I V Characteristics of SCR - Deepakkumar Yadav

I-V Characteristics (Static/Output Characteristics)

Q. Define following terms of SCR and state their typical values : 1. Holding current 2. Latching current.

Q. Sketch V-I characteristics of SCR and label V_BO,
I_L and I_H.

Q. Draw and explain zero gate signal characteristics of SCR.

Q. Draw construction of SCR and explain its operation with V-I characteristics.

Q. State the meaning of holding current and latching current. Label them on the V-I characteristic of SCR.

Q. Draw and explain V-l characteristics of SCR.

Q. State significance of holding current and latching current. Label them on V-I characteristics of SCR.

Q. Define the following terms relating to SCR and discuss their significance:

1. Forward break over voltage.

2. On-state voltage drop

3. Latching current

4. Holding current

Q. State the meaning of holding current and latching current. Label them on the V-I characteristics of SCR.

Definition of IV characteristics of SCR

The I-V characteristics of SCR is a graph of anode current i, on y-axis and anode to cathode voltage plotted on the x-axis as shown in Figure
The characteristics in the reverse direction (anode to o cathode voltage negative) is similar to a reverse biased diode.
The I-V characteristics can be split into two parts namely the forward characteristics and reverse characteristics.
For small reverse voltage a small reverse leakage current flows until the avalanche breakdown takes place at reverse breakover voltage V_BR.
As soon as the reverse breakdown takes place due to avalanche breakdown, a large current flows through SCR whereas the voltage across the device remains constant.
It is dangerous to operate SCR in the reverse breakdown state because it may get damaged due to overheating.
The region from 0 volts upto V_BR Volts in which the SCR is reverse biased and non-conducting is called as “reverse blocking state”. Reverse blocking means that the SCR is reverse biased and in the non conducting (blocking) state,

Forward characteristics of IV characteristics of SCR

1. Forward blocking state

This is the high voltage low current mode of operation in which SCR is in the “OFF” state.
The current through it is “Forward Leakage Current”.
This current flows due to thermally generated minority carriers.

2. Transition state

SCR remains in the blocking state as epsilclong as the forward anode to cathode voltage is less than the breakover voltage.
As soon as V_AK becomes greater than the breakover value, the anode current i_A increases sharply to a high value and the voltage across SCR reduces sharply to a low value (on state voltage).
The switching of SCR from off state to on state and vice versa takes place in a short time. This change over state called as transition state.
As this is an unstable state, it is shown by dotted line in Figure.

3. The low voltage high current mode or the ON state

The voltage across SCR is low in this state. In the on state the SCR anode currents is large and the on state bvoltage drop is small.

Unstable state

These two stable states or modes are connected together by an unstable mode (called as the transition mode) of operation that appears as a negative p resistance on the I-V characteristics. (Dotted line in Figure).

The important voltage and current terms in the I-V characteristics are as follows

The forward breakover voltage (VBo) : This is the maximum forward voltage that can be applied between anode and cathode, without initiating forward conduction. This voltage is defined for a zero gate current. In short this is the maximum forward voltage across SCR in its off state.
The forward leakage current : The small forward current flowing in the forward blocking state of the device. This current is generated due to the minority current carriers and therefore is dependent on the operating temperature.
The holding current (I_H) : It represents the minimum current that can flow through SCR and still “hold” it in the on state. The voltage associated with the holding current is termed as holding voltage V_H. If the forward anode current is reduced below holding current, SCR will be turned off. The holding current is defined for zero gate current (I_G = 0), it is denoted by I_H, and its value is few tens of mA (typically).
ON state voltage : Note that the voltage across SCR in its on state is very low as compared to the off state voltage. It can be shown that the on state voltage is equal to drop across one junction (i.e. of the order of 1 to 1.5 Volts).
Latching current (V_L) : It is minimum anode current that must flow through SCR to latch it into the on state. The latching current is higher than the holding current. The latching current is important when SCR is being turned on.