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25 Basics of MOSFETs and IGBTs for Motor Control These two power-control switches can be used for the application; each brings tradeoffs to the design based on basic performance specifications, maximum ratings, thermal issues, and cost. Today's motors are increasingly driven via electronic controls, which offer better control of speed, position, and torque, as well as much greater efciency, rather than via direct connection to their source of power (whether AC or DC). To do this, the motor-control circuit must switch the current ow to the motor's coils on and off quickly, with minimal switching- time or conduction-period losses in the switch itself. That's where MOSFETs and IGBTs are used. Both of these semiconductor devices serve the needs of motor drive and power control; each is better suited in some application situations. These electrically controllable switches are similar in function and attributes, and have some overlap in internal design, and yet they are quite different in many ways. In most applications, these switches are used in an H-bridge conguration (Figure 1), where they control the current ow path to two or more motor coils. This allows full control of the motor speed and direction. The MOSFET is a eld-effect transistor that, depending on size and design, can switch a few hundred milliamps to tens of amps, and single-digit voltages to thousands of volts. Although there are many ways to draw it on a schematic, the most common symbol is shown in (Figure 2). Note that there are just three connections: source, drain, and gate; the gate controls the current ow from source to drain. Smaller MOSFETs can be fabricated directly on a standard MOS IC die, and so can be part of an integrated, single-chip solution (but only at fairly low power levels, due to die size and dissipation issues). The IGBT is a bipolar transistor, also a three terminal device, but with an emitter and collector as connections for the current path being controlled. Like the MOSFET, it has a gate to control that path, (Figure 3). As a bipolar device, it's very difcult to build an IGBT on a standard MOS IC process; thus, IGBTs are discrete devices. The IGBT combines the simple gate drive of a FET with the high-current/high-voltage handling capability of the bipolar transistor. Figure 1: In the basic H-bridge, a quartet of switches controls current ow and thus motor direction as cross-pairs of switches on or off. Note that the upper switches are oating and not connected to ground. (Source: 48projectsblog) Figure 2: One of the common schematic representations of the MOSFET, with drain (D), gate (G), and source (S) terminals. (STripFET F7 Power MOSFET from ST Microelectronics) Figure 3: One of the common schematic representations of the IGBT, with collector (C), gate (G), and emitter (E) terminals. (H Series 600V IGBT from ST Microelectronics) E C G By Mouser Staff

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