ISSN NO: 0745-6999 JOURNAL OF RESOURCE MANAGEMENT AND TECHNOLOGY Vol12, Issue3, 2021 Page No:408 A New Control Design of Soft-Switching (S6) Boost-Flyback PFC Converter 1 Doddi Satheesh, 2 Er, P.Pedda Reddy, 3 Dr.K. Chithambaraiah Setty 1 M.Tech Student, 2 Asst.Professor, 3 Associate Professor Dept of EEE St John's College Of Eng And Tecnology Abstract—This thesis presents a S6 PFC converter to enhance the current shaping performance and reduce the total harmonic distortion (THD). This improvement is achieved by the aid of an auxiliary winding which is used to lower the input current harmonics and also achieve soft-switching condition. As a result, the switching losses are reduced and harmonic content of the input current is improved noticeably in comparison to the conventional S6 PFC converter. Also the total number of semiconductor elements is reduced in the proposed topology which results in lower cost and higher efficiency. The operating modes of the proposed converter are discussed in details and the design procedure is presented. A 200 kHz prototype of the proposed converter is implemented and the obtained results are provided to verify the converter theoretical analysis and operation. Keywords—power factor correction, soft switching, single-stage, AC-DC converter, DC-DC converter I. INTRODUCTION IN recent years, power conversion equipment connected to the grid are constantly increasing. In order to manage the problems associated with the harmonic pollution of power conversion equipment and fulfill harmonic current limits set by standards like IEC61000-3-2[1], it is imperative to develop power factor correction (PFC) techniques[2]-[4]. Thus, major research has been carried out to address the mentioned issues and develop high performance PFC converters [2]-[10]. Two-stage cascade PFC converter which consists of a PFC stage and DC/DC stage is an approach to achieve a smooth output voltage in conjunction with a high power factor. By using separate controllers, these converters can achieve high performance input current shaping and output voltage regulation. However, the major drawback of this type of PFC is its high cost due to high device count (of at least two switches and a separate controller for each stage) [3]. In order to overcome this problem, single- stage PFC converters are developed in which the current shaping and the DC/DC stages are combined [3]-[10]. In most single stage structures, the DC/DC stage switch, together with other elements act also as the current shaping stage. Single stage PFC converters are commonly used in low power applications like LED drivers [2],[6], ballast circuits [7],[8] and battery chargers [9],[10]. These converters operate under either continuous conduction mode (CCM) or discontinuous conduction mode (DCM) [11],[12]. Operating the converter under DCM allows the input inductor current to depend only on the input voltage and not on the previous cycle parameters which can eliminate the current control loop of current shaping stage and simplify the control circuit and also, make it possible for the DC/DC stage to achieve fast output regulation. On the other hand, operating under CCM condition produces less high order harmonics that means higher efficiency is possible in CCM [12]. In the proposed topology, the DCM operation is selected due to selfpower factor correction characteristic and also other desired features which are discussed. Soft switching methods are applied to single stage PFC converters to improve the efficiency and further increase the operating switching frequency [13]-[22]. However, soft switching characteristic in these converters is mostly achieved by using additional switches and other circuit components which results in more complicated control scheme and extra cost [14]- [18]. In addition to these drawbacks, the loss associated with the newly added circuit is another concern in such methods. Nevertheless, some of these PFC converters like boost flyback converters proposed in [16],[17] are not capable of achieving full soft switching condition. In other to overcome above mentioned shortcomings, the idea of single stage- single switch-soft switching (S6) PFC converters are developed [13],[19]-[22] which achieve soft switching condition without any extra switch and only by adding few passive components and/or extra diodes. An integrated SEPIC-flyback PFC converter is proposed in [19] as a LED driver. However the converter only achieves soft-switching at turn-on. S6 LED drivers proposed in [20],[21] are capable of achieving low THD but they have high number of components. Moreover, they do not achieve soft switching condition at switch turn off which degrades their efficiency. In [22] a singlestage isolated power-factor- corrected power supply (SSIPP) is introduced which uses a regenerative clamping to reduce the voltage stress and to recycle the energy trapped in the leakage inductance. However, same as [20] and [21] it suffers from high number of semiconductor components and not achieving soft-switching condition at turn-off. In [13] a boost flyback S6 PFC converter is proposed in which the same extra elements used to provide soft switching at turn off, are employed to replace the
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ISSN NO: 0745-6999 JOURNAL OF RESOURCE MANAGEMENT
AND TECHNOLOGY
Vol12, Issue3, 2021
Page No:408
A New Control Design of Soft-Switching (S6) Boost-Flyback PFC Converter 1Doddi Satheesh, 2Er, P.Pedda Reddy,3Dr.K. Chithambaraiah Setty
1M.Tech Student, 2Asst.Professor, 3Associate Professor
Dept of EEE
St John's College Of Eng And Tecnology
Abstract—This thesis presents a S6 PFC converter to
enhance the current shaping performance and reduce
the total harmonic distortion (THD). This
improvement is achieved by the aid of an auxiliary
winding which is used to lower the input current
harmonics and also achieve soft-switching condition.
As a result, the switching losses are reduced and
harmonic content of the input current is improved
noticeably in comparison to the conventional S6 PFC
converter. Also the total number of semiconductor
elements is reduced in the proposed topology which
results in lower cost and higher efficiency. The
operating modes of the proposed converter are
discussed in details and the design procedure is
presented. A 200 kHz prototype of the proposed
converter is implemented and the obtained results are
provided to verify the converter theoretical analysis
and operation.
Keywords—power factor correction, soft switching,
single-stage, AC-DC converter, DC-DC converter
I. INTRODUCTION
IN recent years, power conversion equipment
connected to the grid are constantly increasing. In
order to manage the problems associated with the
harmonic pollution of power conversion equipment
and fulfill harmonic current limits set by standards like
IEC61000-3-2[1], it is imperative to develop power
factor correction (PFC) techniques[2]-[4]. Thus, major
research has been carried out to address the mentioned
issues and develop high performance PFC converters
[2]-[10]. Two-stage cascade PFC converter which
consists of a PFC stage and DC/DC stage is an
approach to achieve a smooth output voltage in
conjunction with a high power factor. By using
separate controllers, these converters can achieve high
performance input current shaping and output voltage
regulation. However, the major drawback of this type
of PFC is its high cost due to high device count (of at
least two switches and a separate controller for each
stage) [3]. In order to overcome this problem, single-
stage PFC converters are developed in which the
current shaping and the DC/DC stages are combined
[3]-[10]. In most single stage structures, the DC/DC
stage switch, together with other elements act also as
the current shaping stage. Single stage PFC converters
are commonly used in low power applications like
LED drivers [2],[6], ballast circuits [7],[8] and battery
chargers [9],[10]. These converters operate under
either continuous conduction mode (CCM) or
discontinuous conduction mode (DCM) [11],[12].
Operating the converter under DCM allows the input
inductor current to depend only on the input voltage
and not on the previous cycle parameters which can
eliminate the current control loop of current shaping
stage and simplify the control circuit and also, make it
possible for the DC/DC stage to achieve fast output
regulation. On the other hand, operating under CCM
condition produces less high order harmonics that
means higher efficiency is possible in CCM [12]. In
the proposed topology, the DCM operation is selected
due to selfpower factor correction characteristic and
also other desired features which are discussed. Soft
switching methods are applied to single stage PFC
converters to improve the efficiency and further
increase the operating switching frequency [13]-[22].
However, soft switching characteristic in these
converters is mostly achieved by using additional
switches and other circuit components which results in
more complicated control scheme and extra cost [14]-
[18]. In addition to these drawbacks, the loss
associated with the newly added circuit is another
concern in such methods. Nevertheless, some of these
PFC converters like boost flyback converters proposed
in [16],[17] are not capable of achieving full soft
switching condition. In other to overcome above
mentioned shortcomings, the idea of single stage-
single switch-soft switching (S6) PFC converters are
developed [13],[19]-[22] which achieve soft switching
condition without any extra switch and only by adding
few passive components and/or extra diodes. An
integrated SEPIC-flyback PFC converter is proposed
in [19] as a LED driver. However the converter only
achieves soft-switching at turn-on. S6 LED drivers
proposed in [20],[21] are capable of achieving low
THD but they have high number of components.
Moreover, they do not achieve soft switching
condition at switch turn off which degrades their
efficiency. In [22] a singlestage isolated power-factor-
corrected power supply (SSIPP) is introduced which
uses a regenerative clamping to reduce the voltage
stress and to recycle the energy trapped in the leakage
inductance. However, same as [20] and [21] it suffers
from high number of semiconductor components and
not achieving soft-switching condition at turn-off. In
[13] a boost flyback S6 PFC converter is proposed in
which the same extra elements used to provide soft
switching at turn off, are employed to replace the
ISSN NO: 0745-6999 JOURNAL OF RESOURCE MANAGEMENT
AND TECHNOLOGY
Vol12, Issue3, 2021
Page No:409
switch of current shaping stage (Fig.1 (a)). As a result,
a fully soft switched PFC is obtained with no
additional switch, simple control system while no
additional losses are imposed. Also, this converter has
fewer elements as compared to its counterparts, but its
boost inductor charge time depends on the input
voltage value and thus varies with the input voltage
amplitude. To lower the input current THD, it is
preferred that the charge time of boost inductor be
dependent only on the converter duty cycle [11]. To
overcome the drawback of the S6 converter in [13], a
new S6 converter with fewer semiconductor elements
is proposed in this paper. The harmonic content of the
proposed converter fulfill the IEC61000-3-2 class D
harmonic current limits and is noticeably lower than
that of converter in [13]. The proposed converter is
fully soft-switched and also the leakage inductance
energy is recovered to improve the converter
efficiency. In addition, the leakage inductance of the
flyback transformer is employed as the resonant
inductance while no extra switch is used. Furthermore,
the switch zero voltage switching (ZVS) turn-off is
resulted to eliminating the high voltage spike on
switch at turn-off instant which reduces losses,
electromagnetic interference (EMI) and the switch
voltage stress. A prototype of the proposed converter
is implemented to verify the converter theoretical
analysis and operation.
Fig. 1. (a) The S6 PFC converter of [13] (b) Proposed
S6 PFC converter
II. DC to DC (DC to DC converter):
Dc-dc power converters are employed in a
variety of applications, including power supplies for
personal computers, office equipment, spacecraft
power systems, laptop computers, and
telecommunications equipment, as well as dc motor
drives. The input to a dc-dc converter is an unregulated
dc voltage Vg. The converter produces a regulated
output voltage V, having a magnitude (and possibly
polarity) that differs from Vg.
There are three basic types of dc-dc converter
circuits, termed as (I)Buck , (II)Boost and (III)Buck-
boost. In all of these circuits, a power device is used as
a switch. This device earlier used was a thyristor,
which is turned on by a pulse fed at its gate. In all these
circuits, the thyristor is connected in series with load
to a dc supply, or a positive (forward) voltage is
applied between anode and cathode terminals. The
thyristor turns off, when the current decreases below
the holding current, or a reverse (negative) voltage is
applied between anode and cathode terminals. So, a
thyristor is to be force-commutated, for which
additional circuit is to be used. Earlier, dc-dc
converters were called ‘choppers’, where thyristors or
GTOs are used. It may be noted here that buck
converter (dc-dc) is called as ‘step-down chopper’,
whereas boost converter (dc-dc) is a ‘step-up
chopper’. In the case of chopper, no buck-boost type
was used. With the advent of bipolar junction
transistor (BJT), which is termed as self-commutated
device, it is used as a switch, instead of thyristor, in
dc-dc converters. Now-adays, MOSFETs are used as a
switching device in low voltage and high current
applications. It may be noted that, as the turn-on and
turn-off time of MOSFETs are lower as compared to
other switching devices, the frequency used for the dc-
dc converters using it (MOSFET) is high, thus,
reducing the size of filters as stated earlier.
Buck Converter: A buck converter (dc-dc) is shown in Fig..
Only a switch is shown, for which a device as
described earlier belonging to transistor family is used.
Also a diode (termed as free wheeling) is used to allow
the load current to flow through it, when the switch
(i.e., a device) is turned off. The load is inductive (R-
L) one. In some cases, a battery (or back emf) is
connected in series with the load (inductive). Due to
the load inductance, the load current must be allowed
a path, which is provided by the diode; otherwise, i.e.,
in the absence of the above diode, the high induced
emf of the inductance, as the load current tends to
decrease, may cause damage to the switching device.
If the switching device used is a thyristor, this circuit
is called as a step-down chopper, as the output voltage
is normally lower than the input voltage.
Fig.Buck converter
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AND TECHNOLOGY
Vol12, Issue3, 2021
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Boost Converter (dc-dc):
A boost converter (dc-dc) is shown in Fig..
Only a switch is shown, for which a device belonging
to transistor family is generally used. Also, a diode is
used in series with the load. The load is of the same
type as given earlier. The inductance of the load is
small. An inductance, L is assumed in series with the
input supply. The position of the switch and diode in
this circuit may be noted, as compared to their position
in the buck converter. In this case, the output voltage
is higher than the input voltage, as contrasted with the
previous case of buck converter (dc-dc). So, this is
called boost converter (dc-dc), when a self
commutated device is used as a switch. Instead, if
thyristor is used in its place, this is termed as step-up
chopper. The variation (range) of the output voltage
can be easily computed.
Fig: Boost Converter
Buck-Boost Converter
A buck-boost converter (dc-dc) is shown in Fig. Only
a switch is shown, for which a device belonging to
transistor family is generally used. Also, a diode is
used in series with the load. The connection of the
diode may be noted, as compared with its connection
in a boost converter (Fig. 17.2a). The inductor, L is
connected in parallel after the switch and before the
diode. The load is of the same type as given earlier. A
capacitor, C is connected in parallel with the load. The
polarity of the output voltage is opposite to that of
input voltage here.
When the switch, S is put ON, the supply current
flows through the path, S and L, during the time
interval, . The currents through both source and
inductor increase and are same, with being positive.
Fig: Buck-Boost Converter
The polarity of the induced voltage is same as that of
the input voltage.
Then, the switch, S is put OFF. The inductor current
tends to decrease, with the polarity of the induced emf
reversing. is negative now, the polarity of the output
voltage, being opposite to that of the input voltage, .
The path of the current is through L, parallel
combination of load & C, and diode D, during the time
interval, . The output voltage remains nearly constant,
as the capacitor is connected across the load. This
circuit can be termed as a buck-boost converter. Also
it may be called as step-up/down chopper. It may be
noted that the inductor current is assumed to be
continuous.
III.POWER FACTOR
The cosine of angle between voltage and
current in an a.c. circuit is known as power factor. In
an a.c. circuit, there is generally a phase difference φ
between voltage and current. The term cos φ is called
the power factor of the circuit. If the circuit is
inductive, the current lags behind the voltage and the
power factor is referred to as lagging. However, in a
capacitive circuit, current leads the voltage and power
factor is said to be leading. Consider an inductive
circuit taking a lagging current I from supply voltage
V; the angle of lag being φ. The phasor diagram of the
circuit is shown in Fig. 6.1.
The circuit current I can be resolved into two
perpendicular components, namely ;
(a) I cos φ in phase with V
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AND TECHNOLOGY
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(b) I sin φ 90o out of phase with V
IV.PROPOSED SYSTEM AND CONTROL
DESIGN
A. Description of the Idea Fig. 1(b) illustrates the
schematic of proposed converter. The converter
consists of a boost inductor (LB), dc-link capacitor
(CB), output capacitor (Co), and resonant capacitor
(Cr), power MOSFET (Sw), input bridge diode
rectifier, high frequency diodes (D1, Do) and a three
windings transformer (T) where Np, Ns, and Na
denote the primary, secondary and auxiliary windings
number of turns respectively. To simplify the
theoretical analysis, it is assumed that all
semiconductors components are ideal, the voltage of
DC-link capacitor is almost constant and also during a
switching period, the input voltage is constant due to
high switching frequency and low line frequency. In
the proposed S6 PFC, the auxiliary winding (Na) is
connected in series with the boost inductor. This
formation allows controlling the boost inductor
voltage and shaping its current which results in
lowering the THD. Also the auxiliary winding diode
in [13] is eliminated in the proposed topology which
improves the efficiency and simplifies the converter.
The input inductor (LB) operates in DCM, similar to
the converter of [13] in order to have intrinsic PFC
characteristic. The input voltage-current characteristic
of a boost converter in DCM mode is as below [11]: