LED - Electrical Design
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Transcript of 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
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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)
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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
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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
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Less sensitive to varying Vs; Lower system efficiency
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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
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-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 +
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.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
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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
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_
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
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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
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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
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2X Mode, CS1 and CS2 are both charged to VBATT, then added to VBATT when connected toCOUT