Evaluation of Power Semiconductors Temperature

The temperature of power semiconductor components is a physical quantity that affects the reliability and correct operation of static power converters. Its consideration is necessary to meet expectations in terms of performance, reliability and lifetime. Indeed, the current operating rating of a power component in its environment is linked to the temperature reached by the active part in relation to the conditions of use and the performance of its cooling system. If the cooling system is not sufficiently efficient, the current rating specified by the manufacturer cannot be achieved, as the component's temperature will exceed its maximum permissible value. The temperature of components also varies during operation. Apart from the effects induced by damage mechanisms due to ageing, these variations are due to several phenomena of environmental or operational origin with distinct time scales:

• variations in ambient temperature (day/night cycles, summer/winter, high and low altitudes, etc.) ;

• variation in power dissipation linked to the mission profile (acceleration and deceleration phases, for example, in the case of transport);

• variation in power dissipation over a period of electrical operation. This is the case, for example, in inverters, where the integrated components undergo periodic variations in temperature, the amplitude of which depends on the system's operating point;

• variation in power dissipation and its distribution in the internal structure of the component over a switching period.

Each of these variations generates electrothermal and thermomechanical stresses that can lead to premature aging of the heterogeneous materials that make up a power module. Knowing or estimating the temperature variations linked to an application and to technological choices, for example, makes it possible to estimate indicators that reflect the integrity of the power semiconductor module, as well as to quantify its performance.

In this article, we will first define the notion of junction temperature of the active part, and then explain why it is necessary to develop experimental methods to measure it. Secondly, we present the main methods for measuring junction temperature, particularly for characterizing the thermal performance of power modules. Finally, we will present existing solutions and those under development for measuring junction temperature under converter operating conditions.