A silicon-controlled rectifier (or semiconductor-controlled rectifier) is a four-layer solid state current controlling device. The name silicon controlled rectifier is General Electrics trade name for a type of thyristor. The SCR was developed by a team of power engineers led by Gordon Hall[1] and commercialized by Frank W. Bill Gutzwiller in 1957. Some sources define silicon controlled rectifiers and thyristors as synonymous,[2] other sources define silicon controlled rectifiers as a subset of a larger family of devices with at least four layers of alternating N and P-type material, this entire family being referred to as thyristors.[3][4] According to Bill Gutzwiller, the terms SCR and Controlled Rectifier were earlier, and Thyristor was applied later as usage of the device spread internationally.[5] SCRs are unidirectional devices (i.e. can conduct current only in one direction) as opposed to TRIACs which are bidirectional (i.e. current can flow through them in either direction). SCRs can be triggered normally only by currents going into the gate as opposed to TRIACs which can be triggered normally by either a positive or a negative current applied to its gate electrode. Construction[edit] The Silicon Control Rectifier (SCR) consists of four layers of semiconductors, which form NPNP or PNP structures. It has three junctions, labeled J1, J2, and J3 and three terminals. The anode terminal of an SCR is connected to the P-Type material of a PNPN structure, and the cathode terminal is connected to the N-Type layer, while the gate of the Silicon Control Rectifier SCR is connected to the P-Type material nearest to the cathode.[6] An SCR consists of four layers of alternating P and N type semiconductor materials. Silicon is used as the intrinsic semiconductor, to which the proper dopants are added. The junctions are either diffused or alloyed. The planar construction is used for low power SCRs (and all the junctions are diffused). The mesa type construction is used for high power SCRs. In this case, junction J2 is obtained by the diffusion method and then the outer two layers are alloyed to it, since the PNPN pellet is required to handle large currents. It is properly braced with tungsten or molybdenum plates to provide greater mechanical strength. One of these plates is hard soldered to a copper stud, which is threaded for attachment of heat sink. The doping of PNPN will depend on the application of SCR, since its characteristics are similar to those of the thyratron. Today, the term thyristor applies to the larger family of multilayer devices that exhibit bistable state-change behaviour, that is, switching either ON or OFF. The operation of a SCR and other thyristors can be understood in terms of a pair of tightly coupled bipolar junction transistors, arranged to cause the self-latching action: Thyristor.svg Modes of operation[edit] There are three modes of operation for an SCR depending upon the biasing given to it: Forward blocking mode (off state) Forward conduction mode (on state) Reverse blocking mode (off state) Forward blocking mode[edit] In this mode of operation the anode is given a positive potential while the cathode is given a negative voltage keeping the gate at zero potential i.e. disconnected. In this case junction J1 and J3 are forward biased while J2 is reversed biased due to which only a small leakage current flows from the anode to the cathode until the applied voltage reaches its breakover value at which J2 undergoes avalanche breakdown and at this breakover voltage it starts conducting but below breakover voltage it offers very high resistance to the flow of current and is said to be in off state. Forward conduction mode[edit] SCR can be brought from blocking mode to conduction mode in two ways - either by increasing the voltage across anode to cathode beyond breakover voltage or by application of positive pulse at gate. Once it starts conducting no more gate voltage is required to maintain it in on state. There is one way to turn it off i.e. Reduce the current flowing through it below a minimum value called holding current. Reverse blocking mode[edit] SCR are available with reverse blocking capability. Reverse blocking capability adds to the forward voltage drop because of the need to have a long, low doped P1 region. (If one cannot determine which region is P1, a labeled diagram of layers and junctions can help). Usually, the reverse blocking voltage rating and forward blocking voltage rating are the same. The typical application for reverse blocking SCR is in current source inverters. SCR incapable of blocking reverse voltage are known as asymmetrical SCR, abbreviated ASCR. They typically have a reverse breakdown rating in the 10s of volts. ASCR are used where either a reverse conducting diode is applied in parallel (for example, in voltage source inverters) or where reverse voltage would never occur (for example, in switching power supplies or DC traction choppers). Asymmetrical SCR can be fabricated with a reverse conducting diode in the same package. These are known as RCT, for reverse conducting thyristor.
Posted on: Sun, 07 Sep 2014 16:37:25 +0000
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