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I have a project that requires wireless charging of a small hand-held controller with a single 1S lithium-ion cell. People's initial thought was to use the standard Qi mobile phone chargers to save reinventing the wheel, however they appear to need large, flat coils which isn't ideal for the form factor we have.

What IS ideal is the system used in electric toohbrushes which have a smaller base that drops onto a little charging dock - every teardown I've seen online they appear to use a very similar if not identical little coil of wire and very simple charging circuit, which is perfect.

However, I've not seen any great detail on how they work, if there's a standard system or chipset in use or if it's just something that one manufacturer came up with and everyone's kinda copied.

As this is not my area of expertise it would be really helpful if there's any sort of appnote or design / standard out there for this application, and indeed any off-the-shelf components such as standardised coils that could be incorporated in a design.

Ken Shriff's teardown shows the circuit and describes it briefly, all the others seem to gloss over it.

To charge the toothbrush, it is set on a stand and charges inductively without physically being plugged in. A coil in the stand is magnetically coupled to a coil in the toothbrush, transmitting the power wirelessly. You can see the coil at the bottom of the toothbrush. When set on the stand, the coil picks up about 12 volts, which is used to charge the battery. The power is transmitted at high frequency (80kHz) for efficiency.

The coil is connected to a diode bridge that converts the power to DC. It then goes through a transistor circuit that regulates the charging, as directed by the microcontroller.

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  • \$\begingroup\$ Buy one and strip it down. \$\endgroup\$
    – Andy aka
    Mar 28, 2022 at 9:54
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    \$\begingroup\$ Person asks about technical standards to make their own, people vote to close as "use not design". Seriously... \$\endgroup\$
    – Passerby
    Mar 28, 2022 at 10:52
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    \$\begingroup\$ @Andyaka tearing one down seems a bit pointless given there's documented tear-downs online (and linked in the question) - MY question is about information not (easily) gleaned by teardown - and asking a question here seemed like an easier, cheaper alternative. \$\endgroup\$
    – John U
    Mar 28, 2022 at 11:30
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    \$\begingroup\$ @Jeroen3 the question is not "how does wireless charging work" or even "how does this toothbrush's charging work"... the question is about whether there's some documented industry standard or system that we could pick up and apply rather than re-invent the wheel or reverse-engineer. Asking an electronics Q+A forum seemed like a pretty sensible method for discovering that sort of information to me. \$\endgroup\$
    – John U
    Mar 28, 2022 at 11:31
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    \$\begingroup\$ @Jeroen3 Wipower & WPC both seem to point towards the Qi standard as their main thing, and my question explains reasons for seeking alternatives to that. I don't consider this an overly "open" question given it is directly asking about a specific implementation of wireless charging. \$\endgroup\$
    – John U
    Mar 28, 2022 at 12:26

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Nothing much to it. A fixed drive to a TX coil, then a RX coil with a rectifier. You can use synchronous rectification on the RX coil to cut down rectifier losses if you wish. You get a raw DC voltage you can then regulate, use to drive a charger chip, etc.

The TX coil can detect the load impedance and go from being continuously on to a pulsed operation to conserve power when the battery is fully charged and the RX coil load is light.

If you want a hard power switch, add a little magnet to the RX side, and a reed switch on the TX side. Trivial and works well. Zero standby power consumption when the RX device is away from the cradle.

The drive circuit for the TX coil is pretty much a 556 double timer generating non-overlapping push-pull drive signals, and two darlington transistors as switches, or two mosfets. Or just use a push-pull driver IC if you have a good reason to believe it will not become unobtainium tomorrow.

There’s no need for a “standard”: the voltages and power levels are up to you, since the circuitry is so simple. If you can afford a magnet+reed switch for hard-turn-off then it’s very much foolproof. Otherwise use a mosfet as a power switch for the TX circuit and a low power Hall sensor to control the mosfet. The magnet can be quite tiny then. A pulsed load sense circuit can also be implemented, although those usually use an ASIC or an MCU for control – but it doesn’t take much to do it the “old school” analog way – and it won’t become obsolescent.

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