Driving High Brightness LEDs

1
Driving High Brightness LEDs
2
Outline

• Overview of LEDs and Solid State Lighting
• System Perspective and Driver Requirements
• Example Applications
• Powering LEDs from an offline supply
• Low voltage ac and dc applications
• Portable applications
• Conclusions

3
LED Overview

• The most common white LED is a blue emitting
  LED coated with a phosphor which then emits
  yellow light when excited.
• A white light can also be generated by modulating
  (mixing)
• the light from 3 colors - Red, Green, and Blue
  LEDs

Blue . 470 nm Green . 510 nm Yellow .570
nm Red .650 nm
Relative sensitivity of the human eye to
different colors
4
LED Overview
Energy value and radiation wavelength for various
semiconductor compounds
Material Ge Si GaAs GaP SiC GaAs1-xPx
Eg ( eV ) 0.67 1.11 1.43 2.26 2.86 1.432.26
?R ( mµ ) 1850 1100 867 550 435 867550
Remark Infra red  Far Infrared Red Yellowish green Violet Visible light
5
Solid State Lighting Evolution

• International Energy Agency estimates for 2005
  are that 1900 Mt of C02 were emitted globally for
  lighting (equivalent to 70 of light passenger
  car emissions)
• In 2006, solid state lighting market over 7
  billion USD worldwide, yearly increase by 20
  estimating total 50 100 billion USD for coming
  5 10 year.
• In US, targeting 2010, over 55 of Incandescent
  and Fluorescent lamp are replaced by LED
  lighting, expected energy saving to be 35 billion
  USD.
• In China, if moving 1/3 lighting market into LED,
  30 energy can be saved, equivalent to 100
  billion unit of energy.
• LED product developing location are in Xiamen,
  DaLian, Shanghai, NanChang cities.

6
Solid State Lighting Evolution

• New lighting technologies based on LEDs offer a
  great potential to create new and exciting
  products and reduce global energy impact of
  lighting
• Example products benefiting from this technology
  today
• Exit signs, traffic control, signage, small LCD
  backlighting and torch-lights, notebook
  backlighting
• High brightness LEDs technology has advanced both
  in light output/package (lumens) and conversion
  efficiency
• Unlike traditional lighting, LEDs are inherently
  low voltage devices and require current drive for
  optimum performance due to steep I/V slope

7
Home Lighting Efficiency and Life (av.)
Parameter Incandescent Fluorescent CFL White LED
Light Efficiency (lm/W _at_36W) 7 16 50 60 50 70 30 45
Life (av.) (hours) 1k 2k 2k 12k 6k 8k 50k 80k
8
Home Lighting Energy Saving and Comparison
Parameter Incandescent ( 25W ) CFL (5W) White LED (3W)
Price per lamp (RMB) 2.6 12 89
Average life (hours) 1500 6000 50000
No. of lamp for 50k hours (unit) 34 9 1
Expense for lamp for 50k hours (RMB) 88.4 108 89
Energy consumed (kWh) 1250 350 250
Electric Bill for 50k hours (0.48RMB/unit) 600 168 120
Total Expense for 50k hours (RMB) 688.4 276 209
9
Light Efficiency Status
10
Historical Perspective on Progress
11
Historical Perspective on Progress

• Two important developments of Efficiency of LED
  now
• To increase the brightness of some mid power LED
  of several hundred mW is operating at 70mA
  current only.
• Recently developed 1 to 10W, or greater than 10W
  LED, the operating current is ranged from 350
  700mA, some of those high power LED are required
  1A or above.

12
Lighting power management solutions

Off-Line
Regulated Current Out
Power In
Low Voltage AC and DC
Battery Powered

• Provide solutions for driving LED with any supply
  source
• High voltage ac applications
• Low voltage ac or dc applications
• Battery-powered portable applications

13
Common power sources
Power Source Use/Application Voltage Regulation
Offline AC Regulated Adapter Low to medium volume applications, reduces safety requirements Common voltages of 5, 12, 24 Vdc, regulation to /-10
(Sealed) Lead Acid Battery Automotive or Solar Powered, marine Loose regulation, 8-13 Vdc, plus for automotive, load dump considerations
12 Vdc and 12 Vac Common in interior, track lighting outdoor landscaping applications Loose if magnetic ballast, tight to /-5 if electronic ballast, minimum load may be required, cable losses
14
Solid state lighting requires a system solution

• "Edison was the 38th inventor of a filament based
  lamp. Edison was the 1st to deliver the entire
  lighting system." - Roland Haitz
• The LED Source compact effective light source
  available in a broad range of colors and output
  power
• Power Conversion Efficient conversion ac wall
  plug, battery, solar cell - to safe, low voltage
  dc
• Control Drive Electronics to regulate and
  control the LED
• Thermal Management To achieve long operating
  lifetime, control of the junction temperature is
  critical, heat-sinking needs to be analyzed
• Optics Focusing the light to were it needs to be
  requires lenses or light guides

15
Issues with driving a LED from a voltage source

• Forward voltage of LEDs have wide variation,
  typically gt1 V from lot-to-lot
• Current does not share evenly between LEDs
• Need to select in test or design for worst case
  operation which results in under-driving the LED

If the same voltage is applied across 2 LEDs, the
current will not be equal If the current is not
equal, the luminous intensity will not be
equal If the current is different, the color can
be shifted
16
LED and light output relationships

• Junction temperature, drive current and LED
  forward voltage are all related
• Higher junction temperature, lower the LED Vf
• Higher the current, higher the forward voltage
• Increasing current, increases light output, but
  higher junction temperature, reduces light output
  power

Courtesy
17
Key LED driver circuit requirements

• Constant current regulation
• Linear current source or sink
• Switching regulator topologies
• If dimming of the LED is needed, provisions for
  pulse width modulation techniques are required
• Adequate power handling
• Robust for fault conditions
• Simplicity of implementation

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• Offline applications
• Lighting ballasts
• Traffic signals
• LED light bulbs
• Street and parking lighting
• Architectural lighting
• Obstruction lights
• Signage
• Applications may or may not require galvanic
  isolation for safety

fortunately offline LED drivers have
similarities with battery chargers/adapters
19
Moving to the purely constant current realm
based on proven ac mains solutions
CV Constant Voltage
CC Constant Current
Output can be designed to have tight current
limited
Output is voltage clamped at No Load
20
NCP1014/28 LED driver Gen2 demo board
ON Semiconductor offers the NCP1014 an offline
PWM switcher with integrated high voltage MOSFET
capable of providing up to 8 W when powered from
a universal ac main. The NCP1014 LED driver
board is a fully isolated ac-dc converter
optimized for constant current applications. 350
mA / 22 Vdc transformer design as well as 700 mA
/ 17 Vdc configuration (NCP1014/NCP1027) Note
with an alternate transformer for 230 Vac ac
main, the converter is capable of delivering up
to 19 W (NCP1014) or 25 W (NCP1028 Future
Product 1Q07)

• Key Features
• Input voltage range from 90 to 265 Vac
• Continuous output power up to 8 W
• Output Open Circuit voltage clamping
• Frequency jittering for reduced EMI signature
• Built in thermal shutdown protection

• Applications
• LED ballasts
• Signage and channel lighting
• Architectural lighting
• Display lighting
• Task lighting

21
NCP1014/28 offline Gen2 LED driver
Universal AC Input
EMI Filter
LED String Connected across E1/E3
For NCP1028 only, brownout protection
Zener Clamp For open load Fault protection
Only needed for max power applications Or when
NCP1028 is used
Simple Feedback Circuit
Nominal ILED 0.65/(R3AR3B)
22
NCP1014 LED driver with 700 mA / 17 Vdc
transformer current regulation vs. of LEDs
across line
23
NCP1014 universal offline PLED/Pin efficiency
vs. LEDs _at_ 350 mA nominal
24
200 - 265 Vac version of NCP1014
25
NCP1014 20 W/360 mA LED driver efficiency
26
230 Vac/20W output current regulation
27
Offline power limited LED strobe application
Open loop design takes advantage of NCP101x
safety feature to create strobe effect
28
NCP1216 controller used for higher power
Above 8 W universal (25 W _at_ 230 Vac), we
transition from an integrated converter
(NCP1014/27) to a PWM controller and external FET
EMI Filter and rectification
Open LED Fault Protection
External FET with peak current sense

• 100 kHz Current mode PWM controller
• Direct High Voltage connection, no Aux Winding
• Internal Ramp Compensation
• Frequency jittering for better EMI signature
• 500mA Drive Capability

Current Control ILED 700mA nominal
29
Nonisolated applications

• Depending on safety considerations, a
  non-isolated design may be an option
• Sealed Applications
• Ex within bulb, special enclosure
• Physically isolated
• Ex Street light
• Controllers and converters can be used, selection
  determined by peak current
• Converters have peak currents internally set
  within the device

30
Offline non isolated up to 1 A buck LED driver
This design is for 120 Vac, slight component
changes would be Required for a 230 Vac
application Such as the power FET
Optional PWM Dimming capability
Peak current feedback when 1V is across the sense
resistor, Low power parallel resistors for cost
savings
Since this is a non-isolated AC to DC design,
high voltages are present. This is a floating
design and the IC and LEDs are not referenced to
earth ground. The LEDs MUST be connected to the
board before powering the device
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Vincent Thomas Bridge, California

• Obstruction Lighting requires high availability
  and expensive maintenance costs
• Each of the lower blue lights is a string of 7
  Luxeon III running at 700mA
• NCP1200 operating in a non-isolated line powered
  configuration

32
Alternative non isolated HV architecture
Inductor value set by Vin, LED string voltage and
current
Direct operation from HV rail
RC filter to augment leading edge blanking
NCP1200 Series or NCP1216 Series
Sets peak current
33
PFC regulation
gt 25 W
gt 75 W
34
2 stage single stage PFC topologies
High Voltage DC node
Secondary Side control is not draw for simplicity
35
PFC can establish a dc rail for multiple strings
405 Vdc
36
NCP1216 isolated 350-700 mA LED constant current
drive
PFC Front End Establishes HV DC Power Rail
37
NCP1651 Isolated single chip PFC
NCP1651 integrates the Power Factor Correction
and converter in a single chip solution with an
external FET this allows a scalable solution
depending on the power required
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• Applications for dc-dc products
• Landscape Lighting
• Interior Low Voltage Track Lighting
• Solar Powered Lighting
• Transportation
• Emergency Vehicles
• Display Backlighting
• Marine Applications
• Portable Projectors
• Low Voltage Halogen Replacement

39
Low and high power applications

• In general, if the LEDs are low current, a linear
  driver is the best solution when Vin (min) gt Vout
  (max)
• Simple to use
• No switching regulator design
• - Power not dissipated in the LED is must be
  dissipated in the driver
• As the power increases or if the LED string
  voltage is greater than the input or overlaps,
  then a switching regulator solution is required

40
Switching regulator topologies for LEDs

• Buck (Step Down) when minimum Vin is always
  greater than the maximum voltage of the LED
  string under all operating conditions
• Driving a single 1 W LED from a 12 Vac or 12 Vdc
  supply
• Boost (Step-Up) - when maximum Vin is always
  less than the minimum voltage of the LED string
  under all operating conditions
• Driving 6 LEDs in series from a 5 Vdc supply
• Buck-Boost or SEPIC Input and output voltages
  overlap
• Driving 4 LEDs in series from a 12 Vdc car battery

41
Low power linear driver for 12 Vac

• Applications
• Low Voltage Track Lighting
• Landscape Lighting
• Path Lighting
• Step Lighting

Power not dissipated in the LEDs are dissipated
in the Current Driver
Resistor sets LED current
12 Vac Magnetic Or electronic Transformer
42
NUD4001/NUD4011 PWM dimming
The average current is directly proportional to
the duty cycle If Ipeak 100 mA, then Iavg at
20 duty cycle is 20 mA
43
NUD4001 with transistor assistorto manage
thermal dissipation
Vcc Input
1
8
NUD4001
2
7
D1 MRA4003
6
3
R1 1.0
5
4
NUD4001 acts as a current source controller with
98 of power dissipated in the low cost TIP32
T1 TIP32
R2 2k
T2 2N7000
PWM
44
Nonisolated 40 Vdc regulators
PWM Buck Regulator
CS51413/14 _at_ 1.5 A 520 kHz CS51411/12 _at_ 1.5 A
260 kHz
Advanced Features
1.276 to 37 Vdc
4 to 40 Vdc in
PWM Boost Regulator
CS5171 _at_ 1.5 A 250 kHz CS5173 _at_ 1.5 A 560 kHz
4 to 30 Vdc in
Pure Boost or SEPIC
Maximum Output 40 Vdc
Buck / Boost / Buck-Boost Regulator
NCP3063 _at_ 1.5 A up to 150 kHz NCP3163 _at_ 3.4 A
up to 250 kHz
2.5 to 40 Vdc in
1.25 to 40 Vdc
Flexible Cost Effective
NCP3063 can also be used as a controller
45
Basic burst mode controller architectureNCP3063
and NCP3163

• Oscillator operates at a fixed frequency and a
  fixed duty cycle, similar to a voltage mode PWM
• A low frequency control signal, gates the
  oscillator on and off for small groups of cycles
• The output ripple will contain switching
  frequency ripple as well as lower frequency
  ripple due to the gating circuit

46
MC34063 configured as a step-down LED driver
Current is constant Vout floats based on LED Vf
1.23V Feedback
Lets look at switch current for the buck
configuration Designing for minimum inductor
size (discontinuous operation) for a buck
topology the switch current would be 2x the LED
current, so for a 350 mA LED current, the peak
switch current is 700 mA
Level Shifter Added to reduce power lost in R2
47
Wide 12 V input, 2 LED NCP3063 driver
Nominal 250 mV drop across current setting
resistor

• Approximately 77 efficiency at Vin of 14 Vdc and
  Iout of 750 mA, Vf 6.5 Vdc
• Note compared to the MC34063, output cap
• And inductor have been reduced by a factor of 4

48
NCP3163 boost LED driverLED String Voltage gtgt Vin
EXCEL Design Spreadsheet available for
component selection
49
NCP3163 boost LED configuration 12 Vdc input,
Iout 400 mA Efficiency versus Vout (4-10 LEDs)
4 LEDs
10 LEDs (14W)
10 LEDs
50
NCP3163 boost LED configuration 12 Vdc input,
Iout 400 mAIout versus Vout (4-10 LEDs)
51
Up to 60 Vdc boost for higher power
applicationsNCP3063 as a FET controller
Output Voltage 27.7 Vdc _at_ .662 A Output Power is
18.3 W  
Vin is 12.1 Vdc _at_ 1.85 A Input Power is 22.3 W

• Voltage Conversion Efficiency is 82
• Pled/Pin Efficiency 78.4

52
Buck-boost situations

• Driving strings of LEDs from standard supplies
  rails
• 12 Vac - wide supply variation with magnetic
  transformer and wiring line drops, a perfect 12
  Vac with no rectifier losses is 16.9 Vdc
• Loose 12 Vdc - wall plug transformers,
  automotive, and solar-backed up batteries
• It is quite common to have situations where the
  input voltage and LED load voltage overlap
• 3 White Luxeon K2 _at_ 350 mA can range from
  8.37-12.69 Vdc
• 3 White Luxeon K2 _at_ 700 mA can range from 9.09-15
  Vdc
• Assuming delta TJ of 100 C, these values could
  shift by -0.6 Vdc

53
Noninverting buck / boost operating theory
Switches OFF
Switches ON

• To keep switching currents low, Continuous
  Conduction mode (CCM) is desired
• Switches Q1/Q2 turn on for time DTs (Dduty
  cycle,Tsswitching period) charging inductor from
  Vin
• When Q1/Q2 turn off, diodes D1/D2 deliver
  inductor energy to Vout
• Since this is CCM operation, neglecting losses,
  Vin DTs Vout(1-D)Ts
• So Vout VinD/(1-D) and when D0.5, Vout Vin
• By controlling D, Vout can be greater or less
  than Vin

54
Using the NCP3063 as a buck/boost controller

• 2 power switches are required for this
  architecture
• Although the NCP3063 does contain 1 switch, we
  can use a low VCEsat PNP/NPN pair to get higher
  conversion efficiency
• This is a scalable approach where rectifiers and
  power switches can be sized based on the specific
  input and output voltages and current levels
• Low VCEsat transistors like the NSS40500UW3 offer
  excellent performance in a small 2x2 mm package
• 40 Vdc, 5 A Continuous operation
• Betas of 250 typical and a VCE (sat) of lt 100
  mVdc
• Complementary P and N devices available at
  different voltage ranges

55
Up to 25 W NCP3063 buck/boost implementation
Duty Cycle Feedback Control

• Design is intended for up to 1 A applications,
  Vin can range from 8-28 Vdc
• Efficiency of 72-80 with Vout of 21 Vdc _at_ 700mA
  and Vin of 13-26 Vdc
• Controller drives 2x2 mm low VCEsat PNP and NPN
  (lt80 mVdc _at_1 A)

56
Battery powered portable lighting

• LEDs are commonly used for backlighting of small
  color displays and portable torch lights
• In addition they can be embedded into other
  products to offer new capabilities
• Portable test and medical equipment
• Bike and mining lamps
• Hand tools
• Games and educational toys
• Security devices
• Emergency power loss backup lights
• Solid state traffic flares
• UV dental curing

57
1 2 cell simple LED torch Boost Topology Vout
gtgt Vin
For 1 cell applications, the NCP1400A 5 V design
can supply 50 mA to a single LED from a small
thin SOT23-5 package, resistor R1 is shorted by
J2 For 2 cell applications the driver can source
100 mA to a single LED with the addition of a
current limiting resistor in series with the LED
58
Up to 350 mA torch/flashlight exampleoptimized
for 2 cell primary (Ni-based)
Synchronous rectification boost with true cutoff
to disconnect load from source
NCP1421
2 cell Alkaline/NiCD or 1 cell Lithium Primary
59
Continuous operation for pen torchlight over 2
cell battery life
60
Conclusion

• Solid State Lighting is evolving rapidly with the
  emergence of cost effective ultra high bright
  power LEDs
• Constant current drive architecture is key to
  driving LEDs due to the steep V/I transfer curve
• A wide variety of power solutions is required
  depending on the power source and LED
  configuration
• To achieve a robust product requires a system
  oriented approach taking into account electrical,
  thermal and optical considerations

61
LED application note support
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Thank you!