What is a thyristor?
A thyristor is a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure contains four levels of semiconductor elements, including 3 PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These 3 poles are definitely the critical parts in the thyristor, letting it control current and perform high-frequency switching operations. Thyristors can operate under high voltage and high current conditions, and external signals can maintain their functioning status. Therefore, thyristors are popular in different electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of the silicon-controlled rectifier is normally represented by the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The functioning condition in the thyristor is that whenever 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 in between the anode and cathode (the anode is linked to the favorable pole in the power supply, as well as the cathode is connected to the negative pole in the power supply). But no forward voltage is applied to the control pole (i.e., K is disconnected), as well as the indicator light fails to illuminate. This demonstrates that the thyristor is not really conducting and it 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 (known as a trigger, as well as the applied voltage is referred to as trigger voltage), the indicator light switches on. This means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is switched on, whether or not the voltage in the control electrode is removed (which is, K is switched on again), the indicator light still glows. This demonstrates that the thyristor can continue to conduct. Currently, in order to cut off the conductive thyristor, the power supply Ea must be cut off 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 in between the anode and cathode, as well as the indicator light fails to illuminate at this time. This demonstrates that the thyristor is not really conducting and may reverse blocking.
- In summary
1) When the thyristor is put through a reverse anode voltage, the thyristor is in a reverse blocking state no matter what voltage the gate is put through.
2) When the thyristor is put through a forward anode voltage, the thyristor will simply conduct once the gate is put through a forward voltage. Currently, the thyristor is incorporated in the forward conduction state, which is the thyristor characteristic, which is, the controllable characteristic.
3) When the thyristor is switched on, as long as there is 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 disorder for that thyristor to conduct is that a forward voltage needs to be applied in between the anode as well as the cathode, plus an appropriate forward voltage ought to be applied in between the gate as well as the cathode. To turn off a conducting thyristor, the forward voltage in between the anode and cathode must be cut off, or even the voltage must be reversed.
Working principle of thyristor
A thyristor is basically a unique triode composed of three PN junctions. It may be equivalently thought to be composed of a PNP transistor (BG2) plus an NPN transistor (BG1).
- If a forward voltage is applied in between the anode and cathode in the thyristor without applying a forward voltage to the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor is still switched off because BG1 has no base current. If a forward voltage is applied to the control electrode at this time, BG1 is triggered to create basics current Ig. BG1 amplifies this current, along with a ß1Ig current is obtained in their 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 sent to BG1 for amplification then sent to BG2 for amplification again. Such repeated amplification forms a vital positive feedback, causing both BG1 and BG2 to enter a saturated conduction state quickly. A large current appears in the emitters of the two transistors, which is, the anode and cathode in the thyristor (how big the current is really dependant on how big the burden and how big Ea), therefore the thyristor is totally switched on. This conduction process is done in an exceedingly short time.
- After the thyristor is switched on, its conductive state is going to be maintained by the positive feedback effect in the tube itself. Even if the forward voltage in the control electrode disappears, it is still in the conductive state. Therefore, the purpose of the control electrode is just to trigger the thyristor to change on. After the thyristor is switched on, the control electrode loses its function.
- The only way to turn off the turned-on thyristor would be to reduce the anode current that it is not enough to maintain the positive feedback process. How you can reduce the anode current would be to cut off the forward power supply Ea or reverse the link of Ea. The minimum anode current necessary to keep the thyristor in the conducting state is referred to 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 may be switched off.
What is the difference between a transistor along with a thyristor?
Transistors usually consist of 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 job of the transistor depends on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor needs a forward voltage along with a trigger current in the gate to change on or off.
Transistors are popular in amplification, switches, oscillators, and other facets of electronic circuits.
Thyristors are mainly found in electronic circuits such as 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 manipulating the trigger voltage in the control electrode to understand the switching function.
The circuit parameters of thyristors are based on stability and reliability and in most cases have higher turn-off voltage and larger on-current.
To sum up, although transistors and thyristors may be used in similar applications in some cases, because of their different structures and functioning principles, they may have noticeable differences in performance and make use of occasions.
Application scope of thyristor
- In power electronic equipment, thyristors may be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- Within the lighting field, thyristors may be used in dimmers and lightweight control devices.
- In induction cookers and electric water heaters, thyristors can be used to control the current flow to the heating element.
- In electric vehicles, transistors may be used in motor controllers.
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