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Lecture - 4: Applications of Diode

Diode Approximation: (Large signal operations):

1. Ideal Diode:

  • When diode is forward biased, resistance offered is zero,
  • When it is reverse biased resistance offered is infinity. It acts as a perfect switch.

The characteristic and the equivalent circuit of the diode is shown in fig. 1.

image

Fig. 1

2. Second Approximation:

  • When forward voltage is more than 0.7 V, for Si diode then it conducts and offers zero resistance. The drop across the diode is 0.7V.
  • When reverse biased it offers infinite resistance.

The characteristic and the equivalent circuit is shown in fig. 2.

image

Fig. 2

3. 3rd Approximation:

•  When forward voltage is more than 0.7 V, then the diode conducts and the voltage drop across the diode becomes 0.7 V and it offers resistance Rf (slope of the current)

VD= 0.7 + ID Rf.

The output characteristic and the equivalent circuit is shown in fig. 3.

image

Fig. 3

  • When reverse biased resistance offered is very high & not infinity, then the diode equivalent circuit is as shown in fig. 4.

image

Fig. 4

 

IIT ROORKEE

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Lecture - 4: Applications of Diode

Example - 1:

Calculate the voltage output of the circuit shown in fig. 5 for following inputs

(a) V1 = V2 = 0.

(b) V1 = V, V2 = 0.

(c) V 1 = V2 = V knew voltage = Vr

Forward resistance of each diode is Rf.

image

Fig. 5

Solution:

(a). When both V1 and V2 are zero , then the diodes are unbiased. Therefore,

Vo = 0 V

(b). When V1 = V and V2 = 0, then one upper diode is forward biased and lower diode is unbiased. The resultant circuit using third approximation of diode will be as shown in fig. 6.

image
image
Fig. 6 Fig. 7

Applying KVL, we get

image

(c) When both V1 and V2 are same as V, then both the diodes are forward biased and conduct. The resultant circuit using third approximation of diode will be as shown in Fig. 7.

image


IIT ROORKEE

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Lecture - 4: Applications of Diode

Applications of diode:

Half wave Rectifier:

The single – phase half wave rectifier is shown in fig. 8.

image
image
Fig. 8 Fig. 9

In positive half cycle, D is forward biased and conducts. Thus the output voltage is same as the input voltage. In the negative half cycle, D is reverse biased, and therefore output voltage is zero. The output voltage waveform is shown in fig. 9.

The average output voltage of the rectifier is given by

image

The average output current is given by

image

When the diode is reverse biased, entire transformer voltage appears across the diode. The maximum voltage across the diode is Vm. The diode must be capable to withstand this voltage. Therefore PIV half wave rating of diode should be equal to Vm in case of single-phase rectifiers. The average current rating must be greater than Iavg

Full Wave Rectifier:

 A single – phase full wave rectifier using center tap transformer is shown in fig. 10. It supplies current in both half cycles of the input voltage.

image
image
Fig. 10 Fig. 11

In the first half cycle D1 is forward biased and conducts. But D2 is reverse biased and does not conduct. In the second half cycle D2 is forward biased, and conducts and D1 is reverse biased. It is also called 2 – pulse midpoint converter because it supplies current in both the half cycles. The output voltage waveform is shown in fig. 11.

The average output voltage is given by

image

and the average load current is given by

image

When D1 conducts, then full secondary voltage appears across D2, therefore PIV rating of the diode should be 2 Vm.

IIT ROORKEE

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