[Intro]
To begin with, I am an undergraduate electrical engineering student working on my senior project. I wanted to make a wireless charging pad like that of Tesla's wireless charging platform and Huawei's multi-device smart wireless chargers. I though it would be a cool project (and a cool skill to learn) to show how multi-coil chargers work.
I soon learned that wireless charging technology is much more intricate and deeper than what I have learned about wireless communications in my courses. Furthermore, I have done some research on multi-coil circuits, but it seems that the companies likes to keep it for themselves.
[Background]
For the purpose of communication, I will call the power transmitter PTx and the receiver PRx. This is what they use in Wireless Power Consortium. I am using my phone for PRx and I am using a generic Qi standard wireless chargers you can find in Amazon. Therefore, I do not have any schematics or part numbers other than what I used to hack the hardware and most measurement was done on the oscilloscope.
[Problem]
I am currently using NMOS transistors to control the AC current signal that is entering the inductor coils. The AC signal has a frequency range of 100-250 kHz, mostly sitting around 100-120��kHz. Turning "on" and "off" one coil was very straight forward. I simply placed an NMOS transistor in series between the PTx output and the inductor coil.
After learning that I can activate a coil using NMOS transistors, I made two parallel channels with the transistor and inductor coil coming out of the PTx output. Both channel returns to the input of PTx. I have included the circuit diagram that was used. What is not included in the image are the gate resistors and the box diagram of the PTx, shown as an AC source in the diagram.
When I tried to turn on individual coils in the diagram by turning one transistor on and the other off, for some reason, both coils would activate. I was not able to turn on only one coil no matter what I have tried.
[My research and what I have tried]
Before I begin listing things I have tried, I've notices something interesting going on when both channels in my circuit are active despite only intending to activate one at a time using NMOS transistors. There is an alternating voltage at the "OFF" transistor. My investigation concludes that this oscillating current/voltage comes from the parasitic capacitance of the NMOS. The NMOS I am using is IRFZ44N (Q1, Q2). I used jumper wires and my hands to simulate the switches (S1, S2).
After some extensive research, I have found some potential reason for the leaky(?) transistors. And experimented with the limited resources I had.
- Adjusting gate resistors values to damp the oscillations at the gate --> I have also tried adding small capacitors (at pF range) and the resistors and capacitors have damped the oscillating signal at the gate and source. However they failed to debug the issue.
- Physically separating and insulating the two channels.
- Putting NMOSs on both end of the coils
[Things that I will try in the near future] I have a couple of ideas to try in next. First is to use gate drivers to turn on and off the transistors. Due to the lack of equipment, I am using series of 9V batteries to operate the NMOS. Next is the ground loop bounce. I am still learning about this concept and will be testing if I think it is relevant to the problem in regard.
Let me know what you guys think the source of the problem is or if you think I should revisit some of the things I have tested for better understanding of the problem.
