Wireless Phone Charging Circuits

[ZakW]

Hello,

Can someone help me understand how these circuits function? I have noticed these multi-coil systems and have a few questions:

- Each coil must be driven by its own driver circuit, right?
     - If so, are they all working at the same frequency?
- Why are the coils overlapped as they are? Does that boost the flux density or range?

https://techcrunch.com/2020/01/03/zens-liberty-wireless-charger-review/





Thanks!
Zak

[Zipdox]

I assume the coils are driven in phase with eachother, probably each with their own driver yes.

Let's not mention efficiency...

[ZakW]

So as far as power absorption/induction goes, it has to do with how strong of a field the coil(s) is producing and how much the receiving coil(s) is in that field. Far away = less voltage, Closer = high voltage, right?

They mention in the tech crunch article that the charger uses 60W. That's crazy considering a normal phone charger is around 10-20W.

For better efficiency, couldn't you increase the number of coils and reduce their overall output so that when a receiving coil is brought near by its absorbing many weaker signals for more current? Is part of their design to boost the overall field so the charger works through thicker materials (like phone cases and things).

[klugesmith]

We could guess that the coils are arranged in non-overlapping groups.
The coils in a group could all be wired in series or in parallel.
I think it would not help to drive more than one group at the same time, because in many places their fields would cancel.

The pattern could be studied with a small, movable sense coil. Driving a LED?  If the charger won't run without a power receiver/phone, then do the sense coil survey on back side of the charger, while something is getting charged.

60 watts in? ! ! Whatever fraction does not go into battery will go directly to heat.  Could keep a mug of coffee or tea warm while charging the phone.

[ZakW]

klugesmith,

From the look of the PCB's on some of them, I would say they appear to all be driven separately. What do you mean the fields would interfere with each other? I am assuming they have studied the pattern they chose. I wonder if I can find any research that was published on the topic.

If I wanted to make a simple circuit with multiple coils just to test different arrangements what would I be looking at for parts? Discrete MOSFET, gate drivers, and something to vary the drive frequency for each coil? Could I feed the same drive signal to each gate drive/MOSFET to cut down on components?

It looks as if they all use litz wire too, is that really necessary?

[davekni]

I expect this is a "smart" charger that senses where the phone or other such device is, then drives a single coil with the closest-to-optimum position.

Frequency is likely high enough where litz wire helps with efficiency.  Especially since overlapping un-driven coils will absorb energy from the one coil selected to be driven.

[ZakW]

I expect this is a "smart" charger that senses where the phone or other such device is, then drives a single coil with the closest-to-optimum position.

Frequency is likely high enough where litz wire helps with efficiency.  Especially since overlapping un-driven coils will absorb energy from the one coil selected to be driven.

I didn't even think of that  ! No wonder why they mentioned that "it doesn't matter as much where they device is placed".

In that case, the coils are not laid out like that for any other reason than for coverage of the surface.

[Twospoons]

I expect this is a "smart" charger that senses where the phone or other such device is, then drives a single coil with the closest-to-optimum position.

Frequency is likely high enough where litz wire helps with efficiency.  Especially since overlapping un-driven coils will absorb energy from the one coil selected to be driven.

You are exactly right. I spent a year working for a company developing this kind of tech.  The coil alignment is critical to getting even vaguely reasonable power transfer and efficiency.   I invented a system whereby the coils were built out of multi-layer pcb instead of litz wire - theres a couple of tricks needed to not melt the pcb.

The efficiency of wireless charging is rather poor compared to a cable though.

[Hydron]

The efficiency of wireless charging is rather poor compared to a cable though.

Still, it's handy enough that I would not consider buying a phone without wireless charging these days.
I have a rather early Nokia Qi charger (runs on 12V, 3 coils inside, probably not very efficient) which isn't very fast, but it doesn't need to be when you can just put the phone down on it when going to sleep and wake with an always-full phone (slow charge is probably actually good for battery longevity!).

[ZakW]

Does anyone know of a good starting point for a basic circuit for creating one of these? I have not found very much when searching for a circuit diagram of a wireless charger. It would be cool to see if I could replicate a wireless charger and what efficiencies I could get out of it.

[Zipdox]

Does anyone know of a good starting point for a basic circuit for creating one of these? I have not found very much when searching for a circuit diagram of a wireless charger. It would be cool to see if I could replicate a wireless charger and what efficiencies I could get out of it.

I believe qualcomm has some sort of electrical and communications standard for this. It isn't straightforward at all.

[Twospoons]

There are a few companies making chips for the Qi charging standard.