Silicon Controlled Rectifier (SCR) and TRIAC

 Silicon Controlled Rectifier (SCR)

schematic symbol

Today we will discuss SCR which is one of the types of Thyristor. Recently we discussed two other types, Shockley diode and DIAC. 

SCR is an enhancement device for shockely diode and there’s no much difference between them, SCR definitely like Thyristor family that it’s a type of 4-layer semiconductor and can be modeled as three diodes connected in series back to back or 2 transistors with feedback position but here there’s a third terminal added to cathode and anode and that is the difference between it and Shockely diode, SCR model is shown in the figure below.

(i) Basic Structure                          (ii) BJT Modeling                              (iii) BJT Equivalent Circuit

It supports hysteresis phenomena, the device doesn’t ‎return to its original state even the cause of the action has been removed and it back again under condition.

It can handle high voltage reach some kilovolts therefore it’s suitable in high power application.

Typical SCR Packages

some definition that will help us to read data sheet:

Breakover voltage (V_BO):

It’s the minimum farward voltage -with gate is open- at which SCR start conducting

It rang from about 50 V to 500 V.

Latching current (I_l):

It’s the minimum anode current which must be attained to turn on the device before removing gate voltage.

Holding current (I_H):

It’s the minimum anode current -gate is open- that switch the SCR off and it’s always lower than latching current.

Farward current rating:

The maximum anode current the SCR can pass without damaging it.

Peak reverse voltage:

It’s the maximum reverse voltage that SCR can maintain without damaging it.

Circuit fusing (I²t) rating:

It’s the maximum surge current that SCR can capable ( surge current occur at the time of switching operation and surge current is higher than normal full load current.

We will now move on to conducting condition: 

First, when the gate is open: it acts like Shockley diode exactly on its 3 state:

SCR modeling with gate open

   1) Forward conducting: the state when the SCR work and conduct current and it require sufficient voltage across anode and cathode higher than breakover voltage
V≥V_BO
once SCR starts conducting it conducts until the current decrease to holding current.

   2) Forward blocking: the voltage of anode is higher than cathode but not sufficient to make SCR to open because voltage across anode and cathode is lower than brakover voltage.
V<V_BO

   3) Reverse state: voltage of cathode is higher than anode and in that situation the SCR doesn’t work because the two-outer junction is reverse and the only inner is forward and there’s no current flow.

Second, when the gate is positive with respect to cathode.

SCR model with voltage across gate w.r.t anode

The importance of the gate is to reduce the voltage we must apply to anode and cathode in other words we the gate reduce the breakover voltage and hence we need lower voltage to apply to anode and cathode to make SCR works

And here the question what’s does gate do?

BJT equivalent circuit

it’s better to explain it on BJT equivalent circuit of SCR

when gate is positive w.r.t cathode current flow throw the gate of T2 and hence it works and flow current from collector to emitter and as shown collector of T2 is connected to the base of T1 hence T1 work then current flow from anode to cathode once the SCR work gate terminal doesn’t no longer important and SCR will continue work even if we open gate terminal so we usually apply a pulse across gate for completely turn SCR on.

SCR I-V characteristic:

As shown by increasing the gate voltage the breakover voltage is decrease           

TRIAC

SCR is a unidirectional device so if we need a device like SCR and conduct current in both direction we use TRIAC

TRIAC is a bidirectional device and consist of two SCR connected in parallel with different direction as shown

And I-V characteristic of TRIAC is:

Application of SCR:

     1.   Full-wave rectifier

    2.    Half-wave rectifier

    3.    Overlight detection

    4.    Power control

    5.    Speed control of DC shunt motor

    6.    Protection of voltage-sensitive loads (crowbar)

    7.    Lighting system for power interruption (backup system)

And we will discuss most of them later