Types Of Thyristors And Their Applications
- Jun 06, 2022
Commercially, the first thyristor devices were released in 1956. With a small device Thyristor can control large amounts of voltage and power. The wide range of applications in light dimmers, electric power control and speed control of electric motor. Previously, Thyristors are used as current reversal to turn off the device. Actually, it takes direct current so it is very difficult to apply to the device. But now, by using control gate signal the new devices can be turned on and turned off. Thyristors can be used to fully turn on and fully turn off. But transistor lies between turn on and turn off states. So, the thyristor is used as a switch and it is not suitable as an analog amplifier.Please follow the link for: Thyristor communication techniques in power electronics
What is a Thyristor?
A thyristor is a four layer solid-state semiconductor device with P and N type material. Whenever a gate receives a triggering current then it starts’ conducting until the voltage across the thyistor device is under forward bias. So it acts as a bistable switch under this condition. To control the large amount of current of the two leads we have to design a three lead thyristor by combining the small amount of current to that current. This process is known as control lead. If the potential difference between the two leads is under breakdown voltage, then a two lead thyristor is used to switch on the device.
Thyristor Circuit Symbol
Thyistor circuit symbol is as given below. It has three terminals Anode, cathode and gate.
There are three states in a thyristor
- Reverse blocking mode– In this mode of operation, the diode will block the voltage which is applied.
- Forward blocking mode– In this mode, the voltage applied in a direction makes a diode to conduct. But conduction will not happen here because the thyristor has not triggered.
- Forward conducting mode– The thyristor has triggered and current will flow through the device until the forward current reaches below the threshold value which is known as “Holding current”.
Thyristor Layer Diagram
Thyristor consists of three p-n junctions namely J1, J2, and J3.If the anode is at a positive potential with respect to the cathode and the gate terminal is not triggered with any voltage then J1 and J3 will be under forward bias condition. While J2 junction will be under reverse bias condition. So J2 junction will be in the off state (no conduction will take place). If the increase in voltage across anode and cathode beyond the VBO(Breakdown voltage) then avalanche breakdown occurs for J2 and then thyristor will be in ON state (starts conducting).
If a VG (Positive potential) is applied to the gate terminal, then a breakdown occurs at the junction J2 which will be of low value VAK. The thyristor can switch to ON state, by selecting a proper value VG. Under avalanche breakdown condition, the thyristor will conduct continuously without taking consideration of gate voltage, until and unless,
- The potential VAK is removed or
- Holding current is greater than the current flowing through the device
Here VG– Voltage pulse which is the output voltage of the UJT relaxation oscillator.
Thyristor switching circuits
- DC Thyristor Circuit
- AC Thyristor circuit
DC Thyristor Circuit
When connected to the DC supply, to control the larger DC loads and current we use thyristor. The main advantage of thyristor in a DC circuit as a switch gives a high gain in current. A small gate current can control large amounts of anode current, so the thyristor is known as a current operated device.
AC Thyristor Circuit
When connected to the AC supply, thyristor acts differently because it is not same as DC connected circuit. During one half of a cycle, thyristor used as an AC circuit causing it to turn off automatically due to its reverse biased condition.
Types of Thyristors
Based on turn on and turn off capabilities the thyristors are classified into the following types:
- Silicon controlled thyristor or SCRs
- Gate turn off thyristors or GTOs
- Emitter turn off thyristors or ETOs
- Reverse conducting thyristors or RCTs
- Bidirectional Triode Thyristors or TRIACs
- MOS turn off thyristors or MTOs
- Bidirectional phase controlled thyristors or BCTs
- Fast switching thyristors or SCRs
- Light activated silicon controlled rectifiers or LASCRs
- FET controlled thyristors or FET-CTHs
- Integrated gate commutated Thyristors or IGCTs
For better understanding of this concept,here we are explaining some of the types of thyristors.
Silicon Controlled Rectifier (SCR)
A silicon controlled rectifier is also known as thyristor rectifier. It is a four layered current controlling solid state device. SCRs can conduct current in only one direction (unidirectional devices). SCRs can be triggered normally by the current which is applied to the gate terminal. To know more about SCR. Please follow the link to know more about: SCR tutorial basics and characteristics
Gate turn off Thyristors (GTOs)
One of the special types of high power semiconductor devices is GTO (gate turn-off thyristor). The gate terminal controls the switches to be turned ON and OFF.
If positive pulse applied between the cathode and gate terminals, then the device will be turned ON. Cathode and gate terminals behave as a PN junction and there exists a small voltage relatively between the terminals. It is not reliable as an SCR. To improve the reliability we must maintain a small amount of positive gate current.
If negative voltage pulse applied between the gate and cathode terminals, then the device will turn OFF. To induce the gate cathode voltage some of forward current is stolen, which in turn induced forward current may fall and automatically GTO will transition to the blocking state.
- Variable speed motor drives
- High power inverters and traction
GTO Application on Variable Speed Drive
There are two main reasons for adjustable speed drive is process energy conversation and control. And it provides smoother operation. High frequency reverse conducting GTO is available in this application.
Emitter Turn OFF Thyristor
The Emitter turn OFF thyristor is one type of the thyristor and it will turn ON and turn OFF by using MOSFET. It includes both the advantages of the MOSFET and GTO. It consists of two gates- one gate is used to turn ON and another gate with a series MOSFET is used to turn OFF.
If a gate 2 is applied with some positive voltage and it will turn ON the MOSFET which is connected in series with the PNPN thyristor cathode terminal. The MOSFET connected to the thyristor gate terminal will turn OFF when we applied positive voltage to gate 1.
The drawback of MOSFET connecting in series with gate terminal is that total voltage drop increases from 0.3V to 0.5V and losses corresponding to it.
ETO device is used for the fault current limiter and solid-state circuit breaker because of its high capability current interruption, fast switching speed, compact structure and low conduction loss.
Operating Characteristics of ETO in Solid State Circuit Breaker
When compared to electromechanical switchgear the solid-state circuit breakers can provide advantages in lifetime, functionality and speed. During Turn off transient we can observe the operating characteristics of an ETO semiconductor power switch.
Reverse Conducting Thyristors or RCTs
The normal high power thyristor is different from reverse conducting thyristor (RCT). RCT is not able to perform reverse blocking because of the reverse diode. If we use freewheel or reverse diode then it will be more advantageous for these types of devices. Because the diode and SCR will never conduct and they simultaneously cannot produce heat.
RCTs or reverse conducting thyristors applications in frequency inverters and changers, used in AC controller by using Snubbers circuit.
Application in AC Controller by Using Snubbers
Protecting the semiconductor elements from over voltages is by arranging the capacitors and resistors in parallel to the switches individually. So the components are always protected from the over voltages.
Bidirectional Triode Thyristors or TRIACs
TRIAC is a device for controlling current and it is a three terminal semiconductor device. It is derived from the name called Triode for Alternating Current. Thyristors can conduct only in one direction, but TRIAC is able to conduct in both directions. There are two options to switch AC waveform for both halves- one is using TRIAC and the other is back to back connected Thyristors. To switch ON one half of the cycle, we use one Thyristor and to operate other cycle we use reverse connected Thyristors.
Used in Domestic light dimmers, small motor controls,electric fan speed controls, controlling of small domestic AC power appliances.
Application in Domestic light dimmer
By using the chopping parts of AC voltage the light dimmer will works. It allows lamp to pass only the parts of the waveform. If dim is more than chopping of the waveform is also more. Mainly the power transferred will determine the brightness of the lamp. Typically TRIAC is used to manufacture the light dimmer.
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