What is a thyristor?

A thyristor is a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure includes 4 levels of semiconductor materials, including 3 PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These 3 poles are definitely 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 working status. Therefore, thyristors are widely used in various electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.

The graphical symbol of any silicon-controlled rectifier is normally represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). In addition, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-weight-controlled thyristors. The working condition of the thyristor is the fact that when a forward voltage is applied, 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 of the power supply, and the cathode is attached to the negative pole of the power supply). But no forward voltage is applied for the control pole (i.e., K is disconnected), and the indicator light does not light up. This implies that the thyristor is not really 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 applied for the control electrode (known as a trigger, and 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, 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. Currently, to be able to shut down the conductive thyristor, the power supply Ea has to 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 does not light up at this time. This implies that the thyristor is not really conducting and may reverse blocking.

5. In conclusion

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

2) If the thyristor is exposed to a forward anode voltage, the thyristor will simply conduct if the gate is exposed to a forward voltage. Currently, the thyristor is within the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.

3) If the thyristor is turned on, as long as there exists a specific forward anode voltage, the thyristor will always be turned on regardless of 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, and the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.

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

Working principle of thyristor

A thyristor is basically a distinctive triode made from three PN junctions. It could be equivalently viewed as comprising a PNP transistor (BG2) and an NPN transistor (BG1).

1. In case a forward voltage is applied 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 is still switched off because BG1 has no base current. In case 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 brought to BG1 for amplification then brought 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 large current appears in the emitters of these two transistors, which is, the anode and cathode of the thyristor (the dimensions of the current is actually based on the dimensions of the stress and the dimensions of Ea), and so the thyristor is entirely turned on. This conduction process is completed in an exceedingly short period of time.
2. After the thyristor is turned on, its conductive state is going to be maintained from the positive feedback effect of the tube itself. Even when the forward voltage of the control electrode disappears, it is still in the conductive state. Therefore, the purpose 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 only way to shut off the turned-on thyristor is to reduce the anode current that it is insufficient to keep up the positive feedback process. How you can reduce the anode current is to shut down the forward power supply Ea or reverse the connection of Ea. The minimum anode current required to keep your thyristor in the conducting state is called the holding current of the thyristor. Therefore, strictly speaking, as long as the anode current is lower than the holding current, the thyristor can be switched off.

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

Structure

Transistors usually include a PNP or NPN structure made from three semiconductor materials.

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

Operating conditions:

The work of any transistor depends on electrical signals to control its closing and opening, allowing fast switching operations.

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

Application areas

Transistors are widely used in amplification, switches, oscillators, along with other facets of electronic circuits.

Thyristors are mostly utilized 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 accomplish current amplification.

The thyristor is turned 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 in most cases have higher turn-off voltage and larger on-current.

To summarize, although transistors and thyristors can be used in similar applications sometimes, because of the different structures and working principles, they may have 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.
• Within the lighting field, thyristors can be used in dimmers and light-weight control devices.
• In induction cookers and electric water heaters, thyristors may be used to control the current flow for the heating element.
• In electric vehicles, transistors can 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, which can be fully working in the progression of power industry, intelligent operation and maintenance control over power plants, solar power 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 searching for high-quality thyristor, please feel free to contact us and send an inquiry.