Wireless Charging

[Sterer]

TL;DR

5V DC to 5V AC, 5W

I need a circuit that converts 5V DC to 5V AC and can supply 5W for a wireless charging coil. I am finding incredibly little information online
Edit: It also needs to run at 275kHz, so a standard relaxation oscillator to my knowledge would not work

 

[berkeman]

Welcome to PF.

How are you physically going to place the charging coil and the target device? How close will they be?

Also, keep in mind that the 3rd harmonic of your charging frequency is in the AM radio band, which is regulated by the FCC in the US (and there are similar regulations around the world). If you cause interference with your charging device, you could be subject to some penalties.

What is the application?

 

[AlexB23]

TL;DR: 5V DC to 5V AC, 5W

I need a circuit that converts 5V DC to 5V AC and can supply 5W for a wireless charging coil. I am finding incredibly little information online
Edit: It also needs to run at 275kHz, so a standard relaxation oscillator to my knowledge would not work

What is your use case for this charger? It may not be good for phones

 

[Sterer]

It is for charging a battery connected to a custom external board. It does not need to be too efficient but should deliver some amount of power

 

[berkeman]

but should deliver some amount of power

Over what distance? Can the charging coil be placed in direct external contact with the battery case? Or better yet, can it be designed so the battery power receiver part fits inside the charging coil, kind of like some electric toothbrushes are recharged?

https://www.google.com/url?sa=t&sou...cKEwjQloWF2oSTAxUAAAAAHQAAAAAQCA&opi=89978449

 

[Sterer]

Nothing should be visible externally, but there is a possibility to have the charging coil inside the receiving coil. They will be over a cm apart though.

 

[Averagesupernova]

Also, keep in mind that the 3rd harmonic of your charging frequency is in the AM radio band...

As is the 2nd, 4th, and 5th. Typically the 3rd and 5th are higher in amplitude than the even harmonics. I believe I have this right. Feel free to correct me as I would put more faith in your knowledge of RF than pretty much anyone I know these days.
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At any rate, this is a challenging project for a beginner. Anyone who phrases the original post in the way that it was I would have to assume is in fact a beginner.

 

[DaveE]

TL;DR: 5V DC to 5V AC, 5W

I need a circuit that converts 5V DC to 5V AC and can supply 5W for a wireless charging coil. I am finding incredibly little information online
Edit: It also needs to run at 275kHz, so a standard relaxation oscillator to my knowledge would not work

I would look into resonant SMPS designs. My first guess is a resonant LLC half-bridge topology, but I haven't looked into this for a few decades. There is a lot of information available and probably some control ICs. Look at TI.com and Analog.com (or google) for starters.

If you're not finding info on the web, you're searching for the wrong things. It's out there, lots of it.

 

[berkeman]

Typically the 3rd and 5th are higher in amplitude than the even harmonics.

Agreed. I was just using the 3rd harmonic because it is usually the biggest depending on the signal generation circuit, and was an existence proof that there may be regulatory issues.

 

[berkeman]

I need a circuit that converts 5V DC to 5V AC and can supply 5W for a wireless charging coil.

there is a possibility to have the charging coil inside the receiving coil. They will be over a cm apart though.

What is the application?

What is your use case for this charger?

The problem is not creating a 5W 275kHz sinusoidal waveform with low harmonic content to drive a charging coil. The problem is optimizing your wireless charging setup to help you achieve your goal. If you give us so few details about your application, you are not likely to succeed. What exactly is your application and what are your design constraints?

 

[Baluncore]

You could drive a loop antenna, with the square wave from a microcontroller, through a MOSFET, but that would be full of harmonics.

The solution might be an inductor driven by a power amplifier, driven by a sine wave oscillator.

I would consider a Hartley oscillator, with the resonant tapped inductor as the coupling loop. That would significantly reduce the harmonics generated. Design it to stay on frequency under load, while the voltage amplitude is regulated.
https://en.wikipedia.org/wiki/Hartley_oscillator#Operation

 

[DaveE]

It also needs to run at 275kHz

Why?

a standard relaxation oscillator to my knowledge would not work

I'm not sure what makes a relaxation oscillator "standard", but the operating frequency isn't hard to make whatever it is.

the resonant tapped inductor as the coupling loop

Yes, 100% agree. You'll want the inductor to be part of a high(ish) Q tank. This will help a lot with energy storage (efficiency) in the transmitter field as well as reduce EMI in the harmonics. The EMI from the drive circuit can be filtered and contained, but you don't want anything "extra" in the transmitter coil.

They will be over a cm apart though.

It's not really your circuit question, and you probably don't have much control over the specs. But the coupling efficiency is a huge issue. Anything you can do to reduce the distance or increase the size of the coils will pay off. Can you add a magnetic core to better direct the flux? I'm thinking something like a pot core with a really, really, big gap.

Finally, I assume you've studied existing designs, like Qi. Don't reinvent the wheel if you don't have to.