Integrated RF-EMI transmission line filters for Integrated Power

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					       Integrated RF-EMI transmission line filters for Integrated Power Electronics
                                   J.D. van Wyk jr., *P.J. Wolmarans, *J.D. van Wyk, +W.A. Cronje
   +                                                                                  *
   Industrial Electronics Technology Research Group                                 Center for Power Electronics Systems
  Department of Electrical and Electronic Engineering                  The Bradley Department of Electrical and Computer Engineering
               Rand Afrikaans University                                     Virginia Polytechnic Institute and State University
           Johannesburg, Gauteng 2006 RSA                                              Blacksburg, VA 24061 USA

    Abstract– Conducted EMI filter realization is possible                   frequency paths, it becomes possible to selectively propagate
through interconnect attenuation factor manipulation. Addition               the power frequency and absorb/dissipate the significantly
of new layers into a traditional parallel plate interconnect                 smaller amplitude RF in different locations. This offers
structure, taking into account, and utilizing, the skin- and                 considerable thermo-mechanical advantages in a hybrid
proximity effects, enhance high frequency attenuation while                  integrated structure. Conventional RF-EMI filters have to
maintaining unimpeded low frequency propagation. These
filters can be incorporated into integrated converter input bus
                                                                             pass the full converter power, yet absorb the RF-power in the
bars. This paper describes the development, manufacture and                  same structure, leading to the problem of propagating
initial experimental investigation of such a filter.                         multikilowatts, yet absorbing possibly only some hundred
                                                                             milliwatts of RF-power. Furthermore, the conventional
                                                                             approach to the design of EMI filters for given input/output
                          I.        INTRODUCTION                             impedances have been known to lead to unexpected EMI-
   Integration of power electronic converters is driven by the               amplification [6], something that can be alleviated by the
requirements for improved performance and reliability as                     presently proposed propagation filters [6,7,8].
well as reduced cost [1]. In the increasing process of (hybrid)                 The work presented in this paper will indicate how the use
integration, the power density is also increased appreciably,                of the geometric arrangement of materials within the parallel
leading to increased electromagnetic coupling between parts                  plane transmission line structure (input bus bar) at the input
of the hybrid integrated power electronics module (IPEM), as                 of an IPEM leads to the spatial and frequency dependent
well as between closer coupled sub-systems. This enhances                    propagation constants being utilized as an integrated RF-EMI
the importance of RF-EMI for intra-compatibility and                         filter. The dimensions of the practically constructed filter
intercompatibility of these IPEMs.            The traditional                introduced here makes it compatible with the Embedded
technology for RF-EMI filters, however, negates much of the                  Power technology for integrated power switching stages in
advantages of integration, especially in terms of power                      IPEMs [9]. It is 10mm wide, 1.5mm high, consists of three
density and manufacturing cost, as these types of filters tend               sandwiched planar layers and is realized in a U-form, with
to be bulky as well as use discrete components. In the                       three 50mm long legs, giving a mean interconnect length of
exploration of the technological possibilities for three-                    130mm. In the design and construction of the filters it was
dimensional integration of power electronic converters,                      assumed that the RF-EMI region of interest would be
planar metalization technologies have been explored for the                  between 1 MHz and 50 MHz.
integration of both power switching stages and
electromagnetic power passives [2].          This opens the                                     II.   FILTER STRUCTURES
possibility of utilizing these planar metalized structures as                   The electromagnetic energy flow along a traditional
multi-functional elements in an integrated converter. These                  parallel plate transmission line allows design of a structure
planar metalized interconnections form parallel plate                        where highly conductive materials and more resistive
transmission lines and offers the opportunity of implementing                materials are placed location specific within the conducting
frequency selective propagation as an RF-EMI filter                          structure. The current distribution within the structure is a
mechanism, as recently illustrated for IPEM-application [3].                 major design variable. Prevailing knowledge of the skin and
   Although the idea of using frequency selective propagation                proximity effects is used to determine how and where the
as filters has been known for some time [4-7] and explored                   current will flow within a given structure. For a parallel plate
for a range of applications [8], the recently proposed novel                 transmission line one expects the current distribution to look
use of the input propagation structure of an IPEM as                         similar to that of Fig. 1 at low frequencies. At high
integrated filter establishes the feasibility within the                     frequencies the current distribution would then look more
dimensions required [3] and also exhibits further advantages                 like that of Fig. 2, due to skin and proximity effects.
in terms of integration. Due to different low and high

 Some of this work made use of shared facilities under the ERC Program of
the National Science Foundation, Award number EEC-9731677

                                                                                     involved integration steps. In terms of manufacturing,
                                                                                     Design-11 is easily realizable. A large number of alternative
                                                                                     designs, in addition to those in Fig. 3, were investigated.
                                                                                     These show similar characteristics, but were discounted
                                                                                     based on insufficient attenuation.
                                                                                                                           Design-4              Design-10
                                                                                                                           Design-11             Design-12
   Fig. 1. Cross-section of a planar transmission line showing a qualitative
                                                                                                         1        10          100         1000       10000       100000
        current density distribution in the conductors at low frequency.


                                                                                       Gain [dB]



   Fig. 2. Cross-section of a planar transmission line showing a qualitative                                             Frequency [kHz]
        current density distribution in the conductors at high frequency.
                                                                                         Fig. 4. FEM obtained gain vs. frequency plot for a variety of 0.13m long
   Inserting resistive material into the areas of high frequency                                                       structures.
current concentration, and thus realizing two alternate
                                                                                        Consequently, the manufacturability of Design-11 was
conduction paths, should increase high frequency losses and
                                                                                     evaluated in the CPES Integrated Packaging lab. It was
attenuation. Based on these and other considerations, several
                                                                                     decided to replace the flame sprayed aluminum conductors
structures of varying degrees of complexity were investigated
                                                                                     with nickel because of ease of use and familiarity with the
[3]. A few of these possibilities are shown in Fig. 3. The use
                                                                                     sputtering and electroplating processes. To facilitate a range
of ferrites was investigated and some designs make use of up
                                                                                     of measurements, the different parts were not epoxied
to five different materials.
                                                                                     together. The new structure, Design-17 (Fig. 5), uses the
                                                                                     embedded nickel-plated transmission line as the high
                                                                                     frequency propagation path to achieve the desired
                                                                                     attenuation. A high permittivity ceramic (εr = 12000
                                                                                     minimum) is used as the center dielectric. Nickel conductors
                                                                                     are deposited onto both sides of the ceramic to form the
                                                                                     inside transmission line. The outer transmission line consists
                                                                                     of two alumina substrates, metalized with copper conductors
                                                                                     on the outside.

      Fig. 3. Cross sections of four structures showing different design
        complexities (Not to scale: vertical dimensions exaggerated).

   Each structure1 was evaluated using Finite Element
Method software to evaluate the transmission line parameters
R, L, C and G. From the results of these simulations, the
attenuation factors of the structures were calculated. Most                                          Fig. 5. Cross section of Design-17 as used in FEM simulations.
structures exhibited an increased attenuation as the frequency
increased beyond 1 MHz (Fig. 4). A very simple variation on                             Figure 6 shows a comparison between Design-17 and
the parallel plate transmission line structure, Design-11,                           Design-11. The utilization of nickel, with conductivity
yielded the best results of those evaluated.                                         comparable to flame sprayed aluminum, but a higher
   Attenuation factors comparable to that of Design-11 are                           magnetic permeability, leads to smaller high frequency
attained with some of the structures (Fig. 4), but these                             attenuation. The overall filter behavior, however, was still
involved stepped profiles and ferrites, leading to more                              considered acceptable.

  The type of aluminum used in the simulations is flame sprayed aluminum
with a conductivity of σ = 1.6e6 S/m as opposed to solid aluminum (σ =
3.82e7 S/m).

                          Design-11                Design-17
                                                                                       The above agrees well with the conceptual philosophy. At
                                                                                    10 kHz, energy propagates unimpeded along the copper
           1            10              100            1000           10000
                                                                                    interconnect. At high frequencies, such as 10 MHz and up,
                                                                                    current flows via the alternative resistive nickel path, thus
                                                                                    effecting losses.
                                                                                                             III.    CONSTRUCTION
     -20                                                                               Test structures based on Design-17 were made to be
     -25                                                                            compatible with the dimensions of the active IPEMs in
                                                                                    Embedded Power Technology [9] being constructed in the
                                                                                    CPES-IP-Lab. These IPEMs are made on 50x50mm alumina
                          F re quenc y [k H z]
                                                                                    substrates. A fairly wide edge remains unused. The initial
  Fig. 6. FEM obtained Gain vs. Frequency plot for 0.13 m long structures.          test design was a 10 mm wide transmission line around three
   Comparison showing difference between using flame sprayed aluminum
          (Design-11) and nickel (Design-17) as inside conductors.
                                                                                    edges of the substrate as shown in Fig. 9.

   The copper and nickel conductors are connected in parallel
during simulations, thus implying source current sharing.                                                            Alumina with copper on top side
Fig. 7 illustrates current density for Design-17 at 10kHz, as
obtained from FEM simulations. It is clear that current
distribution is nearly uniform in both copper and nickel
conductors, with the nickel current almost negligible, as
                                                                                                                     Alumina with copper on bottom side
expected. Fig. 8 illustrates the FEM obtained 10MHz current
distribution. Current flows predominantly in the nickel, with
some current in the copper edges, due to the skin- and                                                    Fig. 9a. Exploded view of filter.
proximity effects.

     Fig. 7. Part of a cross section of Design-17 showing current density            Fig. 9b. Constructed filter. A piece of ceramic can be seen between the top
               distribution in the conductive layers at 10 kHz                        and bottom alumina substrates. Copper tabs were attached for measuring
                                                                                                      purposes. (Penny added to show scale).

                                                                                       The high permittivity dielectric ceramic is thoroughly
                                                                                    cleaned to remove any oily deposits, acids and bases and
                                                                                    checked for the existence of defects and pinholes. An RF-
                                                                                    sputtering process is then used as the first metalization step.
                                                                                    A sub-micron layer of titanium is sputtered onto this ceramic
                                                                                    first. This seed layer is necessary to ensure that proper
                                                                                    adhesion is attained between the ceramic and the required
                                                                                    metal. A similar seed layer of nickel is then sputtered onto
                                                                                    the titanium. The ceramic is removed from the chamber after
                                                                                    it has cooled down. The nickel is electroplated to the desired
                                                                                    thickness in a heated electrolytic solution. Finally, the excess
                                                                                    metal is etched away leaving the design as shown in Fig. 9.
                                                                                    On the alumina, copper is used instead of nickel, and only
                                                                                    one side is metalized. The metalized ceramic is then
     Fig. 8. Part of a cross section of Design-17 showing current density           sandwiched between the two alumina substrates, with the
                   distribution in the conductors at 10 MHz                         copper conductors on the outside. The terminals of the

copper and nickel are connected with copper tabs for                              The filter was shorted at one end, while the input terminals
measurements, but in the final realization will be part of the                 were kept as close together as possible to eliminate parasitic
Embedded power integrated structure.                                           influences on the measurements. Figure 11 shows that the
                                                                               impedance is very low and frequency effects only start to
                        IV.     MEASUREMENT                                    show at 300 kHz. Around 2.1 MHz the impedance drops and
                                                                               starts to rise again at 4.9 MHz, encountering another slight
   Gain-Phase measurements were made with a HP3577B
                                                                               dip at 10 MHz.
Network Analyzer. The measurements were made with 50 Ω
terminations and 20 dB attenuators at either end.
   The gain-phase measurement given in Fig. 10 shows that
the required increase in attenuation with increased frequency
in the region above 1 MHz exists.

                                                                                 Fig. 11. Impedance vs. Frequency. Short circuit impedance of Design-17.
                                                                                                          Dark line: Impedance

                                                                                  The inside and outside transmission lines were
                                                                               subsequently evaluated separately. The results of the
                                                                               impedance measurements are presented in Figures 12 and 13.
                                                                                  In Fig. 12, the higher impedance at the low frequency can
                                                                               be ascribed to the higher resistance of the nickel. The first
                                                                               peak in this case is at 2.3 MHz. The high frequency short
                                                                               circuit behavior of the inside transmission line corresponds in
                                                                               some respects to that of the complete filter, except for a small
     Fig. 10. Gain-Phase vs. Frequency plot of filter. Top line: Gain.         upward shift in frequency and slightly lower impedances.
                                                                               Given the mean length of this structure it was determined that
   The gain has a positive peak around 1MHz. The drop-off                      the first minimum point at 5.4 MHz coincided closely with
after this point however, is much steeper than the simulated                   the quarter wave resonant frequency.
results. The measured attenuation at 10 MHz is at least 10                        This was calculated from:
dB better than the simulated results. The cause of the                             f = (λ⋅√(µ0⋅ε0⋅εr))-1                                    (1)
positive peak around 1 MHz might possibly be ascribed to a
parallel resonance that occurs due to a lack of sufficient                     where   λ = 0.127 m,
damping from the 50 Ω measuring system, although other                                 εr = 12000, which gives 21.5 MHz, giving a quarter
causes could exist. The shape of the plot is very similar to                   wave length at 5.38 MHz. The next point is the half wave
that of an underdamped LC low pass filter.                                     resonance, at double the frequency.
   As the effectiveness of the filter is due to the division of
high and low frequency currents between the different parts,
the different subsystems of the filter were also characterized
by impedance measurements. Short circuit impedance
measurements were made in order to identify the possible
quarter wavelength resonances and other transmission line
effects that can influence the filter behavior. It was done
with an HP4194A Impedance/Gain-Phase Analyzer. The
analyzer is limited to 401 sample points during a single
sweep, but these points can be set manually. The impedance
analyzer has a workable range of 10 mΩ up to 100 MΩ with
a resolution of 100 µΩ. In addition, error estimation tables
are provided for various frequency ranges and measurement
setups.                                                                         Fig. 12. Impedance vs. Frequency. Short circuit impedance of ceramic with
                                                                                              nickel conductors only. Dark line: Impedance

   Fig. 13, illustrating the short circuit impedance of the outer                the proximity effect and the number of conductors, and their
transmission line, doesn’t show any resonant effects. The                        material type, into account.
quarter wave resonant frequency was calculated to be at 208                         The behavior of the filter around the low megahertz range
MHz. As expected, the low frequency impedance is very                            as shown in Fig. 10, has to be investigated further, especially
low. The thickness of the alumina substrates and the                             in terms of the design variables and materials, as well as the
permittivity makes the capacitance negligible in this range.                     relation to the input and output impedances. Furthermore,
The high frequency behavior is then as expected, with both                       the preliminary but positive results obtained with this filter
the resistance and inductance causing an increase in                             encourage further work on detailed modeling as well as on
impedance. If the measured impedance of 5 Ω for the outer                        technology developments.
transmission line is translated to an inductance, we obtain                         Firstly, as the impedance measurements indicate clear
153 nH/m. (Neglecting capacitance, resistance and                                transmission line resonances in the frequency range of
conductance). This is very close to the 135 nH/m obtained                        interest, the structure has to be further analyzed from the
from the FEM simulation of the outer transmission line.                          viewpoint of an interconnected (mostly parallel) transmission
Since no theoretical or experimental values for the ceramic                      line system. The transmission line parameters, however, are
conductance at 40 MHz is available at present, it is difficult                   now frequency dependent (especially the value of the
to calculate an impedance for comparison with measurement.                       conductance, G, due to the use of ceramic dielectrics, and the
                                                                                 resistance, R, due to the specifically intended utilization of
                                                                                 the skin and proximity effects). This frequency dependency
                                                                                 can only be properly explored by further FEM work on
                                                                                 structures selected for implementing such filters.
                                                                                    Secondly, the success of this first concept encourages
                                                                                 further exploratory work on the materials to be used, as well
                                                                                 as the processes and integration technology for these filters.
                                                                                 The present concept has been illustrated in terms of a
                                                                                 minimum profile planar sandwich structure that can be
                                                                                 introduced into the input of the power switching stage of an
                                                                                 IPEM. Further work needs to explore the integration into the
                                                                                 power switching stage of the IPEM itself, as well as the
                                                                                 accompanying thermo-mechanical issues.
   Fig. 13. Impedance vs. Frequency. Short circuit impedance of alumina
        substrates with copper conductors only. Dark line: Impedance                                   VI.    CONCLUSION
                                                                                    This paper has introduced the concept of using the parallel
                           V.     DISCUSSION                                     plane input conductive structure of an integrated power
                                                                                 electronic module as a RF-EMI filter. The concept was the
   From the FEM simulations and the measured results it can                      spatial and frequency dependent propagation achieved by
be seen that this planar integrated filter concept is viable. At                 appropriate geometrical arrangement of electromagnetic
low frequencies there is very little attenuation (Fig. 10).                      materials in a multi-conductor transmission line fashion. The
Since the conductivity of copper (σCu = 5.8e7 S/m) is much                       feasibility was first established by FEM simulations and then
higher than that of the nickel (σNi = 1.45e7 S/m), the outside                   tested by construction of an integrated filter compatible with
transmission line has a lower resistance. Fig. 7 shows a                         the 50x50 mm planar dimensions of integrated power
higher current density in the copper than in the nickel. The                     switching stages as developed within CPES.
copper conductors can be designed to handle the appropriate                         In terms of impedance as function of frequency, excellent
low frequency power levels. At higher frequencies, the skin                      correspondence between the FEM calculations and the
effect should develop quicker in the nickel than in the copper.                  measurement results were obtained in the regions where
The relations for skin depth are given by                                        transmission line effects were not dominant. The measured
   δCu = 0.066/√f           [m]                               (2)                insertion loss of the filter exceeded the theoretical
                                                                                 predictions, justifying the original expectations. It underlines
  δNi = 0.0187/√f               [m]                                       (3)    the necessity for refining the work with analytical frequency
The point at which skin depth equals conductor thickness is                      dependent parameter modeling of this class of RF-EMI
1.21 MHz for nickel and 6.97 MHz for copper. The FEM                             integrated transmission line filters. Further FEM and
simulations however, show that the proximity effect                              experimental investigation of alternative resistive materials
dominates at 10MHz, with almost uniform current                                  can also be justified.
distribution in the Ni (Fig. 8). Determining analytical
relations for frequency dependent resistance will have to take

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