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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

hjchiu@mail.ntust.edu.tw

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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