INTRODUCTION need increased autonomy models laptops cellular phones re

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Fast Charging Control with Temperature Sensing
INTRODUCTION need increased autonomy models laptops cellular phones resulted high-energy density power packs Li-ion batteries. These batteries charged quickly, condition that fast charging complies with several criteria. techniques used following cells, quick fast charging operation uses V,d2V/dt2, maximum time, (Temperature Off), techniques. measurement high temperature used protection, temperature variation (T/t) also used monitoring. Li-ion cells, fast charging uses CCCV techniques (Constant Current Constant Voltage). initial temperature measured order allow initiation fast charging. temperature reaches high threshold (TCO), fast charging would stop. sophistication electronic system depends principally upon cost upon requirements batteries. Often, fast charging monitored measuring voltage batteries, charging current sense resistor, measuring temperature batteries several Negative Temperature Coefficient (NTC) thermistor(s). IC's almost always chargers integrated battery pack (Li-ion). thermistors almost always integrated battery packs, sometimes placed charger, and/or final apertures (low cost cellular phones). This application note explains design thermistor from VISHAY BCcomponents BQ2005 from TEXAS INSTRUMENTS dual batteries charging computation methods performed here sufficiently general extended other configurations. FAST CHARGE ALGORITHM BQ2005 Referring notice BQ2005 will focus design part related temperature control charge operation (see figure
PACK Vtco
BQ2005
Figure
PACK
thermistor, together with fixed resistors RT2, used voltage divider between current sense resistor input VSNS beginning charge cycle, checks voltage Vtemp VSNS within limits designed manufacturer (low temperature: high temperature: 0.75 Vtco). Vtco threshold defined external resistors (not represented figure after starting fast charge phase, Vtemp becomes lower than Vtco, then return trickle mode operated. During fast charge period, samples voltage Vtemp return trickle mode also operated when variation time Vtemp going over threshold. This called termination: each Vtemp sampled Vtemp fallen compared value measured samples earlier, then fast charge terminated. following table summarizes voltage levels applicable here:
Average adjustable between 0.75 Vtco period Tolerance
Symbol Vtco Vlow temp Vhigh temp Vtherm
Parameter supply voltage voltage temperature fault high temperature fault input change termination
Document Number 29089 Rev. 1.0, 16-Jan-06
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CONFIGURATION EXTERNAL THERMISTOR/RESISTOR NETWORK voltage around input
Rntc VSNS Rntc +RT2 Rntc -VSNS)
derive from equations (3):
RT22 Rntc
Rntc +RT2 Rntc)2
voltage around fault, high fault, temperatures comply thresholds designed BQ2005. This expressed equations (1a), (1b), (1c). low) VSNS (1a) high) VSNS 0.75 VTCO (1b) off) VSNS VTCO (1c) Normally VSNS order simplicity, will consider here that VSNS Should this approximation valid, then computations hereunder must modified. call Rntc (low temperature fault), Rntc (high temperature fault) Rntc (cut temperature) respectively RnL, RnH, RTCO. Introducing (1a) solving with respect RT2, obtain:
0.666 (RnL 0.66 RT1) (2a)
T/t, Tlow, given battery manufacturer. VTS/Dt defined characteristics thermistor defined VISHAY BCcomponents Tlow values. value found catalog using Steinhart Hart interpolation polynoms calculation. These parameters given appendix several currently used VISHAY BCcomponents thermistors. this base, remaining parameters defined with help relations (2a), (2b), which have verified simultaneously: chosen respect Tlow equation (2a) (2b). Vtco will defined that required (equation will respected. last, high fault will computed with equation (1b). NUMERICAL EXAMPLE Example following data currently applicable batteries: fault /min Then: Using dV/dt sec) Designing sensor VISHAY BCcomponents leaded thermistor 2381 103: B25/85 3977 0.75 Using Vtco arbitrarily derive 2753 2020 Then compute different temperatures from TCO. results shown following table:
Introducing (2a) (1c) obtain:
(RnL RTCO)
(2b)
Once thermistor characteristics Vtco defined, will defined. also have compute speed variation temperature thermistor, which will induce voltage Vtherm operating dT/dt termination. Assuming exponential dependence electrical resistance thermistor function temperature:
Rntc(T) exp( (1/T 1/298.15)
where electrical resistance B25/85 characteristic component (K), absolute temperature (K).
Characteristics fault High fault Temp (°C) 42.5 Rntc 19872 4824 3605 1.999 1.704 1.599
Vthreshold 2.000 1.700 1.600
VTS/T (mV/°C)
(°C/min) 2.57 1.07 0.95
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Document Number 29089 Rev. 1.0, 16-Jan-06
that falls into range °C/min. would cause, then should have VTCO slightly change. tolerances electrical characteristics introduce also variation thresholds: limit case: make calculations value thermistor being limits value 0.75 will also take into account errors introduced tolerances fixed resistors (supposed error thresholds (low fault temperature TCO) these tolerances simply obtained performing calculations fixed temperature comparing these values with requested ones, dividing these differences sensitivity VTS/T. results summarized following tables: Rntc 10500 B25/85 3977 0.75
Temp (°C) fault Rntc (ohms) 20755.49 3814.942 2.027 1.639 Vthreshold 2.000 1.600 VTS/T mV/degC) (degC/min) 2.66 0.97 5.01 2.70
Rntc 9500 B25/85 3977 0.75 RT2=
Temp (°C) fault Rntc 18978.88 3398.598 1.971 1.558 Vthreshold 2.000 1.600 VTS/T (mV/°C) (°C/min) 2.48 0.93 5.12 2.73
With these tolerances: temperature fault will fall range approx. Temperature will fall range approx. such variations should acceptable, then design thermistor with tolerance down (code number: 2381 55103) instead tolerances definition threshold will become negligible compared inherent tolerances Example same calculations thermistors (AgPd NiSn terminations, sizes 0805, 0603, 0402 described appendix) give following results: Adjusting slightly Vtco 1.55 order keep dT/dt nominal °C/min high fault temperature, then compute:
Component 2381 1x103 0805 AgPd 2381 2x103 0603 AgPd 2381 5x103 0805 NiSn 2381 3x103 0603 NiSn 2381 4x103 0402 NiSn fault high fault fault high fault fault high fault fault high fault fault high fault 41.9 41.9 41.8 41.9 41.75 18625 5270 3957 18625 5270 3957 18515 5331 4004 18664 5271 3960 18290 5408 4079 1.999 1.668 1.549 1.999 1.668 1.549 1.999 1.668 1.549 1.999 1.668 1.549 1.999 1.668 1.549 2.000 1.663 1.550 2.000 1.663 1.550 2.000 1.663 1.550 2.000 1.663 1.550 2.000 1.663 1.550 2.00 1.01 0.92 2.00 1.01 0.92 1.98 1.01 0.93 2.01 1.01 0.92 1.95 1.02 0.94 3647 2793 Characteristic Temp (°C) Rntc Vthreshold VTS/T (mV/°C) (°C/min)
3647
2793
3708
2850
3649
2794
3811
2947
Document Number 29089 Rev. 1.0, 16-Jan-06
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CONCLUSION GENERAL COMMENTS their tolerances, cost, high sensitivity, thermistors perfectly suited fast charging monitoring protection battery packs. notes calculations described this note easily extrapolated other IC's, example BQ2954 Li-ion packs. this case, charge termination application, which makes even more simple. greatest care should used when positioning thermistor into pack ensure close contact between thermistor batteries. Otherwise, calculations about tolerances will applicable. Further information different mechanical executions (insulated leads, version) suitable these applications available from VISHAY BCcomponents offices. APPENDIX Different thermistors Steinhart Hart characteristics Formula (R(T)/R25) +C/T2 D/T3 where expressed Kelvins 273.15)
Code number Tol. deg) Type B25/85 3977 Tolerance 2381 5x103 leaded 0.75 14.63372 Steinhart Hart coefficients 4791.842 115334 3730535
2381 1x103
0805 AgPd terminations 0603 AgPd terminations 0805 NiSn terminations 0603 NiSn terminations 0402 NiSn terminations
3620
-13.26396
4245.26
46761
11891500
2381 2x103
3620
13.26396
4245.26
46761
11891500
2381 x103
3570
13.40886
4547.961
176965.9
3861154
2381 3x103
3610
13.40957
4481.799
150521.7
1877103
2381 4x103
3570
12.0714
3503.902
109391
24154454.74
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Document Number 29089 Rev. 1.0, 16-Jan-06

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