Applica ation Note APT0406 U Using NTC Tempe erature sensor inttegrated into pow wer modu ule Pierre--Laurent Doumergue R R&D Engine eer Microsemi Power Mod dule Produccts pilleau 26 rue de Camp 20 Bruges, France 33 52 Introduction: Most po ower modu ules include e a temperrature senssor. Usuallyy it is a Ne egative Tem mperature Coefficie ent (NTC) th hermistor w with resistance that decrreases while temperatu ure increase es. With itss low costt, the NTC C thermistor is the device of cchoice for module tem mperature measure ements an nd over-tem mperature protection, but otherr devices like PTC (Positive Temperrature Coeffficient) resisstors are pre eferable for specific tem mperature co ontrol appliccations. Using the informattion from th he tempera ature senso or is easy, but some care must be taken regardin ng safety co onsideration ns within the e equipmentt. 1. ure. Module interrnal structu The NT TC thermisto or is located close to the power dice, on the same ceramic subsstrate (see figure 1). Fig 1 1: NTC therm mistor locattion on a substrate Because e of negligible self-hea ating, the NT TC thermisttor remains at almost ssame tempe erature as the pow wer module case. Also, since the case to he eat-sink therrmal resista ance RθCS of o a power module is in generral very sma all, the mea asured temp perature is a assumed to o be close to o the heat sink tem mperature. An NTC C thermistorr can never be used to monitor the e junction te emperature of the powe er devices directly; it would n need to be integrated in the pow wer die, which is not tthe case here. The junction temperature can be e estimated ba ased on the e NTC therm mistor temp perature and d case-toermal resista ance, as will be shown. sink the January 2016 © 2011 Miccrosemi Corpora ation 1 Using NTC temperature sensor integrated into power module 2. NTC thermistor features. The NTC thermistor is useful for protecting a power system from overheating or cooling system failures because of the following features: • Low cost • More sensitive response than thermocouples • Easy to use • Immune to noise • Temperature range is well matched to power module operating temperature range An NTC thermistor has a time constant in the range of a few seconds, meaning that it takes at least a few seconds to detect a change in temperature in a module. Due to its slow response, the NTC thermistor is not suited to detect rapid changes in temperature and therefore can only be used to protect the system from slow changes in temperature. The NTC thermistor cannot be used for short circuit or over current protection. The response of the NTC thermistor is exponential. In spite of its nonlinearity, the NTC thermistor is useful for module temperature measurements because: • • A simple threshold circuit can be used to indicate an over-temperature condition, which will be discussed. The exponential response can be processed by analog circuitry or by software in a digitally controlled system. The NTC thermistors used in power modules have the following characteristics. Symbol R25 B25/85 Characteristic Resistance @ 25°C Curve fit constant Value 22kΩ ± 5% 3980K Symbol R25 B25/85 Characteristic Resistance @ 25°C Curve fit constant Value 50kΩ ± 5% 3952K The equation for the NTC thermistor response is: RT = R 25 ⎡ ⎛ 1 1 ⎞⎤ − ⎟⎥ exp ⎢ B25/ 85 ⋅ ⎜ ⎝ T25 T ⎠ ⎦⎥ ⎣⎢ (1) RT is the thermistor resistance, T is its temperature in Kelvin, and T25 is the Kelvin temperature at 25°C (298.15K). 2 Using NTC temperature sensor integrated into power module 3. Circuit Implementation The NTC thermistor can easily be used for module protection without computing the actual thermistor temperature. Comparing the voltage across the NTC thermistor to a reference voltage (see figure 2), and stopping the operation of the module if it becomes too hot reduces the risk of module failure. VREF 1 VREF 2 VDD R1 R3 NULL POWER MODULE 1 -IN 2 +IN 3 -Vs 4 8 7 - +Vs R4 ISOLATED OVER TEMP. PROTECTION OUT 6 + C1 NULL VCC 5 NC NTC R2 Fig 2: Example of NTC thermistor comparator circuit If the NTC thermistor is placed in the bottom leg of a voltage divider as in figure 2, the resulting voltage at the input of the comparator decreases from almost VREF1 to the voltage trigger level VREF2 as the NTC thermistor temperature increases. Assuming the temperature trigger level needs to be set at 100°C and the resulting comparator voltage trigger level is set at half the voltage reference level (VREF1 / 2), the top resistor R1 has to be set to the same value as the NTC thermistor resistance at 100°C. The thermistor resistance at a given temperature can be calculated using equation (1) or looked up in a table provided in a later section. In the case of this example, RT = R1 = 3.43kΩ at 100°C. If the thermistor temperature is lower than 100°C, the output state of the comparator is high. If the thermistor temperature is higher than 100°C, the output state of the comparator is low. The position of the thermistor and R1 can be swapped. In this case, the resulting voltage at the input of the comparator increases from almost zero volts to the trigger level voltage VREF2 as the temperature increases. Whatever the position of R1 and the NTC thermistor time constants and noise immunity level remain the same. In practice, a comparator with hysteresis is used, and resistors R1 and R2 must be adjusted to set the amount of hysteresis. The hysteresis equals the output swing of the comparator attenuated by the resistive divider of R1//RT and R2. Assuming rail-to-rail output swing of the comparator in figure 2, Vhyst = VDD ⋅ R1 ⋅ R T R 2 ( R1 + R T ) + R1 ⋅ R T Solving for R2: R2 = R1 ⋅ R T VDD − Vhyst ⋅ R1 + R T Vhyst 3 Using NTC temperature sensor integrated into power module To increase the noise immunity of the NTC thermistor, which exhibits a resistance of a few thousand Ohms at rated temperature, it is recommended to parallel a capacitor. This capacitor (C1 in figure 2) must be between 10 to 100nF. Even using a 100nF decoupling capacitor, which guarantees a very high noise immunity level, the time constant at 25°C is only 320 microseconds, e.g., τ = ( R1 // RT ).C1 , more than 1000 times lower than the time constant of the NTC thermistor itself. In most cases, a 10nF decoupling capacitor is more than enough to ensure good noise immunity. The maximum power in the thermistor must not exceed 20mW whatever the temperature to not affect temperature measurement by self-heating. 4. Temperature Measurement Solving equation (1) for temperature (in Kelvin) we get: T= B25 / 85 ⋅ T25 ⎛R ⎞ B25 / 85 − T25 ⋅ ln ⎜ 25 ⎟ ⎝ RT ⎠ (2) We know the values of B25/85, T25, and R25: for example 3952K, 298.15K, and 50kΩ respectively. Once we determine the value of RT we can compute the temperature. Referring to figure 2, the voltage VT across the NTC thermistor is VT = VREF1 ⋅ RT R T + R1 (3) Now we can solve for the thermistor resistance. RT = R1 ⋅ VT VREF1 − VT (4) Note that accuracy is improved if R1 has a neutral temperature coefficient. Finally, equation (4) can be substituted into equation (2) to compute the thermistor temperature in Kelvin. T= B25/85 ⋅ T25 ⎛ R ⋅ ( VREF1 − VT ) ⎞ B25/ 85 − T25 ⋅ ln ⎜⎜ 25 ⎟⎟ R1 ⋅ VT ⎝ ⎠ (5) The result from equation (5) can be converted from Kelvin to Celsius by subtracting 273.15. Equation (5) looks fairly complex but can easily be solved by a microprocessor or a DSP in digitally controlled systems. Alternatively, equation (5) can be used in a spreadsheet program to create a lookup table stored in a header file, eliminating temperature computation run-time in a digital controller. The NTC thermistor remains at almost the same temperature as the power module case, so the thermistor temperature can simply be used for the power module base plate (case) temperature TC. 4 Using NTC temperature sensor integrated into power module Knowing the module case temperature TC, the junction-to-case thermal resistance, and the power dissipation for each die, the power die junction temperature can be determined with the formula TJ = (P ⋅ R θJC ) + TC . T The heat sink temperature can be calculated as THS = TC − R θCS ⋅ P , where HS is the heat sink temperature, P is the power dissipation, and RθCS is the case-to-sink thermal resistance. Since the case-to-heat sink thermal resistance RθCS of a power module is generally very small, the thermistor temperature can be assumed to be close to the heat sink temperature. If appropriate, a correction of -5 to -10°C can be subtracted from the temperature measurement to estimate the heat sink temperature. For example, 10°C corresponds to 100W dissipated in a module with 0.1°C/W case-to-heat sink thermal resistance. 5. Safety Issues Severe damage inside the module can lead to the destruction of the power dice, creating under extreme conditions the generation of plasma. The propagation of this plasma is unpredictable and it might be in contact with the NTC thermistor circuit, exposing it to dangerously high voltages. Temperature monitoring using a NTC thermistor presents a potential risk of high voltage exposure of this part of the circuit. It is the responsibility of the system designer to ensure that appropriate measures are taken to provide reliable insulation. Following are some examples to achieve good isolation: • The NTC thermistor is used in a comparator circuit, which is isolated from the control logic by an opto-coupler (see figure 2). Usually other protections like short-circuit, overcurrent, over-temperature, etc. are also performed at the switch level. The resulting fault signals can all be summed together and transmitted via the same opto-coupler. • The complete equipment is covered with an appropriate isolation material or enclosure. Each application is unique and the designer must take the most efficient actions to ensure system operator’s safety. 5 Using NTC temperature sensor integrated into power module 6. NTC Thermistor resistance table 6.1 50kΩ NTC Thermistor Resistance Table The following table is data taken from the NTC thermistor manufacturer. Similar results are obtained by solving equation (1), which is valid for NTC thermistors used in power modules. Using the table 1 or equation (1), it is very easy to determine the NTC thermistor resistance at a specific temperature. Note that the data sheet lists R25 = 50 kΩ. T (°C) Rt/R25 nominal Temp coef (%/°C) B deviation (*)(± %) -50 61.32 6.91 8.96 -45 43.66 6.68 8.18 -40 31.45 6.46 7.41 -35 22.89 6.25 6.67 -30 16.835 6.05 5.95 -25 12.498 5.87 5.25 -20 9.363 5.69 4.57 -15 7.074 5.52 3.90 -10 5.389 5.37 3.26 -5 4.137 5.21 2.63 0 3.199 5.01 2.02 5 2.5 4.86 1.57 10 1.968 4.71 1.15 15 1.56 4.58 0.75 20 1.245 4.45 0.37 25 1 4.32 0 30 0.808 4.21 0.35 35 0.6567 4.09 0.69 40 0.5367 3.98 1.01 45 0.4409 3.88 1.32 50 0.3641 3.78 1.62 55 0.3022 3.68 1.93 60 0.2520 3.59 2.23 65 0.2111 3.49 2.51 70 0.1777 3.41 2.76 75 0.1502 3.32 3.02 80 0.1274 3.24 3.25 85 0.1086 3.17 3.47 90 0.0928 3.09 3.68 95 0.0797 3.02 3.87 6 Using NTC temperature sensor integrated into power module T (°C) Rt/R25 nominal Temp coef (%/°C) B deviation (*)(± %) 100 0.0686 2.95 4.05 105 0.0593 2.88 4.15 110 0.0514 2.82 4.25 115 0.04475 2.75 4.34 120 0.03907 2.69 4.44 125 0.03421 2.63 4.54 130 0.03004 2.57 4.64 135 0.02646 2.51 4.74 140 0.02337 2.46 4.84 145 0.02069 2.41 4.94 150 0.01837 2.38 5.05 155 0.01633 2.32 5.15 160 0.01456 2.27 5.25 165 0.01301 2.22 5.36 170 0.01166 2.17 5.47 175 0.01047 2.12 5.57 180 0.00943 2.08 5.68 185 0.00851 2.03 5.78 190 0.00769 1.99 5.89 195 0.00697 1.95 5.99 200 0.00633 1.93 6.11 205 0.00575 1.90 6.21 Table 1: Data from NTC thermistor manufacturer (50kΩ). (*) The deviation resulting from the tolerance on the material constant Beta. The deviation must be added to the resistance tolerance of the part as specified at 25°C. 7 Using NTC temperature sensor integrated into power module • To calculate Rt/R25 at temperatures other than those listed in the table 1, use the following equation: Rt B C D⎞ ⎛ = exp ⎜ A + + 2 + 3 ⎟ R25 T T T ⎠ ⎝ R25 = 50kΩ Rt = thermistor resistance T = temperature in Kelvin K = °C + 273 .15 Temp range (°C) -50 to 0 0 to 50 50 to 100 100 to 150 150 to 200 200 to 250 • A -1.7718174E+01 -1.6391831E+01 -1.6267345E+01 -1.5586597E+01 -1.4360600E+01 -1.4956600E+01 B 6.9923532E+03 6.3460312E+03 6.3651593E+03 5.8374988E+03 4.5701737E+03 5.1897766E+03 D 3.4307893E+07 3.6804552E+07 4.6929412E+07 4.4005223E+07 1.0155939E+07 3.4668554E+07 To calculate the actual thermistor temperature as a function of the thermistor resistance, use the following equation: 2 ⎛ R ⎞ ⎛ R ⎞ ⎛ R ⎞ 1 = a + b ⎜⎜ ln t ⎟⎟ + c ⎜⎜ ln t ⎟⎟ + d ⎜⎜ ln t ⎟⎟ T ⎝ R25 ⎠ ⎝ R25 ⎠ ⎝ R25 ⎠ Rt/R25 range 61.32 to 3.199 3.199 to 0.3641 0.3641 to 0.06862 0.06862 to 0.01837 0.01837 to 0.00633 0.006331 to 0.00263 8 C -6.2682835E+05 -5.5838575E+05 -6.0889839E+05 -4.9895349E+05 -1.0221320E+05 -3.1375858E+05 a 3.3600620E-03 3.3540176E-03 3.3534734E-03 3.3446840E-03 3.3065226E-03 3.3021333E-03 3 b 2.5313332E-04 2.6025088E-04 2.5896369E-04 2.5229699E-04 2.3663693E-04 2.3643631E-04 c 4.9240651E-06 3.3044941E-06 2.5490046E-06 1.2806632E-06 4.3893009E-08 -9.6846436E-08 d -5.9119386E-08 -8.6084408E-08 -1.0052993E-07 -1.0221063E-07 -2.9026088E-08 -8.2833871E-08 Using NTC temperature sensor integrated into power module 6.2 22kΩ NTC Thermistor Resistance Table NTC R/T Calculation 5,0 Type 805 R/T characte 8502 R at 25°C = 22000,0 [Ohm] T[°C] R nom[Ohm] ‐55 2115500 ‐50 1471600 ‐45 1036800 ‐40 739330 ‐35 533340 ‐30 388990 ‐25 286710 ‐20 213440 ‐15 160430 ‐10 121690 ‐5 93115 0 71846 5 55880 10 43795 15 34575 20 27487 25 22000 30 17721 35 14363 40 11710 45 9601,8 50 7915,9 55 6560,2 60 5464,1 65 4573,2 70 3845,4 75 3247,9 80 2755,1 85 2346,8 90 2007,0 95 1723,0 100 1484,7 105 1284,0 110 1114,2 115 970,15 120 847,43 125 742,54 Ordering code = 'B57421V2223J062' B(25/100) = 4000,0 [K] ± 3,0% R nom at 25°C = 22000 [Ohm] ± 5,0% R min[Ohm] 1696400 1198200 856400 619040 452320 333930 248980 187390 142320 109030 84220 65571 51441 40649 32346 25911 20900 16709 13450 10893 8874,0 7269,7 5987,6 4957,2 4124,6 3448,3 2896,2 2443,3 2070,1 1761,1 1504,2 1289,6 1109,8 958,48 830,62 722,22 629,98 R max[Ohm] 2534500 1745000 1217200 859630 614350 444050 324440 239490 178540 134350 102010 78121 60319 46940 36804 29064 23100 18734 15276 12528 10330 8562,2 7132,9 5971,1 5021,9 4242,6 3599,7 3067,0 2623,6 2252,9 1941,9 1679,8 1458,2 1270,0 1109,7 972,64 855,10 deltaR/R [+‐%] deltaT [+‐°C] alpha [%K] 19,8 2,7 7,4 18,6 2,6 7,1 17,4 2,5 6,9 16,3 2,4 6,6 15,2 2,4 6,4 14,2 2,3 6,2 13,2 2,2 6,0 12,2 2,1 5,8 11,3 2,0 5,6 10,4 1,9 5,4 9,6 1,8 5,3 8,7 1,7 5,1 7,9 1,6 4,9 7,2 1,5 4,8 6,4 1,4 4,7 5,7 1,3 4,5 5,0 1,1 4,4 5,7 1,3 4,3 6,4 1,5 4,1 7,0 1,7 4,0 7,6 1,9 3,9 8,2 2,1 3,8 8,7 2,4 3,7 9,3 2,6 3,6 9,8 2,8 3,5 10,3 3,0 3,4 10,8 3,2 3,3 11,3 3,5 3,2 11,8 3,7 3,2 12,3 4,0 3,1 12,7 4,2 3,0 13,1 4,5 2,9 13,6 4,7 2,9 14,0 5,0 2,8 14,4 5,3 2,7 14,8 5,5 2,7 15,2 5,8 2,6 9 Using NTC temperature sensor integrated into power module 7. PTC Resistor (Positive Temperature Coefficient) As opposed to a NTC thermistor, a PTC resistor increases resistance with temperature, and the variation is linear. The PTC resistance value can easily be determined by the formula below. R T = R 0 ⋅ (1 + αT ) RT = PTC resistance at temperature T T = delta of temperature α = temperature coefficient R0 = resistance at 0°C NTC thermistor is the most common device used for temperature protection in power systems. If fine temperature control is required a PTC device with better accuracy and most of all a linear variation versus temperature may be preferred. 10 Microsemi Corporation (NASDAQ: MSCC) offers a comprehensive portfolio of semiconductor solutions for: aerospace, defense and security; enterprise and communications; and industrial and alternative energy markets. Products include high-performance, high-reliability analog and RF devices, mixed signal and RF integrated circuits, customizable SoCs, FPGAs, and complete subsystems. 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