Switched-Capacitor Boost Converter Design and Modeling for Indoor Optical Energy Harvesting with Integrated Photodiodes

1
Switched-Capacitor Boost Converter Design and
Modeling for Indoor Optical Energy Harvesting
with Integrated Photodiodes
Stanley W. Hsu, Erin Fong, Vipul Jain, Travis
Kleeburg, Rajeevan Amirtharajah University of
California, Davis
2
Outline

• Introduction/Motivation
• Integrated Photodiode
• Switched-Capacitor Boost Converter (SCBC)
• Delta-Sigma Modulator (DSM)
• Supply Ripple Effects on DSM
• Summary

3
Outline

• Introduction/Motivation
• Integrated Photodiode
• Switched-Capacitor Boost Converter (SCBC)
• Delta-Sigma Modulator (DSM)
• Supply Ripple Effects on DSM
• Summary

4
Introduction

• Ultra-low voltage sensor circuits powered by
  free-space optics (Kleeburg, 2010)
• Integrated photovoltaics for optical
• power, data, and clock delivery
• Subcutaneous medical implants
• Ultra-low duty cycle sensor (Ayazian, 2012)
• Integrated photovoltaics
• (2.5 mm x 2.5 mm)
• Off-chip capacitive and resistive
• transducers, and electrodes

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Energy Harvesting from Indoor Lighting

• Low light intensity limits harvested energy
• Issue light flickering

• Rectified AC mains at 120 Hz

• Pulse-width-modulated dimming at gt 200 Hz

Source ksj.mit.edu
Source www.dlsound.net
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Indoor Lighting-Powered Sensor
Light
Cost, Area/Volume
Integrated Photodiode
Bypass Capacitor
Power Electronics
Supply ripple
Circuit performance
Vdd
Vdd Domain Circuits
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Outline

• Introduction/Motivation
• Integrated Photodiode
• Switched-Capacitor Boost Converter (SCBC)
• Delta-Sigma Modulator (DSM)
• Supply Ripple Effects on DSM
• Summary

8
Integrated Photodiode Designs
P/NW
P/DNW
P/NW P/DNW
Voc 523 mV 508 mV
Isc density 134 mA/mm2 52 mA/mm2
Power generated/area 557.5 mW/mm2 210 mW/mm2
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Integrated Photodiode Results
3 P/DNW photodidoes stacked in series

(no bypass capacitor)
Increasing frequency or duty cycle decreases
ripple.
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Outline

• Introduction/Motivation
• Integrated Photodiode
• Switched-Capacitor Boost Converter (SCBC)
• Delta-Sigma Modulator (DSM)
• Supply Ripple Effects on DSM
• Summary

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Switched-Capacitor Boost Converter
S4

• Phase 2 Charge capacitors to VIN
• Phase 1 Boost output to 4x VIN

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Buck Converter Model
Slow Switching Limit
Fast Switching Limit
(Seeman, 2008)
Combined Output Impedance
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Proposed Boost Converter Model
N4
Model accounts for bottom plate parasitic effects
and allows cascading of multiple stages
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SCBC Output vs. Switching Frequency
Model is accurate to within 10, from 500 Hz to 5
MHz
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SCBC Simulated Ripple to Output Ratio
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Outline

• Introduction/Motivation
• Integrated Photodiode
• Switched-Capacitor Boost Converter (SCBC)
• Delta-Sigma Modulator (DSM)
• Supply Ripple Effects on DSM
• Summary

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Conventional 1st Order DSM Design
Comparator
Digital output
Integrator
Pre-Amp
Analog input
error
Latch

-
1-bit DAC
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Proposed 1st Order DSM Design
Removed!
Digital output
Low Pass Filter
Pre-Amp
Analog input
error
Latch

-
1-bit DAC
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Proposed 1st Order DSM Schematic
Switched-capacitor low pass filter
Dynamic Comparator No pre-amplifier
Attenuates input! Gain lt1
1b DAC feedback
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DSM Die Photo and Measured Results
1
SNDR 27 dB
Technology 180 nm 180 nm
Supply Voltage 1.4 V 1.8 V
Sampling Rate 50 kHz 1.6 MHz
Nyquist Rate 4 kHz 4 kHz
SNDR _at_ -7dBFS input 27 dB 50dB
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Outline

• Introduction/Motivation
• Integrated Photodiode
• Switched-Capacitor Boost Converter (SCBC)
• Delta-Sigma Modulator (DSM)
• Supply Ripple Effects on DSM
• Summary

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Supply Ripple Effects on DSM

• Sampling switch behaves as passive mixer (Cook,
  2006)
• Distortion due to passive mixing
• Sampling switch
• Mixing between input and ripple
• 1b DAC feedback switch
• Mixing between ripple and itself

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Measured DSM Lower Sideband Spectrum
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Measured DSM SNDR vs. Ripple
4.5 bits
Vdd 1.4V Sampling Rate 50 kHz Input Amplitude
-7dBVdd
2 bits
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Outline

• Introduction/Motivation
• Integrated Photodiode
• Switched-Capacitor Boost Converter (SCBC)
• Delta-Sigma Modulator (DSM)
• Supply Ripple Effects on DSM
• Summary

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Summary

• P/NW integrated photodiodes achieves
• 557.5 µW/mm2 with Voc523 mV
• Switched-capacitor boost converter model for
  predicting output voltage to within 10 accuracy
  from 500Hz to 5 MHz
• Supply ripple effects on passive delta-sigma
  modulator results in IM2 distortion at

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Summary
Light

• If DSM can tolerate an increased supply ripple
  from 10 to 21 of Vdd, bypass capacitor can be
  reduced from 56.5 nF to 3.86 nF.

Integrated Photodiode
Bypass Capacitor
Power Electronics
Vdd
Vdd Domain Circuits
28
Acknowledgements

• Texas Instruments for chip fabrication
• William McIntyre
• Arun Rao
• Keith Schoendoerfer
• Greg Winter
• Bijoy Chatterjee