Solve the temperature problem for you (4) Ambient temperature monitoring

In the first three articles, we’ve covered the basics of temperature sensors and how to monitor board temperature. In many applications, ambient air temperature monitoring is critical to controlling environmental conditions or ensuring safe operating conditions. Measuring ambient temperature accurately and quickly is often challenging because the sensor may not be fully exposed to the external environment and may be affected by self-heating from other components in the system. TI’s high-accuracy, low-power single- and multi-channel temperature sensors are available in compact packages for faster thermal response.

Layout Considerations for Accurate Measurement of Ambient Temperature

Measuring ambient temperature with surface mount devices can be challenging because heat transfer from other power-hungry Electronic components can affect the sensor’s temperature reading.

Accurate measurement of ambient temperature requires good layout practices such as understanding the major thermal paths, isolating the sensor package, and placing the device away from interfering heat sources. The diagram below shows a simple thermostat design using these methods.

Solve the temperature problem for you (4) Ambient temperature monitoring

Temperature Sensor Thermostat Design Thermal Radiation and Printed Circuit Board (PCB) Layout

In the image above, the passive airflow created by the self-heating of the system draws outside air over temperature sensor A. The sensor is placed at the air intake away from the main heat source (central processing unit) and is insulated to ensure more accurate measurements.

Heat Radiation and PCB Layout

It is important to first understand which components radiate the most heat to avoid routing near heat sources. Below is a thermal image captured using Mentor Graphics’ FloTHERM thermal analysis tool, showing the temperature distribution in the air near the heat source.

Solve the temperature problem for you (4) Ambient temperature monitoring

Heat radiated on the test board

If the components are placed inside the enclosure, the heat distribution may be more concentrated. Remember to place temperature sensors away from heat sources to avoid false temperature readings in open air scenes and inside enclosures. The table below lists the recommended distances between the sensor and the heat source for various heat source temperatures.

Solve the temperature problem for you (4) Ambient temperature monitoring

If the sensor is close to the heat source, it is best to create an isolation island and maximize the air gap between the sensor and the heat source. The larger the air gap, the better the ambient temperature measurement. However, the gap does not provide additional shielding when the sensors are further apart. However, the gap can improve the thermal response time of the sensor.

Solve the temperature problem for you (4) Ambient temperature monitoring

Heat dissipation air gap when groove width is 0.8mm

The top graph shows a temperature reading of approximately 38.5°C when the cut is 0.8mm wide, while the bottom graph shows a temperature reading of approximately 35.5°C when the cut is 1.8mm wide. These images show how larger isolation gaps affect ambient temperature readings.

Solve the temperature problem for you (4) Ambient temperature monitoring

Thermal Air Gap with 1.8mm Slot Width

Solve the temperature problem for you (4) Ambient temperature monitoring

Isolation Island PCB Layout

When designing a PCB for a temperature sensor, it is important to use good layout practices. The image above shows a PCB layout with isolation islands and outline routing, while the image below shows an alternative design with perforations around the area where the temperature sensor is mounted.

Perforated PCB Layout

On these two small boards, the size is so small that only sensors and bypass capacitors can be deployed; the smaller the thermal mass of the isolation island, the better the thermal response. These designs greatly reduce the amount of heat transfer from other components.

temperature difference

In applications that require higher measurement accuracy, consider using a differential temperature design. This type of design adds additional sensors next to the high temperature components, however, this design requires a model of the correlation between ΔT and ambient temperature, and this model will vary depending on the system application. The differential temperature design takes into account the effects of self-heating, providing a more accurate algorithm for estimating ambient temperature.

Device Recommendations

The TMP112 and TMP116 are digital temperature sensors designed for high precision, low power applications such as environmental monitoring and thermostatic control. The TMP112 has an accuracy of ±0.5°C from 0°C to 65°C, while the TMP116 has an accuracy of ±0.2°C from -10°C to 85°C.

Both temperature sensors are highly linear, require no calibration, and feature programmable alarms. The TMP112 is available in a compact 1.60mm x 1.20mm Small Outline Transistor (SOT)-563 package, while the TMP116 is available in a 2mm x 2mm Ultra Thin Small Outline No-Lead (WSON) package.

The Links:   MG600Q1US41 6MBP50RA060

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