The paralleling behavior for
IGBTs deserves of an special attention that has to be given to the drive
circuit, this is due to the variation of the gate threshold voltage of the
different chips, simply connecting the gates is not adequate. As they are not
just a few of them but several, Instead, each gate has to be driven by
its own gate resistor 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.
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 the
gate drive emitter connections. It is recommended to use a resistor in the
range of at least 0.5 Ohm, but not to exceed approximately 1/3 of the total
gate resistance.
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