IGBT Modules - Technologies, Driver and Application (Second Edition) - page 250

6 I
In order to prevent the operation below the range of about 11...12V, many commercially
available drivers have low voltage detection, called "under-voltage lock-out" or just
UVLO. If the gate control supply voltage of the driver output stage or the DC/DC
converter falls below a certain point, the driver turns off the output stage. Preferably, the
driver should automatically produce a low impedance path to ground (which is equal to
the emitter potential of the IGBT), and thus ensure a reliable and steady turn-off of the
IGBT. In case the driver has an output stage with high-impedance, it might otherwise
lead to turning onof the IGBTdue toparasitic effects - albeit briefly (chapte
The UVLO detection of the driver usually has a hysteresis
in order to suppress
inadvertent switching of the output stage. Upon reaching the lower hysteresis threshold
it is switchedoff. Not until theupper hysteresis threshold is reachedwill thedriver output
stage switchback to active. This circuit caneasily be set upwith a comparator.
Drivers with an internal separation of input and output stage (e.g. using an optocoupler
or a pulse transformer) may have aUVLO detection both on the secondary side and for
the supply voltage on theprimary side. In aUVLO fault on the input side, a signal is sent
to the gate drive on the secondary side, which ensures that the output is switched off.
For this it is necessary to ensure (possibly with external circuitry of the driver such as
large buffer capacitors) that the primary-side has sufficient energy in case of a UVLO
event to transmit the error signal to the gate drive. The same should apply to the driver
secondary side output stage to inform the connected microcontroller regarding the
failure of the secondary side supply voltage. Unfortunately, this error signal is not
available in all standarddrivers.
Especially when using a bootstrap circuit to supply the driver output stage at low
switching frequencies one has to take care that the bootstrap capacitor is not
discharged too far. This can occur, for example, when using switched reluctancemotors
inwhich the switching frequency of the IGBTs is equal to the output frequency to control
themotor. Also, a UVLO tripmay occur due to high peak currents of the output stage if
the buffer capacitors of the driver are not dimensioned sufficiently large on the one hand
and, secondly, thepath for the supply voltage is not of sufficiently low impedance.
6.5 Coupling capacitances
Coupling capacitances always take effect where there is a dynamic potential difference,
i.e. where voltages with a high
are switched. In power electronics, this concerns
above all the IGBT drivers, the PWM signal transmission and the associated power
supply. In current and voltagemeasurements, too, coupling capacities show an effect, if
an isolated measurement is performed. The largest coupling capacitances take effect
between the actual switches and their environment. In power electronics, these are
therefore the IGBTs, the diodes or the MOSFETs with respect to their cooling,
separated from eachother electrically by an insulator.
Coupling capacitances cannot be avoided and their effect on the overall system is
sometimes severe. At each switching operation of a power electronics device, a voltage
change is generated. This voltage change causes a displacement current I
via the
coupling capacitanceC
, the so-called commonmode current.
TheGreek term hysteresis, in this context, designates the difference in the response voltages of an electronic
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