Just what is a thyristor?
A thyristor is actually a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure includes four levels of semiconductor components, including 3 PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These 3 poles would be 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 functioning status. Therefore, thyristors are widely used in different electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of the semiconductor device is normally represented by 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 light-weight-controlled thyristors. The functioning condition of the thyristor is that each time 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 between the anode and cathode (the anode is attached to the favorable pole of the power supply, and also the cathode is linked to the negative pole of the power supply). But no forward voltage is applied to the control pole (i.e., K is disconnected), and also the indicator light does not illuminate. This shows 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 (called a trigger, and also the applied voltage is called 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 turned on, even if the voltage on the control electrode is taken away (which is, K is turned on again), the indicator light still glows. This shows that the thyristor can continue to conduct. Currently, in order to cut off the conductive thyristor, the power supply Ea has to 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 between the anode and cathode, and also the indicator light does not illuminate at the moment. This shows that the thyristor is not really conducting and may reverse blocking.
- In summary
1) If the thyristor is exposed to a reverse anode voltage, the thyristor is within a reverse blocking state regardless of what voltage the gate is exposed to.
2) If the thyristor is exposed to a forward anode voltage, the thyristor will only conduct once the gate is exposed to a forward voltage. Currently, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) If the thyristor is turned on, as long as you will find a specific forward anode voltage, the thyristor will always be turned on whatever the gate voltage. That is, following the thyristor is turned on, the gate will lose its function. The gate only functions as a trigger.
4) If the thyristor is on, and also the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The problem for that thyristor to conduct is that a forward voltage should be applied between the anode and also the cathode, as well as an appropriate forward voltage also need to be applied between the gate and also the cathode. To turn off a conducting thyristor, the forward voltage between the anode and cathode has to be cut off, or perhaps the voltage has to be reversed.
Working principle of thyristor
A thyristor is actually a distinctive triode made from three PN junctions. It could be equivalently regarded as comprising a PNP transistor (BG2) as well as an NPN transistor (BG1).
- If a forward voltage is applied 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 switched 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 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 introduced the collector of BG2. This current is brought to BG1 for amplification then brought to BG2 for amplification again. Such repeated amplification forms an essential positive feedback, causing both BG1 and BG2 to get into a saturated conduction state quickly. A large current appears in 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), and so the thyristor is totally turned on. This conduction process is finished in a really short period of time.
- Right after the thyristor is turned on, its conductive state will be maintained by the positive feedback effect of the tube itself. Whether or not the forward voltage of the control electrode disappears, it really is still in the conductive state. Therefore, the function of the control electrode is simply to trigger the thyristor to turn on. Once the thyristor is turned on, the control electrode loses its function.
- The only method to switch off the turned-on thyristor is always to lessen the anode current so that it is inadequate to maintain the positive feedback process. The way to lessen the anode current is always to cut off the forward power supply Ea or reverse the connection of Ea. The minimum anode current needed to maintain the thyristor in the conducting state is called 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 switched off.
What exactly 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 consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The work of the 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 at the gate to turn on or off.
Transistors are widely used in amplification, switches, oscillators, and other facets of electronic circuits.
Thyristors are mainly found 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 attain current amplification.
The thyristor is turned on or off by manipulating the trigger voltage of the control electrode to realize the switching function.
The circuit parameters of thyristors are related to stability and reliability and in most cases have higher turn-off voltage and larger on-current.
To sum up, although transistors and thyristors can be used in similar applications sometimes, due to their different structures and functioning 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.
- Inside the lighting field, thyristors can be used in dimmers and light-weight 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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