vatech-lai 4-19
TRANSCRIPT
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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
dead-time, currentcirculating thru
anti-paralleled diodes
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
dead-time, currentcirculating thru
anti-paralleled diodes
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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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
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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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6. Fuel Cell and Converter
Interactions
• Static modeling• Dynamic modeling
• Fuel cell dynamic response with and without
converters
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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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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
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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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Fuel Cell Modeling with Electrical Circuit
load
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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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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
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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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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
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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
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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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Fuel Cell Responds with a Paralleled
10mF Electrolytic Capacitor
I fc
V fc
I cap
Fuel Cell Voltage (10V/div)
Fuel Cell Current (20A/div)
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
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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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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
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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
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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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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)
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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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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
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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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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
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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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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
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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
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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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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
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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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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
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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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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
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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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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
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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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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
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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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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
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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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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
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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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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.
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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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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
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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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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
Single Phase500W60Hz
Single Phase5 kW50kHz
Three Phase5 kW50kHz
Single Phase500W60Hz
Single Phase5 kW50kHz
Three Phase5 kW50kHz
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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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Where are the Losses?
• Switch conduction
• Diode conduction
• Transformer
• Output rectifier
• Output filter inductor and capacitor
• Input capacitor
• Parasitics
– Copper traces
– Interconnects
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Calorimetry for Accurate LossMeasurement
Calibration with resistor bank
fans
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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
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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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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
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The Beta Version Prototype Converter
• Schematic Circuit Diagrams
• V6 Cost Estimate
• Summary of Beta Version Prototype
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Schematic Circuit Diagrams
Power boardGate drive board
Control board
Digital board
Interface board
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Photographs of V6-Converter Together with DC-AC Inverter Prototype
Front View Rear View
Converter Inverter
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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
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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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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
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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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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
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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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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
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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
Fuel Cell Voltage (10V/div)
Fuel Cell Current (10A/div)
DC Link Current (25A/div)
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)
DC Link Current (25A/div)
AC Load Current (10A/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
Fuel Cell Voltage (10V/div)
Fuel Cell Current (20A/div)
DC Link Current (25A/div)
AC Load Current (10A/div)
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
Fuel Cell Voltage (20V/div)
Fuel Cell Current (20A/div)
AC Load Current (5A/div)
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
Fuel Cell Voltage (20V/div)
Fuel Cell Current (20A/div)
AC Load Current (5A/div)
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