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SCR 1

A Silicon Controlled Rectifier (SCR) is a unidirectional, three-terminal semiconductor device used in power electronics for rectification and switching. It operates in three modes: Forward Blocking Mode (OFF), Forward Conduction Mode (ON), and Reverse Blocking Mode, with the ability to remain ON after the gate current is removed if the anode current is above the holding current. SCRs can withstand a peak reverse voltage of up to 2.5 kV and conduct current only in one direction when triggered.

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23 views5 pages

SCR 1

A Silicon Controlled Rectifier (SCR) is a unidirectional, three-terminal semiconductor device used in power electronics for rectification and switching. It operates in three modes: Forward Blocking Mode (OFF), Forward Conduction Mode (ON), and Reverse Blocking Mode, with the ability to remain ON after the gate current is removed if the anode current is above the holding current. SCRs can withstand a peak reverse voltage of up to 2.5 kV and conduct current only in one direction when triggered.

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chandangpm
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SCR

• A Silicon Controlled Rectifier (SCR) is a four-layer (alternating P-type and N-type


semiconductor materials), three-junction semiconductor device .
• SCR is a unidirectional device.
• It is widely used in power electronics for rectification, switching, and controlling high-
power applications.
• It has three terminals : Anode(A), Cathode(K) and Gate(G)
Working of SCR

SCR operates in three modes:

(i) Forward Blocking Mode (OFF state)


(ii) Forward Conduction Mode (ON state)
(iii) Reverse Blocking Mode
(i) Forward Blocking Mode (OFF state)
Anode is positive w.r.t. cathode but gate current is zero.
Junctions J1 and J3 are forward biased, but J2 is reverse biased, preventing conduction.

Gate is Open Circuited

Details explanation -Since one of the three junctions is reverse biased so there is no current that
can flow through the load, hence the SCR is OFF. However if the applied voltage is gradually
increased, a stage is reached, when reverse biased junction (J2) breaks down. The SCR now, starts
conducting and becomes ON. The value of applied voltage at which the reverse biased junction
breaks down and the SCR becomes ON is known as Breakover Voltage.
(ii) Forward Conduction Mode (ON state)
A small gate current (Ig) is applied.
This reduces the breakdown voltage of J2, allowing conduction.
The SCR remains ON even if the gate current is removed, as long as the anode current is above
the holding current (IH).

(iii) Reverse Blocking Mode


If the anode is negative w.r.t. cathode, junctions J1 and J3 become reverse biased.
The SCR blocks current, acting as an open switch.

Peak Reverse Voltage (PRV) − It is the maximum reverse voltage that can be applied to the SCR
without conducting in the reverse direction. The commercially available SCRs have PRV up to 2.5 kV.
Note -It conducts current only in one direction (from Anode to Cathode) when triggered by a gate pulse.
In the reverse direction, it blocks current just like a diode.

Once turned ON, it remains conducting until the anode current drops below the holding current.

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