WEBENCH LED Tools

Jeff Perry

WEBENCH Manager

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LED selection parameters

How to Use WEBENCH® LED Architect for LED and LED Driver Selection

Hands on examples

Objectives

WEBENCH Update

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LED Drivers in WEBENCH

• LM3402/4(HV)

• LM3405(A)

• LM3406 (HV)

• LM3407

• LM3414(HV)

Buck Integrated Buck Controller Boost/Buck-Boost AC

• LM3401

• LM3409(HV)

• LM3433

• LM3434

• LM3410X/Y

• LM3421/23

• LM3424

• LM3429

• LM3431 (multiple)

• LM3444

• LM3445

• LM3464

• LM3464A

LED Selection Parameters

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Explosion of Applications for LEDs

Automotive:

• Headlights• RCL• CHMSL• Interior Lighting• Instrument Panel• Infotainment Backlighting• Aviation, Marine, and

Rail• Crash Avoidance• Instrument Panel• Interior Lighting

General Illumination:

• Architectural• Residential• Industrial• Portable Consumer• Outdoor Area• Projectors & Copiers• Entertainment Lighting• Retail Display• Medical• Emergency/Safety Lighting• Signs and Channel Lettering

Mobile Devices:

• Display backlighting• Camera flash

Backlighting & Projection:

• Infotainment• Large format TV displays• Laptops• Pocket & Data Projectors

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Quantifying Light From LEDs

Luminosity Function Luminous Flux(Lumens)

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LED Dominant Wavelength

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5

10

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20

25

30

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450 500 550 600 650Dominant Wavelength (nm)

Fre

qu

en

cy

LED Color – Dominant Wavelength

Sampling of color LEDs

OrangeYellow/Amber

GreenCyanBlue Red

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White LEDs – Color Temperature

Color Temperature Of White LEDs

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20

25

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3000 4000 5000 6000 7000Color Temperature (K)

Co

un

t

Warm White White Cool White

Sampling of white LEDsDaylight

RedTint

BlueTint

Incandescent Bulb

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Luminous Flux – Comparison Chart

Application Brightness (lumens)

40W tungsten bulb 500

100W tungsten bulb 1,500

25W compact fluorescent 1,500

55W halogen auto headlight 1,500

35W high intensity discharge auto headlight 3,250

150W halogen projector bulb 5,000

150W high pressure sodium bulb 16,000

Luminous Efficacy

• Measure of the efficiency of the lighting source (lumens/watt)

• Can be for the LED only or LED + Driver (system luminous efficacy

• Increasing efficacy = lower cost

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Luminous Flux for LEDs

Sampling of .35A cool white LEDs:

100 Lumens

Flux

Power0

140

1.2

Most efficient

WEBENCH® LED Architect

Overview of the NEW Webench tool

A Groundbreaking New Tool

• First of it’s kind on the market

• System level approach

• Saves time in LED lighting system design

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WEBENCH® LED Architect Overview

1. Select LED & Driver

2. Analyze & Optimize

3. Simulate

4. Build It

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How to Access WEBENCH® LED Architect

• Use the entry panel on www.national.com

Behavior of LEDs Is Dynamic

• Light output increases vs current

• Light output decreases vs temperature

• Efficacy decreases vs current

• Vf increases vs current

• Need to model these behaviors to give true light output

• Tradeoffs:– High current = more light = fewer LEDs

– High current = higher temperature = less light/shorter lifetime = bigger heat sink

– High current = lower efficacy = no Energy Star approval

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Can You Drive a .35A LED At .5A?And Why?

• LED datasheets typically rate LEDs at a nominal current– Luminous Flux

– Efficacy

• 1W LED is usually .35A nominal current– Lower current = higher efficacy

• The LED can be driven at a higher current which increases the light output per LED– Fewer LEDs may be required

• But:– Temperature goes up

– Efficacy goes down

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Luminous Flux Increases With Current

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.35A Nominal LED can be driven at .5A to get 25% more luminous flux.

This reduces the number of LEDs required

.35A .5A

100%

125%

Luminous Flux Decreases With Temperature

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Luminous flux reduces to 70% of nominal at 125C.This means big heat sinks are needed

50C 125C

92%

70%

Heat Sinks Are Required

• LEDs generate a lot of heat

• Total luminous efficiency of LEDs is only 4% to 22%– Total visible light/input power

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15% of power converted to light

85% converted to heat

LEDThermal vias

Heat sink

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60

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90

0 100 200 300 400 500 600 700 800

Eff

icac

y (l

m/W

)

Drive Current (mA)

LED Efficacy

small hs

medium hs

large hs

custom hs

Efficacy Decreases With Current

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Theoretical maximum efficacy for neutral white is 336 lumens/watt

Decreased efficacy = no Energy Star certification

Initial Input Panel

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Enter parameters here

Enter LED Requirements

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Enter:1) Input voltage2) Ambient temperature3) Desired light output4) LED color

Advanced inputs

Advanced inputs

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Max VoutParallel strings on 1 driverMax heat sink dimensionsManufacturerMax junction temperature

Step 1: Choose The Ideal LED Solution

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LEDs and heat sink required to give the desired light output

Detailed LED Performance

• Click on the details button to get LED performance

• Why does the flux go down with increasing current?

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Visualize the LED choicesWhat is best for the goals?

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Foo

tprin

t of

HS

(cm

2)

Efficacy (lumens/watt)

Bubble size = cost100

70

64 78

Optimize the LED Solution

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Optimization knob1 = Smallest footprint2 = Lowest cost3 = Balanced4 = Higher efficacy5 = Highest efficacy

Example Range of LED Options for 1300 Lumens

1 25cm2

5.2C/W77L/W $44.45

Heat Sink Size Efficacy CostOptimization

12 LEDs

2 58cm2

3.1C/W63L/W $30.558 LEDs

5 1144cm2

.69C/W97L/W $74.1613LEDs

Temp

115C

114C

48C

Osram Oslon LUW CP7PKTLP5C8E

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Hands On Exercise

What is the LED and heat sink combination with the:

Smallest footprintHighest luminous efficacyLowest costNote the following:1)LED manufacturer2)LED part number3)# LEDs4)Heat sink thetaSA5)LED current

Source: 24 – 32VLight output: 2000 lumensNeutral white LEDMaximum string voltage: 60VNo parallel LEDs on a single driver allowed

Design Problem: Goals:

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LED Arrays – Parallel vs Serial

• In order to get the desired amount of light, LEDs must be combined.– Parallel:

• Keeps total Vf low – good for buck driver topology

• But Vf of each LED may not be the same, so some LEDs may get higher current/brightness/temperature

– Series:• No problem with differences in current

and thus brightness/

• But, Vf adds up. If exceeds VinMin, then need to use Boost topology driver

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Driving The LED – Switching Regulator Topology

• Buck (Step Down):– Simple

– Lowest current requirements

– Requires high input voltage (VinMin > Vled)

• Boost (Step Up):– Well known topology

– Requires high current (Vin*In = Vout*Iout/Efficiency)• Ex: Vin: 5V, Vout: 14V, Iout: .35A, Eff: 90%,

– Requires Iin of 1.1A

• Buck/Boost– More complicated/expensive but needed if VinMin < Vout <

VinMax (Battery)

Step 2: View LED + Driver Solutions

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Complete solutions including: LED array Heat sink Driver(s)

Example Range of Driver Topology Options for 1300 Lumens, Vin = 14-22V

Boost 88cm2 69L/W $37.14

Driver+ArrayTotalSize

TotalEfficacy

TotalCostTopology

Buck 91cm2 67L/W $41.62

Buck/Boost 94cm2 60L/W $43.79

#LEDs

1 x 9

3 x 3

2 x 5

Osram Oslon LUW CP7PKTLP5C8E

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LED System tradeoffs

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Foo

tprin

t of

HS

+dr

iver

(cm

2)

System efficacy (lumens/watt)

Buck

Buck-Boost

106

86

59 69

Boost

9 LEDs + HS

Driver

LEDs Dominate the Design

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6cm2

82cm2 $34.24

$2.90

Footprint Size Cost

1300 Lumens, Optimization 3, Boost Driver

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Create and View Design

• Design Dashboard:

– LED System summary

– LED array

– LED / heat sink display

– Charts

– Optimization Graphs

– Bill of Materials Graphs

– Simulation

– Custom Design Report

– Prototyping

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Hands On Exercise

What is the system (including the LEDs, heat sink and driver) with the:

Smallest footprintHighest luminous efficacyLowest cost(Note the LED array and driver topology used)

Source: 24 – 32VLight output: 2000 lumensNeutral white LEDMaximum string voltage: 60VNo parallel LEDs on a single driver allowed

Design Problem: Goals:

Creating A Custom LED Array

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Click on custom LED button

Custom LED Array Configuration

• Manually change the array, heat sink, LED current

• This will change the calculated light output

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Custom LED Array• Increasing current will increase light output, but require heat sink

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© 2011 National Semiconductor Corporation. Confidential.

1) .6A - 1000 lu

2) Lower ThetaSA – 1099 lu3) 1A – 1435 luF

ootp

rint

Efficacy

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Hands On Exercise

Customer wants more light:Source: 24 – 32VLight output: 2000 lumensNeutral white LEDMaximum string voltage: 60VNo parallel LEDs on a single driver allowedUse the custom LED array to increase the light output to 2500 lumens

Design Problem: Goals:

What is the LED and heat sink combination?

Note the following:1) Footprint2) Luminous efficacy3) Cost4) LED manufacturer5) LED part number6) # LEDs7) Heat sink thetaSA8) LED current

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Optimization – Efficiency vs Footprint

Left side:Higher frequencySmaller footprint

Right side:Lower frequencyLower resistance

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Optimization – Power Dissipation

Lower frequency

As freq is decreased:

FET Pdiss improves

L Pdiss may get worse Higher L is required to maintain VoutPP

L = V*dt/di

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Optimization Summary

• To get high efficiency– Decrease frequency to reduce AC losses

– Choose components with low resistance

• To get small footprint– Increase frequency to reduce inductor size

– Choose components with small footprint

• Cost

• These parameters are at odds with each other and need to be balanced for a designer’s needs

• Tools are available to visualize tradeoffs and make it easier to get to the best solution for your design requirements

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Why Do Electrical Simulation?

• Design has already been configured for stable operation, but:

• May want to verify operation under dynamic conditions

• May want to further optimize the design for your requirements:– Improve transient response

– Minimize output ripple

– Improve loop stability

Simulation Controls

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Select sim type and start sim

After sim is completeSelect waveforms here

Waveform viewer

Simulation Waveform Viewer

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Advanced controls

Right click to delete a waveform

Click and drag to zoom

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Model Verification: Sim vs Bench

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0.85

0.9

0.95

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1.1

0 1 2 3 4 5 6 7 8 9 10

Time [µs]

Iload

[A

]

Iload

sim_Iload

-10

-5

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0 1 2 3 4 5 6 7 8 9 10

Time [µs]

Vsw

[V

]

Vsw

sim_Vsw

Inductor Current

Switch Voltage

LED Current

• Spice model verification involves taking bench data at various operating points and comparing to simulation

Example: Effect of Output Cap

• Vin: 24-32V

• Light output: 650 lumens

• LED: 5 x Cree MX6AWT-A1-0000-000D51

• ILED: 0.497A (target)

• LM3402

• What are the advantages/disadvantages of having:

• 1) Standard output cap?

• 2) No output cap?

• 3) Smaller value output cap?

• Use the WEBENCH Advanced Options to check this

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LM3402 with Cout Low Ripple Target

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Cout

LM3402 with No Cout

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No Cout

Larger L1

LM3402 with Small Cout - 30% Ripple Target

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Smaller Cout

Compare Output Cap Options

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With output cap:18mA ripple$1.61, 381mm2, 92%

Compare Output Cap Options

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With output cap:18mA ripple$1.61, 381mm2, 92%

No output cap:53mA ripple$1.64, 375mm2, 91%

Compare Output Cap Options

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With output cap:18mA ripple$1.61, 381mm2, 92%

Small output cap:65mA ripple$1.62, 362mm2, 92%

No output cap:53mA ripple$1.64, 375mm2, 91%

Example: Effect of PWM Dimming Frequency

• Vin: 24-32V

• Light output: 650 lumens

• LED: 5 x Cree MX6AWT-A1-0000-000D51

• ILED: .497A (target)

• LM3402

• Compare default 2kHz dimming frequency to 4kHz

• How will this affect the circuit behavior?

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PWM Dimming Simulation

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PWM Dimming

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Dimming oscillator voltage

LED Current

2 kHz dimming frequency

PWM Dimming Simulation

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Click on Dimming Oscillator

Change PWM Dimming Frequency

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Change pulse width

Change pulse period

PWM Dimming Simulation

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2 kHz dimming

frequency

4 kHz dimming

frequency

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Hands On Exercise

1) Using the default input transient range of 24V – 32V, what is the LED current overshoot and undershoot?

2) Change the input transient to 26V to 30V. What is the LED curent overshoot and undershoot?

Create a design using the following: Source Voltage: 24 – 32VLight output: 650 lumensCool WhiteOptimization 3LED: 5 x Cree MX6AWT-A1-0000-000D51LM3402Run a line transient simulation

Design Problem: Goals:

Summary

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WEBENCH LED Architect: Considers LED and heat sink properties Computes LED ArrayProvides driver configuration/topology based on size, cost, efficiency

LED parameters are dynamic: Environment must be taken into account

WEBENCH Design Tools save you time

Thank You!

Try WEBENCH® LED Architect yourself :http://www.national.com/led_architect

Also FPGA Power Architect: http://www.national.com/fpga_power_architect