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I am looking for a Qi charging compliant transmitter and have found kits from different manufacturers like ST/TI/Microchip. These kits have one receiver and are tuned to work together.

How can I build a transmitter (chip+coil) such that I can transmit power to multiple lower power devices with one transmitter? I haven't been able to get much information on how that can be done.

For example, to use a 15W transmitter to power four 3W receivers. Can the transmitter communicate with all devices under the Qi protocol? Will this boil down to designing the coil appropriately or there needs to be support from the transmitter chip as well?

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  • \$\begingroup\$ Mutual coupling loss is much greater than you expect \$\endgroup\$ – Sunnyskyguy EE75 Jul 11 '18 at 19:57
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The Qi technology doesn't allow for multiple devices on a single transmitter.

There is considerable handshaking between a transmitter and receiver. The receiving device transmits control information to the transmitting device, which then adjust its own power transmission parameters. This repeats, if I remember correctly, every 250 ms. In this way, the power transfer can be optimized with single-direction communications.

There is nothing in the Qi spec that allows for multiple devices or device addressing, and so multiple receiving devices would simply try to talk over one other's transmissions and break the control loop.

The communication method is pretty clever! The receiver varies the impedance of its own coil by switching in capacitors onto the resonant circuit. These perturbations are seen by the transmitter as differences in current through the transmitting coil. Here is an example from TI's BQ51003 datasheet:

qi

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    \$\begingroup\$ @rookie Great! Glad to help :) By the way, you can find Qi transmitters that are for "multiple devices", but actually they contain a number of individual charging coils/controllers in the same package. Basically, you can place a device on each available "target". Take care! \$\endgroup\$ – bitsmack Jul 12 '18 at 4:31
  • \$\begingroup\$ I've seen end products of that kind. Do chip manufacturers provide that kind of product? I imagine there would be considerable effort in designing the coils so as to cause least interference. \$\endgroup\$ – rookie Jul 12 '18 at 16:06
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    \$\begingroup\$ @rookie No, that's up to the product engineer to figure out. The chip manufacturers don't even make coils. They often have example designs that you can use, or may reference coils by another company, but the coil is still just one more design element to figure out when designing a product. A few manufacturers (TDK, for one) now sell coil assemblies, complete with a coil affixed to a disc of shielding material. Some even have openings in the middle of the coil intended for a locating magnet. (so the receiver "snaps" to the transmitter). Try googling "wireless charging coils". \$\endgroup\$ – bitsmack Jul 12 '18 at 17:21

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