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A Tutorial on Battery Simulation - Matching
Power Source to Electronic System
Manish Kulkarni and Vishwani D. Agrawal
Auburn University
Auburn, AL 36849, USA
[email protected], [email protected]
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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
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Introduction: Powering a System
RB
VB
AHr
(capacity)
+
_
IL
VL
RL
Power supplied to load, PL = IL2 RL = (VB2/RB)(RL/RB) / (1+ RL/RB)2
Ideal lifetime = AHr/IL = AHr.RB (1 + RL/RB) / VB
Efficiency = PL / Battery Power = (1+ RB/RL) –1
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Lifetime, Power and Efficiency
1.0
Efficiency
8
0.8
6
0.6
4
PL x
Lifetime
2
0
0.4
VB2/(4RB)
0
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1
2
3
0.2
4
RL/RB
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6
7
8
Efficiency or Power
Lifetime (x AHr.RB /VB)
10
0.0
4
Problem Statement
Battery problem
Solution
• Determine minimum battery
• Battery should be capable of
size for efficiency ≥ 85%
supplying power (current) for
required system performance.
• Increase battery size over the
• Battery should meet the
minimum size to meet
lifetime (time between
lifetime requirement.
replacement or recharge)
requirement.
• How to extend the lifetime of • Determine a lower
selected battery.
performance mode with
maximum lifetime.
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Power Subsystem of an Electronic
System
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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.
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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.maximic.com/en/an/AN2031.pdf
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A DC-to-DC Buck Converter
Vin
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Vout
PWM control;
duty cycle
determines
Vout
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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.
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Critical Path Simulation
• Simulation model: 45nm bulk CMOS, predictive
technology model (PTM), http://ptm.asu.edu/
• Simulator: Synopsys HSPICE,
www.synopsys.com/Tools/Verification/AMSVeri
fication/CircuitSimulation/HSPICE/Documents/
hspice ds.pdf
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Hspice Simulation of 32-Bit RCA, VDD = 0.9V
100 random vectors including critical path vectors
Average total current, Icircuit = 74.32μA, Leakage current = 1.108μA
Critical path vectors
2ns
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Hspice Simulation of 32-Bit RCA, VDD = 0.3V
100 random vectors including critical path vectors
Average total current, Icircuit = 0.2563μA, Leakage current = 0.092μA
Critical path vectors
200ns
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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
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Delay and Current vs. VDD
3.18A
~ 2ns (500MHz)
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Battery Simulation Model
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.
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1008s
Lifetime from Battery Simulation
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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%
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Battery Efficiency vs. Size
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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.
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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.
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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.
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Minimum Energy Operation
16
Battery capacity 3.6AHr
Battery capacity 1.2AHr
Lifetime (x1012 cycles)
14
12
10
8
6
4
2
0
0
0.1
0.2
0.3 0.4 0.5 0.6
0.7 0.8 0.9
1.0
VDD (volts)
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Summary
VDD = 0.9V, 500MHz
Battery size
Lifetime
Effici.
3
11
x10
x10
%
N
AHr
seconds cycles
3
1.2
93
1.263
7.03
9
3.6
103
4.198
22.80
VDD = 0.3V, 5MHz
Lifetime
Effici.
6
11
x10
x10
%
seconds cycles
100+ 1.234
48.60
100+ 3.894 150.30
seven-times
1. Battery size should match the current need and satisfy
the 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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