




Wireless Battery Charger
By: Dave Johnson July 24, 2007 

Exciter Circuit 
Driving a series resonant
network is a little tricky. Both current and voltage in the network can get quite
high. If we start with some of our initial requirements, we can come up with a rough
idea of the coil specs. 
With a good driver design, most
of the power sent to the exciter network will be dissipated in the coil. I estimated
last time that I will need about 1 Amp of current from a 5 volt supply. This says
that the load resistance is about 5 ohms. The Q of a series resonant network is
defined as the reactance of the coil at the drive frequency, divided by the source
impedance. The higher the Q, the higher the efficiency. However, the Q will
drop, when there is another coil nearby, which is also tuned to the drive frequency.
I think a Q of 30 is a good place to start. If the source impedance is about 5 ohms, then
the inductive reactance should be about 5 x 30 or 150 ohms. Using the formula Z =
6.28 x FL, where Z is 150 ohms, F is the 125,000Hz drive frequency and L is the inductance
in henries. Solving for L, we come up with a target inductance of about 200uH.
After calculating the number of turns needed for a circuit with a 4.5” radius, I come up
with about 20 turns needed in the coil. Again, this is a starting place and I can
add more turns or take turns off it I need to. 
Exciter Coil Voltage:
What kind of voltage will be induced across the coil? If we assume an average
current of 1 Amp, the peak to peak current will be about 3 Amps. With an inductive
reactance of 150 ohms, the expected peak to peak voltage across the coil will be about 3 x
150 or 450 volts, with a peak voltage of about 225v. The insulation of the wire used
in the coil will therefore have to be rated at something greater than 300 volts. I might
be able to get by with some standard magnet wire with good enamel insulation. The
series resonant capacitor will also have to have a high voltage rating. If I use a
small gage wire, such as 24ga or smaller, the resistance of the wire might be high enough
to keep the Q within reasonable limits. This would also make it easier to stuff the
wire coil into a pocket I make in a mouse pad. 
Exciter Capacitor: I
calculated that the target exciter coil inductance should be something around 200uH.
To resonant the coil at 125KHz, the series capacitor will need to be carefully selected.
Using the formula: F = 1/[6.28 x (LC)^2] and solving for C, we come up with a
capacitance of 0.008uF. A 8200pF capacitor should be close enough. The capacitor should
be rated at about 500v and should be a low loss type. A high voltage polypropylene,
polystyrene or mica should do the trick. I’ll see what I have in my inventory.
I may have to order some. 
Power Receiver Coil:
The back of my cell phone is about 1.5 inches by 3 inches. If we want to collect
about one watt of power at say 5 volts, the load impedance would then need to be about 25
ohms. The Q of a parallel resonant network is defined as the parallel load
resistance divided by the inductive reactance of the receiver coil at the frequency F.
We don’t want the Q to be too high, since we will not be able to easily tune that coil.
There could be a mismatch between the exciter frequency and the resonant frequency of the
receiver coil network. The mismatch could be something like +10%, so a Q of 5 might
be a good place to start. If the target load resistance is about 25 ohms, then with
a Q of 5, the inductive reactance should be about 5 ohms. Inserting 5 ohms into the
equation Z = 6.28 x FL and solving for L, we come up with a target inductance of 6uH.
With radius of about 1.2” the number of turns for a 6uH coil works out to about 7 turns.
This will be a place to start. I can add more turns if I need to. With a 6uH
coil, the parallel capacitance will need to be about 0.27uF. Unlike the exciter
network, this capacitor can be a low voltage type. A polyester type rated at about
50 volts should work fine. 
Exciter Coil Driver Circuit:
I’d like to try a simple pushpull driver circuit
for my first test circuit. One such circuit is shown below. The weakness of
this circuit is that there may be some power lost during each edge of the square wave
drive. Current can shot through both FETs briefly, as the circuit transitions
between sourcing and sinking current to the load. If I keep the square wave edges
fast, the losses may be acceptable. For testing purposes, I can quickly wire up this
driver circuit and use a good signal generator for the 125KHz source. That way, I
can easily change the frequency to hit the resonant point. The idea is get an
exciter circuit working, so I can see how much power I can pump into the exciter coil.
Then, I can build up a receiver coil and see how much power I can couple between the
coils. After that, I can refine the system for maximum power transfer. 






