1. Wireless Laptop Charger
Onur Cam, Enrique Ramirez, Jason Kao
Group 37

2. Introduction
We aim to reduce the amount of cable traffic in classrooms.
To rectify this problem we decided to attempt to make laptop chargers more wireless.
We experimented with a couple of topologies.
Initial design integrated Qi protocol.
Required advanced communication unit
Concerns over low efficiency
Settled on a DC/DC type topology.
Utilize transformer coils and core to transfer power upon suggestion from Prof.
Our design focuses on transformers.
Transformers usually step-up or step-down voltage, but we explored the possibilities of mainly utilizing the transformer for wireless power transfer, instead sending the power through magnetic waves.

3. Objectives Input some DC power and output some DC power, wirelessly.
Input: 18V
Output: 12V, 3.33A → W
60%+ efficiency
Be able to separate two parts of the circuit.
Implemented using a unfastened transformer.

4. System Blocks Transmitter Unit: Receiver Unit:
Transformer primary side
Clamp circuit
MOSFET switching circuit
Receiver Unit:
Transformer secondary side
Rectifier Circuit
LC Filter

5. Block Diagram

6. Schematic of Transmitter Unit

7. Low Side Gate Driver
6A and noninverting

8. Schematic of Receiver Unit

9. Physical Units
Transmitter Unit
Receiver Unit

10. Units in Contact

11. Transformer Requirements
We needed the transformer to be able to do the following things:
Primary side voltage close to Vin of 18 V
Secondary side voltage is close to double the primary voltage ~36V
Efficiency greater than 70%

12. Transformer Core Specifications
A transformer requires a core to amplify and focus magnetic fields
We decided on a ferrite core for convenience and high frequency range
Effective Magnetic Cross-Section (Ae)
535 mm2
Saturation Magnetization (Bsat)
320 mT

13. Transformer Calculations
Relate PWM, Input Voltage, Output Voltage, Turns Ratio
Given: D = 35%, Vin = 18V, Vout (post-rectification) = 12V
Possibly need to increase duty ratio due to losses
Finding the minimum number of windings
Given: f = 50 kHz, Vin = 18V, D = 35%, Ac = 535 mm^2, Bsat = 320 mT
Np > turns
Decided on Np = 7 to be safe

14. Transformer Calculations
Second equation relates the Turns Ratio with the Voltage Ratio of the coils
means that Ns = 14
Max efficiency of power transfer
Given: Ps = 42.7W, Pp = 45.18W
n = 94.5%

15. Transformer Calculations
Wire Gauge Calculation
Given: VA = 67 (12V * 3.33A / 0.6 PF), efficiency = 0.945, Vp = 18V
I = A
A = mm2
Pick 14 AWG ( mm2) magnetic wire to wind the coils with
Thicker wire is bulkier and has reduced skin depth
Thinner wire cannot handle current for power transmission
14 AWG is common and readily available

16. Transformer Primary IV Waveform

17. Transformer Secondary IV Waveform

18. Switching Circuit(Clamp Circuit) Requirements
Switching Circuit requirements are the following:
Reset the Core
Vc > 30V

19. Switching/Clamp Circuit Calculations
Pdiss= 1/2 (Lm+L1k (Im)^2 =~ 45 uW
-Clamp Circuit(Zener+Diode)
DT+Deltareset <_ T so P(1+(Vin/(|Vin-Vc|) <_1
D+Vin/|Vin-Vc|<_1 where;
D = 49%
Vin = 18V
Vc,min = 30V

20. Switching Circuit(Clamp Circuit) Waveforms 1W

21. Switching Circuit(Clamp Circuit) Waveforms 5W

22. PWM Generator Requirements
We decided to pick 50 kHz as our operating frequency for the PWM.
Comfortable frequency for ferrite core (1-200 kHz)
Any lower could make noise
Any higher could result in suboptimal skin depth in copper wire.
Larger duty cycle results in higher voltage on secondary side.
Maximum rated duty cycle: 50%
We consistently needed more voltage and capped out at 48% to account for fluctuations
Output 5V peak to peak signal

23. PWM Generator Specifications
Purchased PWM that allows us to modify the operating frequency and duty ratio.
Range: 1Hz -150 kHz, 0-100% duty ratio
USB powered
5V pk-pk output
8-30 mA output

24. PWM Voltage Output Waveform
We theorize that the 4V peak to peak is due to parasitics in the MOSFET Module.

25. Power MOSFET Requirements
The following are our power MOSFET requirements.
According to simulation, has a voltage level equal to input voltage.
Has a voltage level equal to 0V
MOSFET specifications can handle stress of circuit(Vc,min = 30V)

26. Power MOSFET Details
Gate driver is powered with voltage range of 4.5V - 18V
Used a 12V power supply for safety and convenience.
Gets 5V square wave input from PWM
Duty Cycle determined by PWM logic
Creates a waveform with sectioned, flat voltage levels
MOSFET minimum stresses
V = Vc = -100V (zener breakdown voltage)
|I| = 10.3A (experimentally found)

27. MOSFET Simulation Waveform
*the real waveform is not captured due to circuit breaking after demo

28. Rectifier Circuit with LC Filter Requirements
Rectifier Circuit and LC filter had the following requirements:
Make a clean output of 12V and 3.33A for load
Within 2% ripple
The rectifier handles voltage coming from secondary side coil
Verified by stress calculations

29. Rectifier Circuit with LC Filter Details
2 schottky diode stress values:
Secondary side diode d1:
V = -97V, I = 4A
Secondary side d2:
V = 18 * (n) = 94V
I = 4.3A
LC Filter
L = µH
C = 1000 µF

30. Rectifier Waveform

31. Rectifier Schematic Waveforms
Free-wheeling Diode 2
Diode 1

32. Load IV Waveform

33. Overall Efficiency Overall Efficiency was calculated as follows:
Pin = 34.74W
Pout = 24.89W
n =~71.6%

34. Conclusion Accomplishments: Failures: Future work:
Modules integrated well to produce some output.
Efficiency higher than expected. (70% compared to 60% expected)
Project operates for a non-trivial amount of time (10s)
Failures:
Clamp circuit unable to handle input power
Zener continuously burned out, had to buy replacements
Unable to retrieve waveforms for certain parts of the circuit
Project stopped working the day after demo
Future work:
Fix the project so it works again
Implement a working PCB version to reduce ringing
Improve our clamp circuit(zener and diode pair, RC circuit)
Increase rated power to meet specification