In the calculation of the capacitor a sufficiently large factor S should be included. This
should be designed so that, when switched on, the voltage drops by less than 5%.
Values of S> 10arenot uncommon.
6.4.2 DC/DC converters
The DC/DC converter generates the isolated supply voltage for the IGBT gate drive.
Mostly used are push-pull or flyback converters. The transmitted power is relatively
small. Typically this is less than 10W, and is dependent on the switching frequency of
power electronic components as well as the gate voltage and gate charge. In special
cases, such as drive units with high switching frequency and high IGBT currents and/or
parallel connection of IGBT modules, a gate power of up to 60W may be necessary.
The gate power and therefore requiredDC/DC converter power is calculated by:
U )U(Q f
G sw GE
∆⋅ ∆ ⋅
U): Gate chargeof the IGBT to be controlled as a function of
If the gate charge Q
is not completely listed (e.g. some manufacturers of IGBT
modules only list Q
for 15V) this can easily be determined by
dt )t( I
: Gate current [A]
For this measurement
U should match the gate voltage of the subsequent gate drive.
The current is integrated until the voltage reaches its final value.
Measurement of thegate charge at IGBT turn-on
Should an external gate-emitter capacitor C
be used in the IGBT gate
drive itmust be included in the calculation of thegatepower: