DN06031/D Design Note – DN06031/D High Brightness LED SEPIC Driver Device Application Input Voltage Output Power Topology I/O Isolation NCP3065 NCV3065 Solid State, Automotive and Marine Lighting 8-25 V <15 W SEPIC NONE Other Specifications Output Voltage Current Ripple Nominal Current Max Current Min Current Output 1 Output 2 Output 3 Output 4 7.2-23 V <15% 0.35, 0.7 A 1A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/ A N/A N/A N/A N/A N/A N/A Minimum Efficiency 70% Circuit Description This circuit is intended for driving high power LEDs, such as the Cree XLAMP™ series, Lumileds Luxeon™ Rebel and K2 and OSRAM, Golden and Platinum Dragon™ as well as the OSTAR™. It is designed for such wide input nominal 12 Vdc applications as automotive and low voltage lighting (12 Vdc/12 Vac). An optional dimming PWM input is included. The circuit is based on NCP3065 operation at 250 kHz in a non-isolated configuration. The primary advantages of this circuit are in the wide input voltage range, wide output voltage range, and in its high efficiency. A pulse feedback resistor (R8) is used to vary the slope of the oscillator ramp, achieving duty cycle control and steady switching frequency over a wide input voltage range. November 2007, Rev.2 Key Features y Buck-Boost operation y Wide input and output operation voltage y Regulated output current y Dimming y High frequency operation y Minimal input and output current ripple y Open LED protection y Output short circuit protection www.onsemi.com 1 DN06031/D Schematic Figure 1 – SEPIC converter schematic Design Notes A SEPIC (single-ended primary inductance converter) is distinguished by the fact that its input voltage range can overlap the output voltage range. The basic schematic is shown in Figure 2. Figure 2 – Generalized SEPIC schematic When switch SW is ON, energy from the input is stored in inductor L1. Capacitor CP is connected in parallel to L2, and energy from CP is transferred to L2. The voltage across L2 is the same as the CP voltage, which is the same as the input voltage. At this time, the diode is reverse biased and COUT supplies output current. If the switch SW is OFF, current in L1 flows through CP and D1 then continues to the load and COUT. This current recharges CP for the next cycle. Current from L2 also flows through D1 to the load and COUT that is recharging for the next cycle. Inductors L1 and L2 could be uncoupled, but then they must be twice as large as if they are coupled. Another advantage is that if coupled inductors are used there is very small input current ripple. Values of coupled inductors are set by these equations: D= VOUT min 7.2 = = 0.47 VOUT min + V IN min 7.2 + 8 ΔI = r ⋅ I OUT L1, 2 = D 0.47 = 0 .8 ⋅ 0 .7 ⋅ = 0.51A 1− D 1 − 0.47 VIN min ⋅ D 8 ⋅ 0.47 = = 15.0μH 2 ⋅ f ⋅ ΔI 2 ⋅ 250 ⋅ 10 3 ⋅ 0.51 where r is the maximum inductor current ripple factor. November 2007, Rev.2 www.onsemi.com 2 DN06031/D For a 0.35 A output current variant of this circuit, the values of inductors are ΔI = r ⋅ I OUT L1, 2 = D 0.47 = 0.95 ⋅ 0.35 ⋅ = 0.3 A 1− D 1 − 0.47 V IN min ⋅ D 8 ⋅ 0.47 = = 25.1μH 2 ⋅ f ⋅ ΔI 2 ⋅ 250 ⋅ 10 3 ⋅ 0.3 The nearest coupled inductor value for the 0.7 A variant is 15 μH. A variant with 0.35 A output current needs to use inductors with value 22 μH. The output current is set by R10 (R11). So this resistor can be calculated by the formula: R10 = 0.235 = 350mΩ . I OUT To protect the circuit against high output voltage under light loads or a fault condition, the output voltage is clamped by a Zener diode (D3) to approximately 24.5 V. Capacitor C7 is used to stabilize feedback, but it impacts line regulation. R3 fixes the line regulation error caused by C7. External power MOSFET is driven by internal NPN Darlington transistor, external diode D2 and PNP transistor Q2. Compensated divider C3, R6 and R7 is used to reduce gate-source voltage, mainly for high input voltage and to keep sharp edges. Maximum gate–source voltage can be calculated by this formula: VGS max = (V IN − VCE − V D 2 ) ⋅ R7 1500 = (27 − 1.4 − 0.4 ) ⋅ = 18.4V R6 + R7 390 + 1500 Maximum MOSFET current can be calculated in this way: V max ⎛ 0.8 ⎞ 23 ⎛ r⎞ I Q 4 max = ⎜1 + ⎟ ⋅ I OUT OUT = ⎜1 + = 2.5 A ⎟ ⋅ 0.7 ⋅ VIN min 2 ⎠ 8 ⎝ 2⎠ ⎝ To minimize power MOSFET conductance losses, it is recommended to select a transistor with small RDSON. To minimize switching losses, it is recommended to select a transistor with small gate charge. Power MOSFET must also have a breakdown voltage higher than: VFETPK = VIN + VOUT = 18 + 23 = 41V Cycle by cycle switch current protection is set by R1 at I PKset = 0.2 R1 A suitable value is higher than maximum switch current. R1 < 0.2 I Q1 max = 0.2 = 80mΩ 2.5 Diode D1 maximum voltage is determined by this equation: November 2007, Rev.2 www.onsemi.com 3 DN06031/D VD1 max = VIN + VOUT = 18 + 23 = 41V and with current I D1 = I OUT = 0.7 A The C4 coupling capacitor is selected based on input voltage and on current D max = I C 4 RMS = VOUT max 23 = = 0.74 VOUT max + V IN min 23 + 8 VOUT ⋅ I OUT V IN 1 − D max 23 ⋅ 0.7 1 − 0.74 = = 1.2 A D max 8 0.74 and its minimal value is C4 > I OUT ⋅ D min 0.7 ⋅ 0.47 = = 2 μF 0.05 ⋅ V IN min⋅ f 0.05 ⋅ 8 ⋅ 250 ⋅ 10 3 The output capacitor’s current is I C 5 = I OUT ⋅ D max 0.74 = 0.7 = 1.2 A 1 − D max 1 − 0.74 VOUT min 7.3 ⋅ I OUT ⋅ D min ⋅ 0.7 ⋅ 0.47 VIN min 8 C5 > = = 1.7 μF f ⋅ r ⋅ VOUT min 250 ⋅ 10 3 ⋅ 0.1 ⋅ 7.3 The value could be much larger for higher stability, but a higher value impacts the dimming function at low duty cycle. The resistor R8 is used to stabilize feedback loop. Used value is compromise for whole input and output voltage range. If this circuit is used for specified load only, it should be tuned by this resistor to better efficiency and line regulation. X1-3 input is used for dimming. The dimming signal level is 2-10 V. The recommended dimming frequency is about 200 Hz. For frequencies below 100 Hz the human eye will see the flicker. The dimming function utilizes the NCP3065’s peak current protection input. The second way to achieve this is to use the FB pin. See figure 10. Conclusion This circuit is ideal in applications with strings of two to six LED chips powered from a power supply with wide input range (8-20V). The advantages of this circuit include its small size, low price, wide input and output voltage ranges, and very small input current ripple. November 2007, Rev.2 www.onsemi.com 4 DN06031/D PC Board Figure 3 – components position on PCB Figure 4 – PCB’s top side November 2007, Rev.2 www.onsemi.com 5 DN06031/D Figure 5 – PCB’s bottom side November 2007, Rev.2 www.onsemi.com 6 R3 R9 R7 C6 C3 R2, R4, R5 C2 R8 C1 C4, C5 C7 R6 X2 X1 D2 Q1 D1 Q2 D3 IC1 Q3 R1 R10, R11 TP1, TP2, TP3, TP4, TP5, TP6 TR1 TR1 Designator 1 1 1 1 1 3 1 1 1 2 1 1 1 1 1 1 1 1 1 1 1 1 2 6 1 1 Resistor SMD Resistor SMD Resistor SMD Ceramic Capacitor SMD Ceramic Capacitor SMD Resistor SMD Ceramic Capacitor SMD Resistor SMD Ceramic Capacitor SMD Capacitor Ceramic Capacitor SMD Resistor SMD Inlet Terminal Block Outlet Terminal Block Schottky Diode 30V General Purpose Transistor NPN Surface Mount Schottky Power Rectifier PNP General Purpose Transistor Zener Diode 500 mW 24 V Constant Current Switching Regulator Power MOSFET 24 Amps, 60 Volts, Logic Level, N-Channel Resistor SMD Resistor SMD Test Point Transformer for 0.35A version Transformer for 0.7A version Quantity Description 1M5 1k 1k5 2n7 6n8 10k 10uF/25V 27k 100nF 120uF/50V 330pF 390R DG350-3.50-02 DG350-3.50-03 BAT54HT1G BC817-40LT1G MBRS260T3G MMBT3906LT1G MMSZ24T1G NCV3065MNTXG NTD24N06LT4G 0R050 0R68 Terminal, PCB Black PK100 PF0553.223 PF0553.153 Value Vishay Vishay Vishay Murata Kemet Vishay Murata Vishay Kemet Koshin Kemet Vishay Degson Degson ON Semiconductor ON Semiconductor ON Semiconductor ON Semiconductor ON Semiconductor ON Semiconductor ON Semiconductor Welwyn Tyco Electronics Vero Pulse Pulse Footprint Manufacturer 1% 0805 1% 0805 1% 0805 5% 0805 10% 0805 1% 0805 +80%/-20% 1210 1% 0805 5% 0805 20% 8x15 5% 0805 1% 0805 SOD-323 SOT-23 SMB SOT-23 5% SOT-123 DFN DPAK 1% 2010 5% 1206 1.02mm - Tolerance Bill of Materials for the NPC3065 SEPIC Demoboard CRCW08051M50FKEA CRCW08051K00FKEA CRCW08051K50FKEA GCM2165C1H272JA16D C0805C682K5RAC CRCW080510K0FKEA GRM32NF51E106ZA01L CRCW080527K0FKEA C0805C104J5RAC KZH-50V121MG4 C0805C331J5GAC-TU CRCW0805390RFKEA DG350-3.50-02 DG350-3.50-03 BAT54HT1G BC817-40LT1G MBRS260T3G MMBT3906LT1G MMSZ24T1G NCV3065MNTXG NTD24N06LT4G LR2010-R05FW RL73K2BR68JTD 20-2137 PF0553.223 PF0553.153 Manufacturer Part Number Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No No No No No No No Yes Yes Yes No No Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Substitution Lead Allowed Free Comments DN06031/D November 2007, Rev.2 www.onsemi.com 7 DN06031/D Measurements NCP3065 SEPIC Converter - Line regulation, IOUT = 350 mA 0,400 0,390 0,380 0,370 IOUT [A] 0,360 0,350 0,340 0,330 0,320 0,310 0,300 8 9 10 11 12 13 14 15 6chip LED, Vf = 22V 16 17 VIN [V] 18 19 4chip LED, Vf = 14V 20 21 22 23 24 25 23 24 25 2 LEDs, Vf = 7V Figure 6 – Line regulation for IOUT = 350 mA NCP3065 SEPIC Converter - Efficiency, IOUT = 350mA 95 90 η [%] 85 80 75 70 65 8 9 10 11 12 13 14 15 6chip LED, Vf = 22V 16 17 VIN [V] 18 19 4chip LED, Vf = 14V 20 21 22 2 LEDs, Vf = 7V Figure 7 – Efficiency for IOUT = 350 mA November 2007, Rev.2 www.onsemi.com 8 DN06031/D NCP3065 SEPIC Converter - Line regulation, IOUT = 700 mA 0,800 0,780 0,760 0,740 IOUT [A] 0,720 0,700 0,680 0,660 0,640 0,620 0,600 8 9 10 11 12 13 14 15 6chip LED, Vf = 22V 16 17 VIN [V] 18 19 4chip LED, Vf = 14V 20 21 22 23 24 25 23 24 25 2 LEDs, Vf = 7V Figure 8 – Line regulation for IOUT = 700 mA NCP3065 SEPIC Converter - Efficiency, IOUT = 700 mA 95 90 η [%] 85 80 75 70 8 9 10 11 12 13 14 15 6chip LED, Vf = 22V 16 17 VIN [V] 18 4chip LED, Vf = 14V 19 20 21 22 2 LEDs, Vf = 7V Figure 9 – Efficiency for IOUT = 700 mA November 2007, Rev.2 www.onsemi.com 9 DN06031/D NCP3065 SEPIC Converter - Dimming Linearity 700 600 IOUT[mA] 500 400 300 200 100 0 0 10 20 30 40 50 D[%] 60 70 80 90 100 Figure 10 – Dimming linearity, dimming frequency 200Hz Figure 11 – PCB’s top side November 2007, Rev.2 www.onsemi.com 10 DN06031/D Figure 12 – PCB’s bottom side 1 © 2007 ON Semiconductor. Disclaimer: ON Semiconductor is providing this design note “AS IS” and does not assume any liability arising from its use; nor does ON Semiconductor convey any license to its or any third party’s intellectual property rights. This document is provided only to assist customers in evaluation of the referenced circuit implementation and the recipient assumes all liability and risk associated with its use, including, but not limited to, compliance with all regulatory standards. ON Semiconductor may change any of its products at any time, without notice. Design note created by Petr Konvičný, Tomáš Tichý, e-mail: [email protected], [email protected] November 2007, Rev.2 www.onsemi.com 11