Components

Silicon Controlled Rectifiers (SCR) InformationSilicon controlled rectifiers (SCR) are four-layer (PNPN) thyristors with three terminals:

 

  • An input control terminal (gate)
  • An output terminal (anode)
  • A terminal common to both the input and output (cathode)

Function 

SCRs are used mainly with high voltages and currents, often to control alternating current (AC) where the change of sign causes the device to switch off automatically. For example, SCRs are used in dimmer switches where the turn-on point occurs at a specific point on the sine curve of the AC supply. The device then remains on for the remainder of that cycle.

 

In devices where the gate is left-floating or disconnected, silicon controlled rectifiers behave like Shockley diodes.

 

Latching is achieved by reaching a breakover voltage or exceeding the critical rate of voltage rise between the anode and cathode.

 

Dropout is achieved by reducing the current until one or both internal transistors fall into cutoff mode.

 

Although SCRs offer faster switching speeds than diodes, both types of devices can conduct in only one direction.

 

 

Selection Criteria

Selecting silicon controlled rectifiers (SCRs) requires an analysis of performance specifications.

 

Silicon Controlled Rectifiers (SCR) InformationPeak repetitive reverse voltage is the maximum reverse voltage that may be applied continuously to the anode and cathode.

 

Root mean square (RMS) on-state current is the maximum RMS value of the principal current when SCRs are turned on.

 

Peak cycle surge on-current is the maximum on-state current of short-term duration that can be applied for one full cycle of conduction without performance degradation.

 

Gate trigger current is the minimum current required to switch silicon controlled rectifiers from the off-state to the on-state at the specified off-state voltage and temperature.

 

Gate trigger voltage is the gate voltage required to produce the gate trigger current.

 

Holding current is the minimum principal current required to maintain SCRs in the on-state.

 

Rate-of-rise of off-state voltage is the minimum rate-of-rise value for the principal voltage that causes switching from the off-state to the on-state.

 

Other performance specifications include:

 

  • Repetitive peak controllable on-state voltage
  • Temperature junction  

Integrated Circuit (IC) Packages

 

Silicon controlled rectifiers are available in a variety of integrated circuit (IC) packages and with different number of pins.

 

Basic IC package types include:

 

  • Dual in-line package (DIP)
  • Diode outline (DO)
  • Silicon Controlled Rectifiers (SCR) InformationTransistor outline (TO)
  • Small outline diode (SOD)
  • Small outline transistor (SOT)

Many packaging variants are available:

 

  • Ceramic (CDIP)
  • Plastic (PDIP)
  • Discrete packaging (DPAK) - large surface mounted package that includes a heat sink
  • Integrated packaging (IPAK) -  plastic package with three leads
  • Power packaging (PPAK)
  • Metal electrode leadless face (MELF)
  • Thin small outline package (TSOP) - type of DRAM package that uses gull wing shaped leads on both sides, is available with both L-shaped leads (TSSOP) and J-shaped leads (TSOJ) 

Standards

 

MIL-PRF-19500/108 - Semiconductor device, thyristors (controlled rectifiers), silicon

 

DSCC-DWG-03021 - Semiconductor device, diode, silicon, rectifier, modeule, high voltage

 

Resources

 

Image Credits: New Jersey Semi-Conductor Products, Inc. | Digi-Key Corporation


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