Circuits and Systems, 2011, 2, 242-248
doi:10.4236/cs.2011.23034 Published Online July 2011 (http://www.SciRP.org/journal/cs)
An Improved Non-isolated LED Converter with Power
Factor Correction and Average Current Mode Contro l
Renbo Xu1,2,4, Hongjian Li1,2, Yongzhi Li3, Changqian Zhang1
1School of Physics Science and Technology, Cent ral So ut h Uni versi t y, Changsha, China
2School of Materials Science and Engineering, Central So uth University, Changsha, China
3College of Physics and Information Science, Hunan Normal University, Changsha, China
4Hunan Information Science Vocational College, Changsha, Chin a
Received May 3, 2011; revised May 28, 2011; accepted June 5, 2011
Abstract
A new type of high power LED drivers is proposed by adopting an improved two-stages non-isolated con-
figuration. In order to improve power factor and achieve accurate average current control under universal
input voltages ranging from 100 Vrms to 240 Vrms, the power factor correction and average current mode
control methods operating in continuous current conduction mode are designed and implemented. With the
LUMILEDS emitter type LEDs, a laboratory prototype is built and measured. And from the measured results,
it could be concluded that the proposed driver has many better performances such as high power factor, low
current harmonic, accurate average current control and switch protection.
Keywords: High Power LED, Power Factor Correction, Average Current Mode Control
1. Introduction
In today’s world of lighting applications, many elec-
tronic engineers are striving to find out a more energy
efficient and cost effective way of driving a lighting
source. A new type of lighting sources that has a great
potential to replace existing lighting sources such as in-
candescent and fluorescent lamps in the future is the
power lighting emitting diode (LED), which is due to its
merits: higher efficiency, superior longevity, continu-
ously-improving luminance and environment friendly
[1-3]. This interest in LEDs has prompted many power
electronic designers to work on driving LED at higher
power factor and output current so that it can be applied
broader in lighting applications.
In general lighting applications, the line current har-
monics should satisfy the limits set by International
Electro technical Commission (IEC) 61000-3-2 class C
regulations [4]. And the input current power factor
should be higher than 0.9 required by the Energy-Star [5].
Moreover, with only a small change in the LED current,
the corresponding luminous flux and luminous efficiency
will change by orders of magnitude. In order to avoid big
luminous flux change and meet those regulations, the
LED driver should have the power factor correction
(PFC) ability and constant average current control.
In view of the development of PFC technology, PFC
could be achieved either by passive circuit or by active
circuit. With passive PFC, which uses only inductors and
capacitors to improve power factor, it is difficult to meet
those requirements and become a good candidate be-
cause of the disadvantages of high total harmonic distor-
tion (THD) and bulky size [6,7]. In order to overcome
these disadvantages, active PFC technology is commonly
used in LED drivers due to many advantages such as low
THD, fast dynamic response, precise voltage control and
universal input voltages. And active PFC method is very
suitable for many applications field with high perform-
ance requirements, low cost and high control accuracy.
An LED driver with active PFC, which is implemented
with two stages, is shown in Figure 1. The two-stages
structure has many advantages: avoiding the Electro
Magnetic Interference (EMI) if only the two stages oper-
ate in trailing edge and leading edge respectively; de-
creasing the capacitance of the output capacitor; reducing
the bulk and extending the longevity efficiently [8-12].
The first stage is used to provide a stable voltage for the
post stage with AC/DC conversion at the universal input
voltage, while the post stage is used for the DC/DC con-
version. Unlike conventional LED drivers, it could be

R. B. XU ET AL.
243
applied to both low and high voltage cases, such as it is
widely used in car applications because the car power
supply voltage is easily changeable [13,14]. With the
goal to achieve higher power factor and LED constant
average current control, the Boost-Buck converter with
higher power rating operating in continuous current con-
duction mode (CCM) with average current mode control
(ACMC) is proposed through adding two control blocks:
PFC control and ACMC block.
2. Circuit Description
From the non-isolated LED Boost-Buck converter circuit
diagram, it can be seen that the converter consists of rec-
tifier bridge, boost circuit, buck circuit, driving signal
and the load. And the Boost-Buck converter uses ca-
pacitor as energy transfer component between the first
and post stage rather than the inductor mostly used in
other conventional converters. It can be known from the
analysis that the Boost-Buck converter features fast tran-
sient response and excellent frequency response, allow-
ing highly stable feedback regulation to be achieved with
simple circuit [15-17]. Two inductors at both input and
output side are working in continuous current conduction
mode. The inductor ripple current is low and continuous,
which can greatly reduce the requirements of input and
output filter capacitor. All switch nodes in the circuit are
isolated between the two inductors, input and output
nodes have no effect on each other, which would make
the radiation EMI (Electro Magnetic Interference) from
the converter minimized. The operational principles are
described and discussed in the next sections.
When the metal oxide semiconductor field effect tran-
sistor (MOS-FET) S1 is turned on, current flows through
the rectifier bridge, the input inductor L1 and the MOS
switch S1. The power supply stores energy in the induc-
tor L1 and the diode 1
D
V is off at the moment. When S1
turns off, current in L1 flows through the diode 1
D
V, into
the capacitor C1 and transfer the energy to storage ca-
pacitor C1 as a power supply for the post stage Buck
converter. When the MOS switch S2 is turned on, current
flows through the capacitor C1, the power switch S2, in-
ductor L2, capacitor C2 and LED strings. Transfer the
energy to the capacitor C2 and provide power supply to
the load LED strings and the diode 2
D
V is turned off
synchronously. When S2 turns off, current in inductor L2
flows through the LED strings and the free-wheeling
diode 2
D
V.
From the analysis it can be known that the average
voltage on the inductor L1 is zero as described in Equa-
tion (1).
111 1
L
onL off
Vt Vt (1)
where 1
on and 1 off represent the turn-on and the
turn-off time of the MOS switch S1 in a switching period.
According to the operational principle described in the
former, Equation (1) can be rewritten as
t t



1
11
1
iSC iS
VDTVVD T  (2)
where D1 is the duty ratio of S1 and TS is the switching
period. Solving Equation (2) for 1
C as shown in Equa-
tion (3), we can see that the output voltage is in-
creased much.
V
1
C
V

1
1
1
i
C
V
VD  (3)
Taking the same switching period TS and the duty ratio
D2 of S2, from the similar analysis of the inductor L2, we
have



12
1
CO SO
VVDTV DT
2S

(4)
Substituting (3) into (4), the relation between output
voltage and input voltage can be obtained by

2
1
1
i
ODV
VD  (5)
Figure 1. LED Boost-Buck converter diagram.

R. B. XU ET AL.
244
From the steady-state operational principle [18], it
could be known that the average current flowing through
the capacitor C1 must be zero defined as
11
C charC unch
I
tIt (6)
where char and unch stand for the charging time and
uncharging time respectively. (6) could be rewritten as
t t

1
1
12
2
L
SL S
I
DT IDT (7)
Solving (7) for 1
L
I

2
1
22
11
11
LO
L
DI DI
IDD 

(8)
3. The Proposed Circuit
From the analysis of the Boost-Buck converter it can be
known that the converter has high stability, fast transient
response and high efficiency. But the power factor is low
because of harmonic distortion and the use of inductors
and capacitors, and it can not satisfy the accurate regula-
tion of luminance due to that the light output is propor-
tional to the current delivered to the LEDs string aver-
aged over the utility period. In order to improve the
power factor of the input side of the grid and provide a
constant average current and lighting output, the tech-
nology of active PFC and ACMC is proposed through
adding a PFC control loop and an average current control
loop as shown in Figure 2.
The boost PFC circuit operates in CCM with trailing
edge modulator, while the leading edge modulation is
adopted in the Buck pulse width modulation (PWM)
constant average current control circuit. Although the
switches work under turned-on and turned-off state al-
ternately, the proposed converter reduces the harmonic
current distortion due to the inductors operated in CCM.
From the system block schematic diagram, it can be
known that the error signal between the sampled voltage
of the capacitor C1 and the reference voltage is amplified
and sent to the analog multiplier for generating a half-
sine reference current signal as same frequency and phase
as input voltage Vi. The current regulator is used to
compare the sensing current Ii with the half-sine refer-
ence current and then generate a current error amplified
signal. Compared with the sawtooth wave, the output of
the comparator is the control signal of the MOS switch
S1. Thus, through adjusting the duty cycle, the current of
inductor L1 will track the half-sine reference current sig-
nal that is to say that the input current Iin tracks the sinu-
soidal input voltage Vin well for a high power factor.
To get the working stability of current loop and a good
dynamic tracking ability of the average inductor current,
the current regulator must be designed to have high
low-frequency gain, wide mid-frequency gain, a reason-
able margin steady and strong switching ripple suppres-
sion ability. A compensation network G(s) with two pole
and one zero is taken as a current regulator as shown in
Equation (9).
 

1
1
i
P
s
Gs ss
Z




 (9)
where

12
1
ii
RC C
 , 12
112
PCC
RCC


,
11
1
ZRC
 .
Design goal is to adjust the three parameters to meet
the system’s open loop frequency domain index. And the
specific implement circuit of the current regulator is
shown in Figure 3.
Figure 2. The proposed LED driver with active PFC and ACMC.

R. B. XU ET AL.
245
To study the discrete control property of the current
regulator, a discrete mathematical mode of current sens-
ing is created and then transformed into He(s) at the
complex frequency domain.

2
2
1
nz n
s
s
He sQ


 (10)
where nS
T


,2
z
Q.
To avoid aberrance in the inductor current, PFC volt-
age error amplifier design can’t seek rapidity excessively
and the output voltage should be relatively constant in a
frequency cycle. From the perspective of frequency
analysis, the voltage regulation loop bandwidth should
be limited. And proportional integral voltage regulator is
a good candidate and its implement circuit is shown in
Figure 4.
The transfer function of the voltage regulator is
 
1
z
g
Gu sss

 (11)
where g is the proportional integral coefficient, and
1
2
z
i
CR
  according to the stability analysis.
From the system block diagram in Figure 2, we can
see that there is a sensing resistor Rs to sense the output
current. After the output current flowing through the high
power LEDs and Rs, the sensing feedback voltage can be
acquired. The integral voltage of error value between the
feedback voltage and the reference voltage Vref is sent to
the slope compensation. Compared with the sensing
feedback voltage, the comparator output is obtained to
control the RS flip-flop and then the output of RS
flip-flop controls the MOS switch S2 and regulates the
output average current precisely.
Figure 3. The implement circuit of current regulator.
U
re
f
Figure 4. The implement circuit of voltage regulator.
In order to gain the accurate average current, the slope
compensation technique is adopted, as shown in Figure
5. r is the reference voltage, and dotted line is
the average voltage of sensing feedback voltage
vavr v
s
v in a
switching period.
s
m

, r are the slope of the slope
compensation voltage and the sensing voltage
m
s
v re-
spectively. From the steady-state waveforms, it can be
seen that the sensing average voltage avr
vpresents the
average current of high power LED.
re
From the steady-state waveforms, we have
1
2rSravrss
mdTv vmdT  (12)
Take small-signal perturbation of the relevant vari-
ables as follows.
rr r avravr avr
rrr
vVvv V v
mM mdDd



 
 
(13)
Among them, the capitalized letters are steady-state
values and the variables with “” are small-signal dis-
turbances.

Substituting (12) into (13), we have
2
Srr
rravravr s
TMmDd
VvVvmDdT

 








s
(14)
With ignorance of second-order small-signal variables,
we can get the characteristics equations of DC steady-
state (15) and AC small-signal (16).

2
1
2
ravr ravr
rS
rS ss
VV
DV
nM T
MT mT



V (15)
Among (15)
s
s
mM

 , 2
1
s
r
M
n
M
 .
22
Ss
rss ravr
TD
r
T
M
MT dvvm
 



 (16)
v
m
r
–
m
s
v
s
v
av
r
r
v
2
s
dT
T
s
dT
s
Figure 5. The steady-state waveforms with constant average
current control.
r
vr
–
ms
vs
vavr
s
dT
mr 2
dTs
Ts

246 R. B. XU ET AL.
As a result of the new control technique, the driver
provides a precise constant average output current and
high power factor under pieces of LED series connection.
Moreover, the proposed converter operating in CCM has
many better performances such as variable input source
voltages, wide frequency bandwidth, high efficiency and
stability.
4. Simulation and Experimental Results
To verify the feasibility of the proposed LED driver, a
laboratory prototype with the following specifications
were designed and tested.
 Input voltage: 100 - 240 Vrms
 Switching frequency: 200 kHz
 LED current: 350 mA
The circuit parameters for the laboratory prototype are
as follows: the rectifier sampling coefficient is 0.0032;
the inductor L1 = 1.47 mH and L2 = 0.22 mH; the ca-
pacitor C1 = 3500 uF and C2 = 470 uF, the sensing resis-
tor Rs is 0.1 Ω and the reference voltage Vref = 0.035 V.
The series connected LUMILEDS emitter type LEDs is
used in this experiment. This LUMILEDS diode is a 1 W
high-luminance LED with a nominal forward voltage of
3.42 V. The laboratory prototype is designed to get a
high power factor and a constant output average current
350 mA when the input source voltage varies from 100
Vrms to 240 Vrms. Figure 6 shows the tested wave-
forms of the input voltage Vin and input current Iin at the
input source voltage of 110 Vrms and 220 Vrms, respec-
tively. It could be seen that Iin has a good near-sinusoi-
dal waveform and in phase with the input source voltage
Vin. Power factor under different input source voltages
variations are shown in Figure 7 and the high power
factor is over 0.95.
(a)
(b)
Figure 6. The tested input voltage and current waveforms at
Vin = 110 Vrms and 220 Vrms. ((a) The tested input voltage
and current waveforms at Vin = 110 Vrms; (b) The tested
input voltage and current waveforms at Vin = 220 Vrms).
Figure 8 shows that the line-current harmonics are
below the limits set by IEC 61000-3-2 class C regula-
tions with enough margin at the input source voltage of
110 Vrms and 220 Vrms, respectively. It can be seen that
these high performances such as phase-following, high
power factor, low THD are the results of the improved
APFC block.
Figure 7. Power factor vs different input voltages.
(a)
(b)
Figure 8. The measured line-current harmonics at Vin = 110
Vrms and 220 Vrms. ((a) Line-current harmonics at Vin = 110
Vrms; (b ) L in e-current h arm onics a t Vin = 220 Vrms).

R. B. XU ET AL.
247
Without the ACMC technique, the LED constant av-
erage current is 320 mA below the reference current as
shown in Figure 9(a). However, with the proposed
method the constant average current of 350 mA can be
achieved as shown in Figure 9(b). It is evident that the
LED average current and luminous flux can be regulated
by adjusting the duty cycle of MOS-FET switch. And the
steady and slowly-rising LED current of the new driver
can protect the MOS switch due to the improved ACMC
block.
All the test results are consistent with expectations
well.
5. Conclusions
In this paper, we proposed an improved non-isolated
LED converter operating in CCM with PFC and ACMC
for driving high power LED lamps. A laboratory proto-
type with LUMILEDS emitter type LEDs is used to ver-
ify the feasibility of the proposed driver. From the meas-
ured results, it can be seen that the proposed LED driver
achieves a power factor of 0.99 and a THD of 12.2% at
input voltage 110 Vrms and a power factor of 0.96 and a
THD of 10.4% at input voltage 220 Vrms. Many better
(a)
(b)
Figure 9. The LED current simulated waveforms and the
switching signal. ((a) The LED current simulated waveforms
and the switching signal Without the ACMC; (b) The LED
current simulated waveforms and the switching signal With
the ACMC).
performances such as high power factor, accurate aver-
age current control, low current harmonic and switch
protection are confirmed and the experimental results
match well with the analysis results.
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