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Teardown: Qi chargers

What

Several Qi inductive chargers were obtained for a set of projects where contactless power transfer was needed.

The cheapest transmitters, or charging stations, come in the shape of a round "puck".

The receivers come in multiple form factors; always as a thin sheet intended to place on the inner side of the phone back housing, in some cases mating to the matching contacts on the phone mainboard, in others with a microUSB connector. The microUSB connectors come in two variants, with the connector in normal and upside-down orientation, as when there are two possible orientations there are naturally two standards. The coils are backed with ferrite, so they work from only one side.

Behavior

The transmitter is sending interrogation pulses. When a receiver is detected in range, the transmitter lights up the blue LED. Then a handshake is performed, the receiver starts communication, and within a few hundreds milliseconds the receiver starts providing power on its output.

The power transfer is done at 100-200 kHz frequency. The actual frequency depends on the load. The receiver communicates with the transmitter by changing its coil load, amplitude-modulating the signal on the transmitter coil. This is amplified, decoded and fed to the charger controller.

The startup delay and the maximum operating distance seem to vary with the module used. The distance seems to be 4 millimeters for the least sensitive module and up to 10 mm for the most sensitive one. At higher distances the output power drops rapidly and the communication with the base unit becomes intermittent or lost quickly.

The transmitter and receiver coils have to be fairly accurately aligned by their centers. Misalignment leads to drop in the energy transfer.

Links

The Qi protocol has a decoder in the Sigrok software.

The protocol and its decoding is described here, in a Freescale application note.

Todo

What's inside

Coils

The coils are backed with ferrite layer. The transmitter coil uses about 2mm thick ceramic board. The receiver uses a thinner layer of plastic ferrite composite. This layer is fairly fragile, but the cracks don't seem to have significant effect on performance.

The ferrite backing serves to close the magnetic circuit when the coils are in proximity. This also makes the coils insensitive to the nature of their backing; they perform as well with free space behind them as when they are placed on ferromagnetic sheetmetal or bulk metal.

Puck (transmitter)

The power is connected by a standard microUSB connector, allowing use of a standard phone charger. The power consumption is rated at 1.5 amps at 5 volts.

The transmitter is equipped with a red-blue LED. The LED shines red when the unit is powered up, and blue when the Qi handshake is initiated and when the power transmission is running.

The transmitter coil is made from a litz wire, wound concentrically in a single layer, with a round backing made of hard ferrite ceramics.

There are two chips on the charger; a LM324 quad op-amp, for sensing the data signal from the coil, and a STM8S003F3P6 microcontroller (8bit 16MHz, 8kB flash, 128b data EEPROM, 1kB RAM). The microcontroller takes care of generation of the signal for the coil, and handling the Qi protocol.

The circuit seems to be similar to the reference circuit in the Qi specifications.


Qi puck, outside, top

Qi puck, outside, bottom

Qi puck, inside

Qi puck, inside

Qi puck, board and coil

Qi puck, board

Qi puck, board

Qi puck, board

Stickers (receivers)

The charger receivers are typically made as thin-ish slightly flexible stickers. The universal ones connect to the phone through a microUSB connector, the type-specific ones use specific positions of contacts that mate to the pads on the phone board.

The receivers are usually placed on the battery or on the inside surface of the phone case back.

The receivers are composed of a front and back adhesive sticker, providing mechanical integrity, sandwiched around the electronics. The electronics has two parts - a coil, backed with a thin fragile ferrite layer, and a small circuitboard with electronics.

The stickers can be carefully peeled apart, exposing the electronics and the coil.

The coil is backed by a thin ferrite-plastic composite layer. This layer is very fragile, but the thin cracks don't seem to have much effect. This also allows use of the coil in only one orientation, with the coil facing the transmitter coil. So the sandwich structure is ferrite-coil-gap-coil-ferrite.

The electronics boards have typically more sets of output terminals, allowing connection of the contacts in multiple orientations, allowing use of the same board with several phone types.

There seem to be several types of electronics boards, all similar.

The electronics typically contains a buck converter, stepping down the voltage from the coil to the output 5 volts, a dedicated chip with a SMD coil nearby.

Samsung S3

42x60 mm, 800 mA, two contacts on the side, 35x30 mm coil


Samsung S3 sticker

Samsung S3 sticker

Samsung S3 sticker

Samsung S3 sticker

Samsung S3 sticker

Samsung S3 sticker

Samsung S5

42x60 mm, 800 mA, two contacts on the side, 30x35 mm coil


Samsung S5 sticker

Samsung S5 sticker

Samsung S5 sticker

Samsung S5 sticker

Samsung S5 sticker

Samsung S5 sticker

Samsung S5 sticker

Samsung S5 sticker

Samsung S5 sticker

Samsung S5 sticker

Samsung S5 sticker

Same electronics as Samsung S3

Samsung Note III

42x60 mm, 800 mA, three contacts on the side (only two connected), 35x30 mm coil


Samsung Note III sticker

Samsung Note III sticker

Samsung Note III sticker

Samsung Note III sticker

Samsung Note III sticker

Samsung Note III sticker

Samsung Note III sticker

Same electronics as Samsung S3

Universal MicroUSB sticker 1

67x43 mm, 1000 mA, male microUSB connector


Universal microUSB sticker

Universal microUSB sticker

Universal microUSB sticker

Universal microUSB sticker

Universal microUSB sticker

Universal microUSB sticker

Universal microUSB sticker

Universal microUSB sticker

Universal microUSB sticker

Universal microUSB sticker

Universal microUSB sticker

Universal MicroUSB sticker 2

67x43 mm, 1000 mA, male microUSB connector


Universal microUSB stickers 1 and 2, side by side

Universal MicroUSB sticker 2

Universal MicroUSB sticker 2

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