172
IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 48, NO. 1, FEBRUARY 2006
An Active EMI Filtering Technique for Improving
Passive Filter Low-Frequency Performance
Wenjie Chen, Xu Yang, Member, IEEE, and Zhaoan Wang, Senior Member, IEEE
Abstract—In recent years, there has been considerable interest
in the development and applications of active electromagnetic interference (EMI) filters. An active EMI filter (AEF) for integrated
power electronics module (IPEM) is proposed in this paper, where
large passive EMI filter is replaced by small passive components
and active op-amp circuit. The technique is appropriated when
improved attenuation is required at relatively low frequencies and
the high-frequency filtering requirements are easily met. The effectiveness of the proposed circuit has been verified by experimental
results. It is demonstrated that the proposed approach is most effective in a case where it is desirable to minimize the amount of
passive components in the filter.
Index Terms—Active filters, current transformers, electromagnetic interference (EMI), passive filters.
I. INTRODUCTION
HE size and performance of electromagnetic interference
(EMI) filter components are important considerations in
integrated power electronics modules (IPEMs). Planar integrated passive EMI filters have been employed to achieve the
necessary degree of ripple attenuation [1], [2]. However, in designing passive filters the compensating bandwidth is comparatively narrow. Only a certain part of noise can be eliminated. The
size, weight, temperature, and reliability limitations of magnetic
core can present a significant design constraint.
Active ripple cancellations provide alternative approaches to
the problem. Some practical forms of AEFs have been reported
recently [3]–[11]. In this paper, a small passive filter is coupled
with an active circuitry to attenuate the noise. The passive filter
serves to limit the ripple to a certain degree. The active circuit
cancels or suppresses the low-frequency ripple components that
are most difficult to attenuate with a small size passive lowpass
filter. The combination of these two schemes can lead to an
improvement in the filter’s attenuation performance over a wide
frequency range. At frequency greater than the bandwidth of the
active circuit, the filter performs as a passive component, which
permits a substantial reduction in the passive filter size, with
potential benefits in IPEM size, weight, and cost.
After a brief comparison between active power filter (APF)
and active EMI filter (AEF), an active filter topology suitable
for an IPEM application is introduced. Its characteristics are analyzed and the current sensor is discussed. Experimental results
T
are presented, demonstrating that good noise attenuation can be
combined with high efficiency.
II. COMPARISON BETWEEN APF AND AEF
The basic idea of active filter has been known for some
time [3]–[11]. Fig. 1 illustrates the principle and classification
of AEF. According to the circuit configuration and connections,
the AEFs are classified as parallel AEF and series AEF. To
reduce the complexity of the AEF, there is also hybrid passive/active EMI filter. Their compensation principle is similar
to that of the APF.
On the other hand, there are great differences between the
AEF and APF, which include the following.
1) Noise frequency. APF deals with the harmonic current or
voltage that is below 9 kHz, and in most cases, the APF
could filter only low order harmonic effectively. While
the EMI noise spectrum of the AEF is from 150 kHz to
30 MHz according to the EMC standard—CISPR22, the
bandwidth of AEF should be much wider than that of APF.
2) Power rating. According to [12], low power application
of APF refers to the power lower than 100 KVA while
the high power application of APF means that power is
greater than 10 MVA. In short, the power rating of the APF
is very high. As for AEF, the power is very small. Consider
the CISPR22 class B, and assume that the attenuation on
LISN is 60 dB µV. The power is only 0.2 µW.
3) Implementation of controlled voltage/current source.
Since the power rating of APF is very high, the efficiency
of the circuit should be higher enough; otherwise, the thermal management will be difficult to deal with. To achieve
high efficiency, it is necessary to use the switching mode
in the implementation of the controlled voltage or current
source. On the contrary, the power rating of the AEF is
very tiny, thus it does not need to use switching mode. The
linear mode is enough.
Therefore, it is entirely feasible to utilize the AEF in the
design of the input EMI module of the IPEMs to restrain the
EMI noise.
III. IMPLEMENTATION OF THE AEF
A. Active Circuit Realization
Manuscript received December 11, 2004; revised September 22, 2005. This
work was supported by the National Natural Science Foundation of China
(NSFC) under Project 50237030.
The authors are with the School of Electrical Engineering, Xi’an Jiaotong University, Xi’an, China (e-mail: cwj@mail.xjtu.edu.cn; yangxu@mail.xjtu.edu.
cn).
Digital Object Identifier 10.1109/TEMC.2006.870803
The active topology in this work is shown in Fig. 2(a). The
noise current is sensed through current transformer and is amplified; compensation current is injected back to the power circuit through the RC branch connected to the op-amp’s output.
Ideally, with infinite gain, the negative feedback would drive
the input voltage and, hence, the noise current to zero.
0018-9375/$20.00 © 2006 IEEE
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CHEN et al.: AN ACTIVE EMI FILTERING TECHNIQUE FOR IMPROVING PASSIVE FILTER LOW-FREQUENCY PERFORMANCE
Fig. 1.
173
Generalized block diagram and configuration for active EMI filter. (a) Basic concept. (b) Parallel configuration. (c) Series configuration.
Fig. 2. Topology of active EMI filter and the equivalent circuit. (a) Proposed
active EMI filter. (b) Equivalent two-port T network model.
A two-port T network model will explain the operational
principle more clearly, as shown in Fig. 2(b). In an ideal filter,
the noise current iq at the quiet port would be zero. Thus, the
unwanted noise current in at the noisy port would be moved
down to the controlled current source leg of the T port and flow
back to the noisy port. To achieve this, a cancellation current
ic = in should be generated in the controlled current source.
In reality ic does not equal to in exactly, so the quiet port
current iq = in − ic is nonzero. The closed-loop current transfer
function can be found as
iq
1
.
(1)
=
in
1+F
For good noise attenuation the current gains of the feedback
should be as large as possible. In practice, to avoid instability
the loop gain must be restricted at high frequency, resulting in
limited noise attenuation bandwidth.
In this filter circuit, the injected current cannot be circulated
within the closed loop without using the coupling capacitor C1
because the closed loop cannot be made. Thus, C1 is used to
provide the low-impedance path of the high-frequency noise
current for the internal circulation. In this analysis there is an as-
Fig. 3. Closed loop system of Fig. 2. (a) Block diagram of feedback system.
(b) Frequency response.
sumption that coupling capacitor has sufficiently low impedance
at the frequency band of interest. The transfer function according
to the block diagram of the feedback as shown in Fig. 3(a) is
s
(s
+
ω
)
+
1
1
ω2
iq
=
.
(2)
ω1
1 2
in
s + 1+
+k k s+ω
ω2
ω2
1 2
1
Fig. 3(b) shows the frequency response of the AEF without
including the effect of coupling capacitor.
B. Current Transformer
The measurement of EMI noise currents makes necessary the
use of current transformer with a very wide frequency bandwidth and without distortion. An elementary structure based on
a toroidal current transformer, a winding, and a resistor load is
used in this paper. A simplified equivalent circuit of the current
transformer is shown in Fig. 4.
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174
IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 48, NO. 1, FEBRUARY 2006
Fig. 4. Description of the current transformer. (a) Toroidal transformer.
(b) Low-frequency model.
Fig. 6.
High-frequency response of current transformer.
Fig. 7.
Configuration of experimental system.
Fig. 8.
No EMI filter installed.
Fig. 5. High-frequency performance of current transformer. (a) Equivalent
circuit. (b) Simplified model.
The low cutoff frequency is expressed by
fL =
Rl
.
2πn2 µ0 µr Ae
(3)
Ae is the area of the magnetic circuit, l is the effective length
of the core, and µr is reversible permeability. To improve fL , it
is necessary to increase n and to have large Ae , a short l, and a
high-permeability magnetic material.
Since the frequency of the EMI noise is as high as 30 MHz, it is
necessary to discuss the high-frequency performance. Fig. 5(a)
shows the high-frequency equivalent circuit of the current transformer [13]. L1 and L2 are the primary and secondary leakage
inductance, C1 and C2 are the primary and secondary winding capacitance, Lµ is the magnetizing transformer inductance,
and Cm1 and Cm2 are the primary and secondary interwinding
capacitance.
Although the interwinding capacitance has a great influence
on the frequency response, the electrostatic shield will divide
the capacitance into Cm1 and Cm2 . For an easier understanding
of the current transformer’s behavior, we ignore Lµ , because for
high frequencies, the current in L is greater than that in Lµ . The
simplified equivalent model is shown in Fig. 5(b). The transfer
function is shown in (4) at the bottom of the page, and the high
cutoff frequency is
fH =
2π 3
1
C1 C2 R (L1
+ L2 )
Io
1
.
= 3
Ii
s C1 (L1 + L2 )C2 R + s2 C1 (L1 + L2 ) + sR (C1 + C2 ) + 1
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.
(5)
(4)
CHEN et al.: AN ACTIVE EMI FILTERING TECHNIQUE FOR IMPROVING PASSIVE FILTER LOW-FREQUENCY PERFORMANCE
Fig. 9.
175
Effectiveness of the active and passive filter. (a) Active control circuit installed. (b) Passive EMI filter installed.
It could be seen that fH has nothing to do with the ratio n. To
improve the high-frequency response, it is necessary to decrease
the product of C1 C2 (L1 + L2 ), which means to minimize the
parasitics. Fig. 6 shows the bode plot of the current transformer.
IV. EXPERIMENT RESULTS
In this section, several experiment results are shown. Fig. 7
shows the configuration of the experimental system. A 250-W
buck converter operating at 100 kHz is used as a high-frequency
noise source. The input filter is composed of the active circuit
and additional passive filtering elements. An LISN is used to
provide the stable source impedance at the high frequency and
the peak detector is used in the measurement of conducted EMI
spectrum.
Fig. 8 shows the conducted EMI spectrum of the system without any EMI filter. The conducted EMI spectrum includes both
common- and differential-mode EMI. Although they should be
separately considered, the differential-mode EMI will not be
discussed in this paper with the assumption that some appropriate differential-mode components are installed for each design
stage. After sufficient differential-mode filtering, the commonmode EMI becomes the bottleneck of the total conducted EMI.
Fig. 9(a) shows the conducted EMI spectrum when the proposed active circuit is added into the system. For comparison,
in Fig. 9(b), we also give the measured spectrum when only a
passive EMI filter is used. As can be seen, although the EMI
spectrum of 400 kHz to 2 MHz is much attenuated by the passive
component, the spectrum of 150–400 kHz is still greater than
the standard. It is also comparatively difficult for the passive
EMI filter to improve its performance during these frequencies
because the size and weight of the passive components would
increase to a great degree, while the proposed active circuit can
do this with ease.
In Fig. 10(a), both an AEF and a passive EMI filter are added
to the system. With the aid of the AEF, the EMI spectrum
of 150–700 kHz is highly attenuated so that it is possible to
meet the standard. Although the level of the EMI spectrum is
increased especially above 1 MHz, the entire level is below
the given limit line. Fig. 10(b) shows the experiment result
when a 4.7 nF Y-capacitor Cy2 is used. By introducing simple
additional filter stage, one can dramatically increase the amount
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176
Fig. 10.
IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 48, NO. 1, FEBRUARY 2006
One active EMI filter (AEF) with one passive EMI filter. (a) Cy2 uninstalled. (b) Cy2 installed.
of noise attenuation attainable for the hybrid system. Because
the additional Y capacitor provides a low impedance path for
high frequency noise current, so that the noise current flowing
into LISN will be reduced, then the conducted CM interference
will be reduced too.
Therefore, as illustrated in Fig. 10, the combination of active
and passive filter systems yields a superior result over either one
alone. At low frequencies, the active filter compensates for the
limitations of the passive filter and at higher frequencies only
passive filtering is effective. These two complementary schemes
allow for the maximum usage of the filtering technique.
V. CONCLUSION
In this paper, we have attempted to compare the characteristics of APF and AEF in order to justify the feasibility of using
AEF into the IPEMs. A hybrid passive/active filter topology
that reduces EMI noise by injecting a compensating current is
built. First, conventional passive filters reduce the EMI noise to a
few percent. Then an active filter gives substantial noise attenuation, especially at low frequencies. Finally, some high-frequency
passive filters take over where the active filtering becomes
ineffective. The experimental results demonstrate the feasibility
and high performance of the new approach and illustrate its potential benefits. It is demonstrated that the proposed approach
is most effective in case where it is desirable to minimize the
passive component size and cost.
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Wenjie Chen was born in Xi’an, China, in 1974.
She received the B.S. and M.S. degrees from Xi’an
Jiaotong University, Xi’an, China, in 1996 and 2002,
respectively, both in electrical engineering. She is
currently pursuing the Ph.D. degree at Xi’an Jiaotong
University, Xi’an, China, with her research focused
on the EMI and integration design of power electronics modules.
She has been a member of the faculty of the School
of Electrical Engineering, Xi’an Jiaotong University,
since 2002, where she is currently a Lecturer. Her
main research interests include soft-switching dc/dc converters and active filters
and power electronic integration.
177
Xu Yang (M’02) was born in China in 1972. He
received the B.S. and Ph.D. degrees from Xi’an Jiaotong University, Xi’an, China, in 1994 and 1999, respectively, both in electrical engineering.
He has been a member of the faculty of the School
of Electrical Engineering, Xi’an Jiaotong University,
since 1999, where he is presently a Professor. From
November 2004 to November 2005, he was with
the Center of Power Electronics Systems (CPES),
Virginia Polytechnic Institute and State University,
Blacksburg, as a Visiting Scholar. He then came back
to Xi’an Jiaotong University and engaged in the teaching and researches in
power electronics and industrial automation area. His research interests include
soft switching topologies, PWM control techniques and power electronic integration, and packaging technologies.
Zhaoan Wang (SM’98) was born in Xi’an, China, on
June 9, 1945. He received the B.S. and M.S. degrees
from Xi’an Jiaotong University in 1970 and 1982,
respectively, and the Ph.D. degree from Osaka University, Osaka, Japan, in 1989.
From 1970 to 1979, he was an Engineer at Xi’an
Rectifier Factory. Starting in 1982, he became a Lecturer at Xi’an Jiaotong University, where he is now a
Professor. He is engaged in research on power conversion system, harmonics suppression, reactive power
compensation and power electronic integration, and
active power filters. He has published over 150 technical papers and has led
numerous government and industry-sponsored projects in the areas of power
and industrial electronics.
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