IGBT Switching Test

      This article is about the switching test of MITSUBISHI CM1200HC-66H (1200A - 3300 V) IGBT, which is used in an H-Bridge (Single Phase Inverter). Actually 1 H-Bridge is a component of 3 phase multi-stage inverter based STATCOM application. For each phase 5 H-Bridge modules are cascaded to form multi-level operation. 

      The procedure for IGBT switching test is simply connecting an RL load to the H-Bridge to be tested and applying the firing procedure. Actually, firing procedure in my application is as follows;

     - First fire the IGBT's in cross position at the same time.

     - Then Turn-Off 1 IGBT so that the load freewheels using the anti parallel diode path.

     - Then Turn-On the same IGBT (previously Turned-Off).

     - After a short while Turn-Off both of them.

      With this test procedure, the Turn-On and Turn-Off graphs of IGBT are obtained. 

Fig 1 - H-Bridge and the Test Setup

Fig 2 - Test Setup Circuit Diagram

System Specifications ; 

DC Link Voltage : 1200 V

Load Resistance : 1 Ohm ( 1200 A operation)

Gate Driver Turn OFF Resistance : 2.88 Ohm

Gate Driver Turn ON Resistance  : 1.52 Ohm

    Switching Results 

     The most important part of the result which is needed to draw attention is Collector-Emitter Voltage overshoot in Turn-OFF graph. The overshoot value gives a strongly related opinion about busbar inductance. Busbar inductance (together with inductance of DC link capacitor) and Vce overshoot are directly related. Thus for a good inverter design, semiconductor to DC link current path must be as short as possible and this path should be a busbar instead of ordinary power cable, coaxial cable or rectangular bar. This argument is also tested and it is observed that the IGBT's located far from the DC link capacitor create more overshoot than the ones in the near side. In this application, sum of busbar inductance and DC link capacitor leakage inductance is calculated to be 150 nH. 

       The Turn-ON and Turn-OFF graphs is as below:

Fig 3 - Turn - ON instant of IGBT

Fig 4 - Turn - OFF instant of IGBT

       Red Curve : Collector - Emitter Voltage

       Green Curve : Emitter Current

       Yellow Curve : DC Link Voltage

   What happens if collector terminal of the gate driver is accidentially forgotten to be screwed 

   During my test I recognized that when I fire the IGBT, the IGBT gate driver never turn-ON the device and resets the health feedback of IGBT (gives an error). After few discussions, I found out that 2 of the screws that connecting gate driver to igbt collector is forgotten. The error signal is due to the fact that gate driver measures the collector emitter voltage different than Vce,sat value (since collector is floating) , although turn-ON signal is sended by control system. For this reason, the gate driver suspects the short-circuit of semiconductor, never turn-ON the device and sends a error message.