Least Squares Method for Identification of IGBT Thermal Impedance Networks Using Direct Temperature Measurements

Abstract

State-of-the-art methods for determining thermal impedance networks for IGBT (Insulated Gate Bipolar Transistor) modules usually involves the establishment of the relationship between the measured transistor or diode voltage and temperature under homogenous temperature distribution across the IGBT module. The junction temperature is recomputed from the established voltage–temperature relationship and used in determining the thermal impedance network. This method requires accurate measurement of voltage drop across the transistors and diodes under specific designed heating and cooling profiles. Validation of the junction temperature is usually done using infrared camera or sensors placed close to the transistors or diodes (in some cases and open IGBT module) so that the measured temperature is as close to the junction as possible. In this paper, we propose an alternative method for determining the IGBT thermal impedance network using the principles of least squares. This method uses measured temperatures for defined heating and cooling cycles under different cooling conditions to determine the thermal impedance network. The results from the proposed method are compared with those obtained using state-of-the-art methods.

Measurements and Computations of Internal Temperatures of the IGBT and the Diode Situated in the Common Case

“…Properties of these devices strongly depend on their internal temperature T j [5][6][7]. This temperature exceeds ambient temperature T a due to thermal phenomena [5,8,9]. An increase in T j temperature causes shortening of lifetime of semiconductor devices and changes in values of their exploitation parameters [10][11][12].…”