Inductive charger

Hi there Roger,

It seems that you are trying to re-invent the wheel :-) SMPS chargers (3-5W) are readily available on the market for just a few dollars. However if the task is related to 'having fun' this is something else. However if you don't have expierence in switch mode power supplies this task is not easy. Remember also that you are dealing with a device which has a safety function. (a user must be able to touch the output voltage safely). You as designer are responisible for this safety function. Given all this, your circuit should be a flyback converter. Getting a suitable (and safe) transformer is difficult, other components are easy to get.

A switching frequency of 60kHz is not uncommon. Your power switch is a MOSFET and the controller is a small 8pin device from a supplier such as ST, ON, etc. If you look at the datasheet of such a controller (UC3842B is old but a generic part) you will find a typical application. You can downscale it for your needs.

If you have specific questions i might be able to help you.

Best regards and lot's of luck

Enrico Migchels

Power Conversion Design Engineer

Heliox. B.V.

Best, The Netherlands

www.heliox.nl

Hi Enrico!

Thank you for your answer.

I'm sorry, but I was maybe clera enough with my question:

I have a lot of experience with power supply designs in general with for examples DC/DC converts with all kind of topologies.

What I want to do now is something that is similar to an inductive charger for an electrical toothbrush, menaing with one part that act as a "transmitter" but that are completly sealed and not possible to touch any electrical parts.

The second part shall act as a "receiver" (toothbrush) and is also completly sealed.

So, I will have two parts that is away from each other during normal use, and only close to each other when the battery shall be charged.

To get a high efficency the physical design of those parts must be optimized so the inductors are placed very close to each other, and probably also a core need to be used. I think it's therefore not possible to use any "off-the shelf" products, but I might be wrong.

Hi Roger,

Ok, it's clear. The design problem is not as big as you think. Just see the distance between transmitter (primary part of your flyback converter) and receiver (secondary part of flyback converter) as the air gap which is normally in the centerleg of the core construction. In your situation the airgap is distributed in both centerleg as return legs. This is no problem. Try to limit the wall thickness of the plastics and try to position the transmitter/reciever are accurate as possible. What you have to do is build a circuit on the primary side which detects the receiver part (increase of inductance). This is the challanging part. I would seek for cores used in planar transformers as they are very low in height and have huge cross sections.

Best regards,

Enrico Migchels

Power Conversion Design Engineer

Best - The Netherlands

www.heliox.nl

Hi Roger,

Ok, it's clear. The design problem is not as big as you think. Just see the distance between transmitter (primary part of your flyback converter) and receiver (secondary part of flyback converter) as the air gap which is normally in the centerleg of the core construction. In your situation the airgap is distributed in both centerleg as return legs. This is no problem. Try to limit the wall thickness of the plastics and try to position the transmitter/reciever are accurate as possible. What you have to do is build a circuit on the primary side which detects the receiver part (increase of inductance). This is the challanging part. I would seek for cores used in planar transformers as they are very low in height and have huge cross sections.

Best regards,

Enrico Migchels

Power Conversion Design Engineer

Best - The Netherlands

www.heliox.nl