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A Tutorial on Battery Simulation - MatchingPower Source to Electronic System
Manish Kulkarni and Vishwani D. AgrawalAuburn University
Auburn, AL 36849, USAmmk0002@auburn.edu, vagrawal@eng.auburn.edu
VDAT10, July 8, 2010 1Kulkarni & Agrawal
Contents• Introduction
• Powering an electronic system• Statement of the battery problem
• Power subsystem, components, characteristics• A Design Example
• Circuit simulation for critical path delay and battery current• Battery simulation for lifetime and efficiency• Finding the smallest battery for required system performance• Finding battery for lifetime requirement• Finding minimum energy mode
• Summary
VDAT10, July 8, 2010 Kulkarni & Agrawal 2
Introduction: Powering a System
VDAT10, July 8, 2010 Kulkarni & Agrawal 3
VB +_
RB
VLRL
IL
AHr(capacity)
Ideal lifetime = AHr/IL = AHr.RB (1 + RL/RB) / VB
Power supplied to load, PL = IL2 RL = (VB
2/RB)(RL/RB) / (1+ RL/RB)2
Efficiency = PL / Battery Power = (1+ RB/RL) –1
Lifetime, Power and Efficiency
VDAT10, July 8, 2010 Kulkarni & Agrawal 4
1.0
0.8
0.6
0.4
0.2
0.0
Eff
icie
ncy
or P
ower
0 1 2 3 4 5 6 7 8RL/RB
Life
time
(x A
Hr.
RB /
VB)
10
8
6
4
2
0
Lifetime
Efficiency
PL x VB2/(4RB)
Problem StatementBattery problem
• Battery should be capable of supplying power (current) for required system performance.
• Battery should meet the lifetime (time between replacement or recharge) requirement.
• How to extend the lifetime of selected battery.
Solution
• Determine minimum battery size for efficiency ≥ 85%
• Increase battery size over the minimum size to meet lifetime requirement.
• Determine a lower performance mode with maximum lifetime.
VDAT10, July 8, 2010 Kulkarni & Agrawal 5
Some Characteristics• Lithium-ion battery
• Open circuit voltage: 4.2V, unit cell 400mAHr, for efficiency ≥ 85%, current ≤ 1.2A
• Discharged battery voltage ≤ 3.0V
• DC-to-DC converter• Supplies VDD to circuit, VDD ≤ 1V for nanometer
technologies.• VDD control for energy management.
• Decoupling capacitor(s) provide smoothing of time varying current of the circuit.
VDAT10, July 8, 2010 Kulkarni & Agrawal 7
DC-to-DC Buck (Step-Down) Converter• Components: switch, diode, inductor, capacitor.• Switch control: pulse width modulated (PWM) signal.• Vout = D · Vin, D is duty cycle of PWM control signal.
• References:• M. Pedram and Q. Wu, “Design Considerations for Battery-Powered
Electronics,” Proc. 36th Design Automation Conference, June 1999, pp. 861–866.
• L. Benini, G. Castelli, A. Macii, E. Macii, M. Poncino, and R. Scarsi, “A Discrete-Time Battery Model for High-Level Power Estimation,” Proc. Conference on Design, Automation and Test in Europe, Mar. 2000, pp. 35–41.
• Power Supply Circuits, Application Note 2031, Maxim Integrated Products, Oct. 19, 2000, http://pdfserv.maxim-ic.com/en/an/AN2031.pdf
VDAT10, July 8, 2010 Kulkarni & Agrawal 8
A DC-to-DC Buck Converter
VDAT10, July 8, 2010 Kulkarni & Agrawal 9
VinVout
PWM control;duty cycledeterminesVout
A Design Example• 70 million gate circuit.• Critical path: 32bit ripple-carry adder (RCA)
• 352 NAND gates (2 or 3 inputs), 1,472 transistors.• 45nm bulk CMOS technology.• Three-step design procedure:
• Circuit characterization – current and delay vs. VDD; find average current for peak performance.
• Battery lifetime simulation – minimum battery size for efficiency ≥ 85% at peak performance; battery size for lifetime requirement.
• Minimum energy mode – maximum lifetime VDD and clock frequency.
VDAT10, July 8, 2010 Kulkarni & Agrawal 10
Critical Path Simulation• Simulation model: 45nm bulk CMOS, predictive
technology model (PTM), http://ptm.asu.edu/ • Simulator: Synopsys HSPICE,
www.synopsys.com/Tools/Verification/AMSVerification/CircuitSimulation/HSPICE/Documents/hspice ds.pdf
VDAT10, July 8, 2010 Kulkarni & Agrawal 11
Hspice Simulation of 32-Bit RCA, VDD = 0.9V
VDAT10, July 8, 2010 Kulkarni & Agrawal 12
Critical path vectors
2ns
Average total current, Icircuit = 74.32μA, Leakage current = 1.108μA100 random vectors including critical path vectors
Hspice Simulation of 32-Bit RCA, VDD = 0.3V
VDAT10, July 8, 2010 Kulkarni & Agrawal 13
Average total current, Icircuit = 0.2563μA, Leakage current = 0.092μA
Critical path vectors
200ns
100 random vectors including critical path vectors
Finding Battery Current, IBatt
• Assume 32-bit ripple carry adder (RCA) with about 350 gates represents circuit activity for the entire system.
• Total current for 70 million gate circuit,Icircuit = (average current for RCA) x 200,000
• DC-to-DC converter translates VDD to 4.2V battery voltage; assuming 100% conversion efficiency,
IBatt = Icircuit x VDD/4.2
• Example: Hspice simulation of RCA: 100 random vectors, VDD = 0.9V, vector period = 2ns, average current = 74.32μA, Ibatt = 3.18A
VDAT10, July 8, 2010 Kulkarni & Agrawal 14
Battery Simulation Model
VDAT10, July 8, 2010 Kulkarni & Agrawal 16
Lithium-ion battery, unit cell capacity: N = 1 (400mAHr)Battery sizes, N = 2 (800mAHr), N = 3 (1.2AHr), etc.
M. Chen and G. A. Rincón-Mora, “Accurate Electrical Battery Model Capable of Predicting Runtime and I-V Performance,” IEEE Transactions on Energy Conversion, vol. 21, no. 2, pp. 504–511, June 2006.
Finding Battery Efficiency• Consider:
• 1.2AHr battery• IBatt = 3.6A
• Ideal efficiency = 1.2AHr/3.6A = 1/3 hour (1200s)• Actual lifetime from simulation = 1008s• Efficiency = (Actual lifetime)/(Ideal lifetime)
= 1008/1200= 0.84 or 84%
VDAT10, July 8, 2010 Kulkarni & Agrawal 18
Minimum Battery Size
• Consider a performance requirement of 500MHz clock, critical path delay ≤ 2ns.
• Circuit simulation gives, VDD = 0.9V and IBatt = 3.18A.
• From battery efficiency simulation, for efficiency ≥ 85%, battery capacity should not be less than 1.2AHr, i.e., three-cell (N=3) Li-ion battery.
VDAT10, July 8, 2010 Kulkarni & Agrawal 20
Battery Lifetime Requirement
• Suppose battery lifetime for the system is to be at least one hour.
• For smallest battery, size N = 3 (1.2AHr), IBatt = 3.18A, efficiency ≈ 93%, Lifetime = 0.93 x 1.2/3.18 = 0.35 hour
• For 1 hour lifetime, battery size N = 3/0.35 = 8.57 ≈ 9.
• We should use a 9 cell (3.6AHr) battery.
VDAT10, July 8, 2010 Kulkarni & Agrawal 21
Minimum Energy Operation• A meaningful measure of the work done by
the battery is its lifetime in terms of clock cycles.
• For each VDD in the range of valid operation, i.e., VDD = 0.1V to 0.9V, we calculate lifetime using circuit delay and battery efficiency obtained from Hspice simulation.
• Minimum energy operation maximizes the lifetime in clock cycles.
VDAT10, July 8, 2010 Kulkarni & Agrawal 22
Minimum Energy Operation
VDAT10, July 8, 2010 Kulkarni & Agrawal 23
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Life
time
(x10
12 c
ycle
s)
16
14
12
10
8
6
4
2
0
Battery capacity 3.6AHrBattery capacity 1.2AHr
VDD (volts)
SummaryBattery size
VDD = 0.9V, 500MHz VDD = 0.3V, 5MHz
Effici.%
LifetimeEffici.
%
Lifetime
N AHr x103 seconds
x10 11 cycles
x106 seconds
x10 11 cycles
3 1.2 93 1.263 7.03 100+ 1.234 48.609 3.6 103 4.198 22.80 100+ 3.894 150.30
VDAT10, July 8, 2010 Kulkarni & Agrawal 24
seven-times
1. Battery size should match the current need and satisfythe lifetime requirement of the system:(a) Undersize battery has poor efficiency.(b) Oversize battery is bulky and expensive.
2 Minimum energy mode can significantly increase battery lifetime.
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