WIRELESS POWER TRANSFER

By: David Chavez and Shu-Hui Cheng

Introduction• Wireless Power Transfer System will help eliminate the

clutter of outlets.

Benefits and Features

Benefits• Wireless transmission of electricity

• Eliminate power cords• Safe wireless power transfer

• Smart charging• Environmental friendly charger• Charging stops automatically when the battery is fully charged• Charging begins automatically when the battery level drops below a threshold value

• Compatibility with USB devices• Support for wide range of devices

Features• Ability to do wireless power in a small range

• Automatic detection of battery life and the need for charging

Wireless Power Transfer System

Battery Detection Communication

Wireless Transmission

System Overview

USB

Edge detector

Battery Indicator

SR Latch

Voltage Regulator

Rectifier

Receiving Antenna Coil

Original Charging Annex Block Diagram

Device

Switch

Sine Wave Generator

Ultrasound transmitter

USB

PIC Microcontroller

Hall effect sensor

Ultrasound

Voltage Regulator

Rectifier

Receiving Antenna Coil

Clock Oscillator

Charging Annex Block Diagram

Device

Ultrasound Receiver

Bridge Rectifier

Amplifier

Transmitting Antenna Coil

Amplifier

Oscillator

Voltage Divider

AC to DC power Supply

Voltage Regulator

Original Charging Platform Block Diagram

Low Pass Filter

Switch

Ultrasound

Bridge Rectifier

Amplifier

Transmitting Antenna Coil

Amplifier

Oscillator

Voltage Divider

AC to DC power Supply

Switch

Charging Platform Block Diagram

Hardware Overview• Power Supply

• Takes 120V from wall outlet and converts to 5Vdc to power components

• Also converts the 5Vdc to an amplified 30V for power transmission.• Battery Detection

• Reads current flowing into battery and sends signal to continue charging if current is high

• Recieves ultrasonic signal and powers on MOSFET to transmit power.• Loop Antenna

• Receives 13.5MHz signal and transmits it through a loop antenna.• Battery Charger

• Takes 13.5MHz signal and converts to stable 4.2Vdc voltage to charge battery.

Battery Detection• Encountered a few

problems with initial design

• New design comprised of two main components• Hall Effect Sensor• PIC 16F887

Battery Level (%) Line Current (A)Past 95 ≈0.05

95 0.25

90 0.35

75 0.6

70 0.7

0 0.85

Battery Detection• Reads current flowing into

the battery.• Can be connected to any

Mini USB device• Outputs 50% 5Vpp PWM

Signal to Ultrasound• Receives Ultrasound signal

and turns on MOSFET

PIC16F887 Flow Chart

3.5cm

Antenna Coil Pair

RF Power Transmission using Antenna Coil Pair

Parallel LC resonant circuit

System architecture

• Oscillator: Generate a sinusoidal signal at 13.5 MHz

• Power Amplifier: Amplify signal to increase transmission range

• Balun: Convert the unbalanced signal to a differential signal (because the antenna coil is a balanced structure - no definite ground)

• Antenna coil: Convert the electrical signal to magnetic field (vice versa)

• Bridge rectifier: Down convert the AC signal to DC

• Voltage regulator: Stabilize DC voltage

RF Power Transmission Principle• Based on Faraday’s Law of Induction:

• At the transmit coil, a time-varying current generates a time-varying magnetic field• The time-varying magnetic field induces a time-varying current on the receive coil

• Testing for two coil antennas

• To maximize the power transmission efficiency, the coil must be tuned to the operating frequency

• The coil is brought to resonance at 13.5 MHz by adding a capacitor in parallel• Resonance frequency testing

where(the radiated field has a cubic roll-off)

System Demo

Measurement Results (Transmitter side)Oscillator output:

1.77 Vpp @13.48 MHz

1st amplifier output:4.44 Vpp

2nd amplifier output:12.74 Vpp

Balun output (fed to transmit coil):33.4 Vpp

8 dB gain 9.2 dB gain

Measurement Results (Receiver side)Receive coil output:7.94 Vpp @13.48 MHz

Bridge rectifier output:

Measurement Summary• Range

• Power transmission efficiency = Received output from coil / transmitted output from coil = 6.8 %

• System efficiency = Power drawn from AC supply / power received to USB = 6.6 %

Distance between Two Coil (cm) DC Voltage (V) to the USB

Closest~0.1 4.17

1 2.88

2 1.26

0 1 2 3 4 5 6 7 80

5

10

15

20

25

30

35

40

Voltage (V)

Testing for Two Coil Antennas

Distance (cm)

• Measured peak-to-peak voltage at the receive coil antenna as separation distance increases

12.5 13 13.5 14 14.5 15 15.50

50

100

150

200

250

300

350

Frequency(MHz)

Voltage (m

V)

Resonance Frequency Testing• Measured peak-to-peak voltage at receive coil as the frequency changes • Confirm the coil resonates at 13.5 MHz

Battery Detection System testingPWM signal from output of PIC and Ultrasound receiving signal

Battery Detection System testingRectified Ultrasound signal in range

Battery Detection System testingAmplifier Signal In Range

Battery Detection System testingAmplifier Signal Out of Range

Battery Detection Recommendation • To properly transmit the Charging signal the ultrasound

needs to be in relative line of sight.• Ultrasound creates cone like region of detection• Reassure sufficient power to PIC

RecommendationsMultilayer coil

• Reduce the size of the coil• Can be made planar for easy integration with device platform

Ferrite loading• Loading the coil with ferrite to concentrate the magnetic field so as to increase range

Impedance matching • Amplifier requires 50 input and output impedance for maximum power transfer• Impedance matching circuit constructed by discrete L and C elements may be inserted between the

balun and the output of the final stage amplifier

Range improvement• Insert more RF power amplifiers• Current amplifier after voltage regulator

Battery detection improvement• Output battery level from cell phone’s operating system

•Successes• The communication part with ultrasound work• Battery detection through current can be adjusted to

needed current• Wireless transmission part can transfer power from

annex to platform and light up an LED

•Challenges• Charging platform needs more power from

transmission• The Range for power transmission

Successes and Challenges

Future Work• Improved loop antenna design

• Get antenna made professionally• Different shape may increase range

• Two AC-DC power converters to be able to transmit more power

• Add more devices to single dock

SWOT Analysis• Strengths

• Reduce outlet clutter• No need to remember to

charge

• Weakness• Charge range limited• Efficiency

• Opportunities• Regular Household• Offices

• Threats• Transformer style designs• Similar Projects (WiTricity)

• With this responsibility, not only will we reassure that the final product will meet its expectations, we will assure it is safe for use and put warning labels on items deemed unsafe if tampered with

• Through thorough testing of each component, we will guarantee that our performance claims are accurate

• Ideally this project will further open the door to exploration in wireless power transfer.

Ethnical Considerations

Acknowledgements

Professor Jonathan MakelaRyan May

Parts Shop Staff

Thank you and Questions?