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Introduction
Introduction
Wireless power transfer is an interesting topic for experimenting.
It requires an oscillator that can drive enough power through a low inductance coil, that is used to transfer the energy to the receiver. To optimize the power transfer, resonance is used in both the transmitter and the receiver.
A resonance circuit only needs a small amount of energy at the right moment to start resonating and causing high amplitude waves. So it is an efficient way to generate and transfer power.
To create resonance, the transmitting and receiving coil are paralleled with a capacitance. For optimal power transfer between the transmitter and receiver, both resonance frequencies have to be tuned so they are equal.
The quality factor of the LC network is pretty high, so the bandwidth around the resonance frequency is small. Therefore, precise tuning of the transmitter and receiver LC networks is important when you don't want to waste energy.
There are many kinds of oscillators that can be used for wireless power transfer.
F.e. a Colpitts oscillator can be used because it is based on the LC network. An advantage of the Colpitts oscillator is that it runs with a coil that does not need a tap. The coil is then used for the power transfer.
I have chosen for a more powerful oscillator, that can be found in numerous inductive heating, high voltage generators and wireless power transfer projects on the web, because of its simplicity and efficiency : namely, the Zero Volt Switching (ZVS) Mazzilli oscillator.
Below you find the schematics for the wireless power transmitter, and it's power supply, and for the wireless power receiver.
Circuits
Circuits
Buck converter to convert +12V - 25V to +5.3V / 1.1A power supply for the wireless power transmitter
Buck converter to convert +12V - 25V to +5.3V / 1.1A power supply for the wireless power transmitter
Wireless power transmitter using Mazzilli oscillator with start-help improvement
Wireless power transmitter using Mazzilli oscillator with start-help improvement
Q1, Q2 and Q3 form a time delay circuit that tackles the start-up problem of the Mazzilli oscillator when using a slow starting power supply.
C1 and C3 are only assembled for tuning of the LC circuit. They can be assembled to make small adjustments when tuning of the resonance frequency is necessary.
The coil is made of 14 turns of 1 mm diameter enameled copper wire, with the winding wound on top of each other. The height of the stacked winding is about 16 mm. The total inductance of the coil is about 16 uH. The diameter of the coil is about 57 mm. The coil is tapped right in the middle by removing the enamel and soldering a tap-wire. You can also wind two separate coils of 7 turns and join 1 side of both coils together, but then pay attention that both coils have the same phase. When the coils are connected in opposite phase, the total power will be close to zero. Together with C4 and C5, with a total capacitance of 200nF, the LC network results in a resonance frequency of about 81 kHz. It does not matter if the resonance frequency is higher or lower when you build this circuit, as long as the transmitter and receiver use the same resonance frequency.
Wireless power receiver
Wireless power receiver
C1 and C2 are only assembled when tuning of LC resonance frequency is necessary and small adjustments are needed.
The coil is made of 14 turns of 1 mm diameter enameled copper wire, with the winding wound on top of each other. The height of the stacked winding is about 16 mm. The total inductance of the coil is about 16 uH. The diameter of the coil is about 57 mm. This coil doesn't need a tap. D1 is a Schottky diode that is used as a half-wave rectifier. You can also use a full wave rectifier, but this will eat away more voltage from the incoming signal because then you lose 2 diode-drops. U1 is a 5V ultra low drop voltage regulator with a typical dropout voltage of 0.6V/1A, so it will work down to an input voltage of 5,6V at 1A.
Power delivery of the wireless power system
Power delivery of the wireless power system
I checked how much power can be drawn from the wireless power receiver when the receiver coil is a few centimeters above the transmitter coil. I was able to draw maximum 250mA before the ultra low drop regulator surrendered.
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