Thermal Model of a PCB

I want to simulate the thermal effects of MOSFETs power losses. I already got thermal models from the manufacturer for Junction-to-Case and Junction-to-Ambient (but mounted on a 1 sq. inch PCB area). As I will not need the 1 sq. inch I need a model from Case (thermal pad) to ambient by using a smaller PCB area as the only heatsink. I need your opinion on my approach.

Parameters:

• 2 layer 35 µm copper PCB
• FR4 with a total PCB thickness of 1.6 mm

When the FET is mounted on the PCB there are 4 path for heat dissipation:

• From top side of the case to ambient --> since the FET manufacturer doesn't provide information about this path, there is no way to consider it? But anyway, the thermal pad will be the main heat conductor so this hopefully has no big influence on the result
• from thermal pad to top side copper and from there to ambient
• from thermal pad to top copper, to FR4 substrate, to bottom copper and from there to ambient
• from thermal pad to top copper, to several thermal vias (Rth,via in parallel), to bottom copper and from there to ambient

The overall thermal resistance would be a parallel circuit of the three paths. Each path is the sum of its components.

If I calculate each thermal capacitance in the same manner, will I get a realistic thermal model of the case to ambient thermal resistance of a PCB? I guess the result will be a Cauer model or is it a forster model?

In Simscapes ideal N-MOSFET Modell the thermal capacitance of the junction must be specified if an external thermal network is selected. Can I use the value of CF1 from the above linked thermal Model from Vishay for that?

Many thanks in advance!

Answer 1

[SANKALP DEV]

Hello Kai,

I understand that you want to accurately model the thermal behaviour of a MOSFET mounted on a PCB that is smaller than the 1 sq. inch, as referenced by the manufacturer.

The heat dissipation from top side is generally a less significant path than the thermal pad dissipation, but it should not be disregarded as it can contribute to overall cooling, particularly with airflow or in cooler environments.

Your approach to calculate overall thermal resistance as a parallel circuit for the main paths is accurate, and calculating thermal capacitance similarly will provide a realistic transient thermal model.

The model is likely a “Cauer” thermal model, accounting for physical layout and heat flow through PCB layers.

You can use the CF1 value from the manufacturer's model as a starting point for the junction's thermal capacitance in your Simscape simulation.

To know more about “Cauer” thermal model and basic thermal blocks and modelling technique in Simscape, please refer to the following MATLAB documentations.

• Cauer thermal model: https://www.mathworks.com/help/sps/ref/cauerthermalmodel.html
• Basic thermal blocks and modelling techniques: https://www.mathworks.com/help/simscape/thermal-models.html

Hope this helps,

Regards,

Sankalp dev