PN Diode as Rectifier

PN DIODE AS RECTIFIER

A rectifier is an electrical device that converts alternating current (ac), which periodically reverses direction to direct current (dc), which flows in only one direction. This process is called as rectification. Rectifiers are used in power supplies for radio, television and computer equipment.

A PN diode is a two terminal device which conducts and current flows though it without any resistance during forward bias condition. When the diode is reverse biased, the diode will not conduct due to high resistance and no current flows through the diode i.e., diode is in OFF condition.

Thus an ideal diode acts as a switch either ON or OFF depending on the voltage applied to the diode, since ideal diode has zero resistance under forward bias and infinite resistance under reverse bias.

HALF-WAVE RECTIFIER

In half-wave rectifier, either positive or negative half of the ac signal is passed, while the other half is blocked. It converts an ac voltage into a dc voltage during one half of the ac cycle only.

PN diode is used for rectification because of its unidirectional property ie conducts during forward bias and does not conduct during forward bias and does not conduct during reverse bias. Fig.1.22 shows the circuit diagram of half wave rectifier. Let V_i be the input voltage to the primary of the transformer, and is given as

Where

During positive half cycle of the input signal, the diode is forward biased and the anode of the diode is more positive with respect to cathode. The diode therefore conducts during positive cycle of the input voltage.

For an ideal diode, the forward voltage drop is zero, thus the applied input voltage will appear across the load resistance R_L.

During negative half cycle of the input signal, the diode is reverse biased i.e., the anode of the diode is negative with respect to cathode. Thus the diode D does not conduct due to high impedance. Hence the input voltage does not appear at the output. Fig.1.23 shows the input and output waveform of half wave rectifier.

Ripple Factor

The ratio of rms value of ac component to the de component in the output is known as Ripple Factor.

Substitute (2) in (1)

Average voltage across load, V_av = V_dc

The rms voltage at the load resistance is

Substitute (3) & (4) in (5)

i.e., amount of ac present in the output is 121% of the de voltage.

Efficiency

The efficiency η is defined as the ratio of dc output power to ac input power.

The maximum efficiency of a half-wave rectifier is 40.6%.

Peak Inverse Voltage

Peak Inverse Voltage (PIV) is defined as the maximum reverse voltage a diode can withstand without destroying the junction. For half wave rectifier, PIV is V_m i.e., peak of the negative half cycle.

Transformer Utilization Factor (TUF)

This indicates the rating of the transformer. This can be found by the ratio of dc power delivered to the load to the ac rating of the transformer.

Form factor

It is defined as the ratio of rms value to the average value

Peak Factor

Peak Factor is defined as the ratio of peak value to the rms value.

Peak factor = peak value/rms value