Q: I'm mostly interested in wireless powering of sensors. What components are available in that area?
TI: Wireless sensor nodes are typically low power, small devices. From a power management point of view, they are comparable to the wearable devices with small internal batteries that we discussed in the presentation. You can consider the bq51003 low-power RX device with a corresponding small coil. Depending on the size of the coil chosen, efficiency may be optimized by changing the transmitter coil to a reduced size especially for very small receiver coils. As we showed in the presentation, efficiency is best when TX and RX coil diameters are the same, and the two coils are aligned in close proximity.
Würth: Depends on the space you have available. Würth Elektronik plans to have small transmitter (diameter 30mm) and receiver coils (diameter 10mm) second half of this year. As explained during the webinar the ratio of active transmitter and receiver area have to have a certain value to have a good efficiency.
Q: Are there chipsets and/or coils for the 2d array of coils near release? I've seen papers on it but no actual devices.
TI:These products (assuming you are referring to the A4WP category of wireless power) are in the R&D phase at TI. Product launches will be driven by market demands. We can’t comment on what other semiconductor manufacturers may have available.
Würth: 2D coil arrays are in the market. But as they are based on printed circuit board ( PCB) , most device manufacturer integrate it in their PCB. So it is mostly not separately available.
Würth Elektronik is not offering PCB based solutions as efficiency and Q-factor of those solutions are not as good as of litz wire coils and ferrite shielding.
Q: Is there any communication between the charger and load as to stop sending power when the load is fully charged? Is that done in any standard?
TI:The receiver continuously provides feedback to the transmitter regarding the needed power level. As the battery nears full charge and the load current decreases, the receiver power demand automatically is decreased and output voltage regulation is maintained as part of the normal error feedback process.
However in the WPC 1.1 standard there is also a specific communication packet that can be sent to indicate charge is complete (“End Power Transfer” or EPT01 in the WPC specification). The effect of this is that the transmitter pad (if so designed) can change its LED indicator to show that charge is done, and also reduce its standby power and go to an idle state (digital ping interval is extended from the normal 5 seconds out to 15 seconds in this mode). There are also other types of “End Power Transfer” packets that may be sent depending on specific operating conditions and events. Please refer to the bq51xxx component data sheets for details.
Würth:Yes, in the Qi standard of WPC the receiver will tell the transmitter to stop energy transfer when the battery is full.
As far as I know the other standards have a solution for that as well.
Q: Will the bq500410A intelligently switch on the coil which couples the best?
TI:Yes. The bq5004xx devices which are designed to use the 3 –coil arrays will sequentially “ping” each coil. The RX circuit sends back a signal strength indicator packet for each one. The coil that results in the best signal strength is chosen, and the other two disabled prior to the start of the power transfer phase.
Q: Is the time interval between analog pings configurable? Or is it dictated by the Qi protocol?
TI:The Qi protocol dictates that the transmitter pad must detect the presence of a receiver object within half a second. The analog ping interval is programmed to a default of 400ms in the TI transmitter devices and is not normally user-configurable.
Q: Is there any problem with very small charging currents ~10mA?
TI:The RX devices will maintain output regulation down to no-load conditions. However, the accuracy of the RX devices is not optimized for measurement of very low currents. If taper current termination is required (typical for Li-Ion battery charging), it may be difficult to accurately control very small termination currents using the bq51050 direct-charge IC. It may be preferable to use a low-power constant voltage receiver (bq51003) and follow that with a precision low current charger such as bq24232.
Würth: From coil perspective no, if the ratio of active area between Tx and Rx coil are sufficient.
Q: What does PET stand for?
TI:"PolyEthylene Terephthalate" – A plastic material that can be used for insulating layers.
Q: Qi solutions are tied to normalized coil shapes. Is it expectable to have different coil shapes? I have a situation where I need a narrower shape and this is annoying me.
TI:Many new coil types are being developed for new requirements. The Qi standard does not dictate a specific coil type, but there are several that have been pre-certified. Wuerth has elliptical and other coil shapes and should be able to comment on this further. We showed examples of longer, thinner coils in the presentation.
Würth: In Qi standard the transmitter coil shapes are fixed. The receivers can be shaped freely. As shown in the webinar e.g. 10x25mm receivers are possible as well.
Q: What research has been done to investigate long term health impact?
TI:The World Health Organization has published studies that document the safe exposure limits to a range of EM frequencies. This is explained at the Wireless Power Consortium website here: http://www.wirelesspowerconsortium.com/technology/emf-limits-icnirp-basic-restrictions.html
Würth: The inductive Qi solution of WPC ensures that there is minimal stray field around the coils due to the ferrite shielding. This stray field is below all limits regarding spurious emmission.
I don’t know the status for the resonant solution of A4WP. Please contact A4WP directly.
Q: For the higher power applications, will the coil sizes increase proportionally with the power?
TI:Coil sizes will increase, however not necessarily in linear proportion to the power level. I will defer to Würth for the more detailed answer.
Würth: For the upcoming 20W no new coils are needed. For the mentioned cordless kitchen the coil size will be adjusted to the appliances (pan, kettle,..) and reach around 22cm coil diameter.
Q: When is the 120W Qi standard REALISTICALLY going to be released and when components are going to be available?
TI:This is not on the near term roadmap at TI. We can’t comment on long term product R&D plans.
Würth: There is no plan in the Qi standard to have a 120W class. The 20W standard will be released during this year. Coils are already available as there is no change in the coils. The current 5W coils are capable of 20W as well.
In the upcoming cordliss kitchen standard a 250W class is planned. Standardisation is ongoing, plan is to have a first version of the standard released end of 2014.
Q: Are there interoperability between the different power standards? Say can Qi work with PMA?
TI: There will be dual-mode receivers so that a single device using this receiver type can be charged on either a Qi or PMA type of charging pad. The two standards are not directly compatible, but it is possible to design multi-mode receiver IC that recognizes the appropriate type of transmitter and adjusts its feedback / communication accordingly. A dual-mode receiver, part bq51221, is presently in “preview” status on the TI website: http://www.ti.com/product/bq51221
Würth: Technically there is no interoperability possible between the inductive and magnetic resonance solution as they are today.
Several companies are working on dual standard receiver solutions for WPC and PMA (as both standards are inductive); means a receiver with such a solution can be charged either on a Qi or a PMA transmitter.
Würth Elektronik plans to have such a receiver coil during this year.
Q: Will there be eddy current problems in the receiving device. What I mean is the heating effects? Will not the device act as an induction heater?
TI: If there is a mis-match between TX coil and RX coil (e.g. RX coil is much smaller than TX coil or misaligned) then it is possible for uncoupled field lines to be absorbed into the metal surfaces in the receiver device such as battery casing or other metal objects inside the portable device. To prevent this, the RX coil should be properly shielded to cover and protect any sensitive areas inside the receiver device. Careful attention to thermal and parasitic heating issues is a key aspect of proper wireless power design.
Würth: As explained during the webinar eddy currents can be handled e.g. by using litz wire in the coils (on both sides: transmitter and receiver). The “heating” effect in current solutions is manageable, so no danger that the coil acts as an induction heater as you say.
Q: How many countries have accepted the standard is it fully international as Australia gets a bit fussy sometimes over standard especially EMC
TI: The WPC-Qi standard specifies intercompatibility and communications protocol between receivers and transmitters. Country-specific EMC standards are required at the product design level. The WPC-Qi standard does not apply here.
Würth: Qi standard of WPC is a global standard and to my knowledge there are Qi products developed and in the market in Australia as well.
The other 2 standards (PMA, A4WP) are global as well, but have few or no products currently. I cannot judge their distribution in Australia.
Q: About the receiver module, is it possible to receive the energy by a device as small as credit card?
TI: Yes, and we have already implemented this. The TI receiver devices are less than 3x3mm footprint. The complete receiver circuit including external components can be packaged on a 5x15mm PCB using only the bq51013B chip and external R/C passive components. The Evaluation Module (EVM) shown on the website is looks larger because it includes connectors, jumpers and test points - but is still not much larger than a credit card.
Würth: Yes, the receivers can be made very small as they need only a minial PCB. The receiver coil has to have a certain size as explained during the webinar to couple to a good efficiency.
Creditcard size is no problem for the current available Qi solutions.
Q: Is it possible to transfer data over this system?
TI: The WPC spec is intended for power transfer. At this time, no user-configured data communication can be added. Future versions of the spec may support limited data communication but it will be very low bandwidth.
Würth: Technically in principle yes, but this would lead to a different certification and approval procedure as then the system would be rated as a radio transmitter and not an energy transfer.
Q: How accessible is the Qi wireless standard for small entities (SMEs) and hobbyists?
TI: The detailed spec documents are downloadable at no cost from www.wirelesspowerconsortium.com
WPC membership is not required to use TI chipsets which are web-orderable at www.ti.com and additional technical information and guidelines can be found at www.ti.com/wirelesspower.
Würth: Specification part 1 is public on WPC website http://www.wirelesspowerconsortium.com/developers/specification.html
With that knowledge and the material provided e.g. at the TI website http://www.ti.com/lsds/ti/power-management/wireless-power-receiver-solutions-overview.page?DCMP=hpa_pmp_general&HQS=Other+OT+wirelesspower. you can normally build a working system.
Test specification is only for members. Small companies can join the WPC on a reduced membership fee.
