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Inductive charging for batteries, is it required to implement LC resonance ?

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Advanced Member level 3
Oct 31, 2009
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Kerala (INDIA)
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Hi, I am trying to make an inductive charger add-on for a 3.7 500mAh li-po battery.
I have two flat circular coils each of inductance 14.6uH.
I programmed an avr to make square wave of different freq and driven one coil with a power mosfet (at 10v) and I could obtain some voltage at the other coil. (rectified and connected to an LED). It is working to an extent but efficiency is very low (the mosfet is taking more current and getting heated up very fast) even if there is no load.

Then I connected a 0.1uF ceramic capacitor parallel to both coil (transmitter and receiver) and calculated the resonance frequency and provided that frequency squarewave at mosfet gate (from avr (couppled using transistor), 50% duty cycle). Now the performance improved drastically and as I expected, current is taken from source only when the receiver coil connected with load is moved near the transmitter coil. Thats cool...;-)
But the 0.1 uF ceramic capacitor is getting heated up and burned out with in 1 minute due to over current at resonance:-? and this will eventually make it out of resonance and will damage the mosfet and other power supply circuitry.

So my question is:

1> Which capacitor (0.1uF) should I select for this? It seems like it should withstand high ac current at resonance, it should not burn out. The current at resonance seems to be like 100mA (average) when the load is not moved near the transmitter coil. But current at LC will be very high, didn't measured.....

2> Seen some inductive charging circuit in internet without any resonance capacitor. Will it be efficient like that of with caps ?

3> Normally in wireless charging for mobile phones, does they use any capacitor along with inductor?

4> I am using it at frequency around 140KHz (and may be some harmonics, didn't considered). Is it ALLOWED TO USE THIS RF FREQUENCY?

5> How to further improve the power transfer efficiency? Right now I am setting same resonance frequency in both transmitter and receiver and providing squarewave to drive mosfet.

6>Both transmitter and receiver coils are similar. Need to make any modifications?
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