William Shenberger, Field Application Engineer at Littelfuse, offers his tips for Designing LED Bulbs using transient and surge protection devices. 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, not to mention the basic challenge of driving the LEDs that produce the light.
Providing protection from transients for both the LEDs and all 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 over current and over voltage 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 they can operate from standard bulb sockets. 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.
The AC line fuse is the bulb’s primary over current 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.
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 the requirements of the application.
The primary over voltage 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 minimise 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.
LED bulb designers need to consider several circuit protection issues including:
Investing the time and resources necessary to ensure adequate circuit protection from the earliest stages of a 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.