led - electrical design

7/29/2019 LED - Electrical Design

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Basic Electrical Design

Linear Forward Voltage ModelingLinear Forward Voltage Modeling

Series and Parallel Drive CircuitsSeries and Parallel Drive Circuits PWM and DimmingPWM and Dimming

LedEngin ConfidentialLedEngin, Inc.


2/30

LZ Series Typical VF Characteristics

1250

1500Typical AlInGaP

VF Curve

Typical InGaN

VF Curve

1000

urrent(m

A)

RD = VF /IF(Red 0.6

RD = VF /IF(Blue 0.5

500

750

-Fo

rwardC

0

250

IF

AlInGaP

Cutin Voltage

InGaN

Cutin Voltage

0 1 2 3 4 5

VF - Forward Voltage (V)


dVf/dTj = 24mV/C


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Derivation of Linear Model

EquationV = V + I

ProcedureMeasure VFversus IF

rap a a on a near sca eDraw straight line tangent to actual data through

points (VF1, IF1) and (VF2, IF2) F O

Note: Model worksover a limited ran e

( )( )12

1221

FF

FFFFO

II

IVIVV

=

of forward current( )( )12

12

FF

FFS

II

VVR

=



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Linear Forward Voltage modeling

TTjj = 25= 25CC

VVff = V= V00 ++ RRss * I* Iff==

300

350

400

Rs=d

Vf-typ for Vf bin a)

ss

VV00 = Threshold Voltage= Threshold VoltageRRss typtyp == dVdVff // dIdIffRR may vary from part to partmay vary from part to part 150

200

250

urrent(mA

)

/dI

=0.3V/0.25

-

TTjj > 25> 25C:C: 50

100

Forward

A=1.2

VVff = V= V00 + (+ (RRss * I* Iff)+ ()+ (dVf/dTdVf/dTjj * (* (TTjj -- 2525C))C))

Example for Red and Amber (Example for Red and Amber (AlInGaPAlInGaP):):

** **

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Forward Voltage (V)

= . .= . . .. -- --VfVf=2.58V+0.42V=2.58V+0.42V--0.19V=2.81V0.19V=2.81V



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Pros and Cons of Linear Model

ProsPros ConsCons

qua on can e so ve orqua on can e so ve or

IIFF == ff((VVFF)) Can be substituted intoCan be substituted into

m e ynam c rangem e ynam c range

(


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Paralleled LEDs

+

+

IF

VF IF1 IF2LED1 LED2 VF



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Ex Vf M hin f r LZ- ri Pr

3.4

2.8

3.0

3.2

2.2

2.4

2.6

F

Ra

tio

1.6

1.8

2.0I

1.0

1.2

1.4

0.10 0.15 0.20 0.25 0.30 0.3


Design Current (Amps)


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Improving Matching of Paralleled LEDs

us ng a anc ng es s ors

IF

R R

VF IF1

IF2LED1 LED2 VF

+



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Effect of Balancing Resistor

Expected matching for paralleled 1%-tile and 99%-tile white Luxe

with 0.75 ohm external series resistor for each LED

3.4

2.8

3.0

3.2

2.0

2.2

2.4

.

IF

ratio

1.2

1.4

1.6

1.8

1.0

LED drive current



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Series String Drive Circuits

IF

Resistor R = Current Limiting Device

R R R R

+V+

V-F

-

Series String Series String Series String Series String

2 LED 3 LED 4 LED 5 LED

F

FS

IR

=

RI FSF

=

More sensitive to var in Vs Hi her s stem efficienc


Less sensitive to varying Vs; Lower system efficiency


11/30

Typical Circuit Configurations

+R

X strings

+

X strings

VIN Y LEDlamps per

strin

Y LED

lamps per

VIN

X strings

Paralleled strings

Configuration B

Series-connected strings

Configuration A

+

Z LED lamps per rung.

Note z = 1,2Y LED

R

IN (z = 1 is illustrated)

string


-Configuration C


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Dimming Using Pulse Width Modulation

Duty Factor (%) = ton/(ton+toff)*100

If

time ms

DF=10%

If DF=50% t t

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 time(ms)

IfDF=100%



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Dimming Operation

Disadvantages of DC driveDisadvantages of DC drive

Light output matching is worse at low currentsLight output matching is worse at low currents Low current operation of paralleled strings ofLow current operation of paralleled strings of

variationsvariations

Benefits of PWM operationBenefits of PWM operation r ve amps at same pea current ut at owr ve amps at same pea current ut at ow

duty cycleduty cycle

Eliminates matchin roblems caused b drivinEliminates matchin roblems caused b drivin

at low currentsat low currents



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Electrical Design Summary

Key PointsKey Points

Forward characteristics of LEDs can be modeled.Forward characteristics of LEDs can be modeled.

LEDs can be driven with either a current source orLEDs can be driven with either a current source or

series with the LED. LEDs should NOT be drivenseries with the LED. LEDs should NOT be driven

directly from a voltage source.directly from a voltage source. LEDs can be connected in parallel, only if theLEDs can be connected in parallel, only if the

LEDs are from the same VLEDs are from the same VFF Bin and driven atBin and driven at

current).current).



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Content

DC/DC Circuit ApproachesDC/DC Circuit Approaches



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Basic DC Circuit Approaches

Resistor Voltage Regulator + Resistor Current Source

Fixed current

RR

Voltageregulator

Current

source

dcsource

(variable)

dcsource

(variable)

csource

(variable)

R

VVI FINF

= R

VVI FREGF

= constantIF



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Resistor Circuit Design Example

Example for VF = 12V, IF = 1000mA, R j-air = 50C/W, VF/T = -3mV/C

R

Vf Bin Bin Range

[V]

Resistor

[ ]

R (5% tol)

[ ]

P_resistor

[W]F 3.2 3.44 8.80 9.1 9.1

G 3.44 3.68 8.56 9.1 9.1

IN

dc input

voltage

F 3.68 3.92 8.32 8.2 8.2

G 3.92 4.16 8.08 8.2 8.2

F 4.16 4.4 7.84 7.5 7.5

nVV

FI=



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Pros/Cons of Resistor Circuit

ProsPros ConsCons

eas expens ve c rcueas expens ve c rcu

Electrical transientsElectrical transientsgenerally do not damagegenerally do not damage

equ res res s or se ec onequ res res s or se ec on

for each LED Vfor each LED VFF categorycategory Forward current throughForward current through

c rcu u may armc rcu u may arm ss s no regu a e we ors no regu a e we orchanging input voltagechanging input voltage

Power inefficientPower inefficient



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Pros/Cons Voltage Regulator/Resistor Circuit

ProsPros

Re ulates forwardRe ulates forward

ConsCons

More expensive than resistorMore expensive than resistor

current throughcurrent through LEDsLEDsdue to input voltagedue to input voltagevariationsvariations

Requires resistor selection for each LEDRequires resistor selection for each LEDVVFF categorycategory

Ma be susce tible to electricalMa be susce tible to electrical More power efficientMore power efficient

than resistor alone atthan resistor alone at

high input voltageshigh input voltages

transients/ reverse voltagestransients/ reverse voltages

Forward current will vary overForward current will vary over

tem erature due totem erature due to VV //T of LEDsT of LEDs

Requires higher minimum input voltageRequires higher minimum input voltagethan resistor due to minimum voltagethan resistor due to minimum voltagedro across re ulatordro across re ulator



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Pros/Cons of Current Source Circuit

ProsPros ConsCons

egu a es orwar currenegu a es orwar curren

through LEDs due to boththrough LEDs due to bothinput voltage variations asinput voltage variations as

oug y same cos asoug y same cos as

voltage regulator plusvoltage regulator plusresistor circuit.resistor circuit.

FF Forward current isForward current is

unaffected by LED Vunaffected by LED VFF

ay e suscep e oay e suscep e oelectrical transients/ reverseelectrical transients/ reverse

voltagesvoltages FF of LEDsof LEDs

Roughly same powerRoughly same power

ay ave g er m n mumay ave g er m n muminput voltage requirementinput voltage requirementthan voltage regulator plusthan voltage regulator plus

regulator plus resistor circuitregulator plus resistor circuitat high input voltagesat high input voltages

across sense resistoracross sense resistor



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Simple Current Source

+

n

RSENSE

VREF

+input

voltage

( )( )VV BEREF

Note: VBE varies by ~ -2mV/C, so current will change over temperature, unless temperature

( )1RSENSEF +


.Note: Minimum input voltage is approximately VREF + nVF.


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DC/DC Converter Circuits

Most useful topologies for LED drivers are Boost, Buck, BuckMost useful topologies for LED drivers are Boost, Buck, Buck--,, -- ..

Some topologies can generate a regulated output voltageSome topologies can generate a regulated output voltagegreater than input voltage.greater than input voltage.

os an comp ex y muc g er an o er es gns.os an comp ex y muc g er an o er es gns.

Generally offer power efficiencies from 60 to 90%.Generally offer power efficiencies from 60 to 90%.

May generate electrical transients that could damageMay generate electrical transients that could damage LEDsLEDs May generate electrical noise that could interfere with otherMay generate electrical noise that could interfere with other

electronics.electronics.



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Boost Topology DC/DC Converter

+

Switch closed

VIN LOADI

_

+

VIN LOAD


INote: VIN < VLOAD

_


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Boost DC/DC Converter

VOUT

VBATT

Control

OUT BATT



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Buck Topology DC/DC Converter

+

ISwitch closed

VIN LOAD

_

I

+

Switch open

VIN LOADI


_

Note: VIN > VLOAD


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Buck DC/DC Converter

VOUT

BATT

Control

Note: VOUT < VBATT



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Cascaded DC/DC Converters

Cascaded Buck Boost DC/DC Converter

Buck BoostVBATT VOUT


van age: OUT < > BATT


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Charge-Pump DC/DC converters

V V

+ Cs COUTCIN

Note: This circuit called a 1X, 2X converter because output



30/30

1X, 1.5X, 2X Charge-Pump DC/DC Converter

BATT OUT

CS1COUT

CS2

Note: 1X Mode, CS1 and CS2 are both charged to VBATT, then connected to COUT

1.5X Mode, CS1 and CS2 are connected in series, so they each charge to VBATT,then added to VBATT when connected to COUT


2X Mode, CS1 and CS2 are both charged to VBATT, then added to VBATT when connected toCOUT

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