JSL1

A High-Efficiency Low-Cost DC-DC Converter for SOFC

The First Introduction of V6 Converter

September 30 October 1, 2003

SECA Core Technology Program Review Meeting

Albany, New York

Presented byDr. Jih-Sheng (Jason) Lai

Virginia Polytechnic Institute and State UniversityFuture Energy Electronics Center

JSL2

Outline

• Introduction of V6 Converter• State-of-the-Art DC/DC Converter Technologies• Hardware Implementation of V6 Converter• Sensorless Control Design and Implementation• Fuel Cell Current Ripple Issue• Conclusion

JSL3

The Role of DC-DC Converter in an SOFC Power Plant

SOFC LoadDC-DCconverter

dc control

Inverter

DC Link

Vdc

Iaccommunication Controller Vac

Vfc

Low voltageDC 22-50V

HighVoltage

ac control

AuxiliaryPower

supplies

The DC/DC converter is the most crucial electrical interface to the fuel cell source

JSL4

Major Issues Associated with the DC-DC Converter

• Cost• Efficiency• Reliability• Ripple current• Transient response along with auxiliary

energy storage requirement• Communication with fuel cell controller• Electromagnetic interference (EMI) emission

JSL5

Why V6 Converter?Analogy to Automotives

• Six Phases (V6)• Mid power converters• >5kW

• Six Cylinders (V6)• Midsize vehicles• >2500 c.c.

• Single Phase • Low power converters• <1kW

• Single Cylinder • Motorcycle• <500 c.c.

Fuel Cell ConvertersAutomotives

JSL6

How to Categorize Converters?

• Single-Phase Converters– Half-Bridge Converter– Push-Pull Converter

• Two-Phase Converters– Full-Bridge Converter

• Three-Phase Converters• Four-Phase Converters• Six-Phase Converters

JSL7

Half-Bridge Converter A Single-Phase Converter

Q1

Q2

Vfc

Cin1

+

–

1 : 2n

Cin2

! Low device count! Low voltage device" Device sees twice current" Split capacitors" Twice transformer turns ratio

JSL8

Push-Push Converter A Single-Phase Converter

1 : n

Q1 Q2

Vfc

+

–

Cin

+ Low component count misleading when power level is high!+ Simple non-isolated gate drives+ Suitable for low-voltage low-power applications– Device sees twice input voltage high voltage MOSFET is needed

# High conduction voltage drop, low efficiency– Center-tapped transformer

# Difficult to make low-voltage high-current terminations # Prone to volt-second unbalance (saturation)

JSL9

A Commercial Off-the-Shelf 1-kW Push-Pull Converter for Fuel Cell Applications

Push-pull dc/dc converter

LoadDC

source

• Input 28 to 35 V • Device voltage blocking level 100 V • Efficiency <85% even with 4 devices in parallel

JSL10

Full-Bridge Converter A Two-Phase Converter

Vfc

Cin

+

–

1 : n

Q1 Q3

Q2 Q4

! Most popular circuit today for high-power applications# Soft switching possible# Reasonable device voltage ratings

" High component count from the look " High conduction losses

JSL11

List of Available Power MOSFETs for up to 50-V Fuel Cell Converters

Manufacturer Part Number VDSS (V) RDS-on (mΩ) Package Fairchild FDB045AN08A0 75 4.5 TO-263 International Rectifier IRFP2907 75 4.5 TO-247 Fairchild FDP047AN08A0 75 4.7 TO-220AB IXYS FMM 150-0075P 75 4.7 ISOPLUS i4-PAC* Vishay Siliconix SUM110N08-05 75 4.8 TO-263 IXYS IXUC160N075 75 6.5 ISOPLUS 220 International Rectifier IRF3808 75 7.0 TO-220AB Fairchild FQA160N08 80 7.0 TO-3P *Note: IXYS FMM 150-0075 is a dual pack (half bridge) device.

JSL12

Efficiency Consideration with Conventional Full Bridge Converter

• At 6kW fuel cell power output condition • Input voltage: 25 V • Input current: 240 A • Device conduction resistance = 4.5mΩ at 25°C and 9mΩ at 125°C• Device conduction voltage drop: Von = 9mΩ×240×2 = 4.32V• Maximum achievable efficiency: 82% (18% loss) • Minimum number of parallel devices to achieve 97% efficiency: 6$ Von = 0.72 V (assuming no parasitic and switching losses and no output stage losses)

• Minimum total number of devices to achieve 97% efficiency: 24$ Equivalently 12 phases are needed to achieve efficiency goal

JSL13

Full-Bridge Converter with Paralleled Devices to Achieve the Desired Efficiency

Voltage clamp

Load6x 6x

LfFuel cell

source

• With 6 devices in parallel, the two-leg converter can barely achieve 97% efficiency

• Problems are additional losses in parasitic components, voltage clamp, interconnects, filter inductor, transformer, diodes, etc.

JSL14

The V6 DC-DC Converter and the Associated the Output Stage DC-AC Inverter

rectifier/filter

isolation xformer

Isolated V6 DC-DC Converter

The

V6C

onve

rter 120V

120V

DC/AC Inverter

+

–

SOFC

!Low-voltage high-current input !High-voltage low-current output !Dual output dc voltages provide neutral point for grounding!Residual current in non-switching phases allows zero-voltage

switching at light load condition

JSL15

Photograph of a Three-Phase Hard-Switched DC-DC Converter

Gate drive board

Power board

Transformer

Rectifier board

DC chokeVoltage clamp

DSP board

Note: The power switch has four TO-263 devices in parallel

JSL16

Photograph of the Soft-Switched V6 DC-DC Converter

Controller chip

Full-bridge gate driver Power board

Transformer

Rectifier board

Note: The power switch is single device without paralleling to avoid parasitic losses

JSL17

Phase Shifted Controller Signals

VA

VB

VC

VAB

VBC

VCA

JSL18

Significance of Parasitic Components

Gate drive with twisted wire Direct plug in type

Potential Problems: • High frequency ringing and EMI • Additional transformer loss• Additional switching loss

JSL19

Voltage Overshoot and Ringing on Primary Side under Hard Switching Condition

Vds

Vgs

Vo

JSL20

Hard Switched Full-Bridge Converter Device Voltage Waveform at Lighter Load

Vds (10V/div)

At lighter load, for the primary side device voltage" Severe parasitic ringing remains " Resulting substantial EMI propagation

JSL21

Simulation Verification of Hard Switched Converter Voltage Overshoot and Ringing

Vds

Causes of voltage overshoot and ringing1. Circuit parasitic inductance2. Output capacitance of the device3. Transformer leakage inductance

JSL22

Conventional Full-Bridge Soft-Switched Converter Voltage and Current Waveforms

device voltage, Vds

voltage before LC filter, Vd

Inductor current, iL

device voltage

!Primary side soft switching without voltage overshoot and parasitic ringing

" Secondary side has high voltage swing from 0 to 2.5Vo andsevere ringing with single-phase operation

" High inductor current requires large inductor to smooth it out

JSL23

Voltage before LC filter, Vd

Phase-a device voltage, Vds

Phase-b device voltage, Vds

Inductor current

(20V/div)

Voltage and Current of the V6 Soft-Switched Converter

!Primary side soft switching without voltage overshoot and parasitic ringing

!Secondary inductor current is rippleless; and in principle, no dc link inductor is needed

!Secondary voltage swing is eliminated with <40% voltage overshoot

JSL24

Significant DC link Inductor Size Reduction as Compared to Full-Bridge Converter

Lf for full-bridge converter

Lf for V6 converter

With V6 converter, an effective 10x reduction in dc link filter inductor in terms of cost, size and weight

JSL25

Input and Output Voltages and Currents at 1kW Output Condition

Vin

Vo Iin

Io

(86%)

Vo

Iin

Vin

Io

(99%)

(b) V6 Converter(a) Full bridge converter

Significant improvement with multiphase converter!Less EMI!Better efficiency (97% versus 87% after calibration)

JSL26

Efficiency Measurement Results

80%82%84%86%88%90%92%94%96%98%

100%

0 1000 2000 3000 4000

70%

75%

80%

85%

90%

95%

100%

0 1000 2000 3000 4000 5000 6000

calculated efficiency

experimental data and prediction

Output power (W)

with reduced parasitics

Experimental data and trend line

• Measurement error: within 1%• Heat sink temperature rise:

<20°C at 2kW with natural convection

JSL27

Current Sensorless Control Design

• To regulate dc bus voltage, current loop may not be necessary. However, with added current loop the control response is faster, and the voltage regulation is more stable.

• The problem with adding a current control loop is the cost associated with the current sensor

• In this project, a novel current sensorless control is developed with superior performance to the conventional voltage loop control system

JSL28

DC Bus Voltage under 15% Load Step without Current Loop

Vdc(50V/div)390V

40ms/div

ILref

IL (28A/div)

Experimental results

(A)

0.0

20.0

40.0

t(s)

0.2 0.24 0.28 0.32 0.36 0.4

(A)

0.0

20.0

40.0

(V)

360.0

380.0

400.0

420.0(0.25996, 400.0)

(0.29146, 383.98)

(A) : t(s)

inductor current

(A) : t(s)

current command

(V) : t(s)

vdcDC bus voltage

Current command

Inductor current

Simulation results

• Without current loop, voltage fluctuates during load transients• Both simulation and experiment agree each other

JSL29

A Novel Current Sensorless Control

–

+

CCMDCM

VmD

Vref

Cm(s)Σ× 1/s

2

2222

),(o

oggg V

VnVDVnDVDk

−=

( )∫ −= dtVVL

i odf

L1

×

–

+ Σ

nVg

1/s

Vo

• A plant model based on known inductance and operating mode, continuous and discontinuous conducting mode (CCM and DCM)

• A simple lead-lag compensator is designed to achieve fast dynamic control without the need for the current sensor

JSL30

Uncompensated Loop Gain PlotPhase Margin = 2.6°

JSL31

Compensated Loop Gain PlotPhase Margin = 67.5°

JSL32

Load Dump with Sensorless Control

0

1

2

3

4

0 1 2 3 4 5 6

100

150

200

250

300

0 1 2 3 4 5 6

Load

Cur

rent

(A)

Out

put V

olta

ge (V

)

Time (ms)

No overshoot

JSL33

Load Step with Sensorless Control

0

1

2

3

4

0 1 2 3 4 5 6

100

150

200

250

300

0 1 2 3 4 5 6

Load

Cur

rent

(A)

Out

put V

olta

ge (V

)

Time (ms)

No undershoot

JSL34

Fuel Cell Current Ripple Issue

DC bus energy storage capacitor

• The inverter output 60 Hz current tends to reflect back to fuel cell source with 120 Hz ripple if there is not enough energy buffer in between.

• On the other hand, the auxiliary energy storage source tends to interact with the fuel cell source given unequal time constant between them.

FuelCell

Source

DC/DCConverter

2

-2

0

-1

1

0 5010 20 30 40

AC Load

0.2

-5

0.

0 5010 20 30 40

Auxiliary energy storage source

JSL35

Solving Current Ripple Problem with Additional Boost Converter

L

Q

D

x6

Multi-leg boost converter

Auxiliary energy storage

DC/DCConverterSOFC

• Fuel cell current ripple can be avoided by adding a boost converter in between dc/dc converter and auxiliary battery.

• The boost converter must be sized equally as the SOFC.• Main problem is additional power conversion losses and cost.

$ Other ways of energy buffering can be better solutions.

JSL36

Steady State Fuel Cell Current and Voltage Ripples with Inverter Load

60 Hz ac load voltage and current

Fuel cell voltage

Fuel cell current

• Significant 120 Hz voltage and current ripple present

JSL37

Fuel Cell Output Voltage During Load Dump

46474849505152

-0.04 0 0.04 0.08 0.12 0.16 0.2

Vol

tage

(V)

-15-10-505

1015

-0.04 0 0.04 0.08 0.12 0.16 0.2

Cur

rent

(A)

Time (s)

Inverter output current

Fuel cell output voltage

• Experiment with a 3-kW PEM fuel cell and a 3.3-F ultra capacitor.

• Use incandescent lamps as the load.

• Ultra cap smoothes the load transient effectively.

• Fuel cell time constant is reasonably fast, in millisecond range.

JSL38

Fuel Cell Dynamic Response During Single-Phase Motor Start-up Transients

48505254

-0.1 0 0.1 0.2 0.3

Vol

tage

(V)

0102030

-0.1 0 0.1 0.2 0.3

Cur

rent

(A)

-20-10

01020

-0.1 0 0.1 0.2 0.3

Cur

rent

(A)

Time (s)

Inverter output current

Fuel cell voltage

Fuel cell current

JSL39

Fuel Cell Current Ripple with Inverter Load30

0

10

20

0 50m10m 20m 30m 40m0.25k

-50

0.1k

0 50m10m 20m 30m 40m0.45k

00.1k

0.2k

0.3k

0 50m10m 20m 30m 40m0.4k

-0.4k

0

-0.2k

0.2k

0 50m10m 20m 30m 40m20

-20

0

-10

10

0 50m10m 20m 30m 40m

Time (s)

fuel cell current ripple (120 Hz)

fuel cell bus voltage (28 V)

per phase converter current ripple

DC Bus Voltage

Midpoint DC Bus Voltage240V

120Va120Vb

i-240

i-120a

i-120b

3kW-240V1kW-120Va1.44kW-120Vb

JSL40

Adding Energy Storages at DC Bus is Another Alternative

0.25k

-50

0.1k

0 50m10m 20m 30m 40m0.25k

-50

0.1k

0 50m10m 20m 30m 40mTime (s)

20% ripple current4-cycle energy backup

8-cycle energy backup 10% ripple current

DC bus energy buffer

Adding energy storage capacitors at the DC bus not only smoothes out the inverter load transient, but also reduces fuel cell current ripple proportionally.

DC/DCConverter

JSL41

Summary

A V6 DC-DC converter has been successfully developed and tested to demonstrate– Soft switching over a wide load range– High efficiency ~97% – Low device temperature $ High reliability– No overshoot and ringing on primary side device voltage– DC link inductor current ripple elimination $ cost and size

reduction on inductor– Secondary voltage overshoot reduction $cost and size

reduction with elimination of voltage clamping– Fast dynamic response with sensorless control $ cost

reduction on current sensor– Significant EMI reduction $ cost reduction on EMI filter

JSL42

Future Work

• Test V6 converter with SOFC• Define fuel cell and converter interface • Develop interface and communication protocol • Design package for the beta version • Develop energy balancing strategy • Facilitate Standardization!