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Switched-Capacitor Converters: Big and Small

Switched-Capacitor Converters: Big and Small. Michael Seeman UC Berkeley. Outline. Problem & motivation Applications for SC converters Converter fundamentals Energy-harvested sensor nodes Energy harvesting technology Power conversion for energy harvesting

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Switched-Capacitor Converters: Big and Small

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  1. Switched-Capacitor Converters:Big and Small Michael Seeman UC Berkeley

  2. Outline • Problem & motivation • Applications for SC converters • Converter fundamentals • Energy-harvested sensor nodes • Energy harvesting technology • Power conversion for energy harvesting • SC converters for microprocessors • Conclusions

  3. Problem & Motivation • Inductor-based Converters: • Efficient at arbitrary conversion ratios • Cannot be integrated • The inductor is often the largest and most expensive component • Causes EMI issues • Switched-capacitor (SC) converters: • Can easily be integrated • No inductors • EMI well controlled • Efficient at a single (or a few) conversion ratios

  4. Flash Memory LED Lighting Microprocessors Sensor Nodes Motor Drive RS-232 Interfaces Applications Existing: Proposed: And more…

  5. Switched-Capacitor Fundamentals Simple 2:1 converter: • The flying capacitor C1 shuttles charge from VIN to VOUT. • Fixed charge ratio of 2:1 • A voltage sag on the output is necessary to facilitate charge transfer • Fundamental output impedance:

  6. Switch Parasitics Charge Transfer Capacitor Bottom-Plate Switch Resistance Performance Optimization Switch Area Switching Frequency

  7. Wireless Sensor Node Converters • Distributed, inexpensive sensors for a plethora of applications • Batteries and wires increase cost and liability • Low-bandwidth and aggressive duty cycling reduces power usage to microwatts • Miniaturization expands application space

  8. Node Structure Energy Harvester Power Conditioning Energy Buffer Power Conversion IC Loads Efficiently convert input energy when it occurs and at varying voltages Efficiently convert buffer voltage to load voltage(s) over a large dynamic range Feb. 20, 2009 Michael Seeman: Harvesting Micro-Energy 8

  9. Environmental Energy AC appliances vibrate at multiples of 60 Hz! S. Roundy, et. al., “Improving Power Output for Vibrational-Based Energy Scavengers,” IEEE Pervasive Computing, Jan-Mar 2005, pp. 28-36. Feb. 20, 2009 Michael Seeman: Harvesting Micro-Energy 9

  10. Energy Harvesters Voltage Considerations Efficiency drops inside due to carrier recombination and spectrum shift 0.6V/cell (outdoors) 0.1V/cell (indoors) Solar Resonance must be tuned to excitation frequency for maximum output, sensitive to variation 1-100V (macro) 10mV-1V (MEMS) Vibrational 1-3 µV/K / junction 1mV-1V / generator Requires large gradient and heat output; low output voltage unless thousands of junctions used Thermal

  11. Ultra-compact Energy Storage Commercial LiPoly batteries only get down to ~5mAh; 300mg Printed batteries and super-capacitors allow flexible placement and size Li-Ion and AgZn batteries under development Christine Ho, UC Berkeley Feb. 20, 2009 Michael Seeman: Harvesting Micro-Energy 11

  12. Example: PicoCube TPMS top bottom storage board uC board sensor board switch/power board radio board 1cm A wireless sensor node for tire pressure sensing: on a dime radio COB die Yuen-Hui Chee, et. al., “PicoCube: A 1cm3 sensor node powered by harvested energy,” ACM/IEEE DAC 2008, pp. 114-119. Feb. 20, 2009 Michael Seeman: Harvesting Micro-Energy 12

  13. Synchronous Rectifier High gain amplifier controls high-side switches to provide lossless diode action VOC (open circuit voltage) VR (loaded voltage) IR (input current) Hysteretic low-side comparator reduces power consumption at zero-input 100 Hz input, 2.1kΩ source impedance Feb. 20, 2009 Michael Seeman: Harvesting Micro-Energy 13

  14. Converter Designs Native 0.13µm NMOS devices used for high performance 30 MHz switching frequency using ~1nF on-chip capacitors Hysteretic feedback used to regulate converter switching frequency Novel gate drive structures used to drive triple-well devices 3:2 Converter (0.7V) 1:2 Converter (2.1V) STMicro 130nm CMOS Fall 2007 Feb. 20, 2009 Michael Seeman: Harvesting Micro-Energy 14

  15. Power Circuitry Performance Power Conditioning: Power Conversion: Synchronous Rectifier Switched-Cap Converters Matched Load RL = RS Ideal diode rectifier (VD=0) This chip, ≤ 1 kHz input This chip, 10 kHz input VD = 0.5V diode rectifier Regulated Peak efficiency of 88% (max possible 92%) Unregulated VDD = 1.1 V NiMH; 2.1 kΩ source Feb. 20, 2009 Michael Seeman: Harvesting Micro-Energy 15

  16. Intel Atom (2008) 45nm, 25mm2 2.5W TDP SC Converters for Microprocessors • Power-scalable on-die switched-capacitor voltage regulator (SCVR) to supply numerous on-die voltage rails • Common voltages:1.05V, 0.8V, 0.65V, 0.3V • From a 1.8V input • On/off capability allows replacement of power gates • Small cells are tiled to provide necessary power for each rail

  17. SCVR: Topology • For low-voltage rails, add an additional 2:1 at the output

  18. SCVR: Performance High-efficiency points aligned with nominal load voltages 20 fF/mm2 MIM Cap; 2.5 W in 2.5 mm2 die area

  19. SCVR: Performance Tradeoffs Efficiency [%] Max. Switching Frequency [MHz] Capacitor Area [mm2] 20 fF/mm2 MIM Cap; 2.5 W in 2.5 mm2 die area

  20. Improving SCVR Efficiency • Improving switch conductance/capacitance • Improving capacitor technology • Higher capacitance density • Lower bottom plate capacitance ratio • Parasitic reduction schemes • Charge transfer switches • Resonant gate/drain • Control tricks can help for power backoff

  21. Switch conductance modulation Pulse-width modulation Frequency modulation Regulation with SCVRs • Regulation is critical to maintain output voltage under variation in input and load. • No inductor allows ultra-fast transient response • Given ultra-fast control logic • Regulation by ratio-changing and ROUT modulation: All methods equivalent to linear regulation (zero-th order)

  22. Regulation and Efficiency Varying frequency & switch size Varying frequency Fixed frequency (unregulated) 3:2 @ 1.05V out; 2.5W using 2.5mm2 area

  23. Example Transient Response 2.2 V Open-loop 1.8 V input Full load current Lower-bound feedback 8% load

  24. Conclusions • Switched-capacitor converters exhibit significant advantages over inductor-based converters in many applications • SC converters can be easily modeled using relatively simple analysis methods • SC converters and CMOS rectifiers make ideal power converters for sensor nodes • Modern CMOS technology allows for high-power-density on-chip power conversion

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