DN05042/D 200-265 Vac Low Ripple Buck LED Driver Overview http://onsemi.com High voltage LEDs such as Philips-Lumileds Luxeon H50−2 offer the opportunity to use a cost effective non-isolated topology such as the NCL30002 based CrM LED driver. This design note shows a compelling implementation for a very simple driver for a string of 3 high voltage LED’s with a nominal forward voltage of 50 V per LED. The basic architecture is scalable to higher current LEDs and can support other HV−LEDs on the market. This makes the CrM buck topology an excellent architecture as an LED driver for the following reasons: Low RMS Current Stress on the FET and Output Diode since the Current is Much Lower for the HV LEDs and the Duty Cycle is Relatively High Low Current Stress Allows the Use of Smaller FETS and Diodes Leading to Optimum Bill-of-material (BOM) Cost Standard Mass Produced Inductors Can Also Be Used which Further Supports a Cost Effective Design Low Output Current Ripple DESIGN NOTE Please note the input bulk capacitor was sized to comply with EN61000−3−2 Class C (EU), but the same basic design can be used for other lower voltage mains regions like US, Canada, and Latin America for example where that standard does not apply. The key reason this controller was selected is that it has a very accurate current sense threshold of 485 mV 2% which is important to achieve good current regulation accuracy. In addition, in this design the inductor was also the focus of change from the standard implementation to eliminate the need for auxiliary winding for VCC power and ZCD (Zero Current Detector). This allows the designer to use a standard off-the-shelf inductor rather than a custom inductor. Key Features Operation of the NCL30002 CrM controller for buck operation is detailed in AND9094D. While that application note describes how the device can be used to implement a high power factor buck implementation, this design note will describe a low ripple configuration. Figure 1. NCL30002 Table 1. DEVICE DETAILS Device Application Input Voltage Output Power Topology I/O Isolation NCL30002 LED Lighting 200 to 265 Vac 3.8 W CRM Buck No Table 2. OTHER SPECIFICATIONS Output Specification Output Voltage Nominal Current 24.5 mA Harmonic Content EN61000−3−2 Class C Efficiency 84.8% Typical LED Luxeon 3 H50−2 Semiconductor Components Industries, LLC, 2013 May, 2013 − Rev. 1 156 V 1 Publication Order Number: DN05042/D 1 AC_L 1 AC_N D6 D4 Rfuse1 D7 D5 R2 C10 C5 R13 L3 R3 C4 2 http://onsemi.com Figure 2. Schematic C9 4 3 2 1 NCL30002 8 VCC 7 Comp Gdrv 6 Gnd CT 5 ZCD CS MFP U1 CVCC D10 Rzcd Rgd D9 D1 R12 Rstart C11 Rsens Qfet Dout L2 Cout 1 1 LED_P 1 LED_N 1 DN05042/D DN05042/D V(vds) 330 V 300 V −I(C3) 60 mA 40 mA VCC Charging Current 270 V 20 mA 240 V 0 mA 210 V −20 mA 180 V −40 mA 150 V −60 mA 120 V −80 mA 90 V −100 mA Drain Voltage 60 V −120 mA −140 mA 30 V 0V −160 mA 195.6 ms 195.9 ms 196.2 ms 196.5 ms 196.8 ms 197.1 ms 197.4 ms 197.7 ms 198.0 ms 198.3 ms 198.6 ms Figure 3. Simulation of Drain Voltage and Charge Current V(vds) 400 V −I(C3) 160 mA 120 mA 320 V 80 mA FET Turn On 40 mA 240 V 0 mA 160 V −40 mA Charge Pump Discharge 80 V −80 mA −120 mA −160 mA 0V 190 ms 191 ms 192 ms 193 ms 194 ms 195 ms 196 ms 197 ms 198 ms 199 ms 200 ms Figure 4. FET Drain Voltage and Charge Pump Capacitor Current http://onsemi.com 3 201 ms DN05042/D Referencing the schematic, VCC power and ZCD now come from a charge pump driven from the drain of the FET. The charge pump consists of C11, D1, D9, and R12. When the FET turns off, drain current charges C11 providing a pulse of current into the VCC capacitor via D9. Figure 3 and 4 show some simulations of the charge pump. The 2 noteworthy items from figure 3 are that the current is limited because it is driven by the inductor. Also the drain voltage has well defined rise time which reduces EMI and reduces the trailing edge power losses. At turn on, the FET is fast but turns on into a low current. The turn on of the FET discharges the charge pump capacitor through R12. The on time needs to be at least 3RC time constants of C11 and R12 to ensure good discharge. R12 should be chosen to provide the lowest discharge current while still allowing for a complete discharge of C11. Since this is a CrM control, the peak to average current is 2:1. So by controlling the peak current by choice of Rsens, we can control the average current. In any open loop control, there are error sources that show up in the regulation. The two major error sources are: 1. Propagation Delay in the Sensing and Control: The delays in the current sense cause the current to overshoot the target value resulting in the output current creeping up with the line voltage. This is a relatively linear effect. Higher frequency operation will show this more than low frequency operation. 2. Charge-pump Operation: The charge pump capacitor causes a delay in rise time of the drain voltage. This effect is more prominent at higher switching frequency which is the case at higher line voltages. Start-up Conclusion The start-up resistor (Rstart) connects to the output. This type of connection has three key advantages: 1. Fast Start-up: The start-up resistor precharges the output capacitor while also charging the VCC capacitor. 2. Low Dissipation: In operation, the output voltage is much lower than the HVDC bulk voltage. 3. Inherent Open Circuit Protection: If the load is open, there is no current available to start switching. The charge pump buck LED driver is best used in a single line range configuration. The charge pump current increases with frequency and voltage. The nature of CrM operation causes both frequency and voltage to increase together. The charge pump capacitor is sized by the lowest operating voltage (which is also the lowest frequency). As the line voltage increases, excess charge pump current is dissipated in D10. The effect of this is seen in the efficiency curves. The effect on efficiency is most noticed in low power applications. While 3% regulation is very good over the extremes of a single line range, the addition of feed forward into the current sense node can further improve the line regulation. This requires the addition of 2 resistors R4 and R5 shown in the Figure 5. Regulation The NCL30002 controller operates as a peak current limit controller with no feedback. The internal error amplifier is bias by R2 and R3 to saturate the error amplifier output high. The error amplifier input cannot be left open as this is detected as an open feedback divider and the controller will shutdown. The value of the timing capacitor (C9) is chosen to be long enough not to limit the on time. DI + (V in * V LED) V @ t on + LED @ t off + I peak L L I LED + I peak 2 http://onsemi.com 4 DN05042/D Table 3. BILL OF MATERIALS Qty Reference Part Manufacturer Part Number RoHS Substitution 1 1 CVCC 4.7 mF Murata GRM188C81E475KE11D Yes Yes Cout 100 nF Kemet C1206C104K2RACTU Yes Yes 1 C4 68 nF, 400 V Epcos B32559C6683+*** Yes Yes 1 C5 1 mF, 400 V Rubycon 400LLE1MEFC6.3X11 Yes Yes 1 C9 10 nF Kemet C0402C103K3GACTU Yes Yes 1 C10 1 nF Kemet C0402C102K3GACTU Yes Yes 1 C11 100 pF Johanson 501R15N101K4T Yes Yes 2 D1, Dout UFM15PL MCC UFM15PL Yes Yes 4 D4, D5, D6, D7 SM4006PL MCC SM4006PL Yes Yes 1 D9 BAS21DW5T1G ON Semiconductor BAS21DW5T1G Yes No 1 D10 NZ9F18VT5G ON Semiconductor NZ9F18VT5G Yes No 1 L2 10 mH Bourns RL875S−103K−RC Yes Yes 1 L3 1.5 mH Wurth 7447462152 Yes Yes 1 Qfet BSS127S−7 Diodes BSS127S−7 Yes Yes 1 Rfuse1 10 W Yageo FRM−25JR−52−10R Yes Yes 1 Rgd 10 W Yageo RC0402FR−0710RL Yes Yes 1 Rsens 9.53 W Stackpole RMCF0603FT9R53 Yes Yes 1 Rstart 1.0 MW Yageo RC0805FR−071ML Yes Yes 1 Rzcd 24.9 kW Yageo RC0402FR−0724k9L Yes Yes 3 R2, R4, R5 100 kW Yageo RC0402FR−07100kL Yes Yes 1 R3 681 kW Yageo RC0402FR−07681kL Yes Yes 1 R12 10 kW Yageo RC1206JR−0710KL Yes Yes 1 R13 220 W Yageo RC0805JR−07220RL Yes Yes 1 U1 NCL30002 ON Semiconductor NCL30002DR2G Yes No http://onsemi.com 5 DN05042/D Rstart C4 Dout D10 D1 L2 D9 R12 C11 CVCC R4 1 2 3 4 U1 MFP Comp VCC Gdrv CT Gnd CS ZCD 8 7 6 5 Qfet Rgd Rzcd NCL30002 R5 C9 Rsens Figure 5. Proposed Line Regulation Feed Forward Improvement http://onsemi.com 6 DN05042/D RESULTS 26.00 25.75 25.50 Output Current (mA) 25.25 25.00 24.75 24.50 24.25 24.00 23.75 23.50 23.25 23.00 200 205 210 215 220 225 230 235 240 245 250 255 260 265 Line Voltage (Vac) Figure 6. Output Current across Input Line Voltage without Line Feed Forward 90% 89% 88% Efficiency (%) 87% 86% 85% 84% 83% 82% 81% 80% 200 205 210 215 220 225 230 235 240 245 250 255 260 Line Voltage (Vac) Figure 7. Output Efficiency across Line (Vf = 156 Vdc Nominal) http://onsemi.com 7 265 DN05042/D 4% 3% Relative Current Error (%) 2% 1% 0% −1% −2% −3% −4% 200 205 210 215 220 225 230 235 240 245 250 255 260 265 Line Voltage (Vac) Figure 8. Normalized Output Current across Input Line Voltage (Vf = 156 Vdc Nominal) Table 4. EN61000−3−2 Fundamental 3rd Harm 5th Harm Class C < 25 W Ref 86% 61% Measured **** 78.8% 49.1% LUXEON is a registered trademark of Philips Lumileds Lighting Company and Royal Philips Electronics of the Netherlands. ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of SCILLC’s product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent−Marking.pdf. SCILLC reserves the right to make changes without further notice to any products herein. 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