Short Circuit Current Rating

Infineon's latest IGBT technology switches in silence

The device features an improved softness during switch-off and a lower overshoot voltage as the result of a reduced turn-off current slope di/dt. It was designed especially for medium- and high-current applications. In comparison with the 600 V IGBT3, the new chip offers a better softness during switch-off and higher blocking voltage capability. Moreover, the short circuit robustness is significantly improved. By contrast, the IGBT3 has been optimised for lower-power applications, or higher power in very low stray inductance applications. utilises a trench MOS-top-cell, thin wafer technology and a field-stop concept as seen in Figure 1. The combination of trench cell and field-stop enables comparatively low on-state and turn-off losses. Compared to the IGBT3, the chip thickness was increased by about 15% and the width of the MOS channel was decreased by about 20%. Thereby, the softness during switch-off is improved to reduce the EMI effort. However, these measures obviously also cause additional losses. So in order to compensate for these side effects, the efficiency of the backside emitter was increased by 50%. In addition to the optimisation of the dynamic behaviour, the blocking voltage was increased by 50 V to 650 V. . A consequence of this inherent IGBT behaviour would be in such a case of a special driver stage with integrated IGBT protection functionalities and/or additional components like snubber capacitors. All these functionalities and components create design effort and cost. High current levels need, due to the high di/dt level, a DC link design with very small parasitic inductances. As an alternative, specially designed IGBTs with a soft switching characteristic like the IGBT4, can be utilised. Not only the turn off characteristics, but also the softness of the IGBT is quite insensitive to the gate resistance. The softness during switch-off is improved to reduce the EMI effort. In Figure 3 the Fourier transformation spectra of a soft and a not soft turn-off waveform are given. The oscillation leads to a five times higher level around the oscillation frequency of roughly f = 20–25 MHz, a frequency which is quite typical for chip DC link oscillations at the given parasitic inductance. Even though such a procedure is not able to predict passing or failing of an EMI qualification, it obviously demonstrates the sensitivity of EMI to snap-off phenomena.

Basic source/load relationships

Previous Power systems engineering - fundamental concepts

Fault level and circuit-breaker ratings

The fault level (sometimes called short-circuit level) is a term used to describe the 'strength' of a power supply: that is, its ability to provide both current and voltage. It is defined as

Fault level = Open-circuit voltage × Short-circuit current [VA/phase]

The fault level provides a single number that can be used to select the size of circuit-breaker needed at a particular point in a power system. Circuit-breakers must interrupt fault currents (i.e. the current that flows if there is a short-circuit fault). When the contacts of the circuit-breaker are separating, there is an arc which must be extinguished (for example, by a blast of compressed air). The difficulty of extinguishing the arc depends on both the current and the system voltage. So it is convenient to take the product of these as a measure of the size or 'power' of the circuit-breaker that is needed. The fault level is used for this. The rating of a circuit-breaker should always exceed the fault level at the point where the circuit-breaker is connected –otherwise the circuit-breaker might not be capable of interrupting the fault current. This would be very dangerous: high-voltage circuit-breakers are often the final means of protection, and if they fail to isolate faults the damage can be extreme – it world like having a lightning strike that did not switch itself off.

Fig. 2.1 Thevenin equivalent circuit of one phase of a supply system (neglecting resistance).

Thevenin equivalent circuit model of a power system

At any point where a load is connected to a power system, the power system can be represented by a Thevenin equivalent circuit ( The Thevenin equivalent circuit is a series equivalent circuit, in which the source is a voltage source and it is in series with the internal impedance. In the Notion equivalent circuit, the source is a current source in parallel with the internal impedance l is directly in phase with both E and V. You might ask, 'what is the use of a load that draws no power?' One example is that shunt reactors are often used to limit the voltage on transmission and distribution systems, especially in locations remote from tap-changing transformers or generating stations. Because of the shunt capacitance of the line, the voltage tends to rise when the load is light (e.g. at night). By connecting an inductive load (shunt reactor), the voltage can be brought down to its correct value. Since the reactor is not drawing any real power (but only reactive power), there is no energy cost apart from a small amount due to losses in the windings and core.