Driving LEDs - Switch Mode Drivers: LED Fundamentals

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In this presentation on Driving LEDs – Switch Mode Drivers we will look at how switch mode drivers work, switch mode driver topologies, and benefits of switch mode drivers in LED circuits and systems.

Driving LEDs - Switch Mode Drivers: LED Fundamentals

  1. 1. LED FundamentalsDriving LEDs –Switch M d D iS it h Mode Drivers12-22-2011
  2. 2. Introduction Normalized Spectrum Behavior at 24, 35, 50, 70 Degree Celsius 1,00 TY INTENSIT Proper driving of LEDs is required to 0,80 address some of the fundamental 0,60 Spectrum Behavior at 24, 35, 50, 70 Degree Celsius variations that all LEDs may have due INTENSITY 0,40 6,00E-05 to manufacturing tolerances. 0,20 5,00E-05 WAVELENGTH [nm] 4,00E-05 0,00 580 590 600 610 620 630 640 650 660 670 680 3,00E-05 There are different methods that are 2,00E-05 1,00E-05 used to drive LEDs. These methods 0,00E+00 , WAVELENGTH [nm] can be very simple or quite 380 430 480 530 580 630 680 730 780 complicated, depending on the application. Some of the key parameters needed to choose a proper driver include, expected Tj (Junction Temperature), expected Vf mismatch between LEDs, t d i t hb t LED color accuracy needed at the system level, and if dimming of the LEDs is required.LED Fundamentals | Switch Mode Drivers | Page 2
  3. 3. Need for Current Regulation in LED Systems The I-V characteristics of an LED plays a key role in deciding what type of regulation, current or voltage, is best suited for driving LEDs. Also current regulation is required in the Also, LED system to control and maintain: » Color shift vs LED current » Flux or light output vs LED current• There are three common methods of driving LEDs: resistor based, linear regulators, and , g , switch mode regulators.LED Fundamentals | Switch Mode Drivers | Page 3
  4. 4. Driving Options LEDs LEDElectrical characteristics Optical characteristics Thermal characteristics Covered in this Driving options presentation Switching mode drivers - Buck mode Discrete component Linear drivers - Boost/ SEPIC mode - Resistor based - Fixed voltage - Buck-boost - Transistor based - constant current - Fly backLED Fundamentals | Switch Mode Drivers | Page 4
  5. 5. Commonly Used Driving Options Resistor driveSwitch moderegulator driving isexplained in this ppresentationLED Fundamentals | Switch Mode Drivers | Page 5
  6. 6. Advantages and Disadvantages of Each Driving Method Before choosing a switch mode driver, refer to this table that outlines some of the advantages and disadvantages of each driving option. Switch Discrete based drive Linear Mode Resistor/ Transistor Regulator Regulator Vf mismatch between LEDs addressed NO/ YES YES YES Vf change due to temperature addressed NO/ YES YES YES Source voltage variation addressed NO/ YES YES YES Tight current regulation NO/ NO YES YES Simple solution YES/ YES YES NO YES (compared Costly solution NO/ NO to resistor YES driver) Efficient solution NO/ NO NO YES Stable over wide-range of temperature g p NO/ YES YES YESLED Fundamentals | Switch Mode Drivers | Page 6
  7. 7. Switch Mode Regulators/Drivers Up to U t 90% efficiency can ffi i Switch mode regulators offer the advantage of be achieved using switching increased efficiencies of power conversion, drivers. especially in high power applications applications. In comparison to linear regulators, switch mode regulators may increase complexity cost and complexity, cost, real estate needed for the driver circuit. In this presentation, various topics of switch presentation mode regulators will be examined at a very high level. This presentation is an overview of various key topics for switch mode regulators. A much more detailed look and analysis is required in an actual design. Efficiency characteristics of ICL8001GLED Fundamentals | Switch Mode Drivers | Page 7
  8. 8. How Do Switch Mode Regulators Work? PWM generated by Driver IC Driver IC that turns on/ off the switch, Q1 In linear regulators, the difference between the input and output voltages is burned as wasted heat. The wider the difference between the input and output voltage, the more heat is produced. With linear regulators, there are cases in which more energy is wasted as heat than delivered t th l d th d li d to the load.LED Fundamentals | Switch Mode Drivers | Page 8
  9. 9. Switch Mode Driver TopologiesThere are four main switch mode driver topologies that are commonly used in LEDsystems, depending on application. Buck Boost Buck-Boost B kB t Fly-backLED Fundamentals | Switch Mode Drivers | Page 9
  10. 10. Buck Mode Operation 24VBuck mode regulation is used when theinput voltage or source voltage is higherthan the expected output voltage.LED Fundamentals | Switch Mode Drivers | Page 10
  11. 11. Typical Application Circuit of HV9910B in Buck ConfigurationThis driver outputs an LED current of 700mA and can achieve up to 94% efficiency whendriving OSRAM Opto Semiconductors’ 6-chip OSRAM OSTAR LED with an input of24Vdc.LED Fundamentals | Switch Mode Drivers | Page 11
  12. 12. Boost Mode Operation The boost configuration is used when output voltage is expected to be higher than the input voltage.LED Fundamentals | Switch Mode Drivers | Page 12
  13. 13. Typical Application Circuit of Boost ConfigurationAs an example circuit, Supertex’s HV9911 in boost mode is shown here.LED Fundamentals | Switch Mode Drivers | Page 13
  14. 14. Shortcoming of Boost Mode OperationIn a boost topology, the input is less than the output. However, what will happen if the inputgoes above the output?Due to circuit arrangement in a boost topology, there will be current flow (shown in red)when input goes higher than the output and the output will not be regulated. This maycause damage to the load and should be prevented. In a situation similar to the above, the SEPIC configuration helps and maintains a regulated positive output when the input is higher or lower than the output. Boost Topology p gyLED Fundamentals | Switch Mode Drivers | Page 14
  15. 15. SEPIC Configuration In SEPIC (Single Ended Primary Inductance Converter) configuration, the output voltage can be either higher or lower than the input voltage.LED Fundamentals | Switch Mode Drivers | Page 15
  16. 16. Typical Circuit Schematics of SEPIC Configuration Note use of two Shown below is Supertex’s HV9911 in SEPIC configuration. inductorsLED Fundamentals | Switch Mode Drivers | Page 16
  17. 17. SEPIC in battery operated applications SEPIC DRIVERLED Fundamentals | Switch Mode Drivers | Page 17
  18. 18. Buck-Boost Mode of Operation + In the buck-boost configuration, the output voltage can be hi h or l lt b higher lower than the input. However, the output is inverted, due to it’s configuration and the way it g y operates.LED Fundamentals | Switch Mode Drivers | Page 18
  19. 19. Typical Circuit Schematic of Buck-Boost Configuration National Semiconductors’ LM22670-ADJ in buck-boost configuration is shown below. Note here the polarity of the output is inverted.LED Fundamentals | Switch Mode Drivers | Page 19
  20. 20. Fly Back TopologyFly back topology is the most versatile because it can have single or multiple outputs.A single output configuration is shown below.LED Fundamentals | Switch Mode Drivers | Page 20
  21. 21. Fly Back TopologyA multiple output fly back topology is shown below For more information on multiple below.output configuration, please consult LED Fundamental “AC-DC Power Supplies.”LED Fundamentals | Switch Mode Drivers | Page 21
  22. 22. Continuous and Discontinuous Conduction Modes (CCM/ DCM)Irrespective of all different topologies explained, there are two conduction modes of operations at p p g p , pwhich any of the topologies can operate.A continuous conduction mode is when the inductor current never reaches zero between on and offcycles of the switch, where as i a di l f th it h h in discontinuous mode of operation th i d t current can reach ti d f ti the inductor t hzero.There are advantages and disadvantages of each mode For more information on each mode please mode. mode,consult the LED Fundamental “AC-DC Power Supplies.” Inductor current Switch ON reaches zero h Switch OFFLED Fundamentals | Switch Mode Drivers | Page 22
  23. 23. Summary There are four different switch mode driver topologies that are widely used: Buck, Boost, Buck-Boost, and Fly back All of the topologies can be operated in Continuous or Discontinuous mode (CCM/ DCM). SEPIC can be considered a variation of Boost configuration. Fl b k t Fly back topology gives th d i l i the designer f d freedom i th d i and i considered th most in the design d is id d the t versatile. S it h mode d i Switch d drivers are much more efficient th di h ffi i t than discrete component b t t based d i d drivers or linear drivers.LED Fundamentals | Switch Mode Drivers | Page 23
  24. 24. Disclaimer All information contained in this document has been checked with the greatest care. OSRAM Opto Semiconductors GmbH can however, not be made liable for any damage that occurs in connection with the use of these contents. OSRAM Opto Semiconductor GmbH makes no representations and warranties as to a possible interference with third parties intellectual property rights in view of products originating f i i ti from one of OSRAM Opto Semiconductor GmbHs partners, or in view of f O t S i d t G bH t i i f products being a combination of an OSRAM Opto Semiconductor GmbHs product and a product of one of OSRAM Opto Semiconductor GmbHs partners. Furthermore, OSRAM Opto Semiconductors GmbH cannot be made liable for any damage that occurs in p y g connection with the use of a product of one of OSRAM Opto Semiconductor GmbHs partners, or with the use of a combination of an OSRAM Opto Semiconductor GmbHs product and a product of one of OSRAM Opto Semiconductor GmbHs partners.LED Fundamentals | Switch Mode Drivers | Page 24
  25. 25. Thank you for your attention.LED Fundamentals | Switch Mode Drivers | Page 25

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