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JSL1
A High-Efficiency Low-Cost DC-DC Converter for SOFC
The First Introduction of V6 Converter
September 30 October 1, 2003
SECA Core Technology Program Review Meeting
Albany, New York
Presented byDr. Jih-Sheng (Jason) Lai
Virginia Polytechnic Institute and State UniversityFuture Energy Electronics Center
JSL2
Outline
• Introduction of V6 Converter• State-of-the-Art DC/DC Converter Technologies• Hardware Implementation of V6 Converter• Sensorless Control Design and Implementation• Fuel Cell Current Ripple Issue• Conclusion
JSL3
The Role of DC-DC Converter in an SOFC Power Plant
SOFC LoadDC-DCconverter
dc control
Inverter
DC Link
Vdc
Iaccommunication Controller Vac
Vfc
Low voltageDC 22-50V
HighVoltage
ac control
AuxiliaryPower
supplies
The DC/DC converter is the most crucial electrical interface to the fuel cell source
JSL4
Major Issues Associated with the DC-DC Converter
• Cost• Efficiency• Reliability• Ripple current• Transient response along with auxiliary
energy storage requirement• Communication with fuel cell controller• Electromagnetic interference (EMI) emission
JSL5
Why V6 Converter?Analogy to Automotives
• Six Phases (V6)• Mid power converters• >5kW
• Six Cylinders (V6)• Midsize vehicles• >2500 c.c.
• Single Phase • Low power converters• <1kW
• Single Cylinder • Motorcycle• <500 c.c.
Fuel Cell ConvertersAutomotives
JSL6
How to Categorize Converters?
• Single-Phase Converters– Half-Bridge Converter– Push-Pull Converter
• Two-Phase Converters– Full-Bridge Converter
• Three-Phase Converters• Four-Phase Converters• Six-Phase Converters
JSL7
Half-Bridge Converter A Single-Phase Converter
Q1
Q2
Vfc
Cin1
+
–
1 : 2n
Cin2
! Low device count! Low voltage device" Device sees twice current" Split capacitors" Twice transformer turns ratio
JSL8
Push-Push Converter A Single-Phase Converter
1 : n
Q1 Q2
Vfc
+
–
Cin
+ Low component count misleading when power level is high!+ Simple non-isolated gate drives+ Suitable for low-voltage low-power applications– Device sees twice input voltage high voltage MOSFET is needed
# High conduction voltage drop, low efficiency– Center-tapped transformer
# Difficult to make low-voltage high-current terminations # Prone to volt-second unbalance (saturation)
JSL9
A Commercial Off-the-Shelf 1-kW Push-Pull Converter for Fuel Cell Applications
Push-pull dc/dc converter
LoadDC
source
• Input 28 to 35 V • Device voltage blocking level 100 V • Efficiency <85% even with 4 devices in parallel
JSL10
Full-Bridge Converter A Two-Phase Converter
Vfc
Cin
+
–
1 : n
Q1 Q3
Q2 Q4
! Most popular circuit today for high-power applications# Soft switching possible# Reasonable device voltage ratings
" High component count from the look " High conduction losses
JSL11
List of Available Power MOSFETs for up to 50-V Fuel Cell Converters
Manufacturer Part Number VDSS (V) RDS-on (mΩ) Package Fairchild FDB045AN08A0 75 4.5 TO-263 International Rectifier IRFP2907 75 4.5 TO-247 Fairchild FDP047AN08A0 75 4.7 TO-220AB IXYS FMM 150-0075P 75 4.7 ISOPLUS i4-PAC* Vishay Siliconix SUM110N08-05 75 4.8 TO-263 IXYS IXUC160N075 75 6.5 ISOPLUS 220 International Rectifier IRF3808 75 7.0 TO-220AB Fairchild FQA160N08 80 7.0 TO-3P *Note: IXYS FMM 150-0075 is a dual pack (half bridge) device.
JSL12
Efficiency Consideration with Conventional Full Bridge Converter
• At 6kW fuel cell power output condition • Input voltage: 25 V • Input current: 240 A • Device conduction resistance = 4.5mΩ at 25°C and 9mΩ at 125°C• Device conduction voltage drop: Von = 9mΩ×240×2 = 4.32V• Maximum achievable efficiency: 82% (18% loss) • Minimum number of parallel devices to achieve 97% efficiency: 6$ Von = 0.72 V (assuming no parasitic and switching losses and no output stage losses)
• Minimum total number of devices to achieve 97% efficiency: 24$ Equivalently 12 phases are needed to achieve efficiency goal
JSL13
Full-Bridge Converter with Paralleled Devices to Achieve the Desired Efficiency
Voltage clamp
Load6x 6x
LfFuel cell
source
• With 6 devices in parallel, the two-leg converter can barely achieve 97% efficiency
• Problems are additional losses in parasitic components, voltage clamp, interconnects, filter inductor, transformer, diodes, etc.
JSL14
The V6 DC-DC Converter and the Associated the Output Stage DC-AC Inverter
rectifier/filter
isolation xformer
Isolated V6 DC-DC Converter
The
V6C
onve
rter 120V
120V
DC/AC Inverter
+
–
SOFC
!Low-voltage high-current input !High-voltage low-current output !Dual output dc voltages provide neutral point for grounding!Residual current in non-switching phases allows zero-voltage
switching at light load condition
JSL15
Photograph of a Three-Phase Hard-Switched DC-DC Converter
Gate drive board
Power board
Transformer
Rectifier board
DC chokeVoltage clamp
DSP board
Note: The power switch has four TO-263 devices in parallel
JSL16
Photograph of the Soft-Switched V6 DC-DC Converter
Controller chip
Full-bridge gate driver Power board
Transformer
Rectifier board
Note: The power switch is single device without paralleling to avoid parasitic losses
JSL17
Phase Shifted Controller Signals
VA
VB
VC
VAB
VBC
VCA
JSL18
Significance of Parasitic Components
Gate drive with twisted wire Direct plug in type
Potential Problems: • High frequency ringing and EMI • Additional transformer loss• Additional switching loss
JSL19
Voltage Overshoot and Ringing on Primary Side under Hard Switching Condition
Vds
Vgs
Vo
JSL20
Hard Switched Full-Bridge Converter Device Voltage Waveform at Lighter Load
Vds (10V/div)
At lighter load, for the primary side device voltage" Severe parasitic ringing remains " Resulting substantial EMI propagation
JSL21
Simulation Verification of Hard Switched Converter Voltage Overshoot and Ringing
Vds
Causes of voltage overshoot and ringing1. Circuit parasitic inductance2. Output capacitance of the device3. Transformer leakage inductance
JSL22
Conventional Full-Bridge Soft-Switched Converter Voltage and Current Waveforms
device voltage, Vds
voltage before LC filter, Vd
Inductor current, iL
device voltage
!Primary side soft switching without voltage overshoot and parasitic ringing
" Secondary side has high voltage swing from 0 to 2.5Vo andsevere ringing with single-phase operation
" High inductor current requires large inductor to smooth it out
JSL23
Voltage before LC filter, Vd
Phase-a device voltage, Vds
Phase-b device voltage, Vds
Inductor current
(20V/div)
Voltage and Current of the V6 Soft-Switched Converter
!Primary side soft switching without voltage overshoot and parasitic ringing
!Secondary inductor current is rippleless; and in principle, no dc link inductor is needed
!Secondary voltage swing is eliminated with <40% voltage overshoot
JSL24
Significant DC link Inductor Size Reduction as Compared to Full-Bridge Converter
Lf for full-bridge converter
Lf for V6 converter
With V6 converter, an effective 10x reduction in dc link filter inductor in terms of cost, size and weight
JSL25
Input and Output Voltages and Currents at 1kW Output Condition
Vin
Vo Iin
Io
(86%)
Vo
Iin
Vin
Io
(99%)
(b) V6 Converter(a) Full bridge converter
Significant improvement with multiphase converter!Less EMI!Better efficiency (97% versus 87% after calibration)
JSL26
Efficiency Measurement Results
80%82%84%86%88%90%92%94%96%98%
100%
0 1000 2000 3000 4000
70%
75%
80%
85%
90%
95%
100%
0 1000 2000 3000 4000 5000 6000
calculated efficiency
experimental data and prediction
Output power (W)
with reduced parasitics
Experimental data and trend line
• Measurement error: within 1%• Heat sink temperature rise:
<20°C at 2kW with natural convection
JSL27
Current Sensorless Control Design
• To regulate dc bus voltage, current loop may not be necessary. However, with added current loop the control response is faster, and the voltage regulation is more stable.
• The problem with adding a current control loop is the cost associated with the current sensor
• In this project, a novel current sensorless control is developed with superior performance to the conventional voltage loop control system
JSL28
DC Bus Voltage under 15% Load Step without Current Loop
Vdc(50V/div)390V
40ms/div
ILref
IL (28A/div)
Experimental results
(A)
0.0
20.0
40.0
t(s)
0.2 0.24 0.28 0.32 0.36 0.4
(A)
0.0
20.0
40.0
(V)
360.0
380.0
400.0
420.0(0.25996, 400.0)
(0.29146, 383.98)
(A) : t(s)
inductor current
(A) : t(s)
current command
(V) : t(s)
vdcDC bus voltage
Current command
Inductor current
Simulation results
• Without current loop, voltage fluctuates during load transients• Both simulation and experiment agree each other
JSL29
A Novel Current Sensorless Control
–
+
CCMDCM
VmD
Vref
Cm(s)Σ× 1/s
2
2222
),(o
oggg V
VnVDVnDVDk
−=
( )∫ −= dtVVL
i odf
L1
×
–
+ Σ
nVg
1/s
Vo
• A plant model based on known inductance and operating mode, continuous and discontinuous conducting mode (CCM and DCM)
• A simple lead-lag compensator is designed to achieve fast dynamic control without the need for the current sensor
JSL30
Uncompensated Loop Gain PlotPhase Margin = 2.6°
JSL31
Compensated Loop Gain PlotPhase Margin = 67.5°
JSL32
Load Dump with Sensorless Control
0
1
2
3
4
0 1 2 3 4 5 6
100
150
200
250
300
0 1 2 3 4 5 6
Load
Cur
rent
(A)
Out
put V
olta
ge (V
)
Time (ms)
No overshoot
JSL33
Load Step with Sensorless Control
0
1
2
3
4
0 1 2 3 4 5 6
100
150
200
250
300
0 1 2 3 4 5 6
Load
Cur
rent
(A)
Out
put V
olta
ge (V
)
Time (ms)
No undershoot
JSL34
Fuel Cell Current Ripple Issue
DC bus energy storage capacitor
• The inverter output 60 Hz current tends to reflect back to fuel cell source with 120 Hz ripple if there is not enough energy buffer in between.
• On the other hand, the auxiliary energy storage source tends to interact with the fuel cell source given unequal time constant between them.
FuelCell
Source
DC/DCConverter
2
-2
0
-1
1
0 5010 20 30 40
AC Load
0.2
-5
0.
0 5010 20 30 40
Auxiliary energy storage source
JSL35
Solving Current Ripple Problem with Additional Boost Converter
L
Q
D
x6
Multi-leg boost converter
Auxiliary energy storage
DC/DCConverterSOFC
• Fuel cell current ripple can be avoided by adding a boost converter in between dc/dc converter and auxiliary battery.
• The boost converter must be sized equally as the SOFC.• Main problem is additional power conversion losses and cost.
$ Other ways of energy buffering can be better solutions.
JSL36
Steady State Fuel Cell Current and Voltage Ripples with Inverter Load
60 Hz ac load voltage and current
Fuel cell voltage
Fuel cell current
• Significant 120 Hz voltage and current ripple present
JSL37
Fuel Cell Output Voltage During Load Dump
46474849505152
-0.04 0 0.04 0.08 0.12 0.16 0.2
Vol
tage
(V)
-15-10-505
1015
-0.04 0 0.04 0.08 0.12 0.16 0.2
Cur
rent
(A)
Time (s)
Inverter output current
Fuel cell output voltage
• Experiment with a 3-kW PEM fuel cell and a 3.3-F ultra capacitor.
• Use incandescent lamps as the load.
• Ultra cap smoothes the load transient effectively.
• Fuel cell time constant is reasonably fast, in millisecond range.
JSL38
Fuel Cell Dynamic Response During Single-Phase Motor Start-up Transients
48505254
-0.1 0 0.1 0.2 0.3
Vol
tage
(V)
0102030
-0.1 0 0.1 0.2 0.3
Cur
rent
(A)
-20-10
01020
-0.1 0 0.1 0.2 0.3
Cur
rent
(A)
Time (s)
Inverter output current
Fuel cell voltage
Fuel cell current
JSL39
Fuel Cell Current Ripple with Inverter Load30
0
10
20
0 50m10m 20m 30m 40m0.25k
-50
0.1k
0 50m10m 20m 30m 40m0.45k
00.1k
0.2k
0.3k
0 50m10m 20m 30m 40m0.4k
-0.4k
0
-0.2k
0.2k
0 50m10m 20m 30m 40m20
-20
0
-10
10
0 50m10m 20m 30m 40m
Time (s)
fuel cell current ripple (120 Hz)
fuel cell bus voltage (28 V)
per phase converter current ripple
DC Bus Voltage
Midpoint DC Bus Voltage240V
120Va120Vb
i-240
i-120a
i-120b
3kW-240V1kW-120Va1.44kW-120Vb
JSL40
Adding Energy Storages at DC Bus is Another Alternative
0.25k
-50
0.1k
0 50m10m 20m 30m 40m0.25k
-50
0.1k
0 50m10m 20m 30m 40mTime (s)
20% ripple current4-cycle energy backup
8-cycle energy backup 10% ripple current
DC bus energy buffer
Adding energy storage capacitors at the DC bus not only smoothes out the inverter load transient, but also reduces fuel cell current ripple proportionally.
DC/DCConverter
JSL41
Summary
A V6 DC-DC converter has been successfully developed and tested to demonstrate– Soft switching over a wide load range– High efficiency ~97% – Low device temperature $ High reliability– No overshoot and ringing on primary side device voltage– DC link inductor current ripple elimination $ cost and size
reduction on inductor– Secondary voltage overshoot reduction $cost and size
reduction with elimination of voltage clamping– Fast dynamic response with sensorless control $ cost
reduction on current sensor– Significant EMI reduction $ cost reduction on EMI filter
JSL42
Future Work
• Test V6 converter with SOFC• Define fuel cell and converter interface • Develop interface and communication protocol • Design package for the beta version • Develop energy balancing strategy • Facilitate Standardization!
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