Tutorials - Technical principles Rth

The Ferraz Shawmut Thermal Management concept is driven by first developing a clear model of the thermal cooling chain between the component and the cooling system.
The main parameter of the model is to maintain an acceptable junction temperature.

A simple representation of the thermal cooling chain is presented to the right. This diagram offers a simple view of the temperature change in the cooling fluid from inlet to outlet. Additionally referenced are the respective thermal resistances of the Components (Rjc), Contact surface (Rc), and the Cooling System.

This theoretical representation works well with power semiconductors for which the power dissipated by Joule effect is mostly removed by conduction towards the contact surface between case and support.

Therefore:
Tj-Tc = Rjc.P , Tc-Tp = Rc.P, Tp-Ts = Rh.P

The thermal protection of the electronic system is defined by several different parameters of the cooling system.

The following must be taken into consideration:

• Electronic Component (Rthjc) and Thermal Losses
• Mounting Type (leads to Rc)
• Required reliability (function of Tj)
• Environment (i.e. Ambient Temperature)

Finally, the cooling device must have a smaller thermal resistance as defined by: