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AND8138/D DC-DC Converter Driving High-Intensity Light-Emitting Diodes with SEPIC Circuit
During conduction pulse, this current increases, energy stored When turns off, current flows into capacitor through diode output. Inductor also contributes current discharging energy that stored earlier. When turns again, cycle repeats. Some energy that stored delivered while diode reverse-biased. understand operation circuit helpful realize that average voltage across inductor always zero (the winding short-circuit). Thus, average voltage across capacitor simply input voltage, since connected input connected ground, current inductor cannot change instantaneously (the current either inductor will same before after turns off). Figure shows waveforms circuit.
-1.5 -2.5
INTRODUCTION light-emitting diodes (LEDs) present some challenges area power converters that drive them. They must driven from "current source" rather than voltage source because, with previous LEDs, their forward voltage varies from part part, with temperature. makes sense that interest stable operation source should remain constant current. second consideration that original source power have voltage that varies above below voltage LEDs that driven. example, input voltage between Vdc, while series string LEDs have voltage nominally Vdc, such would occur with four LEDs. Given that galvanic isolation required between input output, what's needed nonisolated dc-dc converter that handle input that below above output. single-ended primary inductor converter (SEPIC) meets this requirement. inductor input, providing smooth input current, requires only switching transistor, operate over wide range input, both above below output voltage. Figure shows schematic diagram example SEPIC, designed output input.
Input Controller Output
Figure Example SEPIC
function SEPIC follows. controller, produces constant-frequency, pulse-width modulated drive switching transistor, When current flows from input through ground.
Figure Waveforms Example SEPIC
Semiconductor Components Industries, LLC, 2003
October, 2003 Rev.
Publication Order Number: AND8138/D
AND8138/D
J1-1 Input 8-20 Max. J1-2 Vref Output UC3843A NTP18N06L MBR360 Output 8-42 0.35
J2-1
Rt/Ct
2N3904 0.047 0.47
LEDs 0.35
J2-2
Current Sense
1N5941B
Compensation Voltage Feedback (2.5 Ground
0.047
1N5918B (5.1
Figure DC-DC Converter Driving High-Intensity LEDs
Regulating Output Figure shows complete circuit driving high-intensity LEDs 0.35 from wide-range input such rectified output low-voltage transformer. controller popular UC3843 current-mode device, running kHz. Current-mode control achieved current-sensing resistor, that senses current switching transistor stable operation when input voltage less than output, ramp compensation required, duty ratio greater than 50%. This accomplished feeding some ramp signal into current sense input oscillator frequency timing resistor timing capacitor, respectively. Resistor capacitor provide filtering reduce usual spike leading edge pulse across capacitance FET, Resistor provides level shifting compensate voltage offset ramp signal injected resistor Capacitor provides dominant-pole compensation main feedback loop. current regulated sensing voltage across resistors which
series with load. output current regulated (the reference input voltage divided resistance which This produces output current 0.35 Resistor provides input impedance loop compensation pole formed with capacitor Fault Protection circuit inherently short-circuit proof, since output current regulated. There input protection, however, production design should have input fuse perhaps series diode protect against input lead reversal. output protected ways, Zener diodes Zener diode protects circuitry event open-circuit output open-circuit failure. this case, feedback loop closed output will regulate Zener voltage plus internal feedback reference (2.5 Zener diode protects circuitry from positive voltage surge that will occur when output circuit opens energy stored inductors discharged into output.
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AND8138/D
Performance Converter efficiency converter dependent many factors, course. main contributors switching FET, output boost diode, There also losses switch-current-sensing resistor, output current sampling resistors, also characteristic SEPIC that there reasonable amount energy stored within converter. This adds losses. example, unless inductors quite large value, there considerable change current during each portion switching cycle. This causes additional losses inductors, well switching elements. There also loss main internal capacitor, must pass energy from input output converter. output capacitor also subjected reasonable amount ripple current. These must taken into account during design. Figure shows measured performance converter, with equivalent LEDs load. actual load resistor, causing output voltage 0.35 same would obtained from LEDs with forward voltage drop each.
84.0 82.0 Efficiency 80.0 78.0 76.0 74.0 72.0 70.0 68.0 Input Voltage
Figure shows typical waveforms final circuit with input output driving load 0.35 (the equivalent high-intensity LEDs series). Note presence ramp compensation (increased slope) beginning each current ramp. Printed Wiring Board Design Figures show component side silk screen artwork solder side copper traces, respectively.
Figure Component Side, Silk Screen
Figure Efficiency SEPIC Output
Figure Circuit Artwork (Board 2.38,)
Top: V/div. Center: mA/div. Bottom: mV/div. Horizontal Scale: ms/div.
demonstration board been designed easy modification. inductors intentionally overdesigned (rated switching transistor device with heat sink attached, allowing additional power-handling capability. When making significant changes, designer cautioned observe ripple current ratings capacitors Figure gives designer good picture this situation. waveform Figure corresponds current series capacitor final circuit. waveform identical current output capacitor, value nearly identical current seen Figure ripple current identical input inductor current, IL1, which fairly trivial shown Figure
Figure Waveforms 0.35
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AND8138/D
Table Lumiled Demonstration Board CM1, Bill Material
Ref. Qty. Same Description Cap., Electrolytic ***Alternate Above*** Cap., Ceramic ***Alternate Above*** Cap., Ceramic ***Alternate Above*** Cap., Ceramic, ***Alternate Above*** Cap., Electrolytic ***Alternate Above*** Cap., Electrolytic ***Alternate Above*** Current-Mode Controller Jumper, Insulated, Blk. Jumper, Insulated, Blk. Heat Sink, TO-220 Feet, Nylon Centers Centers 18.8 8-Pin Semiconductor UC3843AN Res., Metal Oxide, Res., Carbon Film, Res., Metal Oxide, 0.47 Axial Axial Axial Yageo Yageo Yageo CFR-25JB-330 0.47W-1-ND 330QBK-ND 3.6W-1-ND Transistor, MOSFET, Channel Res., Carbon Film, Res., Carbon Film, Res., Carbon Film, TO-92 TO-220 Axial Axial Axial Semiconductor Semiconductor Yageo Yageo Yageo 2N3904 NTP18N06L CFR-25JB-15K CFR-25JB-2.2K CFR-25JB-1.5K 15KQBK-ND 2.2KQBK-ND 1.5KQBK-ND Inductor Vert. J.W. Miller 1120-151K M6268-ND 16.3 0.76 Diode, Schottky Diode, Zener Diode, Zener Terminal Block 2-Position Radial Radial Radial Radial DO-201AD DO-41 DO-41 Nichicon Panasonic Nichicon Panasonic Semiconductor Semiconductor Semiconductor Phoenix EEU-FC1H221 MBR360 1N5941B 1N5918B MKDSN1.5/2-5.08 277-1247-ND P10325-ND 5.05 10.2 16.5 12.7 12.7 5.08 UPL1V331MPH EEU-FC1V331 P10299-ND Value Package Radial Radial Disk Disk Disk Disk Disk Panasonic ECU-S1H221JCA P4929-ND 0.55 Panasonic ECU-S1H473KBB P4955-ND 0.55 Vendor Illinois Capacitor Panasonic Panasonic ECU-S1H102JCB P4937-ND 0.55 Vendor 476RZS035M EEU-FC1V680 P10292-ND 11.2 Digikey Dia./ Lead Lead Dia.
from item strip 0.25 from each end. Bend install into holes centers. from item strip 0.25 from each end. Bend install into holes centers. Aavid HHSmith 576802B04100 3929 HS211-ND Distr. A3051B-100-ND Note: 0.156 hole
Wire, AWG, Solid, Black Insul. Printed Wiring Board
Roll
Alpha
305/1BK005
Lumiled Demonstration Board CM1, Rev.
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AND8138/D
Notes
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AND8138/D
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AND8138/D

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