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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 31, NO. 8, AUGUST 2016 5837
Single-Stage AC/DC Single-Inductor
Multiple-Output LED Drivers
Yue Guo, Sinan Li, Member, IEEE,AlbertT.L.Lee, Member, IEEE, Siew-Chong Tan, Senior Member, IEEE,
Chi Kwan Lee, Senior Member, IEEE, and S. Y. R. Hui, Fellow, IEEE
Abstract—Various ac/dc LED driver topologies have been pro-
posed to meet the challenges of achieving a compact, efficient,
low-cost, and robust multistring LED lighting system. These LED
drivers typically employ a two-stage topology to realize the func-
tions of ac/dc rectification and independent current control of each
LED string. The choice of having two stage conversions involves ad-
ditional hardware components and a more complicated controller
design process. Such two-stage topologies suffer from a higher sys-
tem cost, increased power loss, and large form factor. In this paper,
a single-stage ac/dc single-inductor multiple-output LED driver
is proposed. It uses only one single inductor and N+1active
power switches (Nbeing the number of LED strings) with reduced
component count and smaller form factor. The proposed driver
can achieve both functions of ac/dc rectification with a high power
factor and precise independent current control of each individual
LED string simultaneously. A prototype of an ac/dc single-inductor
triple-output LED driver is constructed for verification. Experi-
mental results corroborate that precise and independent current
regulation of each individual LED string is achievable with the pro-
posed driver. A power factor of above 0.99 and a peak efficiency of
89% at 30-W rated output power are attainable.
Index Terms—Color control, light-emitting diode (LED),
lighting system, power factor (PF) control, single-inductor
multiple-output (SIMO).
I. INTRODUCTION
LIGHT-EMITTING diodes (LED) are increasingly gain-
ing acceptance in lighting industry with a growing list of
applications, such as general, decorative, and display lighting
applications [1]–[6]. The four major factors supporting their
popularity are 1) preponderant long lifetime; 2) mercury free
and environmental friendly; 3) high luminous efficiency; and 4)
flexibility to perform color mixing and dimming control [7]–
[11]. Depending on the specific application requirements, the
LED can either be arranged in series as a single string (or a
single LED chip), or in parallel forming a multistring structure
Manuscript received July 27, 2015; revised September 16, 2015; accepted
October 16, 2015. Date of publication October 30, 2015; date of current version
March 2, 2016. This work was supported by the Hong Kong Research Grant
Council under Theme-Based Research Project: T22-715/12N, and the patent ap-
plication [45]associated with the invention reported in this paper was supported
by The University of Hong Kong. Recommended for publication by Associate
Editor J. M. Alonso.
Y. Guo, S. Li, A. T. L. Lee, S.-C. Tan, and C. K. Lee are with the Department
of Electrical and Electronic Engineering, The University of Hong Kong, Hong
Kong (e-mail: guoyue3858@163.com; snli@eee.hku.hk; tlalee@eee.hku.hk;
sctan@eee.hku.hk; cklee@eee.hku.hk).
S. Y. R. Hui is with the Department of Electrical and Electronic Engineering,
The University of Hong Kong, Hong Kong, and also with the Imperial College
London, London SW7 2AZ, U.K. (e-mail: r.hui@imperial.ac.uk).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TPEL.2015.2496247
(for medium- and high-power applications). Many LED drivers
achieving small form factor and low cost have been proposed
for the single LED chip/string applications [12]–[14]. However,
achieving a compact and low-cost LED driver design is chal-
lenging for applications where multiple parallel LED strings
are needed. This is because extra functionalities, such as current
balancing, individual string current regulation, or open-/short-
circuit fault protection are typically demanded in such multi-
string LED systems.
For instance, in high-power applications, such as streetlight
and large-scale LCD panels, current sharing between strings
is crucial for providing an evenly distributed light output and
heat. Most importantly, if the current imbalance causes one
or more LED strings to exceed their rated current values, the
lifetime of the LED strings will be drastically reduced [15]–
[19]. In color mixing applications, such as RGB LED lamp and
LED-backlit LCD display, fast and precise current control of
the red, green, and blue LEDs should be guaranteed [20]–[22].
Basically, these functionalities, i.e., current sharing, individual
string regulations, and/or open-/short-circuit fault protection,
can be simultaneously achieved if each of the string current
is regulated independently. In this way, current sharing can be
simply realized by assigning a common current reference for all
strings, while individual current regulation is accomplished by
assigning a different reference command for each string.
Several solutions for driving multistring LED systems with
independent current control have been proposed. They can be
broadly classified into two types, as shown in Fig. 1(a) and (b).
Their major difference lies in the circuit architecture of the
ac/dc stage, which is required to enable an ac voltage input
and/or perform power factor correction (PFC) function. Fig. 1(a)
shows an ac/dc stage which generates a single common output
bus Vothat is shared by all the LED strings [14], [23]–[26],
whereas Fig. 1(b) shows an ac/dc stage which assigns a separate
output voltage for each LED string [15], [27], [28]. To realize
independent current regulation of each LED string, the output
of the ac/dc preregulation stage must be cascaded with an ad-
ditional postregulator for each LED string, which regulates the
current of the string to which it is connected. There are gener-
ally two types of postregulators: linear type [23], [24], [28] and
dc/dc converter type [14], [15], [25].
The linear type of postregulators gives the simplest hardware
configuration, but might incur severe power loss if improperly
designed [23]. On the other hand, the dc/dc converter type of
postregulators is ideally lossless. However, each dc/dc postreg-
ulator introduces additional switches and passive component
such as inductor to the system. This inevitably leads to a higher
0885-8993 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications standards/publications/rights/index.html for more information.
5838 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 31, NO. 8, AUGUST 2016
Fig. 1. Conventional multistring LED systems of which the ac/dc stage gen-
erates (a) a common output bus voltage and (b) a separate output voltage for
each individual LED string.
Fig. 2. System architecture of the existing two-stage ac/dc SIMO LED driver.
system cost and larger form factor that grows as the number
of LED strings increases. Therefore, there is always tradeoff
between efficiency and the system’s cost and size whenever
a postregulator is used. Another problem with the two-stage
configuration is that the two sets of controllers (one for the
ac/dc stage and the other for the postregulators) are required,
which complicates the system design. Additionally, a two-stage
structure requires the use of dc-link capacitor(s) [typically elec-
trolytic capacitors (E-Cap)][Co1for Fig. 1(a), and Co1–CoN
for Fig. 1(b)]. If the dc-link voltage is high, it is hard to select
a proper capacitor that has a long lifetime. The use of short-
lifetime capacitors in the LED drivers reduces the reliability of
the LED driver [29], [30].
In view of the aforementioned issues, in this paper, a single-
stage ac/dc single-inductor multiple-output (SIMO) LED driver
for multistring LED applications, which can simultaneously
achieve PFC and independent current regulation of each LED
Fig. 3. System architecture of the proposed single-stage ac/dc SIMO LED
driving system.
string, is proposed. The system architecture of the proposed
single-stage SIMO driver is illustrated in Fig. 3, in which the
functions of a PFC stage and a conventional dc/dc SIMO topol-
ogy are integrated into a single stage. Therefore, the need for
a postregulator stage is eliminated. As the name suggests, only
one inductor is needed. The total number of switches is also
reduced as compared with the conventional two-stage solution
using dc/dc type of postregulators. Therefore, the proposed LED
driver is compact and cost effective. In addition, it requires only
one controller to regulate the switching sequence of all the power
and output switches. This is made possible by time multiplexing
the control signals of each string. Moreover, by enabling one-
stage operation, the intermediate high-voltage E-Cap is elimi-
nated. It enables the use of low-voltage long-lifetime capacitors
which extends the operating life of the proposed LED driver.
In order to perform a power loss analysis, a nonideal circuit
simulation model, which includes the parasitic resistance, in-
ductance, and capacitance for the major components, has been
created for the proposed single-stage ac/dc SIMO LED driver
topology as well as the two prior arts, namely the conventional
two-stage ac/dc LED driver (with three postregulators) [14],
[23]–[26] and the two-stage ac/dc SIMO LED driver [39], [40]
for comparison purpose. Based on the simulation results, the
total power loss and the power efficiency in each of the three
topologies have been compared and tabulated in Table I.
To summarize, the proposed single-stage ac/dc SIMO LED
driver results in the smallest total power loss, compared with the
prior arts. Specifically, the proposed single-stage ac/dc SIMO
driver results in a 32% reduction in the total power loss, com-
pared with the conventional two-stage driver using three postreg-
ulators and about 18% reduction in the total power loss, com-
pared with the two-stage SIMO. On the other hand, the simu-
lated power efficiency of the proposed SIMO driver is around
91%, compared with 83.68% from the conventional two-stage
driver and 88.96% from the two-stage SIMO. The proposed
single-stage SIMO driver can achieve higher power conversion
efficiency due to the use of only one buck switch as well as one
freewheeling diode in the power stage. As a further improve-
ment in power efficiency of the proposed driver, we can con-
sider replacing the freewheeling diode in the power stage with a
GUO et al.: SINGLE-STAGE AC/DC SINGLE-INDUCTOR MULTIPLE-OUTPUT LED DRIVERS 5839
TAB LE I
COMPARISON OF THE SIMULATED POWER LOSS AND POWER EFFICIENCY OF
THE PROPOSED SINGE-STAGE AC/DC SIMO LED DRIVER AGAINST THE
CONVENTIONAL TWO-STAGE AC/DC LED DRIVER [14], [23]–[26] AND THE
TWO-STAG E AC/DC SIMO LED DRIVER [39], [40]
low-side MOSFET having a small Rds(on) as in a synchronous
buck converter configuration.
II. AC/DC SIMO LED DRIVERS
A. Existing AC/DC SIMO LED Driver
There is growing interest in using dc/dc SIMO converters
for multistring LED applications due to their reduced cost and
smaller form factor. A single-inductor dual-output (SIDO) con-
verter with time-multiplexing control scheme operating in DCM
is first reported in [31] and [32]. Extending from SIDO, a dc/dc
SIMO parallel string LED driver operating in DCM is recently
reported in [33]–[38]. All of these reported SIMO converters can
only realize dc/dc conversion, and a stable dc input is typically
required. To accommodate an ac voltage input, e.g., a 110-V
60-Hz ac mains, a dc/dc SIMO LED driver is often cascaded
behind an ac/dc front-end stage [39], [40], as shown in Fig. 2,
again forming a two-stage configuration, which is similar to that
given in Fig. 1.
In [39], the ac/dc front-stage is simply a diode bridge rectifier
with a large capacitor. An unregulated dc voltage is produced
without performing any PFC. Such a configuration is only use-
ful for low-power LED applications, of which the power factor
(PF) requirement is less stringent [41], [42]. Also, the SIMO
converter in [40] is operating in continuous-conduction mode
(CCM) and suffers from cross-regulation issues. Therefore, in-
dividual current regulation of LED strings is unviable, and only
current sharing function is performed. On the other hand, in
[40], a boost PFC converter is implemented as the ac/dc front-
stage converter, providing a well-regulated dc voltage and a
high PF. Nevertheless, by employing a two-stage configuration,
these existing ac/dc SIMO LED drivers inherently have similar
demerits as those described in Fig. 1.
B. Proposed Single-Stage AC/DC SIMO LED Driver
Fig. 3 shows the configuration of the proposed single-stage
SIMO LED driver.
Fig. 4. Derivation of a buck-type single-stage ac/dc SIMO LED driver. (a)
DCM buck PFC converter. (b) Buck-type dc/dc SIMO converter. (c) Derived
buck-type single-stage ac/dc SIMO LED driver.
Unlike existing ac/dc SIMO LED drivers that are configured
as shown in Fig. 2, the proposed ac/dc SIMO LED driver can
directly drive multiple LED strings off an ac voltage source in
a single stage, without an intermediate dc link. Both PFC and
independent regulation of string currents are simultaneously
viable. This is possible through proper component integration
of a PFC stage and a dc/dc SIMO converter. For example, if a
DCM buck converter is adopted for the PFC stage [see Fig. 4(a)],
and a buck-type dc/dc SIMO is selected for the SIMO stage [see
Fig. 4(b)], by integrating their main power switch Saand S
a,
freewheeling diode Daand D
a, and inductor Land L, a single-
stage buck-type ac/dc SIMO driver can be obtained as shown in
Fig. 4(c).
By employing a time-multiplexing control scheme, at any in-
stance in time, the LED driver depicted in Fig. 4(c) can be oper-
ated to act as a single-input single-output DCM buck converter.
Since a DCM buck converter is naturally an emulated resistor
at low frequencies [43], the averaged input current of the LED
driver over each switching period is inherently proportional to
the line voltage. As a result, the original dc/dc SIMO converter
can be readily turned into a single-stage ac/dc SIMO driver inte-
grated with PFC function through minor hardware modifications
including the addition of the front-end diode rectifier. In contrast
to all previous methods that are two-staged-based, the driver in
Figs. 3 and 4(c) requires no E-Cap between the diode bridge and
the SIMO stage. Clearly, the removal of a short-lifetime high-
voltage E-Cap extends the operating lifetime of the proposed
LED driver. Also, by operating the proposed SIMO driver in
5840 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 31, NO. 8, AUGUST 2016
Fig. 5. Complete circuit diagram with three LED strings.
DCM, cross regulation can virtually be eliminated as the indi-
vidual LED strings are fully decoupled from one another.
III. OPERATING PRINCIPLES OF THE PROPOSED SINGLE-STAGE
AC/DC SIMO LED DRIVER
A. Operating Modes
A single-inductor triple-output (SITO) ac/dc buck converter
as shown in Fig. 5 is used for the sake of our discussions.
As shown in Fig. 5, a total of four switches, i.e., one main
switch Saand three output switches S1–S3, are used in this
converter. Lfand Cfforms the input EMI filter, Cdis the
high-frequency filter capacitor, Dais the freewheeling diode,
and Lis the main inductor. Diis the branch diode in the ith
LED string for preventing reverse flow of the branch current.
Coi and Rsi are the output capacitor and sensing resistor of
the ith LED string. The ac input voltage is Vac, the input volt-
age to the buck converter is represented by Vin and the three
output voltages are Vo1–Vo3.ILis the inductor current and
Ibranch1–Ibranch3 are the branch currents that flows through the
respective output switches. The ideal waveforms of Sa,S1–S3,
IL, and Ibranch1–Ibranch3 are shown in Fig. 6(a), where Tsrep-
resents the switching period of the main switch Sa.
It can be seen that the proposed ac/dc SIMO converter oper-
ates in DCM where ILalways returns to zero at the end of each
switching cycle. Fig. 6(b) depicts the control sequence of the
SITO ac/dc converter under normal operations. In three switch-
ing cycles (0–3Ts), there are a total of nine control sequences
which can be categorized into the following three distinctive
modes of operation:
1) Mode 1 (t0–t1):Main switch Sais ON and freewheeling
diode Dais OFF. The inductor current ILincreases at a
rate of (Vin −Vo1)/L. The output switch S1is ON and S2
and S3are OFF since only the first output is enabled. This
corresponds to (1–1), (2–1), and (3–1) in Fig. 6(b).
Fig. 6. (a) Timing diagram of the main switch Saand output switches S1–S3,
inductor current IL, and branch currents Ibranch1–Ibranch3 and (b) control
sequence of the proposed ac/dc SITO LED driver.
2) Mode 2 (t1–t2):Sais OFF and Dais ON. ILdecreases
linearly at a rate of Vo1/L.Att2,ILdrops to 0 and Mode
2 ends. This corresponds to (1–2), (2–2), and (3–2) in
Fig. 6(b).
3) Mode 3 (t2–t3):Both Saand Daare OFF. ILremains
at zero during this idle period. In order to reduce the
switching loss, for example, S1can be turned OFF with
zero-current switching (ZCS) and S2can be turned ON
with ZCS during this interval. This corresponds to (1–3),
(2–3), and (3–3) in Fig. 6(b).
The same process is repeated in the next two switching periods
for the second and third output in which S1is OFF and S2,S3
take turns to be ON. The energy is transferred from the inductor
to the three outputs in a time-interleaved manner. The same
control sequence can also be scaled conveniently to Noutputs,
where Nis the total number of LED strings. The output switch
corresponding to each LED string, namely S1,S2,...,SN,isON
GUO et al.: SINGLE-STAGE AC/DC SINGLE-INDUCTOR MULTIPLE-OUTPUT LED DRIVERS 5841
Fig. 7. Timing diagrams for different PWM duty ratios using three distinct-
colored LEDs.
only during one of the Nswitching cycles. The output switch is
OFF during the remaining (N−1) switching cycles.
B. Control Schemes
The control circuit of the proposed ac/dc SITO buck LED
driver is a specialized time-multiplexed controller as shown in
Fig. 5. According to the operating principles described in Sec-
tion III-A, the on-instant of Sashould be synchronized with
respective output switches S1–S3. The synchronization is real-
ized by the 75-kHz time synchronization block. A more detailed
explanation will be given in Section III-C. The averaged current
of each LED string is controlled by the respective control loop
that compares the current-sense voltage Vsi (which is equal to
the LED current amplified by ten times) to a reference Irefi.
The error signal VEAi is compensated by a PI compensator and
modulated by a PWM modulator to give the on-time duty ratio
diand command Sa. The signals that are provided by the three-
phase clock generator are used to command S1–S3and select one
of the three channels of the MUX. In practice, there will be a to-
tal of three feedback loops, one for each LED string. The three
PI controllers take turns to use the analog comparator, which
means that, in any instance, the circuit effectively has only one
set of PI controller in operation. In addition, with reference to
[43], by operating the system in DCM, the load is essentially an
emulated resistor connected to the converter input. Although the
emulated resistor, which is determined by the duty cycle di,is
different in three LED strings, in any instance only one emulated
resistor will be connected to the converter input, which means
that the PFC can be achieved. Fig. 7 shows the timing diagram of
the time-multiplexed PWM control using three distinct-colored
LEDs to represent different loading conditions among the three
LED strings.
Note that for different loading conditions and/or with dif-
ferent current reference command, the PI outputs are different,
and, thus, the PWM duty ratios for each string are different. In
order to minimize the hardware resources, the outputs of the
PI compensators are time multiplexed together, while sharing
a common PWM modulator. This enables the subsequent logic
Fig. 8. 555 timer operating in monostable state to generate linear ramp Vsaw
and pulse train Vpulse.
elements beyond the PI compensators to be time shared among
all the SITO outputs. In the SITO topology, the use of time-
interleaving control with multiple energizing phases means that
each of the LED string is independently driven and is decoupled
from the other strings with minimal cross interference. The cur-
rent in each individual LED string can be controlled separately
by assigning a unique current reference in each LED string. It
can be expected that, with different loading conditions and cur-
rent reference commands, the inductor current ILfor respective
string will have different (rising and falling) slopes and dura-
tions. This phenomenon is shown in Fig. 6(a) and is verified
later by experimental measurement. In addition, current balanc-
ing, which is a special case of independent current control, can
be realized by using the same current reference signal across all
the LED strings without the need for additional postregulator
circuits.
C. 75-kHz Timing Synchronization Block
The timing of Saand S1–S3is synchronized using a 555
timer operating in monostable state. The detailed 75-kHz timing
synchronization block is illustrated in Fig. 8.
The bias voltage of the BJT Tis set by RT2and RT3, and
RT1serves to limit the current flowing through Tto charge up
capacitor CT1. The voltage across CT1is
VCT1=Q
CT1
=IT
CT1
t(1)
where Qis the charge of CT1, and ITis the current through the
BJT. Under the given configuration, the 555 timer operates to
generate a linear ramp Vsaw at pin 6. The output pin 3 gener-
ates a trigger pulse which dips every time CT1is discharged.
By inverting this trigger pulse, a pulse train Vpulse, which is
synchronized with Vsaw, is obtained. Vsaw is fed to the PWM
comparator and Vpulse is used to generate the three-phase clock
to enable the SITO operation.
5842 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 31, NO. 8, AUGUST 2016
Fig. 9. Equivalent LED model which comprises of an ideal diode DLED,
small signal resistor RLED, and threshold voltage Vth.
TAB LE I I
PARAMETERS OF THE RGB LEDS
Type Luxeon Rebel
Red
Luxeon Rebel
Green
Luxeon Rebel
Blue
Equivalent Resistance
RLED (Ω)
466
Rated Current
ILED(mA)
350 350 350
Threshold Voltage Vth
(V)
0.7 0.8 0.85
Forward Voltage VF(V) 2.1 2.9 2.95
Rated Power PLED (W) 0.735 1.015 1.0325
Fig. 10. Inductor voltage waveform of a buck converter in a CCM operation.
IV. PARAMETER DESIGN OF THE SIMO LED DRIVER
A. Inductor Design
To minimize the size of the inductor and simplify the con-
troller design for PFC, the converter should be operated in DCM.
Also, the current ripple in the inductor Lshould be limited to
reduce the current stress of the power switches. Thus, the buck
main inductor should neither be too large nor too small.
Fig. 9 shows an equivalent LED model, which comprises a
series connection of an ideal diode DLED, a resistor RLED , and
a threshold voltage Vth. Based on this model, the parameters
of the red (R), green (G), and blue (B) LEDs [44] used in the
experiments are tabulated in Table II.
Fig. 10 shows the inductor voltage waveform of a buck con-
verter in CCM at steady state. Using inductor volt–second bal-
ance
(Vin −Voi)diTs−Voi(1 −di)Ts=0 (2)
where Vin is the input voltage of the buck converter, Voi is the
output voltage of the ith LED string, as described in Fig. 5, and
diis the duty ratio of the ith LED string as shown in Fig. 7. The
output voltage of the ith LED string is
Voi =diVin.(3)
In the first switching interval, the increasing rate of inductor
current ILi is
dILi
dt =VLi(t)
Li
=Vin −Voi
Li
(4)
where Liis the inductance when the ith string is considered.The
peak-to-average current ripple is defined as
ΔILi,pa =Vin −Voi
2Li
diTs.(5)
In steady-state condition, the dc component of the buck ca-
pacitor current should be zero. Therefore, the dc component of
the buck inductor current is
ILi =ILEDi=Voi −WV
thi
WR
LEDi
(6)
where Wis the number of LEDs in one string, and ILEDi,Vthi,
and RLEDiare, respectively, the rated LED current, the LED
threshold voltage, and the LED equivalent resistance in the ith
string. If the system operates in DCM, then ILi <ΔILi, where
ΔILi represents the maximum inductor current ripple when the
buck converter operates in boundary-conduction mode (BCM),
i.e.,
Voi −WV
thi
WR
LEDi
<Vin −Voi
2Li
diTs(7)
where di=Voi/Vin in BCM. Hence, the minimum value of Li
is
Limin =(Vin −Voi)WR
LEDi
2(Voi −WV
thi)diTs
=(Vin −Voi)WR
LEDi
2(Voi −WV
thi)
Voi
Vin
Ts(8)
and the upper boundary of the main inductance is given by
L<min{Limin},i=1,2,3, ...W. (9)
On the other hand, the lower boundary can be obtained by con-
sidering the maximum allowable inductor current ripple ΔILmax
using
ΔILi =Vin −Voi
Li
diTs≤ΔILmax.(10)
In DCM operation, we have
V2
rms
Re i(di)=PLEDi(11)
where Vrms is the RMS value of Vin,Re i (d) is the equivalent
resistance emulated by the DCM buck converter for the ith LED
string given by [43]
Re i(di)= 2Li
d2
iTs
(12)
GUO et al.: SINGLE-STAGE AC/DC SINGLE-INDUCTOR MULTIPLE-OUTPUT LED DRIVERS 5843
Fig. 11. Small-signal block diagram of the ith string in the proposed closed-
loop SIMO converter.
and PLEDiis the power consumed by the LED load in the ith
string given by
PLEDi=WV
FiILEDi(13)
where VFi is the forward voltage of LED in the ith string. From
(11) and (12), the duty cycle dican be represented by
di=2Li
Re i (di)Ts
=2LiPLEDi
V2
rms Ts
.(14)
By substituting (14) into (10), the maximum value of Liis
Limax =Vin −Voi
ΔILmax 22PLEDiTs
V2
rms
(15)
and the lower boundary of the main inductance is given by
L≥max{Limax},i=1,2,3, ...W. (16)
B. Output Capacitor Design
For each LED string, an output capacitor Coi is separately
required. The design of the capacitors can be performed inde-
pendently since the operation of each string is decoupled. The
design approach is the same as that for a dc-link capacitor in
conventional ac/dc rectifying systems since the employed out-
put capacitors have to perform the same functions of ac energy
storage and switching frequency filtering. This is different from
that of the dc/dc SIMO LED driver in which the output capacitor
is designed to handle only switching ripples.
If ΔVi=kVoi, where Voi is the average output voltage in
string i,ΔViis the peak output voltage ripple, and kis the ripple
factor that defines the allowable peak voltage ripple, then with
reference to [43], the lower limit for Coi is
Coi ≥PLEDi
kV 2
oi ×Tac
2π(17)
where Tac =1/(60Hz).
C. Small-Signal Analysis and Controller Design
Due to the time-multiplex arrangement of the three con-
trollers, only one output is effective at any instance. Therefore,
the controller can be designed independently. Take one string
as an example. Fig. 11 shows the small-signal block diagram
of one string. Essentially, the controlled power plant is a buck
converter operating in DCM. A straightforward way to deter-
mine the low-frequency small-signal control-to-output transfer
function of the buck converter in the ith string, denoted by
GBucki(s), is to let the main inductance Ltend to zero. With
reference to [43], GBuck i(s)is given by
GBuck i(s)= ˆ
ioi
ˆ
diˆvg=0
=
2Voi
di×1−Mi
2−Mi×1
WR
LEDi
1+ s
2−Mi
(1−Mi)WR
LEDiCoi
(18)
where Miis the DCM conversion ratio of the ith LED string
given by
Mi=Voi
Vin
=2
1+1+ 8Li
WR
LEDid2
iTs
.(19)
A simple PI controller is used as the compensator. In Fig. 11,
which shows the small-signal control block diagram, the transfer
function of the compensator of the ith LED string is given by
Gci(s)= skp i +kint i
s(20)
where kp i is the proportional gain and kint iis the integral gain.
Here, VMis the amplitude of the sawtooth carrier waveform
and Hi(s) is the sensing gain for the ith string. The output of PI
compensator ˆ
Vci is fed into the PWM modulator with a gain of
1/VMin order to generate a duty ratio di. The averaged current
in each LED string is determined by the corresponding current
reference value Irefi(s).
The loop gain Ti(s)of the system can be represented as
Ti(s)=Gci(s)×1
VM×GBuck i(s)×Hi(s).(21)
By substituting (18) and (20) into (21), the loop gain becomes
Ti(s)= skp i +kint i
s×
2Voi
di×1−Mi
2−Mi ×1
WR
LEDi
1+ s
2−Mi
(1−Mi)WR
LEDiCoi
×1
VM×Hi(s).(22)
D. Design Example
The design parameters given in Table III are adopted for
illustrative purpose. By substituting the values into (8), the upper
limits of the inductance for the three different LED strings can
be found as L1min = 254 μH,L2min = 336 μH,L3min =
341 μH. According to (9), the upper limit of the inductance will
be L<254 μH.
Next, by substituting the same design parameters into (15),
the lower limit of inductance for the three LED strings can
be found as L1max =3.52 μH,L2max =4.48 μH,L3max =
4.53 μH. From (16), the lower limit of inductance is found as
L≥4.53 μH. Therefore, the range of inductance is 4.53 μH≤
L≤254 μH. In order to minimize the size of the main inductor
to achieve a smaller overall form factor of the proposed LED
driver, Lis selected to be 5μH. However, for a practical design,
more design margins of Lare recommended to compensate
for the operating transient, component tolerances, etc. Then,
by referring to (17), the lower limits of Coi for the three LED
strings are Co1≥902 μF,Co2≥653 μF,Co3≥642 μF.For
illustration purpose, Co1,Co2, and Co3are all chosen to be
1000 μF.
5844 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 31, NO. 8, AUGUST 2016
TABLE III
DESIGN SPECIFICATIONS
Design Parameter Value Design
Parameter
Value
Input voltage Vac 110 V Rated LED
Current ILED
350 mA
EMI Filter
(Lf,C
f)
1 mH, 0.1 μF Voltage Ripple
Factor k
7%
Filter Capacitor Cd0.1 μF Sensing Resistor
Rs
1Ω
Main Switch
Frequency fs
75 kHz Power Inductor
L
5μH
Maximum Current
Ripple ΔiLmax
8 A Cross-Over
Frequency fc
2.5 kHz
Output Capacitor
(Co1,Co2,Co3)
1000 μF Same-Colored
LED
String 1: seven
Blue LEDs
String 2: seven
Blue LEDs
String 3: seven
Blue LEDs
Rated Output
Voltage
(Vo1,V
o2,V
o3)
String 1: 14.7 V Distinct-colored
LED
String 1: seven
Red LEDs
String 2: 20.3 V String 2: seven
Green LEDs
String 3: 20.7 V String 3: seven
Blue LEDs
To demonstrate the controller design, string 1 (red LEDs)
is chosen as an example. The input voltage Vin has a peak
value of 110√2V. With reference to Table II, it is desired to
supply a regulated output voltage Vo1=14.7V and LED current
ILED1 = 350 mA. The first step is to determine the feedback
gain H1(s).A1-Ωresistor Rs1is used as the current sensing
resistor. The voltage of Rs1will then be amplified by a factor of
p=10using proportional amplifier, and compared with current
reference Iref1 . Hence, we have
H1(s)=Rs1p=10.(23)
By substituting the related parameters listed in Table III into
(21), the open-loop transfer function of the system before com-
pensation (when Gc1(s)=1) can, therefore, be written as
Tu1(s)= 1
VM×GBuck 1(s)×H1(s)= 56
s
75 +1.(24)
By setting |Tu1(jω)|=1, the cross-over frequency fcu1 of
the uncompensated loop gain Tu1(s)can be obtained as fcu1 =
0.668 kHz. The desired cross-over frequency of the loop gain
after compensation T1(s)is chosen to be fc1=(1/10) ×fo=
2.5kHz, where fois the output switch frequency. From (24) at
2.5 kHz, the magnitude of Tu1(s)is
|Tu1(j×2π×2.5k)|=|56
j×2π×2.5k
75 +1|=−11.46 dB.
(25)
From (20), to obtain a unity loop gain at 2.5 kHz, the com-
pensator should have a 2.5 kHz gain of 11.46 dB, which means
that
|Gc1(j×2π×2.5k)|=|j×2π×2.5k×kp1+kint 1
j×2π×2.5k|
=11.46 dB.(26)
Fig. 12. Bode plots of loop gain T1(s)before and after compensation as well
as the compensator transfer function Gc1(s).
Fig. 13. Hardware prototype of the proposed single-stage ac/dc SITO LED
driver.
By choosing kp1=3.5,kint 1can be calculated as kint 1=
20755. Thus, the compensator transfer function Gc1(s)is
Gc1(s)=skp1+kint 1
s=3.5+20755
s.(27)
Based on (24) and (27), the Bode plots of the open-loop
gain before and after compensation as well as the compensator
transfer function Gc1(s)can be plotted as shown in Fig. 12.
From the figure, the phase margin is 70°, which indicates that
the system is stable.
GUO et al.: SINGLE-STAGE AC/DC SINGLE-INDUCTOR MULTIPLE-OUTPUT LED DRIVERS 5845
TAB LE I V
COMPONENT LIST
Component Model no. Component Model no.
Diode Bridge
Rectifier
GBU10G-BP MUX CD74HC4051E
Main Switch (Sa) IPW50R280CE Comparator AD8561ANZ
MOSFET Gate
Driver
IRS2101PBF Oscillator LM555CN/NOPB
Freewheeling and
Branch Diodes
MUR1540G Operational
Amplifier
OP340PA
Output Switches (S1,
S2,S3)
IRFI4227PbF Output Capacitor
(Co1,Co2,Co3)
UPX1V102MHD
(long lifetime)
Fig. 14. Measured waveforms of the ac line input voltage and current for
30-W output power using same-colored LEDs with a common current reference
of 350 mA.
V. EXPERIMENTAL VERIFICATION
A hardware prototype of the proposed single-stage ac/dc
SITO LED driver has been constructed. Fig. 13 shows a photo
of the prototype.
Experiment verifications are performed based on the hard-
ware prototype shown in Fig. 13 and the design specifications
provided in Table III.
Table IV shows a list of components used in the experiment.
The experiments involve two types of LED loads. In the first
scenario, same-colored LEDs are used for the three strings,
that is, each string consists of seven blue LEDs. In the second
scenario, distinct-colored LEDs are used for the three strings,
that is seven red LEDs are assigned to the first string, seven green
LEDs for the second string, and seven blue LEDs for the third
string. Note that the current in the three strings in either scenario
can be controlled independently to be identical or different.
A. Cicruit Operating Principle
Fig. 14 shows the ac line voltage and input current waveforms
using a 110-V 60-Hz ac source and same-colored LEDs as the
load. It can be seen that the ac line voltage and the input current
are essentially in phase and the PF is measured as 0.99, thereby
verifying the functionality of PFC.
Fig. 15 shows the full view of the inductor current ILand
the three branch currents Ibranch1–Ibranch3 with same-colored
LEDs and a common 350-mA reference current. The maximum
Fig. 15. Measured waveforms of ILand Ibranch1 –Ibranch3 employing
same-colored LEDs with identical LED current of 350 mA.
Fig. 16. Close-up view of (a) driving signals of main switch Saand output
switches S1–S3and (b) the corresponding ILand Ibranch1–Ibranch3 with
same-colored LEDs and a common 350-mA reference command.
current in each LED string peaks at around 7.5 A which falls
within the design specification limit (i.e., ΔiLmax =8A). Figs.
16–18 show the close-up view of ILand Ibranch1 –Ibranch3 , and
the corresponding driving signals of main switch Vdrivemain and
output switches Vdrive1–Vdrive3under different conditions. From
5846 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 31, NO. 8, AUGUST 2016
Fig. 17. Close-up view of (a) driving signals of main switch Saand output
switches S1–S3and (b) the corresponding ILand Ibranch1–Ibranch3 with
distinct-colored LEDs and a common 350-mA reference command.
Fig. 16, same-colored LEDs with a 350-mA common reference
command are used. It shows that 1) the duty cycles of the PWM
signal that drives the main switch Saare similar for different
LED strings; and 2) the peak values of Ibranch1–Ibranch3 are the
same. Also, Sais ON in every switching cycle, but the output
switch Si(where i=1,2,3) is ON in every three switching cycles.
Consequently, ILramps up and down in each switching cycle
but the branch current Ibranchiof each LED string appears every
three switching cycles. In other words, ILis assigned to each of
the three LED strings in a round-robin fashion. The experimental
results verify the functionality of the SIMO topology and the
time-multiplexed control method.
Fig. 17 shows the “distinct-colored LEDs” scenario with a
350-mA common reference. It is important to note that the
PWM duty ratio corresponding to each of the three LED strings
is different. Also, the peak value of the branch current Ibranchi
(also the peak inductor current) is also distinct among the three
LED strings. On the other hand, Fig. 18 shows the “same-
colored LEDs” scenario with different reference values. Similar
to Fig. 17, the duty cycle of the PWM signal which drives Sa
and the peak values of Ibranchiin three LED strings is different
in every switching cycle.
B. Current Balancing and Steady-State
Independent Current Regulation
The averaged current in each of the three individual LED
strings can be independently adjusted for the purpose of
color-mixing and dimming. Also, in order to achieve brightness
Fig. 18. Close-up view of (a) driving signals of main switch Saand output
switches S1–S3and (b) the corresponding ILand Ibranch1–Ibranch3 using
same-colored LEDs and with distinct reference current values (i.e., 250, 350,
450 mA) across the three LED strings.
uniformity, current balancing of different LED strings is re-
quired. The waveforms for these two scenarios are illustrated in
Fig. 19.
Fig. 19(a) shows the individual current control of output cur-
rents ILED1–ILED3 in each LED string in a steady-state condi-
tion. It shows that the average current values in the first, second,
and third LED string are 250, 350, and 450 mA, respectively, due
to different current references being applied to each LED string.
Fig. 19(b) shows the current balancing of ILED1–ILED3 in each
LED string. The average current values in each of the three
LED strings are identical (ILEDi= 350 mA) with a peak-to-
peak ripple within 10% of ILEDi. This demonstrates the current
balancing capability of the proposed driver.
C. Independent Current Control Without Cross Regulation
In order to further demonstrate the independent current con-
trol capability of the proposed ac/dc LED driver, the reference
command Iref3 for String 3 is step changed from 3.5 (350 mA)
to 2.5 V (250 mA) and then back to 3.5 V (350 mA) shown
in Fig. 20, corresponding to 100% to 70% load interchange.
The current references of the other two strings Iref1,2are kept
constant at 350 mA. As shown, the rising and falling transi-
tion times are both around 25 ms and there is no observable
cross-regulation issue for the three LED strings.
GUO et al.: SINGLE-STAGE AC/DC SINGLE-INDUCTOR MULTIPLE-OUTPUT LED DRIVERS 5847
Fig. 19. Output current waveforms of the three LED strings using same-
colored LEDs and with (a) 250-, 350-, 450-mA individual current control and
(b) 350-mA current balancing condition.
Fig. 20. Transient current waveforms and reference control command for (a)
350 to 250 in LED string 3 and (b) 250 to 350 in LED string 3.
Fig. 21. Measured power efficiency versus the output power.
Fig. 22. PF measurements versus the output power.
Fig. 23. Measured THD versus the output power.
D. Measured Efficiency and Performance
The measured power conversion efficiency, PF, and total har-
monic distortion (THD) versus output power are, respectively,
shown in Figs. 21–23.
From Fig. 21, it can be seen that as the output power increases,
the efficiency of the proposed ac/dc SIMO LED driver also in-
creases and peaks at 89% (including driver’s loss) at around
21 W. Fig. 22 shows the variations of the PF across different
5848 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 31, NO. 8, AUGUST 2016
Fig. 24. Comparison of the measured harmonic currents versus their corre-
sponding maximum harmonic current limits defined by the IEC1000-3-2 stan-
dard at (a) 30-W rated output power; (b) 3-W output power (i.e., 10% rated
output power).
values of the output power. The measured PF peaks at 0.996
and the corresponding THD is measured to be 7%, as shown in
Figs. 22 and 23. The measured input current also conforms to
Class C of the IEC1000-3-2 standard [41], as will be discussed
shortly. It should be noted that with an increasing number of
LEDs connected in series or with an increased output power
(i.e., the output voltage becomes larger) at a given ac line in-
put voltage, the PF could potentially drop below 0.99 due to
the larger distortion in the ac line input current Iin at the zero-
crossing point, where there is a short interval when the current
is not conducting. The duration of this nonconducting interval
of Iin is directly related to the output dc voltage. That is, the
larger the output voltage, the longer this interval will be. Hence,
when either more LEDs are connected in series or the out-
put power increases (i.e., higher output dc voltage), both THD
and PF performance will be degraded. From the above analysis,
the proposed LED driver can be designed based on the rated
output power so that the PF can be maintained to be no less than
0.99 over the entire dimming range.
E. IEC1000-3-2 Standard Compliance
The harmonic currents of the proposed LED driver, which be-
longs to the Class C Equipment under the IEC1000-3-2 standard
[41], are measured and compared against the corresponding har-
monic current limit in accordance with the IEC 1000-3-2 stan-
dard. Fig. 24(a) shows the measured harmonic currents against
the harmonic current limits at a 30-W rated output power. Like-
wise, Fig. 24(b) shows the measured harmonic currents against
the harmonic current limits at a 3-W output power (i.e., 10%
of the rated output power). The experimental results clearly
show that all the measured harmonic currents fall within their
corresponding maximum harmonic current limit as defined by
the IEC1000-3-2 standard [41].
V. CONCLUSION
This paper proposes an ac/dc SIMO LED driver which inte-
grates the PFC preregulation and LED current regulation into
a single-stage converter. Unlike the existing two-stage driver
topologies, the intermediate dc-link stage is eliminated in the
proposed single-stage topology. This enables the use of low-
voltage long-lifetime capacitors in the proposed LED driver. In
addition, the proposed driver employs only one single induc-
tor to drive multiple independent LED strings. It can achieve
fully independent current control in each LED string with no
noticeable cross regulation. The major benefits of the proposed
single-stage LED driver include a lower component count, re-
duced BOM cost, simplified control scheme, and ease of im-
plementation. The experimental results demonstrate the effec-
tiveness of the proposed SITO LED driver in attaining precise
and independent current regulation across the three individual
LED strings. It enables flexible color-mixing and wide-range
dimming for high-quality lighting applications.
.
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Yue G uo received the B.Eng. degree in electronic and
information engineering degree from the Hong Kong
Polytechnic University, Hung Hom, Hong Kong, in
2014. He is currently working toward the M.Phil. de-
gree in electrical and electronic engineering at The
University of Hong Kong, Hong Kong.
His research interests include embedded hardware
design and digital signal processor-based applica-
tions.
Sinan Li (M’14) was born in China, in 1986. He
received the B.S. degree in electrical engineering
from the Harbin Institute of Technology, Harbin,
China, in 2009, and the Ph.D. degree in electrical
and electronic engineering from The University of
Hong Kong (HKU), Hong Kong, in 2014.
He is also one of the founding members of the
IEEE-Eta Kappa Nu (HKN) at HKU. He is currently
a Research Associate at the Department of Electrical
and Electronic Engineering, HKU. He has published
more than 20 transaction papers and conference pa-
pers. He also holds three U.S. patents. His current research interests include the
power electronics, LED lighting, control, renewable energy, and smart grids.
5850 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 31, NO. 8, AUGUST 2016
Albert T. L. Lee (M’13) received the B.Sc.(Hons.)
degree in electrical engineering from the University
of Wisconsin-Madison, Madison, WI, USA, in 1994,
the M.Sc. degree in electrical and computer engineer-
ing from the University of Michigan, Ann Arbor, MI,
USA, in 1996, and the Ph.D. degree in electronic and
computer engineering at the Hong Kong University
of Science and Technology, Kowloon, Hong Kong,
in 2014.
In 1996, he joined Intel Corporation, Hillsboro,
OR, USA, as a Senior Component Design Engineer
and was involved in the development of Intel’s P6 family microprocessors. In
2001, he served as a Senior Corporate Application Engineer with the System-
Level Design Group, Synopsys Inc., Mountain View, CA, USA. In 2003, he
joined the Hong Kong Applied Science and Technology Research Institute
Company, Ltd., and served as an EDA Manager with the Wireline Communi-
cations Group. In 2006, he joined the Giant Electronics Limited as a Hardware
Design Manager and became an Associate General Manager in 2008. He is
currently a Research Associate at the Department of Electrical and Electronic
Engineering, The University of Hong Kong, Hong Kong. His research interests
include power electronics and control, LED lightings, and emerging LED driver
technologies.
Siew-Chong Tan (M’06–SM’11) received the
B.Eng. (Hons.) and M.Eng. degrees in electrical and
computer engineering from the National University
of Singapore, Singapore, in 2000 and 2002, respec-
tively, and the Ph.D. degree in electronic and infor-
mation engineering from the Hong Kong Polytechnic
University, Hung Hom, Hong Kong, in 2005.
From October 2005 to May 2012, he was a Re-
search Associate, Postdoctoral Fellow, Lecturer, and
Assistant Professor with the Department of Elec-
tronic and Information Engineering, Hong Kong
Polytechnic University. From January to October 2011, he was a Senior Scien-
tist with the Agency for Science, Technology and Research, Singapore. From
September to October 2009, he was a Visiting Scholar with the Grainger Center
for Electric Machinery and Electromechanics, University of Illinois at Urbana-
Champaign, Champaign, USA, and in December 2011, an Invited Academic
Visitor with the Huazhong University of Science and Technology, Wuhan,
China. He is currently an Associate Professor at the Department of Electri-
cal and Electronic Engineering, The University of Hong Kong, Hong Kong. He
is a Coauthor of the book Sliding Mode Control of Switching Power Convert-
ers: Techniques and Implementation (Boca Raton, FL, USA: CRC, 2011). His
research interests include power electronics and control, LED lightings, smart
grids, and clean energy technologies.
Dr. Tan serves extensively as a Reviewer for various IEEE/IET transactions
and journals on power, electronics, circuits, and control engineering. He is an
Associate Editor of the IEEE TRANSACTIONS ON POWER ELECTRONICS.
Chi Kwan Lee (M’08) received the B.Eng. and Ph.D.
degrees in electronic engineering from the City Uni-
versity of Hong Kong, Kowloon, Hong Kong, in 1999
and 2004, respectively.
From 2004 to 2005, he was a Postdoctoral Re-
search Fellow with the Power and Energy Research
Centre, National University of Ireland, Galway, Ire-
land. In 2006, he joined the Centre of Power Elec-
tronics, City University of Hong Kong, as a Research
Fellow. In 2008–2011, he was a Lecturer of electrical
engineering with the Hong Kong Polytechnic Univer-
sity. Since January 2012, he has been an Assistant Professor at the Department
of Electrical and Electronic Engineering, The University of Hong Kong, Hong
Kong. Since 2010, he has been a Visiting Researcher at the Imperial College
London, London, U.K. His current research interests include wireless power
transfer, clean energy technologies, and smart grids.
Dr. Lee received an IEEE Power Electronics Transactions First Prize Paper
Award for his publications on Mid-Range Wireless Power Transfer in 2015. He
is a Coinventor of the Electric Springs and planar EMI filter.
S. Y. R. Hui (M’87–SM’94–F’03) received the B.Sc.
(Hons.) (Eng.) degree from the University of Birm-
ingham, Birmingham, U.K., in 1984, and the D.I.C.
Ph.D. degree from Imperial College London, Lon-
don, U.K., in 1987.
He currently holds the Philip Wong Wilson Wong
Chair Professorship at The University of Hong Kong,
Hong Kong, and a part-time Chair Professorship at
the Imperial College London. He has published more
than 300 technical papers, including more than 190
refereed journal publications and more than 55 of his
patents have been adopted by the industry.
Dr. Hui is an Associate Editor of the IEEE TRANSACTIONS ON POWER ELEC-
TRONICS and the IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS,andan
Editor of the IEEE JOURNAL OF EMERGING AND SELECTED TOPICS IN POWER
ELECTRONICS. His inventions on wireless charging platform technology un-
derpin key dimensions of Qi, the world’s first wireless power standard, with
freedom of positioning and localized charging features for wireless charging of
consumer electronics. In November 2010, he received the IEEE Rudolf Chope
R&D Award from the IEEE Industrial Electronics Society and the IET Achieve-
ment Medal (The Crompton Medal). He is a Fellow of the Australian Academy
of Technological Sciences and Engineering and he also received the 2015 IEEE
William E. Newell Power Electronics Award.
