72 Power Electronic Switches

An electronic switch is one that can change an electric circuit configuration by switching states from on to off and vice versa. The electronic switch of Figure 7.3a

FIGURE 7.3 Ideal switch and its conduction characteristics. (a) Switch symbol; (b) switch characteristics in four quadrants.

is an ideal four-quadrant switch, which means that it can handle bidirectional current as well as bidirectional voltage without any power loss. In the ideal switch, there is no voltage drop across the device when it conducts. The ideal switch also turns on and off instantaneously. The quadrants are labeled in Figure 7.3b in the counterclockwise direction of the i-v plane of switch conduction characteristics. Ideally, the operating point will be along the axes, with the ideal switches carrying current with zero voltage drop across it or blocking a voltage with zero current flowing through it. The practical semiconductor switch differs from the ideal switch with respect to conduction voltage drop and finite time required to turn on and turn off. Furthermore, practical devices cannot give four-quadrant capability unless combined with one other device.

Each of the practical devices used in power converters has specific characteristics.12The particular application and its power requirements determine the type of device to be used in a converter topology. BJTs have higher power ratings and excellent conduction characteristics, but the base drive circuit is complicated, because these are current-driven devices. On the other hand, MOSFETs are voltage-driven devices and, hence, the gate drive circuits are much simpler. The switching frequency of a MOSFET is much higher compared to a BJT, but the maximum available device power ratings would be much smaller for the former. The IGBT is a device, invented in the early 1980s, that combines the positive features of MOSFET and BJT. IGBTs are the devices of choice today, in most cases, due to their availability in high power ratings. Previously, when DC motors were the primary machine choice, highpower converters were typically made of SCRs, which are available in very high power ratings. However, unlike the other devices, SCRs cannot be turned off through a gate signal, and a commutation circuit is required to turn them off. The GTOs are a kind of SCR that can be turned off through a gate signal, although the current required in the gate signal to turn them off is typically four to five times the current required to turn them on. Attempts to combine the gating characteristics of a MOSFET and the conduction characteristics of an SCR resulted in devices called MCTs in the late 1980s and early 1990s. However, failure of the MCTs under certain conditions did not make these devices popular. In addition to the switches mentioned above, there is an additional two-terminal device called a diode, which is universally used in all converters. Diodes are used in conjuction with other controlled devices in the power converter to provide current paths for inductive circuits or for blocking reverse voltages. The important features of the devices discussed above are summarized in Table 7.1. Further details of the operating characteristics of the devices that are significant in EV and HEV applications are discussed in the following.

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