Behind The Mystery of Electromagnetic Compatibility Design 12
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WE-TI Radial Leaded
Wire-Wound Inductors
WE-PD2 Power
Choke Inductors
Attenuation of 40dB is required to achieve levels below the
permissible interference level of 66dBµV (at 150kHz). This
corresponds to two decades in the logarithmic representation.
One-tenth of the pulse frequency is used as factor for the corner
frequency or further calculation of Y capacitors. The oscillation
equation is now converted and used to determine Y capacitance:
Figure 6: Radio interference voltage with line filter (Source: Würth Elektronik)
As two Y capacitors are needed, the calculated value is divided by
two. Y capacitors conduct common mode interference from the
switch-mode power supply back to the protective earth. Depending
on the device type, only leakage currents of 0.25mA to ≤ 3.5mA
are permissible, no capacitance with a value greater than 4.7nF
should be used. Two Y capacitors with an E12 value of 2.2nF have
been selected for this reason. Figure 6 shows the result of the
measurement with this line filter.
With the filter in circuit, the result of the radio interference voltage
test is a pass. The margin between the respective interference limits
and the quasi peak and average measurements at the frequency of
150kHz is greater than 10dB, and this margin significantly increases
across the frequency range.
