IGBT Modules - Technologies, Driver and Application (Second Edition) - page 243

The capacity of the bootstrap capacitor C
must be sufficiently large to meet
the energy requirement of theupper driver for a complete cycle.
The voltage of the bootstrap capacitor must not fall below a minimum lower
value, as otherwise an under-voltage lock-out (UVLO) trip occurs (chapter
High peak currents might flow especially during the initial charging of the
bootstrap capacitor, which may lead to interference with other circuits. It is
therefore advisable to limit the current with a low-resistance series resistor R
On the one hand the bootstrap diode must be fast, since it works at the same
frequency as the switching of the IGBT. On the other hand, it must have
sufficient blocking capability, at least equal to the blocking voltage of the IGBT.
That means, with 600V IGBTs also 600V bootstrap diodes have to be selected.
In the selection of the diode, next to the voltage rating, sufficiently large
clearance and creepage distances are to be observed, which are determined
by the diodepackage.
When determining the supply voltage, the internal voltage drops in the driver,
across the diode D
and the resistor R
have to be allowed for, but also a
possibly existing booster stage, so that the resulting positive gate voltage on
the IGBT does not turn out to be too low and thus the conduction losses
increase (due to an increased U
voltage). Further still, the saturation
voltage of the bottom IGBT T
has to be subtracted of the resulting voltage.
Since the same voltageU
is used for both the bottom driver as well as the
upper driver, however, thementioned part-voltages have tobe deducted for the
upper driver, different positive switching voltages for IGBTs T
and T
result. The voltage U
is thus to be set that T
is supplied with sufficient
switching voltage, but at the same time the voltage at T
does not become too
When selecting the bootstrap capacitor, a type with low ESR
and ESL
values is preferable (e.g. ceramic capacitors) to provide pulse currents for the
driver. Capacitors with higher capacity (e.g. electrolytic capacitors) can be
connected in parallel to this type depending on requirements and application.
These often have high ESR and ESL values compared to ceramic capacitors.
In general, this requirement applies also to the buffer capacitor C
of the lower
It is not common practice to provide a negative voltagewith a bootstrap circuit,
so one has to pay attention to possible parasitic turn-on of the IGBT (theMiller
clamp can be used as ameasure against parasitic turn-on, which is described
further below).
ESR stands for "Equivalent Series Resistance" and refers to the parasitic series resistance of each capacitor.
Here, the resistive component of the internal conductors and the dielectric change-of-charge losses are added
together. A voltage will drop across this series resistance proportional to the current flowing, which will reduces
the effective external voltage of the capacitor under load. Especially pulse loads with high peak currents may
causeexcessive voltage drops given a largeESR .
ESL stands for "Equivalent Series Inductance" and refers to the parasitic series inductance of each capacitor.
This causes an internal voltage drop with every current change that reduces the effective external voltage of the
capacitor under load.
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