The increasing demand for motor drives with higher power levels is also driving the demand for power modules providing higher currents. The conventional approach to fulfill this requirement is to look for dedicated, high-current power modules. This article describes the alternative approach of paralleling IGBTs and diodes within one power module to extend its power capability, for example, by using a 35 A “sixpack” module as a 100 A half bridge.
The conclusion is that this approach provides an advantage due to the improved thermal behavior of several small chips rather than fewer big ones. The breakthrough in performance is seen when real-life data of parameter variations within one power module are considered, instead of the datasheet values, which suggest a much higher spread than actually seen in real life.
Figure 1 shows how a sixpack can be used as a half bridge.
Figure 1: Sixpack used as a half bridge
The following calculations are based on the P700-F sixpack module from Vincotech, which uses Infineon IGBT3 Low Loss IGBTs and Emcon HE FREDs. Both components feature a positive temperature coefficient for their voltage drop at high junction temperatures. This is important for avoiding thermal runaway of individual components when they are paralleled.
When paralleling IGBTs, special attention has to be given to the drive circuit. Because of the variation of the gate threshold voltage of the different chips, simply connecting the gates is not adequate.
Instead, each gate has to be driven by its own gate resistor and therefore it own current source in order to ensure that the chip with the lowest threshold voltage does not clamp the voltage for the others and carry all the current.
Furthermore, the layout of the emitter circuit has to be very symmetrical in order to minimize differences in emitter inductances and resistances. Even minor, unavoidable differences in the emitter inductances and resistances will generate compensation currents between