Sonja Brown, Director of Product Marketing – Piezo and Protection Devices, EPCOS, a TDK Group Company
It is no surprise that electronic equipment can get hot. This is because electrical currents running through semiconductors and other components create thermal losses. Especially in IT equipment the losses in the form of heat dissipation can be quite significant. As much as 40 percent of the power used by data centers is for temperature control – mainly in the form of cooling the equipment to extend hardware life and increase uptime and efficiency. And multiple companies have recalled notebooks due to poor thermal management.
While the cooling of a notebook or data center is important, thermal management of IT equipment can be assisted through the use of PTC thermistors during the design of the equipment itself, allowing for increased efficiency of the semiconductor and proper heat dissipation.
Due to the non-linear characteristics of PTC thermistors, their resistance is low at low and ambient temperatures. However, their resistance increases sharply as temperature rises. The type of ceramic material used determines the different threshold values based on the reference or limit temperature. Many of the newer temperature sensors based on PTCs use a more homogeneous ceramic material, which improves reliability while permitting processing by reflow soldering. This helps cover a wider range of temperatures and threshold values.
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Figure 1: Typical characteristics of PTC thermistors: The different limit temperatures and resistance curves of PTC thermistors are determined by the ceramic mixture of the PTC thermistors.
At normal temperatures resistance is typically less than 1 kΩ. Although different PTC sensors have different limit temperatures, resistance typically approaches 4.7 kΩ as the specified limit temperature is reached. The PTC resistance increases to 47 kΩ as the temperature increases an additional 15 K, providing an exponential upsurge compared to temperature rise. This is predictably accurate within ±5 K.
Due to the dependability of the sudden increase in resistance as temperature rises, PTC thermistors make the best and most accurate limit temperature sensors for sensitive electronic components. PTC thermistors should be mounted as close as possible to the component they are protecting to ensure the proper thermal contact and fastest response time. When this is done, they can quickly sense the critical temperature, ensuring efficiency and reliability of the components and thereby the equipment.
PTC sensors are usually coupled with a fixed resistor in voltage division circuits to create a temperature-dependent output voltage. In this case, the voltage changes promptly according to the characteristic of the PTC sensor. This directly controls a component such as a switching transistor or comparator and triggers corresponding functions that help avoid overheating and other temperature-related damage. For example, as temperature rises, a fan can be triggered, or other components can be switched off quickly and cost-effectively.
The design of PTC thermistors is such that there is only a very low resistance of a few ohms at their rated temperature. If the current exceeds the defined limit temperature, the thermistor heats up and power dissipation increases, increasing resistance and limiting the current. When the component has cooled down, it returns to its low resistance state.
Thermal monitoring of system components in data center equipment and other IT equipment is essential because convective cooling is insufficient. Most notebooks utilize DC/DC converters – known as points of load (POLs) – instead of a central power supply to provide one or more supply voltages via a bus system. As a result, POLs are distributed across the entire board to generate the required voltage which is needed to close the load.
POLs are highly efficient, however, they generate thermal losses that can lead to local overheating. As a result, POLs need thermal monitoring to circumvent this overheating. Processors, drives, batteries, RAM and other chipsets and system components also require the monitoring of potential hot spots on the board.
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Figure 2. A circuit of PTC sensors in series can be used to monitor the hot spots in IT equipment
The exponential resistance change of PTC sensors allows the monitoring of multiple hot spots with a single, simple circuit with sensors in series. When multiple points on a circuit board need simultaneous thermal monitoring, a single PTC is located at every point that needs to be monitored. The PTCs are connected serially and thereby ensure reliable monitoring of each individual hot spot. The series circuit does not negatively affect the reliability of over-temperature measurement at each hot spot.
Circuits with serially-connected PTC sensors are not only simple and reliable, but certain PTC sensors, such as the EPCOS Superior Series from TDK are available for limit temperatures from 75 to 145°C in increments of 10 K, so each hot spot can be monitored with a reference temperature specific to it. In this case, as long as each of the PTC sensors in circuit remain below the limit temperature, the resistance of each of the sensors will remain below 10 kΩ. If any of the PTC sensors exceeds its limit temperature, the resistance will exponentially rise.
Aside from notebooks and servers, this type of PTC sensor circuit can be used for other systems such as power supplies, UPS, frequency converters, light controllers and automotive electronics.
Thanks to their characteristic resistance curve, PTC thermistors have multiple uses as limit temperature sensors and current limiters. Whether they are used in IT equipment such as servers and notebooks, they can not only help control temperature, but improve the efficiency, reliability and lifespan of the products in which they monitor.