DC-DC Converter for Driving High-Intensity Light-Emitting Diodes with the SEPIC Circuit

AND8138/D
DC−DC Converter for
Driving High−Intensity
Light−Emitting Diodes
with the SEPIC Circuit
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Prepared by: Chuck Mullett
ON Semiconductor
APPLICATION NOTE
INTRODUCTION
During the conduction pulse, this current increases, and
energy is stored in L1. When Q1 turns off, the current in L1
flows into capacitor C1 and through diode D1 to the output.
Inductor L2 also contributes to the current in D1,
discharging energy that was stored in it earlier. When Q1
turns on again, the cycle repeats. Some of the energy that was
stored in C1 is delivered to L2, while diode D1 is
reverse−biased. To understand the operation of the circuit it
is helpful to realize that 1) The average voltage across an
inductor is always zero (the winding is a dc short−circuit).
Thus, the average voltage across capacitor C1 is simply the
input voltage, since L1 is connected to the input and L2 is
connected to ground, and 2) The current in an inductor
cannot change instantaneously (the current in either
inductor will be the same before and after Q1 turns on or off).
Figure 2 shows the key waveforms of the circuit.
The new 350 mA light−emitting diodes (LEDs) present
some new challenges in the area of the power converters that
drive them. They must be driven from a “current source”
rather than a voltage source because, as with previous LEDs,
their forward voltage varies from part to part, and with
temperature. It makes sense that in the interest of stable
operation the source should remain a constant current. A
second consideration is that the original source of power
may have a voltage that varies above and below the voltage
of the LEDs that are to be driven. For example, the input
voltage may be between 10 Vdc and 15 Vdc, while a series
string of LEDs may have a voltage of nominally 12 Vdc,
such as would occur with four 3−V LEDs. Given that no
galvanic isolation is required between the input and output,
what’s needed is a nonisolated dc−dc converter that can
handle an input that is below or above the output. The
single−ended primary inductor converter (SEPIC) meets
this requirement. It has an inductor input, providing smooth
input current, requires only one switching transistor, and can
operate over a wide range of input, both above and below the
output voltage. Figure 1 shows the schematic diagram of an
example SEPIC, designed for 5.0 V output and 10 V input.
IC1
ID1
C1
D1
L1
C2
U1
VQ1
Q1
IL2
15 V
10 V
0V
IL1
1.5 A
1A
0.5 A
1.5 A
1A
Output
5 Vdc
@2A
+
Input
10 Vdc
@1A
IL1
VQ1
IC1
+
0A
−1.5 A
−2 A
−2.5 A
L2
IL2
Controller
2.5 A
2A
1.5 A
4A
3A
Figure 1. Example of a SEPIC
ID1
The function of the SEPIC is as follows. A controller, U1,
produces constant−frequency, pulse−width modulated drive
to the switching transistor, Q1. When Q1 is on, current flows
from the input through L1 and Q1 to ground.
 Semiconductor Components Industries, LLC, 2003
October, 2003 − Rev. 0
2A
0A
Figure 2. Waveforms of the Example SEPIC
1
Publication Order Number:
AND8138/D
AND8138/D
L1
150 H @ 4 A
C6
330
35 V
+
J1−1
+
7
Input
8−20 Vdc
17 W Max.
C1
47
35 V
Output
−
J1−2
C7
100
50 V
U1
UC3843A
VCC
+
Vref (5 V)
Q2
NTP18N06L
6
3
C3
0.047
R4
2.2 k
W2
R3
1.5 k
Voltage
Feedback
(2.5 V)
3 to 10 LEDs
@ 2.5 to 4 V
0.35 A
Q1
2N3904
4
R2
2.2 k
Compensation
J2−1 +
8
C2
1 nF
Current
Sense
+
L2
150 H
@4A
R1
15 k
Rt/Ct
Output
8−42 V
0.35 A
D1
MBR360
R5
0.47
1W
C4
220
pF
1
C5
0.047
J2−2
−
D2
1N5941B
(47 V, 3 W)
W1
R6
330
2
D3
1N5918B
(5.1 V, 3 W)
Ground
5
R7
3.6
1W
R8
3.6
1W
Figure 3. DC−DC Converter for Driving 3 to 10 High−Intensity LEDs
Regulating the Output
Figure 3 shows the complete circuit for driving up to 10
high−intensity LEDs at 0.35 A, from a wide−range input
such as the rectified output of a low−voltage transformer.
The controller is the popular UC3843 current−mode PWM
device, running at 100 kHz. Current−mode control is
achieved via the current−sensing resistor, R5, that senses the
current in switching transistor Q2. For stable operation
when the input voltage is less than the output, ramp
compensation is required, as the duty ratio is greater than
50%. This is accomplished by feeding some of the ramp
signal into the current sense input via Q1 and R2. The
oscillator frequency is set by the timing resistor and timing
capacitor, R1 and C2, respectively. Resistor R4 and
capacitor C4 provide filtering to reduce the usual spike at the
leading edge of the pulse across R5 due to capacitance of the
FET, Q2. Resistor R3 provides level shifting to compensate
for the voltage offset of the ramp signal injected by resistor
R2. Capacitor C5 provides dominant−pole compensation of
the main feedback loop. The LED current is regulated by
sensing the voltage across resistors R7 and R8, which are in
series with the load. The output current is regulated to 2.5 V
(the reference input voltage of U1) divided by the resistance
of R7 and R8, which sum to 7.2 . This produces an output
current of 0.35 A. Resistor R6 provides the input impedance
for the loop compensation pole formed with capacitor C5.
Fault Protection
The circuit is inherently short−circuit proof, since the
output is current regulated. There is no input protection,
however, so a production design should have an input fuse
and perhaps a series diode to protect against input lead
reversal.
The output is protected in two ways, by Zener diodes D2
and D3. Zener diode D2 protects the circuitry in the event of
an open−circuit output or LED open−circuit failure. In this
case, the feedback loop is closed via D2 and the output will
regulate at the Zener voltage (47 V) plus the internal
feedback reference (2.5 V). Zener diode D3 protects the
circuitry from the positive voltage surge that will occur
when the output circuit opens and the energy stored in the
inductors L1 and L2 is discharged into the output.
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2
AND8138/D
Performance of the Converter
The efficiency of the converter is dependent on many
factors, of course. The main contributors are the switching
FET, Q2, and the output boost diode, D1. There are also
losses in the switch−current−sensing resistor, R5, and the
output current sampling resistors, R7 and R8. It is also a
characteristic of the SEPIC that there is a reasonable amount
of energy stored within the converter. This adds to the losses.
For example, unless the inductors are quite large in value,
there is considerable change in current during each portion
of the switching cycle. This causes additional losses in the
inductors, as well as the switching elements. There is also
loss in the main internal capacitor, C6, as it must pass all of
the energy from the input to the output of the converter. The
output capacitor is also subjected to a reasonable amount of
ripple current. These must all be taken into account during
the design.
Figure 4 shows the measured performance of the
converter, with the equivalent of 10 LEDs as a load. The
actual load is a 100 resistor, causing an output voltage of
35 V at 0.35 A − the same as would be obtained from 10
LEDs with a forward voltage drop of 3.5 V each.
Figure 5 shows typical waveforms in the final circuit with
an input of 18 Vdc and the output driving a 100 load at
0.35 A (the equivalent of 10 high−intensity LEDs in series).
Note the presence of the ramp compensation (increased
slope) at the beginning of each current ramp.
Printed Wiring Board Design
Figures 6 and 7 show the component side silk screen
artwork and the solder side copper traces, respectively.
Figure 6. Component Side, Silk Screen Art
84.0
82.0
Efficiency (%)
80.0
78.0
76.0
74.0
72.0
70.0
68.0
8
9
10
11
12
13
14
15
16
17
18
19
20
Input Voltage
Figure 4. Efficiency of the SEPIC at 11 W Output
Figure 7. Circuit Artwork (Board is 3 x 2.38)
The demonstration board has been designed for easy
modification. The two inductors are intentionally
overdesigned (rated at 4.0 A) and the switching transistor is
an 18 A device with a heat sink attached, allowing additional
power−handling capability. When making significant
changes, the designer is cautioned to observe the ripple
current ratings on capacitors C1, C6 and C7. Figure 2 gives
the designer a good picture of this situation. The waveform
of IC1 in Figure 2 corresponds to the current in series
capacitor C6 in the final circuit. The waveform of ID1 is
identical to the ac current in the output capacitor, C7, and the
rms value is nearly identical to the current in C6, as can be
seen in Figure 2. The ripple current in C1 is identical to the
input inductor current, IL1, which is fairly trivial as shown
in Figure 2.
T
T
T
Top: VQ1 at 20 V/div.
Center: IL2 at 200 mA/div.
Bottom: Pin 3 of U1 at 500 mV/div.
Horizontal Scale: 2.5 s/div.
Figure 5. Waveforms − 18 V In, 7.0 V @ 0.35 A Out
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3
AND8138/D
Table 1. Lumiled Demonstration Board CM1, Bill of Material
Ref.
Qty.
C1
1
C2
C3
C4
Same
As
1
2
1
C5
EEU−FC1V680
P10292−ND
6.3
Panasonic
ECU−S1H102JCB
P4937−ND
5.0
3.1
6.5
5.0
0.55
Panasonic
ECU−S1H473KBB
P4955−ND
5.0
2.5
5.5
5.0
0.55
Panasonic
ECU−S1H221JCA
P4929−ND
5.0
2.5
5.0
2.5
0.55
Radial
Nichicon
UPL1V331MPH
Radial
Panasonic
EEU−FC1V331
P10299−ND
10
16
5.0
0.6
Panasonic
EEU−FC1H221
P10325−ND
10
20
5.0
0.6
Description
Value
Package
Vendor
Vendor PN
Cap., Electrolytic
47 F, 35 V
Radial
Illinois
Capacitor
476RZS035M
***Alternate for
the Above***
68 F, 35 V
Radial
Panasonic
Cap., Ceramic
1.0 nF, 50 V
Disk
AVX
***Alternate for
the Above***
1.0 nF, 50 V
Disk
Cap., Ceramic
47 nF, 50 V
Disk
***Alternate for
the Above***
47 nF, 50 V
Tab
Cap., Ceramic,
C0G
220 pF, 50 V
Disk
***Alternate for
the Above***
220 pF, 50 V
Disk
Cap., Electrolytic
330 F, 35 V
***Alternate for
the Above***
330 F, 35 V
Cap., Electrolytic
100 F, 50 V
Radial
Nichicon
***Alternate for
the Above***
220 F, 50 V
Radial
L
mm
H
mm
Lead
Sp.
mm
Lead
Dia.
mm
11.2
2.5
0.5
C3
C6
C7
Digikey #
Dia./
W
mm
1
D1
1
Diode, Schottky
3.0 A, 60 V
DO−201AD
ON
Semiconductor
MBR360
5.05
7.9
16.5
1.3
D2
1
Diode, Zener
47 V, 3.0 W
DO−41
ON
Semiconductor
1N5941B
2.7
5.2
12.7
0.9
D3
1
Diode, Zener
5.1 V, 3.0 W
DO−41
ON
Semiconductor
1N5918B
2.7
5.2
12.7
0.9
J1
2
Phoenix
MKDSN1.5/2−5.08
277−1247−ND
8.1
10.2
10
5.08
1.0
M6268−ND
21
16.3
16
0.76
J2
L1
Terminal Block
2−Position
J1
2
L2
Inductor
150 H, 4.0 A
Vert. TH
J.W. Miller
1120−151K
40 V
TO−92
ON
Semiconductor
2N3904
60 V, 18 A
TO−220
ON
Semiconductor
NTP18N06L
L1
Q1
1
Transistor, NPN
Q2
1
MOSFET,
N Channel
R1
1
Res., Carbon Film,
5%, 1/4 W
15 k
Axial
Yageo
CFR−25JB−15K
15KQBK−ND
10
0.6
R2
2
Res., Carbon Film,
5%, 1/4 W
2.2 k
Axial
Yageo
CFR−25JB−2.2K
2.2KQBK−ND
10
0.6
R3
1
Res., Carbon Film,
5%, 1/4 W
1.5 k
Axial
Yageo
CFR−25JB−1.5K
1.5KQBK−ND
10
0.6
0.47W−1−ND
15
0.8
330QBK−ND
10
0.6
3.6W−1−ND
20
0.8
R4
R2
R5
1
Res., Metal Oxide,
5%, 1.0 W
0.47 Axial
Yageo
R6
1
Res., Carbon Film,
5%, 1/4 W
330 Axial
Yageo
R7
2
Res., Metal Oxide,
5%, 1.0 W
3.6 Axial
Yageo
8−Pin DIP
ON
Semiconductor
R8
CFR−25JB−330
R7
U1
1
IC, Current−Mode
Controller
W1
1
Jumper, Insulated,
Blk. #22
0.8″ Centers
Cut from item X3 to 1.2″ and strip 0.25″ from each end. Bend 90° and install into holes at 0.8″ centers.
W2
1
Jumper, Insulated,
Blk. #22
0.3″ Centers
Cut from item X3 to 0.7″ and strip 0.25″ from each end. Bend 90° and install into holes at 0.3″ centers.
X1
1
Heat Sink,
TO−220
X2
4
Feet, Nylon
X3
1
Wire, 22 AWG,
Solid, Black
Insul.
X4
1
Printed Wiring
Board
18.8 C/W
100 Ft. Roll
UC3843AN
Aavid
576802B04100
HS211−ND
HHSmith
3929
See Web for
Distr.
Alpha
305/1BK005
Lumiled Demonstration Board CM1, Rev. 0
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4
A3051B−100−ND
Note: Use 0.156″
hole
AND8138/D
Notes
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5
AND8138/D
ON Semiconductor and
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