Jan Fuhrmann

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.

Current-direction detection for static MOS-control of the BIGT in the three-level neutral-point-clamped converter

The gate-emitter voltage VGE affects the on-state characteristics of the BIGT in diode forward-conduction mode. While for conventional IGBT with antiparallel free-wheeling diode VGE is at 15 V regardless of current direction, it has to be adapted for the BIGT. The gate-emitter voltage VGE has to be below threshold voltage, in order to operate at low static losses in diode forward-conduction mode. This paper proposes an optimised gate control based on the direction of the load current for a three-phase three-level converter.

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.

Non-destructive potential-free capacitive tap for low and mid voltage smart grid communication

A smart grid can be created with the help of additional glass-fiber wires or with power line communication. For power line communication (PLC) an ohmic contact is needed which results in harming the wire insulation and defining the potential. A capacitive tap can be installed without these disadvantages and allows retrofitting ever grid to a smart grid. A floating tap which reads the high frequency signal uses the capacitance between the wire and the isolated electrode. To prevent all following components from overvoltage conditions, a filter is realized including the capacity for a passive high-pass design. An active band-pass approach is advisable in the case of high frequency interferences. The proposed solution of a non-destructive and potential-free tap is experimentally evaluated.

Active rectifier with extended operating range

To overcome the drawbacks of diode rectifiers in applications without regenerative functionality active rectifiers can be used. Three level rectifiers are already known, but operating range is limited. A new four level rectifier topology that has an extended operating range and avoids all drawbacks of diode rectifiers is presented.