54 Frequency Converters The choice of the inverter switching frequency is a trade-off between losses in the motor (sine shape of motor current) and losses in the inverter. As the switching frequency increases, so do the losses in the inverter, in line with the number of semiconductor circuits. High-frequency transistors can be divided into three main types: • Bipolar (LTR) • Unipolar (MOSFET ) • Insulated Gate Bipolar (IGBT ) Table 2.2 Comparison of power transistor characteristics shows the key diff erences between MOSFET, IGBT and LTR transistors. Table 2.2 Comparison of power transistor characteristics.
IGBT transistors are a good choice for FCs in terms of the power range, the high level of conductivity, the high switching frequency and ease of control. They combine the features of MOSFET transistors with the output properties of bipolar transistors. The actual switching components and inverter control are normally combined to create a single module called an “intelligent power module” (IPM).
Properties Semi-conductor MOSFET IGBT LTR Symbol Design Conductivity Current conductivity Losses Low High High Insignif icant High Insignif ciant Blocking conditions Upper limit Low High Medium Switching conditions Turn-on time Turn-of f time Losses Short Short Insignif icant Medium Medium Medium Medium Low High Control conditions Power Driver Medium Voltage Medium Voltage High Current