Solve the temperature problem for you (2) System temperature monitoring

Solve the temperature problem for you (2) System temperature monitoring

In the previous article, we have covered the basic principles of temperature sensing. This article will continue this topic and address the topic of system temperature monitoring.

For many system designs, it is necessary to monitor high power components (processors, field programmable gate arrays, field effect transistors) to ensure system and user safety. The accuracy of temperature readings is important because it enables designers to improve performance while staying within safe limits, or to reduce system cost by avoiding overdesign elsewhere. Texas Instruments’ family of compact, high-accuracy temperature sensors can be placed closer to these critical components for the most accurate measurements.

Solve the temperature problem for you (2) System temperature monitoring

How to Monitor Board Temperature

Temperature issues in circuits can affect system performance and damage expensive components. Measuring the temperature of areas of a printed circuit board (PCB) where hot spots or power-hungry integrated circuits (ICs) exist can help identify thermal problems and allow for timely preventive or corrective action.

You may want to monitor the die temperature of power-hungry ICs (such as central processing units, application-specific ICs, field programmable gate arrays, or digital signal processors) to dynamically adjust their performance, or you may want to monitor hot spots around the power stage to Control fan speed in your system or initiate a safe system shutdown procedure. The ultimate goal is to optimize performance and protect expensive equipment.

Heat transfer from PCB to temperature sensor

Local temperature sensors measure their own die temperature to determine the temperature of a specific area. Therefore, it is critical to understand the main temperature conduction paths between the die and the object or environment surrounding the sensor. There are two main paths for heat transfer: through the die attach pads (DAP) connected to the package or through the package lead pins. The DAP, if present, provides the most significant thermal path between the PCB and the die.

Solve the temperature problem for you (2) System temperature monitoring

Package with DAP

If the package type does not contain DAP, the leads and pins provide the most significant thermal path.

Solve the temperature problem for you (2) System temperature monitoring

Package without DAP

The molding compound provides an additional thermal path, but due to its low thermal conductivity, any heat transfer through the molding compound itself is slower than through the leads or DAP.

Thermal response

The package type determines how quickly the temperature sensor responds to temperature changes. The graph below shows the relative thermal response rates for different classes of selected surface mount technology package types for temperature measurement.

Solve the temperature problem for you (2) System temperature monitoring

Packages without molding compound (Chip Scale, Die Scale Ball Grid Array) and packages with DAP (Quad Flat No Leads) [QFN] Packaged, Double Flat No Leads [DFN] package) are designed for applications that require fast heat transfer from the PCB, while packages without DAP are designed for applications that require slower response rates. The fast thermal response rate enables the temperature sensor to respond quickly to any temperature change, providing accurate readings.

Design Guidelines – Bottom Mount

The sensor location should be as close as possible to the heat source to be monitored. Holes or cutouts in the PCB between the heat-generating IC and the temperature sensor should be avoided as this may slow or prevent the thermal response. If possible, mount the temperature monitor on the bottom of the PCB directly below the heat source.

Solve the temperature problem for you (2) System temperature monitoring

The sensor is mounted on the other side of the heat source

TI recommends using vias to quickly transfer heat from one side of the PCB to the other because vias have better thermal conductivity of copper compared to FR-4. Use as many parallel vias or filled conduction vias as possible to transfer heat from the heat source to the temperature monitor for fast thermal equilibration between the two ICs. A QFN or DFN package with DAP helps to further shorten the thermal resistance path between the via and the sensor die.

Design Guidelines – Formation Considerations

If placing the temperature sensor on the other side of the heat source is impractical or not cost-effective, place it on the same side as close to the heat source as possible.

Solve the temperature problem for you (2) System temperature monitoring

Shared formation helps achieve thermal balance

The most efficient way to establish thermal balance between the heat source and the temperature monitor is to use the formation. A solid formation should be used that extends from the heat source to the temperature sensor.

in conclusion

Temperature monitoring is critical in PCB designs with thermoelectric regions or power-hungry ICs. The selection of local temperature sensors must be evaluated for compliance with the system requirements and protection schemes of the relevant design.

The sensor location and high thermal conductivity paths should be considered to establish a fast thermal equilibrium between the sensor and the heating element.

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