Linear PFC regulator for LED lighting withthe multi-level structure and low voltage MOSFETs.
Yuichi Noge

Jun-ichi Itoh

Nagaoka University of Technology
Niigata, Japan
noge@stn.nagaokaut.ac.jp

Nagaoka University of Technology
Niigata, Japan
itoh@vos.nagaokaut.ac.jp

Abstract—This paper proposes a linear PFC regulator for LED
lighting applications. The proposed circuit is small in size
because the circuit structure consists of only semiconductors andresistors without any reactors and electrolytic capacitors. The
current bypass circuit which is connected in parallel to the LED
string consists of single MOSFET, two zener diodes and tworesistors. The MOSFET is operated in an active state by a selfbias circuit. Thus, an external controller and high voltage gatedrivers are not required. The proposed circuit is experimentally
validated by using a 15 W prototype. From the experimental
results, the THD of input current is 5.1% and the power factor
is 0.999. In addition, the simulation loss analysis demonstrates
an efficiency of 91.6% for a 15.2 W prototype.

I.

INTRODUCTION

LED lighting market has grown rapidly due to theadvantage of extended life-time and improved luminance, withthe significant improvement in the manufacturing process ofLEDs [1][2]. In the conventional AC LED driver, a converter
is composed of a power factor correction (PFC) rectifier and a
DC-DC converter. The conventional converter requires
electrolytic capacitors in the DC link, however, it is wellknown that the life-time of electrolytic capacitor is shorter
than that of LED. The converter with discontinuous current
mode (DCM) operation that uses no electrolytic capacitor has
been proposed to extend the life-time of a converter [3-5].
However, the switching and conduction losses of thesemiconductor switch are increased due to the large peakcurrent. In addition, the conducted EMI occurs in the input
lines because of the switched mode power supply (SMPS).
Therefore, SMPS requires a bulky EMC filter to suppress
electromagnetic noise.
On the other hand, a diode-clamped linear multi-level
amplifier is proposed to obviate the need for passive EMI
filters and improve efficiency in the AC motor drive [6]. The
linear multi-level concept is possibly applied for an AC LEDdriver as a linear PFC regulator [7-9]. These circuits areconstructed from only semiconductors without any passivecomponents.
However, high voltage rating operational

amplifiers and MOSFETs are required. Hence, the losses will
be increased due to the large quiescent current of theoperational amplifiers and the high on-resistance of theMOSFETs.
In this paper, a linear PFC regulator for LED lighting withthe multi-level current bypass circuit is proposed. The
proposed circuit can be operated by a self-bias circuit without
any external controllers. In addition, the voltage rating of theMOSFETs can be reduced to Vin_max/(n+1) (n: Number of theseries current bypass circuits). Thus, the loss of the MOSFETs
can be reduced.
II.

PROPOSED CIRCUIT

Fig. 1 shows the equivalent circuit diagram. The rectified
input voltage is applied to the LED strings. The current
bypass switches S1~S4 are connected in parallel to the LEDstring. The off state voltage of the switches is clamped to theforward voltage of parallel connected LEDs. The number ofthe required switches is different in each of the string, which isdepending on the on voltage of the string current.
Fig. 2 shows the input current waveforms of the AC sideand the LED strings. The current amplitude of the each stringis limited to constant value. Firstly, in the case that the input
voltage Vin is increased from the zero point, all of the switches
are turned on. Then, the switches S1~S4 are turned off
accordingly which is subjected to input voltage. As a result,the input current is increased in proportional to the input
voltage.
Fig 3 shows loss reduction strategy of current regulator byusing multiple operating points. When the input voltage ischanging, the current bypass switches S1~S4 will be operated
to vary the forward voltage of the LED string.
Fig 4 shows the circuit diagram of the first string in thecase of 5 and 10 strings. The current bypass circuit consists ofa single MOSFET, two zener diodes and a single resistor. Theoperation is as follows.

Table 1. Comparison of LED driver circuits.
Proposed circuit(5 and 10 strings)

Step-down
chopper(4)Single switch
converter(4)Sequential linear(8)(9)Input current

Sinusoidal

Low power factor

Sinusoidal

Sinusoidal

Lighting flicker
Element deciding
life time limit
Efficiency [%]

Large

Large

Large

LEDLED88.3, 91.6

Small
Electrolytic
capacitor
>80

8585

Reactor volume
Capacitor volume
EMI filter-LargeLarge
Required

Small
Small
Required-

Input power factor

0.995, 0.999

0.7~0.8

0.98

>0.95

LED

Figure 2: Current waveforms of the AC input
and the LED strings.

Figure 1: Equivalent circuit diagramof the proposed LED driver in the case of 5 strings.
1, The gate-source voltage Vgs of the MOSFETs is biased
by ZDbias and Rbias. The saturation voltage of thecurrent regulator Von is lower than VZDbias (6.8V > 3.0Vtyp.). Therefore, S1~S9 are turned on.
2, The input voltage Vin becomes higher than the LED
forward voltage VF0, so the LEDs of VF0 are turned on.
3, Vin - VF0 is applied to the drain-source of S1 (Vds1). In thisarea, S1 is operated in an active state.
4, If Vin becomes higher than VF0 + VF1, S1 is completely
turned off due to the negative bias of Vgs1.
5, The operation of 2~4 is repeated until Vin reaches themaximum value.
As a result, the LED string is operated as a constantcurrent and variable voltage.
Table 2 shows the circuit parameters of the proposed
circuit at 5 and 10 strings.
Fig. 5 shows the equivalent circuit diagram of MOSFET
gate driver. The frequency response of a gate drive circuit canbe decided by a time constant of a bias resistor Rbias and input
gate capacitances Ciss. The cutoff frequency fcutoff of the gate
driver is obtained by (1).

Figure 3. Loss reduction of current regulator
by using multiple operating points.

f cut _ off =1Rgate ⎞⎛............................... (1)⎜⎜ Rbias +⎟⎟ ⋅ nCissn⎝⎠

where n is a number of series bypass MOSFETs. Thefrequency response should be determined to be higher than
twice of input power supply frequency. For example, by using
the parameters in the Table 2 (a), fcutoff becomes 27 kHz.
III.

LOSS ANALYSIS

A. Simulation based analysis
The loss analysis of the proposed circuit is now discussed,
which is carried out using a PSPICE circuit simulator (Tina,Texas Instruments). The losses are dominated by theconduction loss of the current source and the switching loss ofthe MOSFETs. The switching losses of the MOSFETs arevaried with the dV/dt rate of Vin. The loss analysis shows anefficiency of 91.6 % at 10 strings including the diode rectifier.

(a) 5 strings. (PMOS)(b) 10 strings. (NMOS)
Figure. 4. Circuit diagram of the first string.
Table 2. Circuit parameters for simulationand experiment.
(a) 5 strings.

4.0

LED

LUW_JNSH.EC (OSRAM)
VF=3.1V, IF=20 mA, 36-series

3.0MOSFET

5LP01SS (ON semiconductor)
Vds=-50 V, Ron=18 Ω, Ciss=7.4 pF

2.0ZDgate, ZDbias

Vz=6.8 V

Rbias, Rgate

1 MΩ

Current regulator

NSI45020T1G (ON semiconductor)
Istr=20 mA, Von=3.5 V

Diode bridge
Vin

MOSFET

No. 2
No. 3
No. 4

ZDgate, ZDbias

Vz=6.8 V

Rbias, Rgate

1 MΩ

Rshunt62.5 Ω

Current regulator

LM1086-ADJ (Texas Instruments)
Istr=20 mA, Von=3.0 V

Current
source0.012

AC 100 V, 50 Hz
LUW_JNSH.EC (OSRAM)
VF=3.1V, IF=20 mA, 40-series
SSM3K15F (Toshiba)
Vds=30 V, Ron=4 Ω, Ciss=7.8 pF

ZenerdiodeResistors

1.0DF08SA (Vishay)(b) 10 strings.
LEDFET No. 134String number5(a) 5 strings
1.5
1.2
0.9
0.6
0.3
0.0
12345678910Figure 5. Equivalent circuit of gate driver.

(b) 10 strings
Figure 6. Loss distribution of each string.

B. Calculation based analysis
Fig. 7 shows the semiconductor loss of the first string.
The hatching areas indicate the losses of the switches and thecurrent regulator. Fig. 8 shows an example of the loss

waveforms of the current bypass MOSFETs and current
regulative diode (CRD). During t1 to t2, S1 is operated inactive region. The time period of the active region is given by(2).

TInput voltage

50V/div0T

String current
No. 1(a) 5 strings
PRegulator
Psw9
Psw8
VF9
Psw7
VF8
Psw6
VF7
VF6
Psw5
VF5
Psw4
VF4
Psw3
VF3
Psw2
VF2
Psw1
VF1
VF0

Vin_max

010mA/div0T

Forward voltageof LED (VFx)and CRD(b) 10 strings
Figure 7. Semiconductor losses.

VCRD

VF40t

T10 T11T1 T510V/div

VF1

T0.2W/div
Ps1

Power lossof MOSFET

Ps4a −1ta =1∑ V ................................................. (2)Fxsin −1 x=0
ωVin _ max

00

where ω is a angular frequency of the input voltage, Vin_max isa peak input voltage, Vfx is a forward voltage of the LEDs
shown in Fig. 4.
(3)

t1

1t2

2
3
Time [ms]45Figure 8: Simulation waveforms.

The active region loss of the MOSFETs are expressed asn−1ta +1⎛⎞Pswa _ linear = 4 fVin I str ⎜Vin _ max ∫ sin ωtdt − (ta+1 − ta )∑VFx ⎟ (3)tax=0⎝⎠
where fVin is a frequency of the input voltage, the first termshows the integrated value of the input voltage, the second
term shows the total voltage of LEDs.

The conduction loss of the MOSFETs are expressed as (4).

0.0

1.0

2.0

3.0

4.0

5.0

2Pswa _ cond = 4 fVin Ron I str (tn+2 − t a ) ................................ (4)The active region loss of the current regulator is expressedas (5)nt n+ 2⎛⎞Pcs _ linear = 4 fVin I str ⎜Vin _ max ∫ sin ωtdt − (tn + 2 − tn +1 )∑VFx ⎟ (5)t n +1x =0
⎝⎠

(6).

The conduction loss of the current regulator is expressed asPcs _ cond = 4 fVin I strVon (t n + 2 − t1 ) ................................. (6)Fig. 9 shows the loss comparison results of the first stringby comparing between the calculation and simulation. The
calculated loss is slightly higher than that of simulation. The
reason is that forward current of the LED is flowed at the peakof Vds of the MOSFET in simulation. Thus, the current isFigure 9. Loss distribution of the first string. (5 strings)
divided into a MOSFET and a LED. Therefore, the draincurrent of the MOSFET is decreased.
Fig. 10 shows the loss comparison between the 5 string
type and the 10 string type. The active region loss can be
decreased by increasing the number of bypass circuits. Whenthe number of bypass circuits increased to 9, the active region
loss is 46% decreased.
IV.

EXPERIMENTAL RESULTS

Table 3 shows the specification of the proposed circuit andthe sample product of step-down chopper circuit.

020406080100

Figure 10. Switch loss comparison of string No. 1.

(a) Input voltage and current

Table 3. Specification of 5 strings prototype
and sample product.
Proposed circuit(5 strings)

Step-down chopper(Sample product)

Input power [W]

7.8 W

11.0 W

Luminous flux [lm]Luminous
efficiency [lm/W]
Power factor

751

810

9674

0.995

0.79

(a) 5 strings.

(b) 10 strings.
Figure 11. Prototype of experiment.
Fig. 11 shows a prototype. The proposed circuit can bemounted on a single sided PCB with no holes. The circuit
parameters are as same as shown in Table 2.
Fig. 12 shows the input current and the string current
waveforms obtained by using the 5 strings type. The input
active power is 7.8 W. Good sinusoidal waveform is achievedin the input current. The input current THD and the power
factor are 9.8% and 0.995, respectively.
Fig. 13 shows the input voltage and current waveforms
obtained by using step-down chopper circuit. The input
current waveform is distorted by capacitor input diode rectifier.

(b) String current
Figure 12. Experimental waveformsof the proposed circuit.
Fig. 14 shows conduction noise in the proposed circuitand the step-don chopper circuit. Red line is the limit ofCISPR Conduction noise in the step-down chopper circuit isattenuated to -19 dB lower than the limited line. On the otherhand, conduction noise in the proposed circuit is almost as
same as the background noise. Therefore, the conduction
noise can be reduced by using the proposed circuit.
Fig. 15 shows the volume of each component that becomposed of the passive components, semiconductors andnoise filters. The component volume of the proposed circuit is
83% smaller than the sample product.
Fig. 16 shows the input current and the string current
waveforms obtained by using the 10 strings type. The input

Figure 13. Experimental waveformsof the step-down chopper circuit.

Figure 16. Experimental waveforms. (10 strings)(a) Proposed circuit (5-strings)(b) Step-down chopper circuit
Figure 14. Conducted emission spectrum.

Figure 17. Experimental results of string current.
0.0

2.0

4.0

6.0

8.0

active power is 15 W. The input current distortion becomes
lower than the 5 strings type. The input current THD and thepower factor are 5.1% and 0.999, respectively.

Fig. 18 shows the drain-source voltage waveforms of thefirst string MOSFETs. In the transient of Vds, the MOSFET isoperated in saturation region. Then, in the constant Vds area, it
is operated in active region. In addition, the maximum Vds islimited by VF of the parallel connected LEDs. Therefore, lowvoltage rating (30 V) MOSFETs can be used to achieve highefficiency.

Fig. 17 shows the current waveforms of the LED strings.
A conduction time of each string is controlled by the numberof current bypass circuits. In addition, the string current islimited at 20 mA by the current regulator.

Fig. 19 shows the input current harmonics from theexperimental results. It can be confirmed that the low orderharmonic components in the input current are lowered byincreasing the number of the current bypass circuit.

Figure 15. Volume of circuit component.

V.

CONCLUSIONSThis paper proposed a linear PFC regulator for LED
lighting with a new multi-level current bypass circuit.
Features of the proposed circuit are the implementation of low
voltage of MOSFETs, no electrolytic capacitors are required
and a simple gate bias circuit without any external controllers.
The experimental results have confirmed the validity of theproposed circuit structure.
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Figure 18: Experimental results of Vds and Von.
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8.0
6.0
4.0
2.0
0.0

Figure 19. Input current harmonics.

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