Types of thyristors and their uses

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What is a thyristor? Types of thyristors and their uses

Thyristors are an interesting class of semiconductor devices. They share similar characteristics with other solid-state components made from silicon, like diodes and transistors. Therefore, distinguishing thyristors from diodes and transistors could be difficult. To add to the difficulty, there are different types of thyristors available on the market.

In some instances, what sets thyristors apart from one another could be just a tiny detail.

Also, depending on the manufacturer, a given thyristor may be known by another name.

To apply thyristors successfully when designing circuits, it is important to know their unique characteristics, limitations, and their relationship with the circuit. That’s why we’re taking some time to sort it all out so that you can have a better understanding of what thyristor is most suitable for your application.

 

What is a thyristor?

A thyristor is a four-layer device with alternating P-type and N-type semiconductors (P-N-P-N).

In its most basic form, a thyristor has three terminals: anode (positive terminal), cathode (negative terminal), and gate (control terminal). The gate controls the flow of current between the anode and cathode.

The primary function of a thyristor is to control electric power and current by acting as a switch. For such a small and lightweight component, it offers adequate protection to circuits with large voltages and currents (up to 6000 V, 4500 A).

It is attractive as a rectifier because it can switch rapidly from a state of conducting current to a state of non-conduction.

In addition, its cost of maintenance is low and, operating under the right conditions, remains functional in the long term without developing a fault.

Thyristors are used in a wide range of electric circuits, from simple burglar alarms to power transmission lines.

 

How do thyristors work?

A thyristor with a P-N-P-N structure has three junctions: PN, NP, and PN. If the anode is a positive terminal with respect to the cathode, the outer junctions, PN and PN are forward-biased, while the center NP junction is reverse-biased. Therefore, the NP junction blocks the flow of a positive current from the anode to cathode. The thyristor is said to be in a forward blocking state. Similarly, the flow of a negative current is blocked by the outer PN junctions. The thyristor is in a reverse blocking state.

Another state a thyristor can exist in is the forward conducting state, whereby it receives a sufficient signal to switch on, and it starts conducting.

Let’s take a minute to highlight the unique properties thyristors bring to a circuit by going further into the nature of the signal and the thyristor’s response.

 

 
   

 

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Thyristor turn-on at-a-glance

 

When a sufficient positive signal current or pulse is applied to the gate terminal, it triggers the thyristor into a conducting state. Current flows from anode to cathode and will continue to do so, even when the gate signal is removed. The thyristor is said to have “latched on”.

 

To unlatch the thyristor, the circuit needs to be reset by reducing the anode to cathode current below a threshold value known as the holding current.

 

Thyristor turn-on at a semiconductor material level

 

The PNPN structure of a thyristor can be interpreted as two transistors coupled together. That is, the collector current from the NPN transistor feeds the base of the PNP transistor. Similarly, the collector current from the PNP transistor feeds the base of the NPN transistor.

 

For the thyristor to latch on and start conducting current, the sum of the common base

current gains of the two transistors must exceed unity.

 

When a positive current or momentary pulse is applied to the gate that sufficiently raises the loop gain to unity, regeneration occurs. This means that the pulse causes the NPN transistor to begin conducting current, which, in turn, biases the PNP transistor into conduction.  If the

initial triggering current on the gate is removed, the thyristor remains in the on state, as long as the current through the thyristor is high enough to meet the unity gain criteria. This is the latching current.

 

A thyristor can also turn on because of avalanche breakdown of a blocking junction. For the thyristor to switch on when the gate current is zero, the applied current must reach the breakover voltage of the thyristor. This is undesirable, since breakdown damages the device. For normal operation, a thyristor is chosen so that its breakover voltage is magnitudes greater than the largest voltage that will be experienced from the power source. This way, switching a thyristor on can only take place after an intentional pulse is applied to the gate, except where the thyristor has been specifically designed to operate in breakover mode. (See types of thyristors with controlled turn off abilities below).

 

Thyristor turn-off

 

To turn off a thyristor that has latched on (switched on/turned on), the current through it must change such that the loop gain is below unity. Turn off begins when the current is reduced below the holding current.

 

Different types of thyristors and their uses

 

Thyristors can be classified depending on the nature of their turn-on and turn-off behavior and their voltage and current characteristics: The different classes are:

 

  1. Thyristors with turn-on capability (Unidirectional control)
  2. Thyristors with turn-off capability (Unidirectional control)
  3. Bidirectional control

 

  1. Thyristors with turn-on capability (Unidirectional control)

 

  1. Silicon controlled rectifier (SCR)

 

SCRs are the most widely known thyristor. As explained in the general thyristor description above, an SCR remains latched on even when the gate current is removed. To unlatch, the anode to cathode current needs to be removed or the anode reset to a negative voltage relative to the cathode. This characteristic is ideal for phase control. When the anode current becomes zero, the SCR stops conducting and blocks the reverse voltage.

 

SCRs are used in switching circuits, DC motor drives, AC/DC static switches and inverting circuits.

 

  1. Reverse conducting thyristor (RCT)

 

Thyristors usually only allow current in the forward direction, while blocking reverse direction currents. However, an RCT consists of a SCR integrated with a reverse diode which eliminates undesired loop inductance and reduces reverse voltage transients. The RCT allows electric conduction in the reverse direction with improved commutation.

 

RCTs are used in inverters and DC drives for high power choppers.

 

  1. Light-activated silicon-controlled rectifier (LASCR)

 

These are also known as light triggered thyristors (LTT). For these devices, when light particles strike the reverse-biased junction, the number of electron-hole pairs in the thyristor increases. If the light’s intensity is greater than a critical value, the thyristor will switch on. An LASCR provides complete electrical isolation between the light source and the switching device of a power converter.

 

LASCRs are used in HVDC transmission equipment, reactive power compensators, and high-power pulse generators.

 

  1. Thyristors with turn-off capability (Unidirectional control)

 

Traditional thyristors like SCRs turn on when sufficient gate pulse is applied. To turn them off, the main current has to be interrupted. This is inconvenient in DC to AC and DC to DC conversion circuits, where current does not naturally become zero.

 

  1. Gate turn-off thyristor (GTO)

 

A GTO differs from a standard thyristor as it can be switched off by applying a negative current (voltage) to the gate without requiring the removal of the current between the anode and cathode (forced commutation). This means the GTO can be turned off by a gate signal with a negative polarity, making it a fully controllable switch.  It’s also referred to as a Gate-Controlled Switch, or GCS. The turn off time of a GTO is approximately ten times faster than an equivalent SCR.

 

GTOs with reverse blocking ability comparable to their forward voltage ratings are called symmetric GTOs. Asymmetric GTOs do not have considerable reverse voltage blocking capability. Reverse conducting GTOs consist of a GTO integrated with an anti-parallel diode. Asymmetric GTOs are the most popular variety on the market.

 

GTOs are used in DC and AC motor drives, high power inverters, and AC stabilizing power.

 

  1. MOS turn–off thyristor (MTO)

 

An MTO is a combination of a GTO and MOSFET to improve the GTO’s turn-off ability. GTO’s require a high gate turn off current to be supplied whose peak amplitude is about 20-35 % of the anode to cathode current (current to be controlled). An MTO has two control terminals, a turn-on gate and a turn-off gate also called the MOSFET gate.

 

To turn on an MTO, an applied gate pulse of sufficient magnitude causes the thyristor to latch on (similar to SCR and GTO).

 

To turn off the MTO, a voltage pulse is applied to the MOSFET gate. The MOSFET turns on, which shorts the emitter and base of the NPN transistor, thereby stopping latching. It’s a much faster process than a GTO (approximately 1-2 µs) in which case, the large negative pulse applied on the GTO’s gate aims to extract enough current from the base of the NPN transistor. In addition, the faster time (MTO) eliminates the losses associated with current transfer.

 

MTOs are used in high voltage applications up to 20 MVA, motor drives, flexible AC line transmissions (FACTs), and voltage source inverters for high power.

 

  1. Emitter turn off thyristors (ETO)

 

Just like an MTO, the ETO has two terminals, a normal gate, and a second gate connected in series with a MOSFET.

 

To turn on an ETO, positive voltages are applied to both gates which results in NMOS turning on and PMOS turning off. When a positive current is injected into the normal gate, the ETO switches on.

 

To turn off, when a negative voltage signal is applied to the MOSFET gate, NMOS turns off and transfers all current away from the cathode. The latching process stops and the ETO is switched off.

 

ETOs are applied in voltage source inverters for high power, Flexible AC line Transmissions (FACTs), and Static Synchronous Compensator (STATCOM).

 

  1. Bidirectional control

 

The thyristors discussed so far have been unidirectional and are used as rectifiers, DC-DC converters, and inverters. To use these thyristors for AC voltage control, two thyristors will need to be connected in anti-parallel, resulting in two separate control circuits that would involve more wire connections. Bidirectional thyristors that are able to conduct current in both directions when triggered were developed specifically to overcome this problem.

 

  1. Triode for alternating current (TRIAC)

 

TRIACS are the second-most widely used thyristors after SCRs. They can provide control on both halves of the alternating waveform thereby using available power more efficiently.  However, TRIACs are generally only used for low power applications because of their inherent non-symmetrical construction. In high power applications, TRIACs present some disadvantages when switching at different gate voltages during each half cycle. This creates additional harmonics that cause an imbalance in the system and affect EMC performance.

 

Low-power TRIACs are used as light dimmers, speed controls for electric fans and other electric motors, and in computerized control circuits of household appliances.

 

  1. Diode for alternating current (DIAC)

 

DIACS are low power devices and are mainly used in conjunction with TRIACS (placed in series with the gate terminal of a TRIAC).

 

Because TRIACS are unsymmetrical by nature, a DIAC prevents any current flowing through the TRIAC’s gate until the DIAC reaches its trigger voltage in either direction. This ensures TRIACS used in AC switches trigger evenly in either direction.

 

DIACs are found in light bulb dimmers.

 

  1. Silicon Diode for Alternating Current (SIDAC)

 

A SIDAC behaves the same way electrically as a DIAC. The main difference between the two is that SIDACs have a higher breakover voltage and greater power handling capability than DIACs. A SIDAC is a five-layer device that can be used directly as a switch instead of as a trigger for another switching device (like DIACs are for TRIACS).

 

If the applied voltage matches or exceeds its breakover voltage, a SIDAC begins to conduct current. It remains in this conducting state even if the applied voltage changes, until the current can be reduced below its rated holding current. The SIDAC returns to its nonconductive state to repeat the cycle.

 

SIDACs are used in relaxation oscillators and other special purpose devices.