Just what is a thyristor?
A thyristor is really a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure includes four levels of semiconductor materials, including three PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These three poles are the critical parts of the thyristor, allowing it to control current and perform high-frequency switching operations. Thyristors can operate under high voltage and high current conditions, and external signals can maintain their operating status. Therefore, thyristors are popular in different electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of a semiconductor device is normally represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). In addition, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-weight-controlled thyristors. The operating condition of the thyristor is the fact that each time a forward voltage is applied, the gate needs to have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage is used between the anode and cathode (the anode is attached to the favorable pole of the power supply, as well as the cathode is linked to the negative pole of the power supply). But no forward voltage is applied to the control pole (i.e., K is disconnected), as well as the indicator light does not glow. This shows that the thyristor is not conducting and has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, along with a forward voltage is applied to the control electrode (referred to as a trigger, as well as the applied voltage is referred to as trigger voltage), the indicator light switches on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is switched on, even if the voltage on the control electrode is taken away (which is, K is switched on again), the indicator light still glows. This shows that the thyristor can still conduct. At the moment, in order to shut down the conductive thyristor, the power supply Ea has to be shut down or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is applied to the control electrode, a reverse voltage is applied between the anode and cathode, as well as the indicator light does not glow at the moment. This shows that the thyristor is not conducting and can reverse blocking.
- In conclusion
1) When the thyristor is put through a reverse anode voltage, the thyristor is in a reverse blocking state regardless of what voltage the gate is put through.
2) When the thyristor is put through a forward anode voltage, the thyristor will simply conduct if the gate is put through a forward voltage. At the moment, the thyristor is within the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) When the thyristor is switched on, provided that there exists a specific forward anode voltage, the thyristor will always be switched on regardless of the gate voltage. That is, right after the thyristor is switched on, the gate will lose its function. The gate only serves as a trigger.
4) When the thyristor is on, as well as the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The problem for that thyristor to conduct is the fact that a forward voltage ought to be applied between the anode as well as the cathode, as well as an appropriate forward voltage ought to be applied between the gate as well as the cathode. To turn off a conducting thyristor, the forward voltage between the anode and cathode has to be shut down, or the voltage has to be reversed.
Working principle of thyristor
A thyristor is actually a unique triode composed of three PN junctions. It may be equivalently thought to be consisting of a PNP transistor (BG2) as well as an NPN transistor (BG1).
- When a forward voltage is applied between the anode and cathode of the thyristor without applying a forward voltage to the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor remains turned off because BG1 has no base current. When a forward voltage is applied to the control electrode at the moment, BG1 is triggered to produce a base current Ig. BG1 amplifies this current, along with a ß1Ig current is obtained in the collector. This current is precisely the base current of BG2. After amplification by BG2, a ß1ß2Ig current is going to be brought in the collector of BG2. This current is brought to BG1 for amplification and then brought to BG2 for amplification again. Such repeated amplification forms an essential positive feedback, causing both BG1 and BG2 to get in a saturated conduction state quickly. A big current appears inside the emitters of these two transistors, which is, the anode and cathode of the thyristor (the dimensions of the current is in fact determined by the dimensions of the stress and the dimensions of Ea), therefore the thyristor is totally switched on. This conduction process is done in a very short period of time.
- Following the thyristor is switched on, its conductive state is going to be maintained from the positive feedback effect of the tube itself. Whether or not the forward voltage of the control electrode disappears, it is still inside the conductive state. Therefore, the function of the control electrode is only to trigger the thyristor to turn on. Once the thyristor is switched on, the control electrode loses its function.
- The only way to switch off the turned-on thyristor is always to lessen the anode current so that it is inadequate to keep up the positive feedback process. The way to lessen the anode current is always to shut down the forward power supply Ea or reverse the link of Ea. The minimum anode current required to maintain the thyristor inside the conducting state is referred to as the holding current of the thyristor. Therefore, as it happens, provided that the anode current is under the holding current, the thyristor may be turned off.
Exactly what is the difference between a transistor along with a thyristor?
Transistors usually contain a PNP or NPN structure composed of three semiconductor materials.
The thyristor is made up of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The work of a transistor relies on electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor demands a forward voltage along with a trigger current in the gate to turn on or off.
Transistors are popular in amplification, switches, oscillators, as well as other facets of electronic circuits.
Thyristors are mostly used in electronic circuits like controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversions.
Means of working
The transistor controls the collector current by holding the base current to accomplish current amplification.
The thyristor is switched on or off by managing the trigger voltage of the control electrode to comprehend the switching function.
The circuit parameters of thyristors are related to stability and reliability and in most cases have higher turn-off voltage and larger on-current.
To summarize, although transistors and thyristors can be utilized in similar applications in some cases, because of their different structures and operating principles, they have got noticeable differences in performance and use occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be utilized in frequency converters, motor controllers, welding machines, power supplies, etc.
- Within the lighting field, thyristors can be utilized in dimmers and light-weight control devices.
- In induction cookers and electric water heaters, thyristors may be used to control the current flow to the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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