The Impact of Low Frequency Ripple Current on LEDs and LED Drivers - Texas Instruments

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


Lighting Equipment• Small Luminaires & LEDs

– Integrating Sphere & Spectroradiometer

• Color, CCT, Lumens, Efficacy

• Spectral Radiance

• NIST traceability

• Large Luminaires– Color, lux, spatial distribution


LED Specifications• LED datasheets specify:

– VF vs. IF

– Color Coordinates

– CCT

– Lumens vs. constant IF

– Relative luminous flux vs. TJ @

constant current


• 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


• 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


• 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


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


Modulated Current Test Circuit


Modulated Current Test Waveform


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


Pe

rce

nt

Ch

an

ge

- %


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


CC

T -

Ke

lvin

s


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


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


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)


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


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


Lu

me

ns

- l

m


Efficacy vs. % Mod. (corrected)


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


Eff

ica

cy

- l

um

en

s/W

att


% Efficacy vs. % Mod. (corrected)


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


Pe

rce

nt

Ch

an

ge

- %


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



• 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)



• Φ CFL = 247mV

• LED = 347mA

plus sinusoidal

Φ LED occurred @

248mApk-pk

• % Mod. Ipk-pk / Iavg =

71%

VCFLVLED

ILED


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.


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


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


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.


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


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.



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



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.



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.


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


TPS92001 PMP4981 Reference

Design Schematic

+

+

TPS92001

AC Line

Filter

Line Filter

Damper

Normal Load

Current

Augmented

Line Current

RMS Voltage

Detect

Slope

Comparator


• 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

)


TPS92010 Design Strategy


– Not required

• Triac Trigger Current:

– Active bleed resistor

• Triac Holding Current:

– Not required

• Triac Conduction Angle Detection:

– Direct AC Line Conduction Angle Monitor


– Dual slope


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

+

-

+

-


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


TPS92210 Reference Design Strategy


– 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)


– Log + Linear


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


TPS92210 Dimming Performance

R20

R25

D9

U2

VCommand

R40

R27

R28

C14Q6

R3

D6

D5

T1

+5Vref

+5Vref

+5Vref

C12

0V

• Dimmer Compatibility


• 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


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


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

[email protected]


mailto:[email protected]

The Impact of Low Frequency Ripple Current on LEDs and LED Drivers - Texas Instruments

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