Temperature measurement accuracy is an essential part of many electronics designs. This is especially true in consumer, industrial and automotive electronics applications where the functionality or efficiency of an entire system is often dependent on temperature-based variables. With recent advances in surface-mounted negative temperature coefficient (SMD NTC) thermistors, designers, engineers and purchasers – all involved in the new product development process - should consider these temperature sensors for their newest product platforms to help prevent overheating and to extend the lifetime of their designs.
In most applications, electronic designs have high packing densities and are pushed to their thermal limits during operation. This poses a challenge for designs where thermal temperature is a consideration – especially where size is a constraint. Accurate temperature measurement is crucial so that corrective action may be taken in enough time to prevent imminent overheating of the electronic module or design. New advances in SMD NTC thermistors enable highly precise measurements required for these designs and very effective control systems when combined with intelligent circuits.
The latest technology of NTC thermistors have a rated resistance of 10 kΩ in tolerance classes of ±1%, ±3% and ±5%. These narrow tolerances are achieved through rugged glass passivation made possible by state-of-the art production technologies. This technology helps to ensure high reliability, degradation stability and consistent efficiency of the thermistors. It also ensures that the slope of the R/T curve with a B value of 3455 K has a narrow tolerance of ±1% on a consistent basis and a rapid response time.
Due to these improvements, the advanced thermistors can now provide a faster and more accurate temperature measurement across a very wide range and operate in applications up to +125 °C. This makes these NTC thermistors and associated modules well-suited for consumer and industrial applications.
Preventing Thermal Overloads
For example, a charge monitoring circuit for rapid charging rechargeable batteries in mobile devices can be created with these NTC thermistors to prevent overheating during the charging process. We’ve all seen photographs of mobile devices that have caught on fire or have been damaged while they are plugged in to rapidly charge their batteries. This can be prevented by taking the right steps during the design phase.
The latest charging techniques necessitate the maximum permissible temperature be maintained for each battery cell. However, it also necessitates that charging does not exceed the maximum permissible charging current at the maximum permissible cell temperature with a high degree of accuracy.
The current must be very accurately reduced as the charge current heats the cell to its limit temperature in order to avoid damage to the cell. The more rapidly and accurately the change in temperature of the cell can be identified and measured, the faster and more precise the charging current can be adjusted to preserve optimal conditions. This technique reduces the risk of temperature extremes and permits the charging to take place in the fastest time possible with a reduced risk of thermal overload.
However, there is still more needed to prevent a hazardous overheating situation. Engineers should also measure the ambient temperature of the device in order to avoid excessive differences between the ambient and battery cell temperatures. For example, the device may heat up if the operator of the device is streaming video while charging. In this example, an additional NTC thermistor should be integrated on the circuit board of the charging device. This enables extreme differences between the battery and ambient temperatures to be identified, measured and adjusted to prevent thermal overheating.
Reducing Semiconductor Overheating
Other semiconductors also need protection from excessive temperatures in order to perform optimally and to ensure reliable operation. Compact thermistors should be integrated into circuit boards in close proximity to microcontrollers, power semiconductors, processors, logic components, and in other hot spots on the circuit board to measure for excessive temperatures. By doing this, thermal shock, expansion, stress and cracking can be prevented in such applications.
To ensure highly accurate thermal monitoring of sensitive semiconductors, a good thermal contact to the circuit board is required through solder connections with simultaneously negligible self-heating. This is often accomplished through reflow soldering processes and wave soldering. Thermistors can be placed in the design on the underside of the board opposite of these sensitive semiconductors thereby providing a good thermal contact. However, not all thermistors are suitable for such applications, and many are not designed against thermal shock. This specification should be taken into consideration during the design process.
Extending LED Life
Another example is in LED lighting systems. Surface-mounted NTC thermistors help LED systems to have a long operating life with maximum luminous efficiency. Because the efficiency of LED lighting depends to a large degree on the temperature of semiconductor junctions, temperature extremes must be circumvented. If not, LED lighting systems will have a shortened lifetime with earlier power degradation, color shifts and lower output in lighting intensity – and in some cases destruction.
If temperatures are too low, the luminous efficiency and lumen per volume ratio are reduced. As a result, the temperature must be maintained at the specified optimum in order to achieve maximum efficiency. This is typically between 70 °C and 90 °C in LED applications.
When a surface-mounted NTC thermistor is integrated into the LED circuit, every deviation from the optimum operating temperature will cause a significant resistance change of the NTC element. This is evaluated by a comparator, so that the current flow through the LED is reduced. Subsequently, the power loss of the LED system drops and the lifetime of the system is extended.
Temperature monitoring significantly extends the lifetime of electronic systems in automotive, consumer and industrial applications and more. New advances in SMD NTC thermistors can be used in these applications to prevent thermal overloads. They can reduce overheating and extend life – all while maintaining the optimal conditions for maximum operational efficiencies.
About the author: Sonja Brown, is director of product marketing – piezo and protection devices, EPCOS, a TDK Group Company.