Download Analysis and Simulation of Parallel AC to DC Boost

Analysis and Simulation of Parallel AC to DC Boost
Converters Based on Power Balance Control Technique
Uthen Kamnarn and Viboon Chunkag
Department of Electrical Engineering, Faculty of Engineering, King Mongkut's Institute of Technology North Bangkok
1518 Piboonsongkram Rd, Bangkok 10800 Thailand Tel. (+66)-02 9132500-24 Ext. 8518, 8519 Fax. (+66)-2585-7350
Email: [email protected], [email protected]
ABSTRACT
A parallel AC to DC converter based on power
balance control techniques is presented. The analysis and
simulation results of such system with nearly unity power
factor using single voltage loop control and inductor
current calculator for input current wave shaping and
output voltage regulation have been shown. The
relationship between the input voltage and input current
of such switching regulators is studied. This converter is
applied in distributed power supply system. Each
converter operates in continuous conduction mode and
constant switching frequency at 100 kHz. The objective is
to keep output voltage constant and current sharing in
each converter is equal, while keeping the power factor
very close to unity. Analysis and simulation results
indicate that such a scheme is effective and parallel AC to
DC converter has a high power density and fast transient
response. The circuit is designed to operate at dc output
voltage 400 V, output power 1,500W.
Keywords: Parallel AC to DC Boost Converters, Power
Factor Correction, Small Signal Analysis.
iL1
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s1
i
v
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i
iload
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o1
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fw1
L2
+
v
g
−
i
C
c
+
v
Load
o
−
o3
D
fw 3
Fig.1: Parallel Connection of Modular Power Circuit.
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+
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−
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−
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1. INTRODUCTION
Active power-factor-correction technique, using a
boost converter, has been successfully implemented to
improve the power factor and reduce input current
distortion in single-phase line current rectification. A near
unity power factor and very low harmonic distortion
along with good output voltage regulation can be
achieved [1-3]. In practice, the power rating of the PFC
circuit has to meet the maximum load requirement since
it transfers the full power required by the downstream
stage. Distributed power systems (DPS) are replacing
centralized power systems as system size and power
increase while voltage levels are decreasing. Power
requirements for large commercial and military systems,
such as large computers and electronic systems, ships and
aircraft, telecommunications, etc., are increasing and
becoming more complex as systems compete for more
computing and data storage and retrieval capability at
higher speeds and lower costs. As the system power
levels are typically from 300W to 3,000W levels, and the
"point of use" voltages include 400V, +/-48V, +/-15V,
12V, 5V, 3V and less.
i
iload
io
C
i
c
Load
+
v
o
−
o3
D
fw 3
Fig.2: Source Splitting of Modular Power Circuit.
The centralized power distribution system is low cost
and simple, but is becoming less able to provide efficient
and quality power, even with remote sense and load
distribution. A single high power design has
disadvantages, such as heat dissipation and expensive
components of high power ratings. Besides, it would take
much time and effort to have a variety of designs for
different power levels. The modular design approach
would relieve these problems. On the other hand, the
modular design takes advantage of standard
manufacturing processes leading to a lower product cost.
In parallel operation of converters, uniform current
distribution among modules is of primary concern. The
modularized boost converters are operated at
discontinuous-current-mode (DCM)
achieved
by
controlling each converter at same duty-ratio, constant
switching frequency. The switching instants of the
operating modules are phase-shifted to each other by an
equal fraction of the switching period [4].
In the approach of this paper, the modularized boost
converters are operated at continuous-current-mode
(CCM), so that equal current sharing among modules can
be easily achieved by controlling each converter at same
duty-ratio. All PFC modules is operated at a constant
frequency to facilitate the design of electromagnetic
interference (EMI) filters.
The proposed approach was designed and simulated
for parallel a AC to DC boost converters based on power
balance control technique with nearly unity power factor.
Each converter operates in continuous conduction mode
and constant switching frequency at 100kHz. The circuit
is designed @500 W to operate at dc output voltage
400 V, power system rating at 1.5 kVA , which is made
up of 5 modular units.
2. SYSTEM DESCRIPTION
Two possible configurations, parallel modules and
source splitting connection, are available when parallel
AC to DC converter cells.
2.1 Parallel Modules
The first schematic diagram of the proposed
approach is shown in Fig. 1. The parallel connection of
modules of the boost type PFC circuits. All power
devices of the PFC circuit, including the bridge rectifier,
and the power circuit components of the boost converter,
are modularized. A single capacitor output, C, is
connected at the output terminals for filtering the power
frequency output voltage ripples. The parallel operation
of modular power circuits is driven by a control unit,
which consists of a current control circuit, and voltage
loop control circuit. The dc-link voltage is fed back to
adjust the duty-ratio of the boost converters by the dutyratio control circuit for output voltage regulation.
2.2 Source Splitting
The second schematic diagram of the proposed
approach is shown in Fig. 2. The three single-phase
Boost-based PFC modules are connected in wye
connection with neutral at the input side, and in parallel at
the output side. The converters operate in the continuous
conduction mode. The current drawn by each of the
individual converters is controlled to be in phase with its
own input voltage (i.e, line to neutral voltage) by dutyratio modulation.
2.2 Power Balance Control Technique
It mainly consists of a single phase switching mode
rectifier, a sinusoidal reference, driver circuit with
constant switching frequency, voltage regulator, and
inductor current calculator. The desired output voltage
can be achieved by the inductor current calculator and the
voltage regulator. The inductor current calculator
computes the desired AC input current according to the
load current, the DC output voltage and the AC input
voltage. Since the basic concept of the inductor current
calculator is that the input power and the output power of
the switching mode rectifier are equal for one cycle
operation, the computed peak value of the inductor
current.
Therefore, the desired input line current can be
directly computed and the transient of switching mode
rectifier can be largely improved due to the response of
the inductor current calculator is faster than the voltage
regulation loop. However, the voltage regulator is also
needed because of the inaccuracy of the inductor current
calculator and the losses in the switching mode rectifier.
3. AVERAGED SMALL SIGNAL MODEL
The averaged small signal model of the switching
mode rectifier is based on a power balance concept.
Assume that the input current is controlled to be within
the reference currents by the current controller which is in
phase to the input voltage, and the AC source and all
components in switching mode rectifier are ideal, that is
the switching mode rectifier system is lossless. Therefore,
the power of input side and output side for one line cycle
in case single-phase is
Vg I L = Vo I o
(1)
and three-phase system is
3Vg I L = Vo I o
(2)
iLrefi = km irefi sin(ωt )
(3)
The inductor current is
iLrefi is reference current.
irefi is magnitude of reference current in other cell.
km is the gain of the multiplier.
i = 1..n
where
irefi = iLpi + ics
(4)
ics is correcting signal from voltage loop (PI controller)
peak value of the inductor current in case single-phase is
iLpi
⎛k v i
= ⎜ s o load
⎜ vg
⎝
⎞⎛ 1 ⎞
⎟⎜ ⎟
⎟⎝ n ⎠
⎠
(5)
dynamic equation of the output voltage is given by
io = C
5. SIMULATION RESULTS
Using the specification shown below several of the
configurations of parallel modules and source splitting
AC to DC boost converter were simulated using
MATLAB Simulink.
Table 1: Specification of Boost Power Factor Circuit
dvo
+ iload
dt
(6)
For derivation of the small signal model, let
v = V + v%
(7)
v is quantities refer to current or voltage value in circuit.
V is quantities refer to steady state values.
v% is quantities refer to small perturbation.
Characteristic
Input Voltage
Input frequency
Rated Module
Power Output
Output Voltage
fSW
Inductance
Capacitance
kc
Parallel Modules
220Vrms
50 Hz
500 W/cell
1.5 kW
400 Vdc
100 kHz
5 mH/cell
1,000 µ F
1.5
Source Splitting
220Vrms/Phase
50 Hz
500 W/cell/phase
1.5 kW
400 Vdc
100 kHz
5 mH/cell/phase
1,000 µ F
1.778
ωz
33
28
5.1 AC to DC Parallel Modules Configuration
~
vref
~
vo
+
GVR( s )
Gk
Simulation results of Fig. (1) when n=3 are shown in
Fig. 4-6.
ZL
−
k fb
Fig.3: Block Diagram of Parallel Modules Circuit.
The closed loop transfer function of Fig. 3 is
v%o =
Gvr Gk Z L
v%ref
1 + ( Gvr Gk Z L ) k fb
(8)
4. VOLTAGE REGULATOR DESIGN
Fig. 3 shows the simplified block diagram of the
voltage regulation loop. Since the main function of the
switching mode rectifier is to keep the input current
sinusoidal and in phase with the input line voltage, the
bandwidth of the voltage regulation loop should be
smaller than the line frequency. A suitable bandwidth is
in the interval of 1/3, 1/2 line frequency. Then, the
controller parameters, k p and ωz must be chosen such
Fig.4: Input Voltage and Current Waveforms.
that the bandwidth is in such interval for the operating
input line voltage range.
Here, a PI controller is chosen for voltage regulation
and it is
Gvr ( s ) =
k p ( S + ωz )
k p is the high-frequency gain.
ωz is the location of the zero.
S
(9)
Fig.5: Inductor Current Waveforms for Three Modules.
Two possible configurations are simulated. Fig. 4
and 7 shows the simulated input voltage and current at
full load. Evidently, the line current is exactly in phase
with the input line voltage and near sinusodail. Thus, the
input power factor approaches unity.
Fig.6: Transient of Output Voltage and Current
Waveforms from 100% to 50% to 100% Rated Power.
5.2 AC to DC Source Splitting Configuration
Simulation results of Fig. (2) are shown in Fig. 7-9.
Fig.7: Simulated Phase Input Voltage and Current
Waveforms Operation at Full Load.
Fig.9: Transient of Output Voltage and Current
Waveforms from 100% to 50% to 100% Rated Power.
6. CONCLUSIONS
Use of three Non-isolated CCM boost converters for
single-state in single and three phase AC to DC converter
is analyzed in this paper. The three modules are connect
in @ 500 W parallel at the single phase source and single
phase output and the three single-phase modules are
connected in wye with neutral at the input and in the
parallel at the output. The desired output voltage can be
achieved by the inductor current calculator and the
voltage regulator.
The proposed approach is found to be suitable for
low to medium power system. Another important
advantage of the proposed approach is its simple control.
A near unity power factor of the ac input along with a
well regulated dc output voltage is obtained by constantswitching frequency variable duty-ratio control. The
Inductor current sharing is achieved by controlling equal
duty-ratio for all modules. System performance remains
good.
7. REFERENCES
Fig.8: Inductor Current Waveforms for Three Inductor
Corresponding to Three-Phase Operation at Full Load
Fig. 5 and 8 show the simulated inductor current at
full load. The results show that converter can share
inductor current in each converter equally, while keeping
the power factor very close to unity. Fig. 6 and 9 show
the simulated transient response of the output voltage at
step load current, respectively.
The characteristics of the power factor and dynamic
response of proposed system has high input power factor
and fast dynamic response.
[1] C. Zhou, R.B. Ridley, and F.C. Lee, “Design and
Analysis of a hysteric boost power factor correction
circuit,” in Proc. IEEE PESC Conf., pp. 800-807,
1990.
[2] A. Kandianis, S.N. Manias, “A Comparative
Evaluation of Single-Phase SMR Converters with
Active Power Factor Correction,” Industrial
Electronics, Control and Instrumentation, 1994.
IECON '94., 20th International Conference on, Vol 1,
pp 244 – 249, 5-9 Sept. 1994.
[3] J.C. Le Bunetel, M. Machmoum, “Control of Boost
Unity Power Factor Correction Systems,” Industrial
Electronics Society, 1999. IECON '99 Proceedings.
The 25th Annual Conference of the IEEE , Vol.1, pp
266 – 271, 29 Nov.-3 Dec. 1999.
[4] C.S. Moo, H.L. Cheng, and P.H. Lin,“ Parallel
operation of modular power factor correction
circuits,” Power Electronics, IEEE Transactions on ,
Vol 17 Issue: 3 , pp 398 –404, 2002