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Future Energy Electronics Center

Power Electronic Technologies

for Fuel Cell Power Systems

Dr. Jih-Sheng (Jason) Lai

Director, Future Energy Electronics Center

Virginia Polytechnic Institute and State University

Blacksburg, VA 24061-0111

TEL: 540-231-4741

FAX: 540-231-3362

Email: [email protected]

Presentation at

SECA 6th Annual Workshop

Pacific Grove, California

April 19, 2005

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Outline

1. Basic Fuel Cell Power Systems

2. Non-isolated DC-DC Converters

3. Isolated DC-DC Converters

4. DC-DC Converter Implementation Issues

5. Basic DC-AC Inverters

6. Fuel Cell and Converter Interactions

7. Fuel Cell Energy Management Issues

8. Advanced V6 DC-DC Converter

9. Fuel Cell Current Ripple Issues

10. Recap

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1. Basic Fuel Cell Power Systems

Membrane

Electrode

Assembly

(MEA)

Membrane

Electrode

Assembly

(MEA)

Membrane

Electrode

Assembly

(MEA)

Membrane

Electrode

Assembly

(MEA)

1. Fuel Cell Control

Flow rate

Pressure

HumidityTemperature

Fuel in

Core of fuel cell

2. Power Conversions

DC-DC for portable

DC-AC for household

DC-variable frequency

AC for automotive

3. Energy storage

Electricity

out

Power ElectronicsBOP

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Automotive Fuel Cell Power System1. Fuel cell control

2. Power conversions

3. Energy storage

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Stationary Fuel Cell Power Plant for

Telecom Applications

• Fuel cell output or converter input is low-voltage

DC with a wide-range variation• Plant output is 48-V DC

• Isolation may or may not be needed

DC-AC

Inversion

AC-AC

Isolation

AC-DC

Rectification

Fuel

Cell

AC-DC

Rectifier

+

Filter

V in

+

–

+

–

Full

Bridge

Converter

HF

Xformer

DC/DC converter LV-DC

48-55V DC

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Stationary Fuel Cell Power Plant for Household Applications

• Plant output is high-voltage ac

• Multiple-stage power conversions including

isolation are needed

DC-AC

LF-HF

AC-AC

LV-HV

AC-DC

HV-HV

DC-AC

HV-HV

DC-AC

Inverter

+

Filter

Fuel

Cell

AC-DC

Rectifier

+

Filter

V in

+

–

+

–

Full

Bridge

Converter

HF

Xformer

DC/DC converter LV-DC HV-AC

120V

120V240V

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Major Issues Associated with the Power

Conditioning Systems

• Cost

• Efficiency• Reliability

• Isolation

• Fuel cell ripple current

• Transient response along with auxiliary energy

storage requirement

• Communication with fuel cell controller

• Electromagnetic interference (EMI) emission

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2. Basic Non-Isolated DC-DCConverters

• Buck Converter – Output voltage is always

lower than input voltage

• Boost Converter – Output voltage is always

higher than input voltage

• Buck-boost Converter – Output voltage canbe either lower or higher than input voltage

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Basic Principle of Buck Converter

L

R+

V inC

+

V o

v gs D1

v gs

V s

V o

Gate on Gate off

DT s D’T s

ino DV V

Average output voltage:

g

sd

where D is the duty ratio .

Because D < 1, V o is always

less than V in buck

converting

+

V s(t )

average V s = V o

V s = V in

V s = 0

T s: switching period = 1/ f s (s)

f s: switching frequency (Hz)

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A Buck Converter Example

L

+

V inC

+

V o

v gs

Q1

D1 g

sd

+

v s(t )48V 24V22F

40H

• Input is 48 V, and output is 24 V

• Duty cycle D = V o/V in = 0.5

• Switching frequency = 100 kHz

• Output power = 150 W

• Inductor and capacitor are designed to limit the current

and voltage ripples

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Basic Principle of Boost Converter

L

R+

V inC

+

vo

v gs

Q1

D1

v gs

V s

V o

Gate on Gate off

DT s D’T s

g

s

d

V s = 0

V s = V oinino V

DV

DV

'

1

1

1

Average output voltage:

where D is the duty ratio,

and D’ = 1 – D. Because

D’ < 1, V o

is always

greater than V in boost

converting

v s

+

V o

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A Boost Converter Design Example L

+

V in C +

V ov gs

Q1

D1

g

s

d

V s

+

6V48V

• Input is 6 V, and output is 48 V• Duty cycle D = 1 – V in/V o = 0.875

• Switching frequency = 100 kHz

• Output power = 180W

• Inductor and capacitor are designed to limit the current

and voltage ripples

10H

100F

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Circuit Diagram of Buck-boost converter

L R+

V inC

+

v

v gs

Q1 D1

g

sd

inin V D

DV

D

DV

'1

+ +

The output voltage can be expressed as

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Synchronous Rectifier

D

G

S

oxide

S G

n+

D

n p+

n p+

n

ii

• MOSFET can be used as a diode by shorting G-S

• However, when running under diode mode, gating

between G-S would allow current to flow through S-D

channel in reverse direction synchronous rectification

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Features of Synchronous Rectification

• MOSFET voltage drop is resistive and can be aslow as possible, such as <0.1 V.

• The voltage drop is very crucial to the converter

efficiency in a low voltage system. For example,a diode with a fixed voltage drop of 0.7 Vrepresents 3.5% loss of a 20-V system.

• Synchronous rectification allows the voltagedrop to be a function of MOSFET resistance andcurrent and cuts the conduction losssubstantially. For example, a MOSFET with 5m running at 20 A condition, the voltage dropis 0.1 V, much less than diode voltage drop.

• Suitable for low-voltage systems.

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Circuit configuration of a Boost Converter with Synchronous Rectification

• D1 and D2 are body

diode of Q 1 and Q 2 .

• Q 1 and Q 2 switch

complimentary

Q2

L

R+

V in

C +

vo

Q1

D2

D1

v gs1 Q1 on Q1 off

DT s D’T s

v gs2 Q2 off Q2 on

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Experimental Results of a DC-DC

Converter with and without SR

0 0.5 1.0 1.5

80.0

82.5

85.0

87.5

90.0

92.5

P o (kW)

With SR

Without SR E f f i c i e n c y ( % )

Test condition:

V b=13V, V o=300V

Efficiency is improved by 7% with SynchronousRectification

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3. Isolated DC-DC Converters

• Why isolation is needed?

• Push-pull DC-DC converter

• Half-bridge DC-DC converter

• Full-bridge DC-DC converter

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Isolation is Required for Most Systems

• High voltage conversion ratios

– Isolation allows better device utilization

• Grounding requirement

– Isolation avoids noise coupling

• Safety requirement

– Isolation allows meeting safety standards

• Multiple outputs

– Isolation transformer allows multiple

secondary windings

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Problems with Isolation

• Magnetic component design and cost are

non-trivial

• Transformer saturation due to unbalance

input

• Additional losses• Additional terminations

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Q1

Basic Operating Principle of a Half-

Bridge Converter Circuit

V inC f

+

–

Q1

Q2

Half-bridge converter

C 1

C 2

t

Q1

Q2 Q2

vab

dead-time, currentcirculating thru

anti-paralleled diodes

t

t

D2 D1

V in/2

0

Switches conduct

alternately

vab

2

inV

2

inV

–

+

vab = V invab = – V in

1:n

b

a

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Half-Bridge DC/DC Converter

Low device count

Low voltage device

Device sees twice current

Unbalance due to split capacitors

High leakage due to twice transformer turns ratio

L

C R vo

+

–

D5

D6

D7

D8

vd

i2

i L

V in

+

–

1 : n

M 1

M 2

b

C 1

a

C 2

i1

10V

>500A

20V

>250A 400V

5 kW

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Basic Operating Principle of a Push-Pull

Converter

1:1:n

Q2 Q1V in

+

–

C

Q1

t

Q1

Q2 Q2

vab



t

t

D2 D1

2V in

0

Switches conduct

alternatelyvab = 2V invab = –2V in

a

b

Push-Pull converter

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L

C R vo

+

–

D5

D6

D7

D8

vd

i2

i L

Push-Pull DC/DC Converter

+ Simple non-isolated gate drives

+ Suitable for low-voltage low-power applications

– Device sees twice input voltage – need high voltage MOSFETHigh conduction voltage drop, low efficiency

– Center-tapped transformer Difficult to make low-voltage high-current terminations

Prone to volt-second unbalance (saturation)

1:1:n

M 1 M 2V in

+

–

a

b

40V20V

>250A

400V

5 kW

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A Push-Pull Converter with Paralleled

Devices

Load

Push-pull

dc/dc

converter

DC

source

• Input – 28 to 35 V

• Device voltage blocking level – 100 V• Efficiency – <85% even with 4 devices in parallel

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A Commercial Off-the-Shelf 1-kW Fuel Cell

Power Plant Using Push-Pull Converter

Push-pullDC/DC

converter

DC

input

117V

DC/AC

Inverter

+

–

F u e l

C e l l

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Basic Operating Principle of a Full-

Bridge Converter Circuit

V inC f

+

–

Q1 Q3

Q2 Q4

a

b

Full-bridge converter

t

Q14

Q14

Q23 Q23

vab



t

t

D23 D14

V in0

Switches conduct

alternately

vab –

+

vab = V invab = – V in

1:n

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Full-Bridge DC/DC Converter

Most popular circuit today for high-power applications

Soft switching possible

Reasonable device voltage ratings

High component count from the look

High conduction losses

L

C R vo

+

–

D5

D6

D7

D8

vd

i2

i L

V in

+

–

1 : n

M 1 M 3

M 2 M 4

a

b

i1

20V

>250A

20V

>250A400V

5 kW

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Full-Bridge Converter with Paralleled Devices to

Achieve Desired Power Levels

Load

Fuel

cell

source

• Multiple devices in parallel to achieve desired high efficiency

• Problems are additional losses in parasitic components,

voltage clamp, interconnects, filter inductor, transformer,

diodes, etc.

L f

Voltage clamp

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Design Considerations for Isolated DC-DC Converters

• Transformer turns ratio

• Transformer core utilization

• Device voltage stress

• Device current stress

• Output diode voltage stress• Voltage clamping

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Pulse Width Modulation for Isolated DC-

DC Converters

The average output voltage

V o = DnV inWhere

n = transformer turns ratio = n2/n1

D = duty ratio = t on/T

and

n1 = number of turns of primary winding

n2 = number of turns of secondary winding

t on = switch turn-on time

T = switching period

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Transformer Core Utilization

Forward: <50%

Flyback: <50%

iM

Half-bridge: 100%

Push-pull: 100%

Full-bridge: 100%

iM-pk

B

H NiM-pk

B

H

iM

Magnetizing current

00

iM-pk

t t

Core is fully utilized

–NiM-pk

NiM-pk

Core is half utilized

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Device Voltage and Current Stresses

• Device voltage stress

Push-pull: 200%

Half-bridge: 100%

Full-bridge: 100%

• Device current stress

Half-bridge: 200%

Push-pull: 100%

Full-bridge: 100%

• Output diode voltage stress

Center tap: 200%Bridge: 100%

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Voltage Clamping

• Problems of diode over voltages

– Full-bridge

– Center-tapped

• Voltage clamping methods

– Passive clamping method

– Active clamping method

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Over-Voltage Caused by the Transformer

Leakage Inductance

L

C R vo

+

–

1 : n Llk

DT s

v1 v2 vd

–

+

vo

v1

v2

vd

vo

v1: Primary side voltage

v2: Secondary side voltage v2, v2 > vo.

vd : Voltage stress of upper diodes ( D5 , D7 ) when lower diodes

( D6 , D8) conduct, or vice versus, vd-pk > v2-pk > vo, due to

leakage inductance voltage drop.

T s D5

D6

D7

D8

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Full-Bridge Diode Rectifier Over-VoltageClamping

L

C R

Llk

v2

D5

D6

C c

Rc

vo

vd

vo

vd

Without clamp

With passive

clamp

vd

–

+

Advantage: Diode voltage stress is significantly reduced.

Disadvantage: Added cost and complexity.

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Fuel Cell System Example for Topology

Selection

Question: With 48 V fuel cell voltage and 400 V dc output,

what topology is the best?

Answer: Intuitively, push-pull converter is the best

because of least parts count. However, with device

availability and cost consideration, full-bridge converter

may be a better choice.

Reason: For low-side power MOSFET, lower voltage is

more cost effective. Similarly, for high-side diode, lower

voltage is more cost effective.

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4. Implementation Issues in HighPower DC/DC Converters

• Controller output duty cycles tend to be unbalanced

due to internal chip layout, resulting transformer

saturation.

• Voltage sensing problem:

– Feedback voltage signal tends to be corrupted by noises

– Hall sensor is expensive

– Common mode and isolation are difficult to deal with

resistor dividers

• Current sensing problems:

– Lossy with resistor sensing

– Difficult to insert Hall sensor for device current

measurement

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Full-bridge Converter Design Example

• Specifications:

– Input fuel cell voltage ranges from 36 V to 60 V

– Output: 400 V, 10 kW

• Current

– Output: 25 A

– Input: 208 A

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Power Stage Design

V in

L

C f

+

–

C R vo

+

–

1 : nQ1 Q3

Q2

Q4

a

b

HF ac

Xformer

Component Design and Selection:• Power MOSFET

• Rectifier diode

• Transformer

• Filter inductor

• Filter capacitor

D5

D6

D7

D8

vd

i1 i2

i L

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Survey of High Current Power MOSFETs

*Note: IXYS FMM 150-0075 is a dual pack (half bridge) device.

Manufacturer Part Number V DSS (V) R DS-on (m) 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

Quantity 1 100 1000 25,000 50,000 100,000

FDB045AN08A0 $3.50 $2.50 $2.40 $2.30 $2.10 $1.60

IRFP2907 $4.49 $3.96 $3.07 $3.07 $3.07 $2.89

FDP047AN08A0 $3.50 $2.50 $2.40 $2.30 $2.10 $1.60

FMM 150-0075P $8.00 $7.00 $6.19 $5.79 $5.30 $5.03

SUM110N08-05 $2.70 $2.50 $2.50 $2.35 $2.19 $2.19

IXUC160N075 $4.00 $3.00 $2.05 $1.65 $1.49 $1.40

IRF3808 $2.29 $2.16 $1.80 $1.50 $1.30 $1.17

FQA160N08 $4.00 $3.00 $2.90 $2.60 $2.50 $2.20

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Survey of Ultrafast Reverse RecoveryDiodes

Quantity 1 100 1000 25,000 50,000 100,000

RHRG5060 N/A, (300 part min) $3.50 $1.75 $1.50 $1.50HFA50PA60C $8.81 $8.22 $7.71 $7.61 $7.25 $4.00

DSEK 60-06A $4.00 $3.00 $2.50 $2.07 $1.99 $1.90

Manufacturer Part Number V F t rr I Package

Fairchild RHRG5060 1.5 V 45ns (max) 50 A TO-247

International Rectifier HFA50PA60C 1.9 V 23ns (typ) 50 A TO-247AC

IXYS DSEK 60-06A 1.6 V 35ns (typ) 60 A TO-247AD

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Output Filter Capacitor Selection- Typically based on the output voltage ripple

• The output filter capacitor needs to handle 120 Hz, 22 A

ripple current generated from the next stage inverter.

• Assume the voltage ripple is limited to 5%. Thecapacitance can be calculated as

mF2.205.0400608

22

8

V f

I C

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Digital Computer Implementation for High Power DC/DC Converters

• Digital computer such as DSP has become a good

option for high power DC/DC converter control

implementation

• Feedback voltage signal can be converted to digital

and through PWM feeding back to DSP to avoid

noise corruption

• Even if commercial PWM or PSM chips are used, the

control signal can be obtained from DSP through

D/A conversion

• Communication with digital signals has become the

essential part between the dc/dc converter and the

fuel cell controller or other power converters

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Controller Design for a Typical Converter

21)(

sQ

s

K sG p

s K K sG i

pc )(

Compensator Converter

(plant)

Sensor

Reference Output

(A standard PI controller)

Gc( s)

G p( s)

s = j = 2 f

1 j

f = frequency

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Digital Controller for a Typical Converter

Digital

Controller A/D

(sample)D/A

(hold)

Converter

plant

Sensor

ReferenceOutput

1

1

2)(

z

z T K K z G s

i pc

+

–

T s = switching frequency

1

12

z

z

T s

s

Gc( s)

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Voltage Loop Controller Block Diagram

Full Bridge

Converter V in

L

+

– C R vo

+

– vd

i L

H

Gc( s) –

+

vref

Pulse-width

modulator d (t )

v sensevc

H : Voltage scaling = 5.1/400

Gc( s): PI or PID Controller

G p( s)

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Phase Margin Shifts due to Fuel CellInput Voltage Variation: 42 to 60 V

0dB from

initial design G a i n ( d B )

P h a s e ( ° )

22 /)/(1)(

oo

invd

sQ s

nV sG

a negative phase

margin with higher gain

or higher input voltage

0dB with higher gain

or input voltage

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DC-DC Converter Control System Design is

Challenging with the Fuel Cell Source

• A typical controller is designed with low input voltage

and heavy load condition.

• When the load is reduced, the fuel cell voltage

increases, and the original controller design may be

inadequate due to input voltage variation.

• Increasing the input voltage is equivalent to increase

the closed-loop gain and tends to worsen the phase

margin, and the system can eventually become

unstable.

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5. DC/AC Inverters

• Single-phase output

– Half-bridge

– Full-bridge

• Dual single-phase outputs

– Dual half-bridge

– Three-leg inverter

• Load Effect

– Linear loads

– Nonlinear loads

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Half-Bridge DC-AC Inverter with Split DC Buses

Q1

Q2

V dc

L+

–

C AC

Load

V dc1

V dc2

vacvab

Maximum output peak voltage V max = V dc /2

• Simple dc-ac Inverter with minimum switch counts

• Split dc buses should be very stiff and balance to avoid dc or

even harmonics at the ac output

• Control is limited to the ordinary sinusoidal pulse widthmodulation (SPWM)

• Cost burden is in passive components

b

a

V dc1 = V dc2 = V dc/2

c

vba vac

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Sinusoidal Pulse Width Modulation

v sin

vc : carrier wave

Gating signal

When v sin > vc, gate signal is high, and IGBT is turned on;

Otherwise, gate signal is low and IGBT is turned off.

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Single-Phase Full-Bridge DC-AC Inverter

Q1 Q3

Q2 Q4

V dc

L

C dc

+

–

C AC Load

Compared with Half-Bridge inverter, FB inverter features

• Simple dc-ac Inverter with more switch counts, but

less bulky capacitors

• Control is more flexible to have phase-shifted SPWMfor two individual legs – Dual Modulation Method.

• Size of passive components may be reduced

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Dual Single-Phase Outputs with Dual Half-Bridge Inverters

Q1

Q2

2V dc

C dc1

+

–

V dc

V dc

vac1

Q3

Q4

L1

C 1

L2 C 2 vac2

Only one set of split dc buses are required

Q1-Q2 and Q3-Q4 pairs need to be switched complementary

so that the total v ac

= v ac1

+ v ac2

.

Possible unbalanced output ac voltages under unbalanced

load conditions

vac

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Three-leg Inverter for Dual AC Outputs with

Single DC Bus

Q1

Q2

V dc

C dc+

–

vac1

Q5

Q6

L1

C 1

L3 C 2 vac2

vac

Q3

Q4

• Similar to full-bridge inverter with more switch counts, but less

bulky capacitors

• Outer legs do SPWM to produce vac output. Middle leg is

controlled to equalize vac1 and vac2

• Control is more complicated to ensure output voltage balance

• Size of passive components may be reduced

L2

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Using Low-Frequency Transformer for Low-Voltage AC Inverter Output

L

C 48V

120V

120V

240V

Features:

• Low-frequency transformer allows low-voltage DC to be

directly converted to AC

• Output can be single or dual

• Size is the major concern

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Inverter Output with Resistive Load

R

V I

V = 4.1%

vac

Voltage and current

are in phase

R

v I ac

R

I R

R

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Dual Output Voltage and Current withUnbalanced and Reactive Loads

Voltage of Leg 1

Current of Leg 1

Voltage of Leg 2

Current of Leg 2

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-1.0

-0.5

0.0

0.5

1.0

0 90 180 270 360 450 540 630 720

Majority of Power System Loads is Inductive

fL j L j X L 2

vac

I RL

R

L

Impedance of inductor is imaginary (90º apart from real

value)

-1.0

-0.5

0.0

0.5

1.0

0 90 180 270 360 450 540 630 720

vac

I R R L

I L I RL

I R

I L

I RL

vac

I L

I R

90º lagging

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-3-2-10123

0 90 180 270 360 450 540 630 720

-3-2-1

0123

0 90 180 270 360 450 540 630 720

-3-2-10123

0 90 180 270 360 450 540 630 720

What about Watt and VAR?

P

Q

P+jQ

I Lvac

Q (VAR)

I R

vac

P (Watt)

VAR is reactive VA,

average VAR = 0, but

reactive current is not

0.

Watt is real VA,

average Watt > 0, and

is the average (VA).

VA = Watt+jVAR , not

Watt+VAR because

they are not in phase.

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Implications of VAR

• Average VAR = 0 No real power output

• VAR loads are typically inductive such as

motors, magnetic ballasts, relays, etc.

• The current associated with VAR causes

additional heat losses in the wiring and the

internal impedance of the source

• Inductive VAR can be compensated with

capacitive VAR, but not without complexity

• Nonlinear loads also introduce VAR

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Majority of Electronic Loads is Nonlinear

Switch

Mode

Power

Supply

DC

Load

1-phase

rectifier

Inverter

DriveAC

Motor

3-phase

rectifier

Used for:

Computers,

Printers, Fax,

most ITE

Electronic Lighting

Communications

Food Preparation

Used for:

HVAC, Battery

Charging, Food

Preparation,

Elevators

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63


Inverter Output Voltage Under Nonlinear

Rectifier Load

• Single-phase nonlinear load current is rich with odd

harmonics (3,5,7,…) especially with the 3rd harmonic• The voltage waveform is flatten up due to nonlinear

current.

I L

vac

JSL

64


Voltage Waveform Quality may be Improvedwith Closed-loop Control

Voltage THD = 4.6%

Closed-loop control can smooth the voltage waveform but

the nonlinear current waveform remains nasty.

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65


6. Fuel Cell and Converter

Interactions

• Static modeling• Dynamic modeling

• Fuel cell dynamic response with and without

converters

JSL

66


SOFC Voltage-Current Characteristic asa Function of Temperature

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

800 ºC

Current (A/cm2)

V o l t a

g e ( V )

750 ºC

700 ºC

Data source: DOE SECA Modeling team report at Pittsburgh Airport, 10/15/2002

PEMFC

SOFC

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67


Power Density of SOFC and PEMFC

0.0

0.2

0.4

0.6

0.8

1.0

0 0.2 0.4 0.6 0.8 1 1.2

800 ºC

Current (A/cm2)

P o w e r d e n s i t y ( W / c m 2

)

750 ºC

PEMFC

SOFC

JSL

68


PEM Fuel Cell Dynamic Characteristics

0

10

20

30

40

50

60

0 2 4 6 8 10 12t (sec)

0

500

1000

1500

2000

0 2 4 6 8 10 12t (sec)

v FC (V)

i FC (A)

p FC (W)

Step load: 1.47kW

Parasitic power: 70W

voltage undershoot (2.5V)

due to compressor delay

150W dip

27.2V

3kW power

overshoot

43V

Nexa 1.2kW Unit

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69


Fuel Cell Modeling with Electrical Circuit

load

JSL

70


Nexa1200 Polarization Curve

I_Ifc

0A 5A 10A 15A 20A 25A 30A 35A 40A 45A 50AV(fc)

0V

5V

10V

15V

20V

25V

30V

35V

40V

45V

50V

simulation results

experimental results

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71


Polarization Curves for Nexa PEM FC

I_Ifc

0A 10A 20A 30A 40A 50A 60AV(fc)

20V

25V

30V

35V

40V

45V

2

3

1

1. Fuel cell runs at 70-W parasitic load condition

Compressor is running at low speed

2. Fuel cell is fully loaded at 1.4 kW Compressor is not immediately responding

to load step, voltage dips

3. Compressor speeds up, fuel cell voltage

picks up to or above nominal level

JSL

72


Time Domain Response

0

10

20

30

40

50

60

0 2 4 6 8 10 12t (sec)

v FC

i FC

voltage undershoot due

to compressor delay

1

2

3

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73


PEM Fuel Cell Dynamic Simulation

Diagram

1st time

constant

+

X

+

+

2nd time

constant+

• High load current high voltage drop

• Low output voltage low voltage drop

Multiplying ratio

parasitic

load

Transientload

V oc

Fuel

cellHigh current

branch

Low current

branch

JSL

74


Fuel Cell Stack Output DynamicSimulation and Experimental Results

Fuel Cell Output Voltage (10V/div)

10s/div

Fuel Cell Output Current (5A/div)

Fuel Cell Output Power (200W/div)

0s 50us 100us0

10

20

30

40

501

0W

0.2KW

0.4KW

0.6KW

0.8KW

1.0KW2

>>

V fc

P fc

I fc

(b) simulation

results

(a) experimental

results

V fc

P fc

I fc

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Fuel Cell Voltage Dynamic with

Converter Load Transient

Input Voltage from Fuel Cell (5V/div)

100ms/div

Simulated

Experimental

Significantly slower time constant (50ms)

due to 30 mF converter input capacitor and

a long cable

JSL

76


Fuel Cell Responds with a Paralleled140F Ultra Capacitor Capacitor

I fc

V fc

I ulcap

Fuel Cell Voltage (20V/div)

Fuel Cell Current (20A/div)

Load Current (5A/div)

Ultra-Capacitor Current (20A/div)

50ms/div

I Load

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77


Fuel Cell Responds with a Paralleled

10mF Electrolytic Capacitor

I fc

V fc

I cap



DC Link Current (25A/div)

Capacitor Current (5A/div)

20ms/div

Note: Fuel cell output voltage response is slowed down

to 30ms. Capacitor takes over the transient current.

I dc=I cap+I fc

JSL

78


Findings of Fuel Cell Modeling andConverter Test Results

• Fuel cell stack shows very fast dynamic,

nearly instantly without time constant

• Perception of slow fuel cell time constant is

related to ancillary system not fuel cell stack

• Output voltage dynamic is dominated by theconverter interface capacitor and cable

inductor

• Output current dynamic is dominated by the

load

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79


Issues to be Resolved in a Fuel Cell

Power Conditioning System

• Energy management system options – Sizing of

converters and auxiliary sources• Advanced Bidirectional dc-dc converter technologies

• Interleaved control and associated technologies

• Digital control for high power dc/dc converters

• Fuel cell voltage standardization

• Fuel cell ripple current specifications

• Fuel cell output voltage dynamic

• Fuel cell and power conditioning interface and

communication protocol

JSL

80


7. Fuel Cell Energy ManagementIssues

• Problems without Slow Fuel Cell Response

and Auxiliary Energy Storage

• Options of Energy Storage Placement

• Energy Management options with

Bidirectional DC/DC Converters

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Why Fuel Cells Need Auxiliary Energy

Source or Energy Storage?

• For standalone power supplies: needenergy storage for load transient

• For grid-connected power supplies: need

auxiliary energy source for start-up

• For all systems: need auxiliary energy

source to provide power for control signals

JSL

82


Problems of a Fuel Cell System withoutEnergy Storage

• Fuel cell does not have storage capability

• Slow response, output voltage fluctuates

with loads

• Source may not be continuously available

• Size (or capacity) needs to be higher than the

peak load

• When sized enough for the maximum load,

excess energy will be wasted

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83


A Slow and Weak Energy Source During

Startup and Large Load Transient

With a slow and weak input source; it dips

significantly during start up and large load

step.

V o= 320 V

V in = 220 VV in (100 V/div)

iin (20 A/div)

V o (100 V/div)

io (10 A/div)

JSL

84


Converter Step Load Response with Stiff Voltage Source and Voltage Loop Control

Load Current (2A/div)

Input Current (20A/div)

Output Voltage (50V/div)

20ms/div

With voltage control loop bandwidth designed at

20 Hz, settling time is about 40ms under load step

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85


Converter Load Dump Response with Stiff

Voltage Source and Voltage Loop Control

Load Current (2A/Div)

Output Voltage (50V/Div)

10ms/Div

Input Current (20A/Div)

With voltage control loop bandwidth design at 20

Hz, settling time is about 40ms under load dump

JSL

86


A Fuel Cell Power Plant with EnergyStorage

Fuel

Cell

Source

DC/DC

Converter

DC/AC

Inverter

AC Line

Filter

Loads/

Grid

Auxiliary Energy storage options

Fuel Cells Need Auxiliary Energy Storage for energy

management

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87


Low-Voltage Ultra-Capacitor Energy

Storage Configuration

DC/DC

converter

48-72V

Ultra-

capacitor

F u e l

C e l l

120V

DC/AC

Inverter

L f

JSL

88


Use High-Voltage Battery as theAuxiliary Energy Storage

Fuel

Cell

Source

DC/DC

Converter

AC

Load

High voltage auxiliaryenergy storage battery

Capacitor filter

120V

Photograph

of a 96V

battery bank

fuse

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89


Potential Problems with Passive Energy

Storage

• Low-side ultracap option:

– Two voltage sources are paralleled – not a goodengineering practice

– Time to reach equilibrium point is too long becausedynamic characteristics of both sources are different

– Ultra capacitor helps transient current sharing, butcreates significant voltage and current ripples due tointeraction between two voltage sources

– Dynamic current sharing problem

• High-side battery option:

– Battery cell voltage balance problem

– Battery state-of-charge management

– Long-term battery life expectancy – Cost of battery is a concern

JSL

90


Energy Management Options with EnergyStorages and Power Electronics

• Optimum energy usage control

• Start-up control

• Load transient control

• Charging and discharging (bidirectional) controls

for auxiliary energy storages

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Design Considerations for Hybrid-

Source Systems

1. Utilization of Primary Source

2. Simple Power Circuit (as simple as possible)3. Voltage ratio

4. Isolation Requirement

5. Energy Storage Requirement

6. Inverter DC Bus Voltage Requirement

7. Cost

JSL

92


Energy Management Using Low VoltageBattery and Bidirectional DC/DC

Fuel cell LoadDC/DC

converter Inverter

DC Bus

V dc

I ac

feedbacks

Bidirectional

dc/dc

converter

V ac

V batt

• Low voltage battery controlling

DC bus through a bidirectional

dc/dc converter

• Avoid battery cell voltage

balancing problem

• Complicated control

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93


Dual-Source Energy Management Using a

Unidirectional Boost Converter

A

B

C

100kW Inverter

+

+

–

300 V

–

D2

S1

V dc

AC

Output

L1

Low voltage

Fuel cell

High voltage

battery

• Battery voltage > Fuel cell voltage

• Simple boost converter regulates the battery state of charge

• DC bus voltage is constant

1 8 0 – 2 4 0 V

F u e l

C e l l

80kW converter

Total power electronics: 80-kW DC/DC + 100-kW DC/AC

Total energy sources: 20-kW battery + 80-kW fuel cell

JSL

94


An Example of Dual Sources with aBidirectional DC-DC Converter

A

B

C

100kW Inverter

+

+

– 2

4 0 – 3 8 0 V

1 8 0 – 2

4 0 V

–

S1

S2

V dc

AC

Output

Battery voltage < fuel cell voltage needs a boost converter to supply

energy during transient load and a buck converter to charge the battery

Fuel cell voltage = dc bus voltage not regulated

L1

Low

voltage

Battery

High voltage

Fuel cell

F u e l

C e l l

20kW Converter

• Total power electronics: 20-kW DC/DC + 100-kW DC/AC

• Total energy sources: 20-kW battery + 80-kW fuel cell

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95


Interleaved Bidirectional DC/DC Converter for

Fuel Cell Energy Management

Ld1

Ld2

S1u S2u

S1d S2d

V FC

V batt

F u e l

C e l l 100 kW

Inverter

20kW converter

• Total power electronics: 20-kW DC/DC + 100-kW DC/AC

• Total energy sources: 20-kW battery + 80-kW fuel cell

80kW

20kW

Interleaved operation for both boost and buck modes• smaller passive components;• less battery ripple current

JSL

96


DC Bus Voltage During 800-W Load Step andLoad Dump Under Boost Mode Operation

V dcV dc

I dc (1A/div)

(50V/div)(50V/div)

I dc (1A/div)

DC bus voltage fluctuates but returns to original setting after

load transients

20ms/div 20ms/div

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97


Battery Voltage During 800-W Battery Charge

Command Step under Buck Mode Operation

V batt

I dc (2A/div)

(20V/div)

(20A/div) I Ld1

V batt

I dc (2A/div)

(20V/div)

(20A/div) I Ld1

Battery voltage keeps constant during severe charging anddischarging current conditions

JSL

98


8. Advanced V6 Converter

• Single-Phase Half-Bridge Converter

• Two-Phase Full-bridge Converter

• Three-Phase Converter

• V6 Converter

• Test Results with V6 Converter • V6 Converter Prototype

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99


Half-Bridge Converter – A Single-Phase

Converter

1 : n

HF-AC

Half-bridge converter

Xformer

Rectifier+LC filter

A c t i v e L o a d

20V

>250A

S o l i d - O x i d e F u e l C e l l

400V

5 kW

10V

>500A

JSL

100


Full-Bridge Converter – A Two-PhaseConverter

1 : n

HF AC

Xformer

Rectifier+LC filter

A c t i v e L o a d

S o l i d - O x i d e F u e l C e l l

Full-bridge converter

20V

>250A400V

5 kW

20V

>250A

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101


Full-Bridge Converter with Paralleled

Devices to Achieve Desired Power Levels

Load

6x 6x

• With 6 devices in parallel, the two-leg converter can barely

achieve 95% efficiency

• Problems are additional losses in parasitic components,

voltage clamp, interconnects, filter inductor, transformer,

diodes, etc.

Voltage clamp S o l i d - O x i d e F u e l C e l l

20V

250A

JSL

102


Fuel Cell Voltage and Current with FullBridge Converter Case

Time

0s 2ms 4ms 6ms 8ms 10msV(Vfc)

10V

20V

30V

-I(Vfc)20A

40A

60A

I(Cin)

-50A

50A

100A

150A

SEL>>

I(Ld3) V(Iac)-15A

-5A

5A

15A

Fuel cell voltage

Fuel cell current

Load step

Input capacitor current

AC load currentFilter inductor current

Load dump

330%

33%

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103


Switching Waveforms with Full-Bridge

Converter

Time

4.998ms 5.000ms 5.002ms 5.004ms 5.006ms 5.008ms 5.010msI(M1:d) I(M3:d)

-25A

25A

50A

75A

100A

SEL>>

V(M1:d)-V(M1:s) V(M1:g)-V(M1:s)-10V

0V

10V

20V

30V

40VI(Cin)

-100A

-50A

0A

50A

100A

150 A

Ripple frequency = 100 kHz

Zero-voltage switching is achieved

JSL

104


A Three-Phase Bridge Converter

vin

L

C f

+

–

C o vo

+

–

1 : n

S 1 S 5

S 4 S 2

a

c

HF AC

Xformer Rectifier+LC filter

D1

D4

D3

D6

i A ia

i L

A

c t i v e L o a d

F u e l C e l l o

r o t h e r v o l t a g e s o u r c e

3-phase bridge inverter

S 3

S 6

b

D5

D2

A

B

C n

–iC

• Hard switching

• With 4 devices in parallel per switch

• Efficiency 95%

20V

>167A

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105


Fuel Cell Voltage and Current with 3-

Phase Bridge Converter Case

Time

0s 5ms 10ms

V(Vfc)

10V

20V

30V

-I(Vfc)

20A

60A

SEL>>

I(Cin)

-50A

0A

50A

100A

150A

I(Ld3) V(Iac)-15A

-5A

5A

15A

Fuel cell voltage

Fuel cell current

Load step

Input capacitor current

AC load currentFilter inductor current

A significant reduction in capacitor ripple current

33%

80%

No reduction in low-freq. fuel cell ripple current

Load dump

JSL

106


Switching Waveforms with 3-PhaseBridge Converter

Time

4.998ms 5.000ms 5.002ms 5.004ms 5.006ms 5.008ms 5.010msI(M1:d) I(M4:d)

-25A

0A

25A

50A

75A

100AV(M1:d)-V(M1:s) V(M1:g)-V(M1:s)

-10V

0V

10V20V

30V

40VI(Cin)

-50A

0A

25A

50A

SEL>>35 A

Ripple frequency = 300 kHz

Zero-voltage switching is achieved

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107


Key Features of Multiphase Converter

• Device is switched at a lower current, while

maintaining zero-voltage switching.

• High-frequency capacitor ripple current isreduced by >4x, and its frequency is

increased by 3x. This translates to

significant capacitor size reduction and cost

saving.

• No effect on low-frequency AC current ripple,

which remains an issue to be solved.

JSL

108


Circuit Diagram of the Virginia Tech V6Converter

S o l i d - O x i d e

F u e l C e l l

A c t i v e L o a d

Rectifier+LC filter Six-phase bridge converter

HF AC

Xformer

20V

>83A

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109


Key Features of V6 Converter

• Double output voltage reduce turns ratio andassociated leakage inductance

• No overshoot and ringing on primary side devicevoltage

• Input side high-frequency ripple current eliminationcost and size reduction on high-frequency capacitor

• Output DC link inductor current ripple eliminationcost and size reduction on inductor

• Secondary voltage overshoot reductioncost andsize reduction with elimination of voltage clamping

• Significant EMI reduction cost reduction on EMIfilter

• Soft switching over a wide load range

• High efficiency ~97%

• Low device temperature High reliability

JSL

110


Waveform Comparison between Full-Bridge and V6 Converters

Full Bridge Converter V6 Converter

i L

i L

vd

vd

• Secondary inductor current is ripple-less; and inprinciple, no dc link inductor is needed

• Secondary voltage swing is eliminated with <40%voltage overshoot as compared to 250%

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111


Significant DC link Inductor Size

Reduction

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

Single Phase500W60Hz

Single Phase5 kW50kHz

Three Phase5 kW50kHz







JSL

112


Input and Output Voltages and Currents at1kW Output Condition

V in

V oV o

I in

V in

I in

I o

I o

(a) Full bridge converter (b) V6 Converter

Significant improvement with V6 converter Less EMI

Better efficiency (97% versus 87% after calibration)

(97%)(87%)

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113


Where are the Losses?

• Switch conduction

• Diode conduction

• Transformer

• Output rectifier

• Output filter inductor and capacitor

• Input capacitor

• Parasitics

– Copper traces

– Interconnects

JSL

114


Calorimetry for Accurate LossMeasurement

Calibration with resistor bank

fans

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115


Test the 160-Liter Calorimeter at 120-W

Loss Condition

0

10

20

30

40

50

6070

80

90

0 100 200 300 400 500 600

Time (min)

T e m p e r a t u r e ( d e g C )

Average probe temperature

Temperature rise

>9 hours for each test points

JSL

116


Temperature Rise Versus Power Loss

y = 0.4086x + 2.9771

15

20

25

3035

40

45

50

55

60

65

30 40 50 60 70 80 90 100 110 120 130

Power Loss (W)

T e m p e r a

t u r e ( d e g C )

TempRise

Linear (TempRise)

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117


Efficiency Measurement Results

80%

82%

84%

86%

88%

90%

92%

94%96%

98%

100%

0 1000 2000 3000 4000

Output power (W)

Experimental data and trend line

• Measurement error: within 1%

• Heat sink temperature rise:

<20°C at 2kW with natural

convection

E f f i c i e n c y

75%

80%

85%

90%

95%

100%

0 500 1000 1500 2000 2500

Output Power (W)

E f f i c i e n c y

Phase-II V6-Converter Efficiency (calibrated)

Phase-I Eff iciency Measured Results

JSL

118


The Beta Version Prototype Converter

• Schematic Circuit Diagrams

• V6 Cost Estimate

• Summary of Beta Version Prototype

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119


Schematic Circuit Diagrams

Power boardGate drive board

Control board

Digital board

Interface board

JSL

120


Photographs of V6-Converter Together with DC-AC Inverter Prototype

Front View Rear View

Converter Inverter

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121


Prototype and Production Cost Estimate

for the 5-kW V6 DC-DC Converter

Quantity 100 1000 10000Material cost $475 $347 $227Tooling, Assembly & Testing $1,424 $347 $114Production Cost $1,899 $694 $341

Key Materials Parts Count Qty 1 Qty 10000Power Circuit 22 $571.00 $154.40Devices 8 $201.00 $38.40Capacitors 6 $84.00 $30.00Transformers 3 $180.00 $45.00Inductors 2 $24.00 $8.00Sensors 2 $32.00 $8.00Contactor 1 $50.00 $25.00

Control Circuit 325 $113.70 $33.22Resistors 164 $18.59 $2.71Capacitors 110 $46.61 $17.41

Discretes 27 $8.00 $2.42IC's 24 $40.50 $10.68

Miscellaneous 55 $174.80 $52.44Total 402 $840.50 $227.05

JSL

122


9. Fuel Cell Current Ripple Issues

L f

C f RA

BV dc

+

–

S ap

S an S bn

S bp

V o

+

–

60Hz

120Hz

• Current ripple propagates from AC load back to DC side

• With rectification, ripple frequency is 120 Hz for 60 Hz

systems

• Low-frequency ripple is difficult to be filtered unless

capacitor is large enough

AC filter

LC

High-side

cap.

DC-DC

converter

120Hz

Fuel

Cell

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123


AC Current Ripple Problems

• Inverter AC current ripple propagates back to fuel cell

• Fuel cell requires a higher current handling capability

Cost penalty to fuel cell stack• Ripple current can cause hysteresis losses and

subsequently more fuel consumption Cost penalty

to fuel consumption

• State-of-the-art solutions are adding more capacitors or

adding an external active filters Size and cost

penalty

• Virginia Tech solution is to use existing V6 converter

with active ripple cancellation technique to eliminate

the ripple No penalty

JSL

124


Circuit Model for AC Current Ripple

N :1 I in

V in I load

R s L f

V dc

+

–

I p I s

iin /N

iload

N 2 R s L f i p /N i s

DC Model

AC Ripple ModelC in /N 2

N 2 RCin

C in

RCin

C f

RCf

C f

RCf

DC/DC

Converter V in

R s L f

I load C in

RCin

C f

RCf

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125


Solutions to Ripple Currents

• Add more capacitors (ultra capacitor) on the

low-side dc bus

• Add more capacitors on the high-side dcbus

• Add one more stage DC-DC converter

• Add an active ripple cancellation circuit

– with a bidirectional DC-DC converter to

stabilize the high-side dc bus voltage

– with a built-in control function in the DC-DC

converter

JSL

126


Benchmark DC/DC Converter

Parameters for Ripple Study

• Input Voltage: 25V

• Output Voltage: 200V

• Input DC Capacitor: 6mF

• Output DC Capacitor: 2200mF

• Filter Inductor: 84mH

• Inverter Modulation Index: 0.86

• Inverter Load Resistor: 16.7

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127


Adding Hide-Side Energy Storages on

DC Bus Helps Reduce Ripple Current

0.25k

-50

0.1k

0 50m10m 20m 30m 40m

0.25k

-50

0.1k

0 50m10m 20m 30m 40m

Time (s)

20% ripple

current

10% ripple

current

DC/DC

Converter

DC bus

energy buffer

4-cycle energy backup

8-cycle energy backup

AC

Load120VFuel

Cell

JSL

128


Simulation Results with LV-Side Capacitor Input Capacitor has Very Little Effect to Current

Ripple Reduction

t(s)0 0.01 0.02 0.03 0.04 0.05

0

10

15

5

200

19080

40

0

20

25

(A)

(V)

(A)

(V)

HV DC

Current

HV DC

Voltage

LV DC

Voltage

LV DC

Current

t(s)0 0.01 0.02 0.03 0.04 0.05

0

10

15

5

200

19080

40

0

20

25

(A)

(V)

(A)

(V)

Input Cap Reduced to 136FInput Cap 6mF

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Output Capacitor can be Used as Passive

Solution to Current Ripple Reduction

– Cost is a Concern

t(s)0 0.01 0.02 0.03 0.04 0.05

0

10

15

5

200

19080

40

0

20

25

(A)

(V)

(A)

(V)

HV DC

Current

HV DC

Voltage

LV DC

Voltage

LV DC

Current

t(s)0 0.01 0.02 0.03 0.04 0.05

0

10

15

5

200

19080

40

0

20

25

(A)

(V)

(A)

(V)

Output Cap Reduced to 820FOutput Cap 2.2mF

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Current Ripple Reduction with High-SideEnergy Storage Capacitor

1 104

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.05

0.1

0.0068 0.055

Capacitance (F)

F u e l C e l l C u r r e n t R i p p l e ( p u )

0

Standard design

Adding 55 mF capacitor

10X capacitor is needed to drop

ripple current under 5%

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Features of High-Voltage DC Bus Energy

Storage Capacitors

• Fuel cell voltage is low, typically from 36 to 60 V.

• High voltage dc bus voltage is split into two 200 V.

• For a single-phase dual ac outputs such as 120/240 V in USresidential systems, the transformer secondary and dc bus

can be split in two halves. Each phase leg of the full-bridge

along with the split-capacitors becomes a half-bridge inverter

to supply 120 V ac output. Summing two 120 V outputs

becomes 240 V.

• Multiple capacitors are paralleled for the high-voltage dc bus

to store more energy and to provide more transient handling

capability during dynamic load change conditions. Energy

storage is proportional to CV2.

• Size and weight are dominated by passive components. With

split DC buses, the volume of energy storage capacitors

becomes an issue.

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Experimental Current Ripples withoutAdding Capacitors or Controls

Fuel cell voltage

(20V/div)

Fuel cell current

(10A/div)

AC Load Voltage (200V/div)AC Load Current (5A/div)

5ms/div

More than 35% ripple current at the input

35%

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Fuel Cell Ripple Current Problem is

Severe During Load Transients

• For single-phase ac loads, 120 Hz (twice the

fundamental frequency) current ripple can

reflect back to fuel cell.

• During load transients such as turning on

light bulbs or starting up motors, the

transient initial current is typically more than

5 times the steady-state current, and the fuel

cell ripple current exceeds more 100% or

even 200% in some cases.

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Fuel Cell Responds to AC Load Steps(without Externally Added Capacitor)

I fc

V fc

iac




AC Load Current (10A/div)

20ms/div

I d-LV

Fuel cell sees severe current transient (spike)

and current ripple in steady state (35%)

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Using Electrolytic Capacitor to Smooth

Fuel Cell Responding to AC Load Steps

I fc

V fc

iac

Fuel Cell Voltage (10V/div)Fuel Cell Current (20A/div)



20ms/div

I d-LV

I cap

I fc

iac

I capV fc

I d-LV

With 20 mF capacitor, fuel cell current transient

is reduced, but the current ripple remains

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AC Load Transient Response for FuelCell with 53-mF Added Capacitors

Fuel Cell Current Ripple is 5% plus Overshoot

V fc





20ms/div

I fc

iac

I cap

I d-LV

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Fuel Cell Output Response with an Ultra

Capacitor Cut-in

V fc




20ms/div

I fc

iac

I ulcap

Capacitor Current (20A/div)

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Fuel Cell Output Response with an UltraCapacitor Dropout

V fc




20ms/div

I fc

iac

I ulcap Capacitor Current (20A/div)

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Experiment with Open-Loop and with

only Voltage Loop Control

No improvement on current ripple reduction

with voltage loop control

DC bus voltage (100V/div)

Fuel cell voltage (10V/div)

Fuel cell current (20A/div)

DC bus current (10A/div)

(a) Open loop (b) With voltage loop control

t (5ms/div)

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Solving Current Ripple with AddedCurrent Loop inside the Voltage Loop

vref +

– Rv2C v1

C v2

Rv1

H v

v sense

V o

+

R L

Gvc

+

–

V m

PWMd

L f

C f

V d

i Lf +

–

Rcf +

– Ri2C i1

H i

iref

Ri1

i sense

C i2

Gic V d = dV in

Adding a current loop to regulate the output current

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Fuel Cell Current Ripple Reduction with

the Proposed Active Control TechniqueFuel Cell Current Ripple is Reduced to 2%

Input Voltage

Input Current

Output Current

Output Voltage

<2%

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Summary of V6 DC-DC Converter withActive Ripple Cancellation

• High efficiency with a wide-range soft switching: 97%

• Cost reduction by cutting down passive components

– Output inductor filter reduction with three-phase interleavedcontrol: 6X

– Input high frequency capacitor reduction: 6X

– Output capacitor reduction with active ripple reduction: 10X

• Reliability enhancement

– No devices in parallel

– Soft-start control to limit output voltage overshoot

– Current loop control to limit fuel cell inrush currents

• Significance to SOFC design

– Stack size reduction by efficient power conversion and ripplereduction: 20%

– Inrush current reduction for reliability enhancement

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10. Recap

1. Basic DC-DC converters and DC-AC inverters are

introduced

2. Circuit topology selection can be misled by schematicdiagram. Some important considerations are

• Device voltage and current stresses

• Number of paralleled devices

• Parasitic components and losses

3. Advanced V6 DC-DC converter not only shows superior

performance but also low production cost

4. Fuel cell current ripple issue can now be solved with

advanced current control developed by Virginia Tech

without adding cost penalty

vatech-lai 4-19

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