Active devices
The MOSFET and IGBT (insulated gate bipolar transistor) are the predominant active power devices in use today, covering virtually all mainstream applications, the principal exception being the GTO (gate turn off thyristor), which is found in specialized high power systems. The device symbols are shown in Fig. 11.9.
The MOSFET and IGBT both have an insulated gate control terminal and enhancement type control characteristics; they are normally off and the gate voltage must be increased beyond the threshold voltage (typically around 4 V) to bring the devices into conduction. Drive voltages of 12 or 15 V are normally used to ensure that the devices are fully switched on. Since the gate drive circuit must only charge and discharge the input capacitance of the device at the switching instants, the power consumption of the drive circuit is low, but pulse currents of several amperes are required to ensure rapid switching.
The MOSFET is a majority carrier device and is characterized by a constant on-state resistance, so the rms current must be used to estimate conduction losses. The on-state resistance has a positive temperature coefficient and typically rises by a factor of 1.5–2.0 for a 100°C temperature rise. The switching speed of the MOSFET is very high, current rise and fall times of tens of nanoseconds being achievable. The MOSFETs rated at a few hundred volts are available with current carrying capabilities of up to a hundred amperes, whilst devices with voltage ratings approaching 1000 V
tend to have current ratings of just a few amperes, the values of on-state resistance being correspondingly higher. Very few MOSFETs are available with voltage ratings in excess of 1000 V. The MOSFET is therefore used in lower power applications with switching frequencies of up to a few hundred kilohertz. The applications include high- frequency power supplies, dc– dc converters and small servo drives.
The IGBT is a minority carrier device, and through the use of conductivity modulation it is able to operate with much higher current densities than the MOSFET. The device is characterized by a constant on-state voltage, typically in the region of 2.0–3.0 V, requiring the use of the average forward current in estimates of on-state losses. The on-state voltage of the IGBT usually has a positive temperature coefficient, rising by approximately 20 per cent for a 100°C temperature rise. Due to the recombination time of the stored carriers within the device, the IGBT exhibits a tail current characteristic at turn off; the current rapidly falls to around 10 per cent of its on-state level then decays down to zero comparatively slowly, the overall switching time being a significant fraction of a microsecond. This effect limits the maximum operating frequency of the device to a few tens of kilohertz. The IGBTs are available with current ratings of up to several hundred amperes and with off-state voltages of up to several kilovolts. The devices are widely used in three-phase inverters and converters and have been used in small high-voltage dc power transmission systems.
Inductive switching waveforms
In the majority of power electronic circuits the operation of an active device results in the commutation of an inductive current to or from the device and a freewheel diode path. The simple equivalent circuit in Fig. 11.10 illustrates this basic switching process. The inductive current path is represented by a constant current element.
Assuming that the voltages and currents change linearly at the switching instants, then the turn-on and turn-off waveforms are as shown in Fig. 11.10. At the turn-on instant, the transistor current must rise beyond the full load current level by an amount Irr, the peak reverse recovery current of the diode, before the diode can support reverse voltage and the transistor voltage can collapse to its on-state level. During the switching transient, the transistor experiences high instantaneous power dissipation and a significant energy loss. A similar effect is seen to occur at turn-off, where the transistor volt- age must rise to the off-state level, forward biasing the diode, before the transistor current can fall to zero.
The average power loss in a transistor due to the inductive switching waveforms increases with operating frequency and this is a limit to the maximum operating frequency of a device. Snubber circuits have been used to shape the switching waveforms and limit the power losses in the devices but since the MOSFET and IGBT are much more robust than the bipolar power transistors that they replaced, these are now less common.