On-line [TJ, Vce] monitoring of IGBTs stressed by fast power cycling tests

This paper describes a part of a larger supervision system able to monitor the on-state voltage V_CE and the junction temperature T_J of IGBT in operation. That system is associated to an ageing test bench stressing IGBT modules by power cycling. All along the ageing test, it is necessary to supervise V_CE, always measured in the same conditions of junction temperature and collector current, in order to detect possible degradations of wire bonds and/or emitter metallization. In addition, the thermal swing amplitude of the power cycling must be adjusted to realize a given ageing protocol. That requires measuring the junction temperature evolution on a power cycle to choose the initial electrical conditions providing the wished temperature swing and then, to regularly verify the stability of this thermal stress during the ageing test. The temperature measurement needed for both monitoring is carried out by means of V_CE measurement at low current level (100mA), that intrinsic on-stage voltage being a well-known thermo sensitive parameter. The first section describes briefly the ageing test bench, that places the power IGBT modules in operating conditions close to those of real world (PWM operations), and presents the thermal stress protocol applied to the devices, the aim being to define the context in which the measurements have to be made. The second section presents, on the one hand, the principle of an automated measurement of V_CE(100A-125°C), made in steady-state, to detect a possible degradation of the top part of IGBT dies, on the other hand, the dynamic measurement of the junction temperature in operation, i.e. in power cycling conditions generated by the PWM modulation. In both cases, experimental results are shown that demonstrate the feasibility and the good accuracy of these monitoring methods.