Evaluating heat in vehicles' instrument panels

The design of instrument panels must optimise quality, reduce costs and lead times, and guarantee flawless product launches. An important aspect of these requirement is thermal integrity.

This article provides an example where an engineer at Visteon [1] used computer fluid dynamics (CFD) simulation during PCB design to investigate the thermal behaviour of the instrument cluster. The numerical results from the PCB layout thermal simulation were compared with experimental results using thermography before full enclosure modelling [2].

A typical instrumentation cluster consists of two analogue gauges on either side of the unit with several LEDs, a significant number of which are bright LEDs. Each system has a thin film transistor (TFT) display (blue section in figure 1), which can only operate 10°C above the maximum ambient air temperature.

The TFT display temperature must be tightly controlled. The cluster is encased in a housing in the dashboard with limited air openings that inhibit ventilation.

Figure 1: Example of a model of an automotive instrumentation cluster. Image courtesy of Visteon.

When designing an instrument cluster, overcoming the temperature sensitivities of most of the components in the assembly is important. For instance, the LED light and colour will degrade if the junction temperature becomes too hot for long periods.

Another important factor to consider is the higher power dissipation of components, such as linear power supplies, reverse protection diodes, and microprocessors.

The automotive requirements dictate that the instrument cluster remain at an ambient air temperature of 85°C, and space is limited for the components to operate below the temperature limit specified by manufacturers.

The instrument cluster is housed in the dashboard, and the primary route for the heat to exit the system is by conduction through the PCB. Working to optimise the heat spread through the PCB will make it an efficient heatsink before other cooling strategies are considered.

This is made possible by changing the component layout as well as adjusting the copper content of the PCB. When possible, holes are placed on the back of the housing to vent the cluster.

Finally, because of the temperature sensitivity of the TFT display, its brightness may be diminished at high ambient temperatures.

This parameter is something that JCI and their customers work closely on together so that the TFT display is expected to operate at 100% power up to a fixed ambient temperature. Once this is reached, the TFT display is then dimmed to preserve its integrity and maximise its performance.

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