By EPCOS 68
EPCOS' electronic modules with high packing densities are often operated up to their thermal limits. Accurate temperature measurement is indispensable in order to initiate countermeasures in good time in the event of impending overheating. Miniaturized EPCOS SMD NTC thermistors permit the highly precise measurements required for this purpose. Together with intelligent circuits, they then enable the design of effective control systems.
These products are available in case sizes EIA 0402 and 0603 and with a rated resistance of 10 kΩ in tolerance classes ±1%, ±3%, and ±5%. Their narrow tolerances were achieved by means of a new production technology as well as a rugged glass passivation, which additionally ensures high reliability and degradation stability. The slope of the R/T curve with a B value of 3455 K has a narrow tolerance of ±1% across the board. Thanks to this and to their short response time, these new NTC thermistors allow accurate and fast temperature measurement across a wide range. The standard series is suitable for applications up to +125°C. The components of the B57230V2103*260 (EIA 0402) and B57330V2103*260 (EIA 0603) series can be used for a wide range of applications in consumer and industrial electronics.
Precise current control thanks to rapid temperature measurement
A typical example is the charge monitoring of rechargeable batteries in mobile electronics. State-of-the-art charging techniques require not only that the maximum permissible temperature of the battery cells be maintained, but also that, as far as possible, charging does not exceed the maximum permissible charging current at the maximum permissible cell temperature. When the charge current heats the cell to its limit temperature, the current must be very accurately reduced to avoid damage to the cell. The more accurately and quickly the temperature change of the cell can be detected, the more precisely and quickly the charging current can be adjusted. This procedure allows charging to take place in the shortest possible time without the risk of thermal overload.
For some applications, such as rapid charging, it makes good sense to additionally measure the ambient temperature in order to avoid excessive differences between the ambient and cell temperatures. For this purpose, a second NTC thermistor can be integrated directly into the circuit board of the charging electronics. Figure 1 shows a typical circuit of this kind.
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Figure 1: Schematic diagram of a charge monitoring circuit with NTC thermistors Power semiconductors, logic components, microcontrollers, and processors must be protected from excessive temperatures to ensure their reliable operation. Thanks to their compact size (e.g. EIA 0402), the new SMD NTC thermistors can be integrated directly in close proximity to microcontrollers and other hot spots on the circuit board. The good thermal contact to the circuit board via the solder connections with simultaneously negligible self-heating assures highly accurate thermal monitoring of sensitive semiconductors. Thanks to the high resilience of the EPCOS SMD NTC thermistors against thermal shock, they are suitable not only for reflow solder processes but also for wave soldering. Designers can consequently place the thermistors on the underside of the board, for instance, opposite microcontrollers, thus producing a very good thermal contact, even for microcontrollers with large dimensions. Figure 2 shows a typical protective circuit for microcontrollers. |
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Figure 2: Thermal monitoring of microcontrollers In LED lighting systems, SMD NTC thermistors assure a great luminous efficiency together with a long operating life. The efficiency of LED lighting depends very much on the temperature of the semiconductor junctions. Temperature extremes must be avoided, as these lead to faster power degradation, reduced intensity, and color shifts, as well as a significantly shortened lifetime. In the worst case, they can even lead to destruction. Temperatures that are too low reduce the luminous efficiency and thus the lumen per volume ratio. In order to achieve maximum efficiency, the temperature must be maintained at the specified optimum; in typical LED applications between 70°C and 90°C. If an SMD 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. The power loss of the LED consequently drops, thus extending its lifetime. Figure 3 shows a corresponding circuit. A sample kit with EPCOS SMD NTC thermistors is available specifically for developers of LED lighting systems. |
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Figure 3: Thermal monitoring of LED lighting In addition to the standard series, an automotive series, which is qualified to AEC-Q200 and is suitable for applications to +150°C, is available. The new NTC thermistors can be used in automotive electronics applications such as ECUs, air-conditioning systems, and the temperature monitoring of batteries or charging systems. |