Silicon Controlled Rectifier

The SCR or thyristor is one type of semiconductor device and using in high-power switching applications is exceptionally planned. The working of this device should be possible in a switching mode only and acts as a switch. When the SCR is triggered by its gate terminal into the transmission, then it will supply the current constantly. While planning a SCR or Thyristor circuit, exceptional focus ought to be expected for enacting the circuit. This article examines various techniques for SCR setting off or SCR turn ON strategies or setting off of Thyristors. There are different setting off strategies are accessible in light of different substances which incorporate temperature, voltage, and so on.

What is a Silicon Controlled Rectifier?

Silicon Controlled Rectifier is a four-layer current-controlling device, which is used in devices like dimmers. These are used in device that require the control of high power and high voltage. The working of the whole region of the SCR circuit predominantly relies upon the method of its triggering. Some of the related topics to SCR are discussed below.

Key Terminologies of SCR

• SCR (Silicon Controlled Rectifier): The complete name of the Device, addressing a four-layer, three-terminal semiconductor device that conducts current in one direction only.
• Anode (A): The Positively charged terminal of the SCR. It is the terminal where the current enters the device.
• Cathode (K): The negatively charged terminal of the SCR. It is the terminal where the current exit the device.
• Gate (G): The control terminal of the SCR. Applying a little current or voltage to the gate controls the conduction of the device.
• Forward blocking Mode: The mode wherein the SCR is in a non-conducting state, and a positive voltage is applied to the anode regarding the cathode. In this mode, the gate signal isn’t applied or is deficient to trigger conduction.
• Reverse Blocking Mode: The mode wherein the SCR is in a non-conducting state, and a negative voltage is applied to the cathode concerning the anode.
• Forward Conduction Mode: The mode wherein the SCR is in a directing state, permitting current stream from the anode to the cathode. This is set off by applying an adequate gate signal in the forward blocking mode.
• Latching Current (I-L): The base current expected to keep the SCR in the leading state after it has been set off. When the SCR is locked, it stays in the ON state regardless of whether the gate signal is taken out.
• Holding Current (I-H): The base current expected to keep up with the SCR in the ON state after it has been triggered. In the event that the current falls underneath the holding current, the SCR will switch off.
• Gate Triggering Current (I-GT): The base current expected at the gate to trigger the SCR into conduction.
• Gate Triggering Voltage (V-GT): The base voltage expected at the door to set off the SCR into conduction.
• Turn off Time (T-OFF): The time expected for the SCR to switch off after the gate signal is taken out.
• Critical Rate of Rise of Off-State Voltage (dv/dt): The most extreme rate at which the off-state voltage can change without triggering the SCR accidentally.

Construction of SCR

Silicon Controlled Rectifier (SCR) is basically a four layered, three junction and three terminal device. The four layer are PNPN, the three Junction are namely J_{1 ,} J₂ and J₃ and the three terminals are anode(A), cathode(K) and the gate (G) as shown in figure (a) and (b).

P-N-P-N Type SCR

A SCR is built with the four layers that comprise of the P-type and the N-type semiconductor material. These are layered so that it will in general frame three intersections that are J1, J2, and J3. The three terminals that are appended to it are known as anode, cathode, and gate. The anode is the essential terminal through which the ongoing streams or enters the gadget. Where the cathode is the terminal through which the entered current leaves the device.

The ongoing entering terminal is of positive extremity and the terminal through which the current is leaving is of negative extremity. In the middle of between the progression of current among the terminals, there should be a terminal that can give the control. This can be given by the terminal door. This terminal is at times additionally alluded to as the terminal of control.

Allow us to consider a SCR is of P-N-P-N type. For this situation, as the anode is associated at the over that is to P-type and the cathode is associated toward the end that is for N-type. Where the terminal gate is likewise associated with the p-type however it will be the subsequent P-type in the succession. Thus the gate terminal is situated so that it is closer to the terminal cathode.

In this, the intersection J1 is in the middle of between the main P-type and the N-type. The second intersection J2 will in the middle of between the N-type and the subsequent P-type layers. The third intersection will in the middle of between the last P-type and the N-type layers. In view of the prerequisite or the need of the applications these layers of the Thyristor are doped. The silicon liked here for its development is of characteristic sort.

Silicon Controlled Rectifier Symbol

Symbolic Representation of SCR

Importance of Gate Signal in SCR

SCR is simply a conventional rectifier controlled by a gate signal. However, the application of a forward voltage is not enough for conduction of SCR. Hence, a gate signal is applied to control SCR from an off state to the on state for a small period of time for which the anode current becomes equal to the latching current. Once the anode current attains this value, the gate losses the control and hence can be removed. If the anode current decreases to a value called as holding current, the SCR will go back to the forward blocking state(region in which SCR does not conduct any current expect a very small value of the leakage current). Under such condition care must be taken that, the gate signal is present until the anode current rises to the latching current, so as to assure that the complete turn-on of the SCR. Hence, the gate signal controls the rectifier conduction.

Operation of SCR

Figure shows the schematic diagram of SCR. It consists of four layers(i.e., p, n, p and n) three terminals A(Anode), K(Cathode) and G(Gate) and three junction J_{1 ,} J_{2 ,} and J_3.

Basic operation diagram of SCR

The anode and cathode terminals are connected to the main power circuit wherever, the gate terminal is connected to the control circuit. Whenever the P layer near the anode is made more positive compared to N layer near the cathode, Junctions J₁ and J₃ get forward biased. Whereas, the middle junction J₂ gets reverse biased. Junction J₂ is known as a junction capacitance as it acts as a capacitor in this mode. Due to the depletion layer formed at this junction, no current flows through the device, but due to the drift of mobile charge carriers, a small amount of leakage current flows through the circuit.

As the leakage current is negligible small, the device does not conducts. This state is know as forward blocking state or OFF state of the device, as it blocks the forward biased voltage.

Whenever the N layer near the cathode is more positive compared to P layer near the anode, junctions J₁ and J₃ are in reverse biased and middle junction J₂ get forward biased. As the junctions J₁ and J₃ are in reverse biased condition, they do not allow any amount of current to flow through the device. This state is know as reverse blocking state or OFF state.

By increasing the voltage across the anode and cathode, the width of the depletion layer at the junction J₂ may be reduced. At some particular voltage, J₂ disappears this is due to the breakdown of the junctions by large voltage gradient also called as avalanche breakdown. As junctions J₁ and J₃ are already in the forward biased condition, there exists a free carrier movement from anode to cathode. As a result, the device starts conducting and hence it is said to be in the conducting state or ON state.

Modes of Operation in SCR

In SCR there are three junctions J_1, J₂ and J_{3 .} These junctions play a very important role in the operation of SCR. Depending on whether the junctions are forward biased or reverse biased, the SCR will either conduct the current or will not be conduct. For SCR to conduct current, all the three junctions must be forward biased. If any one of the junctions is reverse biased the SCR will not conduct any current and hence acts as an open switch. This can be explained by considering the following cases,

• Reverse Blocking: In this Case the Positive Voltage is given to Cathode and Negative Voltage is given to Cathode and Gate is Open Circuited. In this case, the junction J₁ and J₃ are reverse biased, whereas J₂ is forward biased. As the Junctions J₁ and J₃ are reverse biased so there will be no Current, But there will be Small leakages in the Forward Junction J₂ As the leakage current is Very Small which will not Turn on SCR. Hence the SCR will be in OFF State.

Reverse Blocking

• OFF State Forward Blocking mode: In this case, Positive Voltage is Given to Anode and Negative Voltage is Given to the Cathode and Gate is Open Circuited. The junction J₂ is reverse biased, whereas junctions J₁ and J₃ are forward biased, Because of the Reverse Bias Voltage the Width of the Depletion Region which Block the Flow of Current Between J₁ and J₃.Again due to which the SCR does not conduct and acts as an open switch.

OFF State Forward Blocking mode

• ON State Conduction mode: The SCR will Conduct Current By Increasing Forward Bias Voltage Above the Break Down Voltage and By Applying Positive Voltage at gate terminal. In this case, all the three junctions are forward biased and hence SCR gets turned ON and starts conducting. Here, the SCR acts as a closed switch.

ON State Conduction mode

Hence, from the above analysis, it is clear that all the three junctions play important role in the operation of SCR and they all must be forward biased in order to turn ON the SCR.

V-I characteristics of SCR

The VI characteristics are also classified into Static and Dynamic characteristics of SCR as follows:

Static V-I Characteristics of SCR

The V-I characteristics of SCR is a graph between the anode current I_A and the anode-cathode voltage V_A for different values of gate current I_G. This characteristics can be drawn by considering the basic operation of the SCR. The below figure shows the V-I characteristics which is also called as static-cathode characteristics. It basically consist of three regions, They are

• Region 1
• Region 2
• Region 3

Static V-I Characteristics of SCR

Region 1: When the positive terminal of the supply is connected to cathode and the negative terminal is connected to anode with gate circuit open then SCR operates in region 1. In this region junction J₁ and J₃ becomes reverse biased, whereas the junction J₂ becomes forward biased. The reverse biased junctions (J₁ and J₃) acts as open circuit and the forward biased junction(J₂) acts as a short circuit, as shown in figure.

From figure it is clear that, the SCR does not conduct any current and it is said to be in reverse blocking state or OFF state. However, a very small amount of leakage current flows through it. Now, if we increase the reverse voltage to the value called as reverse breakdown voltage, then an avalanche will occur and breaks the junction J₁ and J₃. Due to this a very huge amount of the current flows through the circuit and hence the SCR starts conducting. But this method of conducting SCR or the circuit in which it is working. Hence, we must ensure that the reverse voltage does not exceed the reverse breakdown voltage V_AK.

Region 2: When the positive terminal of the supply is connected to anode and the negative terminal is connected to cathode with gate circuit open then SCR operates in region 2. In this region junction J₁ and J₃ becomes forward biased, whereas the junction J₂ gets reverse biased. The forward biased junctions (J₁ and J₃) acts as short circuit and the reverse biased junction(J₂) acts as a open circuit, as shown in figure. Even in this region, the SCR does not conduct any current expect a very small value of the leakage current. This mode of SCR is called as forward blocking mode. Just as the region 1, i. e., reverse blocking mode, the SCR can be made to conduct in the forward blocking mode by increasing the anode-cathode voltage to a value called as forward breakdown voltage(V_BO). Even this method is not recommended as it may also damage the SCR. Hence, the SCR does not conduct even in this mode and is treated as open switch.

Region 3: When the positive terminal of the supply is connected to anode and the negative terminal to cathode with gate circuit closed the SCR operates in region 3. In this region, all the three junctions (J_1, J₂ and J₃) act as short circuit shown in figure and hence conducts current. In this region SCR is said to be in a forward conduction mode and hence acts as a closed switch. This method of conducting the SCR is the most efficient, as it requires a voltage which is very much less than V_BO. The only extra thing we require is a gate signal for a small period of latching current. Once the anode current attains this value, the gate losses the control and hence can be removed. The removal of the gate signal will not have any effect on the SCR conduction. However, if the anode current decreases to a value called ad holding current, the SCR will once again go back to the forward blocking gate. Hence, care must be taken that, the anode current should not drop below the holding current after the gate signal is removed.

Dynamic or Switching Characteristics of SCR

During turn ON and turn OFF process, thyristor is subjected to different voltages across it and different currents through it. The time variation of the voltage across a thyristor and the current through it during turn ON and turn OFF constitute the switching Characteristics of a thyristor.

Turn ON Switching Characteristics

A forward biased thyristor is turned ON by applying a positive gate voltage between the gate and the cathode, as shown in figure(1).

Circuit Diagram of a Turn ON Switching Characteristics of SCR

Turn ON Switching Characteristics of SCR

Figure(2) , shows the waveforms of the gate current(I_G), anode current(I_A) and anode to cathode voltage(V_AK). The total switching period being much smaller compared to the cycle time, I_A and V_AK before and after switching will appear flat.

As shown in figure , there is a transition time “T_off” from forward OFF state to forward ON state. This transition time is called the thyristor turn ON time and can be divided into three separate intervals namely, They are

• Delay time (T_d)
• Rise time (T_r)
• Spread time ( T_p)

Delay Time (T_d)

It is the time between the instant at which the gate current reaches 90% of its final value and the instant at which the anode current reaches 10% of its final value. It is the time taken by the anode voltage to fall from V_AK to 0.9 V_AK

Rise Time (T_r)

For a resistive load, “rise time” is the time taken by the anode current to rise from 10% of its final value to 90% of its final value. At the same time, the voltage V_AK falls from 90% of its initial value to 10% of its initial value. However, current rise and voltage fall characteristics are strongly influenced by the type of the load. For inductive load the voltage falls faster than the current. While, for a capacitive load, current rises rapidly.

Spread Time ( T_p)

It is the time taken by the anode current to rise from 90% of its final value to 100%. During this time conduction spreads over the entire cross-section of the cathode of the thyristor. The spreading interval depends on the area of the cathode and on the gate structure of the thyristor.

Turn OFF Switching Characteristics

Once the thyristor is ON and its anode current is above the latching current level, the gate losses control. It can be turned OFF only by reducing the anode current below the holding current. The OFF time t_q of a thyristor is defined as the time between the instant anode current becomes zero and the instant the thyristor regains forward blocking capability. If forward voltage is applied across the device, during this period the thyristor turns ON again.

During turn OFF time, excess minority carriers from all the four layers of the thyristor must be removed. Accordingly, t_q is divided into two intervals, the reverse recovery time(t_RR) and the gate recovery time(t_GR), figure shows the variation of the anode current and the anode to cathode voltage with time during turn OFF operation for an applied sinusoidal voltage(V_I).

Turn OFF Switching Characteristics of SCR

The anode current becomes zero at time t₁ and starts growing in the negative direction with the same Di_A / Dt till time t₂. This negative current removes excess carriers from the junctions J₁ and J₃. At time t₂ excess carriers densities at these junctions are not sufficient to maintain the reverse current. The value of the anode current at time t₂ is called as the reverse recovery current(I_RR). The reverse anode current reduces to the level of reverse saturation current. The total charge removed from the junctions between t₁ and t₃ is called the reverse recovery charge(Q_RR). Fast decaying reverse current during the interval t₂ – t₃ coupled with the di / dt limiting inductor may cause a large reverse voltage spike to appear across the device. This voltage must be limited below the V_RRM rating of the device. Up to time t₂ the voltage across the device(V_AK) does not change substantially from its state value. However, after the reverse recovery time, the thyristor regions the reverse blocking capacity and V_AK starts following the supply voltage. At the end of the reverse recovery period(t_rr) trapped charges still exist at the junction J₂ which prevents the device from blocking forward voltage just after t_rr. These trapped charges are removed only by the process of recombination. The time taken for this recombination process to complete between t₃ and t₄ is called the gate recovery time(t_gr). The time interval t_q = t_rr + t_gr is called “device turn OFF time” of the thyristor.

No forward voltage should appear across the device before the time t_q in order to avoid its inadvertent turn ON. While designing an SCR circuit, one must provide a time interval (t_c > t_q) during which a reverse voltage is applied across the device, where t_c is the circuit turn OFF time.

Types of SCR

• Discrete Plastic
• Plastic Module
• Press Pack

Discrete Plastic

A Discrete Plastic SCR (Silicon Controlled Rectifier) alludes to an individual SCR bundled in a plastic lodging. This bundling gives a defensive and protecting nook for the semiconductor gadget, making it reasonable for different electronic applications. Here are a few vital highlights and qualities of a Discrete Plastic SCR

Construction of Discrete Plastic

Semiconductor Layers: Like other SCRs, a Discrete Plastic SCR comprises of exchanging layers of p-type and n-type semiconductor materials, shaping a P-N-P-N structure.

Gate Design: The SCR incorporates a gate structure, commonly made of metal or one more conductive material, put between the anode and cathode.

Packaging of Discrete Plastic

Plastic Enclosure: The semiconductor gadget is housed in a plastic bundle that gives mechanical security, electrical insulation, and ecological isolation.

Lead Design: The plastic package has leads for external associations. Normal lead setups incorporate TO-92, TO-220, or other industry-standard packages.

Plastic Module

A Plastic Module SCR (Silicon Controlled Rectifier) alludes to a SCR semiconductor gadget that is epitomized in a plastic module for security and simplicity of reconciliation into electronic circuits. This kind of bundling joins the vigor of module-style bundling with the advantages of a plastic nook. Here are key elements and qualities of a Plastic Module SCR

Construction of Plastic Module

Semiconductor Layers: Like other SCRs, a Plastic Module SCR comprises of layers of p-type and n-type semiconductor materials organized in a P-N-P-N structure.

Gate Structure: The SCR module incorporates a door structure, frequently made of metal or one more conductive material, situated between the anode and cathode for controlling conduction.

Packaging of Plastic Module

Plastic Module Enclosure: The semiconductor gadget is housed in a module-style plastic bundle that gives security against ecological variables, mechanical burdens, and electrical protection.

Lead Design: The module normally includes terminal leads or pins that work with simple association with outside circuits. The lead setup relies upon the particular module plan.

Module Size and Form Factor: The plastic module bundling can differ in size and structure factor, contingent upon the power rating, expected application, and industry norms.

Press Pack

A Press Pack SCR (Silicon Controlled Rectifier) alludes to a SCR semiconductor gadget that is typified in a powerful and strong press pack lodging. The press pack configuration offers mechanical help and warm administration, making it reasonable for high-power and high-voltage applications. Here are key elements and qualities of a Press Pack SCR

Construction of Press Pack

Semiconductor Layers: Like other SCRs, a Press Pack SCR comprises of layers of p-type and n-type semiconductor materials organized in a P-N-P-N structure.

Gate Structure: The SCR incorporates an entryway structure, frequently made of metal or one more conductive material, situated between the anode and cathode for controlling conduction.

Packaging of Press Pack

Press Pack Enclosure: The semiconductor gadget is housed in a press pack, which is an unbending, generally metal, nook that offers mechanical help and warm dispersal.

Cooling Components: Press pack SCR modules frequently incorporate cooling components, for example, heat sinks or other warm administration parts to disperse the intensity created during activity.

Terminal Leads: The module highlights terminal leads or pins that work with simple association with outside circuits. The lead setup relies upon the particular module design.

Turn ON methods of an SCR

Triggering is nothing but switching the device from a non-conducting(OFF) state to a conducting (ON) state. The various methods of thyristor triggering are as follows,

• Forward Voltage Triggering
• Gate triggering
• Thermal triggering
• Light triggering
• dv / dt triggering

Forward Voltage Triggering

With the gate circuit opens, the thyristor will not start conducting with the normal value of forward anode to cathode voltage. However, if this voltage is increased beyond the forward break over voltage V_B an avalanche will occur and hence the SCR starts conducting. However, this method of triggering is never recommended, as it may damage the SCR or the whole circuit.

Gate Triggering

Gate triggering is the most efficient and the most commonly used method of triggering the SCR. In this method, small gate pulse is applied between gate and cathode along with the normal anode to cathode. With this, a gate current will be established which will J₂ will be forward biased. As the other two junctions J₁ and J₃ was already forward biased, the SCR starts conducting.

Thermal Triggering

During forward blocking mode of SCR, junctions J₁ and J₃ will be forward biased and hence acts as a short circuit; whereas, junction J₂ will be reverse biased, which acts as an open circuit. Due to this, the applied voltage will appear across the open terminals of junction J₂ along with some leakage current. This leakage current will increase the temperature of the junction J₂ (because of junction resistance) which in turn increases the mobile charges and hence the current. This process is cumulative and hence may turn ON the SCR at point.

Light Triggering

In this method, the inner p-layer is radiated with a light ray of an appropriate wavelength and intensity. With this, free charge carriers will be injected in the junction J₂ and the junction J₂ will be forward biased and hence the SCR gets turned ON. SCRS, employing this technique, are called as LASCR (Light Activated SCR).

dv/dt (V) Triggering

We know that under forward blocking mode, junction J₁ and J₃ are forward biased; whereas, junction J₂ is reverse biased. The junction J₂ acts as a capacitor because of the opposite polarity carriers present at its two terminals. Hence, the leakage current through the junction is nothing but the capacitor current given by,

i_c = C dv / dt

Where, C = Junction capacitance

V = Anode to cathode voltage

If the rate of rise of the voltage V is very high i.e., the voltage across the SCR increases within no time i_c will very large (from above equation ). This large value of current will turn ON the SCR immediately.

Advantages of SCR

• Fast switching speed: SCRs can turn on and off somewhat rapidly, which is significant for applications requiring exact timing and control.
• Good thermal stability: SCRs can deal with high temperatures without critical execution corruption, making them reasonable for high-temperature conditions.
• High current and voltage handling capacity: SCRs are equipped for taking care of high current and voltage levels, making them reasonable for power control applications.
• Low conduction losses: When in the “on” state, SCRs have low voltage drop and negligible power scattering, which brings about proficient conduction.
• Easy gate control: The terminating of a SCR is constrained by applying a door current, making it simple to set off and control.
• High reliability: SCRs have a long functional life and can endure cruel ecological circumstances, making them dependable for the overwhelming majority modern applications.

Disadvantages of SCR

• Gate triggering can be sensitive to noise: The door setting off instrument of SCRs can be delicate to electrical commotion, so legitimate assurance and separating might be vital.
• Unidirectional conduction: SCRs lead current in only one bearing, making them unacceptable for applications that require bidirectional control of current.
• Potential for voltage spikes: At the point when a SCR is switched off, it can produce voltage spikes in the circuit, which might require extra defensive parts.
• Lack of inherent turn-off capability: When a SCR is set off (turned on), it will stay in the “on” state until the ongoing through it dips under a specific limit or is switched. This can be a constraint in certain applications.
• Limited frequency response: SCRs are not appropriate for high-recurrence applications because of their mood killer attributes and entryway control constraints.
• Higher cost compared to some alternatives: At times, SCRs might be more costly than other semiconductor gadgets, like MOSFETs or IGBTs, for explicit applications.

Applications of SCR

• Voltage Regulation: In applications like voltage controllers and stabilizers, SCRs are utilized to control the result voltage and keep up with it inside a predetermined reach, guaranteeing a steady power supply to delicate gear.
• Protection Devices: SCRs can act as security gadgets in flood silencers, where they redirect overabundance voltage and current to protect electronic gear from voltage spikes and floods.
• Uninterruptible Power Supplies (UPS): SCRs are utilized in UPS frameworks to change from network capacity to battery power during blackouts, guaranteeing a nonstop and solid power supply to basic gear.
• Power Control: SCRs are broadly utilized in power control applications, for example, dimmer switches for lighting, engine speed control, and stage point control for warming components. They can direct how much power conveyed to a heap by controlling the terminating point of the SCR.
• Rectification: SCRs can be utilized as high-power rectifiers in power supplies and battery chargers, changing over rotating current (AC) into direct current (DC). They are especially valuable in applications that require high voltage and current levels, as modern rectifiers.
• Thyristor Control Panels: SCRs are used in thyristor control boards for the productive control of different modern cycles and apparatus, like electric curve heaters, welding machines, and modern stoves.
• High-Power Heating: SCRs are utilized for exact control of electric warming components in applications like electric ovens, furnaces, and modern heaters.
• Energy Conservation: SCRs are utilized in energy protection frameworks to streamline the power factor and decrease energy utilization in modern plants and offices.
• Soft Starters: SCRs are utilized in delicate starter circuits to step by step increase the voltage and flow to electric engines, diminishing mechanical pressure and limiting inrush flow, which can expand the life expectancy of the engine and decrease mileage.
• Light Dimming: In dramatic lighting and a few modern settings, SCRs are utilized to control the force of lighting apparatuses and lights, giving smooth and exact darkening control.

FAQs on Silicon Controlled Rectifier

What is an SCR?

A SCR, or Silicon Controlled Rectifier, is a semiconductor gadget utilized for controlling and exchanging electrical flows. It is a sort of thyristor and is known for its capacity to deal with high-power applications.

What are the vital components of a SCR?

A SCR normally comprises of three principal terminals: anode, cathode, and door. It likewise has a fourth layer, the p-type or anode entryway district, which recognizes it from other thyristors.

How does a SCR work?

A SCR permits current to move from anode to cathode when a little door current is applied. Once set off, it stays in the directing state until the ongoing falls under a specific level or is switched.

Are SCRs suitable for DC applications?

Indeed, SCRs can be utilized in DC applications, as they are equipped for correcting DC and controlling DC loads.

Can SCRs be used in high-frequency applications?

SCRs are by and large not reasonable for high-recurrence applications because of their mood killer qualities. Other semiconductor gadgets, as MOSFETs and IGBTs, are more qualified for high-recurrence exchanging.