AIAA Propulsion and Energy Forum
August 9-11, 2021, VIRTUAL EVENT
AIAA Propulsion and Energy 2021 Forum
10.2514/6.2021-3333
Design and Analysis of EMI Absorbing
Composites for Electric Aircraft
Omar Faruqe1, Farhina Haque2 and Chanyeop Park3
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Mississippi State University, Starkville, Mississippi, 39762, USA
Wide bandgap semiconductor (WBG) based power converters are suitable for electric
aircraft because they enable high power density designs. However, due to their high switching
frequency operation and high 𝒅𝒗/𝒅𝒕, WBG converters generate electromagnetic interference
(EMI) radiation. These radiated EMI can negatively impact power electronic components and
control system, and pose risk to the overall electric aircraft. Therefore, it is essential to reduce
EMI radiation of power electronic converter modules. In this study, we numerically analyze
composite materials that absorb EMI radiation. Finite element analysis (FEA) was performed
to identify the required electrical and magnetic properties of the composites that enable
maximum EMI absorption at frequencies ranging from 1 MHz to 3 GHz.
I. Nomenclature
E
H
𝑑𝑣/𝑑𝑡
δ
η
d
ω
f
μ
ε
σ
=
=
=
=
=
=
=
=
=
=
=
electric field strength
Magnetic field strength
changing rate of the drain-source voltage during the switching transient.
skin depth of materials
intrinsic impedance of the medium
thickness of shielding material
angular frequency
frequency
magnetic permeability
permittivity
electrical conductivity
II. Introduction
The recent advancements of power and energy technologies brought a strong interest in developing electric
propulsion in aviation [1]. Powering distributed aircraft propulsion motors involves a large number of power electronic
converters in the aircraft. The use of wide bandgap semiconductor (WBG) based power converter is gaining
momentum over the years because of their ability of enabling high power density and for their lower switching loss
with high 𝑑𝑣/𝑑𝑡 [2] – [6]. However, these WBG power converters are a potential source of high-frequency EMI
radiation because of their fast switching frequency and high 𝑑𝑣/𝑑𝑡 [7] – [8]. These radiated EMI can cause severe
problems to aircraft operation by creating crosstalk and disturbing controllers. Therefore, proper shielding is required
to limit the EMI radiation.
Conducted EMI can be designing power electronic converter circuits with filters and other passive elements. The
mitigation of EMI by introducing filter and PWM techniques is proposed in [9] – [12]. However, in real systems where
a large number of power electronic converters are integrated, these methods will be difficult to be implemented
accurately, and also result in higher cost. On the other hand, mitigation of radiated EMI is achieved by shielding.
However, the current methods of shielding with highly conductive monolithic metals cannot solve this problem
completely because they cause more reflection than the absorption of EMI. In reality, in power electronic applications,
1
Graduate Research Assistant, Electrical and Computer Engineering.
Graduate Research Assistant , Electrical and Computer Engineering.
3
Assistant Professor, Electrical and Computer Engineering.
2
1
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this method of shielding would exacerbate the problem by reflecting EMI waves back to the power converter and
causing cross-talk. Epoxy-based composites, on the other hand, can be used to improve shielding efficacy because the
magnetic and electric properties of the composites can be modified by changing the filler type and concentration.
In this study, we present the properties of a new composite material that can be used as an EMI shield that mainly
absorbs radiation originated from power converters based on numerical analysis. The actual fabrication process and
characteristics of magnetic nanocomposites are discussed in [13] – [14]. From the B-H curve presented in [14], the
maximum relative permeability is found to be on the range of 100 to 300 depending on the percentage of a filler. The
process for increasing electrical conductivity of epoxy composites is discussed in [15]. Here, we performed finite
element analysis (FEA) in COMSOL Multiphysics software and show how to increase EMI absorption properties by
changing the electrical and magnetic properties of composite materials. In the simulation, the relative permeability of
135 was chosen for the absorbing material that is comparable to the composites described in [14]. Based on the
simulation results, we expect that if the composite material is tailored properly, it will be able to effectively absorb
EMI radiation and address the emerging issues of EMI in electric aircrafts.
III. Generation of EMI from Power Converter
EMI generated from a power converter is divided into two parts: differential mode (DM) noise and common-mode
(CM) noise [16]. The DM noise current flows along the operation path of the circuit whereas CM noise currents flow
through the earth via stray capacitance. As an example, a buck converter is shown in Fig. 1 that has stray capacitances
C1 and C2 between DC source input and earth ground. In Fig. 1, CM represents the parasitic capacitance between
earth and the hot node, where the potential changes rapidly with time (dv/dt) and the MOSFET (S), the diode (D), and
the inductor (L) are connected.
Fig. 1 EMI in a buck converter.
When the converter operates, the drain to source voltage (Vds) is full of high-frequency harmonics due to switching.
This leads to the flow of high-frequency current through the operation loop that consists of input capacitor C in, diode
(D), and voltage source (DC). This current generates DM noise which is dominant in the low-frequency range. On the
other hand, Vds drives high-frequency current that flows through the parasitic capacitance CM to earth and returns
through C1 and C2, which causes CM noise, the main source of far-field EMI. When these converter circuits are
operating near the power or signal cables, the currents induced on the cables are the primary source of far-field EMI.
The EMI radiation would be very high in modern electric aircrafts because numerous power electronic converters will
be operated in a confined space. Moreover, the use of WBG semiconductors will further aggravate this problem
because of their high-frequency switching and high dv/dt.
IV. EMI Absorbing Materials
Given an EM wave incident on a material, a portion of the wave is reflected to the propagating medium, another
portion is absorbed by the material, and the remaining is transmitted to the propagating medium [17]. Consider an EM
wave (Ei) propagating in air (intrinsic impedance, η0) as shown in Fig. 2. The wave is incident on a planar material
surface having an intrinsic impedance of ηs. The wave is split into two parts: one part is reflected wave E R, another
part Es penetrates the first interface (x = 0). Traveling inside the material, the wave E s loses energy to absorption
before encountering the second interface (x = d). After reaching the second surface, a portion of the wave is
2
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transmitted (ET) and the remaining is reflected inside the material (EIR) and further absorption occurs. If the material
thickness (d) is much greater than the skin depth (δ) then the energy of E IR will essentially be fully absorbed by the
material. However, if the material thickness is small relative to its skin depth, then EIR will travel inside the material
and cause reflection again. This phenomenon is called multiple-reflection which deteriorates the shielding
effectiveness.
Fig. 2 Shielding mechanism by a conductive material [18].
The shielding ability of a material is determined by the value of shielding effectiveness (SE) which consists of
three parts: Shielding by reflection (SER), shielding by absorption (SEA), and shielding by multiple-reflection (SMR)
[18].
(1)
SE = 𝑆𝐸𝑅 + 𝑆𝐸𝐴 + 𝑆𝐸𝑀𝑅
where
Shielding by reflection 𝑆𝐸𝑅 = 20 𝑙𝑜𝑔
η0
4η𝑠
𝑑
Shielding by absorption 𝑆𝐸𝐴 = 20 log 𝑒 𝛿
(2)
(3)
Shielding by multiple-reflection,
2𝑑
𝑆𝐸𝑀𝑅 = 20 log |1 − 𝑒 − 𝛿 |
Intrinsic Impedance, η =
|𝐸|
|𝐻|
(4)
𝑗𝜔𝜇
= √
𝜎+𝑗𝜔𝜀
(5)
where 𝜔 is the angular frequency, 𝜇 is the magnetic permeability, σ is the electrical conductivity and ε is the
permittivity of the medium. If the wave is propagating in air then σ 0 ≈ 0. So, (5) reduces to,
𝜇0
|η0 | = √
(6)
𝜀0
Assuming the conductivity of shielding material is very high i.e. σ >> 𝜔𝜀. Then, (5) simplifies to,
|η𝑠 | = √
𝜔𝜇
𝜎
= √
2𝜋𝑓𝜇
𝜎
Skin depth, δ = (√𝜋𝑓𝜇𝜎)−1
(7)
(8)
Substituting the values of η0 , η𝑠 and δ in (2) and (3), SER and SEA expressions become:
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𝑆𝐸𝑅 = 3.95 + 10𝑙𝑜𝑔
𝜎
2𝜋𝑓𝜇
𝑑
𝑆𝐸𝐴 = 8.7 = 8.7𝑑√𝜋𝑓𝜇𝜎
𝛿
(9)
(10)
From (4), it is clear that if the thickness (d) is large relative to the skin depth (δ), 𝑆𝐸𝑀𝑅 = 0 that means no effect
due to multiple-reflection. In this case, shielding effectiveness will depend on reflection and absorption.
V. COMSOL Simulation
A. Simulation and Geometry
To simulate the model, let us consider a two-port co-axial cable network as shown in Fig. 3. A sample of absorbing
material will be placed between the ports. Then, we will energize port 1 by EM wave and port 2 will be used as a
receiver.
Fig. 3 Model for COMSOL simulation.
In practice, the absorbing materials will be laminated over the metallic enclosure. Therefore, we added an aluminum
surface (1 mm) beneath the sample in the COMSOL model. The cross-section of the model is shown in Fig. 4.
In the model, we used the dimension and material properties of an RG6-CATV co-axial cable. The diameters of
inner (di) and outer conductor (do) were 1.02 mm and 6.8 mm respectively and the characteristic impedance of the cable
was 75 Ω. The characteristic impedance (𝑍0 ) of a co-axial cable is defined by:
𝑍0 =
138
√𝑘
𝑑
log ( 𝑜 )
𝑑𝑖
(11)
Aluminum plate
Fig. 4 Geometry in COMSOL.
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where
d0 = diameter of the outer conductor
di = diameter of the inner conductor
k = relative permittivity of the insulating material between the conductors
The insulating material of RG6 cable is polyethylene foam that has a relative permittivity (k) of 2.3. The intrinsic
impedances of lumped port 1 and lumped port 2 were also 75Ω to avoid impedance mismatch.
B. Absorption power in COMSOL
From (9) and (10), we can infer that shielding effectiveness depends on material conductivity, magnetic
permeability, thickness, and wave frequency. The reflection, transmission, and absorption power are calculated from
the S parameters as follows:
𝑆11 dB = 10 𝑙𝑜𝑔10 (
𝑃𝑟𝑒𝑓𝑙𝑒𝑐𝑡𝑒𝑑
𝑃𝑖𝑛
)
(12)
Therefore, reflected power 𝑃𝑟𝑒𝑓𝑙𝑒𝑐𝑡𝑒𝑑 = 𝑃𝑖𝑛 x10
𝑃𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑡𝑡𝑒𝑑
And, 𝑆21 dB = 10 𝑙𝑜𝑔10 (
𝑃𝑖𝑛
𝑆11 𝑑𝐵
10
(13)
)
So, transmitted power 𝑃𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑡𝑡𝑒𝑑 = 𝑃𝑖𝑛 x10
(14)
𝑆21 𝑑𝐵
10
(15)
Absorption power 𝑃𝑎𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 = 𝑃𝑖𝑛 - (𝑃𝑟𝑒𝑓𝑙𝑒𝑐𝑡𝑒𝑑 + 𝑃𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑡𝑡𝑒𝑑 )
(16)
where
S11 = input port reflection coefficient
S21 = transmission coefficient from port 1 to port 2
Pin = input power at port 1
In our COMSOL model, the magnitude of source voltage at port 1, Vmax = √6/2 V. So, the input power is Pin =
𝑉𝑟𝑚𝑠 2
𝑍0
= 10 mW. The value of S21 and S11 can be obtained by using the COMSOL function “emw.S21” and “emw.S11”
respectively. We plotted 1D graphs using the (13), (15), and (16) and compare the absorption power by changing the
conductivity, relative permeability, and thickness of the sample.
C. Results and discussions
In this section, we will compare the change of reflection and absorption power by changing the properties of the
shielding material. This comparison can be divided into three parts:
a) Simulation 1 – Electrical Conductivity
In this part, for a specific value of relative permeability, the conductivity was varied from 1 to 200 S/m
to see the impact of conductivity in absorption power. The thickness of the sample was 1mm, relative
permeability was chosen as 135 that is similar to the value of μ r calculated from the B-H curve of [14].
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Fig. 5 Reflection power vs frequency (μr = 135, thickness 1 mm) with increasing electrical conductivity.
Fig. 6 Absorption power vs frequency (μr = 135, thickness 1 mm) with increasing electrical conductivity .
From the simulation, for a specific value of relative permeability, we can get an idea about the impact of
conductivity on absorption. From Fig. 5 and Fig. 6, we can see that the absorption power increases with the increase
of conductivity but after a certain value, absorption decreases with the increase of conductivity. Therefore we need to
modify the conductivity of the sample during fabrication process so that the absorption will be higher.
b) Simulation 2 – Relative Permeability
In this part, for a fixed conductivity value i.e. 100 S/m, we plotted power vs frequency for relative permeability
of 135 and 1000. The thickness of the sample was 1mm.
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Fig. 7 Reflection power vs frequency (σ = 100 S/m, thickness 1 mm) with increasing permeability.
Fig. 8 Absorption power vs frequency (σ = 100 S/m, thickness 1 mm) with increasing permeability.
From Fig. 7 and Fig. 8, it is clear that increasing relative permeability makes the curves steeper and results in higher
absorption in the lower frequency range compared to the relative permeability of 135. Therefore, materials with high
magnetic permeability and lower conductivity in the range of hundreds S/m, will be very effective to absorb the EMI
radiation.
c) Simulation 3 – Sample Thickness
To see the effect of multiple reflection, we performed the simulation with very low thickness i.e. t = 0.1mm and
compared the change of absorption power with higher thickness i.e. t = 1 mm. The relative permeability and
conductivity were 135 and 100 S/m respectively.
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Fig. 9 Reflection power vs frequency (σ = 100 S/m, μr = 135) with increasing thickness.
Fig. 10 Absorption power vs frequency (σ = 100 S/m, μr = 135) with increasing thickness.
From Fig. 9 and Fig. 10 we can see that the absorption ability is reduced by multiple reflection when the thickness is
very low compared to the skin depth, δ = (√𝜋𝑓𝜇𝜎)−1 . As the sample is very thin, more EM waves will pass through
it and incident on the aluminum surface. Because of aluminum’s very high conductivity and low relative permeability,
it reflects the waves and it results in higher reflection than absorption.
D. Fabrication Process
In the future, we will follow the following steps to fabricate the EMI absorbing material:
a) We will fabricate the sample by mixing epoxy and magnetic nanoparticles for different wt% and measure the
relative permeability of each sample. By trial and error method we will identify the best composition of
nanofillers that exhibits maximum permeability with less agglomeration.
b) Once we obtained the suitable relative permeability we will conduct FFT analysis of the unfiltered output of
power converter to identify the dominating high-frequency components. Then we will perform “Simulation
1 - Analysis of Conductivity” to see the optimum conductivity range.
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c)
The conductivity of epoxy-magnetic composites might be lower than the optimum conductivity range. To
make the sample more conductive we will prepare another sample by adding CNT with the magnetic
nanoparticles and epoxy resin.
The fabrication process is summarized in the flowchart below:
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xn = wt% of magnetic nano particle (MNP) for new step
xp = wt% of MNP for previous step
y = wt% of CNT
Fig. 11 The sample preparation process of EMI absorbing material.
E. Measurement of Absorption Power
The epoxy composites will be poured into a mold to create a donut shape sample for the absorption power
measurement. The specimen will be placed inside a cone-shaped sampling holder as shown in Fig. 12. The holder will
be connected to network analyzer via co-axial cable to measure the absorption power of the sample for a wide range
of frequencies.
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Fig. 12 Measurement of EM wave absorption property of the composites
VI. Conclusion
In this study, we propose a new approach to address the EMI radiation generated from power electronic converters
in electric aircraft. The numerical results of the composites show that shielding power electronic converters with
composite materials can be very effective for absorbing radiated EMI. The fabrication and practical implementation
of the EMI-absorbing composite materials will be demonstrated in our future work. We expect that these composite
coatings would solve the EMI radiation challenges in power-electronic-converter-driven aircraft power systems and
thus enhance the resiliency of next-generation electric aircraft.
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