How many phones, music players, and other portables have we tossed into the trash because of a flaky power connector? It seems that the need to recharge our devices has made it so that eventually the power connector is going to be stressed to the point where there is an intermittent connection or an open circuit. Given repair costs, it does not pay to fix it (and good luck finding someone willing to handle the repair). One answer gaining momentum is to eschew the connector entirely in favor of wireless charging.
While a novelty at first, wireless charging has advanced to the point where well-known manufacturers are offering pieces and/or entire solutions that can easily be incorporated into your design. No longer is there a need to hand-wind custom coil pads or to be constrained by numerous manual fabrications tasks.
This article will examine the state of wireless charging technologies as well as the parts and solutions available off the shelf. These resources are making it much quicker and easier to add wireless charging to your design without costly iterations and manual fabrication steps. All parts, datasheets, tutorials, and development kits referenced here can be found on Hotenda’s website.
It’s a transformer
Basically, wireless power transfer is a coil next to a coil, or in other words, a transformer. In this case, the transformer is air-coupled rather than wound around a common ferrite core. In all cases, as one side oscillates, its field radiates outward until it slices through another coil which recovers power in AC form.
PCB traces can be used as a coil in some cases. As with transformers, voltage and current ratios in PCB Traces depend on the number of turns in primary and secondary coils. As a result, size and trace density may limit the number of turns possible on a PCB and the maximum amount of current a trace can carry. This makes real wound coils more desirable and effective.
As inductors, these transformer coils will exhibit resonance frequencies, and it is desirable to be able to select and fine-tune the frequencies you want to use for charging. Each primary frequency will generate harmonics that could potentially interfere with other RF links. Depending on your design, the ability to select the harmonic frequencies may help improve receive signal quality on other bands. In some cases, swept frequencies are used to distribute any noise or interference around the band. In addition, receiver sections are typically tuned to the same frequency for maximum power transfer.
While we are free to basically design our own simple inductively coupled (or magnetically coupled) charging links, several competing quasi-standards have emerged that address design issues and try to get attention. Power Matters Alliance (PMA) provides one standard that some manufacturers and retailers have embraced. Starbucks, for example, announced it would be placing PMA chargers in its 8,000 stores, and already the Android handset maker Kyocera supports it. PMA is absorbing the functionality of A4WP, whose Rezence is a wireless power transfer technology and specification based on the principles of magnetic resonance. However, by far the most widely accepted wireless charging technology comes from the Wireless Power Consortium with its Qi standard.
A wide selection of coils and chips
Regardless of the underlying transmit and receive circuitry, wireless charging requires coils and component makers, such as Abracon, TDK, Vishay Dale and Wurth, to offer a broad selection of wireless charging coils (and chips) that do the job. Finding a reliable source of consistent coils is important in supporting high-volume production runs. Single-coil, dual-layer coils and even bifilar windings can be used to send quite a bit of current through the air gap.
For charge cradle types of designs, a single coil can be the most cost-effective solution. A part like the Abracon AWCCA-50N50H40-C02-B is a single-layer coil that can be used as both a transmitter and a receiver. The 6.3 µH coil inductance features a fairly high Q factor of 72 with a self-resonance frequency of 6.4 MHz. Note the 19 mΩ resistance. Transmitter coils always should use protection circuitry since short-circuit conditions will draw over 50 A at 1 V, using this case as an example.
The supplier’s AWCCA-50N50 Series, which includes the AWCCA-50N50H40-C02-B, provides various versions that can support from 5.4 to 11 A of primary current with 2.8 to 6 MHz operation. The company offers a Product Training Module on the AWCCA series wireless charging coils, which is available on Hotenda's website.
The symmetry is nice when you use the same coil for receiving and transmitting energy, and you can benefit from possible higher level volume pricing since your quantities are doubled. However, it may not always be desirable to use the same coils for transmitting and receiving. For one thing, when you have different coils, you can have more than a 1:1 ratio, which means you will have more flexibility with output voltage levels.
Several manufacturers make separate coils for transmitting and receiving power. Take for instance the TDK WT-505060-10K2-A11-G single-layer energy-transmitting coil, also with a 6.3 µH inductance. This part features a resonance Q factor of 70, a DC resistance of 60 mΩ, and is designed to oscillate at a resonance of 100 kHz.
As part of TDK’s wireless charging transmitting coil units, the company provides the complete subassembly stackup ready for mounting (Figure 1).
Figure 1: OEM offerings contain the subassembly stackups ready for direct mounting, thus providing consistent performance and reducing manufacturing steps.
A corresponding receive coil with the same construction and form-factor could be the TDK WT505090-10K2-A11-G or any of the members of the company’s WT Series receiving coils.
Larger charge areas can allow less precise placement, or even charging of more than one small device when a multi-coil setup is used like the TDK WT-1005660-12K2-A6-G. This three-coil single-layer assembly features fairly consistent inductance (12.5, 11.5, and 12.5 µH) among the three coils and is resonant at the 100 kHz frequency range (Figure 2).
Figure 2: Multi-coil assemblies are prefabricated for larger-area charging pads that permit simultaneous charging of multiple units.
TDK's ultra-thin receiving coil unit for wireless power transfer like its WRM483245-15F5-5V-G or similar thin low-profile planar coils can more easily be mounted inside a rear case of a handheld device as long as it is not a shielded material.
Transmitting and receiving chips
Associated ICs can act as simple oscillator controllers, or they can be more advanced. As increased attention is being paid to the Qi interface standard from the Wireless Power Consortium (WPC), more chipmakers are providing integrated, peripheral-style chips that take over all of the transceiving functionality.
Yes, transceiving. With Qi, a bidirectional communications link is formed using a load modulation technique. This is very attractive since battery packs can now report to the charger for faster and more reliable charging.
With over 200 members, the modern Qi standard permits resonant charging at distances up to 35 mm away from the baseplate. The goal is to evolve the standard to allow up to 2,000 W for larger devices and appliances.
Several chipmakers have gotten into the game with transmitting and receiving chips, reference designs, and development kits. For example, the Toshiba TB6860WBG,EL wireless power receiver chip provides a 60 mA, 3.3 V regulated output for local circuitry in feed mode. It can also operate in charge mode where the bidirectional communications can take place with the remote transmitter through the load modulation technique previously discussed. The built-in I2C communications port allows the local micro to read battery status and help do power management by shutting off the transmitter when not needed.
Even encryption is available for secure operation. This may be used to tether devices so overpower isn’t accidentally delivered to a different receiver. Switching frequencies can vary from 110 to 205 kHz to help find sweet-spot resonances of high efficiency.
Freescale is serious about wireless power as well with its automotive-grade single- and multi-coil 5 V controller chips like the MWCT1000CFM wireless power transmitter, which is also Qi compliant. Like other parts, protection is built in for overcurrent and overvoltage as well as thermal shutdowns. A digital demodulator inside reduces external component count and handles abnormal conditions.
Freescale also offers a low-cost wireless charging demo and development kit. The WCT-5W1COILTX helps ease designers into this technology and includes international power adapters and support. It is a complete system solution, containing all of the hardware and software components necessary to quickly implement a single-coil charger solution.
Texas Instruments is also very active in power management, and Qi is no exception. By separating the energy transmitters from the receivers, devices can be optimized for each side’s functions. Parts like the BQ500210RGZT transmitter and receivers such as the BQ51013ARHLR range from 20 µA to 2.5 A transfer capabilities.
In addition, TI couldn’t make it easier to test and evaluate the technology thanks to a couple of reference designs (TI BQ500210EVM-689 and TI BQ500210EVM-689) and development kits. The reference designs are WPC certified and TI, and third-party development kits like the EP5220HPA11-1 from Active Semi, also Qi certified, as is the Wurth wireless plug and play power transfer kit featuring TI parts (Figures 3A and 3B).
Figure 3: Reference designs and test/development kits from TI and from third-party developers provide quick and easy access to transmitters (A) and receivers (B) that are already Qi compliant and certified.
Several good wireless charging technologies now are ready for use and as a result, designing a manufacturable charge controller and transmitter is no longer a complex and involved process. Magnetic resonance has shown to be effective and reliable and as more standardization takes place, charge stations of the future will wirelessly charge all our devices and in time eliminate the need for fragile power connectors.
For more information on the parts discussed in this article, use the links provided to access product pages on the Hotenda website.