Recent Advances in Single-Stage Power Factor Correction
Khalid Rustom and Issa Batarseh
School of Electrical Engineering and Computer Science
University of Central Florida
Orlando, FL 32816
Abstract - This paper presents an overview of various
interesting power factor correction techniques for
single-phase applications. The discussion includes
commonly-used control strategies and various types of
converter topologies. Included is a comparative study
of these strategies, with the major advantages and
disadvantages is highlighted. We will emphasize the
single-stage topologies, its drawhacks and some
It is well known that power supplies connected to AC
mains introduce harmonic currents in the utility. Such
harmonic currents cause several problems including
voltage distortion, heating, radiated and conducted noises
and can reduce the capability of the line to provide energy
[I]. As a result, national and international standards or
recommendations have been adopted that make the use of
power factor correction circuits in power supplies a
Unity power factor is not necessary to meet the
regulations. For example, both IEEE 519 and iEC 1000-3-
2 [2-31, allow the presence of harmonics in the line current.
This fact has lead to the publication of a great number of
papers in recent years, with solutions that range from a
simple LC filter to the two-stage approach. The two-stage
approach to achieve power factor correction requires the
presence of a PFC stage prior to the DCDC regulation
stage. An alternative solution to realize the goal was to
integrate the active PFC stage with the isolated high quality
output DCDC stage into one stage, which is known as a
single-stage converter with least components and simplest
controller. Theoretically, changing the two-stage scheme to
single-stage scheme can substantially alleviate the cost and
complexity of Single-Stage (S’) PFC ACDC.
The question arises, why hasn’t such a concept been
extensively adopted in today’s power supply industry?
Perhaps the answer is because there are still some existing
technical challenges, with respect to the development of
viable Sz PFC ACDC converters. These challenges
include high voltage and current stresses, and low
efficiency, etc. Also, the single-stage PFC is attractive to
Several existing review papers have been focused on the
general PFC topics with some comparison . Other
papers specialized in a single-stage configuration . in
this paper, we present a general classification of the power
factor correction approaches in term of their topological
structures, followed by common control techniques. At the
end, we emphasized the single-stage approach by
highlighting its common drawbacks and recent solutions.
2 CLASSIFICATION OF POWER FACTOR
The general approaches to improve power factor can be
widely classifieds as passive and active approaches .
The passive approaches use capacitive inductive filters to
achieve PCF, while the active approaches use a switched-
mode power supply to shape the input current. These
approaches are discussed briefly next.
2.1 Passive Approaches
In the passive approaches, a full bridge rectifier with an
LC filter is used to reduce the line current harmonic limits.
Generally the LC filter can be placed in either the AC-side
or the DC-side of the rectifier as shown in Figure l(a).
Placing the LC filter in the AC-side will result in more
pure sinusoidal input current.
Passive PFC can meet the regulation with high
efficiency, superior reliability, low cost, and low EM1 [7-
81, On the other hand, the filter capacitor voltage vanes
with the line voltage, which bas a detrimental effect on the
performance and efficiency of the DCDC converter. When
considering a hold-up time for the power supply, the bulk
capacitance has to be increased and becomes very bulky
compared to what it would be without this varying voltage.
As a result, the passive approaches seem to be more
attractive in low-power applications, up to 300Watts, and
are more suitable for narrow line voltage range. Other
drawbacks are the size and weight of the filter choke
inductor. However, the majority of power supplies
manufachlred in low-power and cost-sensitive applications
have adopted the passive PFC approaches.
2.2 Active Approaches
In active PFC approaches, a switched mode converter is
employed to overcome the limitations of the passive
approaches. Assuming unity power factor, the line current
should be sinusoidal and in phase with the line voltage.
That will result in pulsating output power than contains -
in addition to the real (average power) - an alternating
component with double-line frequency. Since the power
demanded by most loads is constant, an energy storage
element is needed. Since the inductor-stored energy cannot
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IClT 2003 - Maribor. Slovenia
match this excessive energy, another storage component is
needed. This storage capacitor is normally located between
the two stages and should handle the double-line frequency
ripple component, which make it bulky.
This second, harmonic problem that presents itself on
the output of the PCF stage cannot be internally solved.
Usually, a compromise between PFC and output voltage
ripple should he made, but most of the time this output
voltage is not good enough to supply the load. As a result,
another DCDC converter is needed, or what is so called
post regulator, to solve this problem and achieve tight
output regulation. The result in the most powerful PFC
configuration and is the active two-stage PFC shown in
2.2.1 Two-Stage PFC converter
This configuration implies the use of two converters to
achieve both power factor correction and output regulation
in addition to the rectification circuit and the input EM1
filter. These converters are independent, which means each
one bas its own switches and control circuit. The PFC
converter performs the input current shaping using one of
the popular converter topologies (buck, boost, buck-boost,
flypack, SEPIC, Cuk, ZETA) in addition to one of the PFC
controlling techniques. The boost converter is widely used
due to its advantages, which include good power factor,
grounded switch, input inductor and simplicity. However,
this PFC converter normally has a low bandwidth control,
which implies a loosely regulated output voltage across the
storage capacitor. In universal line voltage applications, the
DC bus voltage may vary between 380-400V. Because of
the relative high voltage on the storage capacitor, the value
of the capacitance can be optimized to provide the
necessary hold up time. The DCDC converter is connected
to the storage capacitor to provide the necessary tight
output voltage regulation with the appropriate gain and,
most of the time, provides isolation.
2.2.2 Single-Stage PFC converter
The single-stage PFC configuration came about to
reduce the cost and complexity of the two-stage structure,
and it can be viewed more as a modification on the two-
stage PFC rather than a class by itself. As can be seen from
Figure l(c), the PFC and the DCDC cell share the control
circuit and can also share the switches in this
configuration. The energy storage capacitor between the
two stages serves as a buffer for frequency isolation and to
provide the converter with the necessary hold up time.
However, in single stage configuration, the voltage across
the storage capacitor is not regulated, -because the
controller is used to regulate the output voltage. As a
result, this voltage can vaty greatly, usually between 130-
500V in universal line application. This will have a
negative impact on the design and cost of the PFC
converter. More about single-stage converters is presented
in Section IV.
(a) General Struchlres of the Passive PFC Approaches
& & -
@) System Configuration ofTwo-Stage PFC Power Supply
(c) System Configuration of Single-Stage PFC Power Supply
Figure 1 General StnrchlTes ofthe PFC Conveners
2.3 Approach Comparisons
Generally, the passive approach should be considered in
low power applications, especially when designing to meet
the minimum regulation requirements with a narrow line
At low power levels (<300W), the active single-stage
approach offers a great advantage over the passive
approaches due to its simple structure, low cost, minimum
weight and better PFC performance.
Unity power factor and tight output regulation for any
power range can be achieved through the two-stage active
PFC. This structure can guarantee compliance with any
regulation and is compatible with universal line voltage
applications. Some negative factors of the two-stage
scheme include cost, size, and sometimes its lower
Table 1 provides a general relative perfomance
comparison for the passive and active single- and two-
stage approaches .
3 PFC CONTROL TECHNIQUES
We can classify the PFC control techniques into active
control and automatic control of line current. Since the
active control is mostly associated with the Continues
Conduction Mode (CCM) of the inductor current,
sometimes they refer to it as CCM shaping techniques. The
automatic control, on the other hand, is used when the
converter operates in Discontinues Conduction Mode
(DCM); hence we can refer to it as a DCM shaping
technique. In a two-stage active PFC converter, the DCDC
cell regulates the output voltage, while the PFC cell
performs the current shaping using one of the techniques to
be discussed next. In the single-stage configuration, there
is one controller that should perform both tasks, and hence
the automatic control is mostly adopted by single-stage
PFC converter. Figure 2 shows the general classifications
of the PFC control techniques. In general, active
controllers are more efficient in achieving a pure sinusoidal
input current with unity power factor. But on the other
hand, they are more complex to implement and have a
higher cost. The automatic control is simple and easy to
implement, however it produces high peak current and
large EMI. This limits the use of the automatic control to
low- and medium-power applications. Since the primary
purpose of this paper is to analyze the single-stage PFC
converters, these techniques will be briefly introduced with
emphasis on the automatic control.
: ; : ; ;
In active control techniques, the controller controls the
PFC cell to work as a resistor emulator by sensing the line
voltage, the line current; the output voltage then generate
the command signal as shown in Figure 3. If the command
signal is programmed to command a current (i.e. the switch
or inductor current), then we can refer to it as current mode
control. The opposite is the voltage mode control when the
command signal commands a voltage (that is related to the
line current) to follow a certain shape.
~ r t . n , i
W ~ O I
E O n ~ R ~
variation of the switching frequency that makes the design
converter t $ z
“ , I
Figure 3 General Block Diagram of Active Control Techniques
3.1.1 Current Mode Control
Since the primary goal of the PFC controller is to
control the line current, current mode control is more
effective in achieving the desired results. Many methods
have been developed; the most popular among them is the
average current control [lo-111, where the average value
of the line current over one or few switching periods is
controlled to have a sinusoidal waveform that is in phase
with the line voltage. Peak current control [12-131 is
another method, which is suitable for the boost topologies,
where the peak value of the inductor current is commanded
to shape the input line current. Peak current control is
simpler to be implemented, and has good efficiency. On
the other hand, it has higher current distortion and stress
than average current control.
Voltage mode control is used when the converter is
voltage driven, such as the case in the buck converter. It
can be shown that we can control the line current to be in
phase with the line voltage by controlling the voltage of the
input capacitor or the voltage of the input inductor.
Normally, we refer to the voltage mode control as a
Capacitor voltage control [I61 when we control the
capacitor voltage by a sinusoidal command signal to
indirectly adjust the line current to be in phase with the line
voltage. In order to generate the command signal, the
controller has to detect the phase shift, which is not a
simple task. Also the capacitor voltage control is very
sensitive to parameter variation and perhuhations. To
overcome this difficulty, inductor voltage control  was
invented, which is simpler in implementation and more
effective in keep the line voltage and current in phase.
3.2 Automatic Control @CM)
The automatic control approach resulted in the reduced
cost and complexity of the active approaches. Unlike active
approaches, the automatic control is valid only when the
PFC converter works in the DCM. Automatic control can
be used with two-stage or single-stage PFC stage, when the
front-end cell has a constant input port-resistance under
variable input voltage and constant output voltage.
Examples of such topologies are buck-boost, Cuk and
SEPIC converters when operated in DCM. Other
converters can also be used to produce high power factor
with an accepted amount of harmonics distortions, such as
the boost converter. In automatic control, the main goal is
to keep the duty ratio constant (very small variation) then
the peak inductor current will follow the line voltage when
operating in DCM. The average of that current will result
in a sinusoidal input cunent as shown in Figure 4. Hence,
the control loop will not need any feed forward loops. It is
to be noted that as a result of the shape of the input current
a filtering circuit is needed in front of the converter to
smooth the pulsating current. Also, since the inductor
current will operate in DCM, it cannot store any energy.
This is because all the stored energy should be released by
the end of the switching cycle, and a bulk capacitor should
Figure 4 The Line and loductor Current when Operates in DCM
With the increasing dollar market for PFC converters,
the cost-effective single-stage PFC ACDC conversion is
one of the top priority research areas in Power Electronics.
This section addresses the major technical issues in a
single-stage PFC converter and some of the effective
(a) Boost-Flyback Combination Circuit (BIFRED)
I I Line I P
@) Boost-Buck Combination Circuit (BIBRED)
Figure 5 Single-Stage Single-S~tch Power Factor Correction Circuils
A numher of combinations have been studied by the
recent researchers [18-311 to integrate the two-stage
converter into a single-stage converter. The DCM input
technique has been widely used in single-stage PFC
circuits. Using the basic converter, topologies (usually
boost or flyback converter) operate in DCM combination
with another isolation converter to form the converter. A
storage capacitor is generally required to hold the DC bus
voltage. Unlike the two-stage PFC circuit, in which the bus
voltage is controlled, the single-stage PFC converter has
only one feedback loop from the output. The input circuit
and the output circuit must share the same control signal.
Figure 5 shows two examples of well-known two-stage
PFC circuits [lS]. Since the input circuit and the output
circuit are in a single-stage, it is possible for them to share
the same power switch. Thus, it results in single-stage,
single-switch power factor correction (S4-PFC) circuit.
However, most of the proposed topologies were
especially attractive in low power applications, and the
following drawbacks still exist:
1) Under high current and low duty ratio condition, the
bulk capacitor voltage is high. As a result, high
rating capacitor and switch devices must be used,
which will result in increased costs.
2) Because it is a DCM-CCM type, the bulk storage
capacitor voltage is a function of the load, resulting
in wide capacitor voltage variation and a difficulty
in designing for universal input applications.
3) High current stress is due to the DCM operation.
The PFC stage normally operates in DCM mode to
utilize its inherent current shaping. Current stress
not only accompanies increased switching losses
and lower efficiency, hut also EM1 issues.
4) The unavoidable leakage inductance of the power
transformer produces high voltage spikes at
resulting in decreased
In order to improve the performance of the single-stage
PFC converters, many techniques were developed to
overcome some of the disadvantages mentioned above.
The two capacitors topology shown in Figure 6 came to
enhance the PFC capability of the boost circuit and to
relieve the voltage spikes produced by the power
transformer. The voltage across the storage capacitors is
kept at low levels [19-221.
1 : l : "
Figure 6 Circuil Schematic afthe Two-Storage Capacitor Convener
Soft-switching techniques were introduced to the single-
stage PFC converters in an attempt to enhance the power
density and efficiency . In Figure 7, an auxiliq
branch was added in parallel to the power switch of Figure
6 to achieve Zero Voltage Transition (ZVT) at the turn ON.
In a ZVT circuit, an auxiliary switch needs to be turned
ON for a short time before turning the main switch ON. In
this topology, the resonant inductor was coupled with the
power transformer to eliminate circulating energy.
Figure 7 Two-Storage Capacitor Convener with ZVT
With all the attempts to improve the single-stage PFC
converter's efficiency, its performance still cannot compete
with the two-stage scheme because of the high voltage and
current ratings. It was noticed that one of the common
characteristics in the conventional single-stage PFC
topologies is that there is only one power flow path. In
other words, the active switch needs to process all output
power twice, once through the PFC cell and another time
through the DCDC cell to the output. In [24-251, another
approach was proposed. The main idea was to add an extra
winding to the input inductor to get another power flow
path directly to output as shown in Figure 8. Based on this
concept, a PFC cell with direct power transfer branch was
proposed in  and was called the flyboost cell. As
shown in Figure 9, the flyboost PFC cell consists of a boost
converter with an additional winding to the output. It is
clear that the efficiency of this method is higher than the
conventional method. And the current stress is also reduced
since the power processed in the DCDC cell is reduced. In
addition, when instantaneous input voltage goes higher,
and IV;.(t)l is higher than Vbms - nlV,, the voltage across
transformer T, primary winding is clamped to V,lV,(t)l
(nl is the turn ratio of TI). Figure lO(a) shows a successll
implementation of the flyboost cell . Figure 11 shows
an experimental efficiency comparison between the
converters in Figure 6 and 1O(a) at full load of 150W. It
can be seen that the topology with the flyboost PFC cell
achieves the higher efficiency.
Direct Energy Transfer
Figure 8 Block Diagram for the Parallel PFC Scheme
Figure 9 FlybaoslPFC Cell
Despite the advantage of calming the bus voltage, for
universal input voltage application, this voltage is still high
for the bulk capacitor. The peak value of the maximum
universal input voltage is 375V. Normally, n,V. will be
chosen to be equal to the peak value of the minimum input
voltage, which is about 120V. Then the intermediate bus
voltage will be around 495V, and the minimum voltage
requirement for the intermediate bus capacitor is about
500V. Unfortunately, the maximum voltage of an
economical bulk capacitor available in today's market is
only 450vDC and two capacitors connected in a series are
necessaly for that case. The topology in Figure 10 (b) came
with an optimum solution for universal input applications
. This topology has its intermediate bus voltage as low
as the peak input voltage. The maximum intermediate bus
voltage can be less than 400VDC, as shown in Figure 12,
for universal voltage applications. The commercial 450V
bulk capacitor and 600V economical MOSFET can be used
to meet low cost requirements. Some further modifications
to improve the efficiency for this topology can be found in
As an attempt to enhance the power handling
capabilities of the single-stage converter, the Asymmetric
Half-Bridge configuration (AHBC) was used i n the DCIDC
stage  as shown in Figure lO(c). It is distinct from
other soft-switching converters by its inherent Zero-
Voltage-Switching (ZVS) capabilities, where ZVS can be
achieved for both switches by allowing the leakage
inductor of the main transformer to resonate with the
switch parasitic capacitances. On the other hand, while the
converter is able to handle high power, it was found that it
is not suitable for wide input range.
(a) Flyboost-Parallel/Series Forward Single-Stage PFC Converter
(b) Modified BIFP.ED Convener
L I S
(c) Asymmetric Half Bridge Soft-Switching PFC Converter
Figure 10 Several Single -Stage PFC Converter with Direct Energy
Figure 1 I Experimental Efficiency Comparison
--c 265V input
Figure I2 Measured Bulk Capacitor Voltage Versus Output Power at
Different Input Voltage
A review of common power factor correction techniques
is given in this paper. In low power applications and
especially when designing to meet the minimum regulation
requirements with a narrow line voltage range, the passive
approach appears to be a strong candidate. Unity power
factor and tight output regulation for any power range can
be achieved through the two-stage active PFC. For low to
medium power levels (<300W), the active single-stage
offers a great advantage over the passive approaches due to
its simple structure, low cost, minimum weight and better
PFC performance. Depending on the specific applications,
an engineering decision should be made whether to use the
single-stage or the two-stage approach in this power range.
The decision should be made in terms of cost and
performance trade offs. While there are many issues that
still need to be addressed when designing the single-stage
PFC converter, it has potential in new market trends.
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