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I am working on a new project which uses wireless power transfer.

I know that wireless charging in general is not so effective but my circuit is extremely not effective. Can I ask you for some advice?

I attached a picture of my circuit:

enter image description here

Basically it is just modified Royer oscillator, supplied by 5V DC and using logic level transistor to turn on even with 5V.

This circuit has load 47 ohms however later I would like to connect and charge a battery. As a coil I use a planar coil with litz wire with diameter 40mm with resistance less than 0,1 Ohm.

I also attached a picture of my circuit on breadboard. I am limited with frequency which should be 150 kHz in maximum.

enter image description here

I have already tried plenty of experiments but still dont have a solution. When the receiver coil is placed far from transmitter circuit so no load attached, transmitter takes 5V and something around 150-200 mA. When I am move the transmitter coil closer to the receiver coil, at 1 cm distance current flows through 47 ohm load (around 50 mA) but source gives to transmitter almost 600 mA.

I also noticed that when coils are in touch source gives almost 1 A and chokes and transistor heat up rapidly and transmission doesn't work correctly. It seems that transistors don't fully open. I have plenty of coils and I have not reached higher efficiency with any of configurations more than 15 percent.

Maybe the Royer oscillator is not a good candidate for such a circuit, but I really like it for the simplicity and few components, but I have no idea how to increase efficiency.

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    \$\begingroup\$ ah, high frequency electronics and solderless breadboard: never a good idea. These things notoriously misbehave at higher frequencies due to high stray capacitances between the adjacent metal strips inside and due to the high series inductances of the same. So, build this on a proper PCB and try again. \$\endgroup\$
    – Marcus Müller
    Jan 26, 2020 at 22:34
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    \$\begingroup\$ In addition, you should study these Wiki pages and all of the relevant links they provide: The Wireless Power Consortium, Inductive Charging, and Wireless Power Transfer. I'll write a note worth taking heed of, in the next comment below. \$\endgroup\$
    – jonk
    Jan 27, 2020 at 0:33
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    \$\begingroup\$ One of the Wiki pages listed above has this note: "A drawback of resonant coupling theory is that at close ranges when the two resonant circuits are tightly coupled, the resonant frequency of the system is no longer constant but 'splits' into two resonant peaks, so the maximum power transfer no longer occurs at the original resonant frequency and the oscillator frequency must be tuned to the new resonance peak." I don't know the importance of this, as I've never studied nor attempted an implementation. But it strikes me as worth keeping in mind as you read. \$\endgroup\$
    – jonk
    Jan 27, 2020 at 0:34
  • \$\begingroup\$ a better configuration of the same topology: electronics.stackexchange.com/questions/383783/… \$\endgroup\$
    – Pojj
    Jan 30, 2020 at 11:48
  • \$\begingroup\$ @Pojj Thank you very much for your post, but your circuit does not work for me. I am using IRLZ44 transistors, but it should not be a problem, they are also logic level transistor with even better RdsOn. Circuit does not oscillate. I do not see any feedback loop for transistor to know when one should turn on and another turn off. In my circuit diodes provide feedback for transistors \$\endgroup\$
    – Kols
    Jan 30, 2020 at 16:25

2 Answers 2

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but my circuit is extremely not effective

You certainly have problems with your transmitter: -

enter image description here

You have the MOSFET gates connected to ground and this means it can never work.

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  • \$\begingroup\$ Sorry my diagram was wrong, but my circuit was right. Of course not connect gate to ground.I edited my diagram. thank you. Do you have any ideas what can be wrong or is it even possible to have efficiency more than 30 % with requirements like: (5V input and exact this coil)? @jonk: Thank you very much for your articles, but I have already read them... and frequency splitting in tightly coupled coils is visible on the oscilloscope, so you are right about this theory. It just so confusing, why the transfer is so bad with few millimeters gap between the coils. Marcus: Thank you for the tip \$\endgroup\$
    – Kols
    Jan 28, 2020 at 21:46
  • \$\begingroup\$ The basic circuit is quite inefficient because it’s forever dragging the gates down to ground each cycle of oscillation through 100 ohm resistors. Also the MOSFETs never get turned on properly and act as loss agents in the tuned circuit. Maybe a better question is to ask what the most efficient way is of driving a parallel resonant tuned circuit and see what folk come up with. Most engineers on this site have sim tools that will give their results and they will be believable. \$\endgroup\$
    – Andy aka
    Jan 28, 2020 at 22:34
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My reply was delayed. I've just begun to blog about how to make wireless power transafers. The technology of automatic translation is advanced now, so I would like you to read the explanation in Japanese using automatic translation. And questions for the blog are okay in English.

Mechanism of wireless power transfer explored with LTSpice (Initial setting) http://blog.livedoor.jp/neotesla/archives/51610014.html

Mechanism of wireless power transfer explored with LTSpice (using variables) http://blog.livedoor.jp/neotesla/archives/51610205.html

Everyone will be in trouble if I say it now-Explanation of the essential principle of wireless power transfer explored with LTSpice (equivalent circuit edition) http://blog.livedoor.jp/neotesla/archives/51610671.html

Mechanism of wireless power transfer explored with LTSpice (NS method) http://blog.livedoor.jp/neotesla/archives/51610747.html

Mechanism of wireless power transfer explored with LTSpice (inverter circuit edition) http://blog.livedoor.jp/neotesla/archives/51610755.html

Mechanism of wireless power transfer explored with LTSpice (PP, PS method) http://blog.livedoor.jp/neotesla/archives/51610832.html

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