Introduction to SCR

The silicon-controlled rectifier is an SCR abbreviation, a 3-terminal semiconductor switching device. In power and industrial electronics, SCR is the most important circuit element after the diode, the BJT, and the MOSFET/IGBTs. invented in 1957 as a variation of the Shockley diode, an SCR can be used as a controlled and latched switch to perform rectification, inversion, switching, and regulation of current flow. The SCR is an important device in power electronics because it can handle currents up to several thousand amperes and voltages up to more than 1.5kV.
SCR is a member of the Thyristor family. Thyristors are semiconductor-switching devices. SCR is a basic thyristor, so it is also called a Thyristor, thyroid, and unidirectional solid-state switch.
. SCR is a controlled diode switch and is being extensively used in rectifiers to convert AC into DC. And controlled DC output. In this article, we shall learn the working and structure of SCR with its electrical properties and the various applications in power electronics.

What is Silicon-Controlled Rectifier (SCR)

An SCR is a solid-state device that acts as a solid-state switch. It can convert AC into direct current (DC) and can control the amount of current supplied to the load. Thus, SCR can rectify and control the power supplied to load with phase trimming and pulse width modulation techniques. Simple diode rectifiers cannot be switch-off in forward biasing.

Structure of SCR

When a PN junction is added an NPN bipolar junction transistor with silicon PNPN arrangement with a gate terminal from an internal P-type block. this arrangement of three PN junction devices, called a silicon-controlled rectifier figure, shows its construction. It is essentially an ordinary rectifier diode PN junction and a BJT transistor NPN joined in one block to construct a PNPN device. Three terminals are taken; one from the outer P-type material called Anode or A, the second from the outer N-type material called cathode K, and the third from the sandwiched P-type section and is called gate G. In the normal operating conditions of SCR. SCR is a semiconductor alternative to a thyratron tube. The gate, anode, and cathode of Resembled terminals to the grid, plate, and cathode of the thyratron vacuum tube. the anode is kept at a positive supply voltage disconcerting cathode, positive trigger voltage is applied to the gate to the cathode. The figure shows the SCR symbol and voltage polarity.

The silicon-controlled rectifier is a solid-state equivalent of the thyratron. Another question is, why not a rectifier controlled by germanium? The gadget is composed of silicon because the leakage current in silicon is much lower than in germanium. Because the device is utilized as a switch, it will carry a tiny amount of leakage current when turned off. It was given this name because it is a silicon device that is employed as a rectifier with controllable rectification.

Equivalent Circuit of SCR

The SCR can be partitioned into two bipolar junction transistors as shown in the figure

Turning ON SCR Equivalent Circuit

Thus, the equivalent circuit of SCR is composed of a PNP transistor and an NPN transistor connected as shown in the figure. collector of each transistor is connected to the base of the other BJT, this configuration possesses a positive current feedback loop. If a small current passes through NPN base-emitter junction due to beta-DC a large collector current is flown and passed from PNP base-emitter junction. At this condition PNP collector current starts to flow from the NPN base-emitter. Both transistors’ collector current increases iteratively. Consequently, in a short time, both transistors go into saturation mode. this was about gate triggering by the positive voltage method to turn on the equivalent circuit. But this equivalent circuit also has Shockley diode properties. The Shockley diode doesn’t have a gate terminal and is switched on by breakdown voltage.

So, The forward voltage triggering of an SCR equivalent circuit can be done by gradually increasing the anode terminal voltage. Above figure shows the equivalent circuit of SCR with supply voltage V and load resistance RL. Assume the supply voltage V is less than the BJT break-over voltage. When supply voltage increases to break over voltage level. One or both transistor starts conducting with the gate in an open state there is no base current in the transistor before this. But now transistors are conducting and supplying a base current to each other. Therefore, the anode current also is the base current so both transistors are stands in a latched state.

Turning OFF SCR Equivalent circuit

Like SCR, the switched-on thyristor maintains an anode current. To turn it off anode current must be stopped by removing the supply voltage or bypassing the anode current. If the supply voltage is removed for a while. Once the anode current decreases to zero. The circuit needs again gate triggering to switch on the state. A switch in series with load is enough to interrupt anode current to turn-off SCR.

SCR VI characteristics

forward and reverse biasing characteristics are shown below in figure. upper side is showing forward voltage and current characteristics with gate triggering. lower side is showing reverse current and voltage behavior of SCR.

Forward characteristics:

It is the relationship between an SCR’s anode-cathode voltage (V) and anode current (I) at a constant gate current. Figure 20.7 depicts the V-I characteristics of a standard SCR VI Characteristics of the future. When the anode is positive to the cathode, the curve between V and I is referred to as the forward attribute At IG = 0, O-A-B-C is the forward characteristic of SCR in the characteristic graph. When the supply voltage is raised from zero, a point (point A) is reached when the SCR begins to conduct. The voltage across the SCR abruptly declines in this scenario, as illustrated by the dotted curve AB. And The majority of the supply voltage is seen across the load resistance RL. If the correct gate current is allowed to flow, SCRs can shut with significantly lower supply voltages.

Reverse Characteristics:

When the anode is negative to the cathode, the curve between V and I is referred to as the reverse characteristic When the SCR is used, a reverse voltage is applied to it. AC power supply When the reverse voltage is progressively raised, the anode current stays modest at first i.e., At some reverse voltage, avalanche breakdown occurs, and the SCR begins to conduct. strongly in the opposite direction, as indicated by the curve D to E This greatest reverse voltage is measured at the reverse breakdown voltage is the voltage at which the SCR begins to conduct strongly.

How SCR works as a switch

An SCR circuit’s load is connected in series between the anode and DC positive terminal. The anode is always kept at a positive potential with cathode reference. The cathode of SCR is connected to DC supply negative terminal. Two conditions are considered to learn the switching behavior of an SCR. One is when the gate is open circuit and another, is when a positive voltage is applied to SCR’s gate terminal.

Methods of Turning ON a SCR

SCR not only turns on by gate signal but their are many methods and conditions which may initiates switch on anode current in a SCR. here we will discuss some common methods and conditions which are used to switch on a SCR.

DC Triggering

The SCR can be turned ON by applying a small positive voltage to the gate that is shown in Figure. While gate-to-cathode voltage is positive. Total Source voltage V appears as reverse bias voltage across junction J2 as junctions J1 and J3 are forward biased junction J3 is forward biased and junction J2 is reverse biased. The electrons from the n-type material are mobilized across junction J3 towards the anode side whereas holes from the p-type are towards the cathode side. Consequently, the electrons from junction J3 are attracted across junction J2 gate current starts flowing. when the gate current flows anode current also increases. The increased anode current makes more electrons available at junction J2. This process continues for a short time and junction J2 breaks down and the SCR conducts with the maximum current which is limited by the Load resistance. Now J2 is broken down by anode current and if we remove gate voltage at this state J2 still will remain conducted by anode current. if the gate voltage is removed, the anode current does not decrease, and the maximum supply voltage drops at Load. Stopping the conduction is to reduce the applied voltage to zero or break the anode current path with a switch.

Turning ON SCR by forward voltage method

SCR gate is open. The figure shows the s SCR circuit with the gate open, or no voltage applied between the gate and cathode terminals. in this condition, junction J2 is reverse biased while junctions J1 and J3 are forward biased. Hence, the situation junctions J2 and J3 is like an NPN transistor with an open base. Consequently, SCR is Switched off and no current flows through the load RL. However, if the applied voltage is gradually increasing enough avalanche breakdown occurs at the reverse biased junction J2. Due to the breakdown SCR now conducts and is said to be switched ON. The applied voltage at which SCR is switched on without gate voltage is a called break-over voltage which is exceeded from forward blocking voltage. Because J1 and J3 are forward biased and J2 has broken down. After happening of breakover

The voltage across SCR rapidly decreases to a specific forward voltage. Current through RL rises to the maximum level which depends on load resistance and supply voltage. Turning SCR ON with an anode voltage higher than the SCR breakdown voltage rather than Gate triggering is called forward voltage triggering. In forward voltage triggering, the flow of anode current is established, Anode current passes through the J3 junction which is in the forward-biased state. Holes travel to the J2 P-type block from supply terminals and electrons cross the J3 and J2 junctions. Thus, the J2 depletion region is diminished by break-over voltage and a stable anode current is established. Once the anode current starts to flow and voltage at the anode with respect to the cathode drops, the flow of electrons and holes from the J3 junction keeps open the J2 depletion barrier. Consequently, the anode current doesn’t be unstable upon anode voltage reaching down at SCR’s specific forward voltage. Forward voltage is varied from 0.7V to 1.8V with a small load current it can be larger with a high anode current.
Gate triggering by Positive voltage.

AC Triggering

Once The SCR gate receives a trigger voltage and is turned on, then it does not require further gate voltage to retain its switching state. So that gate voltage may be removed once SCR is switched ON. And continual gate current is not necessary to power up the load. So, AC or varying voltage may be used as a gate trigger input. There are two types of AC triggering of SCR one is AC voltage from the main supply, and another is pulse triggering
Triggering from AC supply

When SCR is being used in a circuit where the main voltage source is AC. At the end of each positive half cycle, SCR needs again trigger voltage to maintain load current in the switch on the state. So, in AC circuits, the gate voltage is fed directly or phase-shifted derived from AC main supply at the start of every positive half cycle. Phase-shifted triggering is used to control the output level of the controlled rectifier or the current level of load.

Pulse triggering

When isolation is required between SCR and triggering circuit for example SCR in a high voltage DC or AC circuit, which is triggered by a microprocessor circuit. There, isolation between the High voltage region and the logic circuit is mandatory. For the seeking of isolation, a pulse transformer is used. A current pulse is passed through the primary coil of the pulse transformer and the induced secondary coil current passes from the gate cathode path of SCR, which is subjected to be triggered. Pulse transformer provides isolation and simplicity of triggering circuit.

Pulse triggering is used in both AC and DC supply circuits. Without input voltage pulse transformer’s secondary coil is forced to pull down the gate terminal with respect to the cathode terminal. Which is sometimes a design requirement to avoid false triggering. A valid pulse is generated from the logic circuit to the pulse transformer drive circuit. The driver circuit may consist of BJT or MOSFET.

dv/dt Triggering

Before the turned-on state, anode voltage is applied SCR state is forward blocking mode, positive voltage is applied to the anode vs negative voltage at the cathode. the junctions J1 and J3 are forward biased and the junction J2 is reverse biased. So, due to the depletion region the J2 junction act as a capacitor. The depletion region is like an insulator between two conductive regions like a capacitor. A capacitor is created with reverse biased which is varied with reverse voltage variations. But a high rate of voltage variation or dv/dt causes the flow of current through the J2 junction by its junction capacitance. If the said current is equal to or more than the threshold current, then SCR cannot maintain the forward blocking state and hold a turned-on state. Mathematical I = dQ/dt

assuming the rate of change of junction capacitance is zero or junction capacitance remains nearly constant over variation of voltage

From the above equation, if the rate of change of the applied voltage is high for example a high voltage pulse is applied to the SCR anode, then a sudden change in anode voltage draws a large surge current for a short time. If this surge current meets the holding current level of SCR, then SCR retains this surge current as holding current after the surge event has been passed.
Junction capacitance is always an undesired parameter. Consequences generated by junction capacitance are also undesired. So, Turning on SCR by applying of a high rate of change in voltage is not a requirement of design but this is a malfunctioning of the SCR circuit in practice.

Temperature triggering

Temperature or Thermal Triggering as the SCR is turned ON by raising its temperature. reverse leakage current is directly proportional to junction temperature, due to the positive temperature coefficient of silicon-based semiconductors. when the temperature is increased to a certain level, the number of holes increases in the inner P-Type layer. Free electrons also increase in the inner N-Type layer. In the depletion region charge carriers are increased so that the leakage current is enormously increased, and the depletion region is squeezed. At a certain temperature and anode voltage, there are enough charge carriers to accomplish a regenerative action inside the SCR, and leakage current also approaches SCR’s minimum gate triggering current. Consequently, this leakage current passes through J3 and J2 junctions and is biased with positive anode voltage.
The number of charge carriers crossing the J2 junction increases continually due to anode voltage. As the analogical circuit’s positive gain raises the anode current rapidly at saturation level and the circuit keeps this current held. Thus, sensitive gate SCRs easily be turned ON by raising the temperature or leakage current.
Thermal triggering is not used in design schemes but often happens in some circumstances particularly when the device temperature is already raised by the environment or previous switch ON operation. At high currents, if SCR is not properly cooled then there are chances of thermal triggering. If control of thermal triggering is not employed in the design. A false thermal triggering may keep the SCR in an ON state even after resetting the anode supply voltage. This situation is analogous to transistor thermal runaway.
There are two considerable Solutions to thermal or false triggering. One is to maintain and ensure the proper heat transfer from SCR to the cooling mechanism.
The second method is to reduce the gate sensitivity of SCR by adding a resistor in parallel with the gate and cathode terminals In this is resistor, most of the gate trigger current passes through a resistor. And SCR requires more gate voltage. Regenerative action is suppressed in case of raising leakage current with temperature. With adding a gate resistor, temperature and anode voltage level are elevated, to happen a false thermal trigger.

Light activation

An SCR which may be turned ON by an exposure to light is also called a Light Activated SCR shortly LASCR. LASCR is specifically manufactured with a window to enable the light to reach SCR’s semiconductor portion, particularly for light-radiation exposure to the J2 junction. In the OFF state, J2 is reverse-biased when light is applied to the window, and J2 start to allow a small current in reverse biasing. The said small current is due to a decrease in fermi-level energy and an increase in the number of free charge carriers in an exposed depletion region. This small current is also an anode current which is magnified by the positive gain action of the PNPN structure. Anode current is furthermore amplified. eventually, gain a saturated level of anode current which is limited by the load resistor. Thus, LASCR is turned ON by light triggering.

Light triggering is also called radiation triggering. LASCRs having different can be activated by different wavelengths. Some LASCRs operate within the visible light range and others in IR or infrared range. IR LASCRs specifically trigger with an Infrared beam in an enlightened environment. The application of LASCR is high voltage DC and AC converters with isolation are required between high voltage lines and trigger control circuits.

SCR turn-off methods.

The turning off SCR is more problematic than it’s turning ON process. When SCR is turned ON, then SCR cannot be controlled by the gate. Even a reverse gate voltage may be ineffective to shut down the anode current. In switched-on SCR anode current is independent of the gate current. Methods of Making an SCR switched off are described under. process Turning off SCR is also called commutation. types and methods of commutation are described below.

Natural commutation or AC line commutation

The above method teaches that SCR may be turned off by interruption in anode current. But in the case of SCR being used in an AC circuit, at each negative half cycle, SCR goes in a switch-off state due to reverse anode voltage. Assuming that the input peak reverse voltage is under the specific break-over voltage range. SCR must be retriggered upon each positive half-cycle to switch it on. And SCR goes in the switched-off state when the supply voltage is reduced enough or reversed in polarity. So, in AC circuits SCR does not need any additional circuit or arrangement to turn off. However, to keep the switch-on state, a gate current is required in each forward voltage cycle.
Switching off an SCR can be performed easily and without complexity in AC circuits. So, in AC supply circuits SCRs are widely used. Especially in single and multi-phase-controlled AC to-DC converter applications, SCR is the best option due to its high current and voltage capabilities. In a controlled rectifier, we can adjust the rectifier output voltage by controlling the AC voltage cycle width with gate triggering. In high-power DC supplies thyristors ratings meet the requirements for controlled output voltage operation as compared to transistors and MOSFETs. So high voltage AC to DC controlled converters is mostly built with SCR.  

Load commutation

Load commutation is a type of commutation technique in which the holding current of SCR is dropped below to holding current by the load itself rather than using an external circuit. So, this type of commutation is also known as self-commutation. In this commutation technique load itself oscillates the anode current due to the resonance of added inductive and capacitive elements in the load. When SCR throughs anode current, the RLC circuits step response is a ringing or oscillation in the anode current. As soon as the gate is triggered, the anode current raises and then falls toward zero. When anode current reaches below holding current, then SCR goes into the forward blocking mode or is said to be switched off. An example of load commutation is shown in the figure.

We have two different configurations of series LC resonance circuits. One with a capacitor is parallel to the load or shunt capacitor arrangement, and the other is a capacitor and inductor in a series of loads. when Load and capacitor are paralleled is energy dissipative configuration. And series capacitor allows anode current only in charging even without L2. So, series configuration mitigates the commutation failure. Commutation failure may damage the SCR or load due to its impedance nature. Load commutation is often used in chopper circuits of voltage converters. Chops of anode current are achieved by repeatedly triggering the SCR. And a defined duration of chops is evaluated by rethe sonance frequency of the series inductor and capacitor.

Advantages and disadvantages of self-commutation

The load commutation circuit does not require an additional thyristor or switching device due to its application turn-on time is fixed. And no need to control commutation. Reduction of the requirement of the additional thyristor lowers the system cost.
In the Load commutation circuit, the turn-off time is fixed and we cannot vary the anode current duration after the gate is triggered. Turn-on time depends on inductance and capacitance values. Which are commercially available with lower accuracy tolerances. Which leads to inaccurate results. Another problem of load commutation for high power loads is choosing capacitor value for a large time. Bulky capacitors or inductors may require for some load commutation designs. small and medium power SCR circuits with Load commutation arrangement can be developed with readily available components.

Forced commutation.

Resonance impulse commutation

This kind of commutation uses an LC series resonance circuit across the SCR. Since the commutation circuit has low resistance and is underdamped, meaning that the current in the LC circuit oscillates anytime the SCR T2 is on.
The capacitor is charged to V volts while the SCR T1 is first switched off. When T1 is in the ON state. Diode D act as reverse biased as capacitor negative charging voltage is applied at the diode’s anode terminal. Capacitor C charging voltage remains constant after T1 is switched on. Current passes through the load, and its constancy is assumed. SCR T2 at the same moment A short circuit initiates oscillation in the LC combination at the resonance frequency. A current begins to flow in the forward direction of T1 and T2 illustrated in the schematic.

As the oscillation cycle changes and the oscillation current changes its direction to reverse of SCRs polarity. The resultant current of ringing and load current decreases below the T1 holding current and switches off. At the same time, T2 also switches off due to the reverse anode current. This commutation is also called controlled commutation. This means that SCR can be switched ON and OFF anytime with dedicated inputs.

Anode current interruption in DC circuit.

If SCR anode current path is broken for a while, then the SCR anode current is dropped to no current. And SCR goes to a turn-off state. when the anode voltage has appeared again then SCR behaves as a switched-off state. And turn on SCR required a further gate trigger or any other turning-on methods. When the anode current is reduced below SCR’s holding current, at this stance SCR does not retain a conduction state due to the minimum anode current required to keep alive a current latching effect of thyristor’s four junctions. SCR starts to reduce anode current rapidly, due to the combination of anode current and tendency to break J2 depletion region is not sufficient. A holding current is a minimum current that can keep breaking the J2 depletion region. So, SCR can be turned-off by terminating the anode current or reducing it below its specific holding current.

Forced commutation with a capacitor in parallel

Forced commutation refers to the process of discharging a capacitor in parallel with an SCR to switch it off. schematic depicts the forced commutation of an SCR, with capacitor C doing the commutation.

Assuming the SCRs are switches, current flows via the load and C as shown in the circuit schematic. C is connected across T1 when T2 is ON. When the charge on C is opposite the forward voltage of T1, T1 is shut off and the current is
transferred to the R-T2 channel.

Important Parameters of SCR

In the study of SCR, the following terms are often faced:
(I) Breakover voltage
(ii) Peak reversal voltage
(iii) Holding current
(iv) Forward current rating
(v) Surge current

(a) Peak reverse voltage

It is the highest voltage in the opposite direction (cathode positive with reference to anode) that can be put on an SCR without making it work in the wrong way. When connecting an SCR to an Alternating Current circuit, it is important to keep in mind the peak reverse voltage. During the negative half of an AC supply, the reverse voltage is applied across the SCR. If the reverse voltage is exceeded, avalanche breakdown may happen, and the SCR will be damaged if the current is not limited by an external circuit. SCRs that are sold on the market have PRV ratings of up to 2500V.

(b) Breakover voltage

I Breakover voltage. It is the minimum forward voltage at which the SCR starts to conduct heavily, which means it is turned on. The gate is open. if an SCR’s break-over voltage is 450 V, it can block a forward voltage, the SCR stays open if the supply voltage is less than 450 V. SCR will be turned on if the supply voltage is higher than this value. In practice, the SCR is turned on by applying a small voltage to the gate. This is done when the supply voltage is less than the break-over voltage. Break-over voltages for SCRs sold on the market range from about 40V to 0.5KV.

(c) Holding current

When the gate is open and the anode current is at its highest, the SCR turns off. an SCR can’t be turned off when it’s conducting , even if the gate voltage is taken away. The only way to turn off or open the SCR is to lower the supply voltage to almost zero. At that point, the internal transistor comes out of saturation and opens the SCR. In this case, the anode current is very small and is called the holding current. So, if an SCR has a holding current of 10mA, it means that the SCR will turn off if the anode current is less than 10mA.

(d) Surge current

It is the sum of the square of the forward surge current and the time the surge lasts.
Rating for SCR fuses = I2 x t
The rating of the circuit’s fuses shows how much forward surge current that an SCR can handle. For example, think about an SCR with a rating of 100A for 3s for circuit fusing. If this rating is exceeded in the SCR circuit, the device will be broken because it will lose too much power.

(e) Forward Current rating

It is the most anode current that can pass through an SCR without destroying it. Every SCR can safely handle a certain amount of forward current. If the current goes over this amount, the SCR may be destroyed because the junctions will heat up so much. For example, if an SCR has a forward current rating of 30A, it means that it can safely carry only 30A. SCRs that are sold on the market have forward current ratings between 30A and 200A.