Performance Improvement Method for the
Voltage-Fed qZSI with Continuous Input Current
Dmitri Vinnikov1, Indrek Roasto1, Ryszard Strzelecki2, Marek Adamowicz2
1
Department of Electrical Drives and Power Electronics, Tallinn University of Technology (Estonia)
2
Department of Ship Automation, Gdynia Maritime University (Poland)
1
dm.vin@mail.ee, 2 rstrzele@am.gdynia.pl
Abstract-This paper proposes the performance improvement
method for the voltage-fed continuous input current quasiimpedance source inverter (qZSI) by the introduction of the twostage quasi-Z-source network (qZS-network). The two-stage qZS
is derived by the adding of one diode, one inductor and two
capacitors to the traditional qZSI. The proposed two-stage qZSI
inherits all the advantages of traditional solution (voltage boost
and buck functions in a single stage, continuous input current
and improved reliability). Moreover, the proposed solution
features over the 30% shoot-through duty cycle reduction for the
same voltage boost factor and component stresses as compared to
conventional qZSI. Theoretical analysis of the two-stage qZSI in
shoot-through and non-shoot-through operating modes is
presented. The design guidelines for the two-stage qZS-network
based step-up DC/DC converter are provided. A prototype has
been built to verify the theoretical assumptions. The simulation
and experimental results are presented and discussed.
I.
INTRODUCTION
The voltage-fed Z-source inverter (ZSI, Fig. 1a) has been
reported suitable for different renewable power applications
(fuel cells, solar panels, wind power generators, etc.) because
of the unique capability of voltage boost and buck functions in
a single stage [1-3]. If necessary, the ZSI can boost the input
voltage by introducing a special shoot-through switching
state, which is the simultaneous conduction (cross conduction)
of both switches of the same phase leg of the inverter. This
switching state is forbidden for the traditional voltage source
converters (VSI) because it causes the short circuit of the dc
link capacitors. In the ZSI, the shoot-through states are used to
boost the magnetic energy stored in the dc side inductors L1
and L2 without short-circuiting the dc capacitors C1 and C2.
This increase in inductive energy in turn provides the boost of
voltage seen on the inverter output during the traditional
operating states of the inverter. If the input voltage is high
enough the shoot-through states are eliminated and ZSI begins
to operate as traditional VSI.
The voltage-fed ZSI has such a significant drawback as
discontinuous input current during the shoot-through (boost
conversion) mode. To cope with the problem the voltage-fed
quasi-Z-source inverter (qZSI) with continuous input current
was introduced [4-6] as a modification of a currently popular
voltage-fed Z-source inverter (ZSI). This qZSI could be
derived from ZSI simply by the changing the location of the
input voltage source (Fig. 1b). The voltage-fed qZSI features
all the advantages of the ZSI, moreover, it ensures the
978-1-4244-5794-6/10/$26.00 ©2010 IEEE
continuous input current as well as lower operating voltage of
the capacitor C2, as compared to the ZSI topology.
This paper discusses the performance improvement method
for the voltage-fed qZSI with continuous input current by the
introduction of the two-stage quasi-Z-source network (qZSnetwork). The two-stage qZS-network is derived by the
adding of one diode (D2), one inductor (L3) and two
capacitors (C3 and C4) to the traditional qZSI, as shown in
Fig. 1c. By the implementation of the proposed two-stage
qZS-network the duty cycle of the shoot-through state could
be sufficiently decreased for the same voltage boost factor and
component stresses as compared to traditional qZSI. Due to
decreased shoot-through duty cycle the values of inductors
and capacitors of the qZS network could also be decreased.
From other hand, for the same component ratings and voltage
and current stresses the qZSI with proposed two-stage qZSnetwork will ensure the higher voltage boost factor as
compared to traditional solution.
Fig. 1.Voltage-fed ZSI (a), voltage-fed qZSI with continuous input current (b)
and voltage-fed qZSI with two-stage qZS-network (c)
1459
II. VOLTAGE-FED QZSI-BASED STEP-UP DC/DC
CONVERTER WITH TWO-STAGE QZS-NETWORK
This paper is generally focused on the power conditioning
units for residential power systems with fuel cells. For the
interconnection of a low DC voltage producing fuel cell
(typically, 40...80 V DC) to the residential loads (typically,
230 V AC single phase or 3x400 V AC), a power conditioning
unit (special voltage matching converter) is required. Due to
safety and dynamic performance requirements, the interface
converter should be realized within the DC/DC/AC concept.
This means that low voltage from the fuel-cell first passes
through the front-end step-up DC/DC converter with the
galvanic isolation; afterwards the output DC voltage is
inverted in the three-phase inverter and filtered to comply
with the imposed standards and requirements (second DC/AC
stage). This paper presents a brand new approach to the frontend step-up DC/DC converters with the high voltage gain.
The topology proposed (Fig. 2) utilizes the voltage-fed qZSI
with two-stage qZS- network and continuous input current
drawn from the fuel cell, high-frequency step-up isolation
transformer and rectifier-filter assembly. Although the
operation principle of the transformer-rectifier stage of the
converter remains the same as for traditional isolated fullbridge converters [7-9], the proposed voltage-fed qZSI with
two-stage qZS-network provides a new approach to the
integrated boost-buck converters with increased voltage gain.
operating state); afterwards the isolation transformer is being
supplied with the voltage with a constant amplitude value
(active state). In the proposed shoot-through PWM control
method the shoot-through states (cross conduction of both
switches of the same phase leg of the inverter) are created by
the overlap of active states as shown in Fig. 4. In order to
generate the shoot-through states, two reference signals (Up
and Un) were introduced (Fig. 4c). If the triangle waveform is
greater than Up or lower than Un, the inverter switches turn
into the shoot through state (Fig. 4c). During this operating
mode the current through inverter switches reaches its
maximum. The voltage across the inverter bridge (UDC)
during shoot-through states drops to zero (Fig. 4d) and the
resulting primary winding voltage waveform of the isolation
transformer is presented in Figs. 4e.
The operating period in this control methodology consists
of a shoot-through state tS and an active state tA:
T = t A + tS .
(1)
The (1) could also be represented as
t A tS
+ = DA + DS = 1 ,
T T
(2)
where DA and DS are the duty cycles of an active and shootthrough states, correspondingly.
The unique two-stage qZS-network connected to the
inverter bridge protects the circuit from damage when the
shoot-through occurs and also boosts the DC-link voltage. The
equivalent circuit of the two-stage qZSI in shoot-through
states is presented in Fig. 3b.
Fig. 2. Proposed isolated DC/DC converter based on voltage-fed qZSI with
two-stage qZS-network and continuous input current
To regulate the varying fuel cell voltage the front-end qZSI
has two different operating modes: the shoot-through and nonshoot-through. In the non-shoot-through mode the qZSI
performs only the voltage buck function. This operation mode
is typically used during the light load conditions, when the
output voltage of the fuel cell reaches its maximum. The
inverter is controlled in a same manner as traditional VSI
utilizing only the active states, when one and only one switch
in each phase leg conducts. The transistors in full-bridge
configuration are controlled alternately in pairs (T11 and T14
or T12 and T13) with 1800 phase shifted control signals. The
equivalent circuit of the two-stage qZSI during active (nonshoot-through) states is presented in Fig. 3a.
When the input voltage of the converter drops below some
predefined value the qZSI starts to operate in the shootthrough mode performing both, the voltage boost and buck
functions. Thus, the varying input voltage is first preregulated
by adjusting the shoot-through duty cycle (shoot-through
(a)
(b)
Fig. 3. Equivalent circuits of the two-stage qZSI: during active (non-shootthrough) state (a) and during shoot-through state (b)
1460
U C 3 = U IN
DA
;
D A − 2 DS
(12)
U C 4 = U IN
DS
.
D A − 2 DS
(13)
Based on the control method presented the one switching
period is a sum of shoot-through state time tS and an active
state time tA By replacing DA by 1-DS the Eqs. (10)-(13) could
be overwrited as
U C1 = U IN
1 − 2 DS
;
1 − 3 DS
(14)
U C 2 = U IN
2 DS
;
1 − 3 DS
(15)
U C 3 = U IN
1 − DS
;
1 − 3 DS
(16)
U C 4 = U IN
DS
.
1 − 3 DS
(17)
Fig. 4. Generation of shoot-through by the overlap of active states
III. CIRCUIT ANALYSIS OF THE VOLTAGE-FED QZSI
TWO-STAGE QZS-NETWORK
WITH
In the non-shoot-through mode the inverter bridge viewed
from the DC side is equivalent to a current source (Fig. 3a).
From Fig. 3a for the active states the voltage of the inductors
could be represented as
u L1 = U IN − U C1 ;
(3)
u L 2 = U C 4 − U C 2 ; u L 2 = U C1 − U C 3 ;
(4)
(5)
(7)
u L3 = U C 3 .
(8)
2 DS
;
D A − 2 DS
(18)
1 − B −1
.
3
(20)
In comparison with the traditional qZS-network based on
two capacitors and two inductors (single-stage network,
Fig. 1b) the proposed two-stage qZS-network features 33.3%
smaller shoot-through duty cycle for the same voltage boost
factor B (Fig. 5). In other words, the time of shoot-through
states could be decreased by 33.3% for one switching period.
(9)
(11)
(19)
For the desired input voltage boost factor B the duty cycle
of the shoot-through could be calculated as:
where DS and DA are the duty cycles of shoot-through and
active states per one switching cycle, respectively.
Thus
D A − DS
;
(10)
U C1 = U IN
D A − 2 DS
U C 2 = U IN
u DC
1
;
=
U IN 1 − 3DS
DS =
Let’s consider that duty cycles of an active and shootthrough states are the DA and DS, correspondingly. At steady
state the average voltage of the inductors over one switching
period is zero. From (3) to (8), we can obtain:
⎧U L1 = u L1 = DA (U IN − U C1 ) + DS (U IN + U C 2 ) = 0
⎪
⎪U L 2 = u L 2 = DA (U C 4 − U C 2 ) + DS (U C 4 + U C1 ) = 0
,
⎨
⎪U L 2 = u L 2 = DA (U C1 − U C 3 ) + DS (U C 4 + U C1 ) = 0
⎪⎩U L 3 = u L 3 = DA (− U C 4 ) + DS (U C 3 ) = 0
B=
(6)
u L 2 = U C 4 + U C1 ;
1
;
1 − 3 DS
The resulting boost factor B of the input voltage is:
From the equivalent circuit of the two-stage qZSI during
shoot-through state (Fig. 3b) the voltage of the inductors
could be represented as
u L1 = U IN + U C 2 ;
u DC = U C1 + U C 2 = U C 3 + U C 4 = U IN
Shoot-through duty cycle, Ds
u L 3 = −U C 4 .
The peak DC-link voltage across the inverter bridge is:
0,4
0,3
0,2
0,1
0
0
1
2
3
4
5
Voltage boost factor, B
Proposed two-stage qZS-network
Traditional (single-stage) qZS-network
Fig. 5. Shoot-through duty cycle as a function of voltage boost factor for the
different types of qZS-networks
1461
The shoot-through duty cycle of the qZSI with two-stage
qZS network and positive input voltage should never exceed
the value defined by (21):
1
DS ,max < .
(21)
3
IV.
SOME DESIGN GUIDELINES OF THE TWO-STAGE QZSI
BASED STEP-UP ISOLATED DC/DC CONVERTER
This section provides the general design equations of the
two-stage qZSI based step-up isolated DC/DC converter,
presented in Fig. 2. The desired operating parameters of the
converter are listed in Table 1. The control principle is
presented in Fig. 4. It is assumed, that converter operates in
conditions of changing input voltage and at rated load. To
provide the desired DC-link voltage (80 V) the converter
should operate in the shoot-through mode if the input voltage
drops below 80 V. Depending on the input voltage the shootthrough duty cycle should be respectively changed for
ensuring the demanded gain of the input voltage. In the
operating point corresponding to the maximal input voltage no
voltage boost is required and converter begins to operate in
the non-shoot-trough mode.
TABLE I
DESIRED OPERATING PARAMETERS OF THE INVESTIGATED CONVERTER
Parameter
Value
Minimal input voltage, UIN,min
Maximal input voltage, UIN,max
Desired DC-link voltage amplitude, UDC
Desired voltage ripple of the capacitors C1...C4
Desired peak-to-peak current ripple through the
inductors L1...L3
40 V
80 V
80 V
≤ 1 % (0.01)
P
,
U IN
(22)
U L3
U ⋅U
D
⋅ t S = IN C3 ⋅ S ,
0.6 ⋅ I L3
0.6 ⋅ P
f
(23)
D
2⋅P
⋅ S .
0.01 ⋅ U IN ⋅ u DC f
(24)
Neglecting transients and voltage drops the maximum
voltage across the switches (T11…T14) and the diodes (D1
and D2) will be equal to the maximal input voltage of the
converter. Caused by the stray inductances the voltage
overshoots could occur across the switching devices. In
several cases the amplitude of these inductive overvoltages
could exceed the nominal DC-link voltage level by up to 5
times and the transistors could be easily destroyed. To reduce
the voltage overshoots across the switches the DC-rail clamp
circuits could be implemented [10-12].
The average current through the diodes D1 and D2 equals
the average current through the inductors (Eq. 22). For
ensuring better efficiency of the converter the ultrafast diodes
with minimal possible forward voltage drop should be
selected for D1 and D2.
The design of isolation transformer and rectifier-filter
assembly could be performed as for traditional step-up
isolated DC/DC converter with full-bridge, center-tapped or
voltage-doubler rectifiers and its discussion is beyond the
scope of this paper. The description of design procedure could
be found from [7-9].
V.
where P is the system power rating and UIN is the input
voltage.
The maximum shoot-through duty cycle can be calculated
by (20) and for ensuring the demanded twofold boost of the
input voltage it should be DS,max=0.167.
The maximum current through the inductors L1, L2 and L3
occurs when the maximum shoot-through happens, which
causes maximum ripple current. In current design, the 60%
peak-to-peak current ripple through the inductors during
maximum power operation was chosen. During the shootthrough states the voltages of inductors L1, L2 and L3 will
have the same values and could be easily derived from (8). In
control method to be implemented (Fig. 4) the shoot-through
time is evenly split into two intervals. With the desired peakto-peak current ripple the inductance for L1, L2 and L3 could
be calculated by
L1 = L2 = L3 =
C1 = C2 = C3 = C4 =
≤ 60 % (0.6)
The average current through the inductors in two-stage
qZS-network could be calculated in the same way as for
traditional (single-stage) qZS-network:
I L1 = I L 2 = I L 3 =
where f is the operating frequency of the qZS-network. In
order to limit the voltage ripple on the inverter during active
states by 1 % (0.8 V) at the peak power, the capacitance of dcside capacitors C1, C2, C3 and C4 should be
SIMULATION AND EXPERIMENTAL RESULTS
For the verification of analysis the simulation model was
developed in PSIM simulation software. Moreover, a 1 kW
laboratory prototype of the step-up DC/DC converter with
two-stage qZS-network (Fig. 2) has been built to verify the
theoretical assumptions. The system parameters used for
simulations and experiments are listed in Table II.
TABLE II
SYSTEM PARAMETERS USED FOR SIMULATIONS AND EXPERIMENTS
Parameter
Value
Input voltage, UIN,min
Desired DC-link voltage amplitude, UDC
Capacitance value of capacitors C1...C4
Inductance value of inductors L1...L3
Operating frequency of the qZS-network, f
Operating frequency of the isolation transformer, fTR
Desired output voltage of the converter, UOUT
44 V
80 V
240 uF
50 uH
10 kHz
5 kHz
600 V
As it can be seen from Table II, the converter was studied in
the operating point with minimal input voltage and rated load,
where the shoot-through duty cycle has its maximal value
(0.167) to boost the input voltage to the desired DC-link
value. The duty cycle of active states was respectively
DA=0.833. During the simulations and experiments the qZSI
was controlled without dead time. The DC/DC converter was
loaded with a 900 Ω resistor, thus the system power during
experiment was approximately 400 W.
1462
UIN
UDC
UTR,pr
(a)
(b)
Fig. 6. Simulated (a) and experimental (b) waveforms of the input voltage (UIN), DC-link voltage (UDC) and isolation transformer primary winding voltage
(UTR,pr) at input voltage UIN=44 VDC and shoot-through duty cycle DS=0.167
UC1
UC2
(a)
(b)
Fig. 7. Simulated (a) and experimental (b) waveforms of the operating voltages (UC1 and UC2) of the capacitors C1 and C2 at input voltage UIN=44 VDC and
shoot-through duty cycle DS=0.167
UC3
UC4
(a)
(b)
Fig. 8. Simulated (a) and experimental (b) waveforms of the operating voltages (UC3 and UC4) of the capacitors C3 and C4 at input voltage UIN=44 VDC and
shoot-through duty cycle DS=0.167
UL1
IL1
(a)
(b)
Fig. 9. Simulated (a) and experimental (b) waveforms of the operating voltage (UL1) and current (IL1) of the inductor L1 at input voltage UIN=44 VDC and
shoot-through duty cycle DS=0.167
1463
Figs. 6-9 show the simulation and experimental results for
this case study. As it can be seen from Fig. 6 the qZSI with
two-stage qZS-network operates correctly, thus ensuring the
expected voltage boost factor of B=2 with the shoot-through
duty cycle DS=0.167. The voltages on capacitors C1, C2, C3
and C4 (Figs. 7b and 8b) were 59 V, 26 V, 71V and 15 V
respectively, which is in a good agreement with the calculated
values:
U C1 = U IN
1 − 2 DS
1 − DS
= 58.7 (V); U C 3 = U IN
= 73.5 (V);
1 − 3 DS
1 − 3 DS
U C 2 = U IN
2 DS
DS
= 29.4 (V); U C 4 = U IN
= 14.7 (V).
1 − 3 DS
1 − 3 DS
An average current through the inductor L1 measured
during experiment was 9 A, which is consistent with the
calculated value:
I L1 =
P
400
=
= 9.1 (A).
U IN
44
The measured peak-to-peak current ripple was 12 A, which
corresponds to the selected inductance value of 50 uH. The
measured voltage of the inductor L1 during the shoot-through
and active state was 71 V and -13 V, respectively, which are
consistent with the calculated values:
u L1 _ shoot −through = U IN + U C 2 = 44 + 29.4 = 73.4 (V)
voltage boost factor the number of stages of the qZS-network
could be increased.
The practical part of the paper is mostly focused on the
step-up isolated DC/DC converter with two-stage qZSI, which
could be used as power conditioning unit for the residential
power systems with fuel cells. The paper unveils some design
equations of the inverter side of this converter. The shootthrough PWM control method is presented and discussed. The
theoretical assumptions were verified by the computer
simulations and experimental investigations. It was stated, that
proposed qZSI with two-stage qZS-network ensures the
demanded voltage gain without serious stresses on
components and with the shoot-through duty cycle value
reduced by 33.3% in comparison with the traditional qZSI.
Moreover, the proposed two-stage qZS-network ensures
continuous input current of the converter during the shootthrough operating mode, thus featuring the reduced stress of
the input voltage source, which is especially topical in such
demanding applications as power conditioners for fuel cells.
ACKNOWLEDGMENT
This research work has been partially supported by the
Research and Development Department of Tallinn University
of Technology under Project no. BF113.
REFERENCES
[1]
u L1 _ active = U IN − U C1 = 44 − 58.7 = −14.7 (V).
The current through the inductor L1 has the same
waveshape and value as the input current of the converter.
Thus, as seen from Fig. 9b, the input current of converter
during shoot-through operating mode is continuous, thus
featuring the reduced stress of the input voltage source, which
is especially topical in such demanding applications as power
conditioners for fuel cells.
VI. CONCLUSIONS
This paper presents the performance improvement method
for the voltage-fed quasi-Z-source inverter (qZSI) by the
introduction of the two-stage quasi-Z-source network (qZSnetwork). The two-stage qZS-network could be derived by the
adding of one diode (D2), one inductor (L3) and two
capacitors (C3 and C4) to the traditional qZSI. The novel
configuration inherits all the advantages of traditional solution
(voltage boost and buck functions in a single stage,
continuous input current and improved reliability). Moreover,
the voltage-fed qZSI with two-stage qZS-network features
over the 30% shoot-through duty cycle reduction for the same
voltage boost factor and component stresses as compared to
the conventional qZSI. The proposed qZSI with two-stage
qZS-network can be applied to almost all DC/AC, AC/DC,
AC/AC and DC/DC power conversion schemes. Moreover, to
further decrease the shoot-through duty cycle for the same
Peng, F. Z., "Z-source networks for power conversion", in Proc. of
IEEE Applied Power Electronics Conference and Exposition
APEC’2008, pp. 1258-1265, Feb. 2008.
[2] Peng, F. Z., "Z-source inverter", in IEEE Transactions on Industry
Applications, vol. 39, no. 2, pp. 504-510, Mar/Apr 2003.
[3] Peng, F. Z., Impedance Source power converter, US Patent 7130205,
Oct. 31, 2006.
[4] Anderson, J.; Peng, F.Z., "Four quasi-Z-Source inverters", in Proc. of
IEEE Power Electronics Specialists Conference PESC’2008, pp. 27432749, 15-19 June 2008.
[5] Yuan Li; Anderson, J.; Peng, F.Z.; Dichen Liu, "Quasi-Z-Source
Inverter for Photovoltaic Power Generation Systems", in Proc. of IEEE
Applied Power Electronics Conference and Exposition APEC’2009, pp.
918-924, 15-19 Feb. 2009.
[6] Jong-Hyoung Park; Heung-Geun Kim; Eui-Cheol Nho; Tae-Won Chun;
Jaeho Choi, "Grid-connected PV System Using a Quasi-Z-source
Inverter", in Proc. of IEEE Applied Power Electronics Conference and
Exposition APEC’2009, pp. 925-929, 15-19 Feb. 2009.
[7] Billings K. H., Switchmode Power Supply Handbook, McGraw-Hill,
Inc. 1989.
[8] Abraham I. Pressman: Switching Power Supply Design, 2nd Edition,
McGraw-Hill, Inc. 1998.
[9] Vinnikov, D.; Roasto, I.; Jalakas, T., "An improved high-power DC/DC
Converter for distributed power generation", in Proc. of 10th
International Conference on Electrical Power Quality and Utilisation
EPQU’2009, pp.1-6, 15-17 Sept. 2009.
[10] Strzelecki, R.; Bury, W.; Adamowicz, M.; Strzelecka, N., "New
Alternative Passive Networks to Improve the Range Output Voltage
Regulation of the PWM Inverters", in Proc. of IEEE Applied Power
Electronics Conference and Exposition APEC’2009, pp. 857-863, 15-19
Feb. 2009.
[11] Shen, M.; Joseph, A.; Yi Huang; Peng, F.Z.; Zhaoming Qian, "Design
and Development of a 50kW Z-Source Inverter for Fuel Cell Vehicles",
in Proc. of CES/IEEE 5th International Power Electronics and Motion
Control Conference IPEMC’2006, vol. 2, pp.1-5, 14-16 Aug. 2006.
[12] Strzelecki, R.; Adamowicz, M.; Strzelecka, N.; Bury, W., "New type TSource inverter", in Proc. of IEEE Compatibility and Power Electronics
Conference CPE’09, pp.191-195, 20-22 May 2009.
1464