What is a thyristor?

A thyristor is a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure includes four quantities of semiconductor components, including three PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These three poles are definitely the critical parts from 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 different electronic circuits, including 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”). In addition, derivatives of thyristors also have fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The operating condition from the thyristor is that each time a forward voltage is applied, the gate should have a trigger current.

Characteristics of thyristor

1. Forward blocking

As shown in Figure a above, when an ahead voltage is used between the anode and cathode (the anode is linked to the favorable pole from the power supply, and the cathode is connected to the negative pole from the power supply). But no forward voltage is applied for the control pole (i.e., K is disconnected), and the indicator light fails to glow. This shows that the thyristor is not conducting and contains forward blocking capability.

2. 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 called trigger voltage), the indicator light turns on. Which means that the transistor can control conduction.

3. Continuous conduction

As shown in Figure c above, after the thyristor is excited, even when the voltage in the control electrode is taken off (that is, K is excited again), the indicator light still glows. This shows that the thyristor can continue to conduct. At the moment, in order to shut down the conductive thyristor, the power supply Ea must be shut down or reversed.

4. Reverse blocking

As shown in Figure d above, although a forward voltage is applied for the control electrode, a reverse voltage is applied between the anode and cathode, and the indicator light fails to glow at this time. This shows that the thyristor is not conducting and will reverse blocking.

5. To sum up

1) If 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) If the thyristor is put through a forward anode voltage, the thyristor will only conduct once the gate is put through a forward voltage. At the moment, the thyristor is in the forward conduction state, the thyristor characteristic, that is, the controllable characteristic.

3) If the thyristor is excited, provided that there exists a specific forward anode voltage, the thyristor will stay excited no matter the gate voltage. That is, after the thyristor is excited, the gate will lose its function. The gate only works as a trigger.

4) If the thyristor is on, and the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.

5) The problem for the thyristor to conduct is that a forward voltage ought to be applied between the anode and the cathode, plus an appropriate forward voltage also need to be applied between the gate and the cathode. To transform off a conducting thyristor, the forward voltage between the anode and cathode must be shut down, or even the voltage must be reversed.

Working principle of thyristor

A thyristor is basically a distinctive triode composed of three PN junctions. It could be equivalently regarded as comprising a PNP transistor (BG2) plus an NPN transistor (BG1).

1. If a forward voltage is applied between the anode and cathode from the thyristor without applying a forward voltage for the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor remains turned off because BG1 has no base current. If a forward voltage is applied for the control electrode at this time, BG1 is triggered to generate 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 brought in the collector of BG2. This current is sent to BG1 for amplification and after that sent 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 large current appears in the emitters of these two transistors, that is, the anode and cathode from the thyristor (how big the current is really dependant on how big the load and how big Ea), therefore the thyristor is completely excited. This conduction process is completed in a very short period of time.
2. Right after the thyristor is excited, its conductive state is going to be maintained through the positive feedback effect from the tube itself. Whether or not the forward voltage from the control electrode disappears, it is still in the conductive state. Therefore, the function of the control electrode is simply to trigger the thyristor to change on. Once the thyristor is excited, the control electrode loses its function.
3. The only way to shut off the turned-on thyristor is always to lessen the anode current that it is not enough to keep the positive feedback process. The best way to lessen the anode current is always to shut down the forward power supply Ea or reverse the connection of Ea. The minimum anode current required to maintain the thyristor in the conducting state is called the holding current from the thyristor. Therefore, strictly speaking, provided that the anode current is lower than the holding current, the thyristor could be turned off.

What is the difference between a transistor along with a thyristor?

Structure

Transistors usually contain a PNP or NPN structure composed of three semiconductor materials.

The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.

Functioning conditions:

The work of the transistor relies upon electrical signals to control its closing and opening, allowing fast switching operations.

The thyristor requires a forward voltage along with a trigger current on the gate to change on or off.

Application areas

Transistors are popular in amplification, switches, oscillators, along with other aspects of electronic circuits.

Thyristors are mostly used in electronic circuits including 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 achieve current amplification.

The thyristor is excited or off by managing the trigger voltage from the control electrode to understand the switching function.

Circuit parameters

The circuit parameters of thyristors are related to stability and reliability and usually 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 operating principles, they may have noticeable variations 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.
• In the lighting field, thyristors may be used in dimmers and light control devices.
• In induction cookers and electric water heaters, thyristors could be used to control the current flow for the heating element.
• In electric vehicles, transistors may be used in motor controllers.

Supplier

PDDN Photoelectron Technology Co., Ltd is a superb thyristor supplier. It is actually one from the leading enterprises in the Home Accessory & Solar Power System, which can be fully working in the progression of power industry, intelligent operation and maintenance management of power plants, solar power panel and related solar products manufacturing.

It accepts payment via Credit Card, T/T, West Union and Paypal. PDDN will ship the goods to customers overseas through FedEx, DHL, by air, or by sea. If you are searching for high-quality thyristor, please feel free to contact us and send an inquiry.