© by SEMIKRON
/ 2017-09-07 /
Technical Explanation
/ SKiiP
®
4
Page 65/73
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On the other hand a different root mean square value of the output current should be avoided.
Symmetric effects come from the inductivities (AC choke) and the Ohmic resistors (choke resistor,
wiring resistor and path resistance) as shown in Figure 7.19.
Figure 7.19: Schematic Circuit Diagram
Tolerances of forward voltage drop of IGBT or diodes
The IGBTs used in the SKiiPs have a positive temperature coefficient. The freewheeling diodes are
produced with a small forward voltage tolerance range. Both features limit an inhomogeneous current
distribution. No further selection of forward voltage selection is necessary.
Different DC link voltage levels
To avoid different output voltages caused by different DC link voltages levels, the DC link should be
connected in parallel. Furthermore oscillations between such interconnected capacitor banks must not
occur. For large systems fuses between the capacitor banks are recommended. The paralleled systems
should have the same DC link with the same capacitor type and capacitor values.
Different cooling conditions of paralleled half bridges
The cooling system of the paralleled SKiiP units should be designed in way to avoid thermal stacking.
In spite of all the mentioned actions it has to be taken into account that for remaining imbalanced current
sharing a derating of the nominal current of the power stage has to be considered.
Different production lot
In general besides all the before mentioned constraints it is not recommendable although not impossible to
parallel SKiiP4 of different production lots. Typically the relevant parameters for an advantageous
paralleling differ less when SKiiPs of one production lot are utilized in parallel.
Provisions for minimum switching pulse and conduction time
In a parallel connection of SKiiP4 it is essential to follow the specified minimum durations for on and off
time to ensure error free operation. For further details please refer to chapter 0.
7.12
Prevention of condensation
A condensation of humidity in the environment of the SKiiP must be prevented. This can be ensured by
control of air or water flow so that the heat sink temperature is always higher than ambient temperature.
Hence, the use of a chilled coolant is therefore not recommended. Condensation may also occur because of
a day/night temperature change. If the power electronic equipment was exposed to humidity or moisture
during transport and storage, it should be dried before commissioning. This can be done by air heater in air
cooled systems or by pre-heated water in water cooled systems before connecting the operation voltage to
the SKiiP.
Z
1
I
out1
Inverter 1
Inverter 2
Z
2
Load
I
out2
I
out
t
I
out
I
out1
I
out2