Brief introduction of silicon controlled rectifier

Silicon controlled rectifier is a kind of power supply power control appliance based on thyristor (power electronic power device) with intelligent digital control circuit as the core. It has many advantages such as high efficiency, no mechanical noise and wear, fast response, small size and light weight.

What is silicon controlled rectifier

A silicon controlled rectifier is a common power semiconductor electronic device with the ability to control and switch several kilowatts or even megawatts of electrical power. Structurally, it is a reverse cutoff triode type gate transistor, consisting of three PN junctions (PN-PN four layers). The outer leads of the device have three electrodes: cathode, anode, and control electrode. The reverse characteristics of the device (anode connected to the negative) are similar to those of a PN junction diode. Its forward characteristics, the device is in a certain range of impedance is very high in the closed state (forward blocking state, that is, voltammetric characteristics of a quadrant of the solid line under the dashed line part). When the forward momentary voltage is greater than the turnaround voltage, the device quickly changes to a low-voltage, high-current on-conductor state.

Thyristor rectifier principle

The four-layer structure of the SCR is usually described by a dual triode, seen as an interconnection of NPN and PNP transistors. If the common base current amplification coefficient of the two transistors are α1 and α2 (both as a function of the operating state), the necessary conditions for conduction are α1 + α2 ≃ 1. In the blocked state, the current amplification coefficients of the two transistors are very small; if in the forward blocked state, in the control pole to inject a positive current, the current amplification coefficient of the NPN tube α1 rapidly increases, and lead to α2 to meet the conduction conditions.

Role of silicon controlled rectifier

The SCR rectifier is an extremely important power electronic device that can control megawatts of power with a very small control power, and is commonly used in rectification, switching, inverter and other circuits. High-power SCRs use very large silicon wafers and are packaged in a heat sink housing.

SCR consists of four alternating layers of p and n semiconductor material. An external lead called the gate is connected to the second p layer. The gate is used to provide the trigger voltage that turns the SCR on and starts conduction. The semiconductor wafer is formed as a compact pellet that is contained in a molybdenum or tungsten housing. One end of the housing is equipped with a threaded stud to connect the heat sink to the gate and the other end of the main input lead.

The SCR rectifier will remain inactive or disconnected until a predetermined gate or trigger pulse is received, at which point the device will turn on and allow current to pass. Once turned on, the gate voltage can be removed and the SCR will continue to conduct as long as the current does not drop below a preset level. The preset minimum value is called the device's hold current. Even if the SCR receives a gate pulse, it will not turn on if the main supply current falls below this level. These characteristics make SCR rectifiers and ideal components ideal for strictly controlled current switching.

The gate voltage of the rectifier is also a variable function of its operation. Each SCR rectifier has a rated threshold gate voltage below which the device will not activate. This feature provides circuit designers with additional control and flexibility over rectifier switching conditions. The selector control circuit is usually equipped with a protection device called a buffer. This prevents a sudden increase in the voltage applied to the rectifier from causing capacitive coupling in the gate and accidentally turning it on.

SCRs are often used in circuits with very high current and voltage values. Modern SCRs can comfortably switch power ratings in excess of one megawatt and have become an integral part of high-voltage alternating current (AC) to direct current (DC) conversions. These heavy-duty variants are typically water-cooled and hung in large stacks from the ceiling of power transmission facilities. SCRs may also be found in domestic applications with much lower power levels, such as motor controllers, heating controllers, and large dimmers.