# Wireless Battery Charger Design

#### Brett Smith 1

Joined Jul 1, 2017
13
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.

Code:
| 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.

#### Attachments

• 161.7 KB Views: 11
Last edited by a moderator:

#### Brett Smith 1

Joined Jul 1, 2017
13
I know that comparable resonant charging solutions can get ~50% efficiency. What am I doing so wrong?

#### Alec_t

Joined Sep 17, 2013
11,119
Do the coils have a ferromagnetic core to concentrate magnetic flux?

#### KeithWalker

Joined Jul 10, 2017
794
Have you tried tuning the receiver coil to the frequency of the transmitter?

#### Dodgydave

Joined Jun 22, 2012
9,070
It's important that the capacitors and inductors are resonant at the particular frequency, so you need to know the Coils Inductances to match the capacitors, then maximum transfer will be easier..

With 200nF that's 30 to 50uH at 50Khz

Post pictures of your PCB work..

Last edited:

#### Brett Smith 1

Joined Jul 1, 2017
13
Thank you so much for the help guys.
I've been "tuning" the system by sweeping the transmitter frequency and checking the amplitude of the output on the receiver side. I am in a pretty remote location, but I have a function generator and oscilloscope, so I should be able to measure the impedance of the transmitter coil. And then try to wind a receiver coil that could pair with one of the capacitors I have on site.
Does that sound correct?
Here is the setup, no PCB for the transmitter yet. The receiver PCB is pretty rough.

#### Attachments

• 87.4 KB Views: 4
• 107.7 KB Views: 4
• 94.2 KB Views: 4

#### KeithWalker

Joined Jul 10, 2017
794
That is a good way to start. Do the same with the receiver coil, changing the parallel capacitor until it resonates at approximately the same frequency. When it is tuned, check the voltage of the secondary with the detector and a load connected. Then try matching the load impedance by using taps on the secondary to get the best power transfer at the charging voltage.
Keith

#### Brett Smith 1

Joined Jul 1, 2017
13
Sounds good. I'll do that and report back if there are any problems.
I'm just a little confused about matching the load impedance? I was hoping I could just put a giant capacitor on the output of the rectifier circuit, as I'm powering a variety of mixed signal ICs.

#### KeithWalker

Joined Jul 10, 2017
794
The most efficient way of transferring the power is by using a high Q tuned circuit for a receiver. A load connected directly across the tuned circuit will damp it and lower the Q. By using a coil with a fairly large number of turns and tapping into the coil you are creating an auto-transformer that will give you higher current at a lower voltage with better power transfer..

#### Brett Smith 1

Joined Jul 1, 2017
13
Thank you so much for your help.
I believe I am starting to understand.
So I should basically make the largest coil I can in terms of space & tuning.
Then tap off the coil and rectify the power from the tap?
Something like this:
If so, I will implement & test ASAP.
I've tuned my coils so that the inductance is about the same. Although now I understand that I should use a smaller capacitor and larger coil on the receiver side. Both coils are about ~30 uH.
However, I'm still getting that 1% efficiency figure. I'm hoping that the fact that my capacitors are low tolerance means that system is still not properly tuned?

Best regards.

#### Brett Smith 1

Joined Jul 1, 2017
13
Do the coils have a ferromagnetic core to concentrate magnetic flux?
They do not! They are home spun. Is this a bigger issue than I anticipated?

#### Alec_t

Joined Sep 17, 2013
11,119
Dunno. I have in mind electric toothbrush charging arrangements. I'm pretty sure those have a ferromagnetic core. No idea what frequency they run at or whether resonance is involved.

#### KeithWalker

Joined Jul 10, 2017
794
I re-read the question and noticed that you mention that you have a zener diode on the receiver output. That will short a lot of the available current to ground if the rectified voltage is higher than the zener voltage. Remove it and use higher voltage rectifiers and smoothing capacitors until you have the turns ratio optimized to give you the right voltage because with it in place, you will not see any improvements as you try different turns ratios.