Silicon-Controlled Rectifiers: Empowering Electronics

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So what is a thyristor?

A thyristor is a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure includes 4 quantities of semiconductor elements, including 3 PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These 3 poles are definitely 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 commonly used in different electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.

The graphical symbol of any semiconductor device is normally represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The operating condition in the thyristor is the fact that each time 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 can be used between the anode and cathode (the anode is linked to the favorable pole in the power supply, and the cathode is attached to the negative pole in the power supply). But no forward voltage is used to the control pole (i.e., K is disconnected), and the indicator light fails to glow. This implies that the thyristor will not be conducting and it has forward blocking capability.

  1. Controllable conduction

As shown in Figure b above, when K is closed, and a forward voltage is used to the control electrode (referred to as a trigger, and the applied voltage is called trigger voltage), the indicator light turns on. This means that the transistor can control conduction.

  1. Continuous conduction

As shown in Figure c above, after the thyristor is turned on, even when the voltage around the control electrode is removed (which is, K is turned on again), the indicator light still glows. This implies that the thyristor can carry on and conduct. At this time, in order to shut down the conductive thyristor, the power supply Ea has to be shut down or reversed.

  1. Reverse blocking

As shown in Figure d above, although a forward voltage is used to the control electrode, a reverse voltage is used between the anode and cathode, and the indicator light fails to glow currently. This implies that the thyristor will not be conducting and will reverse blocking.

  1. In summary

1) Once the thyristor is subjected to a reverse anode voltage, the thyristor is at a reverse blocking state regardless of what voltage the gate is subjected to.

2) Once the thyristor is subjected to a forward anode voltage, the thyristor will simply conduct once the gate is subjected to a forward voltage. At this time, the thyristor is in the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.

3) Once the thyristor is turned on, provided that there is a specific forward anode voltage, the thyristor will remain turned on regardless of the gate voltage. That is certainly, after the thyristor is turned on, the gate will lose its function. The gate only works as a trigger.

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

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

Working principle of thyristor

A thyristor is essentially an exclusive triode composed of three PN junctions. It may be equivalently thought to be comprising a PNP transistor (BG2) plus an NPN transistor (BG1).

  1. When a forward voltage is used between the anode and cathode in 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. When a forward voltage is used to the control electrode currently, BG1 is triggered to produce basics current Ig. BG1 amplifies this current, and 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 delivered to BG1 for amplification and after that delivered to BG2 for amplification again. Such repeated amplification forms a crucial positive feedback, causing both BG1 and BG2 to get in a saturated conduction state quickly. A sizable current appears within the emitters of the two transistors, which is, the anode and cathode in the thyristor (the size of the current is actually dependant on the size of the burden and the size of Ea), and so the thyristor is entirely turned on. This conduction process is done in a really short time.
  2. Following the thyristor is turned on, its conductive state will be maintained from the positive feedback effect in the tube itself. Even when the forward voltage in the control electrode disappears, it is actually still within the conductive state. Therefore, the function of the control electrode is just to trigger the thyristor to change on. When the thyristor is turned on, the control electrode loses its function.
  3. The best way to turn off the turned-on thyristor is to reduce the anode current that it is not enough to maintain the positive feedback process. The best way to reduce the anode current is to shut down the forward power supply Ea or reverse the connection of Ea. The minimum anode current needed to maintain the thyristor within the conducting state is called the holding current in the thyristor. Therefore, strictly speaking, provided that the anode current is under the holding current, the thyristor may be turned off.

Exactly what is the difference between a transistor and 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.

Working conditions:

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

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

Application areas

Transistors are commonly used in amplification, switches, oscillators, and 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 attain current amplification.

The thyristor is turned on or off by manipulating the trigger voltage in the control electrode to realize 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 sometimes, because of their different structures and operating principles, they may have noticeable differences 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 lightweight control devices.
  • In induction cookers and electric water heaters, thyristors may be used to control the current flow to the heating element.
  • In electric vehicles, transistors may be used in motor controllers.

Supplier

PDDN Photoelectron Technology Co., Ltd is an excellent thyristor supplier. It really is one in the leading enterprises in the Home Accessory & Solar Power System, which can be fully involved in the growth and development of power industry, intelligent operation and maintenance management of power plants, solar panel and related solar products manufacturing.

It accepts payment via Charge 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.