Power Transfer Over a Capacitive Interface A Wireless Power Technology

Transcription

1 Power Transfer Over a Capacitive Interface A Wireless Power Technology Mitchell Kline Igor Izyumin Prof. Bernhard Boser Prof. Seth Sanders

2 Why Wireless Power?

3

4 Wireless Power Technology Close-coupled wireless power transfer Power Source Load Power Source Load 1. Inductive 2. Capacitive

5 The Powermat 60% efficiency shield up to 1cm gap

6

7 Capacitive Power Transfer Power Source Load Simple Inexpensive Thin

8 What We Want Small Efficient Robust

9 V c V s V load Large capacitor drops little voltage

10 V c V s V load Small capacitor drops significant voltage

11 V c -V c V s V load Inductor compensates for voltage drop

12 V s V s Equivalent circuit at resonance

13 What we want Small Efficient Robust

14 L C V s Load Start with the resonant circuit

15 V S L C C sw Load Drive it with an inverter

16 V S C sw Load At resonance, the output capacitance remains

17 V S C sw Load Turn on the top switch

18 V S Energy Flow V S C sw Load Output capacitor charged to V S

19 V S V S C sw Load Turn on the bottom switch

20 V S Energy Flow C sw Load Energy stored on capacitor is wasted

21 V S Resonant Tank L residual L resonate C C sw Operate above resonance

22 V S L residual C sw Load Equivalent circuit above resonance

23 V S L residual C sw Load Turn on the top switch

24 V S Energy Flow L residual V S C sw I L Load Energy Stored: ½ C sw V S 2 & ½ L residual I L 2

25 V S L residual V S C sw I L Load Open both switches

26 V S L residual Energy Flow V S C sw I L Load Inductor transfers capacitor energy to load

27 V S L residual Energy Flow V S 0 t C sw I L Load Output voltage pulled to zero

28 V S L residual V S 0 t C sw I L Load Close the switch when the voltage reaches zero

29 V S L residual V S 0 t C sw I L Load Known as Zero Voltage Switching (ZVS)

30 What We Want Small Efficient Robust

31 L C Start with the resonant circuit (again)

32 L C f LC Would like to operate at (or near) resonance

33 L C But the capacitance depends on alignment and gap

34 Current Sense L C Tank Build an oscillator with L and C forming the tank

35 Current Sense L C Tank Phase shift around the loop = 0 Current in tank forced in phase with drive voltage

36 Current Sense L C Tank But we need to operate above resonance

37 Current Sense L C Tank phase shift Introduce extra phase shift

38

39 V S L residual C sw I L Load The equivalent circuit above resonance (again)

40 V S ½ L residual I L 2 I L ½ C sw V S 2 Load Light-load condition: not enough current in tank to get Zero Voltage Switching (ZVS)

41 V S ½ L residual I L 2 I L ½ C sw V S 2 Load Require a minimum I L for ZVS

42 DC to AC AC to DC I S V S I S I L I L Filter Capacitor Require a minimum I S for ZVS

43 DC to AC AC to DC I S V S Filter Capacitor Can analyze this with a simple DC circuit I L

44 Variable Load I S V S I OUT Filter Capacitor At full load, I S is large

45 Variable Load I S V S I OUT Filter Capacitor At light load, I S is too small Zero Voltage Switching is lost

46 Variable Load I S V S I OUT Filter Capacitor On/off modulation causes I S to increase when driver is on: Zero Voltage Switching restored

47 What We Want Small Efficient Robust

48 Capacitive Power Transfer System I S C sw + V L - Load

49 Efficiency or Duty Cycle With 6 by 10 cm 2, we transfer 3.8 W at 83% efficiency over a 0.5 mm air gap. 100% 80% 60% 40% 20% 0% Output Power [W]

50 How? 1. Resonant operation Compensate for small coupling capacitance 2. Zero Voltage Switching Improve the efficiency 3. Automatic Tuning Robust to changes in coupling capacitance 4. Duty cycle adjustment without feedback from RX Preserve efficiency at light loads

51 Capacitive Data Transfer 2 Gb/s data rate < 2 mw G.-S. Kim, M. Takamiya, and T. Sakurai A 25-mV-Sensitivity 2-Gb/s Optimum-Logic-Threshold Capacitive-Coupling Receiver for Wireless Wafer Probing Systems, TCAS II, Vol. 56, No. 9, Sept. 2009

52 Thank You! Acknowledgements Dr. Mei-Lin Chan Dr. Simone Gambini Prof. David Horsley Dr. Mischa Megens James Peng Richard Przybyla Kun Wang Prof. Ming Wu This material is based upon work supported by the Defense Advanced Research Projects Agency (DARPA) under Contract No. W31P4Q