AC Powered Driver Topologies

1 1

AC Powered Driver Topologies

2 • Template • Nov-12 Confidential Proprietary2

Outline

• LED Driver Requirements and Regional Standards

• Topology Overview

• Meeting Power Factor/Harmonic Content Requirements

• Comparison of Switching Topologies

• Conclusions


Main LED Power Conversion Topologies

• Linear• Buck• Boost • Buck-boost • Flyback• Resonant Half Bridge

• Initial Considerations– LED Selection– Efficiency and Size– Performance specifications– Features eg:

Dimming/Control

PowerConversion

LED DriverControl

LED(s)

AC Mains

Real World Interface


LED selection depends on Application

Small-chip, dispersed phosphor

• Linear

• Non-directional retrofit lamps (A-lamps)

• Any application where uniformity and color are critical

Large-chip, coated phosphor system

• Outdoor

• Down-lights

• Directional lamps (PAR, MR16)

• Any application that requires a TIR optic or long throw

“Fried Eggs”

• High-output downlights, some non-roadway outdoor (e.g. Wall packs)

• Lower-volume products

• Easy to assembly

High Current/Low Voltage Low Current/Higher Voltage Medium Current/Med Voltage


Driver Challenges for LED Bulbs

• Efficiency is critical since heat sinking is limited as bulb shape is fixed

• Space inside bulbs is limited, especially for higher power bulbs that need more heat sink area

• If driver does not have electrical isolation, bulb mechanical design must address safety isolation

• Optical design may also reduce the space available for the driver

Courtesy: IEEE Spectrum


Key Driver Selection Factors

Efficiency &Parametric

Performance

Control&

“Smart”Operation

OperatingEnvironment

Production Testing and

Maintenance

FormFactor

&Safety

• Current accuracy

• Ripple

• Vout Range

• Power Factor

• Total Harmonic Distortion

• Efficiency

• Startup

• Fault Operation

• Standards

• Dimming• 0-10 V• DALI• Wireless• Triac• Analog

• Thermal Foldback

• Smart Features

• Occupancy/Activity

• Bi-Level Control

• Does the application require a special shape?

- Low Profile - Circular

• Can the driver be remote from LEDs

• Best way to meet safety requirements

• electrical isolation

• mechanical isolation

• Temperature• Extremes• Normal Range

• Both impact lifetime

• Enclosed or Open Air

• Line Surge and Transients • Residential• Industrial• Commercial

• Production Testing Requirements

• What happens if an LED fails open or shorts?

• Does the driver need to be replaced in the field if there is a failure


North America Standards

• ENERGY STAR® is voluntary standard– No driver efficiency specifications, only at system level (lumens/W)

– Power Factor By Application• LED Bulbs < 5 W … No power factor requirement• LED Bulbs ≥ 5 W … PF ≥ 0.7• LED Residential Fixtures … PF ≥ 0.7• LED Commercial Fixtures … PF ≥ 0.9, no THD spec but <20% common

– California Voluntary “Quality LED Bulb”, effective in 2014• Eligible for Utility Rebate• PF ≥ 0.9, Dimmable Range < 10%, Color Rendering Index (CRI) > 90

– Safety standards under UL (US) and CSA (Canada)


Basic UL Framework

UL 8750Light Emitting Diode (LED) Equipment for Use in Lighting Products

SelfBallasted

Lamps

UL 1993

Low VoltageLandscape

Lighting

UL 1838

Stage and Studio

Luminaires

UL 1573

Class 2 Power Units

UL 1310

Power Units Other than

Class 2

UL 1012

Luminaires

UL 1598

PortableElectric

Luminaires

UL 153

TrackLighting Systems

UL 1574

UnderwaterLuminaires

UL 676

Low Voltage Systems

UL 2108

Note: Examples of some general lighting categories


EU Standards (Followed by Other Countries)

• IEC61000-3-2 Class C Lighting, Harmonic Content, < 25W the specifications is pretty simple with special exception

• IEC 61347-2-13 – Lamp Controlgear – Particular requirements for DC or AC supplied electronic control gear for LED modules – Safety

• IEC 62384 DC or AC supplied electronic control gear for LED modules – Performance

• IEC 62838-2-2 – Particular requirements for connectors used with LED modules

• IEC 61547 – EMC immunity requirements

• IEC 62031 – LED modules for general lighting – safety


Outline





• Conclusions


Linear LED Driver Approach

• Current Control Regulator (CCR) can set fixed current to drive LEDs– LED string Vf must match line

voltage for best efficiency– RMS current varies with AC line– 100% Ripple

• Input capacitor drop acts as a voltage divider to improve efficiency/reduce # of LEDs

~VAC

+

-VAK

ICCR

VF(Total)

P

Pg

VfR

VRIC

1

1Re

2

*

R1 = 470kΩ

R2 = 120Ω

Vz = VLED + 4V

VAC

Info in AND8492-D Application Notewww.onsemi.com

http://www.onsemi.com/pub_link/Collateral/AND8492-D.PDF

http://www.onsemi.com/


Buck Topology

• This buck is the simplest of switched-mode topologies, suitable when Vout (LED forward voltage) is less than Vin– Switch closed current flows into the inductor and into the load– Switch open current continues to flow into the load, with the diode closing the

circuit

• Vout = Vin (Ton/Ts), assuming continuous conduction of the inductor

• Input current is chopped, output current (into the capacitor) is smooth

TS

TON TOFF

Duty Cycle = Duty Ratio = D =TON

TS

TON

TON TOFF

=

Load(R)

Vin Vout

Vout = VinD


• Red switch is ON• Green switch is OFF

Example NCL30105 Buck LED Driver

• Inverted buck– MOSFET is referenced to ground– LED string is directly connected to high voltage

• Example shows CCM (continuous conduction), Critical Mode (CrM) is often used and has several advantages


Current in CrM Buck Inductor

in LED LEDon off peak

V V VI t t I

L L

2peak

LED

II

Ivalley = 0

ton toff

ILED ΔI = Ipeak

Ipeak

Ics

td

• Advantages of CrM over CCM• CrM has much smaller inductor than CCM• No need for low Trr for output rectifier, lower switching losses • CrM reduces current error due to inductor, Vin, and VLED variation


The Other Two Basic Switched-Mode Topologies

• Boost– The inductor, switch and diode

have changed places.– Switch on current in switch.– Switch off current in diode.– Vout > Vin (neglecting Vdiode)– Input current is smooth.– Output current is chopped.

• Buck-boost– Again, the three elements have

changed places.– Switch on current in switch.– Switch off current in diode.– BUT direction of current causes the

output to be negative (always).– Input current and output current are

both chopped.– Non-isolated version of a flyback.

Load(R)

Vin Vout

1Vout = Vin

D'

Load(R)

Vin Vout

Vout = VinD'

- D

D' = 1 - D =TON

TOFF

TOFF

TOFF

TS

=


Flyback(transformer-coupled buck-boost)

• Same characteristics as the buck-boost---both input and output currents are chopped.

• Note polarity of windings (positive output is shown).• Transfer function is like the buck-boost, with added turns ratio (n).

Vin

VLED

VLED = VinD

n D'

n 1


Flyback versus Forward

• Custom transformer in both cases, forward requires reset winding

• Flyback is preferred over forward

• Simplicity and lower parts count

• Can provide high power factor or low ripple based on control method

• Can support wide range of output voltage with good efficiency

Flyback Forward


Multi-Stage Topologies

• Commonly used for medium to high power LED drivers• Reasons multi-stage is used (even at lower power)

– Easy to meet PF and THD across wide input line voltage– Low output current ripple– Support for very wide Vf range

– Stable Half Bridge Input

PFC Boost

Buck

LLC needs stable Vin

Boost + BuckPFC + Flyback Stage


Outline





• Conclusions


Power Factor and Harmonic Content

• Active Power Factor control can meet the requirements of all markets and applications– EU min PF for Class C, < 25W is ~ 0.6 plus harmonic requirements– US Residential > 5 W, PF > 0.7, no harmonic requirements

• Single Stage Active Power Factor > 0.9x has real costs– 100/120 Hz Line Ripple requires large output caps or– 200% Ripple means LEDs are overdriven or under-used– Active PF means high peak to average current

• Bigger MOSFET• Increased Inductor/Transformer Losses

– More complex EMI filtering (no bulk)– More input transient protection (no bulk cap) 100 Hz Current

Ripple


Output Ripple Cause Optical Flicker

• All light sources connected to the AC line have time varying response

• Various physiological effects of low frequency light modulation- Visible frequency range 3-70 Hz including photosensitive epilepsy- Studies with fluorescent lights have indicated human performance impact in

100 – 120 Hz range in work efficiency and health

• New Energy Star Bulb specification requires flicker to be reported (9/2014)

• IEEE PAR1789 working on "Recommending practices for modulating current in High Brightness LEDs for mitigating health risks to viewers”


Passive PFC to achieve > 0.7 PF

• One option besides active PFC, is to modify the input filter to improve the Power Factor using a valley fill circuit– Can achieve PF >0.8-0.9 for single line range– Still introduces distortion, but has been used for lighting in North America– Does require additional input filter components– Since energy storage is on primary side, output capacitor size can be reduced

compared to active PF

Input Current

Output Voltage

PF = 0.87


EN61000-3-2 Class C < 25W Exception– 3rd Harmonic < 86%– 5th Harmonic < 61%

• The specification means that only a smaller than normal input bulk capacitor is needed to meet harmonic content requirements


Start of Conduction - 42° (must be < 60 °)

Peak of conduction - 45° (must be < 65 °)

EN61000-3-2 Class C < 25 W Compliance

• Rule of Thumb:– Cbulk < 0.25 µF/Watt (230 Vac)– Example waveform below is 10 W Pout with 2.2 µF Input Bulk

Input Current

Input Voltage


Outline





• Conclusions


Which Switching Topology Is Best?

• Best is subjective!– Efficiency– Lower Bill of Material Cost – Reduced Parts Count– Smaller Space

• General trend especially for bulbs is to move to non-isolated LED Drivers as it addresses all these concerns

• We have analyzed different topologies to identify best region – Figure of Merit Criteria is Voltage* Current Stress– Data normalized to LED VF / Vin Ratio

– Considered Active PF and non-power factor corrected cases


Relative Stress Analysis, Non PF Corrected

• A For very low LED VF, non-isolated flyback has lowest stress

• B Buck and buck-boost are similar in <20% VF/Vin region, buck-boost is limited around 25% as higher voltage MOSFET would be needed

• C Upper limit of buck is only due EN61000-3-2 Class C compliance

A B

C• 600 V MOSFET• 80% derating• Vin= 265 Vac• Pout =10 W


Relative Stress Analysis, PF Corrected

A

B

C• 600 V MOSFET• 80% derating• Vin= 265 Vac• Pout =20 W

• A For high power factor, boost is the lowest stress topology• B To extend Buck-boost to a higher ratio a higher voltage MOSFET is needed• C Upper limit of buck is based on keeping THD < 20%


Summary

• Best topology is strongly driven by LED selection and expected performance needs

• Active power factor correction is not always needed to meet regional PF and harmonic requirements

• Optical flicker is becoming a topic of increased interest due to possible health concerns

• With newer high voltage LEDs coming on the market, boost and buck are becoming more popular topologies and can be very efficient

Technology

AC Powered Driver Topologies