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Circuit Protection Considerations for LED Lighting

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Circuit Protection Considerations for LED Lighting
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Circuit Protection Considerations for LED Lighting Matthew Williams, Global Applications Engineering Manager Tyco Electronics Circuit Protection Business Unit
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Page 1: Circuit Protection Considerations for LED Lighting

Circuit Protection Considerations for LED Lighting

Matthew Williams, Global Applications Engineering Manager

Tyco Electronics Circuit Protection Business Unit

Page 2: Circuit Protection Considerations for LED Lighting

LED Lighting Increasingly Popular

As government agencies, industry and

consumers look for ways to reduce

energy costs, light-emitting diode (LED)

lighting technology is expected to boom.

• Low energy consumption

• Long service life

• Durability

• Help meet safety and green initiatives

• Fully dimmable

• No frequency interference

Page 3: Circuit Protection Considerations for LED Lighting

Thermal Management Requirements

• LED luminaires require precise power and heat management

systems, since most of the electrical energy supplied to an LED is

converted to heat rather than light.

• Without adequate thermal management, this heat can degrade the

LED’s lifespan and affect color output.

• Since LED drivers are silicon devices, they can fail short. This means

fail-safe backup overcurrent protection may be necessary.

Page 4: Circuit Protection Considerations for LED Lighting

Heat Conduction Comparisons

• A fixture using a 60W incandescent

light bulb produces approximately

900 lumens of light and must

dissipate 3 Watts of heat via

conduction.

• Using typical DC LEDs as the light

source to achieve the same 900

lumens would require about 12

LEDs

• Assuming a VF (forward voltage) of

3.2V and current of 350mA, the

input power to the fixture could be

calculated as:

Power = 12 x 3.2V x 350mA = 13.4W

• In this scenario approximately 20%

of input power is converted to light

and 80% to heat. This is dependent

on various factors

Page 5: Circuit Protection Considerations for LED Lighting

Heat Conduction Comparisons

• Of the total heat generated by the LED, 90% is transferred via conduction.

• To dissipate heat from the junction of an LED, conduction is the principal

channel of transfer since convection and radiation only account for about 10%

of overall heat transfer.

Page 6: Circuit Protection Considerations for LED Lighting

Junction Temperature Effect

• The optical behavior of an LED varies

significantly with temperature.

• VF drops as junction temperature rises and

the drive current increases.

• This can lead to thermal runaway, causing

the component to fail.

• Typical solution to controlling junction

temperature is to mount the LEDs on

metal core PCBs to provide rapid heat

transfer.

Page 7: Circuit Protection Considerations for LED Lighting

Other Circuit Design Challenges

• Power line coupled transients and

surges can also reduce LED

lifespan

• Many LED drivers are susceptible to

damage resulting from improper DC

voltage levels and polarity

• LED driver outputs may also be

damaged or destroyed by short

circuits.

• Most LED drivers include built-in

safety features, including thermal

shutdown, as well as open and short

LED detection.

• Additional overcurrent protection

devices may be needed to help

protect integrated circuits (ICs) and

other sensitive electronic

components.

Page 8: Circuit Protection Considerations for LED Lighting

LED Driver I/O Protection

• LEDs are driven with a constant

current.

• Older designs relied on simple

resistors to limit LED drive current.

• If forward voltage drop across the

LED decreases to a value

significantly less than the typical

stated value the driver may

overheat.

• New systems utilize power

conversion and control devices to

control power dissipation from the

LED driver.

• Protecting these interfaces from

overcurrent and overtemperature

damage is frequently accomplished

with resettable PPTC devices.

Page 9: Circuit Protection Considerations for LED Lighting

Circuit Protection Solution for Switch Mode Power Supplies

• PolySwitch PPTC device installed in series with power input helps protect against

damage caused by electrical shorts, overloaded circuits, or customer misuse.

• MOV placed across the input helps provide overvoltage protection.

• PolySwitch device may also be placed after the MOV.

• R1 is a ballast resistor.

Page 10: Circuit Protection Considerations for LED Lighting

Other Circuit Protection Considerations

• LED drivers may be susceptible to damage resulting from improper

DC voltage levels and polarity.

• Outputs may be damaged or destroyed by an inadvertent short circuit.

• Powered ports are also susceptible to damaging overvoltage

transients, including ESD pulses.

Page 11: Circuit Protection Considerations for LED Lighting

Coordinated Circuit Protection

• PolyZen device on the driver input helps provide transient suppression,

reverse bias protection and overcurrent protection in a compact package.

• PolySwitch device on driver output helps protect against damage caused by

short circuits or other load anomalies.

• PolySwitch device can be thermally bonded to circuit board or LED heat sink.

• PESD devices in parallel with LEDs help protect against electrostatic

discharge damage.

Page 12: Circuit Protection Considerations for LED Lighting

PolySwitch Device – How it Works

• Resettable PPTC devices are composed of semi-

crystalline polymer and conductive particles.

• If temperature rises above the device’s switching

temperature the crystallites melt and become

amorphous

• The increase in volume during melting of the

crystalline phase separates the conductive

particles resulting in a large non-linear increase in

the resistance of the device.

Page 13: Circuit Protection Considerations for LED Lighting

PolySwitch Device – How it Works

• The resistance typically increases by three

or more orders of magnitude.

• Increased resistance helps protect the

equipment in the circuit by reducing the

amount of current that can flow under the

fault condition to a low, steady state level.

• The device remains in its latched (high

resistance) position until the fault is cleared

and power to the circuit is cycled.

Conductive composite cools and re-crystallizes, restoring the PPTC to a low resistance state in the circuit and the affected equipment to normal operating conditions.

Page 14: Circuit Protection Considerations for LED Lighting

PolyZen Device – How it Works

• A low resistance, precision Zener diode is thermally coupled to a PPTC “thermal switch.”

• Extended overvoltage or reverse bias conditions will cause the PPTC to “trip” as the

diode begins to heat up.

• A “trip event” causes the PPTC to transition from a low to high-resistance state, helping

protect downstream electronics by generating a series element voltage drop and

preventing thermal runaway of the Zener diode.

Page 15: Circuit Protection Considerations for LED Lighting

Class 2 Power Supply Safety Standards

• Utilizing a Class 2 power source in a lighting system can be an

important factor in reducing cost and improving flexibility.

• Inherently limited power sources – a transformer, power supply, or

battery – may include protective devices as long as they are not relied

upon to limit the output of the Class 2 supplies.

• Non-inherently limited power sources, by definition, have a discrete

protective device that automatically interrupts the output when the

current and energy output reaches a prescribed limit.

Page 16: Circuit Protection Considerations for LED Lighting

Circuit Protection Options

• Coordinated circuit protection strategy employs a metal oxide

varistor (MOV) on the AC input and a PolySwitch device on an

output circuit branch.

• This method can help manufacturers meet the requirements of UL

1310 paragraph 35.1 overload test for switches and controls.

Page 17: Circuit Protection Considerations for LED Lighting

AC Mains LED Lighting Protection

• Metal Oxide Varistors (MOVs) are typically used for transient

overvoltage suppression in AC line voltage applications where

lightning strikes, inductive load switching, or capacitor bank switching

may cause transient overvoltage events.

• A sustained abnormal overvoltage/limited current condition may

cause the MOV to go into thermal runaway resulting in overheating,

outgassing and possibly fire.

• Protecting the MOV from thermal overheating is frequently

accomplished with a thermal cut-off (TCO) device.

• Design may also incorporate a fuse to protect the system from

damage caused by an overload that exceeds a predetermined level.

Page 18: Circuit Protection Considerations for LED Lighting

2Pro Integrated Device

• The 2Pro device combines PPTC technology with

an MOV component into one thermally-protected

package.

• Because the PPTC element is in series with the

MOV, additional overcurrent protection may not

be required.

• This approach helps manufacturers meet industry

requirements, such as IEC61000-4-5 and

IEC60950, and helps reduce component count

and optimize board space.

The 2Pro device helps protect against damage caused by both overcurrent and overvoltage damage.

Page 19: Circuit Protection Considerations for LED Lighting

Integrated Overcurrent/Overvoltage Solution

• 2Pro device’s PPTC element helps

prevent thermal runaway,

maintaining varistor surface

temperature at less than 150°C.

• In the event of an overvoltage

transient the PPTC element of the

2Pro device heats up, trips and

goes into a high resistance state,

helping to reduce the risk of MOV

device failure.

Typical lighting application utilizing a 2Pro device for low-power AC/DC flyback converter protection

Page 20: Circuit Protection Considerations for LED Lighting

Summary

• A coordinated circuit protection scheme can help designers reduce

component count, provide a safe and reliable product, and comply

with regulatory agency requirements.

• Resettable PPTC devices help protect against damage caused by

both overcurrent and overtemperature faults in LED lighting

applications.

• MOV overvoltage protection devices help manufacturers meet a

number of safety agency requirements.

• PolyZen and 2Pro hybrid devices help provide overcurrent and

overvoltage protection in a single device.

• PESD devices provide exceptionally low capacitance in a small form

factor.


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