High Performance 400Hz Power Filters0 pages

APPLICATIONS NOTES

HIGH PERFORMANCE 400Hz POWER FILTERS
MEETING 100dB AT 14kHz
There are two main considerations when designing high performance filters for use on 400Hz
supplies. The first relates to attenuation in the pass band and the second relates to power
dissipation of filter capacitors and inductors at 400Hz and at harmonics of 400Hz.
1. Attenuation Considerations
Most power line filters are based on simple pi or multiple pi filter circuit designs. The attenuation
slope of such filter circuits will be 6dB per octave per element or 20dB per decade per element.
The attached graph shows the slopes provided by a 3 element, a 5 element and a 7 element filter
in order to meet 100dB at 14kHz.
It can be seen that the 3 element filter would have a significant attenuation at the fundamental
frequency of 400Hz which is unacceptable. The graph also shows, for comparison, that a 3
element filter meeting 100dB at 100kHz would not attenuate the 400Hz fundamental.
The more complex 5 and 7 element filters meeting 100dB at 14kHz do not have any attenuation at
the fundamental frequency of 400Hz but they do have significant attenuation at the harmonic
frequencies. Although this may not be a problem as far as the filter performance is concerned, it
will have implications on filter power dissipation.
The above discussions are based on a simplified approach to give an easy comparison of filter
circuits. They do not take into account system impedance considerations or resonance effects.

2. Filter Power Dissipation Considerations
Filter capacitors will normally be connected from each line to earth. These capacitors will be
designed to conduct the normally expected leakage current at 400Hz but they will additionally
have to conduct currents resulting from voltage harmonics on the supply. These currents can be
very substantial because of the reduced capacitor impedance at harmonic frequencies. The
capacitor current, I, for a capacitor of capacitance, C, at each harmonic frequency, f, and harmonic
voltage, V, will be given by
I= 2πfCV
The capacitor heat losses will then be VI x capacitor dissipation factor for each harmonic.
Losses similarly affect the filter inductors and will be based on the magnitude of current harmonics
flowing through the inductors.

©2012 MPE Limited
Application notes category A
High Performance 400Hz Filters iss 3
December 2012

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This information is for guidance only

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sales@mpe.co.uk
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