National Semiconductor Application Note 2102 David Zhang March 31, 2011 Introduction Key Features This evaluation board consists of a National Semiconductor LM5021 based 120Vac 24W AC/DC flyback converter and a LM3464 based dynamic head room control (DHC) four channel LED driver IC. This demonstration platform was developed so that the customers could evaluate the LM3464 with an AC/DC supply. Once familiar with the operation of the LM3464 and its control of the AC/DC converter's control loop, the type of AC/DC power supply and the converter's output power becomes less relevant. The evaluation board contains eight CREE XLamp MC-E LEDs. Each XLamp MC-E LED has four separate LEDs in each package for a total of 32 LEDs. The CREE MC-E LED allows for an easy demonstration of the LM3464 four channel operation with eight packaged LEDs. each LED string is regulated to 200mA. A heat sink is mounted on the back of the PCB to dissipate the heat generated by the LEDs.The LM3464 regulates each LED string current and adjust the secondary output voltage of the AC/DC power supply 0.9V above the LED string forawrd voltage.The LM3464 monitors the output voltage and adjusts the feedback voltage to the LM5021 primary control. This evaluation also demonstreates the LM3464 thermal fold-back feature which reduces the average LED current when the LED temperature is over 80°C. The AC/DC flyback power supply works in the discontinuous conduction mode and achieves power factor (PF) greater than 0.98. The schematic, bill of materials, layout and measured performance characteristics are included. • • • • • • LM3464 and LM5021 DHC 25W Reference Designs LM3464 and LM5021 DHC 25W Reference Designs Drive four strings of LEDs from 120Vac Overall Efficiency greater than 82% DC/LED Efficiency greater than 90% Power Factor greater than 0.98 Dynamic Headroom Control (DHC) Thermal foldback control Performance Specifications Symbol Parameter Min Typ Max VIN Input voltage 90 VRMS 120 VRMS 132 VRMS VOUT LED string voltage 22 V 26 V 30 V ILED LED string average current per string - 200 mA - POUT Output power - 21 W - fsw Switching frequency - 67 kHz - AN-2102 © 2011 National Semiconductor Corporation 301439 www.national.com AN-2102 Demo Board 30143905 www.national.com 2 AN-2102 LM3464 120VAC, 24W Isolated 4 LED Driver Demo Board Schematic 30143901 Warning: The LM3464 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. 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 the mains AC power is applied (without any isolation), the fuse (F1) will fail open. For bench evaluation, either the input AC power source or the bench measurement equipment should be isolated from the earth ground connection. Isolating the evaliation board (using 1:1 isolation line isolation transformer) rather than the oscilloscope is highly recommended. 3 www.national.com AN-2102 Bill of Materials Designator Description Manufacturer Part Number C1 CAP .10UF 305VAC EMI SUPPRESSION EPCOS B32921C3104M C2 CAP, CERM, 0.33uF, 50V, +/-10%, X7R, 0805 MuRata GRM219R71H334KA88D C3, C18 CAP, CERM, 0.22uF, 250V, +/-10%, X7R, 1210 MuRata GRM32DR72E224KW01L CAP, 33UF, 35V, ELECT, 5mmx11mm Panasonic EEUFM1V330 CAP, CERM, 0.22uF, 100V, +/-10%, X7R, 0805 MuRata GRM21AR72A224KAC5L CAP, CERM, 1uF, 25V, +/-10%, X5R, 0805 AVX 08053D105KAT2A C7 CAP, CERM, 47pF, 50V, +/-5%, C0G/NP0, 0805 Yageo America CC0805JRNP09BN470 C8, C16 CAP, CERM, 0.068uF, 50V, +/-10%, X7R, 0805 AVX 08055C683KAT2A C9, C10 CAP, 1000UF, 35V, ELECT, 12.5mmx25mm Panasonic EEU-FC1V102 C11 CAP, CERM, 1000pF, 250VAC, +/-10%, X7R, 1808 Syfer Technology 1808JA250102KXBSY2 C12 CAP, CERM, 0.15uF, 50V, +/-10%, X7R, 0805 Murata GRM21BR71H154KA01L C17 CAP, CERM, 1uF, 50V, +/-10%, X7R, 1206 TDK C3216X7R1H105K C19 CAP, CERM, 220pF, 500V, +/-10%, X7R, 1206 Vishay VJ1206Y221KXEAT5Z D1 Diode, Switching-Bridge, 400V, 0.8A, MiniDIP Diodes Inc. HD04-T D2, D7 Diode, Schottky, 100V, 0.2A, SOD-123 ST BAT41ZFILM Microelectronics D3 DIODE TVS 150V 600W UNI 5% SMA Littlefuse SMAJ150A D4 DIODE, 1000V, 1A, Mini SMA Comchip Technology CGRM4007-G D5 Diode, Schottky, 100V, 2A, SMB Diodes Inc. B2100-13-F D6 Diode, Zener, 10V, 500mW, SOD-123 ON Semiconductor MMSZ4697T1G D8 Diode, Zener, 8.2V, 500mW, SOD-123 Central Semiconductor CMHZ4694 F1 CMHZ4694 Cooper/ Bussman 6125FA1A H1 Heat Sink CUI Inc. VHS-45 Terminal, Turret, TH, Double Keystone Electronics 1502-2 J7, J8, J9, J10, Test Point, SMT J11 Keystone Electronics 5016 JP1, JP2, JP3 Jumper C4 C5, C14,C15 C6, C13 J1, J2, J3, J4, J5, J6, J12 Samtec TSW-102-07-G-S L1 Common Mode Choke, 10mH, 0.3A Coilcraft BU09-103R25BL L2 Inductor, Shielded Drum Core, 1mH, 0.45A,SMD Coilcraft MSS1038-105KLB LED1 - LED8 LED Cree MCE4WT-A2-0000-000M02 Q1 MOSFET, N-CH, 500V, 6A, DPAK Fairchild Semiconductor FDD6N50TM Q2 Transistor, NPN, 40V, 0.2A, SOT-23 Fairchild Semiconductor MMBT3904 Fairchild Semiconducto FDD3682 Q3, Q4, Q5, Q6 MOSFET, N-CH, 100V, 32A, DPAK R1 RES, 150k ohm, 1%, 0.25W, 1206 Vishay-Dale CRCW1206150KFKEA R2 RES, 100k ohm, 1%, 0.125W, 0805 Vishay-Dale CRCW0805100KFKEA R3, R4 RES, 10.0 ohm, 1%, 0.125W, 0805 Vishay-Dale CRCW080510R0FKEA R5 RES, 0.1 ohm, 5%, 0.25W, 1206 Panasonic ERJ-8RSJR10V R7 RES, 10.5k ohm, 1%, 0.1W, 0603 Vishay/Dale CRCW060310K5FKEA R8, R12 RES, 1.00k ohm, 1%, 0.125W, 0805 Vishay/Dale CRCW08051K00FKEA R9 RES, 5.11k ohm, 1%, 0.125W, 0805 Vishay/Dale CRCW08055K11FKEA R10, R11 RES, 10.0k ohm, 1%, 0.125W, 0805 Vishay-Dale CRCW080510K0FKEA www.national.com 4 R13 Description Manufacturer Part Number RES, 105k ohm, 1%, 0.1W, 0603 Vishay-Dale CRCW0603105KFKEA R14, R15 RES, 25.5k ohm, 1%, 0.125W, 0805 Vishay-Dale CRCW080525K5FKEA R16 RES, 2.67k ohm, 1%, 0.125W, 0805 Vishay-Dale CRCW08052K67FKEA R18 RES, 3.01k ohm, 1%, 0.125W, 0805 Vishay-Dale CRCW08053K01FKEA R19 RES, 8.06k ohm, 1%, 0.125W, 0805 Vishay-Dale CRCW08058K06FKEA R20 RES, 0 ohm, 5%, 0.125W, 0805 Vishay-Dale CRCW08050000Z0EA Vishay-Dale CRCW08051R00FKEA R21, R22, R23, RES, 1.0 ohm, 1%, 0.125W, 0805 R24 R25 RES, 0 ohm, 5%, 0.25W, 1206 Vishay-Dale CRCW12060000Z0EA R26 RES, 22.1k ohm, 1%, 0.125W, 0805 Vishay-Dale CRCW080522K1FKEA RT1 Thermistor NTC, 10k ohm, 2%, 0603 Panasonic ERT-J1VG103GA T1 Transformer, Flyback, 540UH Wurth Elektornik 750311959 U1 AC-DC Current Mode PWM Controller National Semiconductor LM5021MM-1 U2 Dynamic Headroom Controller w/Therm Control National Semiconductor LM3464MH U3 Opto Coupler NEC PS2501L-1-Q-A U4 Low-Voltage Adjustable Precision Shunt Regulator National Semiconductor LMV431AIMF Varistor, 150V, 25J, 10mm Littlefuse V150LA5P VR1 5 www.national.com AN-2102 Designator Power Factor vs. Line Voltage 85 1.000 84 0.995 83 0.990 POWER FACTOR EFFICIENCY (%) Efficiency vs. Line Voltage 82 81 80 79 78 0.985 0.980 0.975 0.970 0.965 77 0.960 76 0.955 75 0.950 90 100 110 120 130 INPUT VOLTAGE (VAC) 140 90 100 110 120 130 INPUT VOLTAGE (VAC) 140 30143938 Total LED Current vs. LED Temperature 30143939 Output Voltage & LED Forward Voltage vs. LED Temperature 1.00 26.5 0.95 0.90 OUTPUT VOLTAGES (V) TOTAL LED CURRENT (A) AN-2102 Typical Performance Characteristics 0.85 0.80 0.75 0.70 0.65 0.60 0.56 0.50 77 26.0 25.5 Vout 25.0 VLED 24.5 24.0 23.5 79 81 83 85 87 89 LED TEMPERATURE (C) 91 30 40 50 60 70 LED TEMPERATURE (C) 30143940 30143941 Line Voltage and Line Current (VIN = 120VRMS) Output Voltage and LED Current (VIN = 120VRMS) 30143918 30143920 Ch1: Line Voltage (100 V/div); Ch3: Line Current (200 mA/div); Time (10 ms/div) www.national.com 80 Ch1: Output Voltage (10 V/div); Ch3: Total LED Current (500 mA/div); Time (10 ms/div) 6 AN-2102 Power MOSFET Q1 Drain to Source Voltage (VIN = 120VRMS) Start Up Voltage and Current (VIN=120VRMS) 30143921 30143922 Ch1: Drain Voltage (100V/div) Time (4 µs/div) Ch1: Output Voltage (10 V/div); Ch3: Total LED Current (500 mV/div); Time (100 ms/div) 7 www.national.com AN-2102 PCB Layout 30143907 Top Layer 30143908 Bottom Layer www.national.com 8 1ST STAGE - DCM FLYBACK The evaluation board is considered a two stage LED driver system. The first stage is the AC/DC flyback converter and the second stage is the LM3464 LED currenr regulator. The first stage employs a NSC LM5021-1 in a discontinuous mode flyback and operates at fixed switching frequency and constant duty cycle. The input current follows the rectified AC input voltage and achieves power factor of 0.98 and higher. To ensure LM5021-1 operates at constant duty cycle, R7 and R1 need to be properly sized so that the slope compensation ramp is bigger than the current sense voltage. In this design, R7 is chosend to be 10.5Kohm and R1 is chosen to be 0.1 ohm. At 40% duty cycle, the peak ramp voltage is 246mV and the peak current sense voltage is around 150mV. The transformer is designed to keep the flyback converter operating in discontinuous mode at peak output power. For a single stage PFC flyback converter, the peak output power occurs at peak input voltage and is twice the average output power. The transformer used in the evaluation board can handle up to 48W peak output power without saturation. The feedback loop of the flyback converter includes U4 the LMV431 shunt regulator and U3 the opto coupler. When LM3464 is not controlling the output voltage (during initial power up), the flyback converter output voltage is set by resistor divider R13, R26 and R11 (approximately 17V). Once the LM3464 begins to control the LM5021 and therefore the output voltage by sink/source current through OutP pin and pushes the output voltage up to as high as 33V. In order to have good power factor, the control loop bandwidth has to be lower than 120Hz. For further explanation of LM3464 operation, please refer to thr LM3464 datasheet. JUMPER/FAULT INFORMATION Jumpers JP1, JP2 and JP3 are provided for the customers to short or disconnect LED strings. One string of LED can be disconnected to the power supply by open connector JP1. One LED each on LED string number one can be shorted by short connctors JP2 or JP3. LM3464 will continue to regulate the currents of the remaining strings of LEDs when one or more strings of LEDs are shorted or disconnected. It will also continue to regulate the LED current when one or more LEDs in that string are shorted. Electromagnetic Interference (EMI) 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. 30143932 FIGURE 1. Peak Conductive EMI scan per CISPR-22, Class B Limits 9 www.national.com AN-2102 2ND STAGE - DYNAMIC HEADROOM CONTROL The second stage of the evaluation board features the LM3464 which is a linear LED driver controller. The LM3464 is a four string controller, and in this particular demonstration board the LM3464 controls four strings of eight LEDs at 200mA. The internal ampiliers of LM3464 compare the voltages of the current sense resistors to a 200mV reference voltage and control the gate voltages of four external MOSFETs ( Q3 - Q6 ) to achieve linear current regulations. The voltage headroom which the voltage difference between the seondary output voltage of the AC/DC power supply and the LED string forward voltage is minimized by the LM3464 controller.By minimizing this voltage, the efficiency of the system is maximized. The LM3464 regulates the drain voltage of the external MOSFET to the voltage of the VDHC pin which is equal to 0.9V, therefore the headroom is about 0.9V. LM3464 has thermal fold-back function to prevent the LEDs from overheating. When the voltage at the thermal pin ( pin #3 ) is less than 3.25V, the average LED current is reduced by PWM dimming. For this evaluation board, the thermal foldback function starts to kick in when the resistance of the NTC thermistor RT1 reaches 3.2kohm. Design Information AN-2102 Thermal Analysis The board temperature was measured using an IR camera (HIS-3000, Wahl) while running under the following conditions: VIN = 120 VRMS The results are shown in the following figure. 30143933 FIGURE 2. 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