circuit protection considerations for led lighting
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Circuit Protection Considerations for LED LightingTRANSCRIPT
Circuit Protection Considerations for LED Lighting
Matthew Williams, Global Applications Engineering Manager
Tyco Electronics Circuit Protection Business Unit
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
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.