Just what is a thyristor?
A thyristor is actually a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure includes four levels of semiconductor materials, including 3 PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These 3 poles are the critical parts of 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 working 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 silicon-controlled rectifier 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 working condition of the thyristor is that each time a forward voltage is applied, the gate will need to have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage can be used in between 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 to the control pole (i.e., K is disconnected), and the indicator light does not glow. This implies 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, and a forward voltage is applied to the control electrode (called a trigger, and 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, following the thyristor is excited, even when the voltage around the control electrode is taken off (that 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 stop the conductive thyristor, the power supply Ea must be stop 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, and the indicator light does not glow at the moment. This implies that the thyristor is not really conducting and will reverse blocking.
- In summary
1) When the thyristor is exposed to a reverse anode voltage, the thyristor is within 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 simply conduct when the gate is exposed to a forward voltage. At this time, the thyristor is in the forward conduction state, which is the thyristor characteristic, that is, the controllable characteristic.
3) When the thyristor is excited, as long as you will find a specific forward anode voltage, the thyristor will always be excited no matter the gate voltage. Which is, following the thyristor is excited, the gate will lose its function. The gate only works as a trigger.
4) When the thyristor is on, and the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The problem for the thyristor to conduct is that a forward voltage ought to be applied in between the anode and the cathode, plus an appropriate forward voltage ought to be applied in between the gate and the cathode. To turn off a conducting thyristor, the forward voltage in between the anode and cathode must be stop, or even the voltage must be reversed.
Working principle of thyristor
A thyristor is essentially a unique triode made from three PN junctions. It may be equivalently viewed as comprising a PNP transistor (BG2) plus an NPN transistor (BG1).
- If a forward voltage is applied in 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 continues to be turned off because BG1 has no base current. If a forward voltage is applied to the control electrode at the moment, BG1 is triggered to create basics current Ig. BG1 amplifies this current, and a ß1Ig current is obtained in its 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 get in a saturated conduction state quickly. A large current appears within the emitters of the two transistors, that is, the anode and cathode of the thyristor (how big the current is really dependant on how big the load and how big Ea), therefore the thyristor is totally excited. This conduction process is completed in an exceedingly limited time.
- Following the thyristor is excited, its conductive state will be maintained through the positive feedback effect of the tube itself. Even when the forward voltage of the control electrode disappears, it is actually still within the conductive state. Therefore, the purpose of the control electrode is only 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 would be to lessen the anode current so that it is inadequate to keep the positive feedback process. The way to lessen the anode current would be to stop the forward power supply Ea or reverse the bond of Ea. The minimum anode current necessary to keep the thyristor within the conducting state is referred to as the holding current of the thyristor. Therefore, strictly speaking, as long as the anode current is less than the holding current, the thyristor could be turned off.
Exactly what is the difference between a transistor and a thyristor?
Transistors usually consist of a PNP or NPN structure made from three semiconductor materials.
The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The task of any transistor relies on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor demands a forward voltage and a trigger current on the gate to transform on or off.
Transistors are popular in amplification, switches, oscillators, along with other facets of electronic circuits.
Thyristors are mainly utilized in electronic circuits including controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversions.
Method of working
The transistor controls the collector current by holding the base current to achieve current amplification.
The thyristor is excited or off by manipulating the trigger voltage of the control electrode to understand the switching function.
The circuit parameters of thyristors are related to stability and reliability and usually have higher turn-off voltage and larger on-current.
To summarize, although transistors and thyristors can be used in similar applications in some cases, because of their different structures and working principles, they have got noticeable variations 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.
- In 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 to the heating element.
- In electric vehicles, transistors can be used in motor controllers.
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