“A High Temperature, High Ripple Current DC Bus Film ... · Capacitor Metallization TCR...
Transcript of “A High Temperature, High Ripple Current DC Bus Film ... · Capacitor Metallization TCR...
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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
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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
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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
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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
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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