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AC/DC CRITICAL CONDUCTION MODE BUCK-BOOST
CONVERTER WITH UNITY POWER FACTOR
Abdul Hakeem Memon
IICT, Mehran UET, Jamshoro, Sindh, (Pakistan).
E-mail: hakeem.memon@faculty.muet.edu.pk ORCID: https://orcid.org/0000-0001-8545-3823
Fazul Muhammad Noonari
IICT, Mehran UET, Jamshoro, Sindh, (Pakistan).
E-mail: fazulmuhammad@gmail.com ORCID: https://orcid.org/0000-0003-4252-2782
Zubair Memon
IICT, Mehran UET, Jamshoro, Sindh, (Pakistan).
E-mail: zubair.memon@faculty.muet.edu.pk ORCID: https://orcid.org/0000-0001-5967-3152
Ahmar Farooque
IICT, Mehran UET, Jamshoro, Sindh, (Pakistan).
E-mail: dahri.ahmar@outlook.com ORCID: https://orcid.org/0000-0003-0015-482X
Mohammad Aslam Uqaili
IICT, Mehran UET, Jamshoro, Sindh, (Pakistan).
E-mail: aslam.uqaili@faculty.muet.edu.pk ORCID: https://orcid.org/0000-0002-6102-3623
Recepción:
10/01/2020
Aceptación:
25/03/2020
Publicación:
30/04/2020
Citación sugerida Suggested citation
Memon, A. H., Noonari, F. M., Memon, Z. A., Farooque, A., y Uqaili, M. A. (2020). AC/DC Critical
Conduction Mode Buck-Boost Converter with Unity Power Factor. 3C Tecnología. Glosas de innovación
aplicadas a la pyme. Edición Especial, Abril 2020, 93-105. http://doi.org/10.17993/3ctecno.2020.
specialissue5.93-105
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ABSTRACT
The buck-boost converter operating in critical conduction mode (CRM) is commonly
utilized in various applications because of many advantages like protection against
short circuit, minimum component count, low operating duct-cycle, and low voltage on
MOSFETs. However, its input power factor (PF) is not high while operating in constant on-
time control. To attain unity PF for universal input voltage range, a new control scheme of
variable on-time control (VOTC) is proposed in this paper. The VOTC can be implemented
by modulating the turn-on time of the buck-boost switch. The working principle and
performance comparison of the converter is discussed with both types of control scheme.
The input PF the converter is high in case of VOTC than the COTC. Simulation results
are presented to verify the eectiveness of the proposed control strategy).
KEYWORDS
Buck-boost converter, Power factor, Critical conduction mode.
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1. INTRODUCTION
Power electronic technology is employed in various sorts of modern equipment’s which has
made our life, simpler, easier and comfortable. However, with this comfort and easiness
this technology brings power quality issues because it is centered on solid-state devices.
These issues introduce harmonic contained current or distorted current which has several
drawbacks like more power loss, voltage distortion and EMI compatibility issues etc.
Therefore, the standards are set by various industrious like IEC61000-3-2 limit and IEEE
519 (IEC 61000-3-2:2014, 2014; Langella, Testa, & Alii, 2014) to limit these harmonics.
In order to meet relevant harmonic standard and reducing input current distortion, power
factor correction (PFC) converter has been widely applied (García et al., 2003; Singh et al.,
2011; Memon et al., 2017; Memon et al., 2018; Memon et al., 2019). Generally, conventional
power converter topologies, such as boost, buck-boost and buck converters, can be used to
achieve low cost single-stage PFC, and each converter topology has its own characteristics.
The traditional boost PFC converter, with advantages of low input current ripple, high
eciency and inherent current shaping ability, is a good choice for PFC application.
However, it cannot maintain high eciency at universal input voltage. Buck converter can
maintain high eciency at all input voltages. However, there is no input current when
the output voltage is less than input voltage (Memon et al., 2018). The traditional buck-
boost topology, with advantages of inherent current shaping ability, low cost, step-down
and step-up voltage conversion, is a good choice compared with yback, CUK and SEPIC
converters. It is used in many applications such as wind energy control, Adaptive control
applications, and power amplier applications etc. However, when the on-time is constant,
the power factor (PF) of buck-boost PFC converter is low.
For modifying the performance of buck/boost converter, various researchers have proposed
various techniques and control schemes. In Ghanem, Al-Haddad, and Roy, (1996), a new
control mechanism is presented to increase the PF near to unity for a cascaded buck-
boost converter for the high-power application in continuous conduction mode (CCM).
Comparative analysis between single stage buck converter and the single buck-boost
converter in discontinuous conduction mode is given in Moschopoulos and Zheng (2006).
The work in Wei et al. (2008) has done the comparative study between the bridged buck-
boost PFC converter and bridgeless buck/boost PFC converter and proposed the bridgeless
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buck-boost PFC topology for improving the eciency. In Jayahar and Ranihemamalini
(2011), inductor average current control strategy is proposed for improving PF of CCM
buck-boost converter. The work in Jayahar, Ranihemamalini, and Rathnakannan (2016)
has given the solution to improve PF for CCM buck converter. The bridgeless buck-boost
converter with switched capacitor for low power applications is put forward in Saifullah et
al. (2017) for reducing the conduction losses and improving the eciency.
In this paper, an improved control scheme for buck-boost converter operating under critical
conduction mode (CRM) is proposed to realize unity PF.
This paper is divided into six sections. In section 2, the operation states of CRM buck-
boost PFC converter are analyzed with traditional control. The proposed control scheme
is introduced in section 3 to realize unity PF. Then the comparative analysis is discussed
in section 4 in terms of input PF. In section 5, the eectiveness of proposed topology is
evaluated by simulation results. Finally, some conclusions are drawn in section 6.
2. OPERATING PRINCIPLE OF THE CONVERTER
Figure 1 illustrates the power circuit of buck-boost converter in CRM mode. It comprises
of bridge diode rectier, a buck-boost switch (Q
b-b
), a freewheeling diode (D
fw
), an inductor
(L) and an output capacitor (Co), etc.
3
Figure 1. Power circuit of a buck-boost converter.
The instantaneous and rectified input voltage during half line cycle can be given as
(1)
Whereas ‘’V
pk
’’ represent the input voltage amplitude, θ represent the input voltage
angle.
There are two switching cycles when buck-boost converter works in critical
conduction mode (CRM). In case of first switching cycle, switch (Q
b-b
) is ON, the
inductor is charged as shown in Figure 2 and the value is given as
Figure 2. the Operation of converter during switching pattern 1.
(2)
(3)
During second switching cycle, (Q
b-b
) is OFF; the inductor will discharge through load
and output capacitor as indicated in Figure 3.
The expression for discharge time is
(4)
Also
(5)
C
o
+
-
D
1
D
2
D
3
D
4
Q
b/b
D
fw
i
in
v
in
L
R
L
LC
Filter
v
g
+
-
R
1
R
2
o
kV
R
5
FMMT
560
R
3
R
4
R
6
sin
m
kV
q
R
S
C
s
C
o
+
-
D
1
D
2
D
3
D
4
Q
b-b
D
fw
i
in
v
in
L
R
L
LC
Filter
+
-
sin
pk
L
V
di
dt L
q
=
_
sin
on pk
L pk
tV
i
L
q
=
sin
pk
onoff
t
V
t
L
q
=
s on off
tt t=+
Figure 1. Power circuit of a buck-boost converter.
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The instantaneous and rectied input voltage during half line cycle can be given as:
sin
in a pk
v vV
q
==
(1)
Whereas ‘’V
pk
’’ represent the input voltage amplitude, θ represent the input voltage angle.
There are two switching cycles when buck-boost converter works in critical conduction
mode (CRM). In case of rst switching cycle, switch (Q
b-b
) is ON, the inductor is charged as
shown in Figure 2 and the value is given as
3
Figure 1. Power circuit of a buck-boost converter.
The instantaneous and rectified input voltage during half line cycle can be given as
(1)
Whereas ‘’V
pk
’’ represent the input voltage amplitude, θ represent the input voltage
angle.
There are two switching cycles when buck-boost converter works in critical
conduction mode (CRM). In case of first switching cycle, switch (Q
b-b
) is ON, the
inductor is charged as shown in Figure 2 and the value is given as
Figure 2. the Operation of converter during switching pattern 1.
(2)
(3)
During second switching cycle, (Q
b-b
) is OFF; the inductor will discharge through load
and output capacitor as indicated in Figure 3.
The expression for discharge time is
(4)
Also
(5)
C
o
+
-
D
1
D
2
D
3
D
4
Q
b/b
D
fw
i
in
v
in
L
R
L
LC
Filter
v
g
+
-
R
1
R
2
o
kV
R
5
FMMT
560
R
3
R
4
R
6
sin
m
kV
q
R
S
C
s
C
o
+
-
D
1
D
2
D
3
D
4
Q
b-b
D
fw
i
in
v
in
L
R
L
LC
Filter
+
-
sin
pk
L
V
di
dt L
q
=
_
sin
on pk
L pk
tV
i
L
q
=
sin
pk
onoff
t
V
t
L
q
=
s on off
tt t=+
Figure 2. The Operation of converter during switching pattern 1.
sin
pk
L
V
di
dt L
q
=
(2)
_
sin
on pk
L pk
tV
i
L
q
=
(3)
During second switching cycle, (Q
b-b
) is OFF; the inductor will discharge through load and
output capacitor as indicated in Figure 3.
The expression for discharge time is:
sin
pk
onoff
t
V
t
L
q
=
(4)
Also,
s on off
tt t=+
(5)
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4
Figure 3. The Operation of converter during switching pattern 2.
The inductor and switch current waveforms are shown in Figure 4.
Figure 4. The inductor and switch current waveforms.
From (4) and (5), following relation is obtained
(6)
The duty-cycle of buck-boost switch is expressed as
(7)
With traditional control the input current of buck-boost converter is given as
(8)
The expression of average input power is derived as
(9)
The value of t
on
is calculated from ‘’ (9)’’ by assuming 100% efficiency
(10)
The input PF with traditional control scheme can be got by joining (1) and (8-10).
C
o
+
-
D
1
D
2
D
3
D
4
Q
b-b
D
fw
i
in
v
in
L
R
L
LC
Filter
+
-
i
L_pk
0
i
L
v
gs
0
t
0
t
L
v
g
-
V
o
L
t
on
i
Qb/b
t
off
t
s
t
L
v
g
( )
sin
on
o pk
o
s
t
VV
V
t
q
+=
_
sin
o
bb
o pk
V
D
VV
q
=
+
( )
(__ )
sin
2 sin
in b b COTC on
o pk
pk o
it
L
VV
VV
q
q
+
=
( )
2
2
_
0
sin
sin
2
pk o
in COTC
o
on
pk
t
L
VV
Pd
VV
p
q
p
q
q
=
+
ò
( )
2
2
0
sin
2
sin
pk o
o
pk o
on
VV d
V
LP
t
V
p
q
q
p
q
=
+
ò
Figure 3. The Operation of converter during switching pattern 2.
The inductor and switch current waveforms are shown in Figure 4.
4
Figure 3. The Operation of converter during switching pattern 2.
The inductor and switch current waveforms are shown in Figure 4.
Figure 4. The inductor and switch current waveforms.
From (4) and (5), following relation is obtained
(6)
The duty-cycle of buck-boost switch is expressed as
(7)
With traditional control the input current of buck-boost converter is given as
(8)
The expression of average input power is derived as
(9)
The value of t
on
is calculated from ‘’ (9)’’ by assuming 100% efficiency
(10)
The input PF with traditional control scheme can be got by joining (1) and (8-10).
C
o
+
-
D
1
D
2
D
3
D
4
Q
b-b
D
fw
i
in
v
in
L
R
L
LC
Filter
+
-
i
L_pk
0
i
L
v
gs
0
t
0
t
L
v
g
-
V
o
L
t
on
i
Qb/b
t
off
t
s
t
L
v
g
( )
sin
on
o pk
o
s
t
VV
V
t
q
+=
_
sin
o
bb
o pk
V
D
VV
q
=
+
( )
(__ )
sin
2 sin
in b b COTC on
o pk
pk o
it
L
VV
VV
q
q
+
=
( )
2
2
_
0
sin
sin
2
pk o
in COTC
o
on
pk
t
L
VV
Pd
VV
p
q
p
q
q
=
+
ò
( )
2
2
0
sin
2
sin
pk o
o
pk o
on
VV d
V
LP
t
V
p
q
q
p
q
=
+
ò
Figure 4. The inductor and switch current waveforms.
From (4) and (5), following relation is obtained:
( )
sin
on
o pk
o
s
t
VV
V
t
q
+=
(6)
The duty-cycle of buck-boost switch is expressed as:
_
sin
o
bb
o pk
V
D
VV
q
=
+
(7)
With traditional control the input current of buck-boost converter is given as:
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( )
(__ )
sin
2 sin
in b b COTC on
o pk
pk o
it
L
VV
VV
q
q
+
=
(8)
The expression of average input power is derived as:
( )
2
2
_
0
sin
sin
2
pk o
in COTC
o
on
pk
t
L
VV
Pd
VV
p
q
p
q
q
=
+
ò
(9)
The value of t
on
is calculated from ‘’ (9)’’ by assuming 100% eciency:
( )
2
2
0
sin
2
sin
pk o
o
pk o
on
VV d
V
LP
t
V
p
q
q
p
q
=
+
ò
(10)
The input PF with traditional control scheme can be got by joining (1) and (8-10).
( )
( )
2
0
2
2
0
sin
sin
2
sin
sin
pk o
COTC
pk o
d
VV
PF
d
VV
p
p
q
q
q
p
q
q
q
+
=
+
ò
ò
(11)
The table of input PF with traditional control is drawn in Table 1 with the help of equation
(11) and the specication of the converter. It indicates low PF at high input voltage.
Table 1. Input PF with traditional control.
S.NO VRMS PF(COTC)
1 90 0.968
2 110 0.963
3 130 0.96
4 150 0.956
5 170 0.954
6 190 0.951
7 210 0.949
8 230 0.947
9 250 0.945
10 264 0.945
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Through Fourier analysis, the harmonics of the input current is calculated as:
0
2
sin ( 1,3,5.....)
n in
I i nd n
p
qq
p
==
ò
(12)
Based on (9) and (12), Figure 5 is drawn. It indicates the comparison of measured current
harmonic with IEC Class C limits. It can be observed that the 5th and 7th harmonic for
converter is unable to meet the limit value. Specially, the 5th harmonic cannot meet the
standard for universal input voltage range, while 7th harmonic at high input voltage.
5
(11)
The table of input PF with traditional control is drawn in Table 1 with the help of
equation (11) and the specification of the converter. It indicates low PF at high input
voltage.
Table 1. Input PF with traditional control.
S.NO
V
RMS
PF(COTC)
1
90
0.968
2
110
0.963
3
130
0.96
4
150
0.956
5
170
0.954
6
190
0.951
7
210
0.949
8
230
0.947
9
250
0.945
10
264
0.945
Through Fourier analysis, the harmonics of the input current is calculated as
(12)
Based on (9) and (12), Figure 5 is drawn. It indicates the comparison of measured
current harmonic with IEC Class C limits. It can be observed that the 5th and 7th
harmonic for converter is unable to meet the limit value. Specially, the 5th harmonic
cannot meet the standard for universal input voltage range, while 7th harmonic at
high input voltage.
Figure 5. Input current harmonic.
3. Proposed variable on-time control scheme to improve input PF
To achieve unity PF, the variation rule for t
on
should be
( )
( )
2
0
2
2
0
sin
sin
2
sin
sin
pk o
COTC
pk o
d
VV
PF
d
VV
p
p
q
q
q
p
q
q
q
+
=
+
ò
ò
0
2
sin ( 1,3,5.....)
n in
I i nd n
p
qq
p
==
ò
( )
31
/II
0
0.08
0.16
0.24
0.32
( )
51
/II
( )
71
/II
0.07
0.1
0.3
90 133.5 177 220.5 264
( )
2V
m
V
Figure 5. Input current harmonic.
3. PROPOSED VARIABLE ON-TIME CONTROL SCHEME TO
IMPROVE INPUT PF
To achieve unity PF, the variation rule for t
on
should be:
(_)
sin
pk o
on b b on
o
VV
tk
V
q
+
æö
=
ç÷
èø
(13)
By substituting (13) into (8), we can get average input current with VOTC as:
(__ )
2
sin
pk
onin b b VOTC
V
i
L
k
q
=
(14)
It shows shape of average input current is purely sinusoidal at all input voltage. Thus, unity
PF can be realized.
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From (1) and (15), the average input power is expressed as:
2
_
4
on pk
in VOTC
kV
P
L
=
(15)
Assuming converter to be 100% ecient, then k
on
is calculated as:
2
4
o
on
pk
PL
k
V
=
(16)
4. COMPARATIVE ANALYSIS
From (14), the input PF curve with proposed control scheme is drawn in Table 2, which
also includes the PF values with traditional control scheme of Table. It can be concluded
that the PF of the converter with proposed control is higher as compared to COTC. The
percentage improvement of PF increases as the input rms voltage is increased.
Table 2. Input PF curve with proposed control scheme.
S.NO VRMS PF(COTC) PF(VOTC) % Improvement
1 90 0.968 1 3.30
2 110 0.963 1 3.84
3 130 0.96 1 4.00
4 150 0.956 1 4.40
5 170 0.954 1 4.82
6 190 0.951 1 5.15
7 210 0.949 1 5.38
8 230 0.947 1 5.60
9 250 0.945 1 5.80
10 264 0.945 1 5.82
5. SIMULATION VERIFICATION
For verifying the eectiveness of VOTC strategy, simulations are carried out. The input
voltage range is 90-264VAC, and the output is 24V. For ensuring the current to be in CRM,
L6561 IC is used. All the components in the circuit are selected as idea. The Simulation
results in Figure 9 and Figure 10 shows that vin, and iin, for proposed converter with COTC
and VOTC at 110VAC input, respectively. The input waveform shows that with VOTC
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the input current is sinusoidal as compared with COTC. Hence, the near unity PF can be
realized by using proposed control scheme.
Figure 9. v
in
, and i
in
, with COTC.
Figure 10. v
in
, and i
in
, with VOTC.
5. CONCLUSION
A variable on-time control scheme and the implementation circuit are proposed in this
paper to make the shape of average input current purely sinusoidal for the CRM buck–
boost PFC converter. The analysis and simulation results are given. Compared with that of
the COT control:
1. Input current meets the harmonic standard.
2. PF is high
3. THD is low
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