Behind The Mystery of Electromagnetic Compatibility Design 8
Motivation for the line filter
Switch-mode power supplies result in conducted
interference because they generate radio interference
voltage on the mains side. This can interfere with other
equipment supplied with mains power. Line filters help to
suppress the generated radio interference voltage. These
can easily be designed from passive components such as
current-compensated line chokes and X/Y capacitors. The
following concerns the design of a single-phase line filter.
Parasitic currents at the Switch-Mode
Power-Supply input
Parasitic currents result in radio interference voltage via
impedances. Figure 1 shows the main current flow of
parasitic currents in a switch-mode power supply.
Initially, a high-frequency active current "i
DM
" flows on
the mains side with the pulse frequency of the switching
regulator, which results in differential-mode interference.
STEFAN KLEIN
WÜRTH ELEKTRONIK
Line Filter is the Last
Barrier in Switch-Mode
Power Supply
Figure 1: Parasitic currents at the switch mode power supply input (Source: Würth Elektronik)
