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CRM BUCK CONVERTER WITH HIGH INPUT POWER
FACTOR
Abdul Hakeem Memon
Institute of Information and Communication Technologies (IICT),
Mehran UET, Jamshoro, Sindh, (Pakistan).
E-mail: hakeem.memon@faculty.muet.edu.pk
ORCID: https://orcid.org/0000-0001-8545-3823
Manzoor Ali
Institute of Information and Communication Technologies (IICT),
Mehran UET, Jamshoro, Sindh, (Pakistan).
E-mail: manzoorabro84@gmail.com
ORCID: https://orcid.org/0000-0002-6430-472X
Zubair Ahmed Memon
Institute of Information and Communication Technologies (IICT),
Mehran UET, Jamshoro, Sindh, (Pakistan).
E-mail: zubair.memon@faculty.muet.edu.pk
ORCID: https://orcid.org/0000-0001-5967-3152
Ashfaque Ahmed Hashmani
Institute of Information and Communication Technologies (IICT),
Mehran UET, Jamshoro, Sindh, (Pakistan).
E-mail: ashfaque.hashmani@faculty.muet.edu.pk
ORCID: https://orcid.org/0000-0001-6412-211X
Recepción:
15/12/2020
Aceptación:
10/03/2021
Publicación:
07/05/2021
Citación sugerida:
Memon, H. A., Ali, M., Memon, Z. A., y Hashmani, A. A. (2021). CRM Buck Converter with High
Input Power Factor. 3C Tecnología. Glosas de innovación aplicadas a la pyme, Edición Especial, (mayo 2021),
149-161. https://doi.org/10.17993/3ctecno.2021.specialissue7.149-161
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ABSTRACT
The buck converter is generally utilized in many applications because of various advantages
like maintaining high eciency for the wide range of input voltage, cost reduction, low
output voltage, protection against inrush current and lifetime improvement. However,
when on-time of buck converter is kept constant, the input voltage and current remains
out of phase that cause’s low PF and high total harmonic distortion (THD). Therefore a
power factor improvement technique is implemented in this paper that permits the average
input current sinusoidal without need of extra converter to work in cascade with buck
converter. The critical conduction mode (CRM) buck converter operating with constant
on-time (COT) control scheme have low PF because of harmonic contained input current
waveform. So in order to make the input current waveform as a sinusoidal by changing
the on-time of buck switch, a variable-on-time (VOT) control scheme for CRM buck
converter is proposed in this paper. The theoretical analysis is given, and the simulation
results conrm the advantages of the proposed control scheme. The object of the research
paper is to propose the control scheme to realize high PF for CRM buck converter by only
modulating the on-time of buck switch and without need of extra converter to work with
buck converter.
KEYWORDS
Buck Converter, Critical Conduction Mode (CRM), Power Factor (PF), DC/DC Converters,
Constant On-Time (COT), Variable On-Time (COT).
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1. INTRODUCTION
Power electronic innovation is utilized in dierent types of present day equipment’s
which has made our life easier, lavish and simpler. However, this innovation depends on
semiconductor devices, because of which the shape of average input current is distorted.
The distorted current has dierent disadvantages, for example, increased power loss,
noise and voltage distortion etc. So the industries have formed dierent standards like
IEC61000-3-2 limit and IEEE 519. In order to meet relevant harmonic standard and
reducing input current distortion, various researchers have proposed dierent types of
power factor correction (PFC) converters (Memon et al., 2020a, 2020b, 2020c). The
DC/DC converters or choppers such as buck, boost and buck-boost etc. are normally
employed for PFC application. Each converter has its own advantages and disadvantages.
The buck converter now days have attracted the attention of the many researchers (Endo
et al., 1992; Memon et al., 2019a, 2019b, 2019c 2019d, 2019e). Its advantages include less
voltage stress on the switch, low component cast, low output voltage, low inrush current,
protection against short circuit and high eciency at universal input voltage. However,
the input PF of buck converter in low because of dead zone in the average input current
that is present until input voltage is more than output voltage. It causes the input current to
contain large harmonic disturbances.
For modifying the performance of buck converter, various researchers have proposed
various techniques and control schemes (Endo et al., 2020; Memon et al., 2019a, 2019b,
2019c 2019d, 2019e).
In Endo et al. (1992), buck converter is introduced for PF improvement. The application
along with analysis and modeling operating in discontinuous current mode (DCM) is
discussed in Lee et al. (1997). The work in Spiazzi et al. (2000) has put forward a converter
to compensate the dead zone in buck converter. In Huber et al. (2009), the analysis and
evaluation of DCM buck converter is given. The bridgeless buck converter is introduced
to enhance the eciency in Jang et al. (2010). The research in Wu et al. (2011) has discussed
various aspects regarding the design of buck converter with constant on-time control
scheme (COTC).
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In Lamar et al. (2012), the AC/DC driver working in boundary current mode for tapped
inductor buck converter is introduced to replace incandescent lamp by high brightness
light emitting diode. For the reduction of input current harmonic for high brightness
light emitting diode, a control scheme is proposed in Wu et al. (2012). In Ki et al. (2012),
a new technique is introduced to reduce losses due to transformer in buck converter. The
interleaved critical current mode (CRM) buck converter is introduced in Choi et al. (2012)
to increase eciency.
In Fardoun et al. (2014), a bridgeless buck is proposed to increase the eciency. For
compensating dead zone, double integrated buck converter is introduced for power
supplies (Sichirollo et al., 2014). The integration of another converter with buck converter is
discussed to enhance the PF in Liu et al. (2020). The works in Memon et al. (2016), Memon
et al. (2018a, 2018b) and Memon et al. (2019a, 2019b, 2019c 2019d, 2019e) have introduced
various control schemes and topologies for buck converter to attain high PF and eciency.
In this paper, a PF improvement technique is implemented for CRM buck converter that
permits the average input current to be sinusoidal. It can improve input PF without need
of other converter to work with buck during dead zone time and also need only one feed
forward circuit.
This paper is divided into six sections. In section 2, the operation states of CRM buck
converter are analyzed with traditional constant on-time (COT) control scheme. The
introduced VOT control scheme is discussed in section 3. Then the comparative analysis is
discussed in section 4 in terms of PF. In section 5, the eectiveness of proposed topology is
evaluated by simulation results. Finally, some conclusions are drawn in section 6.
2. RESEARCH METHODOLOGY
The research methodology is based on:
1. Mathematical analysis of the operating principle of the traditional control scheme
for BCM IBBC with the help of MATHCAD software.
2. Analysis of input PF of BCM integrated BBC.
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3. Introducing the proposed control scheme to obtain high PF with low THD.
4. Comparative analysis of the converter for COT control scheme and VOT control
scheme strategy in terms of input PF and THD.
5. Developing the simulation model of the converter with traditional and proposed
control scheme with the help of MATLAB software.
6. Conrming the results.
3. CONVENTIONAL COTCONTROL SCHEME FOR BUCK
CONVERTER
Figure 1 shows the main circuit of the buck converter.
Figure 1. Schematic Diagram of Buck Converter.
Source: (Memon et al., 2019a).
The input voltage and rectied input voltage is expressed as
(1)
Where V
rms
is the rms value
When buck switch is on, the inductor voltage is expressed as
(2)
where
So its peak value is
(3)
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Where t
on
is the on-time of the switch
While Q
buck
is o, the inductor is discharged through load.
(4)
For each switching cycle, the value of o-time is calculated from inductor’s volt-second
balance
(5)
From (3) and (5), the following relation is obtained
(6)
In addition
(7)
Substituting (6) into (7)
(8)
The value of average input current for buck converter is got as
(9)
Substituting (3) and (8) into (9)
(10)
Based on (1) and (10), the input power of the buck converter is expressed as
(11)
Now t
on
can be calculated by assuming the eciency of buck converter as 100%
(12)
And input power factor can be calculated as
(13)
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Based on (11-13) and specication, the curve of the input PF can be drawn and is given in
Figure 2 from which it can be concluded that the input PF is low for universal input voltage
range.
Figure 2. Input PF of COTCS [Mathcad eq:(10-14)].
3. PROPOSED VOT CONTROL SCHEME FOR CRM BUCK
CONVERTER
For obtaining high PF for the buck converter, the on-time of Q
buck
in (10) should vary as
(14)
Where k
on1
is a constant
Substituting (14) into (10), the average input current of the buck converter is
(15)
From (15), it can be seen that if the on-time of the buck converter varies as (14), the input
current waveform looks like pure sinusoidal and the PF is high
Because of the power balance of the input and output, the input current for the unity PF
can be expressed as
(16)
Based on (15-16), the input current at θ
0
can be written as
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(17)
From (17), it can be concluded that
(18)
4. COMPARATIVE ANALYSIS
From (14), the input PF curve with proposed control scheme is shown in Figure 3 which
also includes the PF values with traditional control scheme of Figure 2. It can be concluded
that the PF of the converter with proposed control is higher as compared to COTC. It can
conclude that improvement in PF is at all input voltage.
Figure 3. Input PF comparison between COT & VOT control scheme [Mathcad eq:(10-18)].
The THD comparison of the buck converter with COTCS and VOTCS is depicted in
Figure 4. It indicates that THD is reduced in case of VOTCS as compared to COTCS.
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Figure 4. THD comparison between COT & VOT control scheme [Mathcad eq:(10-18) Relationship between PF
and THD].
5. SIMULATION RESULTS
In order to verify the eectiveness of VOTCS, simulations results are given. The range of
input voltage is 90-264 V
rms
. The output voltage is selected as 80 V. The control IC 6561 will
ensure the current to be in CRM. Ideal components are used in simulation
Figure 5 and Figure 6 show the simulation waveforms of v
in
, & i
in
of buck converter with
COTCS and VOTCS at 220VAC inputs, respectively. It can be conducted that current is
more sinusoidal in case of VOTCS as compared to COTCS. Thus VOTCS can attain high
P F.
Figure 5. Input voltages and current of buck converter with COT control scheme [Saber Software].
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Figure 6. Input voltages and current of buck converter with VOT control scheme [Saber Software].
6. CONCLUSIONS
The buck converter is generally utilized in many applications because of various
advantages. However, when on-time of buck converter is kept constant, the input voltage
and current remains out of phase that cause’s low PF and high total harmonic distortion
(THD). Therefore, in this paper, a power factor improvement technique is implemented
that permits the average input current sinusoidal without need of extra converter to work
in cascade with buck converter. With COTCS, the input PF of the buck converter is low.
In order to attain high input PF, VOT control scheme is proposed. Simulation results are
presented for the verication of the analysis.
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