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Texas Instruments Using Thermistors to Optimize the Thermal Performance of IGBT Modules Application notes
Using Thermistors to Optimize the Thermal Performance
of IGBT Modules
Mina Shawky, Temperature and Humidity Sensing
Introduction
Thermistors for Temperature Monitoring
The Insulated Gate Bipolar Transistor (IGBT) is a
device widely used in switched-mode power supplies
and traction motor systems. IGBTs play a critical role
in application spaces such as electric vehicles,
renewable energy, airplanes, naval ships, CT and MRI
scanners, and building automation.
Thermistors are passive components that change
resistance with a change in temperature. Typically,
thermistors are either placed on the bottom of the
voltage divider or biased by a constant current source
as shown in Figure 2.
IGBTs typically operate in applications that have
switching frequencies of less than 100 kHz, power
consumption greater than 5 kW, and operating
temperatures exceeding 125℃. Thus, IGBTs require
thermal monitoring and protection to ensure the safety
and reliability of the system. Thermistors have been
widely used as thermal management solutions due to
their low cost and reliable performance.
Thermal Safety With IGBTs
An IGBT uses the input voltage controlled feature of a
MOSFET, but has the output switching and conduction
characteristics of a BJT. Multiple IGBT dies are often
closely packaged together, which can increase the
junction temperature. By using thermistors to monitor
the temperature inside the module, the output can be
safely managed and optimized.
Figure 1. Thermistor in an IGBT Module
High temperatures in IGBTs can indicate overloads or
faults, such as phase-to-phase short circuits, phase-toearth short circuits, or shoot-through effects. Typically,
a thermistor is integrated on its own island in the
module for electrical isolation. Use Figure 1 as an
example.
Failure to properly monitor the temperature can
increase safety hazards and reduce the operational
lifetime of the IGBT module system.
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Figure 2. Thermistor Current and Voltage Biasing
Circuits
There are two types of thermistors that can be used
for this application: Negative Temperature Coefficient
(NTC) and Positive Temperature Coefficient (PTC).
NTCs are inherently nonlinear, making it difficult to
achieve high accuracy over the whole temperature
range. At high temperatures, the ratio of volts per
degree diminishes, and the resolution is coarse due to
noise and errors. Such effects of non-linearity are
often managed through the use of hardware and
software-based linearization methods. Unlike NTCs,
silicon-based PTCs have a positive linear shift in
resistance with an increase in temperature.
As a result of its lower resistance at high
temperatures, an NTC draws more current from the
constant biasing voltage. This leads to a higher selfheating error compared to PTCs. These characteristics
are shown in Figure 3, which compares the linearity of
the PTC versus the NTC.
When designing a temperature-monitoring circuit, the
system must account for errors like the tolerance of
the components, system complexity, and the analogto-digital converter (ADC) resolution. These errors can
greatly influence the thermistor output and
compromise the safety of the system.
Using Thermistors to Optimize the Thermal Performance of IGBT Modules
Copyright © 2019, Texas Instruments Incorporated
Mina Shawky, Temperature and Humidity Sensing
1
www.ti.com
If a PTC is disconnected from the system, however,
the MCU/MPU will read a high temperature and take
necessary actions for protection, increasing the overall
reliability and safety of the module.
Conditioning Circuit
Figure 3. Resistance vs Temperature of PTC vs
NTC
There are many factors used to calculate the system
accuracy. A thermal monitoring system error includes
thermistor tolerance and nonlinearity, bias resistor
temperature drift, ADC quantization errors, and
reference voltage fluctuations. Due to the PTC linearity
across the wide operating range, the linearization error
is much lower than that of NTCs. NTCs also require an
extensive lookup table or a high-order polyfit to
accurately convert the system ADC output values to a
human readable temperature. In contrast, siliconbased PTCs have much smaller lookup tables that use
less memory on the MCU/MPU. By using PTCs, errors
and overall system costs are minimized. Figure 5
shows the typical block diagram for a PTCconditioning circuit in an IGBT module system. Using
PTCs to reduce system errors can increase the
performance and reliability of IGBT modules.
At higher temperatures, NTCs are more susceptible to
these errors due to their lower voltage per degree than
PTCs as shown in Figure 4.
Figure 5. PTC-Conditioning Circuit
Device Recommendations
Figure 4. Voltage Biased Output of PTC vs NTC
In some applications, the PTC can be placed on the
high side of the voltage divider circuit if a negative
slope characteristic is preferred. This outputs a linear
inverse slope that requires less calibration than the
nonlinear NTC (see Figure 4).
Furthermore, if an NTC is disconnected from the
system due to poor layout or other mechanical
stresses, the microcontroller (MCU) or microprocessor
(MPU) will read a low temperature due to the open
circuit (high resistance) (see Figure 3).
2
The TMP61 is a silicon-based PTC thermistor
designed for temperature measurement, protection,
compensation, and control systems. The TMP61 has a
tolerance of ±1% between –0°C to 70°C, and a wide
operating range of –65°C to 150°C. Compared to
traditional NTCs, the TMP61 offers enhanced linearity
and consistent sensitivity across the full temperature
range.
Table 1. Related Documentation
COLLATERAL
DESCRIPTION
Data Sheet
TMP61 Silicon-Based Linear Thermistors
Tech Note
Methods to Calibrate Temperature Monitoring Systems
Application
Report
Methods to Reduce Thermistor Linearization Error,
Memory, and Power Requirements Over Wide Operating
Temperature Ranges
Using Thermistors to Optimize the Thermal Performance of IGBT Modules
Mina Shawky, Temperature and Humidity Sensing
Copyright © 2019, Texas Instruments Incorporated
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