July 2016

SMD-PTCs for thermal monitoring

All hot spots under control

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TDK's portfolio of EPCOS PTC thermistors features a series that is particularly suited for the thermal management of IT equipment. This Superior Series is available for various response temperatures.

The Superior Series of EPCOS SMD limit temperature sensors from based on PTCs is available in case sizes 0805, 0603 and 0402 and covers a temperature range from 70 to 145 °C. Compared with the standard series, the new components use a more homogeneous ceramic material, which improves reliability while permitting processing by reflow soldering. Thanks to these properties, the PTCs of the Superior Series are qualified based on AEC-Q200, Rev. C and thus satisfy the rigorous requirements for use in automotive electronics.

PTC thermistors have a nonlinear characteristic: at low temperatures such as ambient, their resistance is low. As the temperature rises, their resistance jumps suddenly depending on the ceramic material used. This threshold value is also known as the reference or limit temperature. Figure 1 shows the typical characteristic of a PTC thermistor.

Figure 1: Various resistance curves of PTC thermistors

The sudden jump in resistance at different limit temperatures can be clearly seen. The different limit temperatures and slopes are determined by the ceramic mixture of the PTC thermistors.

At normal temperatures, the PTC sensor has a low resistance with a typical value of less than 1 kΩ. As the temperature rises, however, its resistance begins to increase. When the specified limit temperature TSense is reached, its resistance assumes a value of 4.7 kΩ. The accuracy is ±5 °C. If the temperature rises another 15 K, the PTC resistance increases tenfold to 47 kΩ, an exponential jump with respect to the temperature rise. This sudden increase in resistance makes PTC thermistors ideal as limit temperature sensors, allowing them to detect the critical temperature of sensitive electronic components in good time. For this purpose, they should be mounted as close as possible to the component they are designed to protect. This assures good thermal contact as well as a fast response time.

As shown in Figure 2, the PTC sensor is normally inserted together with a fixed resistor into a voltage division circuit. This results in a temperature-dependent output voltage Vout, which changes suddenly according to the characteristic of the PTC sensor and directly controls a component such as a switching transistor or comparator. This in turn triggers corresponding functions in order to avoid overheating and consequent damage. In this way, a blower can be switched in or loads and system components switched off very cost effectively.

Figure 2: Circuit for detecting over-temperatures

This simple circuit can be used to monitor a single hot spot cost-efficiently and reliably.

All hot spots under control

In IT equipment such as notebooks, some system components must be thermally monitored, as convective cooling is insufficient in this case. Instead of a central power supply that provides one or more supply voltages via a bus system, in this case local DC/DC converters – known as points of load (POLs) – are distributed over the entire board to generate the required voltage close to the load.

Although today’s POLs have a high level of efficiency, they still generate thermal losses. In order to avoid local overheating, POLs frequently require thermal monitoring. The same applies to the processor, the chipset of the graphics card, the rechargeable battery, and drives, as well as the RAM and other system units. Figure 3 shows a typical configuration of a laptop and the hot spots to be monitored.

Figure 3: Notebook with potential hot spots

A simple circuit of PTC sensors in series can be used to monitor the hot spots in notebooks.

The steep and rapid change in resistance of PTC sensors with temperature allows several hot spots to be monitored with a simple circuit. For instance, if seven different points have to be monitored simultaneously on a circuit board or in an item of equipment, the circuit shown in Figure 4 is an obvious choice. A single PTC is located at every point to be monitored. Thanks to their steep characteristic, all PTCs can be connected in series while assuring reliable monitoring of each individual hot spot.

Figure 4: Temperature measurement at several hot spots

Despite the series circuit, reliable measurement of over-temperatures is possible at each hot spot. This circuit topology permits cost-effective, space-saving and reliable temperature management.

In addition to its simple and nevertheless reliable configuration, this circuit also offers another considerable advantage: the PTC sensors of the Superior Series are available for limit temperatures Tsense from 75 to 145 °C in stages of 10 K, so that each hot spot can be monitored with a reference temperature specific to it.

As long as all seven PTC sensors in the example circuit remain below the limit temperature, the total resistance of all the series-connected sensors will be below 10 kΩ. Even if only a single one of the series-connected PTC sensors exceeds its limit temperature, the resistance of the resistor chain will rise to values significantly above 10 kΩ. For this reason, a voltage divider can also be used here for detecting the over-temperature (Figure 4).

This circuit can also be used for other systems such as power supplies, UPS, frequency converters, servers, light controllers and systems of automotive electronics. Very often, the hot spots at which power losses can lead to the occurrence of over-temperatures are power semiconductors such as MOSFETs or IGBTs, but they may also be inductors, transformers, capacitors and motors.

PTC thermistors as all-rounders

Thanks to their characteristic resitance curve, PTC thermistors have multiple uses: these are illustrated here by limit temperature sensors as well as current limiters and heating elements.

Current limiters

PTC thermistors are designed to have only a very low resistance in the region of a few ohms at their rated temperature. If the current exceeds a precisely defined limit value, their power dissipation increases and the thermistor heats up. Its resistance then rises suddenly according to its characteristic, thus limiting the current. Only when the component has cooled down does it return to its low resistance state. This behavior makes PTC thermistors ideal as current limiters and self-resetting fuses. TDK offers a wide range of these components in leaded or SMD versions. The main applications are in overcurrent and short-circuit protection of telecommunications lines and power supplies.

Heating elements

PTC thermistors can also be used to produce heat in a focused way. Their self-regulation is a particular advantage: when the element heats up, its resistance increases and a constant current flows that is proportional to the heat output. These elements are used as supplementary heaters in motor vehicles and to heat fuel lines and filters or wiper nozzles, for instance.
Thanks to a process developed by EPCOS, they can also be manufactured as injection-molded parts. This permits an enormous diversity of possible designs, including complex three-dimensional shapes. Thus heating nozzles may be produced in hot glue guns or rotor blades in fan heaters.

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