ð‘‰ðµð‘‚=Forward breakover voltage
ð‘‰ðµð‘…=Reverse breakover voltage
ð¼ð‘”=Gate current
ð‘‰ð‘Ž=Anode voltage across the thyristor terminal A,K.
ð¼ð‘Ž=Anode current
It can be inferred from the static V-I characteristic of SCR. SCR have 3 modes of
operation:
1. Reverse blocking mode
2. Forward blocking mode ( off state)
3. Forward conduction mode (on state)
1. Reverse Blocking Mode
When cathode of the thyristor is made positive with respect to anode with switch open
thyristor is reverse biased. Junctions ð½1 and ð½2 are reverse biased where junction ð½2 is
forward biased. The device behaves as if two diodes are connected in series with reverse
voltage applied across them.
ï‚· A small leakage current of the order of few mA only flows. As the thyristor is
reverse biased and in blocking mode. It is called as acting in reverse blocking
mode of operation.
ï‚· Now if the reverse voltage is increased, at a critical breakdown level called
reverse breakdown voltage ð‘‰ðµð‘…,an avalanche occurs at ð½1 and ð½3 and the reverse
current increases rapidly. As a large current associated with ð‘‰ðµð‘… and hence more
losses to the SCR.
This results in Thyristor damage as junction temperature may exceed its maximum
temperature rise.
2. Forward Blocking Mode
When anode is positive with respect to cathode, with gate circuit open, thyristor is said to
be forward biased.
Thus junction ð½1 and ð½3 are forward biased and ð½2 is reverse biased. As the forward
voltage is increases junction ð½2 will have an avalanche breakdown at a voltage called
forward breakover voltageð‘‰ðµð‘‚. When forward voltage is less then ð‘‰ðµð‘‚thyristor offers high
impedance. Thus a thyristor acts as an open switch in forward blocking mode.
3. Forward Conduction Mode
Here thyristor conducts current from anode to cathode with a very small voltage drop
across it. So a thyristor can be brought from forward blocking mode to forward
conducting mode:
1. By exceeding the forward breakover voltage.
2. By applying a gate pulse between gate and cathode.
During forward conduction mode of operation thyristor is in on state and behave like a
close switch. Voltage drop is of the order of 1 to 2mV. This small voltage drop is due to
ohmic drop across the four layers of the device.