Design of a High Temperature EMI Input Filter for a 2

Design of a High Temperature EMI Input Filter for a 2 kW HVDC-Fed Inverter Rémi ROBUTEL, Christian M ARTIN, Hervé M OREL, Cyril B UTTAY, Nicolas G AZEL*, Dominique B ERGOGNE Université de Lyon, F-69622, France CNRS, UMR5005, France, INSA Lyon, Université Claude Bernard Lyon 1 Laboratoire Ampère, bâtiment L. de Vinci, 21 avenue Capelle F-69621, France [email protected]

* Hispano-Suiza - Safran Power Division Safran Group, France Rond Point René Ravaud, BP 42 77551 Moissy-Cramayel, France

A BSTRACT In this paper, a high temperature EMI input filter for a SiC JFET inverter is designed and tested in operating conditions at 200°C. Inverter EMI issues, design procedure and characterizations of selected components are presented in details. The feasability based on industrially available materials and components is demonstrated with a low temperature dependence of the EMI Spectrum.

Keywords « EMC », « High temperature », « EMI Filter », « Choke », « Common Mode », « Differential Mode ».

Introduction Civil and military Aircraft industries are looking for better energy efficiency and higher energy density. Power train for actuators such as brakes or thrust reverser for example, usually powered by pneumatic or hydraulic power, are moving towards electrical power. In this way, semiconductor devices made of silicon carbide (SiC) offer solutions for power converters which can be used into motor drives and operate over 200°C, gradually replacing the hydraulic power train. Several works demonstrated the potential of SiC transistors as high temperature power switches [1, 2]. To build an entire high temperature inverter, an input filter, including the DC-link capacitor, is required as close as possible to the power switches to provide efficient decoupling and filtering [3]. The required input filter is composed of passive components. Today, much work remain to address on passives, especially for high temperature power converters. It could be a limitation as well as packaging or reliability. To study passive components and related materials for high temperature power conversion and ensure ElectroMagnetic Compatibility (EMC) of a 2kW SiC JFET inverter, a first high temperature input EMI filter is presented in this paper. The main ElectroMagnetic Interference (EMI) issues are briefly presented in section 1, for a

voltage inverter topology. Then, in section 2, the step by step design procedure is described with detailed information. Finally, experimental results are shown in section 3.

1

EMI Issues In Motor Drives

In this study, a voltage inverter topology is chosen to drive a 3 phases Permanent Magnet Synchronous Motor (PMSM). This association offers good dynamic performances and power density for aicraft actuators. A global scheme is presented in Figure 1. The inverter is supplied by an HVDC bus with 540V differential nominal voltage. Voltages between positive and negative phases and the ground are respectively at +/- 270V. The Line Impedance Stabilization Network (LISN) sets a standard impedance for high frequency and makes the EMI measurements reproducible.

1.1

EMI Modes and Noise Paths

As shown in Figure 1, there are three main potentials: positive phase, negative phase and ground. Conducted noise or harmonic currents are circulating between this potentials. Two modes, Common Mode (CM) and Differential Mode (DM), are defined

Figure 1: Electrical power train based on a SiC JFET voltage inverter

1.3

in equation 1. 

icm idm

 =

  i1 + i2 i1 − i2

(1)

CM currents are generated by high dV dt transient (occurring during the switching) through parasitic capacitances to the ground, DM currents by the power current ripple on the DC side. At high frequency, LISN inductance provides a high impedance compared to 50Ω. Consequently, most of the high frequency currents goes through the 50Ω impedances of the LISNs. EMI levels defined by standards are applied on the 50Ω LISN voltages or on DC phase currents, i1 or i2 .

1.2

JFET Inverter

The normally-on SiC JFET has been selected as the power switch for high temperature (200°C ambient) operation. This device, recently industrialized, allows fast commutations with low losses. Its switching losses decrease with temperature but its conduction losses increase with temperature, yielding to an overall increase of the losses with the temperature. However, with appropriate gate drivers, the global power losses are reduced compared to silicon devices [1, 4]. Due to the very good dynamic performances of the SiC-JFET, the switching frequency could be increased to reduce the DC link capacitor value. In a conservative way, however, the switching frequency has been kept here at 15kHz, a common value for motor drives. Higher CM current level at higher frequencies is expected by using the SiC JFET because of its switching speed, which is 2 to 10 times faster than for IGBT. The impact on the network must be contained to meet the standard requirements and avoid perturbations. EMI filtering is therefore a very important issue with the SiC devices.

Input Filter Description

The input filter is the main element that ensures the EMC between several converters supplied by a common HVDC bus. Meeting the EMC requirements enables low susceptibility from other converters and ensures low conducted emissions. Another function is to provide, at low frequency, stability over a large range of operating conditions [5–7].

2

Design Procedure

The design procedure is described step by step into the following subsections. The influence of high temperature is discussed regarding usual design considerations and key parameters.

2.1

Standards

A standard environment needs to be defined to evaluate the filter. According to the EMC requirements for airborne equipments, the section 21 from the EUROCAE ED-14F standard [8], known as DO-160F, is selected. This international civil standards specify test procedures for conducted EMI, from 150kHz to 152MHz. Category B, which defines tolerance level for power converters, is chosen for the motor drive and the tested bandwith is reduced to 30MHz. Conducted emissions are measured by a current probe on i1 or i2 , using the LISN.

2.2

Original Noise

In this paper, we assume that temperature has no influence on the original noise spectrum to be able to evaluate the filter behavior independently. In a real system, temperature is an important parameter that influences the commutations, therefore, the EMI spec-

trum (cf. Section 1). Simulations on an inverter leg with accurate component and cabling models [9] were done to show the influence of the temperature. Results on the EMI spectrum at different junction temperatures are shown in Figure 2.

Figure 2: Simulated conducted emissions from an inverter leg to LISN on DC line current (i1 or i2 ) at 540Vdc - 6A

To design properly the EMI filter, the original noise is separated into both modes by measuring i1 + i2 for CM, and i1 - i2 for DM. Each mode is quantified, and, with a defined filter structure, the cut-off frequency is ajusted for each mode. Close to the switching frequency, both modes are at comparable levels but at higher frequencies CM is larger (shown in section 2.4).

2.3

Filter Structure

Several structures are investigated, and a trade-off was made by taking into account four considerations with a given cut-off frequency:

Figure 3: Current attenuations and Output impedances of several filter structures connected to a realistic network impedance, a capacitor bank with HVDC grid line impedance. Inverter is modeled as a power-regulated load with an infinite bandwidth

Simulated results on attenuations and output impedances are compared in Figure 3 between L-C filter, two stages L-C filter and Pi filter. Damping is done by resistors which are designed following the method presented in [5, 7]. They are added in series with a capacitor or paralleled with the main inductor. Only most influent parasitics, like Equivalent Parasitic Capacitance (epc) of chokes or esl of capacitors, are taken into account.

• Attenuation, as the first criterion to meet standards • Component count, to contain volume and take into account higher failure probability at high temperature • Component values, directly linked to the volume of the components

Figure 4: Selected high temperature input EMI filter structure, Ldm : DM choke, Lcm : CM Choke, Ccm : CM capacitors, Cdm and Cdm2 : DC-link capacitors, Rd : damping resistor

• Output impedance, considered from the converter side for stability and large transients [5–7]

The selected structure is presented in Figure 4. L-C topology with a serial damping on capacitor

shows the best trade-off to demonstrate the feasability of a first high temperature filter. The impact of this structure is significant on the volume because a DClink capacitor with equivalent or higher value has to be added, but it brings no additionnal high frequency parasitics and offers robustness against HVDC network fluctuations.

A damping resistor and a capacitor, respectively Rd and Cdm2 in Figure 4, are chosen using the current transfert function in equation 3 based on component impedances. Znetwork (p) could be defined like a LISN or a more realistic network impedance. Fatt (p) =

2.4

Cut-off Frequency and Component Values

From the original noise levels and the standard level, in Figure 5, the required attenuation could be deduced from a basic substraction in frequency domain. A 6dBmargin is often added.

ZCdm (p)∗(ZCdm2 (p)+Rd ) ZCdm2 (p)+Rd +ZCdm (p) ZCdm (p)∗(ZCdm2 (p)+Rd ) ZCdm2 (p)+Rd +ZCdm (p) + ZLdm (p) + Znetwork (p)

(3) Thanks to [5, 7], Cdm2 is designed to be four times the initial DC-link capacitor to damp properly the system. To prevent any resonances between switching frequency harmonics and the DM filter, and to be able to limit at low frequency the ripple current, the cut-off frequency is chosen lower than the switching frequency , almost a decade below. In this case cut-off frequency, fc , is set at 3kHz. Then required inductance is given by equation 4. Ldm =

Figure 5: Conducted Emissions from Inverter to LISN, measured by a high bandwidth current probe (RFI Solar electronics 6741) on power DC line at 400Vdc-4A

For DM, we see the decrease of current harmonics, from the higher spikes at the switching frequency or two time the switching frequency depending on PWM strategy (not plotted in Figure 5 due to a lower frequency (15kHz) than the standard EMC bandwith). The choice of the cut-off frequency is a tradeoff between low frequency voltage and current ripple requirements, stability criteria and high frequency requirements. From 1MHz to 10MHz, DM level is around 10dBµA above the standard, it is much lower than CM level. Equation 2 is used to compute a primary DClink capacitance, Cdm . A triangle waveform voltage is assumed. ∆V dc is the ripple voltage, ic the RMS current in the capacitor and Tpwm the switching period. Cdm =

ic ∗ Tpwm 2∆V dc

(2)

1 (Cdm +Cdm2 ) ∗ (2π fc )2

(4)

For CM, the design is only focused on the 150kHz and 30MHz frequency range. Standards limit the amount of the total capacitance from phases to the ground at 100nF. Impact on DM is negligible because this value is much smaller than the DC-link capacitance and no damping is necessary. In Figure 5, CM current appears to be the most influent at high frequency. From this measurement, required attenuation is derived. With a similar equation to (3), the CM filter attenuation is estimated and a cut-off frequency is ajusted to ensure an higher or equal attenuation than it is required. Because of the better energy density of ceramics than magnetic materials, the maximum CM capacitance is targetted. Then, using equation 4 with CM parameters, the Lcm value is derived. Parameter and result values are indicated in Table 1. f pwm 15kHz Ldm 40µH

f cd m 3kHz Lcm 200µH

f cc m 35kHz Cdm 13µF

Tpwm 66.7µs Cdm2 53µF

ic 4Arms Ccm 50nF

∆V dc 10V Rd 3Ω

Table 1: Selected parameters and components

2.5

Hardware Design

This section describes the materials and components to achieve the value previously presented in Table 1. Temperature issues are mainly discussed in the following paragraphs. Materials and Components Table 2 presents the selected magnetic materials. Low permeability for the

DM and High permability for the CM are chosen because of different excitation modes. µr is the relative permeability, Js the saturation flux density, Hc , the coercitive field and Tc the Curie temperature. Material

µr

HighFlux Kµ Nanophy

160 200 30000

Js (T) 1.5 1.25 1.25

Hc (A/m, 1kHz) 80 10-15 5

Tc (°C) 500 570 570

Possible Use DM choke DM choke CM choke

permeability nanocristalline core, Kµ, is similar to the Nanophy. equation 5 is also true for the powder core. Js

Js Js = tanh( T 0K ) amb Js0K T

(5)

c

Table 2: Selected Magnetic Materials

From the comparison of µr , Js , Hc and Tc , it is possible to evaluate the performance of this materials respectively for: the inductance value with a given number of turn, the energy density (volume), the losses and the operating temperature range. However detailed characterizations are needed to quantify the influence of frequency and temperature. In Figure 6, the permeability changes of the selected magnetic materials are shown over a large temperature range at several frequencies. The powder core, HighFlux 160, is very stable. High and low permeability nanocrystalline cores, Nanophy and Kµ, change in the same proportion, close to 50% at high temperature, but respectively decrease and increase with the temperature increase. HighFlux and Kµ demonstrate also good frequency properties. Impedance measurements (not presented in this paper) show that the Kµ material has a better quality factor between 1MHz and 10MHz.

Figure 6: Relative variation of permeabilities from impedance measurements under small excitation. Reference is set at 10kHz-25°C

Variation of the saturation flux density could be approximated analitycally by the equation 5. Tamb is the ambient temperature and Js0K the saturation flux density at 0°K. Figure 7 shows analytical and experimental curves for the Nanophy. The behavior of low

Figure 7: Analytical and experimental saturation flux density of Nanophy 30000 as a function of Tamb in (°K), Js0K =1.275T,

Magnetic losses are characterized in Figure 8 for the Nanophy 30000. No strong temperature dependence is highlighted for frequencies over 10kHz.

Figure 8: Mass losses of Nanophy 30000 as a function of flux density at several temperatures and frequencies under sinusoidal excitation

The Table 3 presents the capacitors selected. Because of complex manufacturing process of multilayer ceramic capacitors, the selection is done at the component level. Important parameters to be estimated or measured are: insulation resistance, capacitance and losses. Capacitance of NP0 capacitors are very stable in frequency, from -55°C to 230°C and

Manuf. AVX AMC

Nom. Volt. 1kV 500V

Capacitance (1V/1kHz) 10µF 50nF

Dielectric Class X7R NP0

Use DC-link Common Mode

Table 3: Selected Capacitors

under high DC bias voltage. X7R Dielectric capacitors have a strong ferroelectric behavior which is illustrated in Figure 9 and 10. Stressed by high DC bias electric field, the capacitance is significantly reduced. At 200°C, the capacitance is decreased by 50% at 500Vdc. The transition to a paraelectric phase is measured at around 125°C as expected in BaTiO3 based compound. In Figure 10, the dissipation factor represents losses in the capacitors. The ferroelectric to paraelectric transition leads to a decrease of dielectric losses at high temperature. Insulation resistance, measured in Figure 11, is largely affected by temperature increase. Measured X7R’s value at 200°C is just above 1MΩ. The NP0 capacitor drops with only 3 decades to 100MΩ.

Figure 9: Capacitance measured by impedance analyzer as a function of DC bias voltage at several temperatures

Figure 10: Capacitance and dissipation factor measured by impedance analyzer as a function of temperature at 10kHz25°C

Figure 11: Insulation resistance measured by Source Meter Unit (SMU) as a function of temperature at nominal voltage

Design Considerations Nanocrystalline magnetic materials show significant variations but for CM choke, none of other materials, like ferrites or amorphous materials, have better performances for losses and saturation flux density. Moreover they achieve higher permeabilities. Loss stability from ambient to 230°C ensures a robust thermal management. At high frequency, over 100kHz, the natural decrease of permeability due to skin effect and eddy currents reduces impact of the temperature (Fig. 6). Consequently, especially for the CM, the highest noise level, temperature variations on filtering will be contained. For lower permeabilities, in DM, powder cores, like HighFlux or KoolMµ, are challenged by low permeability nanocrystalline materials which offer lower losses and comparable flux density. However, because of the permeability increase at 200°C, it could result an overdesign at 25°C to prevent any saturation, or a lower volume optimization at 200°C than powder cores. Both, HighFlux and Kµ are tested in Section 3. Coupled inductor, as well for DM as CM, presents for filtering lower volume if a single layer winding is used to reduce parasitic capacitances and also a good symetry to limit unbalanced impedance which can cause mode transfert. Generally, for any magnetic materials, the decrease of flux density at high temperature leads to a lower energy density and consequently a larger core. For capacitors, the best energy density is brought by X7R dielectrics at 200°C. The ferroelectric properties are strongly non linear and it causes an over design at 25°C to achieve the targetted values at 200°C. Insulation resistance is the most critical parameter for the large value X7R capacitors. The DC leakage current caused by the resistance drop becomes important at 200°C and power dissipation, with dynamic losses, could reach several W /cm3 . Most of

Figure 12: Tested coatings for nanocrystalline ribbons, from left to right, Silicone DC567, Silicone Nusil, Polyimide tape, and in second line, Duralco 215

Figure 13: Picture of the high temperature EMI filter prototype

the natural thermal cooling is performed by conduction through the leads. A special layout or heat sink can help to prevent a thermal runaway. Fortunatly, because of the ferroelectric to paraelectric transition, dynamic losses decrease with temperature. Packaging Aspects To work at 200°C, the entire assembly must tolerate ambient temperature and self heating of components. So the packaging should be able to work continuously at 250°C. Polyimide printed board and polyimide insulated copper wires are selected. For nanocrystalline ribbons, no high temperature coating is available. A mechanical protection is required because the material is very brittle. Several coatings are investigated in Figure 12. None of them provide good results. Silicones are too soft to bring enough mechanical protection and the Duralco, a ceramic based encapsulant, is quickly eroded. Polyimide tape is used for characterizations only. PEEK encapsulant is also being investigated. Powder cores are composed by a polymer which is able to work up to 200°C, but heavily deteriorated with a long-term exposure [10]. Prototype The built prototype is shown in Figure 13. According to section 2.4, the damping circuit, composed by Ccm2 and Rd is not considered for high temperature prototype because only the 150kHz to 30MHz frequency range is focused.

3 3.1

Experimental Results

Figure 14: Conducted Emissions with EMI filter, HighFlux and Ku as DM choke core materials, from Inverter to LISN, measured by high bandwidth current probe on power DC line at 400Vdc-4A

Measurements

The test bench is composed by DO-160 LISNs, a 3phases-1200V-40A IGBT power module (MicroSemi APTGL40X120T3G), a double stator PMSM and a resistive load. Operating point is controled by "d-q" current loop implemented in a Dspace Autobox with Matlab.

The filter is conditioned at high temperature ambient into a temperature controled enclosure. Thermal insulating layers of glass wool are added to improve the temperature homogeneity. This setup brings no significant modification to noise paths and impedances due to excellent ground homogeneity be-

tween the chassis of the conditioner and the standard copper ground plane. The results on measured conducted emissions by EMC probe (RFI Solar electronics 6741) on current line are presented in Figure 14 with the two selected DM choke core materials.

Acknowledgments

3.2

References

Analysis

At normal load (400Vdc-4A) and at 200°C, the filter works without any excessive self heating or insulation defect. The presented EMI spectrums demonstrate the good filtering performance until 2MHz for both DM choke materials at room temperature and 200°C. Over 2MHz, attenuation decreases due to the permeability drop of CM and DM choke core but the EMI are still at high level. Over 10MHz, the original noise is lower but attenuation becomes very low because of the parasitic inductance of the CM capacitors and stray capacitances of chokes. Spikes between 10MHz and 20MHz are generated by radiated emissions from the test equipment, their amplitude is higher in the second measurement. It should not be measured in a perfect setup. Impact of the temperature is moderate, nearly 6dBuA before 1MHz. The decrease of the CM choke permeability in temperature is the main explanation. The DM capacitor, as shown in Figure 9, has only 10% of capacitance drop at 400Vdc between 40°C and 200°C. The CM capacitors and the DM choke with an HighFlux core are very stables. The Kµ material, according to the Figure 6, provide a compensation from its permeability variation, so temperature effect on spectrum is more reduced on the second measurement. For the higher frequencies, temperature effect is negligible, and Kµ material, as noticed in section 2.5, show a little better attenuation between 3MHz and 5MHz.

Conclusion EMI issues are discussed for an inverter topology. Then the design procedure leading to the required values regarding the standards, the generated noise and the filter structure is detailed. With numerous characterizations of selected materials and components, a first high temperature EMI filter is built and tested in real operating conditions at 200°C. This first results demonstrate the feasibility of a high temperature EMI filter with industrially available materials and components. Moreover, due to a careful design, the effect of the high temperature (200°C) on EMI spectrum is moderate: an increase of 6dBµA at low frequency is measured.

The authors would like to thank the "Direction Générale de l’Armement" (DGA) for their financial support of this work, as well as the Hispano-Suiza company, a part of Safran group.

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