Thyristor (Thyristor) is a kind of switching element that can work under high voltage and high current conditions, and its working process can be controlled and widely used in controlled rectification, AC voltage regulation, non-contact electronic switching, inverter and frequency conversion. In an electronic circuit, it is a typical device that controls a large current with a small current. In 1957, General Electric Company of the United States developed the world's first thyristor product and commercialized it in 1958.
The thyristor conduction conditions are: adding forward voltage and the gate has trigger current; the derivative devices include: fast thyristor, bidirectional thyristor, reverse thyristor, light control thyristor, etc. It is a high-power switching type semiconductor device, which is represented by a letter symbol "V" or "VT" in the circuit (indicated by the letter "SCR" in the old standard).
Second, the thyristor structureIt consists of a PNPN four-layer semiconductor with three PN junctions in between.
In the working process, the anode (A) and the cathode (K) are connected with the power source and the load to form the main circuit of the thyristor, and the gate G and the cathode K of the thyristor are connected with the device for controlling the thyristor to form a control circuit of the thyristor. .
The thyristor is a semi-controlled power electronic device, and its working conditions are as follows:
1. When the thyristor is subjected to a reverse anode voltage, the thyristor is in a reverse blocking state regardless of the voltage applied to the gate.
2. When the thyristor is subjected to the forward anode voltage, the thyristor is turned on only when the gate is subjected to the forward voltage. At this time, the thyristor is in a forward conduction state, which is the thyristor thyristor characteristic and controllable characteristic.
3. When the thyristor is turned on, as long as there is a certain positive anode voltage, regardless of the gate voltage, the thyristor remains turned on, that is, after the thyristor is turned on, the gate loses its effect. The gate only acts as a trigger.
4. When the thyristor is turned on, the thyristor turns off when the main circuit voltage (or current) decreases to near zero.
The thyristor has three lead legs, an anode A, a cathode K, and a control electrode G.
Only when a forward voltage is applied between the thyristor anode A and the cathode K, and the required forward trigger voltage is applied between the gate G and the cathode, the conduction can be triggered. At this time, A and K are in a low-resistance conduction state, and the voltage drop between the anode A and the cathode K is about 1V. After the thyristor is turned on, even if the controller G loses the trigger voltage, as long as the forward voltage is maintained between the anode A and the cathode K, the thyristor continues to be in a low-resistance conduction state. Only when the anode A voltage is removed or the polarity of the voltage between the anode A and the cathode K is changed (AC zero crossing), the thyristor is switched from the low resistance conduction state to the high resistance off state. Once the thyristor is turned off, even if the forward voltage is added between the anode A and the cathode K, it is necessary to re-add the forward trigger voltage between the gate G and the cathode K to be turned on. The on and off states of the thyristor correspond to the closed and open states of the switch, and can be used to make a contactless switch.
Five, the main parameters of the thyristorIn order to correctly select the thyristor component, it is necessary to understand its main parameters. Generally, the average value or limit value of the parameter is given in the catalogue of the product, and the measured data of the component is marked on the product certificate.
(1) Off-state repeating peak voltage UDRM The forward peak voltage applied across the thyristor can be repeated under the condition of the control pole breaking and the thyristor forward blocking, and its value is 100V smaller than the forward turning voltage.
(2) Reverse repeating peak voltage URRM When the control pole is open, the reverse peak voltage applied to the thyristor element can be repeated. This voltage value is specified to be 100V less than the reverse breakdown voltage. The smaller of the UDRM and URRM values ​​is usually labeled as the rated voltage on the part number. Since the transient overvoltage will also damage the thyristor, the rated voltage should be 2 to 3 times the normal operating peak voltage when selected, as a safety factor.
(3) Rated average current (rated forward average current) IT The power sine half-wave current that the thyristor element can continuously pass under the condition that the ambient temperature is not more than 40oC and the standard heat dissipation is all-on (in one cycle) The average value is called the rated on-state average current IT, referred to as the rated current.
(4) Holding current IH The minimum current for maintaining the continued conduction of the element under the conditions of the specified ambient temperature and the control circuit is called the holding current IH. Generally, it is several tens of milliamperes to more than one hundred milliamperes, and its value is inversely proportional to the temperature of the component, and it maintains a current of about half of 25 degrees Celsius at 120 degrees Celsius. When the forward current of the thyristor is less than this current, the thyristor will automatically turn off.
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