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Circuit Protection Considerations for LED Lighting
- 1. Circuit Protection Considerations for LED Lighting Matthew Williams, Global Applications Engineering Manager Tyco Electronics Circuit Protection Business Unit
Notas del editor
- • 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.