What is a thyristor?
A thyristor is actually a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure contains 4 quantities of semiconductor elements, including 3 PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These 3 poles are definitely 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 commonly used in various electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of the semiconductor device is generally represented through 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 light-weight-controlled thyristors. The operating condition of 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 utilized involving the anode and cathode (the anode is attached to the favorable pole of the power supply, and the cathode is connected to the negative pole of the power supply). But no forward voltage is applied for the control pole (i.e., K is disconnected), and the indicator light does not illuminate. This implies that the thyristor will not be 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 (known as a trigger, and 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, following the thyristor is excited, even when the voltage on the control electrode is removed (which is, K is excited again), the indicator light still glows. This implies that the thyristor can continue to conduct. At this time, 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 for the control electrode, a reverse voltage is applied involving the anode and cathode, and the indicator light does not illuminate at this time. This implies that the thyristor will not be conducting and can reverse blocking.
- To sum up
1) When the thyristor is exposed to a reverse anode voltage, the thyristor is at a reverse blocking state no matter what voltage the gate is exposed to.
2) When the thyristor is exposed to a forward anode voltage, the thyristor will only conduct if the gate is exposed to a forward voltage. At this time, the thyristor is in the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) When the thyristor is excited, so long as you will find a specific forward anode voltage, the thyristor will always be excited regardless of the gate voltage. Which is, following the thyristor is excited, the gate will lose its function. The gate only functions as a trigger.
4) When the thyristor is on, and the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The condition for the thyristor to conduct is that a forward voltage needs to be applied involving the anode and the cathode, and an appropriate forward voltage also need to be applied involving the gate and the cathode. To turn off a conducting thyristor, the forward voltage involving the anode and cathode must be cut off, or the voltage must be reversed.
Working principle of thyristor
A thyristor is basically a unique triode made from three PN junctions. It could be equivalently regarded as consisting of a PNP transistor (BG2) and an NPN transistor (BG1).
- In case a forward voltage is applied involving the anode and cathode of the thyristor without applying a forward voltage for 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. In case a forward voltage is applied for the control electrode at this time, BG1 is triggered to generate basics 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 likely be brought in the collector of BG2. This current is delivered to BG1 for amplification and then delivered to BG2 for amplification again. Such repeated amplification forms a vital positive feedback, causing both BG1 and BG2 to get in a saturated conduction state quickly. A big current appears within the emitters of the two transistors, which is, the anode and cathode of the thyristor (the size of the current is actually determined by the size of the burden and the size of Ea), so the thyristor is completely excited. This conduction process is completed in an exceedingly limited time.
- After the thyristor is excited, its conductive state will likely be maintained through the positive feedback effect of the tube itself. Whether or not the forward voltage of the control electrode disappears, it really is still within the conductive state. Therefore, the purpose 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 turn off the turned-on thyristor would be to decrease the anode current so that it is not enough to maintain the positive feedback process. The best way to decrease the anode current would be to cut off the forward power supply Ea or reverse the bond of Ea. The minimum anode current necessary to maintain the thyristor within the conducting state is referred to as the holding current of the thyristor. Therefore, strictly speaking, so long as the anode current is less than the holding current, the thyristor can be turned off.
Exactly what is the distinction between a transistor along with a thyristor?
Transistors usually consist of a PNP or NPN structure made from three semiconductor materials.
The thyristor is composed of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The task of the transistor relies upon electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor demands a forward voltage along with a trigger current in the gate to transform on or off.
Transistors are commonly used in amplification, switches, oscillators, along with other facets of electronic circuits.
Thyristors are mostly utilized in electronic circuits like 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 accomplish 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 related to stability and reliability and often have higher turn-off voltage and larger on-current.
To sum up, although transistors and thyristors can be utilized in similar applications in some instances, because of the different structures and operating principles, they have 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 can be used to control the current flow for the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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