On integrity assessment of IGBT-based power stacks used in magnet power supplies for particle accelerators

Doctoral thesis, 2019

This thesis analyses an electrical testing method for assessing the integrity of an IGBT-based power stack assembly during factory acceptance tests and service stops. The method combines vce measurements with high current in the Zero Temperature Coefficient (ZTC) operating region and with low sensing current within a specific current cycle using a proposed sampling and filtering technique. Two circuits are presented for the vce measurement. The value of this method is the ability to validate the power stack assembly and to detect IGBT aging without the need for power stack modifications for the vce measurement with sensing and load current. Additionally, no dedicated current control of the IGBTs is required. The aging mechanisms that are targeted with this method are the bond-wire lift-off and the solder delamination. As a part of the method, an on-the-stack vce calibration technique at the sensing current level is proposed for the IGBTs avoiding the need to un-mount and characterize them in a thermal chamber. The reference application is a power electronic converter that is used as a magnet power supply in particle accelerators at CERN, the European Organization for Nuclear Research. Based on the specialized application, the levels of ambient air, junction and cooling water temperature change that could have an impact on the method’s precision are defined. Experimental results, which are obtained with the power stack of a power magnet supply, are presented and are compared favorably with results obtained using Finite Element Method (FEM) and Lumped Parameter Network (LPN) simulations to demonstrate the method’s applicability. For the high-current IGBT module of the application, it is shown that the measurement in the ZTC operating region could detect bond-wire lift-offs when more than half of the bond-wires of the chip have been lifted. A measurement in the Positive Temperature Coefficient (PTC) operating region can be used to detect the early stage aging of a bond-wire lift-off, but the measurement precision is, highly, influenced by temperature. Moreover, the detection of manufacturing issues, such as errors in thermal paste application, is proven to be possible with the help of vce measurements with sensing current.

As a potential improvement of the future power stack designs for lifetime prolongation, this work investigates the possibility for IGBT module’s thermal stressing mitigation using the specialized application as a reference. This investigation is based on LPN simulations. Prior to the LPN modeling, the extensive operation of the magnet power supply in the Negative Temperature Coefficient (NTC) operation region is, experimentally, examined. It is demonstrated that the current and thermal stressing unbalances among the chips inside the Soft Punch Through (SPT) IGBT module operating in the NTC operating region can be neglected and do not have to be considered for the thermal modeling. Moreover, the impact of the material, of the thickness and of the heat convection of the cooling plate on the junction temperature variation and maximum junction temperature is evaluated. It can be stated that, for long current cycles of the specialized application, a relatively thick aluminum cooling plate (3cm) with a moderate heat convection coefficient (10kW/(°Cm2)) may exhibit almost the same performance as a copper cooling plate of equal or even greater thickness (5cm) with a high heat convection coefficient (10kW/(°Cm2)).

Two strategies are proposed with the switching frequency and the gate resistance as parameters for online thermal stressing mitigation. The first strategy reduces the switching frequency in parts of the cycle where a high precision requirement for the output current is not imposed, in order to limit the power losses and the thermal stressing of the IGBT. The second strategy combines the switching frequency reduction in one part of the cycle with the increase of switching frequency and gate resistance in another. By increasing the power losses the junction temperature fluctuation can be limited. Using four typical current profiles from the specialized application, it is shown that both strategies could prolong the IGBTs’ lifetime. It is shown that the contribution of the mitigation strategy to the lifetime prolongation depends on the current profile.