di in the diode and reverse recovery currents. This occurs because the
diodes cannot take the full voltage until all the free charge carriers within the
semiconductor have been removed. This leads to a burst of current in the reverse
directionof the diodewhich, although brief, is not negligible.
Use of fast recovery diodes as freewheeling diodeswith IGBTs in an inductive
The IGBT, which is already passing the load current at the time of the turn-on, also has
to deal with the reverse recovery currents that occur when the diode is turning off. This
necessarily limits themaximum useable currents of the IGBT, because of the increased
peak current and because the SOA is almost reached. According to the IEC 60747-9
standard, the SOA, or safe operating area, describes a diagram that represents the
maximum nominal collector current I
of an IGBT after it is turned on. This cannot be
exceeded, even in the very best of cooling conditions. The current is a function of the
adjacent collector-emitter voltage U
before and during the turn-on time. A case
temperature of T
C is assumedwith direct current at different pulsewidths.
The goal is therefore to optimise the switching behaviour of the diode so as to be able to
make themost of the capability of the IGBT. One option is to reduce the carrier lifetime
by doping. Another is to use electron irradiation to shorten the reverse recovery time
and reduce reverse recovery current when the semiconductors are beingmanufactured,
although this usually increases the forward voltage of the diode, producing an
undesirable increase in forward losses. This is a limiting factor, particularly for power
diodeswith blocking voltages great than 1kV. In addition, reduction of the carrier lifetime
can cause abrupt snap-off of the reverse recovery current, which leads to oscillations
and associated EMC problems, and even to the destruction of the diode itself.
Separation of the current is also referred to as "snappy" behaviour of the diode during
the turning off process.
Approaches to achieving soft recovery behaviour have led, among other things, to the
development of diodes with a reduced degree of p-emitter efficiency. There are two
main types of diodes: Those that have a homogenous internal reduced p-emitter
(emitter controlled diodes) and those in which internal structures effect the reduction.
These are frequently used in high power Schottky diodes and makes up so-called
merged PIN Schottky (MPS) diodes.
hows an example. In addition to the
usual design of a Schottky diode, an MPS diode has p-doped islands on the metal
semiconductor junction, which form aPIN diode structure.