SCR TURNOFF (COMMUTATION) Circuits

Group Members:

1. Aisha Kalsoom 11-EE-08

2. Farwa Aslam 11-EE-32

What is Commutation?• The process of turning off an SCR

is called commutation.•• It is achieved by Reducing anode current below holding current Make anode negative with respect to cathode

Types of commutation are: Natural or line commutation Forced commutation

SCR TURNOFF METHODS Diverting the anode current to an alternate

path

Shorting the SCR from anode to cathode

Applying a reverse voltage (by making the cathode positive with respect to the anode) across the SCR

Forcing the anode current to zero for a brief period

Opening the external path from its anode supply voltage

Momentarily reducing supply voltage to zero

(1) CAPACITOR COMMUTATION

SCR turnoff circuit using a transistor switch

SCR turnoff circuit using commutation capacitor

Value of capacitance is determined by:• C> = tOFF• 0.693RL

(2) COMMUTATION BY EXTERNAL SOURCE

The reverse voltage is applied to the load current carrying SCR from an external pulse source therefore it is called “External pulsecommutation”.

When thyristor is conducting ,the drop across the transformer is negligibly small. The supply voltage appears across the load resistance.

When commutation of SCR is desired a pulse of duration equal or slightly greater than the turn off time specification of the thyristor is applied.

Earlier to the commutation, the capacitor voltage remains at a small value of about2V.

When a pulse of voltage from the pulse generator is applied to the primary of the pulse transformer, the voltage Induced in the secondary appears across T1 as a reverse voltage (-Vp) and turns it off.

Mathematical Expression

(3) COMMUTATION BY RESONANCE

Series resonant turnoff circuit:

When the SCR is triggered, anode current flows and charges up C with the dot as positive. The L-C-R form a second order under-damped circuit. The current through the SCR builds up and completes a half cycle. The inductor current will then attempt to flow through the SCR in the reverse direction and the SCR will be turned off.

Series resonant turnoff circuit:

Parallel resonant turnoff circuit

During initial power up, the capacitor C charges to the supply voltage (Edc) and the thyristor is OFF. The initial charging current of the LC branch flows thru the load.When the thyristor is fired, the supply DC voltage is applied across the load, and the voltage across the LC branch goes to almost zero (the forward drop of the thyristor).Then the LC circuit enters a resonant discharge with an angular frequency of the square root of the inverse of LC product.The LC resonant cycle supplies a peak forward current thru the thyristor of Edc times the square root of C/L.This peak current must exceed the load current in order to effect the turn-off of the thyristor during the second half of the discharge resonant cycle. This peak current is usually designed to be twice the load current.The thyristor is reversed biased (and turned OFF) during the first quarter cycle of this second half of the resonant cycle - thus allowing the supply to recharge the capacitor in the LC parallel branch.Then the cycle can repeat.

Parallel resonant turnoff circuit

(4) AC LINE COMMUTATION

THANK YOU!