VISHAY BCCOMPONENTS www.vishay.com Resistive Products Application Note Fast Charging Control with NTC Temperature Sensing 1. INTRODUCTION The need for increased autonomy for new models of laptops and cellular phones has resulted in high-energy density power packs - Ni MH and Li-ion batteries. These batteries can be charged quickly, on the condition that the fast charging complies with several criteria. 2. THE FAST CHARGE ALGORITHM FOR THE BQ2005 Referring to the notice of the BQ2005 IC, we will focus on the design part related to the temperature control of the charge operation (see figure 1). VCC The techniques used are the following: PACK + • For the Ni MH cells, the quick and fast charging operation uses the - V, d2V/dt2, the maximum time, the TCO (Temperature Cut Off), or the T/t techniques. The measurement of high temperature is used as a protection, but the temperature variation (T/t) can also be used for monitoring. • For the Li-ion cells, the fast charging uses the CCCV techniques (Constant Current Constant Voltage). The initial temperature is measured in order to allow initiation of fast charging. If the temperature reaches a high threshold (TCO), the fast charging would stop. The sophistication of the electronic system depends principally upon cost and upon the requirements of the batteries. Often, the fast charging is monitored by an IC, measuring the voltage of the batteries, the char-ging current via a sense resistor, and measuring the temperature of the batteries via one or several Negative Temperature Coefficient (NTC) thermistor(s). The IC's are almost always in the chargers or integrated in the battery pack (Li-ion). The thermistors are almost always integrated in the battery packs, sometimes placed in the charger, and/or in the final apertures (low cost cellular phones). The computation methods performed here are sufficiently general to be extended to a lot of other configurations. BQ2005 RT2 TS RT1 SNS N T C PACK - Figure 1 An NTC thermistor, together with fixed resistors RT1 and RT2, is used in a voltage divider between Vcc and the current sense resistor input VSNS of the IC. At the beginning of a new charge cycle, the IC checks if the voltage Vtemp = VTS - VSNS is within the limits designed by the IC manufacturer (low temperature: 0.4 Vcc and high temperature: 0.1 Vcc + 0.75 VTCO). VTCO is a cut off threshold defined by external resistors (not represented in figure 1): If after starting the fast charge phase, Vtemp becomes lower than VTCO, then the return to trickle mode is operated. During the fast charge period, the IC samples the voltage Vtemp and the return to trickle mode can also be operated when the variation in time of Vtemp is going over a threshold. This is called the T/t termination: each 34 s, Vtemp is sampled and if Vtemp has fallen by 16 mV ± 4 mV compared to the value measured two samples earlier, then the fast charge is terminated. The following table summarizes the voltage levels applicable here: SYMBOL PARAMETER AVERAGE TOLERANCE Vcc Supply voltage 5V ± 10 % VTCO Cut off voltage Adjustable between 0.1 Vcc and 0.2 Vcc Vlow temp Low temperature fault 0.4 Vcc ± 30 mV Vhigh temp High temperature fault 0.1 Vcc + 0.75 VTCO ± 30 mV TS input change for T/t termination 16 mV/period of 2 x 34 s ± 4 mV Vtherm Revision: 16-Sep-14 Document Number: 29089 1 For technical questions, contact: [email protected] THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 APPLICATION NOTE This application note explains how to design an NTC thermistor from Vishay BCcomponents for a BQ2005 from TEXAS INSTRUMENTS dual Ni MH batteries charging IC. VTCO Application Note www.vishay.com Vishay BCcomponents Fast Charging Control with NTC Temperature Sensing We can derive from equations (1) and (3): 3. CONFIGURATION OF EXTERNAL THERMISTOR/RESISTOR NETWORK ΔVTS ΔVTS ΔT - B RT1RT22RNTCVCC ΔT = = Δt ΔT Δt T2 (RT1RT2 + RT1RNTC + RT2RNTC)2 Δt The voltage around the TS input is: VTS - VSNS = RT2RNTC (V - VSNS) (1) RT1RT1 + RT1RNTC + RT2RNTC CC The voltage around the NTC for the low fault, high fault, and cut off temperatures has to comply to the thresholds designed for the BQ2005. This is expressed by equations (1a), (1b) and (1c). VTS (T low) - VSNS = 0.4 Vcc (1a) VTS (T high) - VSNS = 0.1 Vcc + 0.75 VTCO (1b) VTS (T cut off) - VSNS = VTCO (1c) Normally VSNS is of the order of 0.1 V. For simplicity, we will consider here that VSNS = 0. Should this approximation not be valid, then the computations hereunder must be modified. Let us call RNTC (low temperature fault), RNTC (high temperature fault) and RNTC (cut off temperature) respectively RnL, RnH, and RTCO. Introducing obtain: (1) in (1a) and solving with respect to RT2, we RT2 = 0.666 RT1RnL (RnL - 0.66 RT1) RTCORnL (RnL - RTCO) V CC - 2.5 ) ( VTCO The characteristics of the thermistor are defined by Vishay BCcomponents Tlow and TCO values. The B value can be found in the catalog or by using the Steinhart & Hart interpolation polynoms calculation. These parameters are given in the appendix for several currently used Vishay BCcomponents thermistors. On this base, all the remaining parameters can be defined with the help of relations (2a), (2b), and (4) which have to be verified simultaneously: RT1 and RT2 are chosen to respect Tlow and TCO via equation (2a) and (2b). VTCO will be defined so that the required T/t (equation (4)) will be respected. At last, T high fault will be computed with equation (1b). 4. NUMERICAL EXAMPLE Example 1 • T low fault = 10 °C • T cut off = 50 °C (2a) • T/t = 1 °C/min ± 0.3 °C/min Once the thermistor characteristics and VTCO are defined, RT1 and RT2 will be defined. Then: We also have to compute the speed of variation of temperature on the thermistor, which will induce the voltage Vtherm operating the T/t termination. • Designing for the sensor the Vishay BCcomponents leaded thermistor NTCLE203E3103JB0: R25 = 10 k ± 5 % B25/85 = 3977K ± 0.75 % Assuming the exponential dependence of the electrical resistance of the thermistor in function of the temperature: • Using VTCO = 1.6 V arbitrarily • Using Vcc = 5 V, dV/dt = 16 mV / (2 x 34 s) We derive RT1 = 2753 and RT2 = 2020 Then we compute T/t for different temperatures from 10 °C to TCO. The results are shown in the following table: Rntc (T) = R25 exp(B (1/T - 1/298.15)) (3) APPLICATION NOTE T/t, Tlow and TCO are given by the battery manufacturer. VTS/t is defined by TI. The following data are currently applicable to Ni MH batteries: (2a) Introducing (1) and (2a) in (1c) we obtain: RT1 = (4) where R25 is the electrical resistance of the NTC at 25 °C, B is the B25/85 characteristic of the component (K), and T is the absolute temperature (K). Vthreshold (V) VTS/T (mV/°C) T/t (°C/Min) TEMP (°C) RNTC () Low fault 10 19872 1.999 2.000 -5 2.57 High fault 42.5 4824 1.704 1.700 - 13 1.07 50 3605 1.599 1.600 - 15 0.95 CHARACTERISTICS Cut off Revision: 16-Sep-14 VTS (V) Document Number: 29089 2 For technical questions, contact: [email protected] THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Application Note www.vishay.com Vishay BCcomponents Fast Charging Control with NTC Temperature Sensing We see that the T/t falls into the range of 1 °C/min ± 0.3 °C/min. If it would not be the cause, then one should have let the VTCO slightly change. The tolerances on the electrical characteristics introduce also a variation on the thresholds: For the limit case: Let us make the calculations for the value of the thermistor being at the limits ± 5 % and the B value at ± 0.75 %. We will also take into account the errors introduced by the tolerances on the fixed resistors (supposed ± 1 %). The error T in the thresholds (low fault temperature and TCO) due to these tolerances are simply obtained by performing the calculations of the VTS at the fixed temperature (10 °C and 50 °C) and by comparing these values with the requested ones, and dividing these differences by the sensitivity VTS/T. The results are summarized in the following tables: RNTC (25 °C) = 10 500 RT1 = - 1 % B25/85 = 3977K - 0.75 % RT2 = + 1 % TEMP (°C) RNTC () VTS (V) Vthreshold (V) VTS/T (mV/°C) T/t (°C/Min) V (mV) T (°C) Low fault 10 20 755.49 2.027 2.000 -5 2.66 27 - 5.01 Cut off 50 3814.942 1.639 1.600 - 15 0.97 39 - 2.70 VTS/T (mV/°C) T/t (°C/Min) V (mV) T (°C) RNTC (25 °C) = 9500 RT1 = + 1 % B25/85 = 3977K + 0.75 % RT2= - 1 % TEMP (°C) RNTC () VTS (V) Vthreshold (V) Low fault 10 18 978.88 1.971 2.000 -6 2.48 - 29 5.12 Cut off 50 3398.598 1.558 1.600 - 15 0.93 - 42 2.73 With these tolerances: • Low temperature fault will fall in the range 10 °C± 5 °C approx. • Temperature cut off will fall in the range 50 °C ± 2.7 °C approx. APPLICATION NOTE If such variations should not be acceptable, then design a thermistor with R25 tolerance down to ± 1 % (code number: NTCLE203E3103FB0) instead of ± 5 %: The tolerances on the definition of threshold will become negligible compared to inherent tolerances of the IC. Example 2 The same calculations for all the SMD NTC thermistors (NiSn terminations, sizes 0805, 0603, or 0402 described in the appendix) give the following results: Adjusting slightly VTCO to 1.55 V, in order to keep T/t nominal at 1 °C/min at the high fault temperature, we then can compute: CHARACTERISTIC TEMP (°C) RNTC () VTS (V) Vthreshold (V) VTS/T (mV/°C) T/t (°C/Min) RT1 () RT2 () NTCS0805E3103xMT SMD 0805 NiSn terminations Low fault High fault Cut off 10 41.8 50 18 515 5331 4004 1.999 1.668 1.549 2.000 1.663 1.550 -7 - 14 - 15 1.98 1.01 0.93 3708 2850 NTCS0603E3103xMT SMD 0603 NiSn terminations Low fault High fault Cut off 10 41.9 50 18 664 5271 3960 1.999 1.668 1.549 2.000 1.663 1.550 -7 - 14 - 15 2.01 1.01 0.92 3649 2794 NTCS0402E3103xLT SMD 0402 NiSn terminations Low fault High fault Cut off 10 41.75 50 18 290 5408 4079 1.999 1.668 1.549 2.000 1.663 1.550 -7 - 14 - 15 1.95 1.02 0.94 3811 2947 COMPONENT Revision: 16-Sep-14 Document Number: 29089 3 For technical questions, contact: [email protected] THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Application Note www.vishay.com Vishay BCcomponents Fast Charging Control with NTC Temperature Sensing 5. CONCLUSION AND GENERAL COMMENTS Due to their low tolerances, low cost, and high sensitivity, NTC thermistors are perfectly suited for fast charging monitoring and protection of the battery packs. The notes and calculations described in this note can be easily extrapolated to other IC's, for example the BQ2954 for Li-ion packs. In this case, the T/t charge termination is not of application, which makes it even more simple. The greatest care should be used when positioning the thermistor into the pack to ensure close contact between the thermistor and the batteries. Otherwise, all calculations about tolerances on will not be applicable. Further information of the different mechanical executions (insulated leads, SMD version) suitable for these applications are available from the Vishay BCcomponents offices. 6. APPENDIX Different thermistors Steinhart & Hart characteristics Formula : Ln (R(T)/R25) = A + B/T +C/T2 + D/T3 where T is expressed in Kelvins (°C + 273.15) APPLICATION NOTE CODE NUMBER TOL. R (25 °C) TYPE B25/85 TOLERANCE (K) STEINHART & HART COEFFICIENTS A B C D NTCLE203E3103xB0 x=F x=G x=H x=J 1% 2% 3% 5% Leaded 3977 0.75 % - 14.63372 4791.842 - 115 334 - 3 730 535 NTCS0805E3103xMT x=F x=G x=H x=J 1% 2% 3% 5% SMD 0805 NiSn terminations 3570 3% - 13.40886 4547.961 - 176 965.9 3 861 154 NTCS0603E3103xMT x=F x=G x=H x=J 1% 2% 3% 5% SMD 0603 NiSn terminations 3610 1% - 13.40957 4481.799 - 150 521.7 1 877 103 NTCS0402E3103xLT x=F x=G x=H x=J 1% 2% 3% 5% SMD 0402 NiSn terminations 3490 3% - 12.0714 3503.902 109 391 - 24 154 454.74 Revision: 16-Sep-14 Document Number: 29089 4 For technical questions, contact: [email protected] THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000

- Similar pages
- Application Note
- Application Note
- VISHAY TS8542VA-SF-F
- TS8520VA Datasheet
- TS8514VA Datasheet
- TH5 Datasheet
- TNPV e3 Datasheet
- T1610P Datasheet
- TS8510VA Datasheet
- TI BQ2005
- IDTP9036A
- VISHAY NTCLS100E3222H
- Temperature Sensor - Thinking Electronics Industrial Co., Ltd.
- TMP7 IC Thermometer with Seven Segment and Serial
- DIODES ZXCT1009_11
- DIODES ZXCT1009FTA
- Frequently Asked Questions
- DIODES ZXCT1010E5TA
- AN-9080 Motion SPM® 5 Series Version 2 User`s Guide
- AN66477 PSoC® 3 PSoC 4 and PSoC 5LP Temperature Measurement with a Thermistor.pdf