Tips for designing LED bulbs using transient and surge protection devices

BY
WILLIAM SHENBERGER, Field
Applications Engineer
Littelfuse, Inc.
www.littelfuse.com

Designers
of early indoor LED light bulbs faced numerous technical hurdles, including AC-to-DC
conversion, thermal heat-sinking, constraints imposed by current bulb sizes, electrical
transients, and not to mention the basic challenge of driving the LEDs that
produce the light. Providing protection from transients for both the LEDs and
all of the components upstream from them in the circuit represents a significant
design challenge. These transients often result from lightning-induced surges
on the AC input. LED bulbs require both overcurrent and overvoltage protection
from these threats.

The
demand for added functionality and higher light output has increased the number
of LED board components. Higher light output creates a demand for larger heat
sinks to increase heat dissipation. Because LED bulbs must be form-factor-compatible
with current incandescent and CFL bulbs (such as the A19 household bulbs), they
include an AC/DC power supply circuit so that they can operate from standard
bulb sockets (Fig. 1). Anything directly connected to
an AC power source can be damaged by short-circuit and overload conditions
caused by component and/or circuit failures inside the bulb. In addition,
lightning surges or load switching transients (originating outside the bulb)
can create voltage spikes or ring waves that can stress and ultimately damage
components inside the bulb.

Fig. 1: Construction of a typical
residential LED lamp.

0618_Feature_CPD_Fig-2

Fig. 2: Typical LED luminaire driver
circuit with transient and surge energy protection devices.

The
AC line fuse is the bulb’s primary overcurrent protection device. When properly
selected, this fuse will adequately protect all downstream components from
electrical overstress (EOS) damage by safely disconnecting all circuitry from
the AC line input.

0618_Feature_CPD_Fig-3

Fig. 3: Fast-acting subminiature
surface-mount fuses, like the Littelfuse NANO2 470 and 476 Series, provide surge
tolerance and circuit protection against overload conditions for downstream
components.

Given
the tight space constraints associated with LED bulb design, it is critical to
select a highly compact AC fuse for the AC input. A fuse’s function is to
provide protection for components and complete circuits by reliably and
predictably melting under short-circuit and current-overload conditions. The right
fuse in series with the AC line input will provide the needed protection. Today,
AC fuses are available in the smallest of form factors with a wide choice of amperage
and voltage ratings. A range of additional key parameters and surface-mountable
designs are also available to allow design engineers to choose a fuse that will
satisfy all of the requirements of the application.

The
primary overvoltage protection (OVP) device for an LED-based light bulb is an
AC input circuit metal-oxide varistor (MOV). When properly selected for all
required design parameters, it will protect all downstream components from EOS
damage from induced transients and ring-wave effects by clamping short-duration
voltage pulses. MOVs offer a cost-effective way to minimize transient energy
that could otherwise make its way into downstream components. Proper MOV
selection is based upon a number of electrical parameters, including the
voltage rating, peak pulse current, energy rating, disc size, and lead
configuration.

0618_Feature_CPD_Fig-4

Fig. 4: Thermally protected metal-oxide
varistors (TMOVs), including the LV UltraMOV from Littelfuse, act as an AC
input circuit’s primary overvoltage protection device for an LED-based light
bulb.

LED
bulb designers need to consider several circuit protection issues:

  • Answer technical questions about the
    application such as: the bulb’s normal operating current, application voltage,
    ambient temperature, overload current level and length of time within which the
    fuse must open, maximum allowable fault current, and the pulses, surge
    currents, inrush currents, startup currents, and circuit transients. Littelfuse
    offers numerous design resources including the Fuseology
    Selection Guide: Fuse Characteristics, Terms, and Consideration Factors
    and the Littelfuse iDesign Fuse Selection Tool.
  • Know early
    in the design process the market in which the bulb will be sold. Depending on
    the geographic locations where the bulb is intended for use, different
    standards will govern design and testing requirements.
  • Determine
    the size limitations that might affect the fuse that can be used. Fuses are
    available in a variety of packages, but surface-mount designs are the most
    common form factor for LED bulbs. Smaller footprint fuses are now available to
    protect the AC input, some just half the size of the smallest fuses previously
    available.
  • The
    fuse temperature generated by the current passing through the fuse increases or
    decreases with ambient temperature change. Remember, the fuse’s “ambient
    temperature” is not the same as the “room temperature.” Instead, ambient
    temperature is the air temperature immediately surrounding the fuse, which is
    often much higher than the room temperature. For ambient temperatures at about
    25°C, it’s recommended that fuses operate at no more than 75% of
    their nominal current rating. Fuses are temperature-sensitive devices, so even
    small temperature variations can greatly affect the predicted fuse life when it
    is loaded to its nominal value (i.e., 100% of rating).
  • Determine
    the application’s required breaking capacity (i.e., interrupting rating, I2t). This is
    the maximum approved current that the fuse can safely interrupt at rated
    voltage. During a fault or short-circuit condition, a fuse may receive an
    instantaneous overload current many times greater than its operating current.
    Safe operation requires that the fuse remain intact and clear the circuit.
  • Transient
    suppression must be considered at the beginning of the design process. Coordinate
    the fuse with the downstream OVP and the LED string driver circuit. The
    selected fuse must withstand the energy of the transient at the specified level
    so that the LED string driver circuit capabilities are not adversely affected.
    The AC input circuit fuse and MOV provide transient protection that allows the
    needed overvoltage clamping capability to occur without the fuse opening while
    safely protecting the downstream circuitry, resulting in minimal disruption to
    the LED string driver circuit, including the LED string itself.
  • In
    cases in which the bulb’s operational circuitry is not able to survive the
    required transient event levels, consider adding a secondary TVS diode for OVP.
    This is a proven solution that further clamps the “let-through” energy from the
    MOV. For extreme cases, consider additional OCP/OVP devices (see Fig. 2) to provide even more robust circuit protection.
  • Other
    design considerations, such as the coordination of the fuse with the overvoltage
    protection and LED driver, are also important.
  • Allow
    sufficient time for thorough application testing and verification prior to
    production.

Conclusion
Investing the time and resources necessary to
ensure adequate circuit protection from the earliest stages of an LED-based
light bulb design project will be rewarded with successful products and
satisfied customers. Thanks to the latest advances in AC fuses, MOVs, and TVS
diodes, the next generation of LED bulbs might just be around until the next
generation of designers arrives.