SCR is actually a thyristor which is used to control the high voltage supply as compared to the conventional transistors. It is a four layered P-N-P-N semiconductor three terminal device, such that the entire four segments arranged in a single array. It has an anode, cathode and a gate terminal.
There are three modes of operation for an SCR depending upon the biasing given to it.
In this mode (Fig 1:) of operation anode is given a positive potential while cathode is given negative voltage keeping gate at zero potential i.e. disconnected.
In this case junction J1 and J3 are forward biased while J2 is reversed biased due to which only a small leakage current flows from anode to cathode till applied voltage reaches it breakover value at which J2 undergoes avalanche breakdown and at this breakover voltage it starts conducting but below breakover voltage it offers very high resistance to the flow of current through to it and said to be in off state.
In this mode (Fig 2 :), thyristor conducts currents from anode to cathode with a very small voltage drop across it. A thyristor is brought from forward blocking mode to forward conduction mode by turning it on by exceeding the forward breakover voltage or by applying a gate pulse between gate and cathode.
In this mode, thyristor is in on-state and behaves like a closed switch. Voltage drop across thyristor in the on state is of the order of 1 to 2 V depending on the rating of SCR. It may be seen from the voltage drop increases slightly with an increase in anode current.
In conduction mode, anode current is limited by load impedance alone as voltage drop across SCR is quite small. This small voltage drop vT across the device is due to ohmic drop in the four layers.
*Reduce the current flowing through it below a minimum value called holding current.
*Apply a negative pulse at gate which will bring it in off state instantaneously.
When cathode is made positive with respect to anode with switch S open, Fig. 4 (a), thyristor is reverse biased as shown in Fig 3. Junctions J1 J3 are seen to be reverse biased whereas junction J2 is forward biased. The device behaves as if two diodes are connected in series with reverse voltage applied across them.
A small leakage current of the order of a few milliamperes (or a few microamperes depending upon the SCR rating) flows. This is reverse blocking mode, called the off-state, of the thyristor. If the reverse voltage is increased, then at a critical breakdown level, called reverse breakdown voltage VBR, an avalanche occurs at J1 and J3 and the reverse current increases rapidly.
A large current associated with VBR gives rise to more losses in the SCR. This may lead to thyristor damage as the junction temperature may exceed its permissible temperature rise. It should, therefore, be ensured that maximum working reverse voltage across a thyristor does not exceed VBR. When reverse voltage applied across a thyristor is less than VBR, the device offers high impedance in the reverse direction. The SCR in the reverse blocking mode may therefore be treated as an open switch.
V-I characteristic after avalanche breakdown during reverse blocking mode is applicable only when load resistance is zero, Fig. 4 (b). In case load resistance is present, a large anode current associated with avalanche breakdown at VBR would cause substantial voltage drop across load and as a result, V-I characteristic in third quadrant would bend to the right of vertical line drawn at VBR.
Fig 3: Reverse blocking
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