Semiconductor Rectifier Diode Definition – The main devices normally employed for motor control are diode, silicon-controlled rectifier (SCR) or thyristor, triac, diac, power transistor, etc. These devices are categorized as per their structure: 2-layer devices such as diode (silicon rectifier), Slayer devices such as transistors, 4-layer devices such as thyristors etc. The term “thyristor” is normally used for all 4-layer devices including the triac. Every device has certain maximum voltage, current and time response (speed) capabilities. These can very rarely be attained simultaneously in a single device. Only one of these is realized in one device.
The diodes (Fig 11.2(a)) described here are based on the property of p- and n-type materials in conjunction. An ideal diode presents zero impedance to current flow in one direction and infinite impedance in the opposite direction. A practical diode has a very low forward resistance and has a voltage drop of 0.3 to 0.7 V at all current levels and a reverse current of a few milliamperes when blocking voltage (or peak reverse voltage) is around 1 kV. The v-i characteristic of a diode and its idealized form, commonly employed in diode circuit analysis, are shown in Figs. 11.2(b) and (c) respectively. The voltage rating (maximum 5 kV), is fixed by the reverse characteristic in terms of the break-down voltage. The main parameters of a diode are: blocking voltage, average forward current and maximum operating junction temperature (nearly 125 °C for most silicon rectifiers). Protection is necessary against voltage surges, overcurrents and excessive temperature rise.A “free-wheeling” diode is commonly used in motor control systems to provide an alternate path for continuity of current in the inductive circuit following the switching-off of some power device between the motor and the energy source.