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23 | Power Components MOSFETs and IGBTs have traditionally been used to act as switches in inverter designs to deliver power. MOSFETs can potentially switch at up to 100kHz but are more often used at speeds in the tens of kHz. They use a manufacturing process similar to that of the ICs that control them, meaning that a single-chip solution is possible—at least for some lower-power applications. Other advantages of MOSFETs include high input impedance, low on-resistance, low gate power consumption, easy driving, and a wider safe operating area. These traits make MOSFETs ideal for applications requiring an energy- efficient solution for low current densities. However, as the voltage increases, the junction temperature also rises, and the reverse recovery performance of their internal diode deteriorates, causing increased heat and switching losses. At those higher voltage and current levels, IGBTs are usually the preferred choice of switching devices. An IGBT's structure is similar to a MOSFET, but an extra P+ layer is added at the collector, making it act like a MOSFET driving a PNP transistor (Figure 3). They are easy to drive using a low-power signal and operate at lower frequencies of around 20kHz. However, because of the design of the IGBT, it cannot be manufactured on a typical MOS IC process, meaning that it does not have an integrated internal reverse recovery diode like a MOSFET. For motor control purposes, such a diode must be used, either externally on the PCB or packaged as a separate die along with the IGBT. External recovery diodes present both advantages and disadvantages: They can be tailored to the specific application, but doing so will add cost to the design and require extra PCB space Key Parameters For both MOSFETs and IGBTs, current handling and peak- voltage ratings are the primary traits needed to meet the requirements of the motor's load. After those specifications, the two devices have secondary and tertiary requirements. The most important secondary parameters of MOSFETs are drain-source on-resistance (R DS(ON) ) and gate capacitance. Lower on-resistance reduces resistive losses and lowers the voltage drop when the device conducts, directly resulting in greater efficiency. However, it is not as straightforward as it seems. The gate capacitance is a factor in how quickly conductance can be turned off and on, which can be calculated from the equation I = C dV/dt. Along with the switching frequency, it also contributes to losses at the gate. The higher the switching frequency, the greater the losses and the lower the efficiency. A MOSFET designed to have a lower R DS(ON) will usually have a larger gate area, which leads to a higher gate capacitance. So, a trade- off can be achieved between the R DS(ON) and the gate capacitance to get the best performance and lowest losses. Manufacturers almost always give an idea of the overall value as a figure-of-merit (FOM) in the datasheet using the equation FOM = R DS(ON) × QG. For IGBTs, the on-state voltage drop is a critical specification to consider. This drop includes both the diode drop across the P-N junction and the voltage drop across the driving MOSFET. Figure 3: IGBT symbol and equivalent circuit. The collector terminal is the emitter of a PNP transistor. (Source: Infineon