Imperial Journal of Interdisciplinary Research (IJIR)
Vol-2, Issue-7, 2016
ISSN: 2454-1362, http://www.onlinejournal.in
Cost effective AC-DC Buck-Boost Converter
for single phase UPF Loads with Optimum
Dynamic Performance
Chandana D. & Vasantha Kumar S.
Department of Electrical and Electronics Engineering, DSCE, Bengaluru
Abstract: This paper is an attempt to carry out the indepth analysis of a single-stage single-phase AC-DC
Buck-Boost Converter, and to study the control
characteristics (voltage and current) under the
steady state operation. The proposed model is aimed
for the use, in low voltage applications under unity
power factor conditions at all times. This could
maintain the constant output voltage and also
maintain the unity power factor in the source side.
The topology makes use of minimum number of
passive and active components, while achieving good
dynamic performance under buck and boost
operation and makes use of PI controller. The
topology has the benefit of scalability, by changing
few parameters.
The topology has both the current and the voltage
control loop, in order to take care of DC link voltage
and the power factor. Matlab simulation software has
been used for detailed simulation for the closed loop
operation. The results are analyzed for a unity power
factor case, with the application of regulated input
voltage to the DC-DC converter.
1.0 INTRODUCTION
The controllable switches are operated in a switch
mode, where they are required to turn-on and turn-off
the load current, during each switching. In DC-DC
converter topology, power electronic switches are
subjected DC-DC or AC-DC or vice versa. PWM
DC-DC converter results in high switching stresses
and high switching power loss.
Power electronic converters are used in fuel cell
systems to convert the DC electrical power generated
by the fuel cell into usable AC or DC power through
power electronic circuits. The power electronic
converter plays an important role in the interface of
the fuel cell system as power generating system and
grid and many solutions are already presented in the
literature [14-23]. The output voltage of the fuel cell
Imperial Journal of Interdisciplinary Research (IJIR)
varies in the range of 20 V to 50 V DC. The possible
converter topologies that can be used are such as:
DC-DC together with DC-AC.
ZCS topologies can eliminate the switching losses at
turn-off and turn-on. If a relatively large capacitor is
connected across the output diode during resonance,
the converter operation becomes insensitive to the
diode’s junction capacitance or parasitic capacitance.
The major limitations associated with ZCS when
Mosfet’s are used as capacitive turn-on losses. Thus,
the switching loss is proportional to the switching
frequency, during turn-on, considerable rate of
change of voltage can be coupled to the gate drive
circuit through the Miller capacitor, which is
coupling capacitance between input and output, thus
increasing switching loss and noise. Another
limitation is that the switches are under high current
stress, resulting in high conduction loss.
ZVS eliminates the capacitive turn-on loss. It is
suitable for high-frequency operation. For singleended configuration, the switches could suffer from
excessive voltage stress, which is proportional to the
load. The output regulation of the ZCS and ZVS
resonant converters can be achieved using variable
frequency control. The ZCS [20-22] operates with
constant on-time control, while ZVS [24] operates
with constant off time control.
Generally, the mathematical model of any DC-DC
converter system can be developed using a number of
methods, viz., push pull, half, full bridge of dc- dc
converter. The model used in present work [1],[6],[7]
consists of soft-switching topologies (ZCS/ZVS) are
implemented in primary and secondary side of
isolated transformer[6].
2.0 PROPOSED BUCK-BOOST BRIDGE
CIRCUIT
The operating principle of the Buck-Boost converter
highlights its distinct attractive features such as
Page 361
Imperial Journal of Interdisciplinary Research (IJIR)
Vol-2, Issue-7, 2016
ISSN: 2454-1362, http://www.onlinejournal.in
Buck-Boost converter capability in a single-stage,
single-phase with a single switch.
2.1 Proposed open-loop Buck–Boost Bridge
Circuit
Reduction in circuit components and control
complexity of the system due to no isolated gate
drives results in advantage of proposed system. This
gives stable output of DC voltage.
In the Figure 2.1(a) shown basic converter topology
and which includes three major components namely,
an LC filter, a diode rectifier and a Buck-Boost
chopper. The diode rectifier bridge of Buck-Boost
converter is reversed in order to get positive DC
output voltage.
Fig.2.1(a): Proposed open loop single phase BuckBoost bridge circuit.
Operating mode 1: During the positive half cycle of
Buck-Boost converter the supply voltage vs energizes
DC inductor Ldc by
means of diode D1 and D2 through switch S, diodes
D3,D4 and Dbd are reverse biased.
Thus Capacitor Cdc supplying energy to the load and
capacitor acts as energy tank.
Fig.2.1(b): Operating mode 1 of Proposed open loop
single phase Buck-Boost bridge circuit.
Operating mode 2: During positive half cycle of
Buck-Boost converter of the supply voltage vs, switch
S is turned off and rectifier is in off state and diode
Dbd acts as free-wheeling diode.
Imperial Journal of Interdisciplinary Research (IJIR)
The stored energy in the operational mode 1 of DC
inductor Ldc which is used to recharge the capacitor
Cdc through free-wheeling diode Dbd.
DC capacitor Cdc and DC inductor Ldc are sized for
corrective converter operation and also for prevention
of load and inductor currents discontinuity.
Fig.2.1(c): Operating mode 2 of proposed open loop
single phase Buck-Boost bridge circuit.
Operating mode 3: During the negative half cycle of
Buck-Boost converter the supply voltage vs reenergizes DC inductor Ldc by means of diode D3 and
D4 through switch S and diodes D1,D2 and Dbd are
reverse biased. The stored energy in the capacitor Cdc
is supplied to the load.
In operational mode 1 and mode 2, AC source
modulation did using same switching device. Hence,
as same effect on the charging state of the DC
inductor Ldc and DC capacitor Cdc.
Fig.2.1(d): Operating mode 3 of Proposed open loop
single phase Buck-Boost bridge circuit.
Operating mode 4: During negative half cycle of
Buck-Boost converter of the supply voltage vs, switch
S is turned off and rectifier is in off state and diode
Dbd acts as free-wheeling diode.
The stored energy in the operational mode 3 of DC
inductor Ldc which is use to recharge the capacitor Cdc
through free-wheeling diode Dbd hence, operation
mode 2 and mode 4 are identical.
Page 362
Imperial Journal of Interdisciplinary Research (IJIR)
Vol-2, Issue-7, 2016
ISSN: 2454-1362, http://www.onlinejournal.in
Fig.2.1(e): Operating mode 4 of Proposed open loop
single phase Buck-Boost bridge circuit.
2.2 Proposed close-loop Buck–Boost Bridge
Circuit
In closed loop single-phase single-stage Buck–Boost
Bridge Circuit, the gate drive of switch S is driven by
Pulse Width Modulation (PWM), gating signal
generated by PI controller.
Here linear controller deals with only two feedback
loops for control operation. Firstly voltage feedback
loop which regulates the output voltage, it also called
as feedback loop of output voltage.
Secondly current feedback loop, during each
switching cycle current control makes the input
current to follow reference current signal hence it is
called as feedback loop of input current.
current i∗s is generated in current feedback control
loop.
The current feedback control loop ensures unity
power factor for sinusoidal input current, and also
estimates the reference AC side capacitor voltage,
which is required for the input current to follow its
reference control current.
PI controller balance power between AC side and DC
side.
3.0 MATHEMATICAL MODEL OF BUCKBOOST CONVERTER
In the proposed single-phase single-stage Buck-Boost
converter, the IGBT is switched ON during δT with
time interval of limit 0 ≤ δ ≤ 1 with period length T,
during this period DC side inductor Ldc start storing
energy and the diode Dbd is blocked, at the same time
interval the load is supplied by DC side Capacitor
Cdc.
During rest of period says, (1-δ)T interval the IGBT
is switched OFF and DC side inductor start
discharging to the load through blocking diode Dbd.
The variable constant δ is called as duty cycle, by
varying duty cycle the DC output voltage can be
varied considerably.
For mathematical modeling of the converter, apply
Kirchoff’s law to the proposed circuit, during
operational mode 1 and 3, the switch S is conducting
and differential equations are
(1)
vcs = − Ldc
dI L
dt
dI L
vcs
=−
dt
Ldc
(2)
Where vcs is AC side capacitor voltage, Ldc is DC side
inductor and IL instantaneous DC side inductor
current.
Fig.2.2: Proposed closed loop single phase BuckBoost bridge circuit.
A Phase Lock Loop (PLL) in the voltage feedback
control loop, which extract the phase from the input
supply voltage, in order to achieve input current in
phase with the input voltage of the Buck-Boost
converter.
The peak fundamental current im is also estimated by
voltage feedback control loop, and also the DC link
voltage is maintain at desired level by this loop.
The estimated peak fundamental current is
synchronized with PI controller output; the reference
Imperial Journal of Interdisciplinary Research (IJIR)
dVdc
dt
(3)
dVdc
Idc
=−
dt
Cdc
(4)
Idc = −Cdc
Where Idc is average DC load current, Cdc is DC side
capacitor, Vdc is instantaneous DC side output
voltage.
Differential equation (1) to (4) derived from
operational mode 1 and 3 of proposed circuit and
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Imperial Journal of Interdisciplinary Research (IJIR)
Vol-2, Issue-7, 2016
ISSN: 2454-1362, http://www.onlinejournal.in
negative sign due to reversal of Rectifier Bridge in
the proposed single-phase single-stage Buck-Boost
converter.
(5)
dI L Vdc
=
dt
Ldc
π
1
defined by Vdc =   ∫ Vdcdωt then equation
 π 0
(9) reduces to
(6)
dVdc
IL − Idc
=
dt
Cdc
δ
Vdc =
1−δ
×
(10)
2Vm
π
The mathematical relationship between the average
Differential equation (5) and (6) derived from
operational mode 2 and 4 of proposed circuit.
The resistive voltage drop in the proposed circuit is
neglected and AC side source capacitor Cs is given in
the equation (8).
vs = vcs + vLs
where
time, and that the average output dc voltage is
inductor current I L and the average load current
is given by equation
I dc
(11)
π
Ic = ∫ − δ Idc (1 − δ )( IL − Idc )]dωt = 0
(11)
0
vLs = LS dis
dt
(7)
In equation (11), the operational modes 1 and 3 and
operational in modes 2 and 4, the capacitor current is
(8)
vcs = vs − LS dis
dt
expressed
Ic = −δ Idc
as
and
Ic = (1 − δ )( IL − Idc ) respectively.
Reduction in Equation (11) results in,
The AC side source inductor considered small
initially, to make AC side capacitor voltage vcs is
equal to supply voltage vs, without loss of accuracy
in the proposed system.
During the operational mode 1 and 2 the DC side
inductor voltage can be expressed as
Where duty cycle δ = ton/Ts and ton, is the dwell time
of the switch S of IGBT within each switching cycle
of switching period Ts.
Based on the inductor zero average volt–second
principle, the equation (9) shows the average DC
voltage across Ldc is calculated and equated to zero.
A
π
π
∫ [−δV
S
+ (1 − δ )Vdc ]dωt = 0
(9)
0
sinusoidal
∫I
dωt =
dc
(1 − δ )
0
Idc = (1 − δ ) IL
v L = (1 − δ )Vdc
1
π
π
π
π
∫ I dωt
L
(12)
0
Where in equation (12), implies
v L = −δvcs
VLdc =
1
AC
source
voltage
given
by
vs = Vm sin ωt , where Vm is the peak supply phase
(13)
The DC side output current or load current is
proportional to the inductor average current, shown
in equation (13). The average inductor current and
output dc side output current maintained constant.
The DC side output voltage can be expressed in
terms of the average inductor current, for the resistive
load as shown in equation (14)
(14)
Vdc = Rdc (1 − δ ) IL
Depending on the maximum ripple of inductor
current ΔIdc and ripple of output voltage ΔVdc the
passive elements Ldc and Cdc are selected
Therefore DC side inductor and DC side capacitor is
given by
voltage, ω is the supply angular frequency and t is
Imperial Journal of Interdisciplinary Research (IJIR)
Page 364
Imperial Journal of Interdisciplinary Research (IJIR)
Vol-2, Issue-7, 2016
ISSN: 2454-1362, http://www.onlinejournal.in
Ldc =
(1 − δ )Ts × Vdc
∆IL
Cdc =
δTs × Vdc
(15)
5.0 SIMULATION RESULTS
Open loop performance:
(16)
∆Vdc × Rdc
4.0 SIMULINK MODEL FOR PROPOSED
CONVERTER
The simulation is performed for proposed open loop
single phase Buck-Boost bridge circuit shown in
figure 2.1(a). Figure 5.1 is the simulated result with
AC supply voltage at 80V with 50Hz, DC output
voltage at 180V.
The proposed single-phase single-stage AC-DC
Buck-Boost converter of open loop and close loop
using PI controller are designed and simulated using
MATLAB/SIMULINK shown in figure 4.1 and 4.2.
Fig 5.1: Open loop DC output voltage.
Fig.4.1 Simulink model for open loop single-phase
single-stage Buck-Boost converter.
The output DC voltage is operated in five different
stages with different duty cycle in both Buck and
Boost mode shown in figure 5.2.
In stage 1 the duty cycle is ramped from 0 to 25%
and maintained constant for 1s.
In stage 2 the duty cycle is ramped from 25% to 50%
and maintained constant for next 1s.
In stage 3 the duty cycle is ramped from 50% to 70%
and maintained constant for next 1s.
In stage 4 and stage 5, the duty cycle deceases from
70% to 50% and 50% to 25% and maintained
constant for next 1s each.
Fig 5.2: Open loop DC output voltage for different
duty cycle stages.
Fig.4.2 Simulink model for close loop single-phase
single-stage Buck-Boost converter.
Imperial Journal of Interdisciplinary Research (IJIR)
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Imperial Journal of Interdisciplinary Research (IJIR)
Vol-2, Issue-7, 2016
ISSN: 2454-1362, http://www.onlinejournal.in
Close loop performance:
The simulation is performed for proposed closed loop
single phase Buck-Boost bridge circuit shown in
figure 2.2, with AC supply voltage at 80V with 50Hz.
The figure 5.3 shows sinusoidal AC supply voltage
and supply current are in phase due to power factor
correction.
Fig 5.3: sinusoidal AC supply voltage and supply
current.
The Figure 5.4 and 5.5 is the simulated result at 200V
and 150V, where the output voltage follows reference
DC voltage across the load and for any change in the
reference DC voltage the converter operated at unity
power factor and supply voltage and current remains
in phase with increase in DC link voltage.
Fig 5.4: The close loop DC output voltage at 200V.
Fig 5.5: The close loop DC output voltage at 150V.
Imperial Journal of Interdisciplinary Research (IJIR)
CONCLUSION
The investigation carried out in this paper is to
explore the scalability of single stage AC-DC Buck Boost converter for low voltage needs. The Buck Boost converter has been operated as a Pulse Width
Modulation rectifier to achieve unity input power
factor. The report also dealt steady state operation
along with Mathematical relations.
The results of matlab simulation for the Buck - Boost
converter have been presented along with discussion
based on simulation study it is inferred that the
topology has healed good dynamic performance and
both the modes, while ensuring unity input power
factor overall operating range. Based on the in depth
literature survey carried out in these topic revealed
that the said topology have pure passive elements.
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