State of the Art in Medium Voltage Power Semiconductors

Recent technology advances in power electronics have been made by improvements in
controllable power semiconductor devices. Figure 2-3 and Figure 2-4 summarize the most
important power semiconductors on the market and their rated voltages and currents today [14],
[21]. The device characteristics for medium voltage power semiconductors are shown in Table
2-2 [28].
Metal Oxide Semiconductor Field Effect Transistors (MOSFET) and IGBTs have replaced
Bipolar Junction Transistors (BJT) almost completely. A remarkable development in
MOSFETs took place during the last years. Nowadays MOSFETs are available up to a
maximum switch power of about 100kVA [21].
Various new concepts of MOS-controlled thyristors such as the MOS-controlled thyristor
(MCT) and the MOS turn-off thyristor (MTO) have been presented but they do not have any
commercial applications.
Conventional GTOs are available with a maximum device voltage of 6kV in traction and
industrial converters (Table 2-2) [21], [28]. The high on state current density, the high blocking
voltages, and the possibilities to integrate an inverse diode are considerable advantages of
these devices. However, the requiring of bulky and expensive snubber circuits [92], [93] as
well as the complex gate drive are the reasons that GTOs are being replaced by IGCTs and
Gifts [21], [28]. Like GTOs, IGCTs are offered only as a presspack device. The symmetrcial
IGCT is offered by Mitsubishi with a maximum device voltage of 6.5kV (Table 2-2) [21], [28].
An increase of the blocking voltage of IGCTs and inverse diodes to 10kV is technically
possible today [21].
Due to the thyristor latching, a GTO structure offers lower conduction losses than an IGBT of
the same voltage class. To improve the switching performance of classical GTOs, gatecommutated
thyristors (GCTs) with a very little turn-off delay (about 1.5μs) have been
developed [90], [91]. New asymmetric GCT devices up to 10kV with peak controllable
currents up to 1kA have been manufactured but only those devices with 6kV and 6kA are
commercially available.
IGBTs were introduced on the market in 1988. IGBTs from 1.7kV up to 6.5kV with dc current
ratings up to 3kA are commercially available today (Table 2-2) [21], [28]. They have been
optimized to satisfy the specifications of the high-power motor drives for industrial and
traction applications. They are mainly applied in a module package due to the complex and
expensive structure of an IGBT presspack [28].
In IGBT modules, multiple IGBT chips are connected in parallel and bonded to ceramic
substrates to provide isolation. Both IGCTs and IGBTs have the potential to decrease the cost
of systems and to increase the number of economically valuable applications as well as the
performance of high-power converters, compared to GTOs, due to a snubberless operation at
higher switching frequencies (e.g. 500-1000Hz).
Figure 2-5 represents the typical converter voltage as a function of power ratings for both IGBT
and IGCT applications [28], [30], [31]. It can be seen that LV-IGBT modules are commercially
available with a maximum device voltage of 1700V on the entire low-voltage drive market (i.e.
up to 690V). On the other hand, MV-IGBT modules enable converter designs in a voltage range
from 1kV up to 7.2kV with a power range from 200kVA up to 7MVA (Figure 2-4) [28]. MVIGBT
modules have replaced GTOs in recent traction applications.
“IGBT presspacks are applied mainly in self-commutated High Voltage Direct Current
(HVDC) converters (e.g. HVDC light) where a redundant converter design is a main
requirement and each converter switch position consists of a series connection of many IGBTs
(e.g. n ≥ 10) [28].”