Page 176 - Power Electronic Control in Electrical Systems
P. 176

//SYS21/F:/PEC/REVISES_10-11-01/075065126-CH005.3D ± 164 ± [153±176/24] 17.11.2001 10:15AM







               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.
   171   172   173   174   175   176   177   178   179   180   181