A Novel Method For Elimination Of current-Harmonics-In-Single-/' >Line-current Harmonics In Single-stage PFC Switching Regulators - Power Electronics, IEEE Transactions on IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 13, NO. 1, JANUARY 1998
75
A Novel Method for Elimination of Line-Current
Harmonics in Single-Stage PFC
Switching current-Harmonics-In-Single-/' class='doin' >Regulators
Martin H. L. Chow, K. W. Siu, Chi K. Tse,
Senior Member, IEEE,
and Yim-Shu Lee
Abstract This
paper
studies
a
particular
single-stage
power-current-Harmonics-In-Single-/' target='blank' class='doin' >factor-correction (PFC) switching regulator employing a
discontinuous-conduction-mode (DCM) boost-input cell and a
continuous-current-mode (CCM) forward output cell. Although
this single-stage PFC regulator can provide a reasonably high
power current-Harmonics-In-Single-/' target='blank' class='doin' >factor when its PFC stage is operating in discontinuous
mode, substantial reduction in line-current harmonics is possible
by applying a suitable frequency-modulation scheme. This paper
derives a frequency-modulation scheme and proposes a practical
implementation using a simple translinear analog circuit. A
quantitative analysis on the total harmonics distortion (THD)
of the line current when the circuit is subject to a limited
range of frequency variation is presented along with some
considerations for practical design. Experimental data obtained
from a prototype conrms the effectiveness of the proposed
frequency-modulation scheme. The proposed analog translinear
circuit allows custom integrated circuit (IC) implementation,
making it a viable low-cost solution to the elimination of
line-current harmonics in switching regulators.
Index Terms Control, power current-Harmonics-In-Single-/' target='blank' class='doin' >factor correction, switch-mode
power supplies.
I. I
NTRODUCTION
D
UE TO THE increased awareness of the many un-
desirable consequences of harmonic distortions in line
currents drawn by switch-mode power supplies (SMPSs),
high power current-Harmonics-In-Single-/' target='blank' class='doin' >factor and low line-current harmonic distortion are
expected to be mandatory requirements for SMPSs in coming
years. A common approach to satisfy these requirements is
to incorporate an additional power-current-Harmonics-In-Single-/' target='blank' class='doin' >factor-correction (PFC)
stage preceding the normal switching converter stage. In this
approach, the switching converter maintains a regulated output
voltage while the preceding PFC stage ensures a high-input
power current-Harmonics-In-Single-/' target='blank' class='doin' >factor. As such, the combined circuit usually has a lower
efciency and is less cost effective, especially for low-power
applications, due to the use of two separate power stages.
Recently, Redl et al. [1] proposed a family of topologies
for single-stage power-current-Harmonics-In-Single-/' target='blank' class='doin' >factor-corrected power supplies, which
amalgamates a PFC cell with a switching regulator cell to form
a single power stage. This family of single-stage PFC power
supplies naturally achieves high power current-Harmonics-In-Single-/' target='blank' class='doin' >factor by employing a
Manuscript received August 23, 1996; revised February 7, 1997. This work
was supported by the Research Grants Council of the University Grants
Committee, Hong Kong Polytechnic University. Recommended by Associate
Editor, F. D. Tan.
The authors are with the Department of Electronic Engineering, Hong Kong
Polytechnic University, Hong Kong.
Publisher Item Identier S 0885-8993(98)00487-6.
discontinuous-conduction-mode (DCM) boost cell. This sim-
ple boost cell is followed by a second switching cell, e.g.,
a forward cell, which provides regulated output voltage. An
important feature that distinguishes this family of circuits is
the sharing of one active switch, or one set of active switches,
by the otherwise two separate stages.
The general control strategy of the aforementioned circuits
involves a simple constant-frequency duty-cycle modulation
scheme, which mainly provides output regulation, with the
power current-Harmonics-In-Single-/' target='blank' class='doin' >factor maintained reasonably high by virtue of DCM
operation of the boost PFC cell. It is apparent that while these
circuits are simple, they have limited performance in terms
of PFC. Further performance improvements will be necessary
if regulatory bodies impose more stringent restrictions on
harmonic distortion in the future.
In this paper, we discuss a particular type of the single-stage
power-current-Harmonics-In-Single-/' target='blank' class='doin' >factor-corrected power supplies proposed by Redl et al.
[1] in which a DCM boost cell and continuous-current-mode
(CCM) forward cell are used. We will derive an appropriate
model for the single-stage PFC power supply under investi-
gation, based on which we will develop a novel frequency-
modulation scheme for achieving unity power current-Harmonics-In-Single-/' target='blank' class='doin' >factor, i.e.,
a harmonic-free line current [2]. We will also study the
performance of the frequency-modulation scheme when a
limited range of switching frequency excursion is permitted.
Analytical as well as experimental results will be presented.
In constructing the experimental circuit, we implement the
control algorithm with a translinear circuit, which provides the
necessary analog computation [4]. Although our experimental
circuit employs discrete components, the translinear circuit
can be readily implemented in custom IC. Thus, the whole
control circuit for the proposed single-stage PFC regulator can
be integrated in a single integrated circuit (IC) chip, making
it a very practical and low-cost alternative for the design of
unity-power-current-Harmonics-In-Single-/' target='blank' class='doin' >factor power supplies for low-power applications.
II. P
RINCIPLE OF THE
S
INGLE
-S
TAGE
PFC R
EGULATOR
U
NDER
S
TUDY
The simplied schematic of a single-stage PFC regulator,
comprising of a cascade connection of a boost cell and forward
cell, is shown in Fig. 1. The two constituent cells share the
same switch
Nodes
and
serve as both the output
terminals of the boost cell and the input terminals of the
08858993/98$10.00
©
1998 IEEE 76
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 13, NO. 1, JANUARY 1998
Fig. 1.
A single-stage PFC regulator using boost-input cell.
forward cell. The presence of diode
prevents the primary
current of transformer
from circulating through diode
The charging current
of capacitor
is a rectied
sinusoid, which causes a ripple voltage to appear on the output
voltage
of the boost cell. The amplitude of the ripple voltage
would depend on the value of
and the input current of the
forward cell. With a sufciently large
, the ripple voltage
can be kept to a small value compared to the dc component
of
As a compulsory requirement of any PFC power supply, a
storage element is needed to buffer the difference between the
instantaneous input power (square of sine) and the instanta-
neous output power (constant value). In this particular case,
capacitor
serves the purpose, presenting itself as the load
for the boost cell and as the source for the forward cell.
In this particular single-stage PFC regulator, the duty cycle
of switch
is used to regulate output voltage
via a voltage
feedback loop. Under this condition, it can be shown that
the shape of the averaged input current
of the boost cell
is not an ideal rectied sinusoid. In the following section,
we shall establish the averaged model for the single-stage
PFC regulator, which will be used to analyze the performance
in respect of harmonic distortion, power current-Harmonics-In-Single-/' target='blank' class='doin' >factor, and voltage
stress. Analysis based on the averaged model also leads to a
novel frequency-modulation scheme, which can achieve unity
power current-Harmonics-In-Single-/' target='blank' class='doin' >factor, as will be discussed in Section IV.
One important feature is worth mentioning here. In its
originally proposed form [1], this family of circuits operates
both cells in DCM, preventing the voltage stress of the storage
capacitor from uctuating with the load current. However,
this will require a more complicated control when frequency
modulation is applied to improve power current-Harmonics-In-Single-/' target='blank' class='doin' >factor since varying
the frequency will have effects on the output regulation of
the DCM switching cell. Hence, we propose to operate the
forward cell in CCM such that output regulation is unaffected
by frequency variation. As a result, frequency modulation
is used exclusively for achieving unity power current-Harmonics-In-Single-/' target='blank' class='doin' >factor, while
duty-cycle modulation is used for output-voltage regulation.
III. D
ERIVATION OF
A
VERAGED
M
ODEL
FOR THE
S
INGLE
-S
TAGE
PFC R
EGULATOR
In the following discussion, it is assumed that the boost cell
is operating in discontinuous mode, whereas the forward cell is
Fig. 2.
Waveforms of
i
i
and
i
D
:
in continuous mode. The waveforms of input current
of the
boost cell and charging current
of
are shown in Fig. 2,
where
denotes the switching period,
denotes the on-
time of switch
, and
denotes the on time of diode
For the boost cell operating in discontinuous mode, we have
(1)
To avoid confusion, throughout the sequel, overbar
de-
notes averaged values over one switching cycle. By inspection
of the waveform shown in Fig. 2, the averaged input current
is given by
(2)
(3)
The averaged charging current of
is
(4)
Furthermore, with the forward cell operating in continuous
mode, the averaged voltage driving the output lter is equal to
, and the averaged current discharging
is
Thus, a complete averaged model [3] for the single-stage PFC
regulator can be obtained as shown in Fig. 3.
To facilitate steady-state analysis, the following conditions
will be assumed whose validity can be readily veried.
1)
is essentially a dc voltage by virtue of
being
sufciently large. We shall denote this voltage by
in the analysis that follows.
2) The output voltage is well regulated, which implies that
the duty cycle
of switch
is constant because
is
approximately a dc voltage. CHOW et al.: ELIMINATION OF HARMONICS IN PFC SWITCHING REGULATORS
77
Fig. 3.
Averaged behavior model of the single-stage PFC regulator.
Fig. 4.
Averaged input current
i
i
waveform with
^
v
i
=V
c
= 0:75:
For off-line operation, the input voltage is a rectied sinusoid
, where
is