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A Single-Stage SEPIC PFC Converter for Multiple Lighting LED Lamps
*Hsiu-Ming Huang, **Shih-Jen Cheng, **Tai-Hung Wang, **Huang-Jen Chiu, Member, IEEE
*Dept. of Electrical Engineering, Chung-Yuan Christian University**Dept. of Electronic Engineering, National Taiwan University of Science and Technology
AbstractThis paper presents a SEPIC PFC converter for
driving multiple lighting LED lamps. With the aid of thisconverter, high power factor and high efficiency can be
achieved by a simple single-stage circuit with low devicestress features. A burst-mode dimming method is used to
regulate the current and brightness of multiple LED lamps.
A laboratory prototype is also built and tested. With the
prototype, high power factor can remain under universalinput voltage operation and individual lamp brightness
control.KeywordsSEPIC PFC Converter, Multiple Lighting LED
Lamps, Single-Stage, Burst-Mode
I. IntroductionThe rapid advancements in new materials and
manufacturing technologies have facilitated the use of
high-luminance LEDs for lighting applications, and they
have efficacies (lm/W) above those of incandescent lamp,
which are growing to fluorescent efficacy levels [1, 2].Compared with fluorescent lamps, LED lamps have
numerous advantages, such as up to 100,000 hours ofoperation life, a wide range of temperature operation for
-20C to 120C, and their ability to work with low and safevoltages. In general lighting applications, a single-stage
DCM Flyback PFC converter as shown in Figure 1(a) is
commonly used to drive LED lamps for achieving a high power factor and regulating lamp current with a simple
circuit configuration. However, DCM operation as shown in
Figure 1(b) causes high peak current stresses and serious
electromagnetic interference (EMI) problem [3, 4]. MultipleLED lamps also usually need to be connected in parallel for
obtaining enough lighting levels. The current/ voltagecharacteristic variations of high-luminance LEDs as shown
in Figure 2(a) cause brightness difference. Therefore,
dimming control is an important design consideration for
multiple lamp lighting applications. It is usually used toregulate lighting levels for human needs as well as to
achieve energy saving. A transconductance-amplifierdimming method as shown in Figure 2(b) is widely used for
current sharing among paralleled LED lamps. Theconduction losses of the dimming transistors under
dimming operation are very significant such that thethermal problem of the lighting system will be difficult to
solve [5-8]. In this paper, a single-ended primary inductanceconverter (SEPIC) with burst-mode (BM) dimming is
presented for driving multiple lighting LED lamps. High
power factor and high efficiency can be achieved by a
simple single-stage circuit with low device stress
characteristics. A large input filter used for eliminating thecurrent harmonics in the DCM Flyback PFC converter is
unnecessary. Correspondingly, a burst-mode dimming
method is designed to control the current and brightness ofmultiple LED lamps. In the following sections, the
operation principle and design considerations of the studied
LED lamp driver will be analyzed and discussed in detail.
(a)
(b)
Figure 1(a) Single-Stage DCM Flyback PFC Converter and(b) Current Waveform.
(a)
(b)Figure 2(a) LED C/ V Characteristic Variations and (b)
Transconductance-Amplifier Dimming.
II.Operation Principle and Steady-State AnalysisAs shown in Figure 3, the studied LED lamp driver is
implemented by adopting a single-stage SEPIC PFC
converter. For achieving the current adjustment of the LED
lamps, the burst-mode dimmers are connected in series with
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the lamps. Individual lamp brightness can be controlled by
the corresponding dimming transistors. These dimming
transistors are operated as low-frequency switches. Underdimming operations, the conduction losses can be
significantly reduced. Likewise, the thermal problem on the
dimming transistors for the transconductance-amplifier
dimming method can be improved. The single-stage SEPICPFC converter is composed of PFC choke Lp, power switch
Q, DC bus capacitor Cbus, coupled inductor Lc, freewheeling
diode Df, reset diode Dr and output capacitor Co. The BM
dimmers are connected in series with the corresponding
LED lamps to regulate the LED current and brightness.Based on the symbols and signal polarities shown in Figure
3, the theoretical waveforms of the single-stage SEPIC PFC
converter are shown in Figure 4. There are six switching
modes within an operating cycle. Referring to theequivalent circuits in Figure 5, the operating principle of the
single-stage PFC converter can be explained in detail.
Figure 3 Schematic Diagram of the Studied LED Lamp Driver.
Figure 4 Theoretical Waveforms of the Single-Stage SEPIC
PFC Converter.Mode I (t0~t1): At t0, the power switch Q is turned on.
During this time interval, the current ILp flowing through the
PFC choke Lp increases linearly and the freewheeling diode
Df remains off. The negative voltage across the leakageinductance Llk of the coupled inductor Lc forces the current
Ir to decrease rapidly at the rate of
)VVN
N(
L
1-
dt
dIobus
p
s
lk
r += , (1)
Mode II (t1~t2): At t1, the current Ir decreases to zero and
the reset diode Dr is turned off with zero-current switching.The DC bus voltage Vbus forces the reversed current ILm
flowing through the magnetizing inductance Lm of the
coupled inductor Lc to change its direction at t2.Mode III (t2~t3): During this time interval, both the
currents ILp and ILm increase linearly. The freewheeling
diode Df and the reset diode Dr remain off. The energy isstored in the PFC choke Lp and the magnetizing inductance
Lm of the coupled inductor Lc.
Mode IV (t3~t4): At t3, the power switch Q is turned off.
The energy stored in the PFC choke Lp is released to the DC bus capacitor Cbus and the output capacitor Co, while the
energy stored in the magnetizing inductance Lm of the
coupled inductor Lc is transferred to the output capacitor Covia the conducting diode Df. The currents ILp and ILmdecrease linearly at the rates of
p
inbusoLp
L
V-VV-
dt
dI += , (2)
m
oLm
L
V-
dt
dI= , (3)
The positive voltage across the leakage inductance Llk
of the coupled inductor Lc forces the current Ir to increaselinearly at the rate of
op
s
lk
r 1)V-N
N(
L
1
dt
dI= , (4)
Then the decreasing rate of the freewheeling diodecurrent IDfcan be expressed as
dt
dI
N
N-
dt
dI
dt
dI
dt
dILr
p
sLmLpDf += , (5)
Mode V (t4~t5): At t4, the current ILm flowing through the
magnetizing inductance Lm of the coupled inductor Lc
changes its direction. During this time interval, both the
diodes Dfand Dr remain on.
Mode VI (t5~t6): At t5, the current IDf decreases to zero
and the freewheeling diode Df is turned off withzero-current switching. The circuit will then proceed back
to Mode I after completing one operating cycle Ts.
From the theoretical waveforms shown in Figure 4, itis obvious that Equation (6) must be satisfied for achievingthe zero-current turn-off of the freewheeling diode Df.
s
pkDf,Df
)T-(1
I-
dt
dI
, (6)
where IDf,pk is the peak value of the freewheeling diode
current IDf. It is the same as the peak switch current IQ,pkthatcan be represented as follows:
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)L
1
L
1(
2
)T-(1VI
12
4II
mp
soLppk,Qpk,Df ++
+==
, (7)
Based on the volt-second balance of the PFC choke Lp
and the magnetizing inductance Lm, the voltage transferratio of the single-stage SEPIC PFC converter can be
obtained as follows [9-11]:
-1VV
in
o = , (8)
The input voltage Vin can be expressed as
tfsin2VV lpin = , (9)
where Vp and fl are the peak value and line frequency of the
input voltage, respectively. Assuming that the ripple current
on the PFC choke Lp is negligible, the input current I in ofthe single-stage PFC converter can be expressed as follows:
tfsinV
P2tfsinII l
p
llpin 22 ==
, (10)
where Pl is the load power and Ip is the peak value of theinput current.
Mode I (t0~t1)
Mode II (t1~t2)
Mode III (t2~t3)
Mode IV (t3~t4)
Mode V (t4~t5)
Mode VI (t5~t6)
Figure 5 Equivalent Circuits under Different Switching
Modes.
III. Design ConsiderationsFor achieving the current adjustment of the LED lamps,
the burst-mode dimmer shown in Figure 6(a) is connected
in series with the lamp. Figure 6(b) shows the theoretical
waveforms of the LED dimmer. The average LED lampcurrent can be represented as follows:
n2,...,1,j,II jlamp_pk,jlamp,jlamp, == , (11)
where Ilamp_pk,j is the peak value of the LED lamp #j and
lamp,j is the duty ratio of the corresponding dimming
transistor Qd,j. As shown in Figure 6(a), the LED lamp
current is regulated by adjusting the duty ratio lamp,j of the
dimming transistor Qd,j. The operating frequency of thedimming transistors is usually higher than 70Hz, making
them perceivable to the human eye. Considering the
switching loss for the dimming transistors, the burst-modefrequency in this paper is designed at 400Hz. However, any
LED failure will result in the extinguishment of the
corresponding LED lamp. As shown in Figure 6(a), eachLED is connected in parallel with a zener diode. The
voltage across a failed LED reaches the breakdown voltage
of the zener diode and the lamp current flows through the parallel connected zener diode. The LED lamp will not
extinguish even though any LED fails.
(a)
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(b)
Figure 6(a) Circuit Schematic and (b) TheoreticalWaveforms of the Burst-Mode Dimmer.
The PFC choke Lp of the studied single-stage SEPIC
PFC converter can be determined by the given current
ripple ILp at the peak value of the minimum input voltage
Vin,min(pk), the duty ratio pk at that input voltage and the
switching frequency fs as follows:
o
)pkmin(,inopk
V
VV = , (12)
Lps
pk)pkmin(,inp
If
VL
= , (13)
The magnetized inductance Lm of the coupled inductor
Lc can be obtained by the given current ripple ILm asfollows:
Lms
pkom
If
)-(1VL
= , (14)
The DC bus capacitor Cbus can be selected as follows:
)L(Lf4
1C
mp2
r2bus +
=
, (15)
where fr is the resonant frequency of Cbus, Lp, and Lm. This
resonant frequency should be lower than the switchingfrequency fs and much greater than the line frequency fl.This is to assure that the capacitor voltage Vbus is considered
constant in one switching cycle and follows the source
voltage in a line period.For achieving the zero-current turn-off condition of the
freewheeling diode Df, the turn ratio of the coupled inductor
Lc can be obtained from Equations (6) and (7) as follows:
)V(3VVV
)V(VPf24)
L
1
L
1(
2
1-
L
1)-/N(N/NN
ino2
po
2inols
mplk
psps
+
+++>
, (16)
IV.Experimental VerificationsTo verify the feasibility of the studied LED lamp driver,
a laboratory prototype with following specifications wasdesigned and tested.
Input Voltage: 80V~260V
Switching Frequency: 100kHz
Rated Output Current: 1.4A
Rated Output Voltage: 46V
The circuit parameters for the laboratory prototype aresummarized in Table 1. The used LED lamp is composed of
13 pieces of series-connected LUMILEDS emitter-type
LEDs. This LUMILEDS diode is a 1W high-luminance
LED with a nominal voltage of 3.42V at a rated current of
350mA. The prototype is designed to supply four paralleled
LED lamps with a maximum output current of 1.4A.Figures 7(a) and (b) show the measured waveforms of the
input voltage Vin and current Iin at the input voltage of 110V
and 220V, respectively. The input current Iin has a
near-sinusoidal waveform and is in phase with the inputvoltage Vin. Power factor and efficiency variations under
different input voltage are depicted in Figure 8. It is evidentthat high power factor and high efficiency can be
maintained under universal input voltage operation.
Meanwhile, Figure 9 shows the measured gating signal Vgs
of the power switch Q and the freewheeling diode currentIDf. It can be observed that the freewheeling diode Df is
turned off with zero-current switching. In this paper, theburst-mode dimming method is used to control the current
and brightness of multiple LED lamps. Figure 10(a) shows
the measured LED lamp currents with an identical
brightness. The lamp currents shown in Figure 10(b) aremeasured under the operation conditions that lamp #1 and
lamp #2 have the same brightness and that lamp #3 andlamp #4 have varying brightness. It is obvious that the
current and brightness of LED lamps could be regulated by
adjusting the duty ratio of the dimming transistors.
Table 1 Circuit Parameters for the Laboratory Prototype.Component Description Value/Part no.
Power Switch Q IRF740
Power Diodes DfDr S3L60
PFC Choke Lp 750H
Magnetizing Inductance Lm 750H
Turn Ratio of Coupled Inductor Ns/Np 1.06
DC Bus Capacitor Cbus 0.47F/400V
Output Capacitor Co 1000F/63VLED Diodes 1W LUMILEDS Diode
Dimming Transistors Qd,j 2N7000
(a)
(b)Figure 7(a) Measured Input Voltage and Current at (a) 110V
and (a) 220V.
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Figure 8 Measured Power Factor and Efficiency Variations.
Figure 9 Measured Gating Signal Vgs and the Freewheeling
Diode Current IDf.
(a)
(b)Figure 10 Measured LED Lamp Currents (a) with Identical
and (b) Different Brightness.
V.ConclusionIn this paper, we presented a SEPIC PFC converter fordriving multiple lighting LED lamps. A burst-mode
dimming method was also studied to regulate the currentand brightness of multiple LED lamps. A laboratory
prototype was built and tested with individual lampbrightness control. It was found that high power factor and
high efficiency can be achieved by a simple single-stage
circuit with low device stress features. The experimental
waveforms of the laboratory prototype were
correspondingly shown to verify the feasibility of the
proposed scheme.
Acknowledgment
The authors would like to acknowledge the financialsupport of the National Science Council of Taiwan, R. O. C.
through grant number NSC 95-2221-E-011-215.
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