SemiSouth Laboratories, Inc.jeff.casady@semisouth.com

www.semisouth.com201 Research Blvd.Starkville, MS 39759

662-324-7607

Power Electronicsfor High Tech Inverters

Michael S. Mazzola, Ph.D., P.E.

SemiSouth Laboratories, Inc.and

Center for Advanced Vehicular Systems,Mississippi State University

Outline

• Introduction– Components in inverters– Power component R&D

• Silicon Carbide Power Electronics– System Benefits– Capabilities– Trends

Introduction

Components in Inverters

3535

3030

2525

2020

1515

1010

55

00

Cap

acito

rs

Bus

Wor

k

PWBs

Mag

netic

s

Ther

mal

Mgm

t.

Cur

rent

Sen

sors

Con

tact

ors

ALL INVERTERS, CONVERTERS AND MOTOR CONTROLLERSCONSIST OF . . . . . . . . . .

Solid

Sta

te S

witc

hes For 10 – 250kW

Systems

% WEIGHT % VOLUMECourtesy of Dr. James Scofield, Propulsion Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base

270 VDC

To Motor

Flight Packaged Motor Drive

Typical Inverter Topology

• Six-pulse topology. • Switches are typically IGBT’s or MOSFET’s• Each switch has an integral anti-parallel rectifier.

• Six-pulse topology. • Switches are typically IGBT’s or MOSFET’s• Each switch has an integral anti-parallel rectifier.

Courtesy of Dr. James Scofield, Propulsion Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base

Applications Driver – Power Density

• Much of the cumulative improvement in power density is still in the future (Moore’s law for power?).

• Civilian transportation and communication applications will drive much of the technology development (affordable).

• The ultimate is goal is monolithically manufactured power supplies.

Power Density – How to increase it?

• Increase the conversion frequency.– Size and weight of passive components ∝ 1/f.– Typically, 75% of size and weight of power converters is in the

passive components.

• Increase the voltage.– Insulation is always lighter than copper.– Skin depth further leverages voltage at higher frequency.– Mobile platforms have common bus voltages:

• 270 V → 600 V device family (JSF, HE-HMMWV, AUV’s, etc.)• ±300 V → 1200 V device family (insulation the same, ½ the Cu)

• Improve the packaging (get the heat out).

Example: Transformer Size & Weight

• Two transformers rated for 10 kVA.

• Increasing the frequency from 400 Hz to 100 kHzdecreases the transformer weight from 150 lbs. to 5 lbs.

• Two transformers rated for 10 kVA.

• Increasing the frequency from 400 Hz to 100 kHzdecreases the transformer weight from 150 lbs. to 5 lbs.

twice the powerRated for

2Xthe same

Why is an anti-parallel diode needed?

• Bidirectional power flow for dc-ac inverters.– Supplying reactive power.– Conducting harmonic currents.

• Energy recovery in motor drives.– Recover energy stored in parasitic inductances.– PWM in Brushless DC (BLDC) motor drives.

• Conducting resonant currents in soft-switched power supplies and drives.– To continue increasing conversion frequency, minimizing

inductance between switch and anti-parallel diode is essential.– Lowest possible inductance is achieved monolithically.

2

1

Anti-parallel current paths: Two possibilitiesHard-Switched BLDC

Motor DriveHard-Switched BLDC

Motor Drive

+ 300 V

- 300 V

Motor Winding

Motor Current

time

Ave.

Half Bridge

• Switch & Diode work on opposite legs.• Switch & Diode work on opposite legs.

• Switch & Diode work on the same leg.• Switch & Diode work on the same leg.

Inductor Current

time

+ 300 V

- 300 V

Half Bridge

Soft-Switched Resonant Power Supply

Soft-Switched Resonant Power Supply

L r Cr

ZCS

DC-AC Inverter for On-Vehicle Applications

H Bridge

a b c

Vr

+

Ir

Cr

Lr

28 V+

Rev. 2 PLR Converter

nA B C

H Bridge

a b c

Vr

+

Ir

Cr

Lr

28 V+

Rev. 2 PLR Converter

nA B C

Digital Controller Select:

• 50 Hz• 60 Hz• 400 Hz

Gate Drivers

+/– PLRImpulse

Bridge “a” high/lowBridge “b” high/lowBridge “c” high/low

M1 M2 M3 M4 S1 S2 S3 S4 S5 S6• 28-VDC to 208-VAC, 10-kW, 3-φ.•Voltage boost: PLR Converter.• Link: high-frequency ac (no link rectifier).• Inverter: six pulse, bidirectional switches.• Preprogrammed digital control.• Limiting Component: Si MOSFET’s.

• 28-VDC to 208-VAC, 10-kW, 3-φ.•Voltage boost: PLR Converter.• Link: high-frequency ac (no link rectifier).• Inverter: six pulse, bidirectional switches.• Preprogrammed digital control.• Limiting Component: Si MOSFET’s.

Output:• 208 VAC (rms)• 3 phase (line-line & line-neutral)• Frequency Adjustable

The PLR Voltage Pulse in an AC-Link Inverter

time+δ(t)

–δ(t)

r

pVdtt

ωδ

4)(Area == ∫

Vr(t) = Vp sin[(ωr /2)t ]

time

ωr

2πωs

2π

Vr(t) = Vp sin[(ωr /2)t ]

time

ωr

2πωr

2πωr

2πωs

2πωs

2πωs

2π

r

pVω

4Area =Vr(t)

PLR Converter Voltage Waveforms

Impulse Function Model of Resonant Capacitor Voltage

Sine Function Model of Resonant Capacitor Voltage

Components are stressed by resonant voltages and currents well in excess

of inverter terminal ratings.

Components are stressed by resonant voltages and currents well in excess

of inverter terminal ratings.

Peak Inverter MOSFET Stress > 800 V.MOSFET RMS Current = 28 A.

Peak Inverter MOSFET Stress > 800 V.MOSFET RMS Current = 28 A.

Introduction

Power Component Research & Development

MEA Gen II Engine StudySiC Benefits StudyFighter Based Laser StudyAirborne Electric LaserUCAV HPM StudyHEXSBR StudySpace Baseload StudySpace SBL Study (Schafer)Space SBL Study (SAIC)MEA Adv Motor Drive II StudyLRSA StudyDielectric StudyUCAV Requirements

TRANSFORMER/INDUCTOR

HIGH TEMP SiC

HIGH TEMP PASSIVES

CRYO POWER COND

PULSE POWER CAPS

HIGH VOLT/PWR SWITCH

HIGH VOLTAGE D

IODES

AFRL/PRPE TECHNOLOGIES

STUDY

LOW VOLTAGE HIG

H

ENERGY CAPS

HIGH VOLTAGE W

IRIN

G

Air Force Investment Strategy in HighTemperature Electronics Driven by Study Results

LIGHT C

ONTROLLED

DEVICES

X XX XX X

XX X X X X X X X

XXX

X X X X X XX XX X

X X XX

IVHM

X X

X XX

X

X

XX

X

X

What is Silicon Carbide?Silicon forms a stronger chemical bond to carbon than to silicon!

SiC is very hard (third hardest substance known)SiC handles very high temperatures (wafers are made at over 3600°F)SiC handles very high voltages (ten times that of silicon and gallium arsenide)

Single crystal SiC wafers for electronics use commercially available:1990 - 1 inch diameter2003 – 3 inch diameter2006(?) – 4 inch diameter

Early versions of SiC power devices perform better than the bestperforming silicon power devices.

SiC power products are commercially available and in commercial use.

Motivation to DevelopSiC Power Devices

Silicon vs. Silicon CarbidePower Semiconductors

PROPERTY SILICONSILICONCARBIDE

BANDGAP (eV@ 300K)

MAXIMUM OPERATINGTEMPERATURE (K)

BREAKDOWN VOLTAGE(106 V/cm)

THERMAL CONDUCTIVITY(W/cm - °C)

PROCESS MATURITY

INTRINSICALLY RAD HARD

1.1

425

0.3

1.5

HIGH

NO

2.9

>900

4

5

LOW

YES

SiC Power Electronics

System Benefits

Overview of BenefitsLower On-resistance increased system efficiency

Higher Frequency smaller power supplies

Higher Temperature relaxed cooling requirements

Payoff: Improved System Performance

(size, weight, cost, reliability, efficiency)

Current commercial applications

1. High-end server power supplies: improved efficiency means smaller supply with the same or less heat generation

Potential future SiC power switch applications

1. High-temperature motor drives & inverters.2. High-power-density resonant inverters with high voltage stress.

Obstacles to increasing Power Density• Simultaneous increases in voltage and frequency limited by

silicon – Efficiency cannot be compromised!– MOSFET silicon switches are fast but not voltage scalable.

– IGBT’s voltage scalable, but not as fast (∼ 20 kHz).

– Reverse recovery of anti-parallel diode another limitation.

• Parasitic inductances and capacitances require resonant circuits (“soft switching”) above 100 kHz and 1-10 kW.– Soft switching requires teaming fast switches with ultra fast

anti-parallel diodes.

• VJFET is the best available >1 MHz, 600-1800 V SiC switch.– “Normally on” and “Normally off” available

– Has the best possible anti-parallel diode: a SBD

Weight VolumeSavings (lbs) Redn.(in3)

Flight Control EHAs/EMAs 58.5 1,350Utility Control Actuators 15.4 313EPS Equipment 45.1 657Avionics 17.6 245ECS/Fuel/Misc Motors 10.9 252TMS Equipment 30+ 400+

Total 177.5+ 3,217+

MEA F/A-18MEA F/A-18

Vehicle-Level SiC Benefits

Minimum 177 lb Equipment Weight and 3,210 in3 Equipment Volume Savings.

SiC Power Electronics

Capabilities

Candidates for SiC Switches• SiC MOSFET

– Unipolar – Normally off– p-n body diode

• Always present

– Low channel mobility– Oxide reliability issues

• SiC JFET– Unipolar– Normally on

• Normally off and “quasi-on” options

– No body diode– Not as sensitive to

oxide interface and reliability issues

4H-SiC Vertical JFETBasic device cross-section

Finished JFET in a TO-257

SiO2

source contact

drain contact

SiO2

source contact

drain contact

n+

implantedp-type gate

n-type substrate

n- drift

VJFET Technology

SemiSouth 600V SiCFET

0

5

10

15

20

25

0 5 10 15 20Drain Voltage (V)

Dra

in C

urre

nt (A

)

Vg=0Vg=1Vg=2vg=-10

• Hard Switch @ 20MHz• Tjmax ≥ 300 °C• Parallel multiple die for

high total current

Multi-die package600V, 20A

Sept 2003

ron = 6.8 mΩ*cm2

Parts currently being sampled to customersMonitored by Dr. J. Scofield, WPAFB

VJFET Technology

300-V, 5-A, 20-MHz VJFET

0

1

2

3

4

5

6

7

8

9

10

0 2 4 6 8 10 12 14

Vd, [V]

Id, [

A]

Vg=2VVg=1VVg=0VVg=-1VVg=-2V

Vd=150V

Id=5A

t=51ns

f=19.6MHz

Parts currently being sampled to customersMonitored by Dr. J. Scofield, WPAFB

The “Quasi-on” OptionNormally on: VT is negative.Normally off: VT is positive.

Quasi-on: VT is slightly negative

VDS = 10V

1.E-12

1.E-10

1.E-08

1.E-06

1.E-04

1.E-02

1.E+00

-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1VGS (V)

I DS (A

) normally on

quasi-on

normally off

10-10

10- 12

10- 8

100

10- 6

10- 2

10- 4

VDS = 10V

0

0.2

0.4

0.6

0.8

1

-5 -4 -3 -2 -1 0 1VGS (V)

I DS

(A) normally on

quasi-on

normally off

Benefits of a Quasi-on JFET• Improved blocking• Circuit protection

• Acceptable current trade-off

1.73 A

0.24 A

0

0.5

1

1.5

2

0 2 4 6 8 10VDS (V)

I DS (A

)

normally on

quasi-on

VGS = 0 V

VGS = -10 V

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

0 100 200 300 400 500 600VDS (V)

I DS (A

)

normally on

quasi-on

10-10

10- 9

10- 8

10-7

10- 6

10- 5

10- 4

3.5 A

2.6 A

0

0.5

1

1.5

2

2.5

3

3.5

4

0 2 4 6 8 10VDS (V)

I DS (A

)

normally on

quasi-on

VGS = 2 V

DC Characteristics at ElevatedTemperatures

DC IDS at VDS = 10 V as a function of temperature for a 600V 4H-SiC VJFET

DC IDS vs. VDS at VG = -10 V for temperatures up to 250ºC 4H-SiC VJFET

VGS = -10 V

1. E- 10

1. E- 0 9

1. E- 0 8

1. E- 0 7

1. E- 0 6

1. E- 0 5

1. E- 0 4

0 100 200 300 400 500 600VDS (V)

I DS (A

)

250ºC200ºC150ºC100ºC25ºC

10-10

10- 9

10- 8

10-7

10- 6

10- 5

10- 4

VDS = 10 V

0

0.5

1

1.5

2

2.5

3

0 50 100 150 200 250 300T (ºC)

I DS (A

)

2 1.5 1 0.5 0

VGS (V)

Summary

• The 600V SiC VJFETs had typical ron values from 6.5 to 10 mΩ*cm2 at room temperature.

• Devices exhibited an expected decrease in drain current with increasing temperature with IDS at 300ºC = ~ 30% IDS at 25ºC.

• Early generation VJFET had maximum switching frequencies in the MHz range which increased with temperature.

• Next generation VJFET has much reduced gate resistance and RT switching over 20 MHz.

SiC Power Electronics

Trends

SiC VJFET/Diode in DC-DC Buck Converter (VJFET @ 225°C)

Comparison of SiC switch (specific on-resistance)

Blocking Voltage\

InfineonSi switch

InfineonVJFET-A

InfineonVJFET-B

SemiSouth

VJFET

600 V 30 (COOLMOS™)

8 20 6.8

1200 V 400 + 12 22

1800 V NA 14 24

Improving SiC Power Switches

MDA/AFRL Program produced a patented SiC VJFET switch which has:

Lower on-resistanceSimpler (inexpensive) fabrication processLower packaging costs

Increased Requirements forHigh Temperature Electronics

• Near Term (3 - 5 years)– 250°C - 300°C (max) junction temperature– Reduction in requirements for active cooling

• Far Term (5 - 9 years)– 300°C - 350°C (max) junction temperature– Supports “high speed” aircraft

SemiSouth and SiC is in the Commercial Market Place

Epitaxy

SiC MaterialsQuality Control

Power Switch

SiC Devices Power Rectifier

RF Transistor

Device Design

SiC OEMDevice Fabrication

Multiple Sources, Decreasing PricesCree II-VI SemiSouth Dow Corning

Substrates Substrates Substrates↓ ↓

Epi Epi Epi↓ ↓

Devices Devices↓ ↓

End users, VARs End users, VARs“Virtual” Vertical Integration

Provide true direct alternative to CreeBut, provides insulation from competing with customers

Open source of SiC technology – provide industry with true 2nd or alternate source… something many customers want now.

Provides direct market feedback and drive to emerging 4H epi-wafer market.