Author Topic: Wireless Charger Design Help  (Read 845 times)

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Offline waymond91Topic starter

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Wireless Charger Design Help
« on: March 27, 2020, 12:08:17 pm »
Hello All!
I am working on a project that requires a very small wireless charger. I need about 15mm outer diameter max on the reciever coil, and about 43mm minimum inner diameter or the transmitter coil. The design intent is for the device to charge while nested within the transmitter coil.

I don't need to transmit much power at all, only enough to charge a 70 mAh lipo in an hour or maybe 3.
Even 100 mW would be passable, 300 mW would be enough to charge the battery at its rated amperage, and 500 mW would be ideal so the device could operate while charging.

My transmitter coil is 20 turns. I have tried a wide range of receiver coils, varying from 10 to perhaps 100's of turns.
Please see the schematic attached below.
I originally hoped that I could increase the power transmitted simply by increasing the voltage on my transmission coil, however as the voltage is increased, the efficiency of the transfer decreases - even after re-adjusting the transmit frequency. I am operating at around 40-60 kHz.
There is a zener diode on the output of the rectifier to protect components from overvoltage.
Space is at a premium in this design.

| Input Voltage | Input Current | Input Power | Output Voltage | Output Current | Output Power  | Percent Efficiency |
|----------------|-----------------|---------------|------------------|-------------------|-----------------|---------------------|
| 5                 | 0.065              | 0.325         | 1.97                | 0.006116113     | 0.012048742 | 1.88
| 10               | 0.25                | 2.5             | 5.3                  | 0.016454517     | 0.087208941 | 0.65
| 11.3            | 0.245              | 2.7685        | 6.2                  | 0.019248680     | 0.11934181  | 0.69

Any ideas folks?
I can provide any part numbers or datasheets if required.
Or photos of my coils.

Thanks for your the help!
 

Offline waymond91Topic starter

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Re: Wireless Charger Design Help
« Reply #1 on: March 27, 2020, 12:53:41 pm »
I know that comparable resonant charging solutions can get ~50% efficiency. What am I doing so wrong?
 

Offline StillTrying

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Re: Wireless Charger Design Help
« Reply #2 on: March 27, 2020, 08:19:17 pm »
Are all the MA4P7455's in the one IC, I can't work out any way to connect it as a bridge rectifier. :)

cdn.macom.com/datasheets/MA4P7455-1225.pdf
.  That took much longer than I thought it would.
 

Online Marco

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Re: Wireless Charger Design Help
« Reply #3 on: March 27, 2020, 09:13:45 pm »
Does your transmission driver detect zero-crossings? Because if it isn't auto-resonant and just drives it at a fixed frequency you can burn a lot of power doing nothing useful, all the energy in the capacitor at switch on gets burned in the transistor.
« Last Edit: March 27, 2020, 09:21:50 pm by Marco »
 

Offline twospoons

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Re: Wireless Charger Design Help
« Reply #4 on: March 28, 2020, 12:58:58 am »
Also if you push too much energy in during the on phase, then the tank voltage will ring down below 0V and dump energy in the MOSFET body diode.
You really do need  a proper resonant driver to control the transmit coil. Have you considered using one of the many Qi chips out there?  Paired with a Qi receiver it should be easy to get efficiency around 50%.  They operate at 125kHz.
 


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