SMD NTC thermistors, application notes

SMD NTC thermistors, application notes
SMD NTC Thermistors
Application notes
Date:
January 2017
© EPCOS AG 2017. Reproduction, publication and dissemination of this publication, enclosures hereto and the
information contained therein without EPCOS' prior express consent is prohibited.
EPCOS AG is a TDK Group Company.
Application notes
1
Applications utilizing the influence of ambient temperature on resistance
(self-heating negligible)
1.1
Temperature measurement
The high sensitivity of an NTC thermistor makes it an ideal candidate for temperature sensing applications. These low-cost NTC sensors are normally used for a temperature range of 40 °C to
+150 °C.
Selection criteria for NTC thermistors are
temperature range
resistance range
measuring accuracy
environment (surrounding medium)
response time
dimensional requirements.
One of the circuits suitable for temperature measurement is a Wheatstone bridge with an NTC
thermistor used as one bridge leg.
Figure 1
Wheatstone bridge circuit
With the bridge being balanced, any change in temperature will cause a resistance change in the
thermistor and a significant current will flow through the ammeter. It is also possible to use a variable resistor R3 and to derive the temperature from its resistance value (in balanced condition).
An example of a circuit including an NTC thermistor and microcontroller is given in figure 2.
Figure 2
Practical application for a circuit
with NTC thermistor and microcontroller
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and Cautions and warnings.
Page 2 of 13
Application notes
1.2
Linearizing the R/T characteristic
NTC thermistors exhibit a distinctly non-linear R/T characteristic. If a fairly linear curve is required
for measurements over a (wide) temperature range, e.g. for a scale, series-connected or paralleled resistors are quite useful. The temperature range to be covered should, however, not exceed 50 K to 100 K.
Figure
Linearization of B57321V2103J060 NTC
thermistor by a paralleled resistor
Figure 4
Signal voltage and power dissipation curves of
the linearized NTC thermistor
Figure 5
Resistance/temperature characteristic
linearized by a paralleled resistor
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Page 3 of 13
Application notes
The combination of an NTC thermistor and a paralleled resistor has an S-shaped R/T characteristic with a turning point. The best linearization is obtained by laying the turning point in the middle
of the operating temperature range. The resistance of the paralleled resistor can then be calculated by the exponential approximation:
The total resistance of RT RP is:
RT
Resistance value of the NTC thermistors at mean temperature T
(in K temperature in °C +273.15)
B
B value of the NTC thermistor
The rate of rise of the (linearized) R/T characteristic is:
The circuit sensitivity however decreases with linearization.
Figure
Linearization of the R/T characteristic:
simple amplifier circuit
1.3
Figure 7
Linearization of the R/T characteristic:
output voltage at the load resistor as a function
of temperature
Temperature compensation
Virtually all semiconductors and the circuits comprised of them exhibit a temperature coefficient.
Owing to their high positive temperature coefficient, NTC thermistors are particularly suitable for
compensating this undesired response to temperature changes (examples: working point stabilization of power transistors, brightness control of LC displays). Resistors in series or shunt plus
suitable voltage dividers and bridge circuits provide an excellent and easy-to-implement compensation network.
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and Cautions and warnings.
Page 4 of 13
Application notes
It is important to match the temperature of the compensating NTC thermistor to that of the component causing the temperature response.
Figure 8 shows a simple circuit configuration for a thermostat.
Figure 8
Circuit for a temperature controller
1.4
Application examples
NTC thermistors for temperature measurement are suitable for a large variety of applications.
Household electronics:
refrigerators and deep-freezers, washing machines, electric
cookers, hair-dryers, electronic ballast, power tools, LED lighting
etc.
Automotive electronics:
electronic control units (ECUs), e.g. motor management, airbags,
cooling control units, gearbox controls, cylinder head or braking
systems, temperature controls for the battery pack in conventional
and hybrid automobiles
sensor systems, e.g. temperature controls in tire air-pressure
modules, temperature sensors for air-conditioning and passenger
compartment
headlights, e.g. LED lighting
displays, e.g. dashboard, car radio, navigation, GPS, in-car TV
Heating and air-conditioning: heating cost distributors, room temperature monitoring, underfloor
heating and gas boilers, outdoor temperature sensors
Industrial electronics:
temperature stabilization of laser diodes and photoelements,
temperature compensation in copper coils or reference point
compensation in thermoelements, LED and semiconductor
overheating protection
Computer and consumer
electronics:
HDDs, printer and PC main boards, audio and video systems,
medical devices
Telecommunications:
temperature sensing and compensation in mobile phones, e.g.
TCXO
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and Cautions and warnings.
Page 5 of 13
Application notes
1.4.1
Temperature control in mobile phones
The use of mobile phones in a wide temperature range (e.g. from 40 °C up to +85 °C) requires
the control of the temperature-sensitive elements of the system. This includes the crystal oscillator (XO), the LCD, the power amplifier and the battery pack. NTC thermistors as temperature sensors fulfill different tasks e.g. temperature compensation or temperature sensing in an overtemperature protection circuitry.
1.4.2
Battery packs
All rechargable batteries and lithium ion batteries in particular must be controlled and protected
by smart charging circuits, as the mobile phone drawing power from the batteries must operate in
a variety of environments, including low and high-temperature operation.
As preferred temperature detection devices NTC thermistors are used in the protective circuitry.
NTC thermistors can detect the ambient temperature for different purposes, depending on the
battery system. Especially for quick charging the ambient temperature has to be measured, as not
all batteries allow the charging in the hot and cold temperature region. Usually charging temperatures of 0 °C up to 45 °C for slow charging, and 5 °C ... 10 °C up to 45 °C for quick charging are
recommended by the battery pack manufacturers depending on the battery chemistry.
The NTC thermistor is part of a smart charging control unit (see figure 9), which assures that the
ambient temperature is in the range allowing quick charging. During charging the NTC thermistor
repeatedly measures the temperature all 5 to 10 seconds and can detect a rise in the battery
cell's temperature at the end of the charging cycle or precipitated from abnormal charging conditions. During discharging NTC thermistors also perform temperature compensation for the voltage
measurement, which helps to measure the remaining charge in the battery.
Figure 9
Schematic drawing of the charging
control unit of a battery pack using
NTC thermistors as temperature
sensors
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and Cautions and warnings.
Page 6 of 13
Application notes
1.4.3
LCD
Liquid crystal displays (LCDs) are widely used in portable electronics. As the fluid used in liquid
crystal displays is sensitive to temperature, LCD modules have a limited operating temperature
range. If a constant voltage is applied to the LCD, the contrast increases with temperature and
power is wasted at high temperature. Low temperature on the other hand means a low unclear
display.
Figure 10
Schematic drawing of the compensation circuit
of an LCD using an NTC thermistor as
temperature sensor
For these LCD modules often a temperature compensation circuit is used (see figure 10), consisting of NTC thermistors and resistors. The thermistor as main temperature-sensitive device with its
characteristic resistance temperature curve provides a high driving voltage in the cold and a low
driving voltage in the hot temperature region, compensating in this way the LCD temperature
characteristic.
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and Cautions and warnings.
Page 7 of 13
Application notes
1.4.4
Temperature control in hard disk drives (HDD)
An important factor which must be considered in the development of HDDs is reliability. Operating
electronic components such as disk drives at high temperatures can dramatically reduce their reliability. The resulting stress can lead to unexpected failures and even data loss. Continuous or
sustained operation above the normally specified ambient temperature of 5 °C to 55 °C may decrease MTBF (mean time between failures).
Figure 11
HDD reliability:
typical temperature
sensitivity
An NTC sensor can be used to monitor the temperature within the drive and to warn the drive
controller when the drive exceeds its maximum permissible temperature. The NTC thermistor is
mounted on the logic board. The typical set-up point is the maximal operating temperature of
55 °C.
Normally the sensor is designed not only for warning, but also to trigger actions. If the temperature exceeds the configured limits, possible actions may be the activation of a cooling fan, a slowdown of drive activity or even a stop of the drive.
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and Cautions and warnings.
Page 8 of 13
Application notes
1.4.5
LED lighting
Light-emitting diodes (LEDs) are not only used in portable electronic solutions. These components – in most cases high-brightness versions – are also found in building and automotive lighting. Power dissipation in such applications is quite high and temperature control is necessary for
the operation of LEDs due to the high operating temperature in these applications.
LED manufacturers usually recommend LED current derating starting at temperatures between
50 and 80 °C to guarantee sufficient lifetime. Without temperature control the developer would
have to make sure the temperature in an application never exceeds the derating starting temperature of the LED or use only 50 to 65% of maximum permissible LED current, thus sacrificing full
LED brightness.
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and Cautions and warnings.
Page 9 of 13
Application notes
If a LED driver with current control by a shunt resistor Rs is used, a common temperature control
circuit consists of an NTC R(T) and an additional resistor R.
The NTC must be connected to a constant voltage, the Vcc of the LED driver for instance. Many
LED drivers also have a reference voltage of 5 V for this purpose, for example. The driver output
for LEDs is unsuitable because, given the current control, the voltage here is not constant. The
LED current in this configuration is calculated as follows:
Vref
R
Reference voltage of feedback voltage input of LED driver
Rs
Resistance of shunt resistor
R(T)
Resistance of NTC at rated temperature (see chapter 3.1 in “General
technical information”)
V
Voltage applied to NTC
Resistance of additional resistor
The rate at which LED current decreases is determined by the B value of the NTC and the rating
of the additional resistor R. Both a higher B value and a higher additional resistance produce a
steeper drop of LED current. Ratings between 10 and 100 kΩ for R25 of the NTC are possible to
minimize its transverse current. The resistance tolerance of the NTC has a substantially smaller
effect on the accuracy of LED current than the B value tolerance, indicating the possibility of using
an NTC with 5% resistance tolerance for this application.
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and Cautions and warnings.
Page 10 of 13
Application notes
RGB backlight
Recommended part numbers:
For standard applications:
B57230V2103+260
B57330V2103+260
B57374V2104+060
(0402, 10 kΩ ±5%; B25/100 = 3455 K ±1%)
(0603, 10 kΩ ±1%; B25/100 = 3455 K ±1%)
(0603, 100 kΩ ±5%; B25/100 = 4480 K ±1%)
For automotive applications:
B57232V5103+360
B57332V5103+360
B57352V2104+360
(0402, 10 kΩ ±5%; B25/100 = 3455 K ±1%)
(0603, 10 kΩ ±1%; B25/100 = 3455 K ±1%)
(0603, 100 kΩ ±5%; B25/100 = 4480 K ±1%)
Depending on the application, additional ESD protection by a ceramic transient voltage suppressor (CTVS) may be necessary both for the LEDs and Vcc. For more information about ESD protection refer to the Epcos CTVS data book.
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and Cautions and warnings.
Page 11 of 13
Application notes
1.4.6
Thermal protection of semiconductors
Example: IGBT
The IGBT must be turned off when the junction temperature 125 °C is reached so that it does not
become too hot and is subsequently damaged. This temperature control is performed by the SMD
NTC contained in the IGBT package.
Recommended part numbers in case size 0603:
B57230V2103+260
B57330V2103+260
B57374V2104+060
Please read Important notes
and Cautions and warnings.
(0402, 10 kΩ ±5%; B25/100 = 3455 K ±1%)
(0603, 10 kΩ ±1%; B25/100 = 3455 K ±1%)
(0603, 100 kΩ ±5%; B25/100 = 4480 K ±1%)
Page 12 of 13
Application notes
2
Applications utilizing the current/time characteristic
If an NTC thermistor is connected to a voltage source via a series resistor and the current is measured as a function of time, an increase in current will be observed.
At first the thermistor is cold, i.e. in high-resistance mode, and only a low current is flowing
through the device. But this current starts to heat up the thermistor and the wattage increases
with the resistance value of the thermistor approaching that of the series resistor. Thus the increase in current becomes faster and faster till the two resistance values are equal. With further
decreasing NTC resistance the wattage will also decrease due to the growing mismatch and the
current reaches a final value. The entire wattage is consumed in maintaining the overtemperature.
Relay delay
To delay relay pick-up thermistor and relay are connected in series. When applying a voltage Vop
the current flowing through the relay coil is limited to a fraction of the pick-up current by the high
cold resistance of the thermistor. With the thermistor heating up, its resistance decreases and the
current rises until the pick-up value is reached.
To delay relay drop-out relay and thermistor are connected in parallel.
Figure 16
Delay of relay pick-up
Figure 17
Delay of relay drop-out
The operating sequence of a relay delayed by a thermistor depends on the recovery time of the
thermistor. The thermistor has to cool down before it can cause second delay. If the thermistor remains unloaded for a time t = 3 τc (3 times the thermal cooling time constant) between two operations, the time for the second delay will be 80% to 90% of that for the first delay. It is therefore
useful to short-circuit or switch off the thermistor by additional relay contacts, so that the thermistor has sufficient time to cool down (see dashed section in figure 16).
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and Cautions and warnings.
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