Driving LEDs - Switch Mode Drivers: LED Fundamentals

LED Fundamentals

Driving LEDs –
Switch M d D i
S it h Mode Drivers

12-22-2011

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

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.


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


Commonly Used Driving Options

Resistor drive

Switch mode
regulator driving is
explained in this
p
presentation


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


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


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.

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

Buck Mode Operation

24V

Buck mode regulation is used when the
input voltage or source voltage is higher
than the expected output voltage.


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.


Boost Mode Operation

The boost configuration is used
when output voltage is expected
to be higher than the input
voltage.


Typical Application Circuit of Boost Configuration

As an example circuit, Supertex’s HV9911 in boost mode is shown here.


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


SEPIC Configuration

In SEPIC (Single Ended Primary Inductance Converter) configuration, the output voltage
can be either higher or lower than the input voltage.


Typical Circuit Schematics of SEPIC Configuration
Note use of two
Shown below is Supertex’s HV9911 in SEPIC configuration.
inductors


SEPIC in battery operated applications

SEPIC DRIVER


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.

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.


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.


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.”


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


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.


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.


Thank you for your attention.


Driving LEDs - Switch Mode Drivers: LED Fundamentals

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Driving LEDs - Switch Mode Drivers: LED Fundamentals