VPPC 2009Comparative Evaluation and Analysis of the 2008 Toyota Lexus, Camry and

2004 Prius DC Link Capacitor Assembly vs. the SBE Power Ring DC Link

Capacitor---

Terry Hosking, V.P. of EngineeringSBE Inc.

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IntroductionPurpose of evaluation:

1.Determine Trise of DC link capacitor designs used in Toyota hybrid vehicles

2.Compare Toyota capacitor Trise with available SBE and Kemet DC linkcapacitor Trise

3.Consider impact to new PEM designs

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Test Logistics

1. Toyota and Kemet DC Link capacitors were supplied by Oak Ridge National Labs (ORNL), National Transportation Research Center (NTRC) facility in Knoxville, TN

2. Thermocouple placement done at SBE

3. Tests conducted by NTRC staff in Knoxville

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DC Link Capacitors Tested

Toyota Camry Toyota LexusToyota Prius [1 new & 1 used for 160Kmiles]KEMET 2600uF [no part number, appeared to

be designed for Lexus]SBE 500uF 700D349SBE 1000uF 700D348

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Toyota Camry

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Toyota Lexus

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Toyota Prius

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KEMET 2600uF 750V

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SBE 500uF 600V 700D349

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SBE 1000uF 600V 700D348

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Capacitor Description Summary

Source Voltage Rating

DC Link Capacitance

Approximate Dimensions

Volume (cm3) Surface Area (cm2)

1 Camry 750 2629245mm L178mm W73mm H

3300 1535

2 Lexus 750 2098254mm L171mm W51mm H

2215 1302

3 Prius 600 1130222mm L184mm W41mm H

1675 1150

4 Kemet 750 2600254mm L171mm W51mm H

2215 1302

5 SBE 600 500 159mm D48mm H 953 637

6 SBE 600 1000 215mm D48mm H 1743 1050

Thermocouple Placement

1. Challenge is to determine meaningful locations for thermocouples

2. Goals…a) anticipate a hot-spot locationb) investigate terminal heating effectsc) determine case temperatured) look at internal T at other locations

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Thermocouple Placement

1. TC placement via holes drilled into capacitor windings in addition to case surface locations

2. For AC Trise measurements, this is OK for metallized capacitors

3. Capacitors may no longer support much DC, so the test sacrifices capacitor functionality

4. Since the internal construction was unknown, where to drill holes???

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Thermocouple Placement

1. X-Ray examination needed• Local medical X-Rays obtained• For capacitors with single windings some detail observed• For complex assemblies they were of no help

2. CT scans obtained from North Star Imaging• These were supplied with software for 3D imaging and

allowed 2D sections to be obtained and saved as .jpg files• Provided ALMOST enough detail….

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Typical CT Scan Top View

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Typical CT Scan Side View

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Lexus Capacitor Thermocouple Placement

CT scans did not show fine interconnect detail, especially thin copper sheet.

a. This created problems, as after drilling the Lexus capacitor it was shorted!

b. Parallel plate interconnect [to reduce ESL] was not shown on the CT image;

c. Capacitor “repaired” by careful use of larger sharp carbide drill to expose and remove layer to layer short circuits.

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Lexus Capacitor Thermocouple Placement

x8x9

x9 x10 x11

x10

x11

x12

x12

x13

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Lexus Capacitor Thermocouple Placement

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Thermocouple placement [Camry]

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Thermocouple Placement [Prius]

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Simulated Temperature Profile (SBE)

5000 second simulation temperature profile indicating predicted hot spot

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SBE 1000uF 600VThermocouple Placement

SBE 700D348 Prior to Encapsulation

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SBE 500uF 600VWith Thermocouples

SBE 700D349 as Tested on a Bus Assembly

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Test Description

All capacitors tested at 200ARMS, 5KHz

Test frequency [5KHz] defined by capability of NTRC test stand to source 200ARMS into all capacitors

Chamber temperature set to 25C

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Test Description

Example of a capacitor under test inside thermal chamber

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Test Description

NTRC capacitor current source and data acquisition schematic

Sine wave

generatorsignal PA

PA

PA

ESPEC Temperature Chamber

5 total

1Capacitor

undertest

Yokagawa

Power SystemAnalyzer

CT

I V

2

Powertron1000A

LEM LC300S

ITECO

PZ4000

Keithley2701

ThermocoupleData

Acquisition

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Test Description

Amplifier stack at ORNL-NTRC

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Test ObservationsHigher test frequency would have been desirable to represent typical switching frequenciesThe low test frequency masks the following effects:• current hogging [not all windings in cap assy. see the same current]• skin effect conduction losses• eddy current losses

Substantial capacitor heating from connection and terminal conduction losses was seen in Toyota and Kemet Capacitors

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Lexus Data, Thermocouples #8 - #13

-2-1012345678910111213

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25

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31

33

35

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41

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0 500 1000 1500 2000 2500 3000 3500 4000

Del

ta T

( 9

-13

)

Tem

pera

ture

(°C

)

Time (sec.)

Lexus Cap. (5kHz, 200A, 25°C ambient)

8 9 10 11 12 13 Delta T

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Lexus Thermocouple Placement

x8x9

x9 x10 x11

x10

x11

x12

x12

x13

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Highest Temperature Measurements

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Measured internal Trise of tested capacitors

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SBE Capacitor Test Result Comments

1. The highest Trise location was not where simulation predicted!

a) back to the drawing board… needed are:• Better simulation boundary conditions • Better values for materials parameters

b) SBIR funded research has provided those for ongoing and future simulations

2. Distributed connection significantly lowers Trise!

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Simulated Temperature Profile (SBE)

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Summary of Test Results

Trise of the SBE capacitors was substantially lower than any of the Toyota or Kemet designs

Prius capacitor had the highest Trise

Capacitor from the 160,000 mile Prius exhibited no performance degradation

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Summary of Test Results

Lexus capacitor substantially smaller size, yet only a small Trise increase as compared to the capacitor used in the Camry

Kemet capacitor performance was similar to that used in the Lexus, but with higher capacitance and ~2.5°C lower Trise

Capacitor AnalysisDesign: Film: Dielectric stress: (at rated V)

Prius 3.8u 150V/u Camry 3.5u 214V/u Lexus 3u 250V/u Kemet 3u 250V/u SBE 3.8u 150V/u

Capacitor Performance Drivers

Assuming constant capacitance:

+Reliability? Reduce T, V/u [size increase]

Higher T? Reduce V/u [size increase]

Higher I? Change winding shapefrom “paper towel roll”toward “hockey puck”

Capacitor Cost Drivers

Capacitance value! [determines film usage]

Film cost vs thickness, especially below 3.8u [where cost increases faster than the capacitance

gained!]

Capacitor assembly complexity

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ImplicationsThe SBE capacitor design could allow power conversion systems with lower cost, volume, and weight without sacrificing current carrying capability.

The lower Trise could result in:• Less capacitance required, and/or• Higher system reliability, and/or • Higher system operating temperature, and/or• Easier thermal management.

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Future WorkCharacterization of Power Ring Trise at higher current and frequency [SBE Test hardware construction and verification is complete; over 500ARMS continuous @ ~19KHz]Continue with tests comparing other capacitor vendors including AVX, CDE, EPCOS, ECI, and others as we become aware of them.Use of bus structure as a capacitor cooling medium in addition to its reduction of connection losses.High stress Trise data to be gathered as illustrated in following graph….

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Future Work

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VPPC 2009

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