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A thyristor is a semiconductor device that acts as a switch. It has three terminals: the anode, cathode, and gate. The thyristor can be turned on by applying a voltage pulse to the gate terminal, which triggers a process called "latching."
To turn on a thyristor, a voltage pulse is applied to the gate terminal. The voltage must be higher than a certain threshold value called the "gate trigger voltage." Once the gate is triggered, a small current flows between the anode and cathode terminals, which in turn triggers a larger current flow. This process is known as "latching," and once it occurs, the thyristor remains in the "on" state until the current flowing through it drops below a certain value.
To turn off a thyristor, the current flowing through it must be reduced to zero. This can be done by reducing the voltage across the device or by interrupting the current flow. Alternatively, a reverse voltage can be applied to the thyristor to turn it off quickly. This is known as "reverse biasing" and it forces the current to flow in the opposite direction, which turns off the thyristor.
It's worth noting that once a thyristor is turned off, it cannot be turned on again until a new voltage pulse is applied to the gate terminal. This makes thyristors useful for applications where a device needs to be turned on and off in a controlled manner.
switching characteristics of thyristor
SWITCHING CHARACTERISTICS OF THYRISTORS Static and switching characteristics of thyristors are always taken into consideration for economical and reliable design of converter equipment.
There are three main switching characteristics of thyristors:
- Turn-On Time
- Turn-Off Time
- Holding Current
During turn-on and turn-off processes, a thyristor is subjected to different voltages across it and different currents through it.the time variations of the voltage across a thyristor and the current through it during turn-on and turn-off processes give the dynamic or switching characteristics of a thyristor.
1. Switching Characteristics during Turn-on
This is the time required for the thyristor to switch from the off state to the on state. The turn-on time is influenced by the gate current, temperature, and the voltage rating of the device.
Total turn-on time can be divided into three intervals;
(i) Delay time td
(ii) Rise time tr
(iii) Spread time tp
(i) Delay time td
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The delay time may also be defined as the time during which anode voltage falls from V to 0.9Va where Va = value of anode voltage.
Another way of defining delay time is the time during which anode current rises from forward leakage current to 0.1 Ia where Ia= final value of anode current. With the thyristor initially in the forward blocking state, the anode voltage is OA and anode current is small leakage current as shown in Fig. in below. Initiation of turn-on process is indicated by a rise in anode current from small forward leakage current and a fall in anode-cathode voltage from forward blocking voltage OA. As gate current begins to flow from gate to cathode with the application of gate signal, the gate current has non-uniform distribution of current density over the cathode surface due to the p layer. Its value is much higher near the gate but decreases rapidly as the distance from the gate increases, show in fig up . This shows that during delay time td anode current flows in a narrow region near the gate where gate current density is the highest.
The delay time can be decreased by applying high gate current and more forward voltage between anode and cathode. The delay time is fraction of a microsecond.
(ii) Rise time tr
The rise time is the time taken by the anode current to rise from 0.1 Ia to 0.9 Ia. The rise time is also defined as the time required for the forward blocking off-state voltage to fall from 0.9 to 0.1 of its initial value OA. The rise time is inversely proportional to the magnitude of gate current and its build up rate. Thus tr can be reduced if high and steep current pulses are applied to the gate.
The main factor determining tr is the nature of anode circuit.
From the beginning of rise time tr, anode current starts spreading from the narrow conducting region near the gate. The anode current spreads at a rate of about 0.1 mm per microsecond. As the rise time is small, the anode current is not able to spread over the entire cross-section of cathode.show in Fig(b). illustrates how anode current expands over cathode surface area during turn-on process of a thyristor. Here the thyristor is taken to have single gate electrode away from the centre of p-layer. It is seen that anode current conducts over a small conducting channel even after tr -this conducting channel area is however, greater than that during td.
During rise time tr turn-on losses in the thyristor are the highest due to high anode voltage (Va) and large anode current (Ia) occurring together in the thyristor as shown in Fig. As these losses occur only, over a small conducting region, local hot spots may be formed and the device may be damaged.
(iii) Spread time tp
The spread time is the time taken by the anode current to rise from 0.9Ia to Ia. It is also defined as the time for the forward blocking voltage to fall from 0.1 of its value to the on-state voltage drop (1 to 1.5 V). During this time, conduction spreads over the entire cross-section of the cathode of SCR.
The spreading interval depends on the area of cathode and on gate structure of the SCR. After the spread time, anode current attains steady state value and the voltage drop across SCR is equal to the on-state voltage drop of the order of 1 to 1.5 V.
Total turn-on time of an SCR is equal to the sum of delay time, rise time and spread time.
Thyristor manufacturers usually specify the rise time which is typically of the order of 1 to 4 μ-sec. Total turn-on time depends upon the anode circuit parameters and the gate signal wave shapes.
During turn-on, SCR may be considered to be a charge controlled device. A certain amount of charge must be injected into the gate region for the thyristor conduction to begin.
This charge is directly proportional to the value of gate current. Therefore, higher the magnitude of gate current, the lesser time it takes to inject this charge.
The turn-on time can therefore be reduced by using higher values of gate currents. The magnitude of gate current is usually 3 to 5 times the minimum gate current required to trigger an SCR.
When gate current is several times higher than the minimum gate current required, a thyristor is said to be hard-fired or overdriven.
Hard-firing or overdriving of a thyristor reduces its turn-on time and enhances it di/dt capability.
This waveform has higher initial value of gate current with a very fast rise time. The initial high value of gate current is then reduced to a lower value where it stays for several microseconds in order to avoid unwanted turn-off of the device.
2. Switching Characteristics during Turn-off
This is the time required for the thyristor to switch from the on state to the off state. The turn-off time is influenced by the rate at which the anode current is reduced, temperature, and the voltage rating of the device.
Thyristor turn-off means that it has changed from on to off state and is capable of blocking the forward voltage. This dynamic process of the SCR from conduction state to forward blocking state is called commutation process or turn-off process.
Once the thyristor is on, gate loses control. The SCR can be turned off by reducing the anode current below holding current. If forward voltage is applied to the SCR at the moment its anode current falls to zero, the device will not be able to block this forward voltage as the carriers (holes and electrons) in the four layers are still favourable for conduction. The device will therefore go into conduction immediately even though gate signal is not applied. In order to obviate such an occurrence, it is essential that the thyristor is reverse biased for a finite period after the anode current has reached zero.
The turn-off time tq of a thyristor is defined as the time between the instant anode current becomes zero and the instant SCR regains forward blocking capability. During time tq, all the excess carriers from the four layers of SCR must be removed. This removal of excess carriers consists of sweeping out of holes from outer p-layer and electrons from quter n-layer. The carriers around junction J can be removed only by recombination.
The turn-off time is divided into two intervals;(i)Reverse recovery time trr(ii)The gate recovery time tgr
tq = trr + tgr
(i)Reverse recovery time trr
At instant t₁, anode current becomes zero. After t₁, anode current builds up in the reverse direction with the same di/dt slope as before t1. The reason for the reversal of anode current after t1 is due to the presence of carriers stored in the four layers. The reverse recovery current removes excess carriers from the end junctions J1 and J3 between the instants t1 and t3.
In other words, reverse recovery current flows due to the sweeping out of holes from top p-layer and electrons from bottom n-layer.
At instant t2, when about 60% of the stored charges are removed from the outer two layers, carrier density across J1 and J3 begins to decrease and with this reverse recovery current also starts decaying. The reverse current decay is fast in the beginning but gradual thereafter. The fast decay of recovery current causes a reverse voltage across the device due to the circuit inductance. This reverse voltage surge appears across the thyristor terminals and may therefore damage it. In practice, this is avoided by using protective RC elements across SCR.
At instant t3, when reverse recovery current has fallen to nearly zero value, end junctions J1 and J3 recover and SCR is able to block the reverse voltage. For a thyristor, reverse recovery phenomenon between t1 and t3 is similar to that of a rectifier diode.
(ii)The gate recovery time tgr
At the end of reverse recovery period (t3-t1), the middle junction J2 still has trapped charges, therefore, the thyristor is not able to block the forward voltage at t3 The trapped charges around J2. in the inner two layers, cannot flow to the external circuit, therefore, these trapped charges must decay only by recombination. This recombination is possible if a reverse voltage is maintained across SCR, though the magnitude of this voltage is not important. The rate of recombination of charges is independent of the external circuit parameters. The time for the recombination of charges between t3 and t4 is called gate recovery time tgr
At instant t4, junction J2 recovers and the forward voltage can be reapplied between anode and cathode. The thyristor turn-off time tq is in the range of 3 to 100 psec. The turn-off time is influenced by the magnitude of forward current, di/dt at the time of commutation and junction temperature. An increase in the magnitude of these factors increases the thyristor turn-off time. If the value of forward current before commutation is high, trapped charges around junction J2 are more. The time required for their recombination is more and therefore turn-off time is increased. But turn-off time decreases with an increase in the magnitude of reverse voltage, particularly in the range of 0 to -50 V. This is because high reverse voltage sucks out the carriers out of the junctions J1, J3 and the adjacent transition regions at a faster rate. It is evident from above that turn-off time tq is not a constant parameter of a thyristor.
commutation failure
The thyristor turn-off time tq is applicable to an individual SCR. In actual practice, thyristor (or thyristors) form a part of the power circuit. The turn-off time provided to the thyristor by the practical circuit is called circuit turn off time tc.It is defined as the time between the instant anode current becomes zero and the instant reverse voltage due to practical circuit reaches zero, show in Fig. Time tc must be greater than tq for reliable turn-off, otherwise the device may turn-on at an undesired instant, a process called commutation failure.
Thyristors with slow turn-off time (50-100 µsec) are called converter grade SCRs and those with fast turn-off time (3-50 usec) are called inverter-grade SCRs. Converter-grade SCRS are cheaper and are used where slow turn-off is possible as in phase-controlled rectifiers, ac voltage controllers, cycloconverters etc. Inverter-grade SCRs are costlier and are used in inverters, choppers and force-commutated converters.
3. Holding Current
This is the minimum current that must be flowing through the thyristor to maintain it in the on state. The holding current is important because if the current drops below this level, the thyristor will turn off and the circuit will be interrupted.
Conclusion
The switching characteristics of thyristors play an important role in the design and performance of power control circuits. Understanding these characteristics is essential for selecting the appropriate thyristor for a given application and ensuring reliable and efficient operation.
- What is SCR and its working?
- What is the difference between thyristor controlled reactor and thyristor switched capacitor?
- What are 4 applications of SCR?
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FAQ
Q1: What is a thyristor?
A: A thyristor is a solid-state semiconductor device that is used as a switch or as a control element in various electronic circuits. It consists of four layers of alternating N-type and P-type semiconductor material, with three junctions between them.A thyristor is a four-layer PNPN semiconductor device with three terminals, an anode, a cathode, and a gate. It is also known as a silicon-controlled rectifier (SCR). The device can be turned on by applying a small current to the gate terminal. Once it is turned on, the current can flow through the anode and cathode terminals until the current is interrupted or the voltage across the device drops below a certain threshold.
Q2: What are switching characteristics of a thyristor?
A: Switching characteristics of a thyristor refer to the behavior of the device during the switching process. These characteristics include the turn-on time, turn-off time, and the forward and reverse recovery times. The turn-on time is the time required for the thyristor to switch from the off state to the on state, while the turn-off time is the time required for the thyristor to switch from the on state to the off state. The forward and reverse recovery times are the times required for the thyristor to recover from forward and reverse voltage applied to it, respectively.
Q3: What factors affect the switching characteristics of a thyristor?
A: Several factors can affect the switching characteristics of a thyristor, including the gate signal amplitude, the gate signal width, the load impedance, and the temperature. In general, higher gate signal amplitudes and wider gate signal widths result in faster turn-on times, while higher load impedances and lower temperatures result in slower turn-off times.
Q4: What is the role of the gate signal in switching a thyristor?
A: The gate signal is used to trigger the thyristor into conduction, allowing current to flow through the device. The gate signal must exceed a certain threshold voltage, known as the gate trigger voltage, in order to switch the thyristor on.
Q5: What is meant by forward and reverse recovery times in a thyristor?
A: The forward recovery time is the time required for a thyristor to recover from a forward-biased state to an off state, while the reverse recovery time is the time required for the thyristor to recover from a reverse-biased state to an off state. During the recovery process, charge carriers in the thyristor must be removed or neutralized in order to turn off the device.
Q6: What are some common applications of thyristors?
A: Thyristors are commonly used in power electronics applications, such as in motor control circuits, power supplies, and lighting control circuits. They are also used in high-voltage applications, such as in voltage regulators and surge protectors.
Q: What is Thyristor Characteristics
A: The main characteristics of a thyristor include:
- Voltage and Current Handling Capacity : One of the most important characteristics of thyristors is their high voltage and current handling capacity. These devices can handle voltages ranging from a few volts to several kilovolts and currents ranging from a few milliamps to several kiloamps.the maximum voltage and current ratings of a thyristor depend on its construction and the materials used to make it. The maximum voltage rating is determined by the breakdown voltage of the PN junctions in the device, while the maximum current rating is determined by the cross-sectional area of the device.
- Switching Speeds : Another important characteristic of thyristors is their switching speeds. Thyristors can switch on and off very quickly, typically in a few microseconds. This makes them useful in applications that require fast switching speeds, such as motor control and power regulation.the switching speed of a thyristor depends on several factors, including the gate current, the load current, and the external circuitry. In general, higher gate currents and lower load currents result in faster switching speeds.
- Gate Triggering : Gate triggering is the process of turning on a thyristor by applying a small current to the gate terminal. The gate current required to trigger a thyristor is typically very small, typically a few milliamps.there are several ways to trigger a thyristor, including using a voltage pulse, a current pulse, or a combination of both. The choice of triggering method depends on the application requirements and the characteristics of the thyristor.
- Holding Current : Once a thyristor is turned on, it can continue to conduct current even if the gate current is removed. This is known as the holding current. The holding current of a thyristor is typically very small, typically a few milliamps.the holding current is important because it determines the minimum load current that must be applied to the thyristor to keep it turned on. If the load current drops below the holding current, the thyristor will turn off.
- Latching Current : Latching current is the minimum current required to keep a thyristor turned on after it has been triggered. The latching current is typically higher than the holding current and is determined by the device construction and materials.The latching current is important because it determines the minimum gate current required to keep the thyristor turned on. If the gate current drops below the latching current, the thyristor will turn off.
- Forward Voltage Drop : the forward voltage drop of a thyristor is the voltage drop across the device when it is turned on and conducting current. The forward voltage drop is typically very small, typically a few volts.the forward voltage drop is important because it determines the power dissipation of the device when it is conducting current.
- Non-linear current-voltage (I-V) characteristics: Thyristors exhibit non-linear I-V characteristics, meaning that their current flow is not proportional to the voltage applied across them. Once the thyristor is triggered into conduction, it exhibits a low voltage drop across its terminals.
- Unidirectional current flow: Thyristors are designed to conduct current in one direction only, from anode to cathode. Any attempt to reverse the polarity of the applied voltage will result in a high resistance state.
- Voltage controlled switch: Thyristors can be triggered into conduction by applying a voltage pulse to their control terminal, after which they will continue to conduct current until the voltage across their terminals is reduced to a certain level, known as the holding voltage.
- High current handling capability: Thyristors can handle high currents, making them useful for applications where high power is required, such as motor control, power supplies, and lighting.
- High switching speed: Thyristors can switch on and off rapidly, making them suitable for use in high-frequency applications.
- Latching behavior: Once triggered into conduction, thyristors will continue to conduct current even after the control voltage is removed, until the current through them is reduced to zero or below a certain level. This behavior is known as latching and is useful in applications where a holding current is required.
- Gate triggering current: Thyristors require a certain amount of current to be applied to their control terminal in order to trigger them into conduction. This current is known as the gate triggering current and is typically in the range of a few milliamps to several amps, depending on the device's specifications.
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