Design and implementation of EMI filter for high A.Majid, J.Saleem

Design and implementation of EMI filter for high A.Majid, J.Saleem
Design and implementation of EMI filter for high
frequency (MHz) power converters
A.Majid, J.Saleem
H.B.Kotte, R.Ambatipudi, K.Bertilsson
Electronics Design Division
Mid Sweden University
Sundsvall, Sweden
[email protected]
Electronics Design Division
Mid Sweden University
Sundsvall, Sweden
Abstract—The fabrication of emerging power semiconductor
devices and high frequency PCB power transformers has
made it possible to design the power converters in MHz
switching frequency range. However, the higher switching
frequency, di/dt loops and dv/dt nodes in power stages of
these converters generate higher order harmonics which
causes Electro Magnetic Interference (EMI). It is commonly
believed that the EMI has worst affect in the converters
switching in MHz frequency range than the converters
operating below 150 kHz. Thus, it is important research
direction to investigate the consequences of implementing a
line filter to suppress the conducted EMI in high frequency
power converters. In this paper, the measurements, and
analysis of the conducted EMI in emerging power
converters, switching in MHz frequency range, and the
design of the filter for its suppression is presented. The
design of LISN and its PCB implementation for EMI
measurements is presented. The measurement of conducted
EMI of a half bridge DC-DC converter switching at
3.45 MHz and the analysis of the frequency spectrum is
discussed. The design, PCB implementation and
characterization of the EMI filter and the measurement of
the suppressed conducted noise by applying the filter are
also discussed.
Keywords: - Electro Magnetic Interference, Electro
Magnetic Compatibility, Line Impedance Stablization Network,
Pulse Width Modulation
I.
INTRODUCTION
Power supply is an essential part of almost every
electronic device. The miniaturization of electronic devices
demands the smaller sized power supplies. The
development of emerging semiconductor devices, like
Cool MOS, GaN and SiC power MOSFETs, and high
frequency multilayered PCB power transformers have
made it possible to design the compact, high frequency and
power efficient isolated converters. Using these new power
MOSFETs and multilayered PCB power transformers the
power converters are designed in the switching frequency
range of 2-4 MHz and tested output power level up to
40 W [1]-[4]. It is very important to analyze the EMI
generated by these converters and develop them according
to Electro Magnetic Compatibility (EMC) standards.
In recent years, the EMI considerations have become
very important because of very stringent EMC regulations.
The EMI produced by power converters is of broadband
type and falls within the frequency range from operating
frequency to several MHz. It affects EMC of these
converters. The careful design of the power stages of the
high frequency Pulse Width Modulated (PWM) converters
is an important step towards the reduction of EMI. The
cause of EMI induction is the coupling between circuit
elements due to a magnetic field or an electric field. The
conducted EMI is divided two major categories: radiated
and conducted.
The radiated EMI is most often a magnetic field, due to
low voltages and high currents. Power lines can also
radiate EMI, the source of which is the current noise
conducted from converters [5].
The conducted EMI consists of both common mode
and differential mode noise signals. The frequency range
of EMI signals generated by power electronic equipment
extends up to 1GHz [6]. There are various standards e.g.
CISPR, FCC IEC, VDE and military standards that specify
the limit on conducted EMI [7].
The common mode noises are high frequency noises
that are in phase with each other having circuit paths
through ground. Common mode noise is generally the
more difficult noise to deal with. It originates due to
charging and discharging of parasitic capacitances,
primarily, the heat sink and transformer inter-winding
capacitance [8]. The main cause of common-mode EMI is
the parasitic capacitances between those points of the
system that have high dv/dt and ground [9].
The differential mode noise is predominantly caused by
the magnetic coupling L.di/dt where L is the parasitic loop
inductance which experiences high switching current slew
rate di/dt [8]. The parameters such as cable spacing and
filtering determine differential mode coupling. The
differential mode current generated at the input of the
SMPS is measured as an interference voltage across the
load impedance of each line with respect to earth at
measured point [10].
In this paper, the design and implementation of EMI
filter for power converters switching in MHz frequency is
presented. The measurement of conducted EMI under a
constant condition requires Line Impedance Stabilization
Network (LISN) for EMC compliance testing. The LISN is
designed and implemented on PCB. Measurements are
performed by HAMEG HMS3000 spectrum analyzer using
a LISN between input and the converter under test. The
frequency spectrum of input conducted noise is analyzed.
The filter is designed and implemented on PCB and after
application of filter the spectrum is analyzed.
50uH
Main Input
50 Ohm
50 Ohm
8uF
II.
Spectrum Analyzer
Spectrum Analyzer
Power Converter
0.1 uF
8uF
0.1 uF
MEASUREMENT SETUP AND LISN IMPLEMENTATION
The common mode EMI is measured for a half bridge
converter switching at 3.45 MHz. The tested output power
level of the converter is 6 W. The common mode EMI is
measured across 50 Ω resistor of LISN using HAMEG
HMS3000 spectrum analyzer.
50uH
Figure 2 LISN Schematic
The block diagram representation of conducted EMI is
shown in Figure 1.
Figure 3 LISN prototype (top side)
III.
The LISN shown in Figure 1 is required for
measurements of conducted noise on a power line to
separate the high frequency noise signals from the input
current. It allows spectrum analyzer to measure the noise
current through 50 Ω source impedance. The internal
circuit of LISN is a high pass filter. It isolates the
measurements from any high frequency shunting which
might exists in power distribution network. It ensures that
the equipment under test receives the proper dc voltage
and current levels and also sees the controlled impedance
for the ripple frequencies of interest. The mono cell LISN
structure is designed and implemented according to
CISPR standards. It is the simplest and commonly used
topology [11]. It comprises inductors, capacitors and 50 Ω
resistors. The 0.1uF Capacitor and 50 Ω resistor provide a
path for the conducted EMI with constant impedance with
respect to frequency. This characteristic impedance is
defined by standards.
The schematic of LISN designed for EMI
measurements is shown in Figure 2 and its prototype is
shown in Figure 3 and Figure 4.
EMI MEASUREMENTS OF HALF BRIDGE CONVERTER
WITH OUT FILTER
In DC-DC converters, the differential mode EMI can
be reduced by decoupling capacitors [12]. Therefore, only
common mode EMI is required to be suppressed. This
noise is mainly created by parasitic capacitances to
ground. The frequency spectrum of common mode EMI,
measured by spectrum analyzer, is shown in Figure 5. It is
observed that emission from the half bridge converter
does not meet the regulatory requirement. The even and
odd order harmonics are present in spectrum. The
fundamental frequency component at 3.45 MHz has the
highest amplitude of 104 dBµV. Therefore, a filter is
required to suppress this noise in the frequency range of
150 kHz to 30 MHz.
CISPR limit
100
80
dB ( uV)
Figure 1 Measurement setup for EMI
Figure 4 LISN prototype (bottom side)
60
40
20
0
0
5
10
15
20
Frequency (MHz)
25
Figure 5 Frequency spectrum of conducted EMI
30
IV.
•
EMI FILTER DESIGN AND IMPLEMENTATION
The direction of the common mode noise is from load
and into the filter. The EMI filters bypass the noise by
using shunt capacitors and block it by using series
inductors. The common mode inductor becomes high
impedance to this noise. It absorbs the noise and then
dumps it to ground through low impedance Y capacitors.
The commonly used EMI filter topologies used to
attenuate the high frequency conducted noise include LC
low pass filters. To attenuate a certain frequency, band
reject filters can also be used. The single stage EMI filter
topology is shown in Figure 6.
In order to avoid coupling between inductors
and ground plan, common mode choke is
soldered on the top side of PCB without
ground plane. There is ground plane only on
the bottom side of PCB.
The filter is implemented on 15mm x 25mm PCB. The
prototype filter is shown in Figure 7 and Figure 8.
CX Caps
Common Mode Choke
Figure 7 Filter prototype (Bottom Side)
L1
Figure 8 Filter prototype (Top Side)
CY
CX
CX
V.
L2
CY
Figure 6 EMI Filter for power converters
The actual size of the filter depends on the design
approach, the materials and the components used.
However, in general, the size of the filter can be expected
to decrease with increasing cutoff frequency and vice versa
[13]. A single stage common mode filter is implemented.
The value of the common mode choke used in the filter is
80 µH. The self-resonant frequency of common mode
choke is about 7 MHz. The capacitors used in the filter
determine the attenuation behavior of the filter above the
self-resonant frequency of the common mode choke. The
values are given in Table 1.
LCM
CY
CX
80µH
560pF
330nF
The inductors and capacitors used in a filter are
complex components. Their effectiveness is dependent on
material properties and placement. There exist various
parasitic parameters in the filter which cannot be
determined by measurements. Therefore two identical
filters may behave differently in a given application [14].
The electromagnetic couplings among filter
components and circuit layouts play very important roles
in the high frequency performance of EMI filters [15]. The
layout of EMI filter is very crucial, therefore, it needs
special consideration. Following measures are used in the
implementation of EMI filters:
In order to avoid inductive coupling between
capacitors and inductors, the common mode
choke is soldered on the top side of PCB and
capacitors are soldered on bottom side.
EMI FILTER CHARACTERIZATION
To predict the performance of EMI filters they are
characterized by independent network parameters.
Scattering parameters are chosen to characterize the EMI
filters because they are easy to measure accurately in high
frequency range [16]. The reflection coefficients ΓS and ΓL
are given in Equations 1 and 2, respectively. For EMI
filters, insertion voltage gain is defined as the ratio of the
port voltage at load side without the filter to that with the
filter [17]. For any two-port network with its scattering
parameters, the insertion voltage gain with arbitrary
source and load impedances can be calculated by using
Equation 3 [18].
Table 1 Values of passive components for EMI Filter
•
CM
Choke
CY Caps
ΓS =
Z S − Z0
Z S + Z0
(1)
ΓL =
Z L − Z0
Z L + Z0
(2)
Where:
ZS is the impedance of the noise source (device
under test) and ZL is the impedance of LISN. For common
mode filter the insertion loss is calculated by using
arbitrary values of ZS = 1MΩ and ZL=50Ω [18].
AV =
S12 (1 − ΓL ΓS )
(1 − S11ΓS )(1 − S 22ΓL ) − S12ΓS S 21ΓL
(3)
The scattering parameters, S11 and S22 are called
reflection coefficients while S12 and S21 are called
transmission coefficients. These scattering parameters are
measured by network analyzer for common mode EMI
filter. The insertion voltage gain of the common mode
filter is calculated by Matlab according to Equation 3
using scattering parameters and reflection coefficients.
The insertion voltage gain plot is shown in Figure 9.
0
REFERENCES
[1]
Insertion Voltage Gain (dB)
-10
-20
-30
[2]
-40
-50
-60
[3]
-70
0
5
10
15
20
25
30
Frequency (MHz)
Figure 9 Insertion voltage gain of the common mode filter
VI.
[4]
EMI MEASUREMENTS AFTER APPLICATION OF FILTER
It is observed that by using this filter the input noise of
the converter is suppressed according to CISPR 22 class-A
limits. The fundamental frequency component is at
64 dBµV and it is attenuated by 40 dBµV. The EMI plot
after the application of the filter is shown in Figure 10.
[5]
[6]
[7]
[8]
CISPR limit
100
[9]
dB ( uV)
80
[10]
60
[11]
40
20
0
0
[12]
5
10
15
20
Frequency (MHz)
25
30
Figure 10 EMI plot after using filter
From the measurement results of the converter circuits,
it is observed that the converter switching at higher
frequency needs smaller EMI filter. Therefore, the overall
size of the converter can be reduced by increasing the
switching frequency of the converter.
[13]
[14]
[15]
[16]
VII. CONCLUSION
The focus of this paper is to measure and analyze the
conducted EMI of the emerging power converters
switching in MHz frequency range and to design the filter
to keep the noise within regulatory limits.
In order to measure the noise spectrum a LISN is
designed and implemented. The frequency spectrum of
common mode input noise of the converter is plotted and
it is observed that the fundamental frequency components
as well as other higher order harmonics are not within
regulatory limit. A single stage compact common mode
EMI filter is designed and implemented on PCB. It is
observed that the size of the converters as well as the EMI
filter, required for these converters, is reduced by
increasing their switching frequency.
[17]
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