There are four main switch mode driver topologies used for LED systems: buck, boost, buck-boost, and flyback. Switch mode drivers are more efficient than discrete component or linear drivers. The presentation discusses the operation and advantages of each topology, including examples of driver ICs in each configuration. SEPIC configuration can regulate output voltage even when input voltage is higher or lower than output. Flyback topology allows for single or multiple outputs and is the most versatile.

1. LED Fundamentals
Driving LEDs –
Switch M d D i
S it h Mode Drivers
12-22-2011

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. 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. Driving Options
LEDs
LED
Electrical 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 back
LED Fundamentals | Switch Mode Drivers | Page 4

5. Commonly Used Driving Options
Resistor drive
Switch mode
regulator driving is
explained in this
p
presentation
LED Fundamentals | Switch Mode Drivers | Page 5

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 YES
LED Fundamentals | Switch Mode Drivers | Page 6

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
ICL8001G
LED Fundamentals | Switch Mode Drivers | Page 7

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. Switch Mode Driver Topologies
There are four main switch mode driver topologies that are commonly used in LED
systems, depending on application.
Buck Boost
Buck-Boost
B kB t Fly-back
LED Fundamentals | Switch Mode Drivers | Page 9

10. Buck Mode Operation
24V
Buck mode regulation is used when the
input voltage or source voltage is higher
than the expected output voltage.
LED Fundamentals | Switch Mode Drivers | Page 10

11. Typical Application Circuit of HV9910B in Buck
Configuration
This driver outputs an LED current of 700mA and can achieve up to 94% efficiency when
driving OSRAM Opto Semiconductors’ 6-chip OSRAM OSTAR LED with an input of
24Vdc.
LED Fundamentals | Switch Mode Drivers | Page 11

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. Typical Application Circuit of Boost Configuration
As an example circuit, Supertex’s HV9911 in boost mode is shown here.
LED Fundamentals | Switch Mode Drivers | Page 13

14. Shortcoming of Boost Mode Operation
In a boost topology, the input is less than the output. However, what will happen if the input
goes 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 may
cause 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 gy
LED Fundamentals | Switch Mode Drivers | Page 14

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. Typical Circuit Schematics of SEPIC Configuration
Note use of two
Shown below is Supertex’s HV9911 in SEPIC configuration.
inductors
LED Fundamentals | Switch Mode Drivers | Page 16

17. SEPIC in battery operated applications
SEPIC DRIVER
LED Fundamentals | Switch Mode Drivers | Page 17

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. 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. Fly Back Topology
Fly 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. Fly Back Topology
A 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. 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 , p
which any of the topologies can operate.
A continuous conduction mode is when the inductor current never reaches zero between on and off
cycles 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 h
zero.
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 OFF
LED Fundamentals | Switch Mode Drivers | Page 22

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. 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 GmbH's 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 GmbH's product and a
product of one of OSRAM Opto Semiconductor GmbH's 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 GmbH's
partners, or with the use of a combination of an OSRAM Opto Semiconductor GmbH's
product and a product of one of OSRAM Opto Semiconductor GmbH's partners.
LED Fundamentals | Switch Mode Drivers | Page 24

25. Thank you for your attention.
LED Fundamentals | Switch Mode Drivers | Page 25