Static V-I characteristics of SCR - Engineering Notes

Static V-I Characteristics:

An elementary circuit diagram for obtaining static V-I characteristics of SCR is shown in the fig.

Elementary Circuit diagram of SCR

The anode and cathode are connected to main source through the load.

The gate and cathode are fed from another source 'Eg'.

The static V-I characteristics of SCR are shown below.

Static V-I Characteristics of SCR

Where,

Va is Anode voltage

VBO is forward breakover voltage

VBR is reverse breakdown voltage

IG is gate current

IL is Latching current

IH is Holding current

The three basic modes of operation of SCR are

1. Forward Blocking mode
2. Forward conduction mode
3. Reverse Blocking mode

Forward Blocking mode:

• When anode is at a higher potential than cathode, thyristor is said to be forward biased, It is seen from the figure that when the gate circuit is open J1 and J3 are forward biased and junction J2 is reverse bias.
• In this mode a small current, called forward leakage current flows from anode to cathode.
• OM represents the forward blocking mode of SCR
• SCR is treated as an open switch in the forward blocking mode.

Forward Conduction mode:

• When anode to cathode forward voltage is increased with gate circuit open, reverse biased junction J2 will have an avalanche breakdown at a voltage called forward breakover voltage VBO .
• After this breakdown, thyristor gets turned ON with point 'M' at once shifting to 'N'. Here NK represents the forward conduction mode.
• A thyristor can be brought from forward blocking mode to forward conducting mode by applying.

1. A positive gate pulse between gate and cathode or
2. A forward breakover voltage VBO  across anode and cathode.

Voltage drop across the SCR 'VT' increases slightly with an increase in anode current. It can be seen from NK.

Latching Current:

It is defined as the minimum value of anode current which it must attain during turn-on process to maintain conduction when gate signal is removed.

The gate pulse width should be chosen to ensure that the anode current rises above the latching current.

Holding Current:

It is defined as the minimum value of anode current below which the SCR gets turned off.

Latching current is more than Holding current.

Latching current is associated with turn on process.

Holding current is associated with turn off process.

Reverse Blocking mode:

• When cathode is made high potential with respect to anode with gate open, then the SCR is said to be reverse biased.
• J1 and J3 are reverse biased and J2 is forward biased.
• A small current flows through the SCR this is called as reverse leakage current.
• This is reverse blocking mode, called the OFF state of the SCR.
• If the reverse voltage increased, then at reverse breakdown voltage VBR, an avalanche breakdown occurs at J1 and J3 and the reverse current increases rapidly PQ.
• The SCR in the reverse blocking mode may therefore be treated as an open switch.