silicon controlled rectifier (thyristor) (SCR)
The thyristor(SCR), also called a four- layer diode, thyristor diode, flip-flop diode or Silicon Controlled Rectifier (SCR), is a semiconductor device used as an electronic switch. The thyristor knows two states: the conducting and the non-conducting. The conducting state is triggered by a current pulse at the control electrode, the non-conducting state by current reversal.
The thyristor is similar in construction to a series circuit of several diodes, consisting of four layers, hence the name four-layer diode, with alternating p-n-p-n doping. The thyristor has three electrodes, besides the cathode and the anode also the control terminal (G). The control terminal can be n-controlled or p-controlled, i.e. it is functionally connected to the anode (n-controlled) or the cathode (p-controlled). With the p-controlled thyristor, the control pulse is positive. This pulse switches the thyristor to the low-resistance, conducting state, causing the resistance value to drop into the milliohm range. Only when the holding current is undershot do the semiconductor paths become high-impedance and non-conducting. The high-impedance state is when a negative voltage is present at the anode. In the low-impedance state, a positive voltage is present at the anode, which switches the thyristor to the conducting state only when a certain potential between the anode and cathode is exceeded.
Thyristors are used as contactless switches and controllable rectifiers in switching power supplies, dimmers and pulse circuits. The firing voltage operates at about 3 V, and switching currents range to over 1,000 A with reverse voltages of 1,000 V. Power thyristors can switch voltages well in excess of 10 kV and currents of several kilo-amperes (kA). The switching capacity is over 10 mega-volt-amps( MVA), and the switching frequency is several hundred hertz.
There are special thyristor switches such as light triggered thyristors( LTT), electrically triggered thyristors (ETT), MCT thyristors, MOS controlled thyristors (MCT) and gate turn off ( GTO ) thyristors, which are used in insulated gate bipolar transistor ( IGBT) technology. A further development of the GTO thyristors are the IGCT thyristors (Integrated Gate Commutated Thyristors), which are characterized by faster switching times. In addition, the breakover diode( BOD) is a thyristor variant that is used for current leakage and switches to the conductive state in the event of leakage currents.