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Advanced Soft Switching for High Temperature Inverters

Plenary Presentation at

The 5th IEEE Vehicle Power and Propulsion Conference (VPPC'09)

Jih-Sheng (Jason) Lai, Professor

Virginia Polytechnic Institute and State University

Future Energy Electronics Center

106 Plantation Road

Blacksburg, VA 24061-0356

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Outline

1. Background – DOE FreedomCAR Program

2. System Level Design Tradeoff

3. HEV Thermal Management System Design Considerations

4. Power Semiconductor Device Characteristics under Different Temperature Conditions

5. Efficiency and Loss Evaluation for Temperature Prediction

6. Summary

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Background – DOE FreedomCAR Program Inverter Target

Goal: Electric Propulsion System with a 15-year life capable of delivering at least 55 kW for 18 seconds and 30 kW continuous at a system cost of $12/kW peak with a 105°C inlet coolant temperature by 2015*.

55-kW System at 105 °C

2015 Targets

Camry R&D Pathways

Cost ($) 660 3740 858

Weight (kg) 45.8 91.7 45.8

Volume (l) 157 33.3 12.5

Efficiency (%) 93 88 91

Source: DOE Vehicle Technology Program – APEEM 2009 Kickoff Meeting

APEEM R&D Activities:

• Electric Traction Drive• Electric Machines - Motors and

Generators• Power Electronics - Inverter and

Boost Converter (if needed)• Thermal Control – Key Enabling

Technology• Vehicle Power Management

Bi-Directional Multi-Voltage DC-DC Converter

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System and Component Level Cost Trade-off with Consideration of Thermal Management System

Dual cooling loops (70°C plus 105°C) – Need dual thermal management systems, penalty on

system level cost, size and weight

Single cooling loop with 105°C coolant – Need to beef up silicon or use wide bandgap devices,

penalty on component level cost

– Need to use high temperature bulk capacitors, penalty on component level cost, size and weight

– Need to design circuit with high temperature rating, penalty on component level cost

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Possible Solutions with Single 105°C Coolant Loop

Loss reduction by reducing switching frequency will result in high motor current ripple and associated loss thus reducing the entire drive efficiency.

Loss reduction by increasing device switching speedwill result in high dv/dt, di/dt and associated EMI and common mode current issues.

Emerging SiC and wide band-gap devices for high temperature operation is not cost effective today.

Junction temperature reduction by reducing thermal impedance – it helps but not enough.

Advanced soft switching with silicon devices to achieve significant loss reduction – a cost-effective way.

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Proposed Approach

Develop a variable timing controlled soft-switchinginverter for loss reduction.

A hybrid soft switch module for conduction loss reduction.

Develop low thermal impedance module withintegrated heat sink for high temperature operation.

Develop a highly integrated soft-switch module for low cost inverter packaging.

Modeling and simulation for design optimization.

Test the soft-switching inverter with existing EV platform and dynamometer for EMI and efficiency performanceverification.

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Timing Diagram with a Variable tdon Defined as Variable Timing Control

S2

Sx2

S1

Sx1

tdon

tdt

tdoff

t0 t1t2t3 t4

iC2

iC1

ILr

VCE2

ID2

IS2

ILoad

VCE1

t5 t6 t7t8

S2

Sx2

S1

Sx1

tdon

tdt

t0t1 t2 t3t4

iC2

iC1

ILr

VCE2

ID2

IS2

ILoad

VCE1

t5

tdoff

t6 t7t8

(a) light-load (b) heavy-load

Dx6

Dx2

Dx1Sx1

Sx2

Dx5

Dx3C1

S1

S2 C2

Load

Dx4

n

1

n

1

Lr2

Lr1

Vdc Cdc

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Variable Timing Controlled Soft-Switching Verified with Simulation and Experiment

1 V ( X 2 : g a t e ) 2 V ( X 2 : d r a i n )

1 V ( X 2 : g a t e ) 2 V ( X 2 : d r a i n )

1 V ( X 2 : g a t e ) 2 V ( X 2 : d r a i n )

iLr (41Apk)ILoad (10.8A)

VGE2

Time (µs)

ILoad (22.0A)

ILoad (35A)

VGE2VCE2 (325V)

VCE2 (325V)

iLr (61Apk)

VGE2

tztdon

tztdon

tz iLr (90Apk)tdon

VCE2 (325V)

0 321 540.5 4.53.51.5 2.50

0

0

0

0

0

iLr (42Apk)

VCE2 (325V)

ILoad (10.8A)

VGE2

Time (0.5 µs/div)

ILoad (22.0A)

ILoad (34.2A)

VGE2

VGE2

VCE2 (325V)

VCE2 (325V)

iLr (62Apk)

tz

tz

tz

iLr (90Apk)

tdon

tdon

tdon

Turn-on delay tdon controlled by zero-voltage crossing detection. Larger current, longer delay.

Gating time after reaching zero voltage tz is fixed

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Variable Timing Soft Switching over a Wide Load Current Range

• During turn-off, VCE slowly rises after IC drops to zero• Variable timing achieves soft-switching at all current conditions• Bonus – slow dv/dt that will result in low EMI emission

• During turn-on, current IC rises after voltage VCE drops to zero

-1000

100200300400500

1 2 3 4 5 6

-1000

100200300400500

2 3 4 5 6 7

-1000

100200300400500

2 3 4 5 6 7

-1000

100200300400500

1 2 3 4 5 6

IC

VCE

IC

VCE

IC

VCE

at 200A

ICVCEat 300A

0 1 2 3 4 5 0 1 2 3 4 5

Time (µs) Time (µs)

(a) Turn-on (b) Turn-off

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Conduction Loss Reduction with a New Hybrid Soft Switch Module

• Hybrid switch reduces conduction loss reduction over a wide range of current and temperature condition

• Integrated module with direct cooling to reduce thermal resistance Higher temperature, lower voltage drop ideal for high temperature operation Compared with commercial modules: 1.46V versus 1.67V drop @400A (13%

reduction)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0 50 100 150 200 250 300 350 400

Device Current (A)

Dev

ice

Vol

tage

Dro

p (V

) 125°C commercial module25°C

100°C

125°C

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Switching Loss Reduction Using LPT IGBT

For hard switching, as compared to 25°C operating condition,– Device switching loss is increased by 40% at 100°C – Device switching loss is reduced by 80% under soft switching

Losses in soft switching are due to layout parasitics with discrete components – necessary to integrate the soft switch module

0

5

10

15

20

25

0 50 100 150 200 250 300 350 400

Eoff at 100°C

Eon at 25°C

Eon at 100°C

Eoff at 25°C

Eon

Current (A)

Sw

itchi

ng e

nerg

y (m

J) IGBT rating = 600 V, 400 A Vdc = 300 V

Soft switching

Eoff (100°C)Eoff (25°C)

Hard switching

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Direct Cooled Module Based Soft-Switching Inverter Assembly

• Direct cooled module – no heat sink is required, but a custom-made water manifold is needed

• DC power bus bar and capacitors are placed on top of modules to reduce parasitic inductance

• Gate drive board snapped on top of the modules to avoid parasitic ringing

Water manifold

Water inlet

Coupled magnetics

AC bus Gate drive

Interface board

Soft switch module

DC bus bar

DC capacitor

Current sensor

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Power Meter Measured Peak Efficiency Exceeds 99% at Low Relatively Low Power Condition

• Using well calibrated power meter, the measured peak efficiency of the inverter exceeded 99% at room temperature condition.

Test condition: Vdc = 325 V, fsw = 10 kHz (PWM), f1 = 83.3 Hz, Ta = 25°CAccuracy of Instrumentation: ±0.2%

95.0%95.5%96.0%96.5%97.0%97.5%98.0%98.5%99.0%99.5%

1 2 3 4 5 6 7 8 9 10Output power (kW)

Fixed PF=0.83

Fixed RL 9+4.5mH

Fixed RL 4.5+4.5mH

Fixed RL 18+4.5mH

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Projected Efficiency Using Loss Measurement with Inductor Load Test

Test condition: DC bus voltage: 325 V dcSwitching frequency: 10 kHzLoad inductance: 2.4 mH per phaseModulation index: 0.2 to 1.15

96.5%

97.0%

97.5%

98.0%

98.5%

99.0%

0 5 10 15 20 25 30 35 40 45

Power Output (kW)

Effi

cien

cy

83.3Hz, 25°C83.3Hz, 90°C60Hz, 25°C60Hz, 90°C

Efficiency Measurement using Ratiometric Calorimeter

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Calorimeter Tested Efficiency Plots over a Long Period of Time

• Using integrated module with light-weight water manifold for the full-version soft-switching inverter.

• Calorimeter chamber inlet and outlet temperatures stabilized after 6-hour testing. Chamber temperature differential was 1.6 C under 0.3 GPM flow rate.

• Efficiency exceeded 99% at full speed, 33% load torque condition.

96.0%96.5%97.0%97.5%98.0%98.5%99.0%99.5%

100.0%

0 60 120 180 240 300 360Time (minutes)

Effi

cie

ncy

96.0%96.5%97.0%97.5%98.0%98.5%99.0%99.5%

100.0%

0 60 120 180 240 300Time (minutes)

Effi

cie

ncy

Motor speed: 2750 rpmMotor torque: 50 Nm

(a) Test results at 12.5 kW

Motor speed: 2650 rpmMotor torque: 33 Nm

(b) Test results at 18 kW

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Soft-Switching Inverter Testing with Motor Dynamometer

• The soft-switching inverter has been tested with the 55-kW motor dynamometer set

• Rigorous test with different torque commands and instant speed reversal

200ms/div

speed reversal with negative torque commanded

Using FEA to Predict Temperature for Soft-Switching Inverter

1

119.7112.1104.596.9789.4081.8374.2666.6959.1251.5543.98

• Simulated hot spot junction temperature consistent with theoretical calculation: 120C or 15C temperature rise

• Circuit components inside the chassis see temperature between 65C and 85C

Boundary conditions: Ambient temperature: 45CHeat sink temperature: 105C

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Summary

The advanced soft-switch module demonstrates – 13% conduction loss reduction– 80% switching loss reduction – 60% thermal impedance reduction

The advanced soft-switching inverter with variable timing control demonstrates high efficiency over a wide speed and torque range – Soft switched inverter power losses are roughly a factor of 3

less than that of the hard switched inverter – Calorimeter test verifies that peak efficiency exceeds 99%

FEM analysis and projection indicate – Less than 15°C junction temperature rise– 105°C coolant operating at full load is possible

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