Thermal management is the ability to control the thermal environment of an electronic system. It is often associated with cooling heat-generating electronics, but it also encompasses generating heat in cold environments to maintain optimal system operation or power wax-based linear motors. As a result of the many uses for thermal management, a wide variety of devices and components are designed for its implementation, including positive- and negative-temperature coefficient thermistors (PTCs and NTCs), thermocouples, etc. resistance-temperature detectors (RTDs).

This FAQ begins by looking at various heating applications that use PTCs, then digs into the uses of NTC thermistors for temperature measurement and thermal protection, and closes with a comparison of NTC thermistors, thermocouples, and RTDs.

PTC thermistors can operate over a range of voltage and dissipation conditions to produce a nearly constant temperature. They are self-regulating with no thermostat needed and are available in many shapes, including squares, rectangles, discs, and cylinders (Figure 1). Several PTCs can be paralleled to provide heating over a larger area. PTC thermistor-based heating solutions are low-cost, efficient, highly reliable with no moving parts, have long service lives, and can be mounted to various surfaces. Silicon PTC thermistors have a highly linear temperature coefficient (typically about 0.7%/°C). When needed, a linearization resistor can be added to enhance linearization.

Important considerations when specifying PTC thermistors include the switch temperature (Ts), which typically ranges from 50°C to 135°C, resistance at 25°C (R25), the surface area, and the maximum rated voltage (Vmax) (Figure 2). Ts is critical in heater designs. The maximum surface temperature of the PTC thermistor is only a few degrees higher than Ts, and the maximum heating temperature is directly related to Ts. The R25 must balance the need to minimize inrush currents upon start-up and be low enough to supply the power needed to heat the PTC thermistor to Ts. The thermistor cold resistance is an important factor in determining the temperature ramp-up rate. A lower resistance produces higher I²R heating.

The power dissipated, and the surface area influences the heat-up and cool-down rates. Multiple PTC thermistors can be used to increase the effective surface area. The Vmax for these devices is typically specified for DC or 60Hz AC. These devices are used in a variety of automotive, communications, aerospace, consumer, and industrial applications such as:

• Providing additional heat inside the cabin of a car or truck with a diesel engine or heat diesel fuel in cold operating conditions before injection into the cylinders
• In temperature-compensated synthesizer voltage-controlled oscillators and crystal oscillators for temperature compensation (Figure 3)

• PTCs can provide the heat necessary to expand the wax in electrically actuated linear wax motors. Wax motors are widely used in aerospace to control fuel, hydraulic, and other oils. They are also used across a variety of systems where humidity or moisture negatively impacts the reliability and performance of electromagnetic-based solutions, including self-actuating thermostatic fluid mixing valves, door lock assemblies on washing machines, control valves in water heating systems, releasing the detergent dispenser door latch in dishwashers, opening and closing vents in greenhouses, and in paraffin microactuators in MEMs devices.
• Electric motors and power transformers often include PTC thermistors in their windings to provide over-temperature protection and prevent insulation damage in the case of overheating. In this application, a thermistor with a non-linear response curve is used. The thermistor resistance rises rapidly at the maximum allowable winding temperature, triggering an external relay and turning off the current flow.
• Polymeric positive temperature coefficient (PPTC) devices can be used to provide overcurrent protection in electronic systems.

NTC thermistors

The resistance of NTC thermistors decreases exponentially with increasing temperatures. The steeper the resistance-temperature (RT) curve, the faster the resistance change. NTC thermistors have various uses, including temperature sensing and measurement, temperature protection devices and temperature compensation, and inrush current control.

An NTC thermistor placed near a heat-generating component such as a DC/DC converter or a CPU can be used to monitor the temperature and initiate temperature compensation actions as needed to protect sensitive devices from overheating. The temperature measurement circuit is typically a voltage divider composed of an NTC thermistor and a fixed-value resistor connected in series (Figure 4).

 NTC thermistors are placed on the substrate inside IGBT and MOSFET power modules to monitor the heatsink temperature and provide thermal protection. With the adoption of wide bandgap materials such as gallium-nitride (GaN) and silicon-carbide (SiC), the operating temperatures of power modules are rising, making it even more important to monitor the temperature accurately.

The contrast of LCDs changes as the ambient temperature changes. In applications that need to control LCD contrast, an NTC thermistor-based voltage divider is often used to adjust the drive voltage to compensate for changes in the ambient temperature.

Temperature-compensated crystal oscillators (TCXOs) are another example where NTC thermistors can be used to maintain stable operation as the ambient temperature changes. Just as in the case of using a PTC thermistor, an NTC thermistor can be used in certain TCXO applications to compensate for temperature changes. The oscillating frequency deviation can be controlled by inserting a compensation circuit with temperature properties that are the opposite of the crystal resonator. Separate compensation is needed for low-temperature and high-temperature operation and is provided by networks of an NTC thermistor, a capacitor, and a resistor (Figure 5).

Where do thermocouples and RTDs fit in?

While an NTC thermistor exhibits a continuous, small, incremental change in resistance correlated to temperature variations, thermocouples are voltage-based devices and reflect proportional changes in temperature through the varying voltage created between two dissimilar metals. Both are good for temperature sensing and control but for different sets of applications. Most NTC thermistors have an operating temperature range of about -50 to 250 °C, while thermocouples operate from about -200 to 1750 °C.

Compared with thermistors, thermocouples have lower accuracy and can be more difficult to use since they require a conversion of mV to temperature. An NTC thermistor can be used as part of a Wheatstone Bridge for applications that need higher accuracy. For measuring temperature, a Wheatstone Bridge is structured as an out-of-balance comparator where the out-of-balance voltage, ΔV, can be measured and related to the thermistor’s resistance, thereby measuring the temperature.

Resistance temperature detectors (RTDs), also called resistance thermometers, consist of a length of fine wire typically wrapped around a ceramic or glass core. The wire is a pure material, such as platinum, nickel, or copper, with an accurate resistance/temperature relationship used to measure temperature. RTD elements are fragile and are often housed in protective probes.

Stable operation is important in many long-term applications. Each of these temperature sensor technologies can drift over time, depending on their materials, construction, and packaging. Epoxy-coated NTC thermistors typically drift of about 0.2 °C per year. Hermetically-sealed NTC thermistors have a smaller 0.02 °C per year drift, while thermocouples have the largest drift and can drift up to 2 °C per year, largely due to chemical changes, especially oxidation.

RTDs have higher accuracy and repeatability and can replace thermocouples in some industrial applications below 600 °C. Because of their accuracy and good stability, NTC thermistors are often used in applications such as thermometers and fire detectors. Thermocouples are more durable and lower in cost, making them suitable for many industrial applications.

Summary

Thermal management is important in a wide variety of applications for an equally wide variety of purposes. In some cases, designers are concerned with cooling hot components; in other instances, generating heat to maintain optimal system operation or power wax-based linear motors is necessary. The diverse uses of thermal management have resulted in numerous thermal management components, including positive- and negative-temperature coefficient thermistors (PTCs and NTCs), thermocouples, and resistance-temperature detectors (RTDs), and numerous application circuit implementations.

References

Ceramic PTC Thermistor Heaters, Bourns
Crystals with Integrated Thermistors, ECS, Inc.
Design Considerations for PTC Heaters, Thermistors Unlimited
How to use temperature protection devices: chip NTC thermistors, TDK
Resistance thermometer, Wikipedia
Thermistor, Wikipedia
Thermistors vs. Thermocouples, Ametherm