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164 Power semiconductor devices and converter hardware issues
power it is possible to operate the converter in switching frequencies in the MHz
region.
This is by far the fastest switching power semiconductor device and this is due to
the gate-controlled electric field required to turn the device on and off. In the case of
a BJT, a current pulse is required to control it. It is also a slower device when
compared with the MOSFET. Although its applications are limited with the lower
power handling capability, it is important to understand its operation and structure
as many of the new popular devices commercially available are based on MOSFET
technology. Figures 5.8(a) and (b) show the circuit symbol for an n-channel and a
p-channel MOSFET. The device is controlled by a voltage signal between the gate
(G) and the source (S) that should be higher than the threshold voltage as shown in
Figure 5.8(d).
5.2.7 Insulated-gate bipolar transistor
The IGBT is the most popular device for AC and DC motor drives reaching power
levels of a few hundred kW. It has also started to make its way in the high voltage
converter technology for power system applications. It is a hybrid semiconductor
device that literally combines the advantages of MOSFETs and BJTs. Specifically, it
has the switching characteristics of the MOSFET with the power handling capabil-
ities of the BJT. It is a voltage-controlled device like the MOSFET but has lower
conduction losses. Furthermore, it is available with higher voltage and current
ratings. There are a number of circuit symbols for the IGBT with the most popular
shown in Figure 5.9(a). The equivalent circuit is shown in Figure 5.9(b). The basic
structure is then shown in Figure 5.9(c). The typical i±v characteristics are plotted in
Figure 5.9(d).
The IGBTs are faster switching devices than the BJTs but not as fast as the
MOSFETs. The IGBTs have lower on-state voltage drop even when the blocking
voltage is high. Their structure is very similar to the one of the vertical diffused
MOSFET, except the p layer that forms the drain of the device. This layer forms a
junction (p±n).
Most of the IGBTs available on the market are two types as follows:
1. Punch-through IGBTs (PT-IGBTs).
2. Non-punch-through IGBTs (NPT-IGBTs).
Figure 5.10 shows the two basic structures of the two kinds of devices mentioned
above.
When comparing the two kinds of devices the following observations can be made.
1. The PT-IGBTs do not have reverse blocking voltage capability.
2. The NPT-IGBTs have better short circuit capability but higher on-state voltage
drop. They also have a positive temperature coefficient, which is a great benefit
when paralleling devices.
There exist vertically optimized NPT structure based IGBT modules with 6.5 kV dc
blocking voltage with rated currents up to 600 A. They have positive temperature
coefficient of the on-state voltage, short circuit capability and high ruggedness
against overcurrent.