Overview of The SCR: What It is & What it Does

SCR - transistors
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The diodes permit electric current in a single direction and block electric current in the alternative direction. In other arguments, the diode converts the AC into DC. The diodes’ unique behavior makes it possible to build different rectifiers such as half-wave, full-wave, and bridge rectifiers. The rectifiers convert the Alternating Current into Direct Current.

The half-wave, full-wave, and bridge rectifiers use standard p-n junction diodes (two-layer diodes). If the voltage provide to these diodes is high enough, then the diodes might get damaged or totally destroyed. Thus, the rectifiers cannot be operated at high voltages.

To overcome these drawbacks, scientists have developed a particular type of rectifier known as Silicon Controlled Rectifier. These rectifiers can withstand high voltages.

What is Silicon Controlled Rectifier (SCR)

The SCR is a 3 terminal and 4-layer semiconductor current controlling device. It is mainly used in the apparatus for the control of high power. The silicon-controlled rectifier is sometimes referred to as the SCR diode, FOUR-layer diode, FOUR-layer device, or Thyristor. It is made up of a silicon material that controls high power and converts high AC into DC (rectification). Hence, it is named a silicon-controlled rectifier.

SCR is a unidirectional current controlling device. Like a standard p-n junction diode, it allows electric current in only one direction and blocks electric current in another direction. A standard p-n junction diode is made of two semiconductor layers, namely P-type and N-type. However, an SCR diode is made of 4 semiconductor layers of alternating P and N-type elements.

The working policy of p-n-p-n switching was developed by Tanenbaum, Goldey, Moll, and Holonyak of Bell Laboratories in 1956. The silicon-controlled rectifier was created by a team of power engineers led by Gordon Hall and marketed by Frank W. “Bill” Gutzwiller in 1957. Names like SCR and controlled rectifiers often refer to the early days of this device development. However, nowadays, this device is often referred to by Thyristor.

Silicon-controlled rectifiers are used in power control applications such as power delivered to electric motors, relay controls, or induction heating elements. The electric power provided has to be controlled.

Construction of Silicon Controlled Rectifier

The schematic symbol of a silicon-controlled rectifier is shown in the below figure. An SCR diode consists of three terminals, namely Anode (A), Cathode (K), Gate (G). The diode arrow represents the direction of the conventional current.

symbol of SCR

A silicon-controlled rectifier is made up of 4 semiconductor layers of alternating P and N-type materials, which form NPNP or p-n-p-n constructions. It has 3 P-N junctions, namely J1, J2, J3, with three terminals attached to the semiconductors’ materials, called Anode (A), Cathode (K), and gate (G). The Anode is a positively charged electrode through which the conventional current enters into an electrical device, and Cathode is a negatively charged electrode through which the traditional current leaves an device. Gate terminal controls the current flow between Anode and Cathode pin. Gate terminal is also occasionally referred to as the control terminal.

The anode terminal of the SCR diode is connected to the first p-type material of a PNPN structure, the cathode terminal is connected to the last n-type material, and the gate terminal is linked to the second p-type material of a PNPN structure which is nearest to the Cathode.

Construction of SCR

In a silicon-controlled rectifier, silicon is used as a basic semiconductor. At the time of pentavalent impurities addition to this Silicon, an N-type semiconductor is designed. When trivalent impurities are added to Silicon or an intrinsic semiconductor, a P-type semiconductor is formed.

When four semiconductor layers of alternating P and N-type materials are placed one over another, three junctions are formed in the p-n-p-n structure. In a p-n-p-n, the junction J1 is created between the first P-N layer, the junction J2 is created between the N-P layer and the junction J3 is formed between the last P-N layer. The doping of PNPN structure depends on the application of SCR diode

How SCR works

It is a little difficult to understand how SCR works. Depending on the polarity of the voltage applied and the gate pulse given to the SCR, it can work in three different modes such as

  • Forward Blocking mode
  • Forward Conduction mode
  • Reverse Blocking mode

Now, let’s understand the SCR working by looking at separately the operating modes by its circuit diagram.

Forward Blocking Mode 

In this mode, the positive voltage is applied to the Anode and the negative voltage applied to the Cathode, and there will not be any pulse applied to the gate. It will be kept in an open state. When the voltage is applied, the junctions J1 and J3 will be forward biased and the junction J2 will be reverse biased. Since J2 is reverse biased, the depletion region’s width increases, and it acts as an obstacle for conduction, so only a little current will be flowing from J1 to J3.

Forward Blocking Mode

At what time the voltage applied to the SCR is increased, it reaches the breakdown voltage of the SCR. J2 junction gets depleted because of avalanche breakdown. Once it occurs, the current starts flowing through the SCR. In forward blocking operation, the SCR is forward biased.  In this condition, no flow of current will occur.

Forward Conduction Mode 

In this mode,  the SCR will be in the ON state. SCR will be conducting current. The SCR conduct in two different ways, we can increase the applied forward bias voltage beyond the breakdown voltage, or in the gate terminal, a positive voltage is applied.

Forward Conduction Mode

When it rises the applied forward bias voltage between the Anode and Cathode, the J2 junction will be depleted due to the avalanche breakdown voltage level and the SCR will start conducting. We cannot do this for all the applications and this technique of triggering the SCR will eventually reduce the lifetime of the SCR.

If someone needs to use the SCR for low voltage applications, you can apply a positive voltage to the SCR gate. The applied positive voltage will support the SCR to move to the conduction state. During this operation mode, the SCR will be operating in forwarding bias and current will be flowing through it.

Reverse Blocking Mode 

In this mode, the positive voltage is applied to the Cathode (-) and the Negative voltage is given to the Anode (+), Here will not be any pulse given to the gate, will be kept as an open circuit. During this operation mode, the J1 and J3 Junctions will be reverse biased and the junction J2 will be forward biased. In the meantime, the J1 and J3 junctions are reverse biased, there will not be any current flowing through the SCR. While there will be a small leakage current flowing because of the drift charge movers in the forward-biased J2, it is not sufficient to turn on the SCR.

Reverse Blocking Mode

Different Types of SCRs and Packages

There are several types of SCR available based on the specification and application. They are available in different kinds of packages that can be used for different types of applications.

Discrete Plastic: It package is a normally known type of SCRs with 3-pins linked to a plastic-covered semiconductor material. These are of planar type construction and they are the cheapest type of SCR related to other packages. Discrete Plastic is available for up to 25A and 1000V applications, they can be easily mounted on any circuit with many different components.

Plastic Module: It provides similar features compare to Discrete Plastic package SCRs. The plastic module also covers more than one device and available in the current range up to 100A. Using a Plastic module will give a circuit a better finishing because it can be mounted to the boards by bolting the heatsink to the circuit panel.

Stud Base: This device will be having a screwed base, it delivers the dual advantage of low thermal resistance and ease of mounting. Stud Base is presented between the current range of 5 to 150 A and a full range of voltage. Stud Base disadvantage, it posses is that it cannot be easily isolated from the heat sink

Flat Base: This delivers the same overview of stud base SCR. An extra advantage is that flat base SCRs being isolated from the heatsink by a tinny layer of insulation medium. The work perfectly under the current range between 10 to 400A.

Press Pack:  This press pack SCRs are suitable for higher power applications, the current rating can be of 200A voltage level of 1200V or higher. The SCR construction and the electrodes are packed within a ceramic enclose that carries the essential isolation between the Anode and Cathode. Both the surface is braced to the heat sink. Press Pack offers better electrical contact resistance and minimum thermal resistance.

Advantages of the SCR

  • As compared with an electromechanical or mechanical switch, SCR has no moving parts. Hence, with high efficiency, it can deliver noiseless operation.
  • Higher switching speed can perform 1 nano operation per second.
  • Can be functioned at high voltage and current ratings with a small gate current.
  • More suitable for AC operations because at every zero position of the AC cycle the SCR will automatically switch OFF.
  • Small in size, hence easy to mount and trouble-free service.

The SCRs behave like a switch as non-conducting or conducting. Three modes are available in SCR – forward blocking, forward conduction, and reverse blocking mode. The SCRs are the most important and commonly used part of the thyristor group. The SCR can be suitable for diverse applications like rectification, regulation of power and inversion, etc. Like a diode, SCR is a unidirectional device that allows the current in one direction and opposes it in another direction. The SCRs have the capability to control high values of current and voltage hence they are used in most industrial applications.

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