National Semiconductor Application Note 2082 Clinton Jensen December 7, 2010 Introduction Key Features This demonstration board highlights the performance of a LM3444 based Flyback LED driver solution that can be used to power a single LED string consisting of 4 to 8 series connected LEDs from an 90 VRMS to 135 VRMS, 60 Hz input power supply. The key performance characteristics under typical operating conditions are summarized in this application note. This is a two-layer board using the bottom and top layer for component placement. The demonstration board can be modified to adjust the LED forward current, the number of series connected LEDs that are driven and the switching frequency. Refer to the LM3444 datasheet for detailed instructions. A bill of materials is included that describes the parts used on this demonstration board. A schematic and layout have also been included along with measured performance characteristics. • • • Line injection circuitry enables PFC values greater than 0.99 Adjustable LED current and switching frequency Flicker free operation Applications • • • Solid State Lighting Industrial and Commercial Lighting Residential Lighting Performance Specifications Based on an LED Vf = 3.57V Symbol Parameter Min Typ Max VIN Input voltage 90 VRMS 120 VRMS 135 VRMS VOUT LED string voltage 12 V 21.4 V 30 V ILED LED string average current - 350 mA - POUT Output power - 7.6 W - fsw Switching frequency - 79 kHz - LM3444 - 120VAC, 8W Isolated Flyback LED Driver LM3444 -120VAC, 8W Isolated Flyback LED Driver Demo Board 30131168 AN-2082 © 2010 National Semiconductor Corporation 301311 www.national.com AN-2082 LM3444 120VAC, 8W Isolated Flyback LED Driver Demo Board Schematic 30131101 Warning: The LM3444 evaluation board has exposed high voltage components that present a shock hazard. Caution must be taken when handling the evaluation board. Avoid touching the evaluation board and removing any cables while the evaluation board is operating. Isolating the evaluation board rather than the oscilloscope is highly recommended. Warning: The ground connection on the evaluation board is NOT referenced to earth ground. If an oscilloscope ground lead is connected to the evaluation board ground test point for analysis and AC power is applied, the fuse (F1) will fail open. The oscilloscope should be powered via an isolation transformer before an oscilloscope ground lead is connected to the evaluation board. Warning: The LM3444 evaluation board should not be powered with an open load. For proper operation, ensure that the desired number of LEDs are connected at the output before applying power to the evaluation board. www.national.com 2 AN-2082 LM3444 Device Pin-Out 30131102 Pin Description 10 Pin MSOP Pin # Name Description 1 NC No internal connection. 2 NC No internal connection. 3 NC No internal connection. 4 COFF 5 FILTER 6 GND Circuit ground connection. 7 ISNS LED current sense pin. Connect a resistor from main switching MOSFET source, ISNS to GND to set the maximum LED current. 8 GATE Power MOSFET driver pin. This output provides the gate drive for the power switching MOSFET of the buck controller. 9 VCC Input voltage pin. This pin provides the power for the internal control circuitry and gate driver. 10 NC No internal connection. OFF time setting pin. A user set current and capacitor connected from the output to this pin sets the constant OFF time of the switching controller. Filter input. A capacitor tied to this pin filters the error amplifier. Could also be used as an analog dimming input. 3 www.national.com AN-2082 Bill of Materials Designator Description Manufacturer Part Number AA1 Printed Circuit Board - 551600530-001A C1 CAP .047UF 630V METAL POLYPRO EPCOS Inc B32559C6473K000 C2 CAP 10000PF X7R 250VAC X2 2220 Murata Electronics North America GA355DR7GB103KY02L C3, C4 CAP 330UF 35V ELECT PW Nichicon UPW1V331MPD6 C6 CAP .10UF 305VAC EMI SUPPRESSION EPCOS B32921C3104M C7 CAP, CERM, 0.1µF, 16V, +/-10%, X7R, 0805 Kemet C0805C104K4RACTU C8 CAP CER 47UF 16V X5R 1210 MuRata GRM32ER61C476ME15L C11 CAP CER 2200PF 50V 10% X7R 0603 MuRata GRM188R71H222KA01D C12 CAP CER 330PF 50V 5% C0G 0603 MuRata GRM1885C1H331JA01D C13 CAP CER 2200PF 250VAC X1Y1 RAD TDK Corporation CD12-E2GA222MYNS D1 DIODE TVS 150V 600W UNI 5% SMB Littlefuse SMAJ120A D2 RECT BRIDGE GP 600V 0.5A MINIDIP Diodes Inc. RH06-T D3 DIODE RECT GP 1A 1000V MINI-SMA Comchip Technology CGRM4007-G D4 DIODE SCHOTTKY 100V 1A SMA ST Microelectronics STPS1H100A D5 DIODE ZENER 30V 1.5W SMA ON Semiconductor 1SMA5936BT3G D7 DIODE ZENER 12V 200MW Fairchild Semiconductor MM5Z12V D8 DIODE SWITCH 200V 200MW Diode Inc BAV20WS-7-F F1 FUSE BRICK 1A 125V FAST 6125FA Cooper/Bussmann 6125FA J1, J2, J3, J4, TP8, TP9, TP10 16 GA WIRE HOLE, 18 GA WIRE HOLE 3M 923345-02-C J5, J6 CONN HEADER .312 VERT 2POS TIN Tyco Electronics 1-1318301-2 L1, L2 INDUCTOR 4700UH .13A RADIAL TDK Corporation TSL0808RA-472JR13-PF Q1 MOSFET N-CH 600V 90MA SOT-89 Infineon Technologies BSS225 L6327 Q2 MOSFET N-CH 600V 1.8A TO-251 Infineon Technology SPU02N60S5 R1, R3 RES 200K OHM 1/4W 5% 1206 SMD Vishay-Dale CRCW1206200KJNEA R2, R7 RES, 309k ohm, 1%, 0.25W, 1206 Vishay-Dale CRCW1206309KFKEA R6, R24 RES, 10.5k ohm, 1%, 0.125W, 0805 Vishay-Dale CRCW080510K5FKEA R12 RES 4.7 OHM 1/10W 5% 0603 SMD Vishay-Dale CRCW06034R70JNEA R13 RES 10 OHM 1/8W 5% 0805 SMD Vishay-Dale CRCW080510R0JNEA R14 RES 1.50 OHM 1/4W 1% 1206 SMD Vishay-Dale CRCW12061R50FNEA R15 RES 3.48K OHM 1/10W 1% 0603 SMD Vishay-Dale CRCW06033K48FKEA R16 RES 191K OHM 1/10W 1% 0603 SMD Vishay-Dale CRCW0603191KFKEA CRCW080540R2FKEA R22 RES 40.2 OHM 1/8W 1% 0805 SMD Vishay-Dale RT1 CURRENT LIMITOR INRUSH 60OHM 20% Cantherm MF72-060D5 T1 Transformer Wurth Electronics 750311553 Rev. 01 TP2-TP5 Terminal, Turret, TH, Double Keystone Electronics 1502-2 TP7 TEST POINT ICT - - U1 Offline LED Driver, PowerWise National Semiconductor LM3444MM www.national.com 4 AN-2082 Demo Board Wiring Overview 30131143 Wiring Connection Diagram Test Point Name I/O Description TP3 LED + Output LED Constant Current Supply Supplies voltage and constant-current to anode of LED string. TP2 LED - Output LED Return Connection (not GND) Connects to cathode of LED string. Do NOT connect to GND. TP5 LINE Input AC Line Voltage Connects directly to AC line of a 120VAC system. TP4 NEUTRAL Input AC Neutral Connects directly to AC neutral of a 120VAC system. Demo Board Assembly 30131169 Top View 30131170 Bottom View 5 www.national.com (Note 1) Efficiency vs. Line Voltage Original Circuit Efficiency vs. Line Voltage Modified Circuits 86 86 84 8 LEDs EFFICIENCY (%) EFFICIENCY (%) 84 82 6 LEDs 80 4 LEDs 78 76 80 90 100 110 120 130 Original Mod A 82 80 Mod B Mod C 78 76 140 80 LINE VOLTAGE (VRMS) 90 100 110 120 130 140 LINE VOLTAGE (VRMS) 30131187 30131188 LED Current vs. Line Voltage Original Circuit LED Current vs. Line Voltage Modified Circuits 1.0 1.0 0.8 0.8 0.7 6 LEDs ILED (A) ILED (A) Mod C 4 LEDs 0.4 0.2 0.7 Mod B 0.4 0.2 Mod A 8 LEDs 0.0 80 90 100 110 120 130 0.0 140 Original 80 LINE VOLTAGE (VRMS) 90 100 110 120 130 140 LINE VOLTAGE (VRMS) 30131189 30131190 Power Factor vs. Line Voltage Original Circuit Output Power vs. Line Voltage Original Circuit 1.000 15 0.996 12 POUT (W) POWER FACTOR AN-2082 Typical Performance Characteristics 0.992 0.988 4 LEDs 90 100 110 120 130 3 140 LINE VOLTAGE (VRMS) 80 90 100 110 120 130 140 LINE VOLTAGE (VRMS) 30131191 www.national.com 6 LEDs 6 0.984 0.980 80 8 LEDs 9 30131193 6 Power MOSFET Drain Voltage Waveform (VIN = 120VRMS, 6 LEDs, ILED = 350mA) 15 Mod C POUT (W) 12 Mod B 9 6 Mod A 3 30131196 Original 80 90 100 110 120 130 140 LINE VOLTAGE (VRMS) 30131194 Current Sense Waveform (VIN = 120VRMS, 6 LEDs, ILED = 350mA) FILTER Waveform (VIN = 120VRMS, 6 LEDs, ILED = 350mA) 30131197 30131198 Note 1: Original Circuit: R14 = 1.50Ω; Modification A: R14 = 1.21Ω; Modification B: R14 = 1.00Ω; Modification C: R14 = 0.75Ω 7 www.national.com AN-2082 Output Power vs. Line Voltage Modified Circuits AN-2082 PCB Layout 30131109 Top Layer 30131110 Bottom Layer www.national.com 8 AN-2082 Transformer Design Mfg: Wurth Electronics, Part #: 750311553 Rev. 01 30131199 30131114 9 www.national.com AN-2082 The 120 Hz current ripple flowing through the LED string was measured to be 170 mApk-pk at full load. The magnitude of the ripple is a function of the value of energy storage capacitors connected across the output port. The ripple current can be reduced by increasing the value of energy storage capacitor or by increasing the LED string voltage. The LED driver switching frequency is measured to be close to the specified 79 kHz. The circuit operates with a constant duty cycle of 0.28 and consumes 9.25 W of input power. The driver steady state performance for an LED string consisting of 6 series LEDs is summarized in the following table. Experimental Results The LED driver is designed to accurately emulate an incandescent light bulb and therefore behave as an emulated resistor. The resistor value is determined based on the LED string configuration and the desired output power. The circuit then operates in open-loop, with a fixed duty cycle based on a constant on-time and constant off-time that is set by selecting appropriate circuit components. Performance In steady state, the LED string voltage is measured to be 21.38 V and the average LED current is measured as 357 mA. Measured Efficiency and Line Regulation (6 LEDs) VIN (VRMS) IIN (mARMS) PIN(W) VOUT (V) ILED (mA) POUT (W) Efficiency (%) Power Factor 90 60 5.37 20.25 216 4.38 81.6 0.9970 95 63 5.95 20.47 238 4.87 81.8 0.9969 100 66 6.57 20.67 260 5.38 81.9 0.9969 105 69 7.23 20.86 285 5.94 82.1 0.9969 110 72 7.89 21.05 309 6.50 82.3 0.9968 115 75 8.59 21.23 334 7.09 82.5 0.9967 120 77 9.25 21.38 357 7.65 82.7 0.9965 125 80 9.94 21.53 382 8.23 82.8 0.9961 130 82 10.62 21.68 406 8.80 82.9 0.9957 135 84 11.26 21.80 428 9.34 83.0 0.9950 LED Current, Output Power versus Number of LEDs for Various Circuit Modifications ( VIN = 120 VAC) # of LEDs Original Circuit (Note 2) Modification A (Note 2) Modification B (Note 2) Modification C (Note 2) ILED (mA) POUT (W) ILED (mA) POUT (W) ILED (mA) POUT (W) ILED (mA) POUT (W) 4 508 7.57 624 9.55 710 11.05 835 13.24 6 357 7.65 440 9.58 500 11.02 590 13.35 8 277 7.69 337 9.59 382 11.00 445 13.00 Note 2: Original Circuit: R14 = 1.50Ω; Modification A: R14 = 1.21Ω; Modification B: R14 = 1.00Ω; Modification C: R14 = 0.75Ω design also exhibits low current harmonics as a percentage of the fundamental current (as shown in the following figure) and therefore meets the requirements of the IEC 61000-3-2 Class-3 standard. Power Factor Performance The LED driver is able to achieve close to unity power factor (P.F. ~ 0.99) which meets Energy Star requirements. This 30131195 Current Harmonic Performance vs. EN/IEC61000-3-2 Class C Limits www.national.com 10 AN-2082 Electromagnetic Interference (EMI) The EMI input filter of this evaluation board is configured as shown in the following circuit diagram. 30131167 FIGURE 1. Input EMI Filter and Rectifier Circuit In order to get a quick estimate of the EMI filter performance, only the PEAK conductive EMI scan was measured and the data was compared to the Class B conducted EMI limits published in FCC – 47, section 15. 30131177 FIGURE 2. Peak Conductive EMI scan per CISPR-22, Class B Limits If an additional 33nF of input capacitance (i.e. C6) is utilized in the input filter, the EMI conductive performance is further improved as shown in the following figure. 30131178 FIGURE 3. Peak Conductive EMI scan with additional 33nF of input capacitance 11 www.national.com AN-2082 ILED = 350 mA # of LEDs = 6 POUT = 7.3 W The results are shown in the following figures. Thermal Analysis The board temperature was measured using an IR camera (HIS-3000, Wahl) while running under the following conditions: VIN = 120 VRMS 30131175 FIGURE 4. Top Side Thermal Scan 30131176 FIGURE 5. Bottom Side Thermal Scan www.national.com 12 Injecting line voltage into FILTER (achieving PFC > 0.99) If a small portion (750mV to 1.00V) of line voltage is injected at FILTER of the LM3444, the circuit is essentially turned into a constant power flyback as shown in Figure 6. 30131118 FIGURE 7. FILTER Waveform For this evaluation board, the following resistor values are used: R2 = R7 = 309kΩ R15 = 3.48kΩ Therefore the voltages observed on the FILTER pin will be as follows for listed input voltages: For VIN = 90VRMS, VFILTER = 0.71V For VIN = 120VRMS, VFILTER = 0.95V For VIN = 135VRMS, VFILTER = 1.07V Using this technique, a power factor greater than 0.99 can be achieved without additional passive active power factor control (PFC) circuitry. 30131117 FIGURE 6. Line Voltage Injection Circuit The LM3444 works as a constant off-time controller normally, but by injecting the 1.0V rectified AC voltage into the FILTER pin, the on-time can be made to be constant. With a DCM Flyback, Δi needs to increase as the input voltage line increases. Therefore a constant on-time (since inductor L is constant) can be obtained. 30131116 FIGURE 8. Typical Operation of FILTER Pin 13 www.national.com AN-2082 By using the line voltage injection technique, the FILTER pin has the voltage wave shape shown in Figure 7 on it. Voltage at VFILTER peak should be kept below 1.25V. At 1.25V current limit is tripped. C11 is small enough not to distort the AC signal but adds a little filtering. Although the on-time is probably never truly constant, it can be observed in Figure 8 how (by adding the rectified voltage) the on-time is adjusted. 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