The insulated gate bipolar transistor（IGBT for short）

the high voltage power element in MMC submodule

becomes gradually thermal aging at long-term operating conditions and eventually leads to failure of the element.Therefore

the study of status monitoring technology of IGBT is of great significance for reliable operation of MMC.The study on evolution law of conduction voltage drop of clamping type IGBT in the present literature is less.In this paper

a power cycle test platform for the clamping type IGBT is set up

the conduction voltage drop and gate current of the element at different thermal aging degree is monitored simultaneously during the power cycle test period.The linear relation between the junction temperature and the gate current

which is obtained by the test

is used to normalize the conduction voltage drop into the same junction temperature

thus removing the influence of junction temperature on the conduction voltage drop.It is shown by the result that conduction voltage drop shows the upward trend with the degree of thermal aging and the variation range is within approximately 10%.Finally

two aged and failed IGBTs are disassembled and it is found that the internal physical structure of the devices has changed and local ablation or even fusion happened on the chip surface is founded.