Tutorial
Power Semiconductor Switches
Power semiconductors employed in power management systems include power switches, gate drivers and rectifiers (diodes). Power switches include MOSFETs, IGBTs, and BJTs (bipolar junction transistors). Gate drivers provide sufficient drive voltage for MOSFET and IGBT power switches to ensure the switch turns on and off. Rectifiers are used primarily to convert ac to dc and also to impede the flow of current in their reverse direction.
Monolithic power semiconductors may be discrete devices, that is, only a single type in a package, or integrated with other circuits in a package. Monolithic MOSFETs and BJTs can be discrete devices or integrated with other circuits in a single package. IGBTs are usually only discrete devices, or may have an integrated diode. Semiconductor gate drivers usually contain several circuits within a package. In addition, various types of power semiconductors may combined in a hybrid package, that is, interconnected with other monolithic discrete devices in the same package.
Of the available power switches, MOSFETs are the power semiconductor of choice in power supplies. IGBTs are used extensively in UPSs (uninterruptible power supplies).
The individual power semiconductor switch (Fig. 1) applies power to a load when a control signal tells it to do so. The control signal also tells it to turn off. Ideally, the power semiconductor switch should turn on and off in zero time. It should have an infinite impedance when turned off so zero current flows to the load and zero impedance when turned on so that the on-state voltage is zero. Another idealistic characteristic would be that the switch input consumes zero power when the control signal is applied. These idealistic characteristics are unachievable with the present state of the art.
In the real world, actual power semiconductors do not meet these ideal characteristics. For example, Fig. 2(a) shows a control signal applied to an ideal power semiconductor switch whose output exhibits zero transition time when turning on and off (Fig. 2(b)). When the transistor is off (not conducting current) power dissipation is very low because current is very low. When the transistor is on (conducting maximum current) power dissipation is low because the conducting resistance is low. In contrast, an actual power switch exhibits some delay when turning on and off, as shown in Fig. 2(c). Therefore, some power dissipation occurs when the switch goes through the linear region between on and off. This means that the most power dissipation depends on the time spent going from the off to on and vice versa, that is, going through the linear region. Thus, the faster the device goes through the linear region, the lower the power dissipation.
Linear vs. Switching
Power management subsystems can use either switching or linear techniques. Switching circuits only have two states, on or off, whereas analog circuits can have an infinite number of states that lie anywhere from on to off. A good mechanical analogy is that a thermostat is digital because it is either on or off. However, a thermometer is analog because it can have an infinite range of values from its high to low temperature. Power supplies can employ either linear or switching techniques.
The switching or switchmode power supply (SMPS) accepts a dc input and employs a power switch to turn on and off at speeds that can range from hundreds of kilohertz to 1 or 2 megahertz. The switched power is then rectified and filtered to provide a dc output. For the same output power delivered to a load, the SMPS dissip