known as Active Clamping
hows two variants of Active Clamping. In all
variants, the voltage between the collector and the emitter is restricted to a certain
The idea of ActiveClamping according t
is based on the fact that as soon as
a certain voltage threshold U
at the collector is exceeded, the unidirectional TV
becomes conductive and enables current to flow. This happenswhile the IGBT
is being turned off, which means that T
is turned on. If now the U
higher than the avalanche voltage of the diode D
, a current I
flows through D
. A voltage has to built up across the gate resistor R
that is greater than the
IGBT threshold voltage U
, in order to turn the IGBT on again and so to reduce the
of the IGBT during the switch-off process. Consequently, high currents must be
flowing inorder to increase the voltageat the gate.
min GE )TO(GE
U U U
: Voltage until the IGBT is turned on [V]
(Comprises the negative gate voltage or 0V in a unipolar drive and
the threshold voltage of the IGBT.
must be greater than the
difference between these two voltages.)
: External gate resistor [
: Current through diodeD
For example, for an external gate resistor of 1
, a negative gate voltage of -15V and a
threshold voltage of the IGBT of 6V a current I
> 21A is required in order to turn the
IGBT on again, i.e. to control it. The TVS diode (D
) and the blocking diode (D
therefore have to be designed for thesepulse currents, whichhasmany disadvantages:
Such diodes are usuallywiredand takeup a lot of space.
As TVS diodes are very powerful high voltage diodes, they are expensive.
Commonly usedare the diodes of type1.5KExxx.
The junction capacitances are huge, so that current is impressed by the
every time the IGBT is switched.
The breakdown voltage is highly temperature-dependent.
Unfortunately, the term Active Clamping is not only used for collector-emitter clamping but also for other
clamping variants. In this book, the termActiveClamping is used exclusively for collector-emitter clamping.
TVS (transient voltage suppressor) describes a component that can be used to suppress voltage peaks of
short duration. For example, diodes and varistors can be used as TVS components. Two types of TVS diodes are
available: unidirectional and bidirectional. Unidirectional diodes suppress only positive voltage peaks, while
bidirectional diodes suppress both positive and negative voltage peaks that exceed a certain value. See also