Optimizing a Wirelessly Powered AC-DC Booster for Biomedical Implants

1. Optimizing a Wirelessly Powered AC-DC Booster for Biomedical Implants 2013 NASA CIPAIR Summer Research Internship Program Electrical Engineering Group Marissa Buell, NehadDababo, Rene Figueroa, Peter Moala Supervised by SFSU Student Kang Bai and SFSU Advisor Dr. Hao Jiang

2. Biomedical Implants • Cardiac pacemakers and defibrillators • Neurological stimulators • Muscle Stimulators • Cochlear implants • Monitoring devices • Drug pumps http://www.dvclub.info/il-pacemaker-biologico-tutto-naturale/

3. Biomedical Implants • Medical implant batteries require replacement every 5-10 years • Effectivepower storage requires a larger battery • Increased risk with multiple surgeries

4. Biomedical implants • Cost of surgery for replacement ranges from $2000-$45,000+ • Insurance companies do not always cover replacement costs • “She (Plaintiff Paige Riley) alleges that Blue Cross & Blue Shield of Mississippi refused to cover an operation to replace the batteries of a stomach-pain device she had surgically implanted in 2005. As a result, Riley had to fork over the $43,364.27 in cash” http://www.forbes.com/2009/11/23/hmo-medical-implants-business-health-care-batteries.html

5. The wireless solution • Wireless power transfer for charging of Internal Medical Devices (IMDs) decreases the need for periodic, invasive surgery. • Dramatically reduces battery size • Currently, San Francisco State University has developed the most efficient AC-DC Booster • An open circuit input voltage of 200mV yields an output voltage of 5V

6. CIPAIR PROGRAM GOAL • Optimize, miniaturize, and redesign the circuit by removing jumper wires and extrinsic components • Minimize parasitic capacitance, inductance and resistance • LT SPICE software—used to simulate testing. • Printed Circuit Board (PC Board)—used to perform testing experimentally. Simulation cannot account for parasitic aspects, so the usage of a PC Board is mandatory to attain realistic results.

7. Transferring wireless power • Time varying magnetic field induces an electric current in the receiver coil Self Induction • SETUP • Alternating current passing through the transmitter coil induces an electromagnetic field. Faraday’s Law Baker, et al., "Feedback analysis and design of RF power links for low-power bionic systems." Biomedical Circuits and Systems, IEEE Trans. on 1(1): 28-38.

8. AC-DC Boost converterOperating principle How does 200mV become 5V? • Switch is thrown open, large voltage drop occurs across the other circuit components. • The coil experiences an abrupt, large decrease in current, generating a large voltage • With the switch open, the coil provides the load with the voltage

9. Voltage across an inductor Current Time

10. Full Wave rectifier • Transforms input AC voltage to output DC voltage required to charge the battery • Uses two diodes and two MOSFET transistorsto avoid the large turn on voltage

11. AC VOLTAGE

12. TO FULLY RECTIFIED DC VOLTAGE

13. What controls the switch?

14. Power Coil Transmitter Coil Signal Coil Analog Signal Conditioning Receiver Coil composed of two coils AC-DC Boost Converter DC Output

15. Signal conditioner • Receives and cleans the the input frequency signal for use by the microcontroller, which converts the analog signal to the digital

17. Power Coil Transmitter Coil Signal Coil Analog Signal Conditioning Logic Control Circuit AC-DC Boost Converter DC Output

18. microcontroller • Small CPU that controls the switch’s on and off time • Uses input signal from the auxiliary coil to control when switch opens and closes • The switch regulates the current passing through through the circuit

19. Current setup • Uses a transmitter coil to wireless transfer power • Allows for high frequency input and an adjustable waveform Why do we want an adjustable waveform and high frequency input? Higher frequency input allows for a higher duty cycle

20. The Square wave advantage • After full-wave rectification, square wave maintains nice steady output line. • Sinusoidal wave requires some manipulation Allows us to increase current input—greater duty cycle, less wasted current

21. Duty cycle vs output voltagefor sinusoidal waveform Peaks at 62%

22. Square Wave 78% Duty Cycle Sinusoidal Wave 62% Duty Cycle Green Wave – Input AC Voltage Blue Wave – Control Signal Rising edge of control signal must be properly aligned with input AC to maximize current input

23. Output voltage vsopen circuit input voltage Sinusoidal Wave Vout=10.9Vin Square Wave Vout=25Vin Smaller input yields same output

24. Designing a pc board

25. The previous schematic OBSOLETE

26. Recreating the current schematic • Working backwards from the manufacturer’s file, the schematic for the presently utilized PC board was generated • Extra components, headers, and jumpers were then removed  Recreation of the layout

27. Microcontroller Signal Conditioner Boost Converter Layout for the most recent pc board

28. Current schematic of circuit

29. Generation of the layout

31. PC Board Design

32. What’s next? • Manufacturing of the board takes 2 weeks • Wind coils of different wire lengths and measure resulting inductance and resistance to minimize input voltage • Run measurements on the old PC Board • After receiving the new PC Board, solder components and rerun testing

33. Pc board measurements Data for 1.43 mH coil

34. Duty cycle vs. output voltage Peaks at 76%

35. Load resistance vs. output voltage

36. Load resistance vs. output voltage The output voltage is dependent on the load resistance; their relationship is somewhat linear.

37. Power Efficiency Crude Power Efficiency for the 1.43mH coil using a 76% duty cycle.

38. Output voltage vs. frequency ƒ=n/τ n=1,2,3,4 τ=L/RS Output voltage vs frequency for various coil sizes. An input voltage of 0.89 V is held constant at 1kHz for each coil.

39. References • H. Jiang, D. Lan, D. Lin, J. Zhang, S. Liou, H. Shahnasser, M. Shen, M. Harrison, and S.Roy, “A Feed-Forward Controlled AC-DC Booster for Biomedical Implants”, in Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC), 2012 • H. Jiang, B. Lariviere, J. Zhang, S. Liou, H. Shahnasser, M. Shen, M. Harrison, and S. Roy, “A Low Switching Frequency AC-DC Boost Converter for Wireless Powered Miniaturized Implants”, 2012 • http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1502062/ • http://www.forbes.com/2009/11/23/hmo-medical-implants-business-health-care-batteries.html • http://www.dvclub.info/il-pacemaker-biologico-tutto-naturale/ • http://www.atlantichealth.org/gagnon/our+services/treatment+services/cardiac+surgery/pacemaker+and+defibrillator+implantation • Baker, et al., "Feedback analysis and design of RF power links for low-power bionic systems." Biomedical Circuits and Systems, IEEE Trans. on 1(1): 28-38.

40. Questions?