27-06-2012, 04:58 PM
WIRELESS POWER TRANSFER
WIRELESS POWER TRANSFER.docx (Size: 243.96 KB / Downloads: 52)
Inductive charging
Inductive charging uses an electromagnetic field to transfer energy between two objects. This is usually done with a charging station. Energy is sent through inductive coupling to an electrical device, which then can use that energy to charge batteries.Because there is a small gap between the two coils employed in each of the sender and receiver of the energy within the respective devices, inductive charging is considered a short-distance "wireless" energy transfer,it frees the user from having to deal with wires between the two devices. This is advantageous for three reasons.
Induction chargers typically use an induction coil to create an alternating electromagnetic field from within a charging base station, and a second induction coil in the portable device takes power from the electromagnetic field and converts it back into electrical current to charge the battery. The two induction coils in proximity combine to form an electrical transformer.
Quality Factor
The ratio of the inductance L to the resistance R of a coil remains constant for different winding arrangements in the same volume and shape. It makes sense to define this value as a figure of merit to distinguish different coil structures. The quality factor Q is defined by this ratio.
The voltage, which is induced by the same current in an inductor scales with the frequency f and thus the apparent power in the device. The general definition of the quality factor is based on the ratio of apparent power to the power losses in a device. From this definition, the quality factor of a coil results to:
Components Functioning Inside The Circuit
Full Wave Bridge Rectifier:
The full bridge rectifier is a circuit that uses four diodes, and AC source. A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output. Full-wave rectification converts both polarities of the input waveform to DC (direct current), and is more efficient. Diodes are used due to the high frequency nature of the signal. Using diodes, the voltage is rectified and passed through a PI filter. The original design specified a 1 % voltage ripple, but this ripple requirement was excessive and difficult to meet at such a low frequency. The final design chosen had a voltage ripple of less than 5 % and was more than suitable.
Transistor:
A transistor is a semiconductor device used to amplify and switch electronic signals. It is made of a solid piece of semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be much more than the controlling (input) power, the transistor provides amplification of a signal. As transistors are fast switching devices frequency of AC becomes 20 KHz.
High Frequency Transformer:
A transformer is used to scale down the voltage to around 12 V. This is done before the signal is converted to DC because high frequency transformers are small and relatively efficient.
The high frequency step down transformer has primary and secondary coils. The secondary coil output is fed to a tuned 20 KHz coil forming primary of an air core transformer.
Coil and Air Gap:
The coils are each made out of 100 turns of 20 magnet wire. They are separated by about 2 m and have a diameter of about 1 m. The power transfer between them is done through resonant magnetic coupling.