Wojciech WOJTKOWSKI
Bialystok University of Technology, Faculty of Electrical Engineering
SEPIC converter for high power LED lighting
Abstract. The SEPIC converter for high brightness LED lighting applications supplied from the lithium-ion batteries is discussed. It was built using
specialized integrated chip MCP1651 and additional microcontroller from AVR family Attiny13, for additional functionality. Typical application of
MCP1651 was extended of a couple of elements. The converter was tested experimentally and some results are presented.
Streszczenie. W artykule omawiany jest zmodyfikowany przekszta tnik typu SEPIC w zastosowaniu do zasilania diod LED du ej mocy. Do budowy
przekszta tnika wykorzystano specjalizowany uk ad MCP1651 oraz mikrokontroler z rodziny AVR Attiny13, co pozwoli o uzyska dodatkowe funkcje.
Aplikacja uk adu scalonego MCP1651 zosta a rozszerzona o dodatkowe elementy. W artykule prezentowane s wybrane wyniki bada
eksperymentalnych. (Przekszta tnik SEPIC do zastosowa o wietleniowych z u yciem diod LED du ej mocy).
Keywords: LED, SEPIC, DC/DC converter.
S owa kluczowe: zasilanie LED, przekszta tnik SEPIC, przekszta tnik DC/DC.
Introduction (L3), additional potentiometer allowing for dimmable LED
Recently, high brightness LED lighting in residential, current, microcontroller which can control turn on, turn off
automotive, industry and many other applications becomes the SEPIC converter and signalize too low battery voltage.
feasible and in many cases can replace the incandescent It can also be used to periodically turn on or off the LED
bulbs, halogen bulbs or compact fluorescent light bulbs as lighting according to the applications needs. Presented
well. In such applications, high efficiency, high power factor converter can be easily controlled with analog voltages and
and low harmonics are particularly important. It can also be can deliver dimmable LED light thanks to additional resistor-
significant to step up or step down the input voltage with potentiometer R2. The R2 resistor can be used to regulate
possibly simple single-stage conversion for low cost and the feedback voltage of the MCP1651. The discussed
high efficiency. A single-ended primary-inductance circuit has certain advantages, like only one stage of power
converter (SEPIC) can meet all the needs especially when conversion and no need to sense the input voltage. It can
the output current is relatively not to high. SEPIC converter step up and down the supply voltage and has dimmable
has the ability to regulate an output voltage that is either LED current. A practical evaluation board based on the
larger or smaller in magnitude than the input voltage. specialized device MCP1651 and AVR microcontroller
The SEPIC converter is a good solution in applications ATtiny13 was developed to verify the proposed design and
supplied from a battery. In such cases, voltage can be some experimental results are presented.
above and below that of the applications needs. For
example, a single lithium-ion battery typically discharges SEPIC converter
from 4.2 volts to below 3 volts, if some application requires General scheme of SEPIC converter is presented in Fig.
for example 3.5 volts, the SEPIC would be effective. This 1.
kind of applications requires very efficient power conversion
in order to improve the battery life.
The SEPIC converter is a DC/DC-converter topology
that provides a positive regulated output voltage from an
input voltage that varies from above to below the output
voltage. This type of conversion is very convenient for
supplying devices from an unregulated input power supply
(which sometimes consists of a transformer, rectifying
bridge and a capacitor only) or from different kinds of
accumulators and batteries. There are some disadvantages
also. The SEPIC topology requires two inductors and
Fig.1. SEPIC converter
expensive ceramic capacitors with relatively huge
capacitance with very small ESR. Instead of using two
The Fig.1 shows a simple circuit diagram, consisting of
separate inductances, a coupled inductors can be used,
an input capacitor Ci, an output capacitor CO, inductors L1
thanks to similar shape of currents. Coupled inductors are
and L2 (which can be coupled), an AC coupling capacitor
sold in a single package at a cost only slightly higher than
CC, a power MOSFET transistor Q1 and a diode D1. During
that of the comparable single inductor. The coupled inductor
steady-state and continuous conduction mode of operation,
not only provides a smaller footprint but also, to get the
pulse-width modulation operation, and neglecting ripple
same inductor ripple current, requires only half the
voltage, coupling capacitor CC is charged to the input
inductance required for a SEPIC converter with two
voltage Vi. When Q1 transistor is turned off, the voltage
separate inductors. When the converter operates with high
across L2 is equal VO. Since Ci is charged to Vi, the voltage
switching frequency, passive elements can be very small
across Q1 (when Q1 is turned off) is Vi + VO. The voltage
which can be important in many portable applications.
across L1 is VO. When Q1 is on, capacitor CC, which is
In this article, SEPIC converter for high brightness LED
charged to Vi, is connected in parallel with L2, so the
lighting applications supplied from the lithium-ion batteries
voltage across L2 is equal Vi. When Q1 is on, energy is
is discussed. It was built using specialized integrated chip
being stored in L1 from the input and in L2 from CC. When
MCP1651 and additional microcontroller from AVR family
Q1 turns off, L1 s current continues to flow through CC and
Attiny13, for additional functionality. Typical application of
D1 into CO and the load. Both CO and CC get recharged so
MCP1651 was modified of a couple elements. There is
that they can provide the load current and charge L2,
added additional inductance which filters the LED current
respectively, when Q1 turns back on.
260 PRZEGL D ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 86 NR 10/2010
Such simple SEPIC circuit (Fig. 1) requires for normal the inductance. The output ripple voltage will vary
operation a couple of additional elements to control the depending on the input voltage, the load current, the
output voltage or current and to controll the duty cycle of Q1. hysteresis voltage and the inductance. The external switch
In this article, the SEPIC converter for high power LED peak current is sensed on the CS pin across an optional
lighting applications is described, which was built using external current sense resistor. If the CS pin falls more than
specialized integrated chip MCP1651 and additional 122 mV below Vi, the current limit comparator is set and the
microcontroller from AVR family Attiny13. Additional pulse is terminated. This prevents the current from getting
inductance L3 filters the LED current while additional too high and damaging the N-channel MOSFET. In the
potentiometer allows to dim the LED current. The AVR described application, the current limit resistor is not used.
microcontroller controls turn on, turn off of the SEPIC The EXT output pin is designed to directly driver external N-
converter and signalize low battery voltage. It can also be channel MOSFETs and is capable of sourcing 400 mA
used to periodically turn on or off the LED lighting according (typical) and sinking 800 mA (typical) for fast on and off
to the applications needs. The proposed application of the transitions. The top side of the EXT driver is connected
SEPIC converter for high power LED lighting is presented in directly to VIN, while the low side of the driver is tied to
Fig. 2. GND, providing rail-to-rail driver capability. To control the
The output voltage regulation is accomplished by speed of the turn on and off an external resistor can be
comparing the output voltage (sensed through an external connected in series with the N-channel MOSFET Q1. By
resistor divider R1, R2 and R3) to a internal reference slowing the transition speed down, there will be less high
inside the MCP1651. When the sensed output voltage is frequency noise generated. Speeding the transition up
below the reference, the EXT pin pulses the external N- produces higher efficiency. The For bootstrap configuration,
channel MOSFET Q1 on and off at the maximal 750 kHz the higher-regulated boost output voltage is used to power
gated oscillator frequency. Energy is stored in the boost the MCP1651. This provides a constant higher voltage used
inductor when the external N-channel MOSFET is on and is to drive the external MOSFET. The R-option device can be
delivered to the load through the external Schottky diode used for applications that need to start up with the input
when the MOSFET is turned off. Several pulses may be voltage below 2.7V. For this type of application, the
required to deliver enough energy to pump the output MCP1651 will start off of the lower 2.0V input and begin to
voltage above the upper hysteretic limit. Once above the boost the output up to its regulated value. As the output
hysteretic limit, the internal oscillator is no longer gated to rises, so does the input voltage of the MCP1651. This
the EXT pin and no energy is transferred from input to provides a solution for 2-cell alkaline inputs for output
output. voltages that are less than 6V (because of possible damage
of MCP device).
The SEPIC components selection
Components selection of the SEPIC converter and
calculations are based on [1, 2 and 3]. General
assumptions: input voltage (VIN): 2.8 4.2 V, output voltage
(VO): 3.2 V, output current (IO): 1.1 A, switching frequency fs:
750 kHz, VD: 0.5 V, peak-to-peak ripple current
approximately 35% of the maximum input current at the
minimum input voltage.
Assuming 100% efficiency, for a SEPIC converter
operating in a continuous condition mode, where the
inductors current never falls to zero, the maximal and the
Fig.2. SEPIC converter for high power LED lighting
minimal duty cycle under given assumptions are equal (1)
and (2):
The range of input voltage for the MCP1651 family of
VO VD
devices is specified from 2.0V to 5.5V. For the S-option
(1)
D(max) 0.57
devices, the under voltage lockout (UVLO) feature will turn VIN (min) VO VD
the boost controller off once the input voltage falls below
2.55V. The MCP1651 can operate over a wide input voltage
where: VD is the forward voltage drop of the diode D, which
range (from 2.0V for the R-option devices to 5.5V) to
should be as small as possible to keep the performance on
accommodate multiple primary-cell and single-cell Li-Ion
the high level.
battery-powered applications. The the MCP1651 R-option
devices are recommended for use when bootstrapping the
The Schottky diode is recommended in such purpose.
output voltage back to the input. Bootstrap application
The minimum duty cycle is under maximum input voltage:
means that the input of the MCP1651 device is supplied by
the output voltage during boost operation. This can be used
VO VD
to derive output voltages from input voltages that start up at (2)
D(min) 0.47
approximately 2V. The output voltage is fed back through a VIN (max) VO VD
resistor divider to the FB pin. It is then compared to an
internal 1.22V reference. When the divided-down output is
where: VD is the forward voltage drop of the Schottky diode.
below the internal reference, the EXT pin pulses the Q1 N-
channel MOSFET on and off, to transfer energy from the
For inductors selection, the peak-to-peak ripple current
source to the load at 750 kHz. This will cause the output
should be known:
voltage to rise until it is above the 1.22V threshold, thereby
gating the internal oscillator off. Hysteresis is provided
(3) IL IIN 35% 0.39A
within the comparator and is typically 12 mV. The rate at
which the oscillator is gated on and off is determined by the
The inductors values can be calculated as follows:
input voltage, the load current, the hysteresis voltage and
PRZEGL D ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 86 NR 10/2010 261
VIN (min)
The coupling capacitor must handle the maximum RMS
(4)
L1 L2 D(max) 4,5 H
IL fS output current:
where: fS is the Q1 switching frequency.
VO VD
(12)
ICs(RMS ) IO 1.26A
VIN (min)
If L1 and L2 are wound on the same core, the value of
inductance L1 and L2 can be divided by two due to mutual
inductance. To ensure the inductors does not saturate, the
After discussed calculations, the following elements was
peak currents in the inductors can be calculated:
selected: coupling capacitor: 4 F ceramic, Q1 transistor:
IRLL014NPBF, diode BYS10-45.
VO VD 0.35
(5) IL1( peak ) IO 1 1.7A
Experimental Verifications
VIN (min) 2
To verify the feasibility of the studied LED driver, a
laboratory prototype with following specifications was
designed and tested: input voltage adequate to Li-ion
0.35
battery: 2.8 4.2 V, switching frequency: 750 kHz, maximal
(6) IL2( peak ) IO 1 1.3A
2 output current: 1.1 A, rated output voltage: 3.2 V. Some of
experimental results are presented in Figures 3, 4, 5, 6.
To select the MOSFET transistor Q1, a couple of
parameters should be considered. Namely, the on-
resistance (RDS(on)), the minimum threshold voltage (Vth(min))
which is very important especially under battery power
supply and the maximum drain to source voltage (VDS(max)).
The peak switch voltage is equal to:
(7) VDS ( peak ) VIN (max) VO 7.4V
The peak switch current is given by:
(8) IQ1( peak ) IL1( peak ) IL2( peak ) 3A
The RMS current through the switch is given by:
(VO VIN (min) VD) (VO VD )
(9)
IQ1(RMS ) IO 1,9A
2
VIN (min)
Fig.3. Q1 control signal -1 and output current -2, switching
The output diode must be selected to handle the peak
frequency fs=758 kHz, Vi = 3 V, Ii=1.3 A.
current and the reverse voltage. In a SEPIC converters, the
diode peak current is the same as the switch peak current
(8). The average diode current is equal to the output
current. The minimum peak reverse voltage the diode must
withstand is:
(10) VDR VIN (max) VO 7.4V
The power dissipation of the diode is equal to the output
current multiplied by the forward voltage drop of the diode,
so to minimize the power losses, Schottky diodes are
recommended.
Very important for the overall performance is the
coupling capacitor Cs. The capacitor must be rated for a
large enough RMS current relative to the output current.
This property makes the SEPIC much better suited to lower
power applications where the RMS current through the
capacitor is relatively small (relative to capacitor
technology). The voltage rating of the SEPIC capacitor must
Fig.4. Q1 control signal -1 and output current -2, switching
be greater than the maximum input voltage. Ceramic
frequency fs=382 kHz, Vi = 3 V, Ii=0.36 A.
capacitors are the best choice in such purpose.
The peak-to-peak ripple voltage on Cs (assuming low
ESR ceramic capacitor):
IO D(max)
(11) VCs 0.17V
CS fs
262 PRZEGL D ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 86 NR 10/2010
The output current was regulated using R2 during the
experiment. When not bootstrap configuration is
considered, a transistor with very low VGS threshold voltage
should be selected.
Conclusions
The SEPIC converter for high brightness LED lighting
applications supplied from the lithium-ion batteries is
discussed. It was built using specialized integrated chip
MCP1651 and additional microcontroller from AVR family
Attiny13, for additional functionality. Typical application of
MCP1651 was extended of a couple of elements. The
converter was tested experimentally and some results are
presented. The converter works well, especially at higher
input voltages, when the efficiency is higher. Lover input
voltages like 2,7 or less are also possible, but the efficiency
will be worse. For very low input voltages, proposed
Fig.5. Q1 control signal -1 and output current -2, switching
configuration can be simply modified for bootstrap
frequency fs=781 kHz, Vi = 4.1 V, Ii=0.76 A.
operation. The bootstrap applications will work with output
voltage not higher than 6 V. Additional potentiometer R2
allows for easy regulation of the output current which can
be used for dimming LED light intensity.
This paper was supported by S/WE/1/06.
REFERENCES
[1] Application Note 1051: http://www.maxim-ic.com/an1051
[2] Application Note 1484: http://www.national.com
[3] Falin J., Designing DC/DC converters based on SEPIC
topology, Analog Applications Journal, 4Q 2008,
http://www.ti.com/
[4] Ye Z., Greenfeld F., Liang Z., Single-Stage Offline SEPIC
Converter with Power Factor Correction to Drive High
Brightness LEDs, Applied Power Electronics Conference and
Exposition, 2009. APEC 2009. Twenty-Fourth Annual IEEE,
Fig.6. Q1 control signal -1 and L2 current -2, not CCM mode, 546 - 553
switching frequency fs=394 kHz, Vi = 4.1 V, Ii=0.19 A, IO=0.1 A.
Author: Wojciech Wojtkowski Ph.D., Bialystok University of
Technology, Faculty of Electrical Engineering, Wiejska 45, 15-351
Bialystok, E-mail: w.wojtkowski@we.pb.edu.pl;
PRZEGL D ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 86 NR 10/2010 263
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