Semiconductor Controlled Rectifier

6This notation, where n-type refers to an electron conductor, and retype refers to a hole (lack of electron) conductor, indicates how a semiconductor was processed. For example, a p-type semiconductor material layer grown on an n-type substrate forms a (p — n) diode, which can be used to rectify alternating current. A p — n — p device may be used as a transistor, and is useful as a signal amplifier or for other applications. 

The external instruction to the SCR module is conventionally a 4 to 20 milliampere dc current from a control microprocessor, where 4 mA corresponds to zero power and 20 mA corresponds to full power. This signal is then translated by 

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F. E. Gentry, F. W. Gutzwiller, N. Holonyak, and E. E. Von Zastrow, Semiconductor Controlled Rectifiers Principles and Applications of p-n—p-n Devices, Prentice-Hall, Englewood Cliffs, NJ, 1964. 

A triac may be perceived as opposing SCR s (semiconductor controlled rectifier or silicon controlled rectifier) in parallel, each activated by a gate current (Figure 2.12). Current can flow in only one direction through the diodes in Figure 2.12. Electrical power from a wall socket, conventionally 110 or 220 ac volts is fed into the device. The SCR s act like diodes in the sense that current is allowed to flow in only one direction. During one half of the cycle, the current may flow through one... 

SCR semiconductor-controlled rectifier silicon-controlled rectifier... 

The exciter is an AC generator with a stator-mounted field. Direct cur rent for the exciter field is provided from an external source, typically u small variable voltage rectifier mounted at the motor starter. Exciter oui put is converted to DC through a three-phase, full-wave, silicon-diode bridge rectifier. Thyristors (silicon-controlled rectifiers) switch the cur rent to the motor field and the motor-starting, field-discharge resistors These semiconductor elements are mounted on heat sinks and assembled on a drum bolted to the rotor or shaft. 

Uses. The metal is used in electroplating, in solder for aluminum, as a constituent of easily fusible alloys, as a deoxidizer in nickel plating, in process engraving, in cadmium-nickel batteries, and in reactor control rods. Cadmium compounds are employed as TV phosphors, as pigments in glazes and enamels, in dyeing and printing, and in semiconductors and rectifiers. 

Common failure modes of discrete semiconductors are zener diode 50% short, 50% open junction diode 60-70% high reverse, 20-25% open, 10-15% short sflicon controlled rectifier (SCR) 2% open, 98% short and transistor 20% high leakage, 20% low gain, 30% open circuit, 30% short circuit (Fig. 7.158). 

Type 2 protection. When Type 2 protected, the equipment is not damaged during a fault/short circuit, and the equipment is protected by semiconductor, rectifier, and in some instances RK-1 fuses. Note that the semiconductor and rectifier fuses do not qnahfy as Underwriters Laboratories (UL) or NEC branch-circuit protection devices. However, occasionally a control manufacturer will use SCRs that can be protected by RK-1 fuses. Consult the manufacturer to be certain. 

The thyristor was previously known as a silicon controlled rectifier since it is a rectifier which controls the power to a load. It consists of four pieces of semiconductor material sandwiched together and connected to three terminals, as shown in Fig. 3.103. 

NoU It is possible that at some loealioiis there is no a.e. source available, such as (or battery-operated lifts iirul motor vehicles,. Such applications may also call for a variable d.e. source. When it is so. it can be achieved with the use of a chopper circuit which uses the conventional semiconductor devices. The devices are switched at high repetitive frequencies to obtain the required variation in the output voltage as with the use of a phase-controlled lliyristor rectifier, A typical chopper circuit is shown in Ingure 6.2, i. using diodes and a controlled unidirectional semieonduetor switch, which can be a thyristor or tin IGBT. 

Solid-State DC Drives. The controlled-thyristor rectifier and separate-field DC motor is the solid-state motor drive in greatest use. The combination provides control over at least a 10 1 speed range, plus an additional two to three times by field weakening. Depending upon the power level, the rectifier is operated directly from the AC supply lines, or via a transformer. Typical speed regulation of 2% can be accomplished with a single control system. The horsepower and speed limitations are set by the DC motor, not by the semiconductor rectifiers. The DC motor and rectifier can be combined to any required power level. 

Use The oxide ferrites in rectifiers on memory and record tapes, for permanent magnets, semiconductors, insulating materials, dielectrics, high-frequency components, and various related uses in radio, television, radar, computers, and automatic control systems. 

Thyristors used in rectifier and inverter bridges are usually of two types. The first type is a three-terminal semiconductor that can only be turned on by a control or firing signal applied to its... 

A thyristor rectifier uses four silicon semiconductors of alternating types in a pack. Figure 8.7 shows that without some outside influence the three junctions can not simultaneously be conductive. When the cathode side is negative, two junctions are conductive, but the center, or control, junction is not. Applying a current pulse to the interior P-layer neutralizes the bias in that layer and allows current to flow through the rectifier pack. 

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