Harmonic Voltage in Single-phase full wave rectifier, Continouse dan Discontinouse mode

Harmonic in Single-phase full wave rectifier, Continouse dan Discontinouse mode

Dear all hand…

As we know, basic configuration for single phase rectifier depicted below:




In this configuration, we use LC filter in DC side
Two common reasons for including the dc-side L-C filter:
• Obtain good dc output voltage (large C) and acceptable ac line
current waveform (large L)
• Filter conducted EMI generated by dc load (small L and C)

Harmonic distortion in rectifier circuit, was also determined by L, and there are 2 mode o rectifier operation:

a. Continuose Current Mode (CCM), below image of an sinusoid Voltage source (Vg), that being loaded by a load with large L, that draw Continuose current in rectifier .




Image above for Large L

As L → ~, ac line current approaches a square wave, in this situation, distortion factor become:



b. Discontinous Current Mode (DCM)
Small L Typical ac line waveforms for DCM :As L approaching zero , current approaches impulse functions (peak detection).
As the inductance is reduced, the THD rapidly increases, and the distortion factor decreases.
Typical distortion factor of a full-wave rectifier with no inductor is in the range 55% to 65%, and is governed by ac system inductance.




So in this situation, specially for rectifier that uses small L like in a SMPS, will introduce large THD.


Sometimes the L-C filter is present only to remove high-frequency conducted EMI generated by the dc load, and is not intended to modify the ac line current waveform. If L and C are both zero, then the load resistor is connected directly to the output of the diode bridge, and the ac line current waveform is purely sinusoidal .

An approximate argument: the L-C filter has negligible effect on the ac line current waveform provided that the filter input impedance Zi has zero phase shift at the second harmonic of the ac line frequency, 2 fL.
An the approximation graph is:


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