Most of the power conversion applications consist of an AC to DC conversion stage and this DC output is then used for further stages. An AC to DC converter is an integral part ofmost of the electronic equipment as most of these equipment are DC powered [1]. Generally to convert AC toDC a diode bridge rectifier is used. To reduce the ripple in the output voltage a suitable filter capacitor is used at therectifier output. The filter capacitor draws a peaky currentfrom the supply, as it charges only for a short duration in a half cycle. Hence the current drawn by a diode rectifier is non-sinusoidal in nature. Due to the non-sinusoidal nature of the input current, THD will be very high and input power factor also will be low.

A single phase rectifier consists of four diodes connected in a closed loop bridge configuration so as to produce the rectified output voltage is shown in Fig.1

The four diodes are connected in series pairs with only one diode pair conduct during each half cycle. The rectifier with filter capacitor is called a conventional AC-DC utility interface which is given in Fig.2 (a). The filter capacitor reduces the ripples present in the output voltage. Although a filter capacitor significantly suppresses the ripple from theoutput voltage, it introduces distortions in the input current and draws current from the supply discontinuously, in short pulses as shown in Fig.2 (b).

THD of such waveform will be very high and power factor also will be very low. This introduces several problems including reduction of available power and increased loss.

Total Harmonic Distortion:

Total harmonic distortion of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. THD is used to characterize the power quality of electric power systems. The THD in terms of current can be written as:

%THD = √(I_s/I_s1)² -1

Where,I_sis the rms value of input current I_s1is thefundamental frequency component.

Power Factor:

Power factor is the ratio between the actual power (kW)and the apparent power (kVAr) drawn by an electrical load.It is a measure of how effectively the current is being converted into useful work output and also shows the effect of load current on the efficiency of the supply system. True power factor (PF) of the system can be written as:

PF=cos f ×Distortion power factor (2)

Wherecos f is the displacement power factor. In an AC circuit, power is used most efficiently when the current is in phase with the voltage. Distortion power factor describe show the harmonic distortion of a load current decreases the average power transferred to the load. Distortion power factor (DPF) can be written as:

DPF I1/I

Where, I1 is the fundamental component of the current and Iis the total current. In diode rectifier with filter capacitor at output draws non-sinusoidal current hence distortion power factor is less than unity resulting in poor power factor. Due to the presence of high total harmonic distortion and poor input power factor generated by conventional converters,the simple diode rectifiers should not be used. There is a need to achieve rectification at close to unity consider pulse width modulated rectifier (PWM) is an improved version ofthe conventional AC to DC diode rectifier.

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