The advent of LED lighting in the past few years has now matured to the point where sweeping changes exist in the way we think about illumination. LED lighting was once the darling of scientists and “early adopters” but has now gone “mainstream”. The technology promises a virtual end to skyrocketing electric bills, the cold glow of fluorescent bulbs, and a life expectancy that will make changing a light bulb so infrequent that we may require instructions on how to do it properly! But does LED lighting actually deliver on these promises?
On a tour of a recently renovated industrial space earlier this year, the host was detailing some of the features of the LED light fixtures that were providing general lighting to the factory floor. His statement was something on the order of “They are really good lights – and they have a seven year warranty!” There was not a hint of doubt in his voice, but then he's not on the hook for repair/replacement costs if the lights don't actually last that long. If those LED lights fail early – well, that's somebody else's problem – unless it was your employer that sold the LED lighting fixture, then maybe that “somebody else” is you!
Let's look at five LED lighting applications with large markets and discuss the threats to the lighting system as well as the various circuit protection strategies that can be deployed to “harden” the design for maximum reliability.
The primary focus will be to invest in circuit protection where there is value. Of course, the safety rating agencies (UL, ETL, etc.) will demand the minimum of fire protection and personnel safety measures. The real goal of this article is to cost effectively extend the useful life of the protected equipment (an issue not addressed by the safety folks). Value goes beyond just the retail value of the light fixture (although that is surely a factor) and includes valuable function (where failure may put people or other equipment in danger), difficult access (where repair or replacement access costs may be an order of magnitude larger than the equipment cost), or equipment sold into markets where reliability is a point of competition between vendors.
AC General Lighting
Figure 1
The block diagram in Figure 1 shows a typical industrial lighting fixture. The third block shows a transformer-less rectification of the AC line (BR), power factor correction (PFC) circuitry, and most industrial lighting fixtures include a capacitor bank to smooth the 120Hz ripple to an acceptable level. Fire safety also requires a fuse on the input, but the addition of overvoltage protection throughout the design will help assure that the fuse will never blow by protecting the critical electronic elements that would otherwise fail and cause excessive current flow at the input. The diodes in the bridge rectifier (BR) are often the first casualties of a voltage surge.
Without input overvoltage protection, AC input surges will find their way into the DC/DC voltage-to-current converter block where the switching electronics chop the high voltage DC input into usable current packets for the LEDs. These electronic devices are rated for the hundreds of volts found in these circuits, but are easily destroyed by a single event that exceeds their rating – and nearby lightning strikes or industrial motors being switched off can cause voltage spikes of several thousand volts.
A network of high capacity metal oxide varistor (MOV) devices right at the AC input are the solution of choice to limit voltage surges. MOVs do age a bit with every surge absorbed, so make sure you size them generously. In streetlight applications, where the electronics are essentially sitting atop a lightning rod, MOV devices in conjunction with line voltage silicon TVS diode devices are usually recommended. The Littelfuse AK10-170C is one choice for 120V line voltages. TVS devices provide precise voltage clamping with no aging characteristic, but TVSs also represent a substantial investment. If repair access costs are expected to be very high and utility service trucks and crews need to be called, the additional expense for designing with the best in circuit protection can often be justified. These high repair costs may have to be borne by the manufacturer if the unit is still under warranty.
In some industrial designs, the AC input fuse must be sized to withstand the surge currents of the very large MOV devices, as in Figure 1. In these cases, the fuse may not be sensitive enough to open on faults in the switching section (DC/DC conversion block in the same block diagram.) However, a downstream fuse can be placed after the line rectification stage (the BR + PFC block) that is a closer match to the operating current. But since this is now a DC circuit, specialized high voltage DC fuses must be used. The Littelfuse 477 Series fuses were designed with this application in mind.
In some designs, the LEDs themselves are actually remote with relation to the driver electronic circuitry. There are advantages to keeping the heat from the LEDs away from the driver electronics. In some streetlight applications, the electronics are in the base and the LEDs are atop the pole. In these designs it may be appropriate to provide additional TVS devices to protect the LED strings from induced high voltages, since long wires from the driver circuitry to the LEDs will act as antennae (see TVS block after the DC/DC conversion, Figure 1).
Designs that utilize wire-bonded LED structures that are wired into long, high voltage series strings have an additional failure mode where heat cycles, or mechanical vibration causes the wire bond to break, resulting in an open circuit LED. The solution is to use open LED bypass devices (Figure 1) that will keep the balance of the string lit, bypassing the open LEDs. Without these devices, a single LED failing as an open circuit would extinguish the entire string. Some warranties may allow the failure of a certain percentage of the LEDs as long as the fixture maintains a specified minimum light output.
AC-sourced LED Light Fixtures (LED Light “Bulbs”)
Figure 1 is also the block diagram for a typical LED AC light fixture but some of the protection elements would not be used. LED lighting fixtures have all of the same operational circuits as their larger and much more expensive industrial counterparts, but the threats and motivation for circuit protection are quite different.
LED light fixtures are sold with minimal warranties and compete largely on price. Therefore, the pressure is on the designer to drive out costs, and circuit protection is usually an easy place to cut corners because it isn't essential to the function of the light bulb. Fire safety requirements can sometimes be met by using a thin circuit board trace as a “poor man's fuse”. This may meet the safety tests where components are shorted or opened, but they will not interrupt overload conditions caused by leaking electrolytic capacitors on the input stage. The resulting failure modes are scary with smoke and bad smell as a real possibility. The use of an actual fuse is the only real solution.
As in the industrial lighting solution, an MOV on the input is a must, as the voltage withstand margins on the electronic components are much smaller on these price competitive goods. Space constraints and budget issues will likely drive the selection to a much smaller MOV device. A smaller MOV will not protect as well, but no protection at all can lead to early failures at the first lightning storm. After investing in an LED lighting fixture, consumers will likely switch preferences to another brand if they have a bad initial experience.
AC Task Lighting
Incandescent light is emitted from a wire suspended in space. Incandescent bulb-designers spent a lot of time and energy trying to redirect the light into a particular direction and beam pattern. One natural advantage of LEDs is that LED light is emitted from a surface and is already very directional – so they are a natural fit into under-counter lighting and desktop lamps. Usually the LEDs are integrated into the fixture – so there are no replaceable parts. Figure 1 is again the block diagram for the above. An MOV is once again the guardian of the AC input, and are usually sized as a percentage of the total design cost. Thus, more expensive units will warrant a larger and more effective MOV device.
One important aspect of task lighting is that it can be unplugged and is often portable. Typically housed in plastic materials that do not drain away static charge, ESD becomes a potential issue. The most vulnerable components are the LEDs themselves. Usually ICs and other electronics are protected by various components on the controller PCB, although not always. One scenario for disaster is that someone decides to move an LED desk lamp from one side of a carpeted room to the other. It is unplugged, grasped by the light-emitting end, walked across the carpet and when the user goes to plug it in, the static charge built up on the journey is discharged from their hand, through the LEDs, through the driver electronics, through the cord and back to earth. (“Honestly! It worked over there, but now it's dead!”) ESD protection around the LEDs is key to the survival of these designs.
Indicator Lighting
Yes, you can argue that using LEDs as indicators isn't really a lighting application. But there are so many devices in some well-equipped houses that nightlights have been completely eclipsed by an array of LED indicators – especially now that so many pieces of electronics have an LED that indicates that the power is off! For LED indicators, a DC circuit that uses a simple resistor to drive the LED with the appropriate current from a fixed DC voltage supply is usually employed. While not particularly power efficient, we're only talking about 2 – 10 mW. The key is that these indicators are placed on front in control panels and sometimes inside control buttons – making them prime targets for ESD. Some LEDs come equipped with an integral ESD suppressor – but many don't, so it's a good idea to check the data sheets carefully.
Automotive Lighting
The LED is a natural fit in automotive applications:
- LEDs are driven by low voltage DC currents – so simple DC-DC converters are all that is needed. Buck converters are often chosen, as input/output isolation is not required.
- LEDs are solid-state, which makes them more reliable in a high vibration environment.
- LED's lighting efficiency helps reduce demand on automotive electrical systems that have become very large and complex.
LED fixtures will still need replacement after an accident or failure; therefore they still come in a bulb-like form – sometimes with the driver electronics integrated into the bulb form factor, too. This makes them the DC-input cousin to the AC LED replacement bulbs. However, lacking the AC input circuitry, these automotive bulbs need to have ESD protection on their inputs.
Also, consider the need for reverse polarity protection because of the possibility of being installed backwards or due to an incorrect jump-start procedure that momentarily drives reversed voltages throughout the vehicle. Note that unidirectional TVS diodes automatically provide reverse polarity protection.
Finally, the system DC voltage in a vehicle is not as steady and stable as you might expect. There are surge requirements for transient system overvoltage conditions; conditions often caused by a sudden removal of a large load such as a window defroster or a seat motor. The lighting systems must be able to withstand such overvoltage conditions. TVS diodes are the typical solution. For sustained overvoltage conditions, resettable PTC devices are often used in conjunction with the TVS diodes. TE Connectivity offers the PolyZen line of circuit protectors that thermally integrate a combination of TVS and PTC devices, producing a pre-matched protection solution for a given operating voltage and current. There are several models that work well at automotive voltages and currents, and are equally suitable for several other electronic applications.
Figure 2
Figure 2 shows a typical arrangement of protection devices for an automotive circuit.
In summary, circuit protection is an insurance policy, to be included in all good designs. Cost savings by omitting it can result in a shortened life and a reputation for poor quality. LEDs have a very long life if they are sufficiently protected. One last item to consider is making sure you get genuine circuit protection devices. Mouser is an authorized distributor for electronic components, which means that stringent procedures ensure genuine parts from original sources, and there is a large selection of genuine circuit protection components at Mouser Electronics. Design with circuit protection in mind, and you can make designs that last.
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