The Impact of Low Frequency Ripple Current on LEDs and LED drivers OSRAM LED Light For You San Diego, California October 28 th , 2010
The Impact of Low Frequency Ripple
Current on LEDs and LED drivers
OSRAM LED Light For You
San Diego, California
October 28th, 2010
The Impact of Low Frequency Ripple Current
on LEDs and LED drivers
Abstract
• The effect of LED ripple current on chromaticity, color shift, spectral radiance and
efficacy is analyzed and related to dimmable AC-DC LED driver solutions. Traditional
wall dimmers are based on TRIAC (triodes for alternating currents). The TRIAC has
been used extensively in residential lighting applications with incandescent lamps.
Since LED lighting consumes much less power for the same amount of light compared
with incandescent sources, the current through the TRIAC wall dimmer is much less.
This training will discuss the basic circuits and operation of a TRIAC. The pros and cons
of several LED lighting TRIAC interface solutions will be presented. The solutions will
include solutions based on TI's TPS92001, TPS92010 and TPS92210 LED lighting
driver controllers.
October 28, 2010 2OSRAM LLFY 2010
Agenda
• LED characteristics with DC current
• LED characteristics with DC and AC current
– 120 Hz sine wave ripple (60 Hz rectified)
• CFL vs. LED
• Basic TRIAC dimmers
• TRIAC Dimmer Practical Considerations
• TPS92001 Reference Design
• TPS92010 EVM
• TPS92210 Reference Design
• Summary
October 28, 2010 3OSRAM LLFY 2010
Lighting Equipment• Small Luminaires & LEDs
– Integrating Sphere & Spectroradiometer
• Color, CCT, Lumens, Efficacy
• Spectral Radiance
• NIST traceability
• Large Luminaires– Color, lux, spatial distribution
October 28, 2010 4OSRAM LLFY 2010
LED Specifications• LED datasheets specify:
– VF vs. IF
– Color Coordinates
– CCT
– Lumens vs. constant IF
– Relative luminous flux vs. TJ @
constant current
October 28, 2010 5OSRAM LLFY 2010
• Linear – constant current
• Switching – constant current + some % ripple
• How do the LEDs perform with modulated current?
• What characteristics change?
• What amount of change is acceptable?
vs vs
LED Drivers
October 28, 2010 6OSRAM LLFY 2010
• Set IF = 700mA
• Allow 30 minutes for thermal settling
• Measure TA and TSP
• TDC = TSP – TA
• TDC is crucial because it is proportional to PDIS of LED
• Measure:
– PDIS, Spectral Radiance
CCT, color coordinates
• Calculate:
– Lumens, efficacy
LED Power Dissipation vs. Time
14.5
14.6
14.7
14.8
14.9
15.0
15.1
15.2
15.3
15.4
15.5
0 5 10 15 20 25 30
Time - Mintues
Po
wer
- W
att
s
Baseline: Constant Current
October 28, 2010 7OSRAM LLFY 2010
• TA = 27.5ºC, TSP = 105ºC
• TDC = 77.5ºC
• CCT = 3780 K (neutral white)
• x = 0.3935, y = 0.3919
• Lumens = 604.6 lm (6 LEDs ~ 100 lm each)
• PDIS = 14.68W
• Efficacy = 41.2 lm/W
Baseline: Constant Current Results
October 28, 2010 8OSRAM LLFY 2010
Modulated Current Test: Sine Wave
• Rectified Sine Wave
• Maintain TMOD = TDC
• How?
• As % Modulation ↑ IAVG ↓
• Why?
50% duty cycle
120 Hz ripple
time
I F
time
I F
time
I F
October 28, 2010 9OSRAM LLFY 2010
Modulated Current Test Circuit
October 28, 2010 10OSRAM LLFY 2010
Modulated Current Test Waveform
October 28, 2010 11OSRAM LLFY 2010
Sine Wave Results
Absolute Temperature
Normalized Change
Delta Temperature vs. % Modulation
75.0
75.5
76.0
76.5
77.0
77.5
78.0
78.5
79.0
79.5
80.0
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160
% Modulation (Ipk-pk to Iavg)
Te
mp
era
ture
- C
% Delta Temperature Change vs. % Modulation
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160
% Modulation (Ipk-pk to Iavg)
Pe
rce
nt
Ch
an
ge
- %
October 28, 2010 12OSRAM LLFY 2010
Sine Wave Results
CCT vs. % Modulation
3700
3750
3800
3850
3900
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160
% Modulation (Ipk-pk to Iavg)
CC
T -
Ke
lvin
s
October 28, 2010 13OSRAM LLFY 2010
Results
CIE 1931 (x,y)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
x
y
October 28, 2010 14OSRAM LLFY 2010
Sine Wave Results
CIE 1931 (x,y) diagram Zoomed in
0.380
0.385
0.390
0.395
0.390 0.391 0.392 0.393 0.394 0.395
x
y
Black Body
0
9.2
20.6
31.6
39.9
49.2
57.1
71.3
82.6
92.4
100.9
108.9
120.8
129.5
141.2
150.6
October 28, 2010 15OSRAM LLFY 2010
Sine Wave Results
Spectral Radiance
0.000
0.002
0.004
0.006
0.008
0.010
0.012
380 430 480 530 580 630 680 730 780
Wavelength - nm
Sp
ec
tra
l R
ad
ian
ce
- W
/(m
2*s
r*n
m)
October 28, 2010 16OSRAM LLFY 2010
IF Reduction vs. % Modulation
3.3% of 700mA = 23mA. A 3.3% change in forward current = 4.4% change in lumen output (26.4 lm).
So, at 157% modulation, I need to add 26.4 lumens to correct for the IAVG reduction.
Average Current Decrease vs. % Modulation Increase
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
4.0
5.0
0 20 40 60 80 100 120 140 160
% Modulation - pk-pk to Average
% A
vera
ge C
urr
en
t
October 28, 2010 17OSRAM LLFY 2010
Lumens vs. % Mod. (corrected)
Uncorrected = Effects of Ripple & Lowered IAVG Corrected = Effects of Ripple Only
Corrected Lumens vs. % Modulation
550
560
570
580
590
600
610
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160
% Modulation (Ipk-pk to Iavg)
Lu
me
ns
- l
m
October 28, 2010 18OSRAM LLFY 2010
Efficacy vs. % Mod. (corrected)
Uncorrected = Effects of Ripple & Lowered IAVG Corrected = Effects of Ripple Only
Corrected Efficacy vs. % Modulation
38.0
38.5
39.0
39.5
40.0
40.5
41.0
41.5
42.0
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160
% Modulation (Ipk-pk to Iavg)
Eff
ica
cy
- l
um
en
s/W
att
October 28, 2010 19OSRAM LLFY 2010
% Efficacy vs. % Mod. (corrected)
Uncorrected = Effects of Ripple & Lowered IAVG Corrected = Effects of Ripple Only
Corrected % Change in Efficacy vs. % Modulation
-8
-6
-4
-2
0
2
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160
% Modulation (Ipk-pk to Iavg)
Pe
rce
nt
Ch
an
ge
- %
October 28, 2010 20OSRAM LLFY 2010
CFL vs. LED: Photometric Ripple
• CFL Basics
– CFL lamps operate from line voltage
– Their drivers contain some amount of 120Hz electrical ripple
– This electrical ripple translates into photometric ripple
• Your eyes do not detect it, due to the amplitude & frequency
• This amount of photometric ripple has been “good enough”
• So, the 60W replacement CFL became our “reference” lamp
October 28, 2010 21OSRAM LLFY 2010
CFL vs. LED: Photometric Ripple
• How do you measure light ripple?
– Cannot use a spectroradiometer – long measurement intervals
– Wide-spectrum phototransistor – fast response time (6us)
• Test Method:
– Power-up CFL and let it settle thermally/electrically
– Measure its light ripple (relative value) Φ CFL
– Drive the LED at rated DC current (350mA)
– Superimpose a 120 Hz sine wave
– Adjust the modulation amplitude until Φ LED = Φ CFL
– Let the LED settle thermally/electrically
– Measure the LED current ripple (Ipk-pk)
October 28, 2010 22OSRAM LLFY 2010
CFL vs. LED: Photometric Ripple
• Φ CFL = 247mV
• LED = 347mA
plus sinusoidal
Φ LED occurred @
248mApk-pk
• % Mod. Ipk-pk / Iavg =
71%
VCFLVLED
ILED
October 28, 2010 23OSRAM LLFY 2010
Summary
• Current ripple does not have a major effect on:
– Color coordinates, or CCT
• Current ripple does effect:
– Lumens, Efficacy – but very minor effect
– If PDIS is not regulated (as it was in this experiment),
increasing the % ripple will increase the LED TJ
• Current CFL technology has high levels of optical
ripple compared to typical LED levels.
October 28, 2010 24OSRAM LLFY 2010
The TRIAC
• TRIAC: five layer semiconductor with gate
• Functions as two SCR’s connected in inverse parallel
• Bipolar device driven to conduction by gate current or applied voltage
• Conducts provided the holding current requirements are met
• Can be triggered by fast dv/dt
P
N
P
N
P
P
N
N
N
Triac Equivalent
CircuitTriac Symbol
P
N
P
N
=
Inverse parallel
SCR’s
MT2
MT1Gate
N
Gate MT1
MT2 MT2
MT1Gate
G
MT1
MT2MT2
MT1G
+Voltage
+Current
on state
forward blocking
region on state
reverse blocking
region
October 28, 2010 25OSRAM LLFY 2010
Practical TRIAC Dimmer
• Removes a portion of the leading edge of each line half-cycle
• Will trigger at any angle where line voltage exceeds Diac threshold
• More symmetrical triggering
• Resistance to Line Voltage Transients
• Compatible with resistive and inductive loads such as magnetic low-voltage
halogen transformers (large inductive loads may damage dimmer)
G
MT1
MT2
AC
Source
t
Vload
Iload
Diac VBO
October 28, 2010 26OSRAM LLFY 2010
TRIAC Dimmer w/60W Incandescent Lamp
60W Incandescent Lumen Output vs. Triac Dimming %
0
100
200
300
400
500
600
700
800
900
1000
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Triac Dimming %
Lu
men
Ou
tpu
t -
lum
en
s
•Dimming profile of a 60W incandescent lamp connected to a
triac dimmer.
October 28, 2010 27OSRAM LLFY 2010
Trailing Edge Dimmer
• Requires a solid state switch that can be turned-off during
the line half-cycle (MOSFET, IGBT, not a TRIAC).
• Compatible with resistive and capacitive loads such as
electronic low-voltage halogen transformers
AC
Source t
Vload
Iload
October 28, 2010 28OSRAM LLFY 2010
T
MT1
MT2
Simple
Triac
Dimmer
AC Hot
AC Neutral
Load
T
MT1
MT2
Simple
Triac
Dimmer
AC Hot
AC Neutral
Load
Trigger
Current
T
MT1
MT2
Simple
Triac
Dimmer
AC Hot
AC Neutral
Load
Trigger
Current
Holding
Current
Practical Design Considerations
for Triac Compatibility
• Triac Trigger Requirements
– The load is an integral and necessary part of the trigger circuit. Load impedance must remain sufficiently low at all times or maximum conduction angles will not be reached.
– Trigger circuit must supply sufficient current to initially latch-on Triac
• Triac Holding Current Requirements
– Typically 10-20mA, Triac will resume
blocking if minimum holding current is not
maintained continuously. Room
temperature holding current requirements
nearly double at -45C.
October 28, 2010 29OSRAM LLFY 2010
Practical Design Considerations
for Triac Compatibility• AC Line Filter
─ A low pass filter is necessary to prevent high frequency switching
currents from reaching the AC line as conducted emissions.
─ The low pass filter must have low resistive losses and a high Q and so
will ring when subjected to the fast rising edge of the Triac dimmer turn-
on.
─ Line filter ringing may result in line current reduction or reversal which in
turn may cause the Triac to cease conduction.
─ The simple solution is resistance in series or parallel with the AC line
but will be accompanied by losses, particularly for 100VAC operation.
AC
L1
C1C2
T1BR1
D1
Q1CDS
PWM
Controller
LED 1-N
Vdd
AC Line
Filter
October 28, 2010 30OSRAM LLFY 2010
Practical Design Considerations
for Triac Compatibility
•AC Line FilterAn example of an un-
damped AC line filter
causing a TRIAC
dimmer to oscillate:
After each TRIAC
trigger the ringing line
filter causes the line
current to reverse
which results in TRIAC
turn-off. The cycle
repeats for as long as
the TRIAC trigger
requirements are
satisfied.
October 28, 2010 31OSRAM LLFY 2010
Practical Design Considerations
for TRIAC Compatibility
• TRIAC Conduction angle detection
– Dimming base on actual TRIAC conduction angle
rather than RMS line voltage will provide rejection to
line voltage variations
• Dimming profile
– Like the human ear, the human eye has a log
response to intensity. Consequently, a log dimming
profile is more pleasing than a linear dimming profile.
October 28, 2010 32OSRAM LLFY 2010
TPS92001 Reference Design Strategy
• AC Line Filter Damping:
– Passive R-RC
• TRIAC Trigger Current:
– Normal or augmented line current
• TRIAC Holding Current:
– Active line current augmentation
• TRIAC Conduction Angle Detection:
– Filtered RMS Line Voltage
• Dimming Profile:
– Dual slope
October 28, 2010 33OSRAM LLFY 2010
TPS92001 PMP4981 Reference
Design Schematic
+
+
TPS92001
AC Line
Filter
Line Filter
Damper
Normal Load
Current
Augmented
Line Current
RMS Voltage
Detect
Slope
Comparator
October 28, 2010 34OSRAM LLFY 2010
• Operation with existing TRIAC
dimmers requires a minimum
current to maintain conduction
• Dual slope feature improves low
angle dimming
• Audible noise can occur with
PWM dimming, particularly with a
high 120Hz ripple content.
However, this reference design
takes steps to minimize this
noise.
• Color shift is negligible
• LED efficacy reduction is low
TPS92001 PMP5163 LED Driver Dimming
UCL64001 Reference Design Dimming Control
0
0.2
0.4
0.6
0.8
1
1.2
0% 20% 40% 60% 80% 100%
Percent Triac Conduction
No
rma
lize
d o
utp
ut cu
rre
nt (A
)
October 28, 2010 35OSRAM LLFY 2010
TPS92010 Design Strategy
• AC Line Filter Damping:
– Not required
• Triac Trigger Current:
– Active bleed resistor
• Triac Holding Current:
– Not required
• Triac Conduction Angle Detection:
– Direct AC Line Conduction Angle Monitor
• Dimming Profile:
– Dual slope
October 28, 2010 36OSRAM LLFY 2010
TPS92010EVM Schematic
AC Line
Filter
No Damping
Required!
Active bleed
resistor
No holding current required!
Line conduction angle monitor
CBULK
FEEDBACK
RCS
RPL
TL431
2
1
6
SS
VDD
4 GND 5GD
FB
8
3 PCS
7VCS
LPM
TPS92010
PRIMARY SECONDARY
CBULK
RCS
+
-
+
-
October 28, 2010 37OSRAM LLFY 2010
TPS92010EVM Dimming Performance
• Traditional problems TRIAC triggering
and holding currents with LED lighting
are solved.
• Dimmer triggering provides loading of
the TRIAC at AC line crossover for
proper dimmer operation.
• DC current during dimming
– No Stroboscopic effect
– No audible noise
– No ripple current efficacy loss
• Steady deep dimming
– Two Step dimming pleasing to eye
October 28, 2010 38OSRAM LLFY 2010
TPS92210 Reference Design Strategy
• AC Line Filter Damping:
– Active RC Damper… low loss
• Triac Trigger Current:
– Normal line current or source follower
• Triac Holding Current:
– Active line current augmentation (secondary side)
• Triac Conduction Angle Detection:
– Reconstructed Line Voltage (secondary side)
• Dimming Profile:
– Log + Linear
October 28, 2010 39OSRAM LLFY 2010
TPS92210 Reference Design
SchematicAC Line Filter Line Filter
Damper
Active
Damper
Clamp
Trigger Current Path
Triac conduction angle
detect & log dimming
profile generator
Line current
augmentationHolding Current Path
October 28, 2010 40OSRAM LLFY 2010
TPS92210 Dimming Performance
R20
R25
D9
U2
VCommand
R40
R27
R28
C14Q6
R3
D6
D5
T1
+5Vref
+5Vref
+5Vref
C12
0V
• Dimmer Compatibility
• Dimming Profile:
• The TPS92210 EVM dimming angle detection circuit has a
logarithmic duty-cycle to voltage transfer function accurate over
more than a decade and then responds linearly to near zero. An
offset on the measured output current allows dimming the LED to
total darkness. Logarithmic dimming closely matches the response
of the human eye.
• Ripple in LED current causes negligible color shift and efficacy reduction
TPS92210 Normalized LED Current vs. Triac Conduction Angle
0
0.2
0.4
0.6
0.8
1
1.2
0% 20% 40% 60% 80% 100%
Triac Conduction Angle
No
rmali
zed
Ou
tpu
t C
urr
en
tI_LED
October 28, 2010 41OSRAM LLFY 2010
Conclusions
• LED drivers must provide a path for TRIAC
dimmer trigger current
• LED drivers must meet the minimum holding
current requirement of the TRIAC dimmer to
insure full conduction for each line half-cycle
• AC line filters must be critically damped to avoid
erratic TRIAC behavior.
• Non power factor corrected LED drivers can
ignore the holding current requirements of the
TRIAC dimmer
October 28, 2010 42OSRAM LLFY 2010
Thank You
• Special Thanks to Joel Brassfield and Gary
Guenther with Texas Instruments for putting
together the content of this presentation.
Contact:
Peter Di Maso
603 222-8574
October 28, 2010 43OSRAM LLFY 2010