1

Off-line LED applications

From PFC basics to constant current LED drive

CompelFest 2013

EU Design Services

Roberto Scibilia

2

AGENDA

Review of Power Factor

EU Specs and Energy Star®

Limits for lighting equipments: EN61000-3-2 Class C

Selecting the right topology:

Buck in average current mode

Buck + Charge pump Boost

Buck + Voltage Feed Forward

Continuous, Discontinuous and Transition Mode

Boost

Flyback

Closing the loop on the output current

DC current sensing

Transformer on secondary side

Peak current stabilizing

Low and Mid-Power LED driver Portfolio

3

Review of Power Factor

Power Factor is the Ratio of Real Power (Watts) to Apparent

Power (RMS Volt-Ampere product)

Power Factor has two components –

Displacement Factor (DispF)

Distortion Factor (DF)

Power Factor PF is the product of DF and DispF

)(Power Apparent

Power Real

VA

(W)PF

2

1

1

1

THDI

IDF

rms

cosDispF

cos1

12

THDPF

Vin

Iin

Current Lags Voltage by

(Displacement Factor)

Vin

Iin

Current has high harmonic

content (THD)

4

Review of Power Factor

Reactive currents, either capacitive or inductive, including reactive harmonics result in

circulating currents and associated I2R losses in the power transmission system but do not

develop power in the load.

Loads presenting the AC line with high power factor minimize unnecessary power losses

in the transmission system.

Loads presenting the AC line with low current THD minimize losses and interference with

adjacent loads.

Ideal resistive loads have a power factor of 1.0 and generate no harmonics (THD = 0).

Legacy incandescent lamps are nearly ideal resistive loads. (temperature coefficient of

filament causes some distortion,.

AC

Line Voltage

Line Current

Volt/

Amp/

AC

Line Voltage

Line Current

Volt/

Amp/

PF = 0.90

THD = 43.5%

PF = 1.0

THD = 0

5

E U Specs and Energy Star ®

Classes

EN-61000-3-2

sets harmonic content for any power supply sold in EU

4 classes

D ~ personal computers and TVs

C ~ lighting equipment

B ~ portable tools

A ~ everything else

Energy Star®

Power supplies with greater than or equal to 100-W input power

must have a true power factor of 0.9 or greater at 100% of rated

load when tested at 115 V, 60 Hz

6

EN61000-3-2 and Energy Star ®

Limits

Power Factor not important!

Only harmonic currents are limited, up to 39th harmonic, nominal

230 VAC

Limits depend on Class: Absolute limit in amps (Class A, B)

Percentage of fundamental (Class C lighting)

Amps/watt up to absolute max limit (Class D 75 W ≤ Pin ≤ 600 W)

Energy Star®

Power factor drives the limit here

7

Lighting limits: EN61000-3-2 Class C

8

Lighting limits: EN61000-3-2 Class C (P<25W)

For class C equipment with an input power smaller or

equal than 25W either:

1. The limits of table 3 (column two) apply

2. Or the third harmonic current shall not exceed 86% and

the fifth harmonic current shall not exceed 61% of the

fundamental current (for further details refer to the

standard).

9

Fixing the maximum limits (P>25W)

1st, 3rd, 5th, 7th, 9th Harmonic (7th is 180 deg. shifted)

10

Fixing the maximum limits

Approximation with a simple waveform:

A trapezoidal line

11

Fixing the maximum limits

The trapezoidal line fulfill the class C limit since it’s

an approximation of the built waveform

12

Is it enough also for Energy Star® ?

The THD of the waveform generated from the

EN61000-3-2 limits is ~ 11.5%

Consequently the PF, assuming there is no

displacement factor, is 94.7%

This value is well higher than the 0.9 dictated from

the Energy Star® regulations

BUT: how do we get this waveform?

13

Selecting the right Topology: Buck

An average current mode Buck converter might do the job, but

the conduction angle is not enough to fulfill Class C

14

Selecting the right Topology: Buck

Peak current mode, sensing on Mosfet’s current

Constant output current: you get the “smiling” input current

15

Selecting the right Topology: Buck

Constant output current and voltage = Constant Power

The “smiling current” has even worse PF (~0.55) and bad THD Real Measurement

17

Solution with Feed Forward injection

Absorbed current almost sinusoidal (PF>0.85)

Low cost, no feedback loop

Output current slightly dependant on input AC voltage (±20% on Vin translates into ± 10% on Iout)

This dependence can be reduced by injecting a DC bias ∩ Vin

18

CCM, TM, DCM…what’s the difference?

IPEAK

IAVERAGE

(C) DRM

(b) DCM

IPEAK

IAVERAGE

IAVERAGE

(a) CCM

Some very important

differences:

Ripple Current

Drives filtering requirements, ac

losses in magnetics

Peak current

Drives semiconductor stress,

losses, peak flux in magnetics

Frequency

CRM is variable frequency

Helps on EMI, but enters quickly

the 150KHz lower limit

19

CCM Loss Analysis

In CCM diode experiences large reverse recovery

current

PFC boost is the worst case for diode

MOSFET losses are increased

Ripple currents are small

low ac losses

low peak current stress

RMS currents are lower

conduction losses are lower

t (a) CCM

t (b) CRM

Diode Current

Switch Current

Diode Current Switch Current

I L_ pk_ccm

IL_valley_ccm

C

V OUT L

Q1

I L I D

I Q

I L_ pk_crm

20

TM and DCM Loss Analysis

Peak current in TM is 2 x CCM (even higher in DCM)

High ac losses

High peak current stress on Mosfet and Diode

TM and DCM benefit: the diode stars conducting

always when the energy in the inductor is zero: no

reverse recovery issue

Reduced risk of MOSFETs failure due to shoot through by

reverse recovery of the Diode

RMS currents are higher than CCM

conduction losses are higher

Worst case in DCM

21

TM Current Loop

TM current loop employs hysteretic type control

Lower boundary is zero

Upper boundary is set by multiplier

No “loop” to design

Simply chose sense resistor based on large signal considerations

Need to sense when zero current crossing occurs

Signal taken from already existing auxiliary winding

Small inductance of TM is traded-off for increased

filter size

Use low losses cores and Litz wire, when possible ($$$)

Variable frequency operation can help

22

CCM: Closing the loop

The inner current loop corrects the Power Factor

The outer voltage loop regulates the output voltage

The input voltage feed-forward speeds-up the voltage regulation

23

Input Average Current: why don’t we get PF=1?

The current loop tries to

stabilize a constant

output voltage (or

current)

Since we have 100Hz

ripple on output cap, the

error signal (Verr) will

have also that ripple

This error signal will add

distortion on the

absorbed current while

trying to regulate “inside”

it

The consequence is a

reduced Power Factor

and higher THD

24

Boost Topology: TM mode

t

(a) CCM

t

(b) CRM

Diode Current

Switch Current

Diode Current

Switch Current

I L_ pk_ccm

IL_valley_ccm

C

V OUT

L

Q1

I L I D

I Q

I L_ pk_crm

Ripple current in CCM is

“small”

Peak current in CRM is 2x

CCM

If TON is constant, the peak

input current is proportional to

the sinusoidal input voltage

The cycle-average input

current is half the triangular

switching waveform area

resulting in a sinusoidal input

current

L

Vin*|sin(ωt)| ILCRM_PK

*TON

25

Flyback Topology: TM mode

If TON is constant, the peak input current is proportional to the sinusoidal input voltage

The cycle-average input current is the average of the switch current, which is NOT sinusoidal because TOFF depends on ILCRM_PK

26

Duty Cycle and Frequency vs. Time

The output power is then calculated by integration of the transferred energy cycle by cycle

POUT0

x1

2LP IPKprim x( )

2 FS x( )

d

0 0.314 0.628 0.942 1.257 1.571 1.885 2.199 2.513 2.827 3.14250

90

130

170

210

250

0.2

0.36

0.52

0.68

0.84

1

212.869

78.646

F.S x( )

1000

1

0.369

Duty_Cycle x( )

0 x

27

Input Average Current: why don’t we get PF=1?

The average input current is NOT a half of the peak inductor current but it is averaged with duty-cycle

Iinavg x( )1

2IPKprimx( )

TON x( )

TON x( ) TOFF x( )

0.435

0

Iinavgvminx( )

0 x

IinavgvminIinavgvmin

0 0.5241.0471.5712.0942.6183.1420

0.083

0.167

0.25

0.333

0.417

0.5

0.435

0

I.inavg x( )

0 x

28

Typical application LED Driving

The loop is closed to the output current, so VSENSE pin needs only a bias voltage

Primary aux. winding used for zero current switching

Secondary aux. winding for biasing and sensing the reflected input voltage (Triac dimming)

29

UCC28810 Transition Mode Controller

Suitable for Boost, Flyback, Sepic, Buck, as PFC controller and constant output current generator as Buck controller.

30

Closing the loop on the output current:

Current sense resistor + op-amp + TL431

R5 senses the output current

U1-B is a differential amplifier

U1-A has a TL431 (pin 3) + op-

amp inside

U3 transfers the error signal to

the EAOUT pin of UCC28810

D9, Q3 provide the Bias

voltage for U1: note the dots

on transformer; the forward

voltage is rectified, which is

independent of the output

voltage (variable from 40V to

120V)

31

Closing the loop on the output current:

Current sense transformer + TL431

C6 + Llk of T1

define the

resonant

frequency (LLC)

T3 senses the

output current

R11 contains the

information of

the output

current

U3 + U4 close the

loop and create

the error voltage,

which modulates

the switching

frequency

U2, D5 work as

OVP

32

Closing the loop on the output current: Stabilizing

the peak current on the output inductor (TM)

R1 senses the switch current (same peak value of the inductor)

The value is compared inside UCC28811 with a fixed reference

An hysteretic mode allows the converter to work into Transition Mode (Bang-Bang modulation)

The average output current is half of the stabilized peak

33

Closing the loop on the output current: Stabilizing

the peak current on the output inductor (CCM)

The peak current through R4 is compared to the internal maximum limit

CCM and duty-cycle > 50% need slope compensation (R5)

C4, D3, D4, supply the Bias

34

Flyback Loop Compensation: Power Stage Gain

35

Loop Compensation: Power Stage Gain

Calculate the power stage gain at 95Vdc input and full

load of a 80Vout@2A, Flyback converter

Transformer primary inductance Lp=70uH and turns

ratio 1:1

The input power is always:

If the efficiency is 89%, Pin = 180W

VINPK = 120.2V, the 95Vdc is reached at 0.91 radians

From the switching frequency graph we pick the

equivalent frequency at 0.91 rad.: ~ 60KHz

“Flying” back to the TL431, the total

gain is: Gps = 30.5dB

Pin1

2Lp Ipk

2 Fsw

VIN t( ) VINPKsin t( )VIN t( ) 95VDC

36

Loop Compensation: Closing the loop

Choose a crossover frequency << 100/120Hz

Place the zero to gain enough phase margin

18Hz, 17.9dB

4.23Hz

18Hz, -17.9dB 160Hz

7.24Hz

-30

-25

-20

-15

-10

-5

0

5

10

15

20

25

30

35

1 10 100 1000 10000

Frequency (Hz)

Gain

(d

B)

Power Stage Compensation Total Open Loop

37

Loop Compensation: Closing the loop

ΦEA = -180º TL431 inverting input (error

amplifier)

ΦINT = -90º due to the integration of the

same amplifier

θZ = Tang-1(Fc/Fz) Fc= crossover

frequency, Fz = zero freq.

θP = Tang-1(Fp/Fc) - 90º Fp = pole frequency

The phase margin without compensation is:

Mφ = 360+ ΦEA + ΦINT + θP = 13.2º …..not enough!

We start choosing a pole at 160Hz (we loose here 6.4º)

We need then a zero that has a phase lead, at least:

θZ = 75º - (13.2º - 6.4º) = 68.2º, where 75º is our ideal

phase margin

38

Loop Compensation: Real Measurement

The real measurement has only a slightly higher

phase margin than calculated (85º instead of 75º)

The crossover frequency and the -1 slope match the

measurement

39

Test results on a 42W isolated LED string driver

89.5

90.0

90.5

91.0

91.5

92.0

92.5

40.0 50.0 60.0 70.0 80.0 90.0 100.0 110.0 120.0

Output Voltage (V)

Eff

icie

nc

y (

%)

180Vdc 230Vdc 265Vdc

40

Driven by a constant current power supply with

ballast resistors

Unbalanced channel current

Poor efficiency

Driven by a constant voltage power supply with

constant current linear LED drivers

Balanced channel current, but

Poor efficiency

TRADITIONAL LINEAR DRIVING

METHOD

40

41

DRIVING LED BY A CONSTANT

CURRENT POWER SUPPLY

41

VLED6

VLED5 VLED4

VLED3

VLED2

VLED1

Constant

Current

Power Supply

Largest current through the

lowest LED voltage channel

Lowest current through the

highest LED voltage channel

Unbalance current through each channel

42

DRIVING LED BY LINEAR CONSTANT

CURRENT DRIVERS

42

VREG6

VLED6

VLED5

VREG5

VLED4

VREG4

VLED3

VREG3

VLED2

VREG2

VLED1

VREG1

Fix Rail Voltage

Constant

Voltage

Power Supply

Linear

constant

current

driver

Significant

power

dissipation

across

linear

drivers

=>Poor

efficiency

43

LM3466 is a linear LED driver which acts like an intelligent ballast

resistor. Each IC communicates with other IC’s to equalize the current in

each channel which derives from a constant current power supply, i.e.,

divides the current equally. Thus it is very easy to construct a high

power lighting fixture by combining an off-the-shelf constant current

power supply and LM3466’s.

DEVICE HIGHLIGHT

43

44

Works with a constant current power supply

Equalizes the current of every active LED string automatically

Maintains constant output power if some strings open (inactive), and the

current of remaining active LED strings will be equalized automatically

No communication to/from the constant current power supply is required

Operating with minimum voltage overhead to maximize power efficiency

(up to 99%)

Wide operating voltage from 6-70V, drives up to 20 LEDs per string

Up to 1.5A driving current

Protections : Thermal shutdown, fault status output

Linear circuitry does not deteriorate EMI

Package: PSOP8

Target application : Street lamp, tunnel lamp, parking lot lamp,

panel lamp

LM3466 OVERVIEW

44

45

LM3466 BUILDING BLOCK

45

46

TYPICAL APPLICATION

46

47

Lighting Power Products – Combined Portfolio

48

AC-DC LED Driver Controller

< 50W PAR 38, Industrial

Fixtures, Area Lights

• Dimming, dissipative

• PF >80 %

• THD < 40%

• Eff > 65%

Perf

orm

an

ce

Output Power

< 25W

A19, PAR

20/38

< 10W

A19, PAR 20,

downlights

< 5W

GU10, E14

Candles

TPS92210

LM3445

LM3444

TPS92070 (Q3/11) Market Base

Requirements

LM3450

TPS92001

TPS92010

UCC28810

Integrated Dimmer Detection

49

TPS92070 LED lighting Driver controller

System Block Diagram (Isolated driver)

Near Lossless Dimmer Triggering

TRIAC Dimmer detect

Dimmer Angle detect

Valley Fill

50

LM3445

TRIAC Dimmable Offline LED Driver

Features Benefits TRIAC Dimming Decoder for LED Dimming Integrated TRIAC Detection Reduces

Component Count and Solution Size

Master/Slave Operation Single TRIAC Controls Multiple Strings with

Consistent Dimming Performance

Application Voltage Range (80-277Vac) Supports Residential and Commercial LED

Lighting Applications

Controls LED Currents of Greater than 1A

Adjustable Switching Frequency

Adaptive, Programmable Off-Time Control

Thermal Shutdown, UVLO, Current Limit

Applications Dimmable Residential LED Lighting Drivers:

A19 (E26/27, E14), PAR30/38, GU10

Lighting Applications: Light Bulb

Replacement, Wall Sconces, Wall Washers,

Architectural and Display Lighting,

Commercial Troffers and Downlights

51

LM3444 AC/DC Offline LED Driver

Features Benefits Application Voltage Range (80-277Vac) Supports Residential and Commercial

LED Lighting Applications

Controls LED Currents of Greater than

1A

Supports a Wide Variety of LED

Configurations

Adjustable Switching Frequency

Adaptive, Programmable Off-Time

Control

Thermal Shutdown, UVLO, Current Limit

Applications Non-Dimming Residential LED Lighting

Drivers: A19 (E26/27, E14), PAR30/38,

GU10

Lighting Applications: Light Bulb

Replacement, Wall Sconces, Wall

Washers, Architectural and Display

Lighting, Commercial Troffers and

Downlights

52

TPS92210

PFC Offline LED Lighting Driver Controller

Features Benefits Flexible Operating Modes: Peak Primary

Current, Constant On-Time, or both

Constant On-Time implements Single

Stage Power Factor Correction (PFC)

Cascoded MOSFET Configuration Fast start up; Line Surge Ruggedness

Better Than Internal HV FET

Works with TRIAC Dimmers Continuous Exponential Dimming

Transformer Zero Energy Detection High Efficiency, Low EMI

Discontinuous Conduction or Transition

Mode Operation

No Reverse Recovery Loss in Output

Rectifier

Advanced Over-Current Protection and

Integrated Over-voltage Protection

Protects Driver Against Fault Conditions

Applications Residential LED Lighting Drivers: A19

(E26/27, E14), PAR30/38, GU10

Lighting Applications: Light Bulb

Replacement, Sconces, Wall Washers,

Architectural and Display Lighting,

Commercial Troffers and Downlights TOOLS

•TPS92210EVM-647 (110V)

•TPS92210EVM-613 (230V)

53

TPS92010 High Efficiency Offline LED Lighting

Driver Controller

Features Benefits High Efficiency LED Lighting Current

Quasi resonant and low power modes

87% Achievable Efficiency – Higher than

Standard Flyback Topologies

High Performance TRIAC dimming with

application circuit

Less than 400mA Standby Power Allows

Efficient Deep Dimming

Programmable Overvoltage Protection 20% More Efficient Dimming Compared with

Other Methods

Internal Over-temperature Protection Safely Shuts Down Driver if Open or Over

Temperature is Condition is Present

TrueDrive Gate Drive 1A sink, 0.75A Source Lower Switching Losses Reduces System

Cost

Current Limit Protection Cycle-by-cycle Power Limit

Primary Side Over-current Hiccup

Restart Mode

Protects Driver from Fault / Abnormal

Conditions

Applications Residential LED Lighting Drivers

Lighting Applications: Wall Sconces, Pathway,

Overhead Lighting, wall washing and display

lighting TOOLS

•TPS92210EVM-592 (110V)

•TPS92210EVM-631 (230V)

54

TPS92001/2General Purpose LED Lighting Driver

Features Benefits Ideal for Single Stage LED Driver

Designs

Power Factor >0.7

Isolated and Non-Isolated Topologies Supports Wide Configuration of LED

Loads

TRIAC Dimmable Application Circuit

with Low External Component Count

Low Cost Deep Dimming Solution with

Small Form Factor

Convenient 5V Reference Power for MCU or Linear Circuits

Two Under-Voltage Lockout Options

(10V or 15V)

Protection from Abnormal Operating

Conditions

Integrated Gate Drive: 0.4A Source /

0.8A Sink

External Gate Drivers Not Required

Applications Residential LED Lighting Drivers: A19

(E26/27, E14), PAR30/38, GU10

Lighting Applications: Light Bulb

Replacement, Wall Sconces, Wall

Washers, Architectural and Display

Lighting, Commercial Troffers and

Downlights

55

Single Stage Flyback AC-DC Controller:

Primary side flyback LED current regulation

Doesn’t require opto-coupler or secondary side circuitry

Adaptive ON-time control with inherent PFC

Critical-Conduction-Mode (CrM) with Zero-Current-Detect (ZCD) for valley

switching

Reduces EMI filter design complexity

LED current setting with external sense resistor

No loop compensation required

Gate driver with slew rate control

Eases EMI filter design

Output voltage protection (OVP) through ZCD

VCC Under Voltage Lock Out (UVLO)

Thermal Shutdown

SOIC-10 package

TPS92310 Features Description

55

56

Typical schematic (COT)

56

57

Typical schematic (PCM)

57

58

What is difference between COT and

PCM operation.

58

Constant ON time

>> High power factor

Peak Current Mode (Pull MODE1 pin to GND)

>>Low output ripple current

IAC

VAC

59

Design consideration

59

Current sense

Resistor

OVP

Resistor

Snubber

Fly wheel Shottky diode

60

Design example and component

selection

1. Snubber circuits: use 600V ultra fast diode.

2. Flywheel diode : use the 100V…200V 2A schottky.

3. ZCD / OVP resistor : Suggest set the Normal voltage is ~3.5V . Current is 1mA.

4. Current sense: IOUT = N*IREF/ RCS

N is turn ratio of transformer (primary : aux ): (3.8:1)

REF = 0.14

0.35 mA = 3.8 x 0.14 /1.5Ω. (*PS: 0.14 is Internal reference.)

5. Power MOSFET :

600V or upper. 2A e.g. 3N60E

6. TDLY resistor selection: 1. First step : 1/4 (TDLY )= 1/(2 Pi x SQRT(Lm x C )) . Assume Cds= 200p, L = 5mH.

2. Calculation : TDLY = 2/(pi*SQRT(Lm x C) =636ns = 8K.

3. Based on the MOSFET drain to source waveforms. Fine tune RDLY.

60

61

Design example and component

selection

7. Pull up resistor (R4):

The current through resistor R4 must less than 1mA. Otherwise OVP can’t

restart. (can not pull low by device itself)

8. Input capacitor (C2):

C2 can’t too small, We assume the VIN is constant within a switching

cycle. If value of C2 too small. The input current can’t estimate the output

current correctly.

For 220VAC the base is 0.15uF > C2> 0.1uF, The C2 too large will impact

the power factor, too small will impact the current regulation.

9. Output capacitor (C3):

C3 can smooth the IOUT current more smooth. Bigger C3 value can

reduce the output current, but will impact the start up time. 430uF or

330uF are typical.

10. Diode (D7,D6): Diode D7 can limit the ZCD pin > -0.3V, it can avoid a negative current

through the IC device. D6 is a zener diode for protection the VCC voltage.

61

62

Design transformer consideration:

Transformer:- Use a better couple transformer.

Consider the transformer layout. e.g. use the interleaved

windings.

Interleaved winding has low eddy current loss.

Full cover the bobbin length.

Switch-node pin should be connect the transformer inside for reduce

the EMI.

62

63

TPS92310 line regulation (ILED

=

350mA)

63

0,250

0,275

0,300

0,325

0,350

0,375

0,400

0,425

0,450

90 95 100 105 110 115 120 125 130 135 140

Iou

t (A

)

AC Input (V)

Iout vs AC Input Voltage LIM_HI LIM_LO

64

TPS92310 I_LED vs Temperature

64

-3%

0%

3%

-30 -15 0 15 30 45 60 75 90

I_L

ED

(%

)

Temperature (°C)

I_LED vs Temperature

65

Input voltage and current

65

VAC

IAC

PF = 0.91, Efficiency = 83%

66

Start up operation waveforms

66

VLED

IOUT

VIN

VCC

Test condition: - Apply VDC = 0 to 250VDC, measure the output LED voltage and current.

CH1 :- Controller input Voltage VCC

CH2:- VLED voltage

CH3 :- VDC Input DC Supply Voltage,

CH4 :- ILED output current.

Test result: - Normal, In upper start up case, LED light up time is 0.6sec. In typical operation, (220VAC ) start up time within 1.2

sec by current silicon.

67

Typical operation waveforms

67

V_Lx

I_TX

Test condition :- Apply 250VDC to light bulb supply pin, measure from drain to source voltage and

Transformer primary side current.

CH1 : NC,

CH2 : NC,

CH3: Drain to source voltage at external main MOSFET,

CH4 : Main switch current,

Test result :- Normal. The convertor can achieve CRM switching and zero current switch. Thus, design

EMI and EMC filter are easy.