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13 High Efficiency And High Power Essay

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High efficiency and high power factor single-stage
balanced forward-flyback converter
Yoon Choi, Moon-Hwan Keum, and Sang-Kyoo Han†

Jeong-il Kang

Power Electronics System Laboratory, POESLA
Kookmin University
Seoul, 136-702, Republic of Korea
E-mail : †[email protected]

Visual Display, R&D Team
Samsung Electronics. Co. Ltd.
Suwon, Republic of Korea
E-mail : [email protected]

Abstract— In this paper, a high efficiency and high power
factor single-stage balanced forward-flyback converter merging a foward and flyback converter topologies is proposed. The
conventional AC/DC flyback converter can achieve a good power factor but it has a high offset current through the transformer magnetizing inductor, which results in a large core loss and low power conversion efficiency. And, the conventional forward

converter can achieve the good power conversion efficiency with the aid of the low core loss but the input current dead zone near zero cross AC input voltage deteriorates the power factor. On the other hand, since the proposed converter can operate as the

forward and flyback converters during switch on and off periods, respectively, it cannot only perform the power transfer during an entire switching period but also achieve the high power factor due to the flyback operation. Moreover, since the current

balanced capacitor can minimize the offset current through the transformer magnetizing inductor regardless of the AC input
voltage, the core loss and volume of the transformer can be
minimized. Therefore, the proposed converter features a high efficiency and high power factor. To confirm the validity of the proposed converter, theoretical analysis and experimental results from a prototype of 24W LED driver are presented.

between AC input and DC output. Even though the two-stage
configuration can provide the high power factor, good output regulation and excellent ripple voltage, it has several
significant disadvantages such as a large system size, high cost of production and low energy conversion efficiency [8].
Therefore, it is common that the two-stage driver is mainly
used for high power applications and single-stage driver is
adopted as a low power LED driver [9, 10].

(a) Single-stage flyback converter

Keywords—single stage; forward-flyback; LED driver;

I.

INTRODUCTION

Recently, light-emitting diodes (LEDs) have become one of
the most promising candidates for displays and lighting
applications, because LEDs have several favorable advantages such as a high efficiency, long life time and echo-friendliness. Therefore, traditional lighting devices such as a light bulb and fluorescent lamp tend to be replaced by LEDs [1, 2]. To drive LEDs, two types of drivers are generally used, that are a linear and switch-mode regulators [3]. Although the linear driver

features a simple circuit configuration, fast transient response and accurate current regulation, it has fatal drawbacks such as a low efficiency and serious heat generation. Therefore, the
switch-mode driver is widely used in LED applications due to its high efficiency and high power density [4, 5].
Meanwhile, since drivers for LED lightings have been
composed of two power conversion stages (ie. a power factor
corrector and isolated DC/DC converter) [6]. The first stage provides a near unity power factor and low total harmonic
distortion (THD) over an entire range of universal inputvoltage (90-270 Vrms) and the second DC/DC stage is used to provide a tight output regulation and galvanic isolation

978-1-4799-0224-8/13/$31.00 ©2014 IEEE

(b) Single-stage forward converter
Fig. 1. Conventional single-stage PFC converter circuits

Fig.1 shows conventional single-stage PFC (power factor
correction) LED drivers, which are well known as most costeffective solutions. Fig. 2 shows their transformer magnetizing inductor currents. As shown in this figure, the magnetizing
inductor offset current of flyback converter is larger than that of forward converter as followings
< iLM , flyback >=
< iLM , forward >= (1 +

822

IO
n(1 - D )

N C VIN
)
D 2TS
N P 2 LM

(1)
(2)

V
IN
L
m

-

...

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