A Tutorial on Battery Simulation Matching Power Source
A Tutorial on Battery Simulation - Matching Power Source to Electronic System Manish Kulkarni and Vishwani D. Agrawal Auburn University Auburn, AL 36849, USA mmk 0002@auburn. edu, vagrawal@eng. auburn. edu VDAT 10, July 8, 2010 Kulkarni & Agrawal 1
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 VDAT 10, July 8, 2010 Kulkarni & Agrawal 2
Introduction: Powering a System RB VB AHr (capacity) + _ IL VL RL Power supplied to load, PL = IL 2 RL = (VB 2/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 VDAT 10, July 8, 2010 Kulkarni & Agrawal 3
Lifetime, Power and Efficiency 1. 0 Efficiency 8 0. 8 6 0. 6 4 PL x V B 0 VDAT 10, July 8, 2010 0. 4 B) Lifetime 2 0 2/(4 R 1 2 3 0. 2 4 RL/RB Kulkarni & Agrawal 5 6 7 8 Efficiency or Power Lifetime (x AHr. RB /VB) 10 0. 0 4
Problem Statement Battery problem Solution • Battery should be capable of • Determine minimum battery 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. VDAT 10, July 8, 2010 Kulkarni & Agrawal 5
Power Subsystem of an Electronic System VDAT 10, July 8, 2010 Kulkarni & Agrawal 6
Some Characteristics • Lithium-ion battery • Open circuit voltage: 4. 2 V, unit cell 400 m. AHr, for efficiency ≥ 85%, current ≤ 1. 2 A • Discharged battery voltage ≤ 3. 0 V • DC-to-DC converter • Supplies VDD to circuit, VDD ≤ 1 V for nanometer technologies. • VDD control for energy management. • Decoupling capacitor(s) provide smoothing of time varying current of the circuit. VDAT 10, 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. 36 th 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/AN 2031. pdf VDAT 10, July 8, 2010 Kulkarni & Agrawal 8
A DC-to-DC Buck Converter Vin VDAT 10, July 8, 2010 Vout PWM control; duty cycle determines Vout Kulkarni & Agrawal 9
A Design Example • 70 million gate circuit. • Critical path: 32 bit ripple-carry adder (RCA) • 352 NAND gates (2 or 3 inputs), 1, 472 transistors. • 45 nm 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. VDAT 10, July 8, 2010 Kulkarni & Agrawal 10
Critical Path Simulation • Simulation model: 45 nm bulk CMOS, predictive technology model (PTM), http: //ptm. asu. edu/ • Simulator: Synopsys HSPICE, www. synopsys. com/Tools/Verification/AMSVer ification/Circuit. Simulation/HSPICE/Documents/ hspice ds. pdf VDAT 10, July 8, 2010 Kulkarni & Agrawal 11
Hspice Simulation of 32 -Bit RCA, VDD = 0. 9 V 100 random vectors including critical path vectors Average total current, Icircuit = 74. 32μA, Leakage current = 1. 108μA Critical path vectors 2 ns VDAT 10, July 8, 2010 Kulkarni & Agrawal 12
Hspice Simulation of 32 -Bit RCA, VDD = 0. 3 V 100 random vectors including critical path vectors Average total current, Icircuit = 0. 2563μA, Leakage current = 0. 092μA Critical path vectors 200 ns VDAT 10, July 8, 2010 Kulkarni & Agrawal 13
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. 2 V battery voltage; assuming 100% conversion efficiency, IBatt = Icircuit x VDD/4. 2 • Example: Hspice simulation of RCA: 100 random vectors, VDD = 0. 9 V, vector period = 2 ns, average current = 74. 32μA, Ibatt = 3. 18 A VDAT 10, July 8, 2010 Kulkarni & Agrawal 14
Delay and Current vs. VDD 3. 18 A ~ 2 ns (500 MHz) VDAT 10, July 8, 2010 Kulkarni & Agrawal 15
Battery Simulation Model Lithium-ion battery, unit cell capacity: N = 1 (400 m. AHr) Battery sizes, N = 2 (800 m. AHr), N = 3 (1. 2 AHr), 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. VDAT 10, July 8, 2010 Kulkarni & Agrawal 16
1008 s Lifetime from Battery Simulation VDAT 10, July 8, 2010 Kulkarni & Agrawal 17
Finding Battery Efficiency • Consider: • • • 1. 2 AHr battery IBatt = 3. 6 A Ideal efficiency = 1. 2 AHr/3. 6 A = 1/3 hour (1200 s) Actual lifetime from simulation = 1008 s Efficiency = (Actual lifetime)/(Ideal lifetime) = 1008/1200 = 0. 84 or 84% VDAT 10, July 8, 2010 Kulkarni & Agrawal 18
Battery Efficiency vs. Size VDAT 10, July 8, 2010 Kulkarni & Agrawal 19
Minimum Battery Size • Consider a performance requirement of 500 MHz clock, critical path delay ≤ 2 ns. • Circuit simulation gives, VDD = 0. 9 V and IBatt = 3. 18 A. • From battery efficiency simulation, for efficiency ≥ 85%, battery capacity should not be less than 1. 2 AHr, i. e. , three-cell (N=3) Li-ion battery. VDAT 10, 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. 2 AHr), IBatt = 3. 18 A, 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. 6 AHr) battery. VDAT 10, 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. 1 V to 0. 9 V, we calculate lifetime using circuit delay and battery efficiency obtained from Hspice simulation. • Minimum energy operation maximizes the lifetime in clock cycles. VDAT 10, July 8, 2010 Kulkarni & Agrawal 22
Minimum Energy Operation 16 Battery capacity 3. 6 AHr Battery capacity 1. 2 AHr Lifetime (x 1012 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) VDAT 10, July 8, 2010 Kulkarni & Agrawal 23
Summary VDD = 0. 9 V, 500 MHz Battery size Lifetime Effici. 3 11 x 10 % N AHr seconds cycles 3 9 1. 2 3. 6 93 103 1. 263 4. 198 7. 03 22. 80 VDD = 0. 3 V, 5 MHz Lifetime Effici. 6 11 x 10 % seconds cycles 100+ 1. 234 3. 894 48. 60 150. 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. VDAT 10, July 8, 2010 Kulkarni & Agrawal 24
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