What is a thyristor?
A thyristor is really a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure includes four levels of semiconductor elements, including 3 PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These 3 poles would be the critical parts in 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, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of any Thyristor is usually represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The operating condition in the thyristor is that each time a forward voltage is applied, the gate should have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage is used involving the anode and cathode (the anode is linked to the favorable pole in the power supply, and the cathode is linked to the negative pole in the power supply). But no forward voltage is applied for the control pole (i.e., K is disconnected), and the indicator light does not light up. This shows that the thyristor is not conducting and contains forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, along with a forward voltage is applied for the control electrode (called a trigger, and the applied voltage is known as trigger voltage), the indicator light switches on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is switched on, whether or not the voltage around the control electrode is taken off (that is certainly, K is switched on again), the indicator light still glows. This shows that the thyristor can continue to conduct. At this time, to be able to cut off the conductive thyristor, the power supply Ea should be cut off or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is applied for the control electrode, a reverse voltage is applied involving the anode and cathode, and the indicator light does not light up at the moment. This shows that the thyristor is not conducting and will reverse blocking.
- To sum up
1) If the thyristor is subjected to a reverse anode voltage, the thyristor is in a reverse blocking state regardless of what voltage the gate is subjected to.
2) If the thyristor is subjected to a forward anode voltage, the thyristor will simply conduct when the gate is subjected to a forward voltage. At this time, the thyristor is within the forward conduction state, which is the thyristor characteristic, that is certainly, the controllable characteristic.
3) If the thyristor is switched on, as long as there is a specific forward anode voltage, the thyristor will remain switched on whatever the gate voltage. Which is, following the thyristor is switched on, the gate will lose its function. The gate only works as a trigger.
4) If the thyristor is on, and the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The disorder for that thyristor to conduct is that a forward voltage ought to be applied involving the anode and the cathode, as well as an appropriate forward voltage also need to be applied involving the gate and the cathode. To change off a conducting thyristor, the forward voltage involving the anode and cathode should be cut off, or perhaps the voltage should be reversed.
Working principle of thyristor
A thyristor is essentially an exclusive triode made up of three PN junctions. It can be equivalently regarded as composed of a PNP transistor (BG2) as well as an NPN transistor (BG1).
- In case a forward voltage is applied involving the anode and cathode in the thyristor without applying a forward voltage for the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor remains turned off because BG1 has no base current. In case a forward voltage is applied for 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 will 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 enter a saturated conduction state quickly. A large current appears inside the emitters of the two transistors, that is certainly, the anode and cathode in the thyristor (how big the current is in fact dependant on how big the stress and how big Ea), and so the thyristor is totally switched on. This conduction process is completed in a very short period of time.
- After the thyristor is switched on, its conductive state will be maintained from the positive feedback effect in the tube itself. Whether or not the forward voltage in the control electrode disappears, it is actually still inside the conductive state. Therefore, the function of the control electrode is simply to trigger the thyristor to turn on. After the thyristor is switched on, the control electrode loses its function.
- The only way to switch off the turned-on thyristor is to decrease the anode current so that it is not enough to keep the positive feedback process. The way to decrease the anode current is to cut off the forward power supply Ea or reverse the bond of Ea. The minimum anode current required to keep the thyristor inside the conducting state is known as the holding current in the thyristor. Therefore, as it happens, as long as the anode current is less than the holding current, the thyristor can be turned off.
Exactly what is the difference between a transistor along with a thyristor?
Transistors usually consist of a PNP or NPN structure made up of three semiconductor materials.
The thyristor is composed of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The job of any transistor depends on electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor requires 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 aspects of electronic circuits.
Thyristors are mostly used in electronic circuits including controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversions.
Way of working
The transistor controls the collector current by holding the base current to attain current amplification.
The thyristor is switched on or off by manipulating the trigger voltage in the control electrode to realize the switching function.
The circuit parameters of thyristors are based on stability and reliability and often have higher turn-off voltage and larger on-current.
To sum up, although transistors and thyristors can be used in similar applications in some cases, because of their different structures and operating principles, they may have noticeable variations in performance and make use of occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- Within the lighting field, thyristors can be used in dimmers and lightweight control devices.
- In induction cookers and electric water heaters, thyristors can be used to control the current flow for the heating element.
- In electric vehicles, transistors can be used in motor controllers.
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