A thyristor is a new type of high-Power Semiconductor Device discovered after a semiconductor diode or a triode. It is a controllable silicon rectifying component, also known as a thyristor.
The shape of the thyristor is divided into two types: a bolt shape and a flat plate shape. The end of the bolt shape with the bolt is the anode A, which can be fixed to the heat sink, and the thick lead at the other end is the cathode K, and the thin wire is controlled. The pole (also known as the gate) G, this structure replacement component is very convenient, for components below 100A. The plate shape, the middle metal ring is the control electrode G, the far side from the control is the anode A, and the near side is the cathode K. This structure has better heat dissipation effect and is used for components above 200A.
The thyristor is composed of four layers of semiconductors.
Briefly, the structure of a thyristor is composed of four layers of semiconductor material stacked into three PN junctions and three electrodes are drawn on the corresponding semiconductor material. These three electrodes are called: A-anode, G-gate, K-cathode.
From the internal structure analysis of the thyristor, the thyristor can be equivalent to two transistors VT1 and VT2 of NPN and PNP connected as shown. Let the magnifications of the transistors VT1 and VT2 be β1 and β2, respectively.
When a forward voltage is applied between A and K: VT1 and VT2 are forward biased. If a forward voltage is applied between G-K, an initial trigger current is generated at the base of transistor VT1. Since the respective collectors between VT2 and VT1 are connected to the base of the other side, the current amplified by VT1 returns to the base of VT2, thereby forming a strong positive current feedback. The thyristor can complete the conduction process in a few microseconds.
When a reverse voltage is applied between AK: VT1, VT2 are reverse biased, regardless of whether the GK terminal is positive or negative. The thyristor cannot be turned on.
Through experimental verification, the following conclusions can be drawn:
(1) Under normal circumstances, if the control pole does not add a forward voltage, the thyristor will not conduct regardless of whether the anode is applied with a forward voltage or a reverse voltage, which indicates that the thyristor has positive and negative blocking capabilities;
(2) When the anode and the control pole of the thyristor are simultaneously applied with a forward voltage, the thyristor can be turned on. This is the two conditions that the thyristor must be connected at the same time;
(3) After the thyristor is turned on, its gate is out of control. To restore the thyristor to a blocking state, the anode forward voltage must be reduced to a certain value (or disconnected, or reversed).
(4) After the thyristor is turned on, the two triodes are saturated and turned on, and the voltage drop between the anode and the cathode is about 1V, and the power supply voltage is almost entirely distributed on the load resistor. The PN junction of the thyristor can pass a current of several tens of amps to several thousand amps. The thyristor triggers conduction for a few microseconds.
