Hans-Guenter Eckel

IGBT and Diode Behavior During Short-Circuit Type 3

A short circuit during inverter operation can result in the so-called short-circuit types 2 and 3. The short-circuit type 3 results in an interaction with several current commutations between the Insulate-Gate Bipolar Transistor (IGBT) and the antiparallel diode. The switched ON IGBT, conducting no current before the short circuit occurs, has no plasma inside and can block any voltage. It behaves like a voltage-controlled current source, which can stress the diode by commutating the complete short-circuit current into the diode. An avalanche arises inside the diode, which can lead to a destruction of the diode. The current through the diode determines dv/dt and makes the diode a dominating semiconductor during the short circuit. The current source behavior of the IGBT can be verified with semiconductor simulations and measurements on a high-power 3.3-kV IGBT. In the simulations, the IGBT is replaced by a current source, which gives similar results. For the measurements, the gate-emitter voltage is varied to change the IGBT current without influencing the collector-emitter voltage.

Effect of the miller-capacitance during switching transients of IGBT and MOSFET

The feedback from the drain to the gate of MOSFET resp. from the collector to the gate of an IGBT during switching transients is described by the miller-capacitance. This is an appropriate approach for MOSFET, where a positive dvDG/dt leads to a positive current through the miller-capacitance. The aim of this paper is to explain, why during turn-off of a bipolar device like an IGBT, this is not always the case. A positive dvCG/dt may lead to a negative current through the miller-capacitance. For a better understanding of the switching behaviour of the IGBT, it is helpful to take the time derivative of the electrical field peak as a measure for the current through the miller-capacitance.

Turn-off behaviour of high voltage NPT- and FS-IGBT

A simple but physical based one dimensional model is used to characterize the turn-off of high voltage IGBTs. The dependence of the overvoltage and the peak electric field on the gate driving conditions is analyzed. The transition from a triangular to a trapezoidal electric field has a major impact on the turn-off behaviour. If this transition occurs during the voltage slope, the dv/dt increases significantly. If the field-stop layer is reached during the current slope, the current snaps off, which leads to a second voltage spike with a high absolute voltage but only a moderate peak field.

Short-circuit behavior of series-connected high-voltage IGBTs

In operation of series-connected IGBTs a voltage balancing is necessary, during short circuits it is even more important. Within this paper the behavior of series-connected IGBTs and diodes during the short-circuit type 2 and 3 is analyzed. The dv/dt during the short circuit is determined by one of two equivalent circuit capacitances. One is the plasma capacitance and the second one is the Miller capacitance. Depending on the gate-drive unit the dv/dt is intrinsic, when the gate current is sufficiently high, and the plasma capacitance determines the desaturation process during short circuit. Or the gate current controls via the Miller-capacitance feedback the dv/dt. The results are obtained with the help of measurement on 3.3 kV-IGBTs.

High-inductive short-circuit Type IV in multi-level converter protection schemes

To build a redundant medium-voltage converter, the semiconductors must be able to turn OFF different short circuits. The most challenging one is a hard turn OFF of a diode which is called short-circuit type IV. Without any protection measures this short circuit destroys the high-voltage diode. Therefore, a novel three-level converter with an increased short-circuit inductance is used. In this paper several short-circuit measurements on a 6.5 kV diode are presented which explain the effect of the protection measures. Moreover, the limits of the protection scheme are presented.

Short-circuit behavior of high-voltage IGBTs

A load fault or a semiconductor breakdown in a high-power inverter results in very challenging shorts for the remaining semiconductors. Understanding the short-circuit behavior improves the robustness of the semiconductor and the inverters. However, this important topic is not very well analyzed and only few literature exist. Up to now, five different short-circuit types are known which can occur in two or multilevel inverters. The five short-circuit types differ in the starting condition and the dominating semiconductor during the short. Despite the different initial conditions, the behavior of the high-voltage IGBT and diode during short-circuit can be explained with the help of a capacitive equivalent circuit. The resulting dv/dt, the current distribution between the semiconductors, and the influence of the gate-drive unit on the short-circuit behavior can be explained. To validate the results retrieved from the equivalent circuit, measurements on a high voltage IGBT and simulations with a semiconductor simulator are presented.

Interaction between IGBT, diode and parasitic inductances during short-circuit type 3

During short-circuit type 3 which occurs when the freewheeling diode conducts current and the anti-parallel IGBT is switched on the current commutates from the diode to the IGBT and vice versa. This commutation between the semiconductor includes parasitic inductances on the module substrate which are not optimized. During the short-circuit type 3 the avalanche of the diode, the collector-emitter voltage at the beginning of the short-circuit, the voltage stress of the semiconductor and the current mismatch between the IGBTs are influenced by the parasitic inductances. In this paper the inductance on substrate level is measured and the influence on the short-circuit behavior is presented with the help of measurements and simulations. The behavior of the IGBT during the short-circuit especially at the beginning is explained with the help of the parasitic inductances. Moreover an equivalent circuit for the substrate inductances between IGBT and diode is discussed.

High-voltage diode robustness during short-circuit type III

A harsh high-voltage diode commutation as a result of a short circuit can destroy the diode. The ruggedness of the diode is given by the cathode design which can suppress an cathode-side filament. Measurements on 3.3-kV and 6.5-kV diodes with and without improved cathode design show the diode behavior during the short. The results are compared and similarities are found. Without an improved cathode a failure in succession of a cathode-side filament can be observed. This behavior is simulated and confirms the destruction mechanism.

Implementation of a space vector modulation for an advanced ANPC three level inverter

In this paper an implementation of a modified three level space vector modulation on a microcontroller with a floating point unit is described. The space vector diagram is grouped to avoid commutations between both zero states and skipping of states which results in high inductive commutations. The calculation time for the space vector modulation is evaluated, therefore the execution time for different operations and functions is measured and compared with the times needed without any optimisations or floating point unit. Moreover a split of the source code is shown to simulate the inverter function with the help of Matlab/Simulink.