LM3444

LM3444 - 120VAC, 8W Isolated Flyback LED Driver LM3444 -120VAC, 8W Isolated Flyback LED Driver National Semiconductor Application Note 2082 Clinton Jensen December 7, 2010 Introduction 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. Key Features • • • 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 VIN VOUT ILED POUT fsw Parameter Input voltage LED string voltage LED string average current Output power Switching frequency Min 90 VRMS 12 V Demo Board Typ 120 VRMS 21.4 V 350 mA 7.6 W 79 kHz Max 135 VRMS 30 V - 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 # 1 2 3 4 5 6 7 8 9 10 Name NC NC NC COFF FILTER GND ISNS GATE VCC NC Description No internal connection. No internal connection. 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. Circuit ground connection. LED current sense pin. Connect a resistor from main switching MOSFET source, ISNS to GND to set the maximum LED current. Power MOSFET driver pin. This output provides the gate drive for the power switching MOSFET of the buck controller. Input voltage pin. This pin provides the power for the internal control circuitry and gate driver. No internal connection. 3 www.national.com AN-2082 Bill of Materials Designator AA1 C1 C2 C3, C4 C6 C7 C8 C11 C12 C13 D1 D2 D3 D4 D5 D7 D8 F1 J1, J2, J3, J4, TP8, TP9, TP10 J5, J6 L1, L2 Q1 Q2 R1, R3 R2, R7 R6, R24 R12 R13 R14 R15 R16 R22 RT1 T1 TP2-TP5 TP7 U1 Description Printed Circuit Board CAP .047UF 630V METAL POLYPRO CAP 10000PF X7R 250VAC X2 2220 CAP 330UF 35V ELECT PW CAP .10UF 305VAC EMI SUPPRESSION CAP, CERM, 0.1µF, 16V, +/-10%, X7R, 0805 CAP CER 47UF 16V X5R 1210 CAP CER 2200PF 50V 10% X7R 0603 CAP CER 330PF 50V 5% C0G 0603 CAP CER 2200PF 250VAC X1Y1 RAD DIODE TVS 150V 600W UNI 5% SMB RECT BRIDGE GP 600V 0.5A MINIDIP DIODE RECT GP 1A 1000V MINI-SMA DIODE SCHOTTKY 100V 1A SMA DIODE ZENER 30V 1.5W SMA DIODE ZENER 12V 200MW DIODE SWITCH 200V 200MW FUSE BRICK 1A 125V FAST 6125FA 16 GA WIRE HOLE, 18 GA WIRE HOLE CONN HEADER .312 VERT 2POS TIN INDUCTOR 4700UH .13A RADIAL MOSFET N-CH 600V 90MA SOT-89 MOSFET N-CH 600V 1.8A TO-251 RES 200K OHM 1/4W 5% 1206 SMD RES, 309k ohm, 1%, 0.25W, 1206 RES, 10.5k ohm, 1%, 0.125W, 0805 RES 4.7 OHM 1/10W 5% 0603 SMD RES 10 OHM 1/8W 5% 0805 SMD RES 1.50 OHM 1/4W 1% 1206 SMD RES 3.48K OHM 1/10W 1% 0603 SMD RES 191K OHM 1/10W 1% 0603 SMD RES 40.2 OHM 1/8W 1% 0805 SMD CURRENT LIMITOR INRUSH 60OHM 20% Transformer Terminal, Turret, TH, Double TEST POINT ICT Offline LED Driver, PowerWise Manufacturer EPCOS Inc Murata Electronics North America Nichicon EPCOS Kemet MuRata MuRata MuRata TDK Corporation Littlefuse Diodes Inc. Comchip Technology ST Microelectronics ON Semiconductor Fairchild Semiconductor Diode Inc Cooper/Bussmann 3M Tyco Electronics TDK Corporation Infineon Technologies Infineon Technology Vishay-Dale Vishay-Dale Vishay-Dale Vishay-Dale Vishay-Dale Vishay-Dale Vishay-Dale Vishay-Dale Vishay-Dale Cantherm Wurth Electronics Keystone Electronics National Semiconductor Part Number 551600530-001A B32559C6473K000 GA355DR7GB103KY02L UPW1V331MPD6 B32921C3104M C0805C104K4RACTU GRM32ER61C476ME15L GRM188R71H222KA01D GRM1885C1H331JA01D CD12-E2GA222MYNS SMAJ120A RH06-T CGRM4007-G STPS1H100A 1SMA5936BT3G MM5Z12V BAV20WS-7-F 6125FA 923345-02-C 1-1318301-2 TSL0808RA-472JR13-PF BSS225 L6327 SPU02N60S5 CRCW1206200KJNEA CRCW1206309KFKEA CRCW080510K5FKEA CRCW06034R70JNEA CRCW080510R0JNEA CRCW12061R50FNEA CRCW06033K48FKEA CRCW0603191KFKEA CRCW080540R2FKEA MF72-060D5 750311553 Rev. 01 1502-2 LM3444MM www.national.com 4 AN-2082 Demo Board Wiring Overview 30131143 Wiring Connection Diagram Test Point TP3 TP2 TP5 TP4 Name LED + LED LINE NEUTRAL I/O Output Output Input Input Description LED Constant Current Supply Supplies voltage and constant-current to anode of LED string. LED Return Connection (not GND) Connects to cathode of LED string. Do NOT connect to GND. AC Line Voltage Connects directly to AC line of a 120VAC system. AC Neutral Connects directly to AC neutral of a 120VAC system. Demo Board Assembly 30131169 Top View 30131170 Bottom View 5 www.national.com AN-2082 Typical Performance Characteristics Efficiency vs. Line Voltage Original Circuit 86 84 EFFICIENCY (%) (Note 1) Efficiency vs. Line Voltage Modified Circuits 86 84 EFFICIENCY (%) Original 8 LEDs Mod A 82 6 LEDs 82 80 Mod C 80 78 76 4 LEDs Mod B 78 76 80 90 100 110 120 130 140 80 90 100 110 120 130 140 LINE VOLTAGE (VRMS) 30131187 LINE VOLTAGE (VRMS) 30131188 LED Current vs. Line Voltage Original Circuit 1.0 0.8 0.7 4 LEDs LED Current vs. Line Voltage Modified Circuits 1.0 0.8 Mod C ILED (A) ILED (A) 6 LEDs 0.7 0.4 0.2 Mod B 0.4 0.2 8 LEDs Mod A Original 0.0 80 90 100 110 120 130 140 0.0 80 90 100 110 120 130 140 LINE VOLTAGE (VRMS) 30131189 LINE VOLTAGE (VRMS) 30131190 Power Factor vs. Line Voltage Original Circuit 1.000 0.996 POWER FACTOR Output Power vs. Line Voltage Original Circuit 15 12 POUT (W) 8 LEDs 0.992 0.988 0.984 0.980 80 9 6 LEDs 6 4 LEDs 90 100 110 120 130 140 3 80 90 100 110 120 130 140 LINE VOLTAGE (VRMS) 30131191 LINE VOLTAGE (VRMS) 30131193 www.national.com 6 AN-2082 Output Power vs. Line Voltage Modified Circuits 15 Mod C Power MOSFET Drain Voltage Waveform (VIN = 120VRMS, 6 LEDs, ILED = 350mA) 12 POUT (W) Mod B 9 6 Mod A 30131196 3 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 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 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. 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. Measured Efficiency and Line Regulation (6 LEDs) VIN (VRMS) 90 95 100 105 110 115 120 125 130 135 IIN (mARMS) 60 63 66 69 72 75 77 80 82 84 PIN(W) 5.37 5.95 6.57 7.23 7.89 8.59 9.25 9.94 10.62 11.26 VOUT (V) 20.25 20.47 20.67 20.86 21.05 21.23 21.38 21.53 21.68 21.80 ILED (mA) 216 238 260 285 309 334 357 382 406 428 POUT (W) 4.38 4.87 5.38 5.94 6.50 7.09 7.65 8.23 8.80 9.34 Efficiency (%) Power Factor 81.6 81.8 81.9 82.1 82.3 82.5 82.7 82.8 82.9 83.0 0.9970 0.9969 0.9969 0.9969 0.9968 0.9967 0.9965 0.9961 0.9957 0.9950 LED Current, Output Power versus Number of LEDs for Various Circuit Modifications ( VIN = 120 VAC) # of LEDs 4 6 8 Original Circuit (Note 2) ILED (mA) 508 357 277 POUT (W) 7.57 7.65 7.69 Modification A (Note 2) ILED (mA) 624 440 337 POUT (W) 9.55 9.58 9.59 Modification B (Note 2) ILED (mA) 710 500 382 POUT (W) 11.05 11.02 11.00 Modification C (Note 2) ILED (mA) 835 590 445 POUT (W) 13.24 13.35 13.00 Note 2: Original Circuit: R14 = 1.50Ω; Modification A: R14 = 1.21Ω; Modification B: R14 = 1.00Ω; Modification C: R14 = 0.75Ω 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 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. 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 Thermal Analysis The board temperature was measured using an IR camera (HIS-3000, Wahl) while running under the following conditions: VIN = 120 VRMS ILED = 350 mA # of LEDs = 6 POUT = 7.3 W The results are shown in the following figures. 30131175 FIGURE 4. Top Side Thermal Scan 30131176 FIGURE 5. Bottom Side Thermal Scan www.national.com 12 AN-2082 Circuit Analysis and Explanations 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. 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. 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. 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