BoP: Electrical Conversion & Connection
DC/DC and DC/AC converters in grid
interfacing
Vesa Väisänen
LUT?
− Lappeenranta University of
Technology
− Established in 1969
− Located in Lappeenranta,
South Carelia, Finland
− Faculty of Technology
− Faculty of Technology
Management
− School of Business
− Number of students ~ 5000
− Number of staff ~ 929
Our project
− Project started in 2007
− 1 professor and 3 researchers
− Partners in co-operation: ABB, Wärtsilä, VTT
Objectives
−
−
−
−
−
Feed the energy from a SOFC stack into electric grid
High efficiency (>95 %)
Reliability
Manufacturability and price
Paying attention to the fuel cell characteristics
Fuel Cell
DC
Current
reference
Low voltage
DC-link
DC/DCconverter
Grid
converter
DC-link
Grid filter
Prototype testing at VTT, results
−
−
−
−
10 kW Power conversion unit successfully integrated to a SOFC system at VTT
Operated over 3000 h
Grid connection is done with ABBs grid converter
Measured losses for power electronics (DC/DC + DC/AC) were 1.1kW,
corresponding to about 43% of total system losses [1].
Requirements
− The requirements for a power conversion unit arise from three major
sources:
• Fuel cell (or any other power source)
• The supplied load or network
• General requirements such as economical constraints,
efficiency requirements, expected operating life, standards,
patents…
GENERAL
REQUIREMENTS
Fuel
Cell
FUEL CELL
REQUIREMENTS
Power
Electronics
Load /
Network
LOAD
REQUIREMENTS
Fuel cell requirements
Ideal voltage
1.0
Region of activation losses
Total loss
Region of gas transport losses
0.5
Region of Ohmic losses
0
Current density (mA/cm2)
Effect of high-frequency current ripple on inductor costs (an example)
6,0
5,5
5,0
Relative inductor cost
Cell voltage
− Fuel cell voltage drops as a
function of current density 
need for voltage regulation
− Current reference must be
accurately followed to avoid
stack overloading  need for
accurate current control
− Low frequency current ripple
must be low to avoid process
oscillation and overloading 
ripple mitigation by the controller
− Effects of long term high
frequency (> 10 kHz) ripple still
unclear?  the lower the
allowed ripple, the more
expensive the filter
4,5
250 A/cm^2
4,0
300 A/cm^2
350 A/cm^2
3,5
400 A/cm^2
500 A/cm^2
3,0
2,5
2,0
1,5
1,0
1%
2%
3%
4%
HF current ripple %
5%
6%
Fuel cell requirements
− The voltage produced by the fuel cell stack can be low (for example 40-60
V), but the DC/AC converter requires a higher input voltage depending on
number of phases and the modulation method:
• One-phase (230 V) Voltage Source Inverter (VSI) 
VDC-link > 255 V (for preferred linear modulation ≥ 325 V)
• Three-phase (400 V) VSI with Space Vector PWM 
VDC-link > 628 V (for preferred linear modulation ≥ 693 V)
 Need for considerable voltage boost
− Fuel cell has high electrical efficiency, so high efficiency is desired also
from the power conversion unit to maintain high overall efficiency 
converter topology and component selection
How to interface a fuel cell?
− Most DC loads require
regulated DC voltage.
Therefore the DC/DC
converter is typically essential.
− Galvanic isolation with a
transformer is preferred for
safety reasons and for voltage
boosting.
− High frequency transformer on
DC side is much smaller than
a low frequency transformer on
AC side.
− For example a 10 kVA, 50 Hz
commercial transformer can
weigh 72 kg, while a 50 kHz
transformer weighs about 2 kg!
Fuel cells
Unregulated
DC voltage
Regulated
DC voltage
DC/DC
converter
3-phase ACvoltage
DC/DC
converter
DC/AC
converter
DC/AC
converter
Low AC-voltage
50/60 Hz
Non-isolated DC/DC converters
Boost converter topology
− Simple, non-isolated topology for voltage step-up.
•
•
•
•
Inductor L1: stores energy and
limits the input current rate of
change
Transistor S1: acts a switching
element
VDC
Diode D1: allows inductor
current to flow to load while
transistor S1 is closed and
prevents current flow from load
to input.
Capacitor C1: feeds energy to
load while transistor S1 is
conducting.
L1
D1
S1
C1
Vout
Non-isolated DC/DC converters
Boost converter operating principle
vL
L1
vL
iL
VDC
C1
S1
ton
toff
ton
VDC
Vout
t
Circuit during ton
VDC-Vout
L1
vL
D1
iL
iL
C1
VDC
Vout
t
Circuit during toff
Ideal relation between input and output voltage is
where
D
ton
ton  toff
Vout ton  toff
1


VDC
toff
1 D
Non-isolated DC/DC converters
Interleaving of Boost converters
− The basic boost converter is often scaled to higher power levels by
paralleling two or more boost converters.
− The stages are controlled in opposite phases (the transistors do not conduct
at the same time), so the total input current ripple is reduced compared to a
single converter.
L1
D1
S1
VDC
L2
D2
S2
C1
Vout
Isolated DC/DC converters
Phase-shifted full-bridge
− Very common topology capable of zero voltage switching  low switching
losses in primary transistors.
− Suitable for higher input voltages.
S
0.5T-tsafe
tsafe
1
S3
S1
D1
Lout
S4
D4
S4
Llk
a
VDC
D5
c
Cin
b
d
D6
S3
D7
D3
S2
D2
D8
Cout Rload
S2
DT
Vab
ILout
ILlk
DeffT
Vcd
t0 t1 t2 t3 t4
Isolated DC/DC converters
Full-bridge boost
− High voltage conversion ratio
− Low input current ripple even without
input capacitors.
− Low inrush current.
Mode1 Mode2
t0 t1
t2
Mode 3 Mode 4
t5
t4
S1 & S2
on
off
S3 & S4
off
on
BTX1
ΔIL1
IL1
L1
Vsec
S1
S3
D1
D3
Llk
ΔIL1/n
Cout Rload
VDC
Isec
TX1
S4
S2
D2
D4
ID1
ΔIL1
IL1/2
IS1
DT
(1-D)T
T
Isolated DC/DC converters
Resonant push-pull boost
− Twice the voltage conversion ratio
compared to full-bridge boost
− Low input current ripple even without
input capacitors.
− Low inrush current.
− Near sinusoidal current waveforms and
zero current switched secondary.
Cc1
S3
S4
L1
VDC
DS1
S2
ipri2
DS2
S1
on
off
S2
off
on
S3
off
on
S4
on
off
IL1
D1
IS4
Cr1
Llk
-IL/2
N1
S1
Mode 4 Mode 5 Mode 6
t4
t5 t6
IS1
DS4
ipri1
N1
Mode 3
t2 t3
IL/2
Cc2
DS3
Mode1 Mode2
t0
t1
Cout Rload
N2
D2
Isec
Cr2
DT
(1-D)T
Isolated DC/DC converters
Sources of power losses
− The losses in switching converters can be divided into three categories:
• Conduction losses
• Switching losses
• Core losses in magnetic components
− The dominating loss mechanism depends on the voltage and current as well
as converter topology (capability of zero voltage or zero current switching
etc.)
− As a rule of thumb:
• Low voltage, high current  conduction losses dominate
• High voltage, low current  switching and core losses dominate
• High voltage, high current  depends strongly on the converter design
Isolated DC/DC converters
Conduction losses
− Conduction losses are caused by the conductor resistances and the intrinsic
resistances in semiconductor junctions.
− Dissipated power P is the product of resistance R and the current I squared.
P  RI
2
− Example: We have a 10 kW converter and two different stack voltages: 50 V
and 250 V. Let us assume that both converters have 3 mΩ of resistance in
the primary circuit.
50 V  I = 200 A  P = 0.003*2002 = 120 W
250 V  I = 40 A  P = 0.003*402 = 4.8 W
− There is a 96% reduction in conduction losses, when the input voltage
changes from 50 V to 250 V!
Isolated DC/DC converters
Transistor switching losses
− Switching losses arise from two major sources:
• Overlapping of current and voltage during switching
• Charging/discharging of parasitic capacitances in components
Hard switching
− In ZVS the transistor body
diode conducts before gate
voltage is applied.
− Voltage across the transistor
is limited to body diode
forward voltage during diode
conduction.
− There is no Miller plateau in
the gate-source voltage and
thus the gate drive losses are
also decreased.
vGS
Zero voltage switching (ZVS)
vMiller
vGS
vGS(th)
vGS(th)
t
t
iDS
iDS
t
vDS
t
vDS
t
t
PSW
PSW
t
t
Isolated DC/DC converters
Transistor switching losses
Ld
Cgd
Cds
Cgs
Ls
120V
100
100V
80
60
50V
40
20
0
0V
-20
-20V
248.7607ms
248.7700ms
V(X_SLPS.va)
248.7800ms
248.7900ms
0.5
1
1.5
2
2.5
3
3.5
4
Time
Switching with ideal MOSFET
No EMI, minimal losses
x 10
Switching with unideal MOSFET
Increased EMI and losses!
Isolated DC/DC converters
Diode switching losses
− Switching losses in diodes are caused by forward recovery and reverse
recovery phenomena, as a diode requires a finite time to switch from
conducting state to non-conducting state and vice versa.
− Forward recovery loss is typically small compared to reverse recovery loss.
− Charge Qrr must be swept away from the junction during the recovery time trr.
− Voltage VR and current IRM behavior during the off-transition defines the
switching losses.
− Silicon Carbide (SiC) diodes do not experience reverse recovery effects.
− Voltage dependent junction
capacitance Cj causes additional
switching losses also in SiC diodes.
[2]
1
Psw  QrrVr f sw  C jVr f sw
2
Isolated DC/DC converters
Magnetic component core losses
− Magnetic field strength H is related on current I flowing through N turns of
conductor surrounded by a magnetic core having a magnetic path length of
lm.
− The flux density B in a magnetic material depends on the material
permeability µ and the magnetic field strength H.
H
NI
lm
B  H
− Flux density B can be plotted as a function
of H to form a hysteresis loop.
− The loop shape depends on the core
material.
− The area inside the loop is the energy
dissipated in the core material.
Isolated DC/DC converters
Magnetic component core losses
− Core losses depend on the difference between the maximum and minimum
flux density (ac flux). The larger the ac flux, the larger the losses.
− The higher the operating frequency, the higher the core loss at certain ac flux.
− In transformers there is a trade-off between the number of turns (conduction
losses) and the core losses. An optimal design is found near the point where
winding losses and core losses intersect.
Isolated DC/DC converters
Examples of loss distributions
− Example loss distributions are given for a 3 kW full-bridge boost [3] and a 10
kW resonant push-pull converter [4].
− The component stresses are dependent on the input/output parameters,
selected topology and component optimization!
FB boost loss distribution example
Total loss 100 W, efficiency 96.7%
RPP loss distribution example
Total loss 663 W, efficiency 93.4%
Capacitors
0%
Diodes
12 %
Misc
14 %
MOSFETs
38 %
Diodes
30 %
Inductive
components
23 %
MOSFETs
65 %
Inductive
components
18 %
Isolated DC/DC converters
Examples of prototype costs
− In modular converters the cost of auxiliary components may be higher in
proportion than in single unit converters.
− Magnetic components can be smaller and cheaper in modular systems, but it
is easier to achieve higher efficiency with larger components.
− Semiconductor efficiency is typically much better in modular converters due to
smaller currents.
Cost distribution in a 10 kW RPP converter
Total cost € 827
Control
17 %
Control
21 %
Magnetics
30 %
Capacitors
12 %
Cooling
13 %
Cost distribution in 2 x 5 kW RPP converter
Total cost € 916
Magnetics
21 %
Capacitors
7%
Semiconductors
28 %
Cooling
16 %
Semiconductors
35 %
DC/DC converters
Bidirectional converters
− Process control backup powering is often implemented with UPS systems
connected to the grid side.
− In emergency shutdown the excess stack power is dissipated in resistors.
− Bidirectional DC/DC converters can interface the fuel cell to battery packs,
that act as small time constant energy storages.
− Some of the stack energy could be recovered also during shutdown.
Resistor bank
FC stack
FC stack
Grid
Grid
Hydrogen
Hydrogen
Oxidant
Oxidant
a
n
o
d
e
E
L
E
C
T
R
O
L
Y
T
E
c
a
t
h
o
d
e
a
n
o
d
e
DC/DC
DC/AC
E
L
E
C
T
R
O
L
Y
T
E
c
a
t
h
o
d
e
Unused
hydrogen
Unused
hydrogen
DC/DC
DC/AC
Air, heat
and water
Air, heat
and water
Battery pack
Resistor bank
DC/DC
BoP
BoP
UPS
Plant
controller
DC/AC
Plant
controller
DC/DC converters
Summary
− A DC/DC converter is an essential component in the power supply chain,
unless the voltage levels between the power source and the load are
directly compatible.
− It is more efficient to transfer certain power with high voltage and low
current than vice versa.
− If galvanic isolation is not needed for safety or voltage step-up, the
conversion efficiency is likely to increase and less complex converter
topologies can be used.
− If the fuel cell output has a high tolerance for high frequency ripple (> 10
kHz) the DC/DC converter input filter requirements can be less stringent 
smaller, cheaper and more efficient components.
− Higher efficiency often results in higher initial costs, so the total cost
efficiency is dependent on the projected system life time.
DC/AC converters
Single phase topologies
Vd
2
Vd
TA+
C+
Vd
2
TA+
C+
A
o
VAo
TB+
A
Vd
o
VAo-VBo
B
Vd
2
C-
TAN
− Half-bridge inverter
− Simple structure and control
− Output peak voltage is ma *
Vd/2, where ma is the
modulation index (ma ≤ 1 in
the linear region) [5]
Vd
2
C-
TA-
TB-
N
− Full-bridge inverter
− Output peak voltage is ma * Vd,
where ma is the modulation index
(ma ≤ 1 in the linear region)
− Bit more complex than the one-leg
inverter
There are lots of other variants too especially in wind and solar
applications!
DC/AC converters
Single phase modulation methods
− Bipolar PWM [5]
− Half-bridge and full-bridge
inverter
− Unipolar PWM [5]
− Only full-bridge inverter
− Lower harmonic content
DC/AC converters
Three phase topologies
− Able to supply all three phase-loads such as motors or electric grid.
− Can be implemented either as voltage source inverter (VSI) or current
source inverter (CSI).
− CSI converters are able to boost voltage from input to output.
− Input inductor in CSI reduces the ripple current taken from the source.
L1
TA+
Vd
2
TA+
C+
TB+
TB+
TC+
TC+
A
Vd
B
o
A
B
C
Vd
C
Vd
2
C-
TA-
TB-
TA-
TC-
TB-
N
VSI
N
CSI
TC-
DC/AC converters
Three-phase modulation methods
− Three-phase PWM for VSI
− Triangular wave is compared with
sinusoidal waveforms that are 120°
out of phase.
− With linear modulation (ma ≤ 1) the
maximum line-to-line rms voltage is
3
2 2
maVd  0.612maVd
− The maximum obtainable line-to-line
rms voltage with overmodulation is
6

Vd  0.78Vd
[5]
DC/AC converters
Three-phase modulation methods
− Space vector PWM for VSI
− Eight discrete voltage vectors based
on the logic states of power switches.
− Other voltage vectors in a sector can
be produced by using the active
vectors and zero vectors for a certain
time during the switching period Ts.
− Maximum radius of the red circle
(linear region) is
Vd
3
 0.577Vd
− Theoretical maximum output voltage is
2

Vd  0.637Vd
[6]
DC/AC converters
Multilevel converters
− In two-level inverters the available
voltages at output are Vd and –Vd.
− By adding levels to the inverter, more
output voltages can be produced
(diode-clamp multilevel converter).
− A three-level inverter could provide
also the neutral voltage N.
− Additional voltage levels reduce the
harmonic distortion, so a filter could be
omitted.
− Other types of multilevel converters
are flying capacitor converters and
cascaded converters with separate DC
sources [7].
Vd
2
C+
S1
S3
S5
S2
S4
S6
A
B
C
Vd
N
Vd
2
S’1
S’3
S’5
S’2
S’4
S’6
C-
DC/AC converters
Losses in a VSI inverter
− Loss example of a 10 kW application with Vd = 700 V and fsw = 6 kHz [8].
− IGBTs having larger rated current exhibit smaller conduction losses (smaller
junction resistance) but larger switching losses (slower switching).
− Typical VSI power losses range between 1-2% of rated power (depending
on the operating point).
− Galvanic isolation or grid filter cause additional losses (typically few percent
of rated power).
SKiM120GD176D, rated current 120 A
Total losses 168 W --> Efficiency 98.3%
Diode
conduction
3%
Diode switching
24 %
Conduction
23 %
Switching
50 %
SK35GD126ET, rated current 35 A
Total losses 128 W --> Efficiency 98.7 %
Diode switching
12 %
Diode conduction
5%
Switching
47 %
Conduction
36 %
DC/AC converters
Summary
− DC/AC converter converts DC voltage to grid frequency AC voltage.
− The required DC link voltage depends on the converter topology and the
modulation method.
− Linear modulation requires higher DC link voltage than overmodulation, but
with linear modulation the output voltage has less harmonics and thus the
waveform is closer to pure sine.
− The better the voltage quality, the smaller and more efficient filters can be
used.
− DC link voltage and switching frequency can often be adjusted in
commercial inverters. The selection is a trade-off between voltage quality
and switching losses.
System interconnection
Process signaling
−
−
−
−
−
Case LUT & VTT
If electrical grid is OK, inverter charges the DC link.
DC/DC initializes and activates PCU OK signal.
If DC/DC is OK  current reference is set  PCU ON signal is activated.
Inverter active signal is activated  inverter running signal is received.
PCU ON, Current reference
(from PLC)
2
Inverter active
Control unit
PCU OK (to PLC)
Inverter running
30-70 V
VDC
10 kW
Isolated
DC/DC
660-700 V
ABB ACSM-204AR016A Regen Supply
Module
230/400 V
Inverter running
Inverter active
System interconnection
Control of DC/DC converter
− Reference current is given from the fuel
cell plant controller.
− Actual current is measured from the
converter input.
− The error between the reference and the
measurement is fed to a current
controller.
− The current controller increases or
decreases the converter duty cycle in
order to force the current error to zero.
− Attention is paid to mitigation of the 150
Hz grid harmonic.
[9]
System interconnection
Control of DC/AC converter
− Outer control loop controls the DC link
voltage to maintain the power balance of
the system.
− Voltage controller gives a d-axis current
reference to the current controller.
− Current controller compares the current
reference to measured values and forces
the error to zero.
− The output of the current controller is a dq voltage reference.
− The d-q voltage reference is transformed
into α-β reference and given to the
modulator together with phase angle.
− The modulator produces the switching
vectors for the DC/AC power stage.
[9]
System interconnection
Coordinate transforms
− Three phase grid voltages and currents are transformed into 2-dimensional
rotating coordinates (d-q) through Clarke and Park transforms.
[10]
System interconnection
Control overview
− DC/DC controller controls only the input current with as small low frequency
ripple and steady-state error as possible.
− DC/AC converter maintains power balance by keeping the DC link voltage
constant.
[9]
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
Halinen, M., et al. (2011). Performance of a 10 kW SOFC demonstration unit. ECS Transactions, 35, pp.
113-120.
Walters, K. (n.d.). Rectifier reverse switching performance. MicroNote Series 302, Tech. Rep. Microsemi.
Nymand, M. and Andersen, M.A.E. (2009). New primary-parallel boost converter for high-power high-gain
applications. In: Applied Power Electronics Conference (APEC), 2009, pp. 35-39.
Väisänen, V., Riipinen, T., Hiltunen, J., and Silventoinen, P. (2011). Design of 10 kW resonant push-pull DC-DC
converter for solid oxide fuel cell applications. In: Proceedings of the 14th European Conference on Power
Electronics and Applications (EPE 2011).
Mohan, N., Robbins, W.P., and Undeland, T.M. (2003). Power Electronics: Converters, Applications and Design,
Media Enhanced Third Edition, 3rd ed. John Wiley & Sons.
Sarén, H. (2005). Analysis of the voltage source inverter with small dc-link capacitor. Lappeenranta University of
Technology.
Lai, J.-S. and Peng, F.Z. (1996). Multilevel converters – a new breed of power converters. IEEE Transactions on
Industry Applications, 32(3), pp. 509-517.
Semikron SemiSel thermal calculator and simulator. url: http://www.semikron.com.
Riipinen, T. (2012). Modeling and control of the power conversion unit in a solid oxide fuel cell environment, D.Sc.
thesis. Lappeenranta: Acta Universitatis Lappeenrantaensis. In peer review.
Ross, D., Theys, J., and Bowling, S. (2007). Using the dsPIC30F for vector control of an ACIM. Application note
AN908. Microchip Technology Inc. url: http://ww1.microchip.com/downloads/en/AppNotes/00908B.pdf
Thank you! Any questions?