IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 24, NO. 5, MAY 2009 1399
Means of Eliminating Electrolytic Capacitor in
AC/DC Power Supplies for LED Lightings
Linlin Gu, Xinbo Ruan, Senior Member, IEEE, Ming Xu, Senior Member, IEEE,andKaiYao
Abstract—This paper proposes two methods of reducing the stor-
age capacitance in the ac/dc power supplies for light emitting diode
(LED) lighting. In doing so, film capacitors can be adopted instead
of electrolytic capacitors to achieve a long power supplies’ life-
time. The voltage ripple of the storage capacitor is intentionally
increased to reduce the storage capacitance. The method of deter-
mining the storage capacitance for ensuring that the boost power
factor correction converter operates normally in the whole input
voltage range is also discussed. For the purpose of further reducing
the storage capacitance, a method of injecting the third harmonic
current into the input current flow is proposed. While ensuring
that the input power factor is always higher than 0.9 to comply
with regulation standards such as ENERGY STAR, the storage
capacitance can be reduced to 65.6% of that with an input power
factor of 1. A 60-W experimental prototype is built to verify the
proposed methods.
Index Terms—Harmonic current injection, LED, power factor
correction (PFC), power supply, voltage ripple.
I. INTRODUCTION
T
HE RAPID development of LED over the last few years
has opened up new opportunities in the general illumina-
tion market, thanks to its distinct advantages such as high effi-
cacy, long lifetime, environmental friendliness, and small size
over incandescent and fluorescent lamps [1]–[4]. The power
supply for LED lighting is an ac/dc converter, which converts
a regular ac voltage to a low dc voltage for an LED driver.
The input power factor is an important requirement of the ac/dc
converter. It needs to be higher than 0.9 for most commercial
luminaries [5]. Thus, the ac/dc converter must typically have
the function of power factor correction (PFC). In a PFC con-
verter, the input current is forced to be in phase with the input
voltage, leading to a pulsating input power, while the output
power is constant. To achieve this, a storage capacitor with large
capacitance is required for balancing the instantaneous power
difference. Due to the high capacity required for capacitance,
an electrolytic capacitor is often used as the storage capacitor.
Manuscript received November 14, 2008; revised January 15, 2009. Current
version published May 8, 2009. Recommended for publication by Associate
Editor J. A. Pomilio.
L. Gu and K. Yao are with the Aero-Power Sci-Tech Center, Col-
lege of Automation Engineering, Nanjing University of Aeronautics and
Astronautics, Nanjing 210016, China (e-mail: nuaa_gulinlin@nuaa.edu.cn;
yaokai@nuaa.edu.cn).
X. Ruan is with the College of Electrical and Electronic Engineering,
Huazhong University of Science and Technology, Wuhan 430074, China
(e-mail: ruanxb@mail.hust.edu.cn).
M. Xu was with FSP-Group R&D center, Nanjing, 210042, China. He is
now with the Center for Power Electronics Systems (CPES), Virginia Poly-
technic Institute and State University, Blacksburg, VA 24060 USA (e-mail:
mixu3@vt.edu).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TPEL.2009.2016662
However, it is well known that because of its liquid electrolyte,
the lifetime of an electrolytic capacitor is quite limited with
only several thousand hours under rated operating conditions.
Even with a conservative design, the theoretical lifetime of elec-
trolytic capacitors is only about 30 000 h (at a high operating
LED’s temperature) [6]. This is much shorter than the potential
lifetime of LEDs (50 000 h). Thus, the electrolytic capacitor is
an obstacle to the overall long-term reliability of the LED and
its power supply.
A review of literature shows that a variety of LED power
supplies and driver solutions, which can accurately control the
current of the LED while achieving a near-unity input power
factor, have been proposed [7]–[10]. However, an electrolytic
capacitor is required in these applications.
PFC converters can be classified into two types: two-stage and
single-stage. Two-stage PFC converters consist of a PFC stage
and a dc/dc stage. They have been widely applied in adaptors for
laptops and silver box [11]–[14]. Single-stage PFC converters
integrate the PFC stage and the dc/dc stage, leading to simple
topology and low cost. They are s uitable for low-power appli-
cations [15]–[18]. Unfortunately, no effective method has been
proposed to significantly reduce the storage capacitance in a PFC
converter such that a long lifetime of a converter can be achieved.
The objective of this paper is to propose methods to signif-
icantly reduce the requirement for a high storage capacitance,
so that film capacitors instead of electrolytic capacitors can
be adopted to achieve a long lifetime of the PFC converter.
Section II analyzes the relationship between the voltage ripple
and the storage capacitance. It also describes how the voltage
ripple of the storage capacitor can be intentionally increased to
significantly reduce the storage capacitance. The method of de-
termining the storage capacitance to ensure that the boost PFC
converter operates normally in the whole input voltage range is
also discussed. For the purpose of further reducing the storage
capacitance, in Section III, we propose a method of injecting
a third harmonic current into the input current flow to reduce
the pulsation of the input power. A 60-W prototype has been
built and tested, and the experimental results are presented in
Section IV.
II. I
NCREASE OF THE VOLTAGE RIPPLE TO REDUCE THE
STORAGE CAPACITOR
A. Relationship Between Voltage Ripple and Storage
Capacitance of a Two-Stage PFC Converter
Fig. 1 shows a schematic diagram of a two-stage PFC con-
verter, which consists of a PFC stage and a dc/dc stage, where
C
B
is the storage capacitor.
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