SwitchedCapacitor Converters Big and Small Michael Seeman UC

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 • SC converters for microprocessors • Conclusions

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

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

Switched-Capacitor Fundamentals Simple 2: 1 converter: • The flying capacitor C 1 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:

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

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

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

Environmental Energy Power Source Power [µW/cm 3] Notes 15, 000 (per square cm) Solar (inside) 30 (per square cm) Temperature 40 -5, 000 Solar (outside) Air flow 380 Pressure variation 17 Vibrations 375 Vibration Source (per square cm, 5 K gradient) (5 m/s, 5% efficiency) AC appliances vibrate at multiples of 60 Hz! Frequency [Hz] Peak Acceleration [m/s 2] Clothes Dryer 121 3. 5 Small Microwave Oven 121 2. 25 HVAC vents in office building 60 0. 2 -1. 5 Wooden Deck (with people walking) 385 1. 3 External Windows (next to busy street) 100 0. 7 Refrigerator 240 0. 1 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

Energy Harvesters Thermal Vibrational Solar Voltage Considerations 0. 6 V/cell (outdoors) 0. 1 V/cell (indoors) Efficiency drops inside due to carrier recombination and spectrum shift 1 -100 V (macro) 10 m. V-1 V (MEMS) Resonance must be tuned to excitation frequency for maximum output, sensitive to variation 1 -3 µV/K / junction 1 m. V-1 V / generator Requires large gradient and heat output; low output voltage unless thousands of junctions used

Ultra-compact Energy Storage • Commercial Li. Poly batteries only get down to ~5 m. Ah; 300 mg • Printed batteries and supercapacitors allow flexible placement and size • Li-Ion and Ag. Zn batteries under development Christine Ho, UC Berkeley Feb. 20, 2009 Michael Seeman: Harvesting Micro-Energy 11

Example: Pico. Cube TPMS A wireless sensor node for tire pressure sensing: top bottom on a dime storage board u. C board sensor board switch/power board radio COB die radio board 1 cm Yuen-Hui Chee, et. al. , “Pico. Cube: A 1 cm 3 sensor node powered by harvested energy, ” ACM/IEEE DAC 2008, pp. 114 -119. Feb. 20, 2009 Michael Seeman: Harvesting Micro-Energy 12

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 Feb. 20, 2009 100 Hz input, 2. 1 kΩ source impedance Michael Seeman: Harvesting Micro-Energy 13

Converter Designs 3: 2 Converter (0. 7 V) 1: 2 Converter (2. 1 V) STMicro 130 nm CMOS Fall 2007 • Native 0. 13µm NMOS devices used for high performance • 30 MHz switching frequency using ~1 n. F on-chip capacitors • Hysteretic feedback used to regulate converter switching frequency • Novel gate drive structures used to drive triple-well devices Feb. 20, 2009 Michael Seeman: Harvesting Micro-Energy 14

Power Circuitry Performance Power Conditioning: Power Conversion: Synchronous Rectifier Switched-Cap Converters Matched Load RL = RS Ideal diode rectifier (VD=0) This chip, ≤ 1 k. Hz input This chip, 10 k. Hz input VD = 0. 5 V diode rectifier Regulated Peak efficiency of 88% (max possible 92%) Unregulated VDD = 1. 1 V Ni. MH; 2. 1 kΩ source Feb. 20, 2009 Michael Seeman: Harvesting Micro-Energy 15

SC Converters for Microprocessors • Power-scalable on-die switchedcapacitor voltage regulator (SCVR) to supply numerous on-die voltage rails • Common voltages: 1. 05 V, 0. 8 V, 0. 65 V, 0. 3 V – From a 1. 8 V input Intel Atom (2008) 45 nm, 25 mm 2 2. 5 W TDP • On/off capability allows replacement of power gates • Small cells are tiled to provide necessary power for each rail

SCVR: Topology • For low-voltage rails, add an additional 2: 1 at the output Switch 3: 2 2: 1 3: 1 S 2 S 3 S 4 S 5 S 6 S 7 S 8 S 9 Φ 1 Φ 2 Φ 1 — Φ 1 Φ 2 Φ 1 Φ 2 — Φ 2 Φ 1

SCVR: Performance High-efficiency points aligned with nominal load voltages 20 f. F/mm 2 MIM Cap; 2. 5 W in 2. 5 mm 2 die area

Efficiency [%] Max. Switching Frequency [MHz] SCVR: Performance Tradeoffs Capacitor Area [mm 2] 20 f. F/mm 2 MIM Cap; 2. 5 W in 2. 5 mm 2 die area

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

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: Frequency modulation Switch conductance modulation Pulse-width modulation All methods equivalent to linear regulation (zero-th order)

Regulation and Efficiency Varying frequency & switch size Varying frequency Fixed frequency (unregulated) 3: 2 @ 1. 05 V out; 2. 5 W using 2. 5 mm 2 area

Example Transient Response Open-loop 2. 2 V 1. 8 V input Full load current Lower-bound feedback 8% load

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 highpower-density on-chip power conversion