Power-Line Filters




The power-line filter is a low-pass L–C topology.
The source (the power supply) and the load (the LISN) impedances
determine the exact configuration of the filter.
Because filter attenuation is a function of impedance mismatch, the role
of a power-line filter is to maximize the mismatch between the source
and load impedances.
For common-mode noise,
JHLin, AppEMC; Conducted Emission
28
Power-Line Filters



The maximum value of the line-to-ground capacitors is limited because
of leakage requirements imposed by various safety agencies.
To obtain the large inductance required to suppress the lower order
harmonics of the switching frequency, L1 is wound on a high
permeability core.
Common-Mode Filtering



In actual practice, the line-to-ground capacitors usually have a value of one half the
maximum allowable by the leakage requirements.
Typical values for the choke are from 2 to 10 mH.
Differential-Mode Filtering


To differential-mode noise, the two Y-capacitors are connected in series.
These capacitors only contribute to the differential-mode attenuation above about 10
MHz where it is usually not required. Therefore, they are usually ignored with respect
to differential-mode filtering.
JHLin, AppEMC; Conducted Emission
29
Power-Line Filters




To provide a significant amount of differential-mode capacitance, a line-to-line
capacitor C3 (X-capacitor) is added to the power line filter. Typical values for this
capacitor range from 0.1 to 2  F .
For safety reasons, a resistor, typically 1 M , is sometimes added in parallel with
this capacitor.
A second X-capacitor located across the power line and located on the power supply
side of the common-mode choke can be helpful.
Leakage Inductance




Leakage inductance of the common-mode choke is important in power line filters
because it determines the degree of differential-mode inductance present.
As a result of leakage inductance, each winding of the choke will have in series with it
a small differential-mode inductance. This differential-mode inductance along with the
X-capacitor forms an L-C filter, which provides differential-mode filtering.
Too much leakage inductance, however, can cause the common-mode choke to
saturate at a low value of ac power current.
Typical power line chokes will have leakage inductances somewhere between 0.5 and
5% of their common-mode inductance.
30

The leakage inductance of a common-mode choke can easily be measured by
shorting one of the windings and measuring the inductance across the other winding.
31
Power-Line Filters

The common-mode filter is usually designed first and then the differential mode filter
is designed by starting with the leakage inductance of the common-mode choke, and
choosing a value for the line-to-line capacitor C3 , to provide the required
attenuation.

If additional differential-mode attenuation is required,

Values for these differential-mode inductors are typically a few hundred microhenries.
32
JHLin, AppEMC; Conducted Emission
33
Power-Line Filters

Filter Mounting


The performance of this filter is as much,
if not more, a function of how and where
it is mounted, and how the leads are
routed, as it is of the electrical design of
the filter.
Problem 1: the filter is not mounted close
to the point where the power line enters
the enclosure.

JHLin, AppEMC; Conducted Emission
Problem 2: the wire grounding the
filter to the enclosure has a large
inductance, which decreases the
effectiveness of the Y-capacitors in
the filter.
34
Power-Line Filters



Problem 3: capacitive coupling occurs between the noisy power-supply-to-filter wiring
and the ac power line.
The cable between the filter and the power supply should be routed close to the
enclosure to minimize any pickup.
The filter’s input leads should also be kept away from any signal cables (especially
digital cables) and should not be routed over, or near, a digital logic PCB.
JHLin, AppEMC; Conducted Emission
35
Power-Line Filters


An additional improvement over the arrangement shown in Fig. 13-23 is to mount the
power supply directly adjacent to the power line filter.
A power-line filter having an integral ac power cord connector as shown in Fig 13-24.
JHLin, AppEMC; Conducted Emission
36
Power-Line Filters

Power Supplies with Integral Power-Line Filters






Some switched-mode power supplies have the power-line filter built into the supply
on the same PCB as the power converter. This is usually done to reduce size and
costs.
However, this arrangement often violates some, if not all, rules for proper filter
mounting and wiring discussed above.
Problem 1: long traces (too much inductance) connecting the Y-capacitors to the
enclosure.
Problem 2: Magnetic coupling to the unshielded common-mode choke. This problem
can be overcome by proper layout and orientation of the common-mode choke on the
board, or by placing a shield over the choke or power line filter portion of the board.
Problem 3: Input and output traces to the filter, routed in such a way as to maximize
the parasitic capacitance between the two, thus coupling noise around the filter to
the power line.
Filters integral with the power supply can be effective, but only if all the issues
previously discussed relating to proper filter mounting and layout are considered
during the design process.
37
Power-Line Filters

High-Frequency Noise (> 10 MHz)


The high- frequency attenuation of the power supply noise is limited primarily by the
interwinding capacitance of the common-mode choke and the inductance in series
with the Y-capacitors.
The best way to deal with this problem is at the source, the digital logic PCB.
JHLin, AppEMC; Conducted Emission
38
Power-Line Filters


The capacitor value should be chosen to have an impedance less than a few ohms at
the lowest frequency of interest. A 1000-pF capacitor is usually satisfactory, if filtering
is only required above 30 MHz. Below 30MHz, a 0.01-F capacitor would be
preferred.
The ferrite bead material should be chosen to provide about 50  of impedance at
the lowest frequency of interest and the ferrite must be capable of carrying the
output current without saturation.
JHLin, AppEMC; Conducted Emission
39
Primary-to-Secondary Common-Mode
Coupling

JHLin, AppEMC; Conducted Emission
40
Primary-to-Secondary Common-Mode
Coupling

JHLin, AppEMC; Conducted Emission
41
Primary-to-Secondary Common-Mode
Coupling



Usually, a Y-capacitor is used with a value of 1000 to 4700 pF.
To be effective, the bridge capacitor must be placed on the PCB in a
location that minimizes the trace inductance (use short, wide traces) in
series with it, and the traces must maintain a small loop for the
common-mode current.
Other ways to eliminate or minimize this problem are by using a
transformer that contains a Faraday shield, or by adding a commonmode choke in the dc output-leads to reduce the common-mode current.
JHLin, AppEMC; Conducted Emission
42