Diode equivalent circuit

It is generally profitable to replace a device or system by its equivalent circuit. Once the device is replaced by its equivalent circuit, the resulting network can be solved by traditional circuit analysis technique.

The forward current I_f  flowing through the diode causes a voltage drop in its internal resistance r_f. Therefore the forward voltage V_F applied across the actual diode has to overcome

1. potential barrier Vo
2. internal drop I_f r_f

V_f = Vo + I_f r_f

For silicon diode Vo=0.7V whereas for Germanium diode Vo = 0.3 V.

 For ideal diode r_f =0.

24.1 Basic Definitions

1.Knee voltage or Cut-in Voltage.

It is the forward voltage at which the diode starts conducting.

2. Breakdown voltage

It is the reverse voltage at which the diode (p-n junction) breaks down with sudden rise  in reverse current.

3. Peak-inverse voltage (PIV)

It is the max. reverse voltage that can be applied to a p-n junction without causing damage to the junction.

If the reverse voltage across the junction exceeds its peak-inverse voltage, then the junction exceeds its Peak-inverse voltage, then the junction gets destroyed because of excessive heat. In rectification, one thing to be kept in mind is that care should be taken that reverse voltage across the diode during –ve half cycle of a.c. doesnot exceed the peak-inverse voltage of the diode.

4. Maximum Forward current

It is the Max. instantaneous forward current that a p-n junction can conduct without damaging the junction. If the forward current is more than the specified rating then the junction gets destroyed due to over heating.

5.Maximum Power rating

It is the maximum power that can be dissipated at the junction without damaging it. The power dissipated across the junction is equal to the product of junction current and the voltage across the junction.