheel controls or voice-command. The same phone placed on a hotel guest room charge spot will immediately stream favorite photos to the bedside digital picture frame, stream video and music to the room’s TV and music system, link to HVAC or room service… Restaurants will offer charging and new services like at-table ordering and payment, loyalty programs, etc.
This vision of ubiquitous wireless charging is driving the investment in numerous startup companies around the globe and is already being realized in cars, hotels, and restaurants. Toyota, Chrysler, Audi, and other automakers now sell models with wireless phone charging. Restaurants and coffee shops around the world are making it available, and we will see other hospitality locations offering wireless charging soon. Major airports around the world (Atlanta, Beijing, Detroit, and Tokyo, for example) now have wireless charging stations, too.
Slowing the adoption of wireless charging are issues common to new technology. There are three competing standards that are not interoperable. Some of the earliest products were, let’s say, best suited to early adopters (they were sometimes quirky or hard to use). Some implementations had technical problems. All the above causes confusion and hesitation with those that would determine large-scale deployments. Because of consumer and commercial demand for wireless charging and its associated benefits, these barriers are being knocked down, and, as I mentioned above, we will discuss these issues in more detail in later posts.
To get us all on even footing, let’s have a look at the basic functionality of a wireless charging system. The following is a generic system, and the functional blocks are common to all modes of wireless charging systems -- single or multi-coil, inductive or resonant.
Most of the functional blocks in this diagram are common to the present wireless charging systems. A basic system will have a power stage, a communication stage, and controller engine. Ancillary functions are required to monitor and respond to operational parameters (voltage, current, temperature) and support the communications and control systems (RTC, PLL, etc.).
Note that communications can be in-band (by modulating the load current and demodulating the reflected signal) or out-of-band (through, say, Bluetooth). This data stream is used to communicate authentication, foreign object detection, charge status, and temperature. In this diagram, the receiver is controlling the charge by communicating its needs back to the transmitter.
Some transmitters are designed to connect to a local or wide-area network (Ethernet, automotive bus, etc.) and may have additional connectivity blocks like a network interface function. The transmission frequency can be between 100 MHz and 6.78 MHz, depending on the control function and the standard. Each of these blocks and architectural choices will be the stuff of upcoming articles. Our next article will dive into the specific design challenges and functionality of an inductive wireless charging system.