AC Powered Driver Topologies
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The document outlines various LED driver power conversion topologies such as linear, buck, boost, and flyback, emphasizing the importance of efficiency and performance in selecting drivers for different applications. It also discusses regional standards pertaining to power factor and harmonic content, particularly in North America and Europe, while highlighting challenges faced in LED bulb design. Additionally, the report compares different switching topologies and addresses rising concerns regarding optical flicker and health impacts.
AC Powered Driver Topologies
- 1. 11 AC Powered Driver Topologies
- 2. 2 • Template • Nov-12 Confidential Proprietary2 Outline • LED Driver Requirements and Regional Standards • Topology Overview • Meeting Power Factor/Harmonic Content Requirements • Comparison of Switching Topologies • Conclusions
- 3. 3 • Template • Nov-12 Confidential Proprietary3 Main LED Power Conversion Topologies • Linear • Buck • Boost • Buck-boost • Flyback • Resonant Half Bridge • Initial Considerations – LED Selection – Efficiency and Size – Performance specifications – Features eg: Dimming/Control Power Conversion LED Driver Control LED(s) AC Mains Real World Interface
- 4. 4 • Template • Nov-12 Confidential Proprietary4 LED selection depends on Application Small-chip, dispersed phosphor • Linear • Non-directional retrofit lamps (A-lamps) • Any application where uniformity and color are critical Large-chip, coated phosphor system • Outdoor • Down-lights • Directional lamps (PAR, MR16) • Any application that requires a TIR optic or long throw “Fried Eggs” • High-output downlights, some non- roadway outdoor (e.g. Wall packs) • Lower-volume products • Easy to assembly High Current/Low Voltage Low Current/Higher Voltage Medium Current/Med Voltage
- 5. 5 • Template • Nov-12 Confidential Proprietary5 Driver Challenges for LED Bulbs • Efficiency is critical since heat sinking is limited as bulb shape is fixed • Space inside bulbs is limited, especially for higher power bulbs that need more heat sink area • If driver does not have electrical isolation, bulb mechanical design must address safety isolation • Optical design may also reduce the space available for the driver Courtesy: IEEE Spectrum
- 6. 6 • Template • Nov-12 Confidential Proprietary6 Key Driver Selection Factors Efficiency & Parametric Performance Control & “Smart” Operation Operating Environment Production Testing and Maintenance Form Factor & Safety • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •
- 7. 7 • Template • Nov-12 Confidential Proprietary7 North America Standards • ENERGY STAR® is voluntary standard – No driver efficiency specifications, only at system level (lumens/W) – Power Factor By Application • LED Bulbs < 5 W … No power factor requirement • LED Bulbs ≥ 5 W … PF ≥ 0.7 • LED Residential Fixtures … PF ≥ 0.7 • LED Commercial Fixtures … PF ≥ 0.9, no THD spec but <20% common – California Voluntary “Quality LED Bulb”, effective in 2014 • Eligible for Utility Rebate • PF ≥ 0.9, Dimmable Range < 10%, Color Rendering Index (CRI) > 90 – Safety standards under UL (US) and CSA (Canada)
- 8. 8 • Template • Nov-12 Confidential Proprietary8 Basic UL Framework UL 8750 Light Emitting Diode (LED) Equipment for Use in Lighting Products Self Ballasted Lamps UL 1993 Low Voltage Landscape Lighting UL 1838 Stage and Studio Luminaires UL 1573 Class 2 Power Units UL 1310 Power Units Other than Class 2 UL 1012 Luminaires UL 1598 Portable Electric Luminaires UL 153 Track Lighting Systems UL 1574 Underwater Luminaires UL 676 Low Voltage Systems UL 2108 Note: Examples of some general lighting categories
- 9. 9 • Template • Nov-12 Confidential Proprietary9 EU Standards (Followed by Other Countries) • IEC61000-3-2 Class C Lighting, Harmonic Content, < 25W the specifications is pretty simple with special exception • IEC 61347-2-13 – Lamp Controlgear – Particular requirements for DC or AC supplied electronic control gear for LED modules – Safety • IEC 62384 DC or AC supplied electronic control gear for LED modules – Performance • IEC 62838-2-2 – Particular requirements for connectors used with LED modules • IEC 61547 – EMC immunity requirements • IEC 62031 – LED modules for general lighting – safety
- 10. 10 • Template • Nov-12 Confidential Proprietary10 Outline • LED Driver Requirements and Regional Standards • Topology Overview • Meeting Power Factor/Harmonic Content Requirements • Comparison of Switching Topologies • Conclusions
- 11. 11 • Template • Nov-12 Confidential Proprietary11 Linear LED Driver Approach • Current Control Regulator (CCR) can set fixed current to drive LEDs – LED string Vf must match line voltage for best efficiency – RMS current varies with AC line – 100% Ripple • Input capacitor drop acts as a voltage divider to improve efficiency/reduce # of LEDs ~VAC + - VAK ICCR VF(Total) P Pg VfR VRI C 1 1Re 2 * R1 = 470kΩ R2 = 120Ω Vz = VLED + 4V VAC Info in AND8492-D Application Note www.onsemi.com
- 12. 12 • Template • Nov-12 Confidential Proprietary12 Buck Topology • This buck is the simplest of switched-mode topologies, suitable when Vout (LED forward voltage) is less than Vin – Switch closed current flows into the inductor and into the load – Switch open current continues to flow into the load, with the diode closing the circuit • Vout = Vin (Ton/Ts), assuming continuous conduction of the inductor • Input current is chopped, output current (into the capacitor) is smooth TS TON TOFF Duty Cycle = Duty Ratio = D = TON TS TON TON TOFF = Load (R) Vin Vout Vout = VinD
- 13. 13 • Template • Nov-12 Confidential Proprietary13 •Red switch is ON •Green switch is OFF Example NCL30105 Buck LED Driver • Inverted buck – MOSFET is referenced to ground – LED string is directly connected to high voltage • Example shows CCM (continuous conduction), Critical Mode (CrM) is often used and has several advantages
- 14. 14 • Template • Nov-12 Confidential Proprietary14 Current in CrM Buck Inductor in LED LED on off peak V V V I t t I L L 2 peak LED I I Ivalley = 0 ton toff ILED ΔI = Ipeak Ipeak Ics td •Advantages of CrM over CCM •CrM has much smaller inductor than CCM • No need for low Trr for output rectifier, lower switching losses • CrM reduces current error due to inductor, Vin, and VLED variation
- 15. 15 • Template • Nov-12 Confidential Proprietary15 The Other Two Basic Switched-Mode Topologies • Boost – The inductor, switch and diode have changed places. – Switch on current in switch. – Switch off current in diode. – Vout > Vin (neglecting Vdiode) – Input current is smooth. – Output current is chopped. • Buck-boost – Again, the three elements have changed places. – Switch on current in switch. – Switch off current in diode. – BUT direction of current causes the output to be negative (always). – Input current and output current are both chopped. – Non-isolated version of a flyback. Load (R) Vin Vout 1 Vout = Vin D' Load (R) Vin Vout Vout = Vin D' - D D' = 1 - D = TON TOFF TOFF TOFF TS =
- 16. 16 • Template • Nov-12 Confidential Proprietary16 Flyback (transformer-coupled buck-boost) • Same characteristics as the buck-boost---both input and output currents are chopped. • Note polarity of windings (positive output is shown). • Transfer function is like the buck-boost, with added turns ratio (n). Vin VLED VLED = Vin D n D' n 1
- 17. 17 • Template • Nov-12 Confidential Proprietary17 Flyback versus Forward •Custom transformer in both cases, forward requires reset winding • Flyback is preferred over forward • Simplicity and lower parts count • Can provide high power factor or low ripple based on control method • Can support wide range of output voltage with good efficiency Flyback Forward
- 18. 18 • Template • Nov-12 Confidential Proprietary18 Multi-Stage Topologies • Commonly used for medium to high power LED drivers • Reasons multi-stage is used (even at lower power) – Easy to meet PF and THD across wide input line voltage – Low output current ripple – Support for very wide Vf range – Stable Half Bridge Input PFC Boost Buck LLC needs stable Vin Boost + BuckPFC + Flyback Stage
- 19. 19 • Template • Nov-12 Confidential Proprietary19 Outline • LED Driver Requirements and Regional Standards • Topology Overview • Meeting Power Factor/Harmonic Content Requirements • Comparison of Switching Topologies • Conclusions
- 20. 20 • Template • Nov-12 Confidential Proprietary20 Power Factor and Harmonic Content • Active Power Factor control can meet the requirements of all markets and applications – EU min PF for Class C, < 25W is ~ 0.6 plus harmonic requirements – US Residential > 5 W, PF > 0.7, no harmonic requirements • Single Stage Active Power Factor > 0.9x has real costs – 100/120 Hz Line Ripple requires large output caps or – 200% Ripple means LEDs are overdriven or under-used – Active PF means high peak to average current • Bigger MOSFET • Increased Inductor/Transformer Losses – More complex EMI filtering (no bulk) – More input transient protection (no bulk cap) 100 Hz Current Ripple
- 21. 21 • Template • Nov-12 Confidential Proprietary21 Output Ripple Cause Optical Flicker • All light sources connected to the AC line have time varying response • Various physiological effects of low frequency light modulation - Visible frequency range 3-70 Hz including photosensitive epilepsy - Studies with fluorescent lights have indicated human performance impact in 100 – 120 Hz range in work efficiency and health • New Energy Star Bulb specification requires flicker to be reported (9/2014) • IEEE PAR1789 working on "Recommending practices for modulating current in High Brightness LEDs for mitigating health risks to viewers”
- 22. 22 • Template • Nov-12 Confidential Proprietary22 Passive PFC to achieve > 0.7 PF • One option besides active PFC, is to modify the input filter to improve the Power Factor using a valley fill circuit – Can achieve PF >0.8-0.9 for single line range – Still introduces distortion, but has been used for lighting in North America – Does require additional input filter components – Since energy storage is on primary side, output capacitor size can be reduced compared to active PF Input Current Output Voltage PF = 0.87
- 23. 23 • Template • Nov-12 Confidential Proprietary23 EN61000-3-2 Class C < 25W Exception – 3rd Harmonic < 86% – 5th Harmonic < 61% • The specification means that only a smaller than normal input bulk capacitor is needed to meet harmonic content requirements
- 24. 24 • Template • Nov-12 Confidential Proprietary24 Start of Conduction - 42° (must be < 60 °) Peak of conduction - 45° (must be < 65 °) EN61000-3-2 Class C < 25 W Compliance • Rule of Thumb: – Cbulk < 0.25 µF/Watt (230 Vac) – Example waveform below is 10 W Pout with 2.2 µF Input Bulk Input Current Input Voltage
- 25. 25 • Template • Nov-12 Confidential Proprietary25 Outline • LED Driver Requirements and Regional Standards • Topology Overview • Meeting Power Factor/Harmonic Content Requirements • Comparison of Switching Topologies • Conclusions
- 26. 26 • Template • Nov-12 Confidential Proprietary26 Which Switching Topology Is Best? • Best is subjective! – Efficiency – Lower Bill of Material Cost – Reduced Parts Count – Smaller Space • General trend especially for bulbs is to move to non-isolated LED Drivers as it addresses all these concerns • We have analyzed different topologies to identify best region – Figure of Merit Criteria is Voltage* Current Stress – Data normalized to LED VF / Vin Ratio – Considered Active PF and non-power factor corrected cases
- 27. 27 • Template • Nov-12 Confidential Proprietary27 Relative Stress Analysis, Non PF Corrected • A For very low LED VF, non-isolated flyback has lowest stress • B Buck and buck-boost are similar in <20% VF/Vin region, buck-boost is limited around 25% as higher voltage MOSFET would be needed • C Upper limit of buck is only due EN61000-3-2 Class C compliance A B C •600 V MOSFET • 80% derating • Vin= 265 Vac • Pout =10 W
- 28. 28 • Template • Nov-12 Confidential Proprietary28 Relative Stress Analysis, PF Corrected A B C • 600 V MOSFET • 80% derating • Vin= 265 Vac • Pout =20 W • A For high power factor, boost is the lowest stress topology • B To extend Buck-boost to a higher ratio a higher voltage MOSFET is needed • C Upper limit of buck is based on keeping THD < 20%
- 29. 29 • Template • Nov-12 Confidential Proprietary29 Summary • Best topology is strongly driven by LED selection and expected performance needs • Active power factor correction is not always needed to meet regional PF and harmonic requirements • Optical flicker is becoming a topic of increased interest due to possible health concerns • With newer high voltage LEDs coming on the market, boost and buck are becoming more popular topologies and can be very efficient
Editor's Notes
- #10: Can be moved to the appendix if the speaker needs more time.Important point is there are multiple standards that have to be considered