A thyristor can be considered as a controllable diode. Either the thyristor is completely open or completely closed. Closest to the anode of the thyristor, the material is P-doped, then it is N-doped, thereafter there is a P-doped section at the gate and most close to the cathode the material is N-doped again. In opposite to the diode, a thyristor has a forward blocking mode besides the reverse blocking mode. The thyristor is triggered by a fast pulse on the gate and continues to conduct until the forward voltage has dropped to zero and changed polarity. This is called self-commuting. The largest thyristors can handle several kV and currents of several kA. They are made very rugged in order to handle high dv/dt and di/dt. Power thyristors are porcelain pucks, with a top and bottom side in metal as a connector for anode and cathode. The way of turning on and off a thyristor create square waveforms and many harmonics. Therefor PCB mounted thyristors are most often replaced with MOS-transistors.
Gate Turn Off Thyristors
The Gate Turn Off thyristor was the first controllable thyristor for highpower. It can actively be turned off by gate control. The GTO’s dramatically changed the drive system in locomotives. The Swedish X2000 train engines are driven by converters with GTO thyristors. The GTO thyristor is turned on with a positive pulse and turn off with a negative pulse. The turn off pulse has to be at least 30% of the current through the transistor. IGBT’s are sometimes replacing thyristors because they are easier to drive.
The TRIAC is a three-connector part in the thyristor family. TRIAC is an abbreviation for ”TRIode for Alternating Current”. The current running through a TRIAC is triggered by a positive or negative current on the gate. When a TRIAC has started to conduct, it continues to do so until the current drops below the so-called holding current. A TRIAC can control high power with milliampere currents on the gate. A trigger pulse at a certain phase angel in an alternating current cycle enable control of the current running through the TRIAC to the load. This so-called phase control is used for controlling the speed of electric motors for instance.