APPLICATION NOTE
18
These responses would also depend upon feedback loop response and bandwidth. You can use these
measurements to fine-tune feedback loop or compensator parameters to get desired response from the
system.
Line Harmonics
Any conventional SMPS has capacitive filter connected at input rectifier stage. Due to this, current drawn
from mains is very irregular in shape, contain high amplitude, and are of short duration. This nature of
current waveform creates severe stress on the input mains system and can lead to potential system
issues or non-compliance to regulatory standards. Two parameters Power factor (PF), and Total
Harmonic Distortion (THD) are used as qualitative measure for this purposes. Both of these
measurements can be performed using PSpice.
One can perform Fourier analysis using PSpice to get harmonic distortion of VRM input current. This can
be done by selecting the Fourier analysis option under simulation setting (refer figure 18). You can define
voltage node or current through a device, for which this analysis need to be performed. The results of this
analysis can be viewed in the simulation output file. The Fourier analysis result for this VRM at full load
condition is given below.
FOURIER COMPONENTS OF TRANSIENT RESPONSE I(V_V6)
Figure 18: Fourier Analysis result at Full load
