What is a thyristor?

A thyristor is a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure contains four quantities of semiconductor components, including 3 PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These 3 poles are the critical parts in 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 operating status. Therefore, thyristors are widely used in various electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.

The graphical symbol of the Thyristor is generally represented by the text symbol “V” or “VT” (in older standards, the letters “SCR”). In addition, derivatives of thyristors include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The operating condition in the thyristor is that when a forward voltage is used, the gate needs to have a trigger current.

Characteristics of thyristor

1. Forward blocking

As shown in Figure a above, when an ahead voltage is utilized between the anode and cathode (the anode is attached to the favorable pole in the power supply, as well as the cathode is attached to the negative pole in the power supply). But no forward voltage is used for the control pole (i.e., K is disconnected), as well as the indicator light does not light up. This shows that the thyristor will not be 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 used for the control electrode (known as a trigger, as well as the applied voltage is called trigger voltage), the indicator light switches on. Which means that the transistor can control conduction.

3. Continuous conduction

As shown in Figure c above, following the thyristor is turned on, whether or not the voltage on the control electrode is removed (that is, K is turned on again), the indicator light still glows. This shows that the thyristor can continue to conduct. At the moment, so that you can cut off the conductive thyristor, the power supply Ea should be cut off or reversed.

4. Reverse blocking

As shown in Figure d above, although a forward voltage is used for the control electrode, a reverse voltage is used between the anode and cathode, as well as the indicator light does not light up currently. This shows that the thyristor will not be conducting and can reverse blocking.

5. In summary

1) If the thyristor is subjected to a reverse anode voltage, the thyristor is at a reverse blocking state no matter what voltage the gate is subjected to.

2) If the thyristor is subjected to a forward anode voltage, the thyristor is only going to conduct once the gate is subjected to 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 turned on, so long as you will find a specific forward anode voltage, the thyristor will always be turned on no matter the gate voltage. Which is, following the thyristor is turned on, the gate will lose its function. The gate only serves as a trigger.

4) If the thyristor is on, as well as 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 between the anode as well as the cathode, as well as an appropriate forward voltage also need to be applied between the gate as well as the cathode. To turn off a conducting thyristor, the forward voltage between the anode and cathode should be cut off, or perhaps the voltage should be reversed.

Working principle of thyristor

A thyristor is essentially a unique triode composed of three PN junctions. It can be equivalently viewed as composed of a PNP transistor (BG2) as well as an NPN transistor (BG1).

1. If a forward voltage is used between the anode and cathode in 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 used for the control electrode currently, 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 will likely be brought in the collector of BG2. This current is sent to BG1 for amplification then sent to BG2 for amplification again. Such repeated amplification forms a vital positive feedback, causing both BG1 and BG2 to enter a saturated conduction state quickly. A large current appears within the emitters of these two transistors, that is, the anode and cathode in the thyristor (the dimensions of the current is really based on the dimensions of the stress and the dimensions of Ea), and so the thyristor is totally turned on. This conduction process is done in a very short time.
2. Right after the thyristor is turned on, its conductive state will likely be maintained by the positive feedback effect in the tube itself. Even if the forward voltage in the control electrode disappears, it really is still within the conductive state. Therefore, the purpose of the control electrode is only to trigger the thyristor to turn on. After the thyristor is turned on, the control electrode loses its function.
3. The only method to turn off the turned-on thyristor is always to reduce the anode current so that it is insufficient to maintain the positive feedback process. How you can reduce the anode current is always to cut off the forward power supply Ea or reverse the link of Ea. The minimum anode current needed to keep your thyristor within the conducting state is called the holding current in the thyristor. Therefore, strictly speaking, so long as the anode current is lower than the holding current, the thyristor could be turned off.

Exactly what is the difference between a transistor along with a thyristor?

Structure

Transistors usually include 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.

Operating conditions:

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

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

Application areas

Transistors are widely used in amplification, switches, oscillators, as well as other aspects of electronic circuits.

Thyristors are mostly used in electronic circuits such as 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 accomplish current amplification.

The thyristor is turned on or off by managing the trigger voltage in the control electrode to realize the switching function.

Circuit parameters

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 may be used in similar applications in some instances, due to their different structures and operating principles, they have noticeable variations in performance and use 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 in the leading enterprises in the Home Accessory & Solar Power System, which is fully involved in the development of power industry, intelligent operation and maintenance handling 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 looking for high-quality thyristor, please feel free to contact us and send an inquiry.