User's Guide SNOA555C – April 2011 – Revised May 2013 AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver 1 Introduction This demonstration board highlights the performance of a LM3448 based Flyback LED driver solution that can be used to power a single LED string consisting of seven to eleven series connected LEDs from a 180 VRMS to 265 VRMS, 50 Hz input power supply. This is a two-layer board using the bottom and top layer for component placement.
Performance Specifications www.ti.com Figure 1. Demo Board Figure 2. LED Current vs.
Typical Performance Characteristics www.ti.com 5 Typical Performance Characteristics TJ=25°C and VCC=12V, unless otherwise specified. 86 1.00 11 LEDs 10 LEDs 9 LEDs 11 LEDS 10 LEDs 9 LEDs POWER FACTOR EFFICIENCY (%) 84 82 80 76 0.80 170180190200210220230240250260270 INPUT VOLTAGE (VRMS) 170 180 190 200 210 220 230 240 250 260 270 INPUT VOLTAGE (VRMS) Figure 3. Efficiency vs. Line Voltage Figure 4. Power Factor vs.
EMI Performance 6 www.ti.com EMI Performance 230V, 6W Conducted EMI Scans Figure 9. LINE – CISPR/FCC Class B Peak Scan Figure 10. NEUTRAL – CISPR/FCC Class B Peak Scan Figure 11. INE – CISPR/FCC Class B Average Scan Figure 12. NEUTRAL – CISPR/FCC Class B Average Scan 230V, 6W THD Measurements Harmonic Current as Percentage of Fundamental 30% 25% Measured 20% Limits 15% 10% 5% 0% Harmonic Order Figure 13.
Circuit Operation With Forward Phase TRIAC Dimmer www.ti.com 7 Circuit Operation With Forward Phase TRIAC Dimmer The dimming operation of the circuit was verified using a forward phase TRIAC dimmer. Waveforms captured at different dimmer settings are shown below: Figure 14. Forward phase circuit at full brightness Figure 15. Forward phase circuit at 90° firing angle Figure 16.
Thermal Performance 8 www.ti.com Thermal Performance The board temperature was measured using an IR camera (HIS-3000, Wahl) while running under the following conditions: VIN = 230VRMS, ILED = 226mA, # of LEDs = 9, POUT = 6.12W. NOTE: Thermal performance is highly dependent on the user's final end-application enclosure, heatsinking methods, ambient operating temperature, and PCB board layout in addition to the electrical operating conditions.
Thermal Performance www.ti.com • • • • • Cursor Cursor Cursor Cursor Cursor 1: 47.3°C 2: 55.4°C 3: 59.2°C 4: 59.8°C 5: 51.5°C Figure 18.
LM3448 Device Pin-Out 9 www.ti.com LM3448 Device Pin-Out SW 1 16 SW SW 2 15 SW NC 3 14 NC BLDR 4 13 ISNS GND 5 12 GND VCC 6 11 FLTR2 ASNS 7 10 COFF FLTR1 8 9 DIM Figure 19. Device Pin-Out Table 1. Pin Description 16 Pin Narrow SOIC 8 Pin # Name 1, 2, 15, 16 SW Description Drain connection of internal 600V MOSFET. 3, 14 NC No connect. Provides clearance between high voltage and low voltage pins. Do not tie to GND. 4 BLDR Bleeder pin.
Demo Board Wiring Overview www.ti.com 10 Demo Board Wiring Overview TP10 J2 J1 LED + NEUTRAL LINE LED TP9 Figure 20. Wiring Connection Diagram Table 2. Test Points 11 Test Point Name I/O Description TP10 LED + Output LED Constant Current Supply Supplies voltage and constant-current to anode of LED string. TP9 LED - Output LED Return Connection (not GND) Connects to cathode of LED string. Do NOT connect to GND.
Demo Board Assembly www.ti.com Figure 22.
Design Guide www.ti.
Design Guide www.ti.com By injecting a voltage proportional to the line voltage at the FLTR2 pin (see Figure 24), the LM3448 circuit is essentially turned into a constant power flyback converter operating in discontinuous conduction mode (DCM). V+ R3 LM3448 R8 11 FLTR2 R20 C18 Figure 24. Direct Line-Injection Circuit VFLTR2 t Figure 25. FLTR2 Waveform with No Dimmer The LM3448 normally works as a constant off-time regulator, but by injecting a 1.
Design Guide www.ti.com 750 mV 50k DIM DECODER ASNS As line voltage increases, the voltage across the inductor increases, and the peak current increases. 370k Tri-State 4.9V Nearly a constant ontime as the line varies RFLTR1 PWM I-LIM FLTR1 RAMP LED Current 1.27V CFLTR1 RAMP GEN. 5.9 kHz 3V 1V 1k ISNS 1V RSNS DIM LEADING EDGE BLANKING The PWM reference increases as the line voltage increases. FLTR2 GND 125 ns CFLTR2 Figure 26.
Design Guide www.ti.com The worst-case average input current is calculated at the minimum peak input voltage and targeted converter efficiency η, (7) where, (8) Next the worst-case peak input current iIN-PK(MAX) is calculated. From Figure 27, the area of the triangle (highlighted with the dashed oval) is the average input current. Therefore, (9) iIN-PK(MAX) IIN-AVE t D TS Figure 27.
Design Guide www.ti.
Design Guide www.ti.com Transformer Geometries and Materials The length of the gap necessary for energy storage in the flyback transformer can be determined numerically; however, this can lead to non-standard designs. Instead, an appropriate AL core value (a value somewhere between 65nH/turns2 and 160nH/turns2 is a good starting point) can be chosen that will imply the gap size. AL is an industry standard used to define how much inductance, per turns squared, that a given core can provide.
Design Guide www.ti.com A clamp circuit is necessary to prevent damage to SW FET from excessive voltage. This evaluation board uses a transient voltage suppression (TVS) clamp D1, shown in Figure 29. V+ R1 C3 D1 TVS T1 D4 VLED+ D3 C12 + D6 VLED± SW SGND Figure 29. TVS Diode Clamp When the LM3448’s internal SW FET is on and the drain voltage is low, the blocking diode (D3) is reverse biased and the clamp is inactive.
Design Guide www.ti.com 12.3 Bias Supplies and Capacitances The bias supply circuits shown in Figure 30 and Figure 31 enables instant turn-on through Q1 while providing an auxiliary winding for high efficiency steady state operation. The two bias paths are each connected to VCC through a diode (D7, D9) to ensure the higher of the two is providing VCC current. T1 V+ VLED + VAUX R2 SW VLED- R7 D7 R15 D8 Q1 Q2 C13 VCC R13 R14 D10 D9 D5 C14 R11 C15 LM3448 VCC 6 Figure 30.
Design Guide www.ti.com PassFET Bias Circuit The passFET (Q1) is used in its linear region to stand-off the line voltage from the LM3448 regulator. Both the VCC startup current and discharging of the EMI filter capacitance for proper phase angle detection are handled by Q1. Therefore Q1 has to block the maximum peak input voltage and have both sufficient surge and power handling capability with regards to its safe operating area (SOA).
Design Guide www.ti.com 12.4 COFF Current Source The current source used to establish the constant off-time is shown in Figure 32. VCC R23 COFF C20 Figure 32. COFF Current Source Circuit Capacitor C20 will be charged with current from the VCC supply through resistor R23.
Design Guide www.ti.com VCC R17 DIM R18 Q4 C17 FLTR2 Figure 33. TRIAC Holding Circuit 12.6 Overvoltage Protection The circuit described in Figure 34 provides over-voltage protection (OVP) in case of LED open circuit failure. The use of this circuit is recommended for stand-alone LED driver designs where it is essential to recover from a momentary open circuit without damaging any part of the circuit.
Design Guide www.ti.com where, VZ is the zener diode threshold, NA and NS are the number of transformer auxiliary and secondary turns respectively, and VOVP is the maximum specified output voltage. 12.7 Input Filter Background Since the LM3448 is used for AC to DC systems, electromagnetic interference (EMI) filtering is critical to pass the necessary standards for both conducted and radiated EMI. This filter will vary depending on the output power, the switching frequencies, and the layout of the PCB.
Design Calculations www.ti.com 12.8 Inrush Limiting and Damping Inrush With a forward phase dimmer, a very steep rising edge causes a large inrush current every cycle as shown in Figure 36. Series resistance (R5, R9) can be placed between the filter and the TRIAC to limit the effect of this current on the converter and to provide some of the necessary holding current at the same time. This will degrade efficiency but some inrush protection is always necessary in any AC system due to startup.
Design Calculations www.ti.com SW FET VDS(MAX) = 600V SW FET RDS-ON = 3.5Ω Vf(D4) = 0.8V VRING = 50V POUT(MAX) = 6.5W VOUT = 26.5V VOVP = 47V VAUX = 13V η = 85% n=5 AL = 90nH/turns2 Ae = 19.49mm2 VCC = 12V VZ(D5)=12V R11=49.9kΩ VGS(Q1)=0.7V 13.2 Preliminary Calculations Nominal peak input voltage: (42) Maximum peak input voltage: (43) Minimum peak input voltage: (44) Maximum average input current: (45) Duty cycle: (46) Maximum peak input current: (47) 13.
Design Calculations www.ti.com Maximum drain-to-source voltage: (49) Maximum peak MosFET current: (50) Maximum RMS MosFET current: (51) Maximum power dissipation: (52) 13.4 Current Sense Current Limit: (53) Sense resistor: (54) Power dissipation: (55) Resulting component choice: (56) 13.
Design Calculations www.ti.com 13.6 Transformer Calculated primary inductance: (62) Chosen primary inductance: (63) Number of primary turns: (64) Chosen primary turns: 154 turns Number of secondary turns: (65) Number of auxiliary turns: (66) (67) Maximum flux density: (68) Resulting component choice: (69) 13.
Design Calculations www.ti.com 13.8 PassFET Calculate maximum peak voltage: (72) Calculate current: (73) Calculate power dissipation: (74) Resulting component choice: (75) 13.9 Input Capacitance Minimum capacitance: (76) AC Voltage rating: (77) DC Voltage rating: (78) Resulting component choice: (79) 13.
Design Calculations www.ti.com 13.11 Overvoltage Protection Zener Diode Calculate Zener diode: (83) Resulting component choice: (84) 13.
Evaluation Board Schematic www.ti.
Bill of Materials www.ti.com 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 LM3448 evaluation board should not be powered with an open load.
Bill of Materials www.ti.com D7, D8, D9 Diode, Schottky, 100V, 150 mA, SOD-323 STMicroelectronics BAT46JFILM D11 F1 DIODE ZENER 17V 500MW SOD-123 Diodes Inc. DDZ9704-7 Fuse, 500mA, 250V, Time-Lag, SMT Littelfuse Inc RST 500 L1, L2 Inductor, Shielded, 4.7mH, 130mA, 7.5mm Radial TDK Corporation TSL0808RA-472JR17-PF Q1 MOSFET, N-CH, 600V, 200mA, SOT-223 Fairchild Semiconductor FQT1N60CTF_WS Q2 TRANSISTOR NPN 300V SOT23 Diodes Inc.
Transformer Design 16 www.ti.com Transformer Design Mfg: Wurth Electronics Midcom, Part #: 750815045 Rev.
PCB Layout www.ti.com 17 PCB Layout NOTE: Spacing between traces and components of this evaluation board are based on high voltage recommendations for designs that will be potted. Users are cautioned to satisfy themselves as to the suitability of this design for the intended end application and take any necessary precautions where high voltage layout and spacing rules must be followed. Figure 37. Top Layer Figure 38.
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