IGBT Modules - Technologies, Driver and Application (Second Edition) - page 36

zone. Conversely, in NPT diodes, the electrical field does not punch through the drift
zone. This is achieved by having an appropriate doping profile and a further extending
drift zone.
In general: The higher the blocking voltage required for the diode, the longer the drift
zonewill be. This, in turn, creates considerable ohmic resistance, due to the low doping
concentration. This resistance cannot be ignored, as it sharply increases the losses
incurred by the diodes in forward operation. However, this resistance is effectively less
than it appearswhen looking only at the numbers. This is because during operation, it is
the holes emitted by the p-layer draw in not only electrons from the n
-layer but also
electrons from the n
-layer to the drift zone. This process, known as double injection or
conductancemodulation, increases the charge carrier concentrationwithin the drift zone
and is therefore higher than theN
value. The result is amuch lower ohmic resistance.
Fig. 1.15
Layer sequence, circuit symbols and I/U diagram of a high power diode (not to
1.2.1 Fast recoverydiodes
Fast recovery diodes are used in high frequency applications, often in conjunction with
controllable power semiconductors. Their reverse recovery time t
for currents of up to
several 100A is in the order of a fewmicroseconds. Reverse recovery time refers to the
time required before the diode moves from a conducting to a blocking state with its
stationary reverse current value. Because of the charge carriers in the semiconductor, a
certain amount of time is required before these are completely removed and the diode
can takeupblocking voltage.
One special aspect of the application of these diodes is the soft turn-off behaviour
known as soft recovery. The various approaches to the internal design of the diodes are
In the vast majority of applications with inductive load, IGBTs require diodes connected
anti-parallel, in order to provide a freewheeling circuit for the current after the IGBT is
turned off. Otherwise, the high voltage transient generated by the inductance would
invariably destroy the IGBT. The anti-parallel diode is therefore called a freewheeling
diode (FWD). With a topology of this type, the turn-off behaviour of the diode is
determined by the switching behaviour of the corresponding IGBT (in a half-bridge
configuration as shown i
these are T
aswell as T
). Because
of this, the fast-switching IGBTs now in common use experience significant current
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