National Semiconductor Power Products - Seminar 3 (LED Lighting)

Transcription

1 National Semiconductor Power Products - Seminar 3 (LED Lighting) Dr. Iain Mosely Converter Technology Ltd. Slide 1

2 Overview Background on LEDs Power Electronics for Driving LEDs LED Driver Specific Solutions Dimming Application Specific LED Drivers System Lifetime Slide 2

3 Background on LED s Behaves like a constant voltage load Light output (Luminous Flux) is proportional to drive current Relatively high efficiency but still produce a lot of heat Careful thermal management is required Luminous Devices PhlatLight Slide 3

4 LED Drive Requirements For controlled Lumen output and operating junction temperature, the LED should be driven by a constant current source The current source should be protected so that disconnection of the LED doesn t lead to a dangerous over-voltage situation A standard constant voltage DC/DC converter will generally not work properly due to the shape of the LED V-I curve The driver should be designed to match the potential lifetime of the LED Mains power systems will often require galvanic isolation Slide 4

5 LED Drive Requirements -How can we make a high efficiency current source? Still need a switched mode DC/DC converter for high efficiency Feedback system is modified so that it delivers constant current rather than constant voltage Any traditional voltage output PWM feedback system can be turned into a constant current system VOUT VOUT Rf b1 Vcc D2 LED Vcc Vref V_ERROR Vref V_ERROR Rf b2 Rf b2 Standard PWM Feedback System for Constant Output Voltage Modification of Feedback System for Constant Output Current Slide 5

6 LED Drive Requirements -Design Methodology Decide on power topology based on maximum LED string voltage (Vledmax) and minimum input voltage (Vinmin) If Vinmin > Vledmax use a buck or synchronous buck converter If Vledmax > Vinmin use a boost converter If the output LED string voltage sits between the min and max input voltage, use a SEPIC, flyback or buck-boost Vin LED DRIVER Topology?? Vled Slide 6

7 Example LED Driver - LM5020 Boost Converter Drive 10 series connected LEDs at 1A from a 12-24Vdc input Each LED has a nominal Vf of 3.5V at 1A (i.e. boost to ~35Vdc) Other than the feedback system, the design of switched mode converter systems for LED drivers is the same as for standard regulated voltage systems LM5020 Controller 36mm x 40mm For details on this design (RD009), visit :- /index.php?s=file_download&id=29 Slide 7

8 Example LED Driver - Sensing LED Current VOUT D2 LED Vcc Vref is typically between 0.6V and 1.2V Rfb2 will experience a power loss of (Iled x Vref) Rf b2 Vref V_ERROR E.g. For 1A drive current and 1.2V, Rfb2 dissipates 1.2W LED Specific IC s tend to use a lower Vref voltage or utilise an internal op-amp to give some gain VOUT D6 LED Rf b Vcc 7 Vref Vcc 7 V_ERROR GAIN STAGE Slide 8

9 Feedforward Control Previous slides show full feedback control i.e. the LED drive current is directly measured and used to form an error signal in a closed loop controller With feedforward techniques, knowledge of the system behaviour is used to infer and control the LED current through control of a system parameter which is already available VIN ILED Controlling peak switch current directly controls the peak inductor current Peak inductor current is very closely related to the DC LED current.. ISWITCH Peak switch current control will indirectly control the DC LED current..! Slide 9

10 Feedforward Control -PoE LED Light RD005 implements a 10W PoE LED driver using feedforward buck converter Efficiency ~ 87% Drives three Luxeon K2 LEDs at 750mA Slide 10

11 National LED Drivers -LM34xx Leverages existing proven power design technology and process technology to provide a range of parts specifically tailored to driving high brightness LEDs Most LED driver IC s are Webench enabled, speeding up part selection and initial design LM34xx are LED specific parts Includes major topologies (Buck, Boost, Buck-Boost) Includes controllers and regulators Slide 11

12 DC Voltage DC Voltage DC Voltage Opportunity Qualification Need to ask input voltage range, output voltage range and required drive current Assumes we drive a single series string of LED s Vinmax? Vinmin? Input Voltage Range Vledmax? Vledmin? LED Voltage Range Vinmax? Vledmax? LED Voltage Range Vledmax? LED Voltage Range Vinmax? Input Voltage Range Input Voltage Range Vledmin? Vledmin? Vinmin? Vinmin? Vinmin > Vledmax Vinmax > Vledmin Any overlap between LED voltage and input voltage range Need Buck Topology Need Boost Topology Need Buck-Boost Capable Topology (Buckboost, SEPIC, Flyback) Slide 12

13 Buck LED Drivers - Integrated MOSFET Extends up to 67V/1.5A capability (100W) Current groupings cover popular LED requirements (350mA/700mA/1000mA/ 1400mA) Scalable product families Buck is the most commonly used topology for LED driving Slide 13

14 Buck LED Drivers - External MOSFET (Controllers) Synchronous Buck Controllers Buck controllers based systems limited to around 3A LED current (Diode Loss) Buck Controllers Synchronous buck controllers extend drive capability to >20A High current systems generally have lower output voltage requirements Slide 14

15 Buck LED Drivers - Example #1 - LM3404HV Uses constant on-time hysteretic controller LED current sense voltage is 200mV Almost constant switching frequency in CCM operation PWM Dimming Capable Slide 15

16 Buck LED Drivers - Example #1 - LM3404HV (8V/1A) Reference Design RD007 25mm x 25mm 320kHz Switching Frequency 10V to 60V Input (i.e. automotive load dump suitable) 1A output with Vf up to 8V LED Current Regulation Conversion Efficiency D2 implements OV protection Slide 16

17 Buck LED Drivers - Example #1 - LM3404HV Operating Waveforms 10Vdc Input 60Vdc Input Duty Cycle =64%, Fs=319kHz Duty Cycle =13%, Fs=309kHz Slide 17

18 Buck LED Drivers - Example #1 Dimming Waveforms Dimming works through PWM (i.e. Output is enabled/disabled with duty cycle determining perceived LED brightness) Hysteretic control and minimal output capacitance allows for fast turn on and high PWM dimming frequency Example here uses 2kHz PWM Power Stage Switching Node Voltage (Input voltage of 60Vdc) Inductor Current (Dimming duty cycle of 30%) Slide 18

19 Buck LED Drivers - Example #2 LM3409 vs LM3433 Both are controllers with external MOSFETs LM3409 is buck and LM3434 is synchronous buck Example runs from 12Vdc Input Drives Luminous SST-50 HB LED Vf is typically 3.6V with current up to 5A LM3433 can drive up to 9A Conversion Efficiency with 12Vdc Input 86% at 4V/5A LM3409 (Buck) Reference Design RD018 LM3433 (Synchronous Buck) 92% at 4V/5A Slide 19

20 Buck LED Drivers - Example #3 LM3401 MR16 Replacement Reference Design RD017 14mm x 19mm Runs from low voltage AC or DC Drives 750mA at up to 10V Vf PFET allows 100% Duty Cycle Fs=300kHz typical Slide 20

21 Boost LED Drivers - Integrated MOSFET (LM3410) Input voltage 2.7 to 5.5Vdc Output LED voltage up to 24Vdc Output power up to ~7W (24V/0.3A) Vref is 190mV Suitable for driving display backlight LEDs Efficiency with Vled of 11.4V Slide 21

22 Boost LED Drivers - External MOSFET LM342x Family LM3421, LM3423, LM3424, LM3429 Can operate with input voltage up to 75Vdc These parts are LED controllers which can be used to build Buck, Boost or Buck- Boost systems LM3423 is the same as the 3421 except it includes diagnostic capability to flag system faults and status on LED output Slide 22

23 Boost LED Drivers - External MOSFET - LM3424 LM3424 builds on the LM3421/3 to add thermal foldback capability A thermal sense device is placed near the LED The LM3424 will reduce the LED drive current if the sensed temperature exceeds a certain level Foldback profile can be modified for different applications Thermal Feedback Input Prevents LED failure if the local ambient temperature rises too high or the cooling strategy of the luminaire is compromised Slide 23

24 Buck-Boost LED Drivers - LM342x Family The LM342x family (and LM3410) can be used to implement Buck-Boost behaviour Useful if the input voltage range overlaps the LED drive voltage range VIN Slide 24

25 Dimming -PWM vs Analog Analog Dimming LED current is constant DC and the magnitude is varied to change brightness PWM Dimming The LED drive current is maintained at the normal drive level but enabled/disabled at a frequency of typically 100Hz up to a few khz. The duty cycle of the PWM is used to change perceived LED brightness Analog Dimming No flicker due to beat frequency interference PWM Dimming Colour temperature of LED is generally conserved Colour temperature will change with DC drive current System efficiency may be poor for low LED drive currents Can introduce perceptible flicker if PWM dim frequency beats with other displays or lights Conversion efficiency is maintained down to low dim PWM duty cycle Slide 25

26 STANDARD MAINS AC/DC CONSTANT OUTPUT VOLTAGE LED Driving - Application Specific Parts LM3464 Uses a low cost standard mains AC/DC PSU to provide isolation and bulk voltage rail LM3464 controls the AC/DC unit output voltage to set bulk rail voltage dynamically VAC 50/60Hz BULK VOUT (CONSTANT VOLTAGE) LM3464 linearly controls an external MOSFET to give accurate current control of each LED string Volt VFB LM3464 Volt VFB Power loss in the external MOSFET is minimised through the dynamic headroom control function ISOLATION Slide 26

27 LM3464 Example - 53W Streetlight Uses commercially available front end AC/DC converter Need access to feedback/control circuit in AC/DC stage LM3464 power stage operates at very high efficiency of 98.1% Slide 27

28 LM3450 Mains Triac Dimmable Solution Operates with leading and trailing edge legacy dimmers Implements full PFC operation to give low mains current harmonics Provides galvanic isolation Designing LED ballasts with traic dimmer support is challenging and generally support intensive so the opportunity needs to be high to justify the design effort. Slide 28

29 LED Drivers - Lifetime Considerations With careful thermal design, LEDs can last in excess of 50,000 hours LED driver can limit the life of the system unless care is taken with the design Avoid using electrolytic capacitors since they dry out over time Keep converter electronics away from hot LEDs and their heatsinks E.g. a 105 C/2000 hour capacitor will only last 16,000 hours (1.8 years) if it runs in a local ambient of 75 Deg C Each drop in ambient temperature of 10 C will double expected lifetime (Arrhenius Equation) Slide 29

30 Support Material Use Webench to help with part selection based on customer requirements Basic topologies can be simulated to check design before prototyping Lots of reference designs are available at Further LED reference designs can be found at National LED Drivers Solutions Guide Slide 30

31 Thanks for Listening! Slide 31