IGBT Modules - Technologies, Driver and Application (Second Edition) - page 351

Fig. 8.12
Charging and discharging circuit in indirect gate drive Galvanically isolatedgatedrive inparallel connection
The IGBTsmust turn on and off as simultaneously as possible. This is critical to current
balancing in parallel connection. While one control signal is transmitted in direct and
indirect gate drive in parallel connection, when the control for each IGBT connected in
parallel is isolated galvanically, one galvanically isolated signal is transmitted to each
IGBT. The differences in transition time that occur in this process are crucial for parallel
Transition time tolerances (jitter) can be caused by temperature, or by aging of the
insulation or component tolerances. They are not usually critical in direct and indirect
gate drive, as the logic and signal transmission constitute a central unit. This is quite
different for the galvanically isolated gate drive. Each IGBT has its own galvanically
isolated control circuit. This type of control has many advantages, but only if the
transition time tolerances are minimal. Conventional optocouplers are therefore
unsuitable for this type of application (chapter
because they are subject to
significant fluctuations in transition time of up to several 100ns. Fibre optic cables
are also unsuitable, due to their transition time tolerances. Instead,
inductive transducers have proved to be the best solution, as they are not subject to the
aging process and their transition time tolerances are very small. Two examples are
Infineon Technologies' Coreless Transformer IC and CONCEPT SCALE-2 integrated
solutions. All solutions transmit signalswith very small tolerances.
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