Drive Component Overview: SCR Definition and Purpose

Drives that control motor speed such as Variable Frequency Drives (VFDs), AC Drives, and Variable Speed Drives (VSDs) include a rectifier unit, in which the AC voltage is supplied to the drive is converted into DC voltage. In most of these motor drives, this rectifier unit makes use of standard P-N junction diodes (two-layer diodes) to build different types of rectifier circuits like half-wave, bridge, and full-wave rectifiers.

However, if high voltage is applied to any of these two-layer diode rectifiers, its diodes are likely to get damaged. For this reason, motor drives using rectifier units made of P-N junction diodes cannot operate reliably at high supply voltages. To overcome this limitation, a special type of rectifier that could withstand high voltages was developed. This rectifier is the Silicon Controlled Rectifier (SCR). SCRs are often used in applications where a large amount of power needs to be switched, though it’s necessary to keep their control voltage and current low enough for simple and reliable operation.

What is a Silicon Controlled Rectifier? 

A Silicon Controlled Rectifier is a three-terminal unidirectional current regulating device, whose conductor is controlled by an input current. Just like a standard P-N junction diode, an SCR allows the flow of electric current in a single direction only while blocking that flow in another direction. But unlike the standard P-N junction diode that is made of two (2) semiconductor layers (P-type and N-type), an SCR diode is made of four (4) semiconductor layers of alternating P- and N-type materials. It’s thus a 4-layered semiconductor device that forms either P-N-P-N or N-P-N-P structure, and that contains a blocking N-P junction.

A typical SCR device is made up of silicon material that can withstand high-power operations. That’s why SCRs are capable of handling very high values of voltage and current associated with most industrial applications.

In addition, an SCR combines the features of a transistor and a rectifier. Hence, it can convert high-voltage Alternating Current (AC) into Direct Current (a process known as rectification) while controlling the amount of power supplied to the connected load. Due to this, Silicon Controlled Rectifiers find application in different industrial processes like rectification, power regulation, inversion, etc. You’ll often come across them in high-power control devices such as electric motor drives, relay controls, or controllers for induction heating elements.

Note: The SCR is the most popular member of the thyristor family compared to other thyristors like SCS (Silicon-Controlled Switch), DIAC (Diode for Alternating Current), and TRIAC (Triode for Alternating Current), etc. That’s why most people use the words Thyristor and SCR interchangeably. Also, sometimes the Silicon Controlled Rectifier is referred to as a 4-layer diode, an SCR diode, or a 4-layer semiconductor device.

Schematic Symbol of a Silicon Controlled Rectifier

The SCR Symbol 

In a circuit diagram, the SCR symbol is similar to that of a normal diode with an anode and a cathode; but being a three-terminal device, the SCR has an additional gate terminal, as shown below.

From the schematic diagram shown above, you can notice that the SCR symbol consists of three terminals, namely Anode (A), Cathode(C), and Gate (G). The diode arrow used in the SCR symbol indicates the direction of conventional current flow. As stated earlier, the SCR is a unidirectional semiconductor that allows current to flow in a single direction only and opposes it in another direction. Also, using an in-built feature the SCR can be switched ON and OFF by controlling the input applied to the gate terminal or the forward and reverse bias conditions, as we’ll discuss later.

Again the SCR symbol is exactly the same as the Thyristor symbol. Now that we know how a Silicon Controlled Rectifier is represented in a circuit diagram, let’s look into its construction and its operating methods to understand more about it.

Construction of a Silicon Controlled Rectifier 

Construction of a Silicon-Controlled Rectifier (SCR)

When a normal P-N junction diode is added to a standard N-P-N junction transistor the resulting unit is a four-layered semiconductor device with a P-N-P-N structure, which eventually forms three junctions J1, J2, and J3. This device is the SCR and it has three terminals.

Among the three terminals, the Anode (A) is the positive electrode and is taken from the outer P-type material, the Cathode (K) is from the outer N-type material and forms the negative electrode. Whereas the third terminal which is referred to as the Gate is taken from the base of the standard N-P-N junction transistor. The Gate acts as a control element of the Silicon Controlled Rectifier. The figure below illustrates how an SCR is constructed.

The outer P- and N-layers where the positive and negative electrodes are placed are heavily doped, whereas the middle P- and N-layers are lightly doped. In a P-N-P-N SCR structure, the gate terminal is connected to the middle P-layer as illustrated above. SCR construction can take three different forms, namely Planar type, Press pack type, and Mesa type.

Working Principle of a Silicon Controlled Rectifier 

The SCR conducts current between the anode and cathode terminals, but this is only possible when proper current control is applied to the gate connection. Also, for the SCR to conduct, the gate terminal has to be made positive with respect to the cathode terminal. When not conducting the SCR acts like an open switch and when conducting it acts like a closed switch. For this reason, SCRs may be considered ‘switchable’ diodes.  

Normally, the SCR starts to conduct electric current in the forward direction after a control signal is applied to the gate connection. Under normal circumstances, it’s not possible to turn it OFF until the forward current drops to zero. The voltage drop across the anode and cathode terminals depends on the size of the SCR and the amount of conventional current flowing through it. Also, when the anode and cathode terminals are reverse biased, the current does not flow through the SCR. 

Thus, depending on the type of biasing (Forward or Reverse), the working principle of a typical Silicon Controlled Rectifier is divided into three modes of operation. These modes include: 

  • Forward Blocking Operating Mode 
  • Forward Conducting Operating Mode  
  • Reverse Blocking Operating Mode 

A) Forward Blocking Operating Mode 

SCR Forward Blocking Operating Mode

In this operating mode, the connection of the Silicon Controlled Rectifier is such that the anode terminal is made positive in relation to the cathode terminal, whereas the gate terminal is kept open, as shown in the diagram below. Hence, positive (+ve) voltage is applied to the anode (A) and negative voltage to the cathode (K) while no pulse is applied to the gate connection and so it remains in an open state. When voltage is supplied to the SCR while in this state, junctions J1 and J3 become forward biased while junction J2 becomes reverse biased.

Due to the reverse bias voltage at junction J2, the width of the junction’s depletion region increases significantly and the region acts as a hindrance to electric conduction between J1 and J3. Therefore, most of the applied current does not flow through the SCR. But a small amount of leakage current is able to flow from J1 to J3.

If the voltage supplied to the SCR is increased to reach the SCR’s breakdown voltage (VB), junction J2 gets depleted due to Avalanche breakdown caused by high energy minority carriers. Once the Avalanche breakdown takes place at junction J2, the current starts to flow through the SCR. But below the SCR’s breakdown voltage (VB), the SCR presents a very high resistance to electric current flow. It, therefore, acts as an open switch and is said to be in turned OFF. So, even if the SCR is forward biased, there will be no current flowing through it; hence, the name Forward Blocking Mode.

B) Forward Conducting Operating Mode

SCR Forward Conducting Operating Mode

In this mode, the Silicon Controlled Rectifier is made to conduct electric current through either of the following ways: 

  • By increasing the forward bias voltage or the voltage applied across the anode and cathode terminals to exceed the SCR’s breakdown voltage (VB). 
  • By applying a positive voltage (VG) to the gate terminal. 

If the applied forward bias voltage is increased beyond the SCR’s breakdown voltage, avalanche breakdown happens at junction J2. As a result, the SCR turns into full conduction mode and acts as a closed switch, allowing current to flow through it. The current at which the Silicon Controlled Rectifier turns from forward blocking mode into forwarding conducting mode is called Latching Current (IL). 

Gate forward biasing is preferred for low voltage applications, as you only need to apply a small positive voltage (VG) to the gate terminal for the SCR to start conducting, as illustrated in the diagram below. 

Note: Forward Conducting Mode is the only operating mode in which the SCR is in the ON state and conducting. Also, activating the SCR by increasing the forward bias voltage applied between the anode and cathode is only restricted to some applications. In addition, this method eventually reduces the lifetime of the Silicon Controlled Rectifier.

C) Reverse Blocking Operating Mode

SCR Reverse Blocking Operating Mode

In the reverse blocking mode, a positive voltage is applied to the cathode (negative terminal) and a negative voltage is applied to the Anode (positive terminal), while no pulse is applied at the gate terminal; instead, it’s kept as an open circuit as illustrated in the circuit diagram below. In this state, junctions J1 and J3 become reverse biased and J2 becomes forward biased.

The reverse-biased voltage across junctions J1 and J3 drives the SCR into a reverse blocking region; thus, no current can flow through the SCR circuit and it acts as an open switch. However, due to the drift of charge carriers in the forward-biased J2 junction, a small leakage current will flow but it’s not enough to turn ON the SCR. Thus, the Silicon Controlled Rectifier will remain in the OFF state.

Purpose of Silicon Controlled Rectifiers 

Silicon Controlled Rectifier circuits are widely used for both AC and DC power control. These circuits employ a variety of different methods to control AC or DC current flow, but they all require a control signal for firing the gate connection. And they are turned OFF by removing the voltage across the anode and cathode terminals, which stops current flow.

With an AC voltage supply, the SCR functions as a controlled half-wave rectifier, since it blocks both the positive and negative half-cycles until a positive control signal is applied to its gate terminal. Then, as long as the applied control signal is present, the SCR will conduct electric current during the positive half-cycle and block it during the negative half-cycle. But when the control signal is withdrawn, the SCR will block the two half-cycles again; since it automatically turns OFF after each positive half-cycle.

However, through proper timing of the control signal applied to the gate terminal, you can make the SCR conduct current for all or part of the positive half-cycles. This allows proportional control of the output current and ON-OFF switching of the SCR also becomes possible. This ON-OFF switching operation enables the SCR circuit to be used in the rectifier section of Variable Speed Drives (VSDs) and Variable Frequency Drives (VFDs) to convert incoming AC voltage into DC output voltage.

In addition, Silicon Controlled Rectifiers can be used in the secondary circuit of a DC power supply to regulate the voltage output. By controlling the phase of the control signal applied to the gate terminal with respect to the phase of the supply voltage, the firing angle of the gate terminal can be held at any point in the power cycle up to about 180°. This varies the delay between the gate control signal and the normal onset of SCR conduction. Thus, by adjusting the SCR’s firing angle, it’s possible to control the average power delivered from the DC power supply to the connected load.

Along with the above applications, SCR switches provide an effective means of controlling the value of rectified DC voltage output in VFDs. SCR drives also offer cost-effective solutions for controlling current flow to DC motors in power transmission applications. In fact, for several years now, SCRs have been used in simple speed controls for DC motors, particularly in offshore drilling applications.

SCRs are also the most widely used solid-state power devices for high-power drive control applications, with voltage ranges between 2.4 to 11kV. Such power devices are available at high voltages and currents, but with a limited maximum switching frequency and they require complex commutation circuits for use in Voltage-Source Inverter (VSI) drives. Hence, SCRs are more popular in drive applications where natural commutation is possible like in Cycloconverters and in LCI (Load Commutated Inverter) current source converters.

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