Fundamental of PN Junction Diode - Application Point of View

Introduction of the Course Content.

Rectifier Circuits: AC to DC conversion

Role of different circuit components

Transformer:

Step down AC voltage amplitude to the desired DC voltage (by selecting an

appropriate turn ratio N1/N2 for the transformer)

Isolate equipment from power-line

Rectifier:

converts an ac input to a unipolar output

Filter:

convert the pulsating input to a nearly constant dc output

Regulator:

Reduce the ripple of the dc voltage

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.

PIV= Vs

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.

PIV= 2Vs-VD

• Current always flows in one direction through the load resistance R,

regardless of whether vS is positive or negative.

• In the positive half cycle, D1 & D2 are forward biased and D3 & D4 are

reverse biased. In the negative half cycle, D3 & D3 are forward biased, and D1

& D2 are reverse biased.

The effectiveness of a rectifier depends upon the magnitude of ac

component in the output; smaller the ac component, the more effective is the

rectifier.

Ripple factor is a measure of effectiveness of a rectifier circuit and

defined as a ratio of rms value of ac component to the dc component in

the rectifier output.

Clippers are the circuit that employ diodes to remove a portion of an input

signal without distorting the remaining part of the applied waveform.

• A clipper can serve as a protective measure, preventing a signal from

exceeding the clip limits.

• A practical application of a clipper is to prevent an amplified speech signal from

overdriving a radio transmitter. Over driving the transmitter generates spurious

radio signals which causes interference with other stations. The clipper is a

protective measure.

Clamping is a process of introducing a dc level into a signal.

• Clamper circuit consist of diode and capacitor that shifts the input waveform to

different dc level without changing the appearance of the applied waveform.

• When the diode is forward biased, it will

conduct and charge the capacitor. The output

voltage across the diode is zero (0.7V).

• The capacitor is charged to peak input

voltage quickly because of small time

constant of the circuit.

• During the –ve cycle when diode is reverse bias, the diode becomes to its

off state. In this case, the output voltage equals to the sum of the input

voltage and the voltage across the terminals of the capacitor which have the

same polarity with each other

Voltage Multipliers:

• Voltage multipliers are the circuit which provide a dc output that is

multiple of the peak input ac voltage.

• Voltage doubler will provide a dc output that is twice the peak input ac

voltage and voltage tripler will provide a dc output that is three times the

peak input ac voltage.

• The basic idea in voltage multiplier circuit is to charge each capacitor to

the peak input ac voltage and to arrange the capacitor so that their stored

voltages will add.

Voltage doubler:

• During the positive half cycle, diode D1 is forward biased and diode D2 is

reverse biased, that will charge capacitor C1 to peak value of input voltage.

• During the negative half cycle, diode D2 is forward biased and diode D1 is

reverse biased, that will charge capacitor C2 to the twice the peak value of

input voltage because capacitor C1 ( charged to Vp) and input voltage (Vp )

now act as series aiding voltage source.

• When input voltage returns to its original polarity, diode D2 is again reverse

biased (off), and then the capacitor C will be discharged through the load R

• The time constant (RLC2) is so adjusted that C2 has little time to loose any

of its charge before the input polarity reverses again.

• During the negative half cycle, diode D2 is turned on , capacitor C2 will be

recharged again until voltage across it is again equal to 2Vp.

Quiz of the Course.