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2008 Motor, Drive & Automation 2008 Motor, Drive & Automation Systems ConferenceSystems Conference

“A High Temperature, High Ripple Current DC Bus Film Capacitor in an Annular

Form Factor”

Presented by:Terry Hosking, V.P. Engineering

SBE Inc.Power Ring Division

February 14, 2008 • Atlanta, Georgia

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Areas of DiscussionAreas of Discussion

• Review 2007 presentation of thermal issues, simulation & testing results

• Dielectric choices, why Polypropylene?

• Recent R&D focus on Polypropylene film

• Findings, summary and direction of future work

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2007 Motor & Drives Review2007 Motor & Drives Review

• In 2007 we described thermal considerations for using the Power Ring in high ripple current inverter applications:

• Advantages of a thin ring form factor; low losses, low T rise

• Optimize interface between capacitor and bus structure

• SBE’s segmented end spray approach to mitigating thermal mismatch of metal end spray and plastic film

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Power Ring’s Power Ring’s Segmented End SpraySegmented End Spray

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2007 Motor & Drives Review2007 Motor & Drives ReviewFindings, SummaryFindings, Summary

• Confirmed FLUX 2D simulation with test results:• T rise and thermal profiles for various film

metallization resistances and cooling conditions

• 1000 µF, 600 VDC unit built and tested

• 100 KHz/240 Amp RMS test indicated only a +2°C temperature rise, surface to hot spot

• Test data confirmed modeling accuracy

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2007 Test Unit2007 Test Unit

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So What is Driving the So What is Driving the Choice of Dielectric Film?Choice of Dielectric Film?

• DoE’s FreedomCAR program outlines the demand for low cost, high temperature capacitor technology

• Upper temperature limit goal of +150°C

• Rationale for +150°C rating was to use +105°C engine coolant for inverter thermal management, and allow for a +45°C capacitor T rise

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Why Polypropylene?Why Polypropylene?

• High temperature films are available, but all are EXPENSIVE

• PPS, 10x the cost of Polypropylene, self-healing issue

• Polyimide, varied electrical performance over frequency and low dielectric strength

• Teflon, difficult to metallize, has low dielectric strength and is difficult to wind in large capacitor sections

• In all cases there is a size penalty to pay when compared to Polypropylene

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Why Polypropylene?Why Polypropylene?

• Polypropylene presents best overall value• Plentiful, Generally Inexpensive

• Long proven history of reliability

• Excellent electrical characteristics

• The combination of these attributes, and the minimal temperature rise of the ring form factor, has led SBE to further research the limits of Polypropylene

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PolypropylenePolypropylene

• Polypropylene resin is composed of 2 parts:

• Atactic material which is non-crystalline (amorphous)

• Isotactic material which is crystalline

• Highly isotactic material has a higher melting point

• Highly isotactic polypropylene is referred to as High Crystallinity Polypropylene (HCPP)

• All capacitor grade polypropylene film, including HCPP, is produced by stretching in two directions, Biaxially Oriented Polypropylene (BOPP)

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BOPP vs. HCPPBOPP vs. HCPP

• HCPP is alleged to have improved performance at high temperature relative to BOPP

•Does it?

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Polypropylene Polypropylene CharacteristicsCharacteristics

• Test data presented here on both BOPP and HCPP for:

•Dielectric strength vs. temperature

•Relative leakage current vs. temperature

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Dielectric StressDielectric Stress

• Test data for dielectric strength vs. temperature

•Test data results not as expected!

•Data taken twice with same results

•3.8 micron film thickness

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600

700

800

900

1000

1100

85 105 110 115 125Temperature (deg C)

DC

Vol

tage

B

reak

dow

n

Dielectric StressDielectric Stress

BOPP

HCPP

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LeakageLeakage

• It was expected that HCPP would have lower leakage

• Essentially no difference, although BOPP was slightly lower

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Relative LeakageRelative Leakage

10

100

1000

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

Temperature (deg C)

Rel

ativ

e Le

akag

e

HCPP

BOPP

1

10

100

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Capacitor ESR at High Capacitor ESR at High TemperaturesTemperatures

• Capacitor Temperature Coefficient of Resistance (TCR) vs. Temperature

• TCR is a characteristic of the metallization on the film

• Relates to ESR increases at higher temperature

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Capacitor Metallization Capacitor Metallization TCRTCR

• Metallization is Aluminum• TCR of bulk aluminum is ~3400 ppm/°C

• Current flow in a capacitor is across the film (Transverse)

Current Flow

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Capacitor Metallization Capacitor Metallization TCRTCR

• Transverse TCR test data gathered • Capacitor-like device constructed to measure TCR using

DC source

• Expected TCR was ~3400 ppm/°C (one would expect ESR to behave the same)

• SURPRISE, SURPRISE, SURPRISE!!

• Test data indicates TCR ~80-90 ppm/°C• Consistent from run to run

• ESR will rise less than expected at high temperature

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TCRTCR

30

40

50

60

70

80

90

100

110

120

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95100 105 110 115 120 125 130 135

Temperature (deg C)

TCR

(ppm

/deg

C)

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Implications of Test Implications of Test DataData

• Advancements in quality of base polypropylene resin has closed the gap between BOPP and HCPP as shown by data

• The form factor of the wound capacitor section has surfaced as a critical parameter in the design regardless of choice of Polypropylene grade!

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Capacitor Form FactorsCapacitor Form Factors

Toilet PaperPaperTowel

Power Ring

Capacitor end spray surfaceCapacitor end spray surface

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Capacitor End Spray Capacitor End Spray Surface AreaSurface Area

1

1.5

2

2.5

3

3.5

Rel

ativ

e su

rface

are

a

0.1 1 10 100 Diameter/axial length

Surface Area vs Diameter/Axial LengthConstant capacitance & film thickness

PaperTowel

ToiletPaper

Power Ring

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ESRESR

0.01

0.1

1

10

Rel

ativ

e E

SR

0.1 1 10 100 Diameter/axial length

ESR vs Diameter/Axial LengthConstant capacitance & film thicknes

ToiletPaper

PaperTowel

Power Ring

25

0

0.2

0.4

0.6

0.8

1

Rel

ativ

e di

stan

ce h

ot s

pot t

o su

rface

0.1 1 10 100 Diameter/axial length

Thermal path vs Diameter/Axial LengthConstant capacitance & film thickness

Thermal Path LengthThermal Path Length

PaperTowel

ToiletPaper

Power Ring

Shortest path to OD Shortest path to end spray

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End Spray Surface AreaEnd Spray Surface Area

• The Thermal Advantage

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The Thermal AdvantageThe Thermal Advantage

• Temperature rise of conventional array greater than 5 times that of the Power Ring for a given ripple current

• Power Ring’s large, flat surface area allows for greater connection area and short thermal path to heat sink, chill plate, etc.• Thus further reducing temperature rise

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Comparative Temperature Comparative Temperature Rise EnvelopesRise Envelopes

Temperature Rise (°C) vs. Ripple Current (Amps)

Data assumes both sides of the capacitor held at the same temperature

Approximation envelope for conventional array

Approximation envelope for Power Ring

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Findings, ConclusionsFindings, Conclusions

• No significant performance observed between BOPP and HCPP. DAMN!

• Polypropylene film capacitors will not be rated to operate at +150°C, HOWEVER, the ring form factor improves performance as temperatures elevate

• Functional upper temperature limit not easily defined, BUT….Power Ring’s low T rise allows higher system temperature for a given capacitor hot spot temperature

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Future WorkFuture Work

• Continued research of High Crystalline Polypropylene mechanical and thermal behavior

• Interconnect and Packaging options

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Thank You!Thank You!

Thank you!Please visit usin Booth #303