What is a thyristor?
A thyristor is really a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure includes four quantities of semiconductor components, including 3 PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These 3 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 various electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of a semiconductor device is usually represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The operating condition of the thyristor is that each time a forward voltage is used, the gate needs to have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage can be used involving the anode and cathode (the anode is connected to the favorable pole of the power supply, and also the cathode is connected to the negative pole of the power supply). But no forward voltage is used to the control pole (i.e., K is disconnected), and also the indicator light will not illuminate. This demonstrates that the thyristor is not really conducting and has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, along with a forward voltage is used to the control electrode (referred to as a trigger, and also the applied voltage is called trigger voltage), the indicator light turns on. Because of this the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is excited, even when the voltage around the control electrode is taken away (which is, K is excited again), the indicator light still glows. This demonstrates that the thyristor can carry on and conduct. At this time, so that you can 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 used to the control electrode, a reverse voltage is used involving the anode and cathode, and also the indicator light will not illuminate currently. This demonstrates that the thyristor is not really conducting and will reverse blocking.
- To sum up
1) Once the thyristor is put through a reverse anode voltage, the thyristor is within a reverse blocking state regardless of what voltage the gate is put through.
2) Once the thyristor is put through a forward anode voltage, the thyristor is only going to conduct if the gate is put through a forward voltage. At this time, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) Once the thyristor is excited, so long as you will find a specific forward anode voltage, the thyristor will remain excited regardless of the gate voltage. Which is, right after the thyristor is excited, the gate will lose its function. The gate only functions as a trigger.
4) Once the thyristor is on, and also the primary circuit voltage (or current) decreases to seal 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 also the cathode, as well as an appropriate forward voltage also need to be applied involving the gate and also the cathode. To transform off a conducting thyristor, the forward voltage involving the anode and cathode should be cut off, or the voltage should be reversed.
Working principle of thyristor
A thyristor is actually a unique triode composed of three PN junctions. It can be equivalently viewed as comprising a PNP transistor (BG2) as well as an NPN transistor (BG1).
- When a forward voltage is used involving 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 continues to be turned off because BG1 has no base current. When a forward voltage is used to the control electrode currently, BG1 is triggered to create a base current Ig. BG1 amplifies this current, along with a ß1Ig current is obtained in its collector. This current is precisely the base current of BG2. After amplification by BG2, a ß1ß2Ig current is going to be introduced the collector of BG2. This current is delivered to BG1 for amplification and then delivered 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 big current appears in the emitters of these two transistors, which is, the anode and cathode of the thyristor (the size of the current is really dependant on the size of the load and the size of Ea), therefore the thyristor is completely excited. This conduction process is finished in a very short time.
- After the thyristor is excited, its conductive state is going to be maintained through the positive feedback effect of the tube itself. Even if the forward voltage of the control electrode disappears, it is still in the conductive state. Therefore, the function of the control electrode is just to trigger the thyristor to transform on. When the thyristor is excited, the control electrode loses its function.
- The best way to shut off the turned-on thyristor is to decrease the anode current so that it is insufficient to keep the positive feedback process. How you can 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 in the conducting state is called the holding current of the thyristor. Therefore, as it happens, so long as the anode current is under the holding current, the thyristor could be turned off.
What exactly 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 composed of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The job of a transistor depends on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor demands a forward voltage along with a trigger current on the gate to transform on or off.
Transistors are popular in amplification, switches, oscillators, and other facets of electronic circuits.
Thyristors are mostly found 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 achieve current amplification.
The thyristor is excited or off by controlling the trigger voltage of the control electrode to realize 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 summarize, although transistors and thyristors can be used in similar applications sometimes, because of the 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 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 may be used to control the current flow to the heating element.
- In electric vehicles, transistors can be used in motor controllers.
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