What exactly is a thyristor?

A thyristor is really a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure contains 4 quantities of semiconductor components, including three PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These three poles would be the critical parts of 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 working status. Therefore, thyristors are widely used in a variety of electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.

The graphical symbol of a semiconductor device is normally represented by the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors also have fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-weight-controlled thyristors. The working condition of the thyristor is that whenever a forward voltage is used, the gate should 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 of the power supply, and also the cathode is connected to the negative pole of the power supply). But no forward voltage is used for the control pole (i.e., K is disconnected), and also the indicator light fails to light up. This shows that the thyristor is not conducting and it has 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 (referred to as a trigger, and also the applied voltage is referred to as trigger voltage), the indicator light switches on. Which means that the transistor can control conduction.

3. Continuous conduction

As shown in Figure c above, right after the thyristor is switched on, even when the voltage on the control electrode is removed (that is, K is switched on again), the indicator light still glows. This shows that the thyristor can continue to conduct. At the moment, to be able to stop the conductive thyristor, the power supply Ea should be stop 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, and also the indicator light fails to light up at this time. This shows that the thyristor is not conducting and will reverse blocking.

5. In summary

1) When the thyristor is exposed to a reverse anode voltage, the thyristor is in a reverse blocking state whatever 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 the moment, the thyristor is within the forward conduction state, which is the thyristor characteristic, that is, the controllable characteristic.

3) When the thyristor is switched on, provided that there is a specific forward anode voltage, the thyristor will always be switched on whatever the gate voltage. That is, right after the thyristor is switched on, the gate will lose its function. The gate only serves as a trigger.

4) When 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 the thyristor to conduct is that a forward voltage should be applied between the anode and also the cathode, plus an appropriate forward voltage should also be applied between the gate and also the cathode. To transform off a conducting thyristor, the forward voltage between the anode and cathode should be stop, or the voltage should be reversed.

Working principle of thyristor

A thyristor is actually an exclusive triode composed of three PN junctions. It can be equivalently regarded as composed of a PNP transistor (BG2) plus an NPN transistor (BG1).

1. In case a forward voltage is used between the anode and cathode of the thyristor without applying a forward voltage for 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. In case a forward voltage is used for the control electrode at this time, BG1 is triggered to create a base current Ig. BG1 amplifies this current, along with a ß1Ig current is obtained in their 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 delivered to BG1 for amplification and then delivered to BG2 for amplification again. Such repeated amplification forms a crucial positive feedback, causing both BG1 and BG2 to get into a saturated conduction state quickly. A sizable current appears in the emitters of these two transistors, that is, the anode and cathode of the thyristor (how big the current is really dependant on how big the stress and how big Ea), so the thyristor is totally switched on. This conduction process is done in a really short time.
2. Following the thyristor is switched 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 is still in the conductive state. Therefore, the function of the control electrode is only to trigger the thyristor to change on. After the thyristor is switched on, the control electrode loses its function.
3. The only way to turn off the turned-on thyristor would be to decrease the anode current that it is inadequate to keep up the positive feedback process. The way to decrease the anode current would be to stop the forward power supply Ea or reverse the connection of Ea. The minimum anode current necessary to maintain the thyristor in the conducting state is referred to as the holding current of the thyristor. Therefore, as it happens, provided that the anode current is less than the holding current, the thyristor could be switched off.

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

Structure

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

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

Working conditions:

The job of a 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 at the gate to change on or off.

Application areas

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

Thyristors are mostly found 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 attain current amplification.

The thyristor is switched on or off by managing the trigger voltage of the control electrode to understand the switching function.

Circuit parameters

The circuit parameters of thyristors are related to stability and reliability and often have higher turn-off voltage and larger on-current.

To summarize, although transistors and thyristors may be used in similar applications sometimes, because of their different structures and working principles, they 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.
• Inside the lighting field, thyristors may 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 for 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 is one of the leading enterprises in the Home Accessory & Solar Power System, that is 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 Credit Card, T/T, West Union and Paypal. PDDN will ship the goods to customers overseas through FedEx, DHL, by air, or by sea. Should you be looking for high-quality thyristor, please feel free to contact us and send an inquiry.