Basic knowledge of silicon controlled rectifier (thyristor)

The Pioneer in the Strong Electric Impact—Silicon Controlled Rectifier

Thyristor is the abbreviation of thyristor rectifier, which is a high-power semiconductor device with a four-layer structure and three PN junctions. In fact, the function of thyristor is not only rectification, it can also be used as a contactless switch to quickly connect or disconnect the circuit, realize the inversion of DC to AC, and convert AC of one frequency to AC of another frequency, etc. Thyristor, like other semiconductor devices, has the advantages of small size, high efficiency, good stability, and reliable operation. Its appearance has enabled semiconductor technology to enter the field of strong electricity from the field of weak electricity, and has become a component that is widely used in industry, agriculture, transportation, military scientific research, and even commercial and civilian electrical appliances. 1. Structure and characteristics of thyristor Thyristor is mainly divided into three types in terms of appearance: spiral type, flat plate type and flat bottom type. The spiral type is more widely used. Thyristor has three electrodes - anode (A), cathode (C) and control electrode (G). It has a four-layer structure composed of a P-type conductor and an N-type conductor overlapping the core, with a total of three PN junctions. The structure of thyristor is very different from that of silicon rectifier diode with only one PN junction. The four-layer structure of thyristor and the use of control electrode lay the foundation for its excellent control characteristic of "controlling large with small". When using thyristor, as long as a small current or voltage is added to the control electrode, a large anode current or voltage can be controlled. At present, thyristor components with current capacity of hundreds of amperes or even thousands of amperes can be manufactured. Generally, thyristors with less than 5 amperes are called low-power thyristors, and thyristors with more than 50 amperes are called high-power thyristors. Why do thyristors have the controllability of "controlling large with small"? First of all, we can see that the first, second and third layers from the cathode upward are an NPN transistor, and the second, third and fourth layers constitute another PNP transistor. The second and third layers are overlapped and shared by two tubes. When a positive voltage Ea is applied between the anode and cathode, and a positive trigger signal is input between the control electrode G and the cathode C (equivalent to _{the base-emitter of BG1} ) , BG1 will generate a base current Ib1 _. After amplification, _{BG1 will have a} collector current Ic1 amplified by β1 _times . Because the collector _{of BG1} is connected to the base of _BG2 , Ic1 is the base current Ib2 of BG2. _BG2 sends the collector _{current Ic2 amplified by β2 compared with Ib2 ( Ib1} ) _back to _the base of BG1 for amplification. This cycle of amplification continues until BG1 _{and BG2 are} fully turned on. In fact , _{this process is a "trigger-once" process. For} thyristors , the trigger signal is added to the control electrode, and _the thyristors are turned on immediately. The turn-on time is mainly determined by the performance of the thyristors . Once the thyristor is triggered to conduct, due to the loop feedback, the current flowing into _the base _{of BG1} is not just the initial Ib1 _, but the current amplified by BG1 and BG2 ( β1 _* β2 * Ib1 ₎ . _This current is much larger than Ib1 _{, which} is enough to keep BG1 Continuous conduction. At this time, even if the trigger signal disappears, the thyristor remains in the conduction state. Only when the power supply Ea is disconnected or Ea is reduced, so that the _collector current in BG1 and BG2 is less than the minimum value to maintain conduction, the thyristor can be turned off. Of course, if the polarity of Ea is reversed, _BG1 and _{BG2 will be in the cut-off state due to the reverse voltage. At this time, even if the trigger signal is} input, the thyristor cannot work. Conversely, if Ea is connected in the forward direction and the trigger signal is negative, the thyristor cannot be turned on. In addition, if no trigger signal is added, and the forward anode voltage exceeds a certain value, the thyristor will also be turned on, but it is an abnormal working condition. The controllable characteristic of the thyristor that controls the conduction (large current passing through the thyristor) through the trigger signal (small trigger current) is an important feature that distinguishes it from ordinary silicon rectifier diodes. 2. Main parameters of thyristors The main parameters of thyristors are: (1) Rated on-state average current IT The average value of a 50 Hz half-sine wave current that can continuously pass between the anode and cathode under certain conditions. (2) Forward blocking peak voltage V _PF The forward peak voltage that can be repeatedly applied to the two ends of the thyristor when the control gate is open and no trigger signal is applied, and the anode forward voltage has not exceeded the energy conduction voltage. The forward voltage peak that the thyristor can withstand cannot exceed this parameter value given in the manual. (3) Reverse cathode blocking peak voltage VPR When the thyristor is in reverse shutdown state with reverse voltage applied, the reverse peak voltage that can be repeatedly applied to the two ends of the thyristor. When used, it cannot exceed this parameter value given in the manual. (4) Control trigger current I _g1 and trigger voltage VGT The minimum control current and voltage required for the thyristor to change from the shutdown state to the conduction state when a certain voltage is applied between the anode and the cathode at the specified ambient temperature. (5) Maintaining current IH at a specified temperature, controlling the electrode to be open circuit, and maintaining the minimum anode forward current required for the thyristor to be turned on. In recent years, many new thyristor components have been introduced, such as fast thyristors suitable for high-frequency applications, bidirectional thyristors that can be controlled to conduct in both directions with positive or negative trigger signals, thyristors that can be turned on with a positive trigger signal and turned off with a negative trigger signal, etc.