LM3448-230VFLBK/NOPB

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. 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 LM3448 Phase Dimmable Offline LED Driver with Integrated FET (SNOSB51) data sheet for detailed instructions. A schematic and layout have also been included along with measured performance characteristics. A bill of materials is also included that describes the parts used on this demonstration board. 2 Key Features • • • • 3 Applications • • • • 4 Drop-in compatibility with TRIAC dimmers Line injection circuitry enables PFC values greater than 0.90 Adjustable LED current and switching frequency Flicker free operation Retrofit TRIAC Dimming Solid State Lighting Industrial and Commercial Lighting Residential Lighting Performance Specifications Based on an LED Vf = 3V Symbol Parameter Min Typ Max VIN Input voltage 180 VRMS 230 VRMS 265 VRMS VOUT LED string voltage 21 V 27 V 33 V ILED LED string average current - 226 mA - POUT Output power - 6.1 W - fsw Switching frequency - 73 kHz - All trademarks are the property of their respective owners. SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated 1 Performance Specifications www.ti.com Figure 1. Demo Board Figure 2. LED Current vs. Line Voltage (using TRIAC Dimmer) LED CURRENT (mA) 250 200 150 100 50 0 40 60 80 100 120 140 160 180 200 220 240 INPUT VOLTAGE (VRMS) 2 AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback 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. Line Voltage 10 11 LEDs 10 LEDs 9 LEDs 11 LEDs 10 LEDs 9 LEDs 8 300 250 POUT(W) LED CURRENT (mA) 350 0.90 0.85 78 400 0.95 200 150 100 6 4 2 50 0 0 170 180 190 200 210 220 230 240 250 260 270 INPUT VOLTAGE VRMS 170 180 190 200 210 220 230 240 250 260 270 INPUT VOLTAGE (VRMS) Figure 5. LED Current vs. Line Voltage Figure 6. Output Power vs. Line Voltage Figure 7. SW FET Drain Voltage Waveform (VIN = 230VRMS, 9 LEDs, ILED = 226mA) Figure 8. FLTR2 Waveform (VIN = 230VRMS, 9 LEDs, ILED = 226mA) SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated 3 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. EN-61000-3 Class C Limits 4 AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback 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. Forward phase circuit at 135° firing angle SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated 5 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. This LM3448 evaluation board is optimized to supply 6W of output power at room temperature without exceeding the thermal limitations of the LM3448. However higher output power levels can be achieved if precautions are taken not to exceed the power dissipation limits of the LM3448 package or die junction temperature. Please see the LM3448 data sheet for additional details regarding its thermal specifications. • • • • • Cursor Cursor Cursor Cursor Cursor 1: 56.2°C 2: 55.1°C 3: 55.4°C 4: 54.8°C 5: 51.1°C Figure 17. Top Side - Thermal Scan 6 AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback 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. Bottom Side - Thermal Scan SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated 7 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. Provides the input signal to the angle detect circuitry. A 230Ω internal resistor ensures BLDR is pulled down for proper angle sense detection. 5, 12 GND Circuit ground connection. 6 VCC 7 ASNS PWM output of the TRIAC dim decoder circuit. Outputs a 0 to 4V PWM signal with a duty cycle proportional to the TRIAC dimmer on-time. 8 FLTR1 First filter input. The 120Hz PWM signal from ASNS is filtered to a DC signal and compared to a 1 to 3V, 5.85 kHz ramp to generate a higher frequency PWM signal with a duty cycle proportional to the TRIAC dimmer firing angle. Pull above 4.9V (typical) to TRI-STATE® DIM. 9 DIM Input/output dual function dim pin. This pin can be driven with an external PWM signal to dim the LEDs. It may also be used as an output signal and connected to the DIM pin of other LM3448/LM3445 devices or LED drivers to dim multiple LED circuits simultaneously. 10 COFF 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. 11 FLTR2 Second filter input. A capacitor tied to this pin filters the PWM dimming signal to supply a DC voltage to control the LED current. Could also be used as an analog dimming input. 13 ISNS Input voltage pin. This pin provides the power for the internal control circuitry and gate driver. Connect a 22µF (minimum) bypass capacitor to ground. LED current sense pin (internally connected to MOSFET source). Connect a resistor from ISNS to GND to set the maximum LED current. AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback 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. J1-1, (or J5) LINE Input AC Line Voltage Connects directly to AC line or output of TRIAC dimmer of a 230VAC system. J1-2, (or J6) NEUTRAL Input AC Neutral Connects directly to AC neutral of a 230VAC system. Demo Board Assembly Figure 21. Top View SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated 9 Demo Board Assembly www.ti.com Figure 22. Bottom View 10 AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback Design Guide www.ti.com 12 Design Guide V+ C1 R1 R3 R2 R8 R7 D1 T1 C3 C2 D4 VLED+ D3 C12 D7 + D6 R14 VAUX Q1 VLED± C13 VCC R13 D9 D5 PGND SGND C15 C14 R11 R16 C16 LM3448 DIM 9 DIM FLTR1 8 10 COFF ASNS 7 11 FLTR2 VCC 6 12 GND GND 5 13 ISNS BLDR 4 COFF FLTR2 R20 C18 R22 R21 14 NC NC 3 15 SW SW 2 16 SW SW 1 VAUX R15 D8 Q2 D10 VCC VCC AUXILIARY BIAS CIRCUIT R4 VCC LINE F1 R5 VCC VAUX L1 R17 R23 C8 C9 VR1 C6 C10 C4 FLTR2 D2 C5 R10 NEUTRAL C7 V+ DIM R18 Q4 D11 COFF R6 Q3 C17 R9 FLTR2 C20 C11 L2 R12 INPUT EMI FILTER AND RECTIFIER CIRCUIT TRIAC HOLDING CIRCUIT COFF CURRENT SOURCE OVP CIRCUIT Figure 23. Evaluation Board Schematic The following section explains how to design an isolated flyback converter using the LM3448. Refer to the LM3448 datasheet for specific details regarding the function of the LM3448 device. All reference designators refer to the Evaluation Board Schematic in Figure 23 unless otherwise noted. 12.1 DCM Flyback Converter This 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. Like an incandescent lamp, the driver is compatible with both forward and reverse phase dimmers. A key aspect of this design is that the converter operates in discontinuous conduction mode (DCM). DCM is implemented by ensuring that the flyback transformer current reaches zero before the end of the switching period. SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated 11 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 1.0VPK rectified AC voltage into the FLTR2 pin, the on-time can be made to be constant. With a DCM flyback converter the primary side current, iL(t), needs to increase as the rectified input voltage, V+(t), increases as shown in the following equations, (1) or, (2) Therefore a constant on-time (since inductor L is constant) can be obtained. By using the line voltage injection technique, the FLTR2 pin has the voltage wave shape shown in Figure 25 on it with no TRIAC dimmer in-line. Peak voltage at the FLTR2 pin should be kept below 1.25V otherwise current limit will be tripped. Capacitor C18 in conjunction with resistor R20 acts a filter for noise. Using this technique a power factor greater than 0.90 can be achieved.Figure 26 shows how a constant on-time is maintained. 12 AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback 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. Typical Operation of FLTR2 Pin Turns Ratio The first step with an isolated design is to determine the transformer turns ratio. This can be an iterative process that will depend on the specified operating conditions, maximum stresses allowed for the LM3448 SW FET and re-circulating diode as well as transformer core parameters. For many LM3448 flyback designs, an integer turns ratio of 4 or 5 is a good starting point. The next step will be to verify that the chosen turns ratio results in operating conditions that do not violate any other component ratings. Duty Cycle Calculation The AC mains voltage at the line frequency fL is assumed to be perfectly sinusoidal and the diode bridge ideal. This yields a perfect rectified sinusoid at the input to the flyback. The peak nominal input voltage VINPK(NOM)is defined in terms of the input voltage VIN(NOM), (3) Duty cycle is calculated at the nominal peak input voltage VIN-PK(NOM). Note that this is the duty cycle for flyback operation at the boundary of continuous conduction mode (CCM) operation. In order to ensure that the converter is operating in DCM, the primary inductance of the transformer will be adjusted lower (refer to "Transformer" section). (4) Peak Input Current Calculation Due to the direct line-injection, the flyback converter operates as a constant power converter. Therefore average input power over one line cycle will approximately equal the output power, (5) However since the input power has 120Hz ripple, the “peak” input power PIN-PK will be equal to twice the output power, (6) Figure 27 illustrates the input current going into the primary side winding of the flyback transformer over one-half of a rectified input voltage line cycle. SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated 13 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. DCM Flyback Current Waveforms Switching MOSFET (SW FET) From its datasheet, the LM3448’s SW FET voltage breakdown rating VDS(MAX) is 600V. Due to a transformer’s inherent leakage inductance, some ringing VRING on the drain of the SW FET will be present and must also be taken into consideration when choosing a turns ratio. VRING will depend on the design of the transformer. A good starting point is to design for 50V of ringing while planning for 100V of ringing if additional margin is needed. The maximum reflected voltage VREFL based on a turns ratio of “n” at the primary also needs to be calculated, (10) The maximum SW FET drain-to-source voltage is then calculated based on the maximum reflected voltage VREFL, ringing on the SW FET drain and the maximum peak input voltage VIN-PK(MAX), (11) where, (12) and the following condition must be met, 14 AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback Design Guide www.ti.com (13) Peak and RMS SW FET currents are calculated along with maximum SW FET power dissipation based on the SW FET RDS-ON value, (14) (15) (16) Current Limit The peak current limit ILIM should be at least 25% higher than the maximum peak input current, (17) The parallel sense resistor combination will need to dissipate the maximum power, (18) Re-circulating Diode The main re-circulating diode (D4) should be sized to block the maximum reverse voltage VRD4(MAX), operate at the maximum average current ID4(MAX), and dissipate the maximum power PD4(MAX) as determined by the following equations, (19) (20) (21) 12.2 Transformer Primary Inductance The maximum peak input current iIN-PK(MAX) occurring at the minimum AC voltage peak VIN-PK(MIN) determines the worst case scenario that the converter must be designed for in order to stay in DCM. Using the equation for inductor voltage, (22) and rearranging with the previously calculated parameters, (23) provides an inductance LCRIT where the flyback converter will operate at the boundary of CCM for a switching frequency fSW. In order to ensure DCM operation, a general rule of thumb is to pick a primary inductance LP at 85% of the LCRIT value. SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated 15 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. With the initial chosen AL value, the number of turns on the primary and secondary are calculated, (24) (25) Given the target operating frequency and the maximum output power, a core size can be chosen using the vendor’s specifications and recommendations. This choice can then be validated by calculating the maximum operating flux density given the core cross-sectional area Ae of the chosen core, (26) With most common core materials, the maximum operating flux density should be set somewhere between 250mT and 300mT. If the calculation is below this range, then AL should be increased to the next standard value and the turns and maximum flux density calculations iterated. If the calculation is above this range, then AL should be decreased to the next standard value and the turns and maximum flux density calculations iterated. With the flux density appropriately set, the core material for the chosen core size can be determined using the vendor’s specifications and recommendations. Note that there are core materials that can tolerate higher flux densities; however, they are usually more expensive and not practical for these designs. The rest of the transformer design can be done with the aid of the manufacturer. There are calculated trade-offs between the different loss mechanisms and safety constraints that determine how well a transformer performs. This is an iterative process and can ultimately result in the choice of a new core or switching frequency range. The previous steps should reduce the number of iterations significantly but a good transformer manufacturer is invaluable for completion of the process. Clamp Figure 28 shows a large ringing (VRING) on the SW FET drain due to the leakage inductance of the transformer and output capacitance of SW FET. VRING VDS VOUT x NP NS VIN t Figure 28. Switch Node Ringing 16 AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback 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. When the SW FET is turned off, the drain voltage rises past the nominal voltage (reflected voltage plus the input voltage). If it reaches the TVS clamp voltage plus the input voltage, the clamp prevents any further rise. The TVS diode (D1) voltage is set to prevent the SW FET from exceeding its maximum rating and should be greater than the "output voltage x turns ratio" but less than the expected amount of ringing, (27) This clamp method is fairly efficient and very simple compared to other commonly used methods. Note that if the ringing is large enough that the clamp activates, the ringing energy is radiated at higher frequencies. Depending on PCB layout, EMI filtering method, and other application specific items, the clamp can present problems with regards to meeting radiated EMI standards. If the TVS clamp becomes problematic, there are many other clamp options easily found in a basic literature search. SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated 17 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. Bias Supply Circuits VAUX R15 D8 Q2 D10 VCC Figure 31. Auxiliary Winding Bias Circuit 18 AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback 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). The design equations are, (28) (29) (30) Note that if additional TRIAC holding current is to be sourced through Q1, then the transistor will need to be sized appropriately to handle the additional current and power dissipation requirements. Auxiliary Winding Bias Circuit For high efficiency during steady-state operation, an additional winding is used to establish an auxiliary voltage VAUX used to provide a VCC bias voltage. A minimum value of 13V is recommended for VAUX. An auxiliary transformer turns ratio nAUX and corresponding turns calculation is used to size the primary auxiliary winding NA, (31) (32) The minimum primary bias supply capacitance (C14||C15), given a minimum VCC ripple specification at twice the line frequency f2L, is calculated to keep VCC above UVLO at the worst-case current, (33) Input Capacitance The input capacitor of the flyback (C2) has to be able to provide energy during the worst-case switching period at the peak of the AC voltage input. C2 should be a high frequency, high stability capacitor (usually a metallized film capacitor, either polypropylene or polyester) with an AC voltage rating equal to the maximum input voltage. C2 should also have a DC voltage rating exceeding the maximum peak input voltage + half of the peak to peak input voltage ripple specification. The minimum required input capacitance is calculated given the same ripple specification, (34) Output Capacitance C12 should be a high quality electrolytic capacitor with a voltage rating greater than the specified overvoltage protection threshold VOVP. Given the desired voltage ripple, the minimum output capacitance is calculated, (35) SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated 19 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. The COFF pin threshold will therefore be tripped based on the following capacitor equation, (36) where, (37) Solving for off-time tOFF results in, (38) and we also know that the tOFF is calculated where Ts is the switching period, (39) Re-arranging and substituting equations results in the following equation where COFF is typically chosen as value around 330pF, (40) 12.5 TRIAC Holding Circuit An optional TRIAC holding current circuit is also provided on the evaluation board as shown in Figure 33. The DIM pin signal is applied through an RC filter as a varying DC voltage to Q4 such that the voltage on the FLTR2 pin is adjusted and additional holding current can be sinked. 20 AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback 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. In the case of an integrated LED lamp (where the LED load is permanently connected to the driver output) a simple zener diode or TVS based overvoltage protection is suggested as a cost effective solution. The zener diode/TVS offers protection against a single open circuit event and prevents the output voltage from exceeding the regulatory limits. Depending on the LED driver design specifications, either one or both techniques can be used to meet the target regulatory agency approval VAUX FLTR2 D11 Q3 C11 Figure 34. OVP Circuit The OVP threshold is programmable and is set by selecting appropriate value of zener diode D11. The capacitor C11 across the base of transistor Q3 is used to filter the voltage ripple present on the auxiliary voltage and prevent false OVP tripping due to voltage spikes caused by leakage inductance. The circuit operation is simple and based on biasing of transistor Q3 during fault conditions such that it pulls down the voltage on the FLTR2 pin to ground. The bias current depends on how much overdrive voltage is generated above the zener diode threshold. For proper circuit operation, it is recommended to design for 4V overdrive in order to adequately bias the transistor. Therefore the zener diode should be selected based on the expression, (41) SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated 21 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. There are two major components to EMI: differential noise and common-mode noise. Differential noise is typically represented in the EMI spectrum below approximately 500kHz while common-mode noise shows up at higher frequencies. R4 LINE F1 R5 L1 C8 C9 VR1 C6 C10 C4 D2 C5 R10 NEUTRAL C7 V+ R6 R9 L2 R12 Figure 35. Input EMI Filter Conducted Figure 35 shows a typical filter used with this LM3448 flyback design. In order to conform to conducted standards, a fourth order filter is implemented using inductors and "X" rated AC capacitors. If sized properly, this filter design can provide ample attenuation of the switching frequency and lower order harmonics contributing to differential noise. A "Y" rated AC capacitor (C1) from the primary ground to the secondary ground is also critical for reduction of common-mode noise (refer to "Evaluation Board Schematic". This combination of filters along with any necessary damping can easily provide a passing conducted EMI signature. Radiated Conforming to radiated EMI standards is much more difficult and is completely dependent on the entire system including the enclosure. C1 will also help reduce radiated EMI; however, reduction of dV/dt on switching edges and PCB layout iterations are frequently necessary as well. Consult available literature and/or an EMI specialist for help with this. Several iterations of component selection and layout changes may be necessary before passing a specific radiated EMI standard. Interaction with Dimmers In general input filters and forward phase dimmers do not work well together. The TRIAC needs a minimum amount of holding current to function. The converter itself is demanding a certain amount of current from the input to provide to its output, and the input filter is providing or taking current depending upon the dV/dt of the capacitors. The best way to deal with this problem is to minimize filter capacitance and increase the regulated hold current until there is enough current to satisfy the dimmer and filter simultaneously. 22 AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback 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. The size of R5 and R9 are best found experimentally as they provide attenuation for the whole system. Triac Fires Æ Inrush Spike Iin(t) 0 t Potential Misfire Figure 36. Inrush Current Spike Damper The inrush spike can also excite a resonance between the input filter of the TRIAC and the input filter of the converter. The associated interaction can cause the current to ring negative, as shown in Figure 36, thereby shutting off the TRIAC. A TRIAC damper can be placed between the dimmer and the EMI filter to absorb some of the ringing energy and reduce the potential for misfires. The damper is also best sized experimentally due to the large variance in TRIAC input filters. Resistors R5 and R9 can also be increased to help dampen the ringing at the expense of some efficiency and power factor performance. 13 Design Calculations The following is a step-by-step procedure with calculations for a 230V, 6.5W flyback design. 13.1 Specifications fL = 50Hz fSW(MIN) =72kHz VIN(NOM) = 230VAC VIN(MIN) = 180VAC VIN(MAX) = 265VAC ILED = 245mA ΔvOUT = 1V ΔvIN-PK = 35V SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated 23 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) 13.3 SW FET Maximum reflected voltage: (48) 24 AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback 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) 13.5 Re-Circulating Diode Maximum reverse blocking voltage: (57) Maximum peak diode current: (58) Maximum average diode current: (59) Maximum power dissipation: (60) Resulting component choice: (61) SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated 25 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) 13.7 COFF Current Source Calculate off-time, (70) Choose capacitor C20: 330pF Calculate R23, (71) 26 AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback 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) 13.10 Output Capacitance Minimum capacitance: (80) Voltage rating: (81) Resulting component choice: (82) SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated 27 Design Calculations www.ti.com 13.11 Overvoltage Protection Zener Diode Calculate Zener diode: (83) Resulting component choice: (84) 13.12 Transil Clamp TVS clamp voltage: (85) Resulting component choice: (86) 28 AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback Evaluation Board Schematic www.ti.com 14 Evaluation Board Schematic V+ C1 R1 R3 R2 R8 R7 D1 T1 C3 C2 D4 VLED+ D3 C12 D7 + D6 R14 VAUX Q1 VLED± C13 VCC R13 D9 D5 PGND SGND C15 C14 R11 R16 C16 LM3448 DIM 9 DIM FLTR1 8 10 COFF ASNS 7 11 FLTR2 VCC 6 12 GND GND 5 13 ISNS BLDR 4 COFF FLTR2 R20 C18 R22 R21 14 NC NC 3 15 SW SW 2 16 SW SW 1 VAUX R15 D8 Q2 D10 VCC VCC AUXILIARY BIAS CIRCUIT R4 VCC LINE F1 R5 VCC VAUX L1 R17 R23 C8 C9 VR1 C6 C10 C4 FLTR2 D2 C5 R10 NEUTRAL C7 V+ DIM R18 Q4 D11 COFF R6 Q3 C17 R9 FLTR2 C20 C11 L2 R12 INPUT EMI FILTER AND RECTIFIER CIRCUIT TRIAC HOLDING CIRCUIT COFF CURRENT SOURCE OVP CIRCUIT WARNING The LM3448 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. SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated 29 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. For proper operation, ensure that the desired number of LEDs are connected at the output before applying power to the evaluation board. 15 30 Bill of Materials Part ID Description Manufacturer Part Number C1 Ceramic, X7R, 250VAC, 10% Murata Electronics North America DE1E3KX332MA5BA01 C2 Polypropylene Film Capacitors 400V .033uF 5% PCM 10 WIMA MKP1G023303F00JSSD C3 CAP, CERM, 330pF, 630V, +/-5%, C0G/NP0, 1206 TDK C3216C0G2J331J C4 CAP FILM MKP .0047UF 310VAC X2 Vishay/BC comp BFC233820472 C5 CAP, Film, 0.033µF, 630V, +/-10%, TH EPCOS Inc B32921C3333K C6, C7 CAP CER 68000PF 630V X7R 1210 TDK C3225X7R2J683M C8 DNP - - C9 DNP - - C10 DNP - - C11, C13 CAP, CERM, 1uF, 35V, +/-10%, X7R, 0805 Taiyo Yuden GMK212B7105KG-T C12 CAP ALUM 680UF 50V 20% RADIAL Nichicon UPW1H681MHD6 C14 CAP, CERM, 0.1µF, 25V, +/-10%, X7R, 0603 MuRata GRM188R71E104KA01D C15 CAP, CERM, 22uF, 25V, +/-10%, X5R, 1210 MuRata GRM32ER61E226KE15L C16 CAP, CERM, 0.47µF, 16V, +/-10%, X7R, 0603 MuRata GRM188R71C474KA88D C17 CAP, CERM, 0.22µF, 16V, +/-10%, X7R, 0603 TDK C1608X7R1C224K C18 CAP, CERM, 2200pF, 50V, +/-10%, X7R, 0603 MuRata GRM188R71H222KA01D C20 CAP, CERM, 330pF, 50V, +/-5%, C0G/NP0, 0603 MuRata GRM1885C1H331JA01D D1 Diode, TVS, 250V, 600W, UNI, 5%, SMB Littelfuse Inc P6SMB250A D2 Diode, Switching-Bridge, 600V, 0.8A, MiniDIP Diodes Inc. HD06-T D3 Diode, Silicon, 1000V, 1A, SOD-123 Comchip Technology CGRM4007-G STPS1H100A D4 Diode, Schottky, 100V, 1A, SMA STMicroelectronics D5, D10 Diode, Zener, 13V, 200mW, SOD-323 Diodes Inc DDZ13BS-7 D6 Diode, Zener, 47V, 550mW, SMB ON Semiconductor 1SMB5941BT3G AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback 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. MMBTA42-7-F Q3 TRANS GP SS NPN 40V SOT323 ON Semi MMBT3904WT1G Q4 MOSFET, N-CH, 60V, 0.24A, SOT-23 Vishay-Siliconix 2N7002E-T1-E3 R1 RES, 221 ohm, 1%, 0.25W, 1206 Vishay-Dale CRCW1206221RFKEA R2, R7 RES, 200k ohm, 1%, 0.25W, 1206 Vishay-Dale CRCW1206200KFKEA R3, R8 RES, 309k ohm, 1%, 0.25W, 1206 Vishay-Dale CRCW1206309KFKEA R4, R12 RES, 10k ohm, 5%, 0.25W, 1206 Vishay-Dale CRCW120610K0JNEA R5, R9 RES, 22 ohm, 10%, 2W, Axial, Fusible WELWYN EMC2-22RK R6 RES, 820 ohm, 5%, 1W, 2512 Vishay/Dale CRCW2512820RJNEG R10 DNP - - R11 RES, 49.9k ohm, 1%, 0.125W, 0805 Vishay-Dale CRCW080549K9FKEA R13 RES, 33 ohm, 5%, 0.25W, 1206 Vishay-Dale CRCW120633R0JNEA R14 RES, 75 ohm, 5%, 0.125W, 0805 Vishay-Dale CRCW080575R0JNEA R15 RES, 10.0k ohm, 1%, 0.1W, 0603 Vishay-Dale CRCW060310K0FKEA R16 RES, 280k ohm, 1%, 0.1W, 0603 Vishay-Dale CRCW0603280KFKEA R17 RES, 475k ohm, 1%, 0.1W, 0603 Vishay-Dale CRCW0603475KFKEA R18 RES, 49.9k ohm, 1%, 0.1W, 0603 Vishay-Dale CRCW060349K9FKEA R20 RES, 1.91k ohm, 1%, 0.1W, 0603 Vishay-Dale CRCW06031K91FKEA R21 RES 3.60 OHM 1/4W 1% 1206 SMD Vishay/Dale CRCW12063R60FKEA R22 RES, 21.0 ohm, 1%, 0.125W, 0805 Vishay-Dale CRCW080521R0FKEA R23 RES, 294k ohm, 1%, 0.1W, 0603 Vishay-Dale CRCW0603294KFKEA T1 Transformer Wurth Electronics Midcom 750815045 Rev 00 U1 LED Driver Texas Instruments LM3448 VR1 Varistor 275V 55J 10mm DISC EPCOS Inc S10K275E2 SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated 31 Transformer Design 16 www.ti.com Transformer Design Mfg: Wurth Electronics Midcom, Part #: 750815045 Rev.00 32 AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback 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. Bottom Layer SNOA555C – April 2011 – Revised May 2013 Submit Documentation Feedback AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver Copyright © 2011–2013, Texas Instruments Incorporated 33 IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. All semiconductor products (also referred to herein as “components”) are sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment. TI warrants performance of its components to the specifications applicable at the time of sale, in accordance with the warranty in TI’s terms and conditions of sale of semiconductor products. Testing and other quality control techniques are used to the extent TI deems necessary to support this warranty. Except where mandated by applicable law, testing of all parameters of each component is not necessarily performed. TI assumes no liability for applications assistance or the design of Buyers’ products. Buyers are responsible for their products and applications using TI components. To minimize the risks associated with Buyers’ products and applications, Buyers should provide adequate design and operating safeguards. TI does not warrant or represent that any license, either express or implied, is granted under any patent right, copyright, mask work right, or other intellectual property right relating to any combination, machine, or process in which TI components or services are used. Information published by TI regarding third-party products or services does not constitute a license to use such products or services or a warranty or endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual property of the third party, or a license from TI under the patents or other intellectual property of TI. Reproduction of significant portions of TI information in TI data books or data sheets is permissible only if reproduction is without alteration and is accompanied by all associated warranties, conditions, limitations, and notices. TI is not responsible or liable for such altered documentation. Information of third parties may be subject to additional restrictions. Resale of TI components or services with statements different from or beyond the parameters stated by TI for that component or service voids all express and any implied warranties for the associated TI component or service and is an unfair and deceptive business practice. TI is not responsible or liable for any such statements. Buyer acknowledges and agrees that it is solely responsible for compliance with all legal, regulatory and safety-related requirements concerning its products, and any use of TI components in its applications, notwithstanding any applications-related information or support that may be provided by TI. Buyer represents and agrees that it has all the necessary expertise to create and implement safeguards which anticipate dangerous consequences of failures, monitor failures and their consequences, lessen the likelihood of failures that might cause harm and take appropriate remedial actions. Buyer will fully indemnify TI and its representatives against any damages arising out of the use of any TI components in safety-critical applications. In some cases, TI components may be promoted specifically to facilitate safety-related applications. With such components, TI’s goal is to help enable customers to design and create their own end-product solutions that meet applicable functional safety standards and requirements. Nonetheless, such components are subject to these terms. No TI components are authorized for use in FDA Class III (or similar life-critical medical equipment) unless authorized officers of the parties have executed a special agreement specifically governing such use. Only those TI components which TI has specifically designated as military grade or “enhanced plastic” are designed and intended for use in military/aerospace applications or environments. Buyer acknowledges and agrees that any military or aerospace use of TI components which have not been so designated is solely at the Buyer's risk, and that Buyer is solely responsible for compliance with all legal and regulatory requirements in connection with such use. TI has specifically designated certain components as meeting ISO/TS16949 requirements, mainly for automotive use. In any case of use of non-designated products, TI will not be responsible for any failure to meet ISO/TS16949. Products Applications Audio www.ti.com/audio Automotive and Transportation www.ti.com/automotive Amplifiers amplifier.ti.com Communications and Telecom www.ti.com/communications Data Converters dataconverter.ti.com Computers and Peripherals www.ti.com/computers DLP® Products www.dlp.com Consumer Electronics www.ti.com/consumer-apps DSP dsp.ti.com Energy and Lighting www.ti.com/energy Clocks and Timers www.ti.com/clocks Industrial www.ti.com/industrial Interface interface.ti.com Medical www.ti.com/medical Logic logic.ti.com Security www.ti.com/security Power Mgmt power.ti.com Space, Avionics and Defense www.ti.com/space-avionics-defense Microcontrollers microcontroller.ti.com Video and Imaging www.ti.com/video RFID www.ti-rfid.com OMAP Applications Processors www.ti.com/omap TI E2E Community e2e.ti.com Wireless Connectivity www.ti.com/wirelessconnectivity Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265 Copyright © 2013, Texas Instruments Incorporated