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

Circuit Protection Considerations for LED Lighting

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  • • Low energy consumption retrofit bulbs range from 0.83 to 7.3 watts • Long service life LED bulbs can last up to 50,000 hours • Durable LED bulbs are resistant to thermal and vibrational shocks and turn on instantly, making them appropriate for applications that are subject to frequent on-off cycling • Help meet safety and green initiatives LEDs remain cool to the touch and contain no mercury • Fully dimmable LEDs do not change color when dimmed, unlike incandescent lamps that turn yellow • No frequency interference no ballast to interfere with radio and television signals
  • For example, an LED may convert close to 10.72 watts of heat (13.4 * 0.8). Of this, 9.648 Watts (10.72 * 0.9) of heat is transferred or removed from the junction via conduction.
  • The optical behavior of an LED varies significantly with temperature. The amount of light emitted by the LED decreases as the junction temperature rises and, for some technologies, the emitted wavelength changes with temperature. If drive current and junction temperature are not properly managed, the LED’s efficiency can drop quickly, resulting in reduced brightness and shortened life. Another LED characteristic, related to junction temperature, is the forward voltage of the LED (Figure 2). If only a simple bias resistor is used to control the drive current, VF drops as temperature rises and the drive current increases . This can lead to thermal runaway , especially for high-power LEDs, and cause the component to fail. It is common practice to control junction temperature by mounting the LEDs on metal core PCBs to provide rapid heat transfer.
  • LEDs are driven with a constant current , with the forward voltage varying from less than 2 to 4.5V, depending on the color and current. Older designs relied on simple resistors to limit LED drive current, but designing an LED circuit based on the typical forward voltage drop as specified by a manufacturer can lead to overheating of the LED driver. This may occur when the forward voltage drop across the LED decreases to a value significantly less than the typical stated value . During such an event, the increased voltage across the LED driver can result in higher total power dissipation from the driver package, which may degrade performance or lifespan. Today, most LED applications utilize power conversion and control devices to interface with various power sources, such as the AC line, a solar panel or battery power, to control power dissipation from the LED driver. Protecting these interfaces from overcurrent and overtemperature damage is frequently accomplished with resettable polymeric positive temperature coefficient (PPTC) devices.
  • PPTC circuit protection devices are made from a composite of semi-crystalline polymer and conductive particles . At normal temperature, the conductive particles form low-resistance networks in the polymer. However, if the temperature rises above the device’s switching temperature (TSw) either from high current through the part or from an increase in the ambient temperature, the crystallites in the polymer 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.
  • The resistance typically increases by three or more orders of magnitude. This 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 – at which time the 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 . Because PPTC devices transition to their high impedance state based on the influence of temperature, they help provide protection for two fault conditions – overcurrent and overtemperature. Overcurrent protection is provided when the PPTC device is heated internally due to I2R power dissipated within the device. High current levels through the PPTC device heat it internally to its switching temperature causing it to “trip” and go into a high impedance state. The PPTC device can also be caused to trip by thermally linking it to a component or equipment that needs to be protected against overtemperature conditions.
  • The polymer protected Zener diode microassembly consists of a low resistance, precision Zener diode that is thermally coupled to a PPTC (polymeric positive temperature coefficient) “thermal switch.” In operation, 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 “thermal switch” to transition from a low to high-resistance state, helping protect downstream electronics by generating a series element voltage drop. Also, by limiting the current, it helps to prevent Zener diode failure due to thermal runaway.
  • 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. Under normal operating conditions the AC line voltage applied to an MOV is not expected to exceed the device’s maximum AC root mean voltage (VAC RMS ) rating and, provided that the transient energy does not exceed the MOV’s maximum rating, short-duration transient events are clamped to a suitable voltage level. However, a sustained abnormal overvoltage/limited current condition, such as a loss of neutral, 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, placed in series with the MOV. A typical line voltage transient protection scheme may also incorporate an overcurrent protection element, such as a fuse, to protect the system from damage caused by an overload that exceeds a predetermined level.
  • Tyco Electronics has developed a new integrated device that helps provide resettable protection in case of overcurrent or overvoltage events.
  • Standard unprotected MOVs are typically rated to 275VAC RMS for a universal input voltage range. In a loss of neutral condition they can overheat with negative consequences, even if a fuse or power resistor is used upstream.

Circuit Protection Considerations for LED Lighting Circuit Protection Considerations for LED Lighting Presentation Transcript

  • 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 V F (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.
    • V F 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.