Powering and protecting LED lighting
31 August 2017
From high bay and architectural lighting to outdoor street lighting, and even 75W incandescent bulb replacement, LEDs are fast becoming the lighting source of choice – despite costing more than incandescent lighting by a factor of 13! This article discusses why this is, and considers what LEDs need to power them – and also, how to protect them from the hazards they face within their respective implementations.
This article originally appeared in the September 2017 issue of Electronic Product Design & Test; to view the digital edition, click here – and to register to receive your own printed copy, click here.
LEDs are not ‘heaters’ like incandescent bulbs, which are essentially resistors; they are more comparable to diodes. As such, they need tightly regulated current and voltage, which is provided by an LED driver circuit. This circuit is susceptible to open or short circuit conditions, however: so, care must be taken during the design process to protect against potential occurrences. Also, high temperatures impair an LED’s useful light output, thus demanding careful driver circuit and housing design.
Unlike incandescent bulbs, which can take AC mains voltage directly, LEDs need regulated DC voltage and current, meaning AC to DC conversion may be required (depending on the power source). Whenever a conversion process occurs, there is always the potential for mishaps. Often, these events can harm or disable the LED driver circuit – without which, the LEDs cannot light themselves! Therefore, it is necessary to protect the LED driver circuit from such occurrences.
LEDs versus the alternatives
Table 1 shows a comparison between LEDs, CFLs and incandescent light bulbs. The ‘green’ highlights help you identify the desirable properties for each respective light source.
Notably, one key feature of LEDs is their efficacy. At 80 lumens per watt of light output, LEDs are at least 8 times more power efficient than incandescent bulbs. Or, to put it another way, LEDs use one eighth of the electrical power of an incandescent bulb to attain the same light output. This equates to considerable energy cost savings over the installation’s lifespan.
LEDs can last at least 50 times longer than their incandescent counterparts. Therefore, there is a point at which the higher initial cost of LED bulbs is offset by reduced energy costs. In fact, by simply replacing the five most used light bulbs in the average home, these energy cost savings can be realised in less than one year. Of course, there are other benefits to using LEDs, such as their containing no toxic materials and having no failure modes – provided that their power supply is designed to power and protect them correctly!
Care and feeding of LEDs
To reiterate, an LED is the electrical equivalent of a diode. As such, it is a current driven device; therefore, simply applying the correct amount of current through the LED will allow it to attain its specified light output in lumens per watt. Correspondingly, supplying less current will limit the amount of light output that it is capable of delivering. For a single LED, this may not seem significant; however, if there are many LEDs in a series string, and they are not receiving uniform current, the light output will vary unmistakably.
Of course, you also have to overcome an LED’s inherent forward voltage drop, which can vary depending on the type of LED and the end fixture configuration. For a typical white LED, this forward voltage drop is usually around 3.5V, but can be slightly higher at elevated temperatures. Depending on the input power source, multiple conversion topologies will likely be needed. This can be further complicated by different LED string configurations, including series (LED forward voltage drop additive), parallel (LED current additive), or a combination of series and parallel strings (both LED forward voltage and current additive).
Having now covered what LEDs need for correct operation, let’s consider what can negatively impact their operation and those of its driver circuits. Many things can adversely affect an LED’s output, or even lead to its catastrophic failure. These consist of overvoltage, usually occurring during an open LED event, or an overcurrent condition, which normally happens during short circuits or the re-plugging of an LED string. Of course, poor thermal environments can also adversely affect an LED’s useful life, so good thermal design is critical too. Once again, the LED driver circuit is critical, and so it is equally imperative that care be taken on its design – so that it can protect itself, and therefore the LED, from opens, shorts and poor thermal environments (issues commonly encountered in most automotive, avionic and industrial applications).
A critical requirement when powering an LED is to correctly deliver a tightly regulated current. The LED driver circuit is key here, since it must take whatever the input power source is (which can vary widely, given the broad range of applications), and convert it to the required voltage and current levels to best optimise the LED’s performance. Overvoltage or current can impair both the LED’s life or its light output over time; so, the tighter they can be regulated, the more robust the system will be. Therefore, having no more than +/–5% voltage and current regulation is beneficial for a long and trouble-free operating life.
Another key criterion is to provide overvoltage protection. Therefore, LED driver circuits that can handle higher transient voltage conditions above those required for normal operation is a prerequisite (such as in an automotive environment, where load dump may be over 42V).
Protecting LEDs from thermal overstress, however, is more challenging. Quite often, the LED deployment fixture is small and compact with very little in the way of heat sinking –and usually no fans to provide air cooling. Therefore, most of the heat must be dealt with via conduction. As a result, good heat sinking must be considered at the design stage. Also, having an LED driver circuit that operates with very high conversion efficiency is important here: with higher conversion efficiency, less heat is produced via conversion power losses. Thus, LED drivers with low to mid-90% efficiencies significantly aid good thermal design.
Another way to help with thermals is to include an on-chip temperature sensor as part of the LED driver circuit: a system microcontroller could monitor this temperature signal, and throttle back the current so as to allow less heat generation. While this would obviously lead to reduced light output, this is preferable to complete system failure. Once the fault condition goes away, normal operation can resume.
LED driver solutions
The following section highlights LED driver ICs which incorporate some of the protection mechanisms discussed above.
LT3795 is a boost topology-based LED driver which incorporates special features to provide enhanced performance. One is spread spectrum frequency modulation, whereby the system clock is dithered to lower the noise floor. Another is short circuit protection, which for a boost converter is hard to implement – unlike a buck converter, where it is inherent, due to its step-down topology and its switch between VIN and VOUT. For a boost converter to provide short circuit protection, a disconnect FET must be added above the LED string. This P-channel MOSFET allows monitoring of the current flowing through it, since if a short circuit were to occur it would rise rapidly. This would potentially disable the LEDs in the downstream string. To prevent this from happening, we need to monitor its current, which can be done with the sense resistor RLED above it. Thus, when a short occurs, to protect the LEDs, M2 needs to be
turned off in less than 1 microsecond, which the LT3795 can achieve (see Figure 1).
In Figure 1, the top trace is the current flowing through the LED string and the middle trace is the current through the P-MOS. The bottom trace represents a transient short between rails. As you can see, the LT3795 turns off the P-FET, which rises to 12A peak in just 500 nanoseconds, thereby protecting the LED string from an overcurrent condition. Without this fast response, this current could go as high as 50A.
The LT3797 is a multi-topology triple output LED driver with an integrated rail-to-rail current sense amplifier and a voltage range of 0V to 100V. Each of its three channels can be configured for Buck, Boost or SEPIC mode of operation, and each output can be operated autonomously with one another.
This IC adds a few features that give additional system protection, including short-circuit protection when a channel is operated in boost mode. Nevertheless, it also incorporates open LED protection, which is afforded by the FBH voltage feedback pin. The LT3797 has a wide input range of 2.5V to 40V, making it ideal for industrial, avionic, medical and automotive LED lighting applications. Furthermore, because it is a controller, current levels in the multiple amps range are attainable.
Figure 3 shows the LT3797 configured as a triple boost LED driver, so each of its three channels are boost controllers. As can be seen, a P-MOS MOSFET and sense resistor are configured above each LED string. Thus, as with the LT3795, when a short-circuit condition occurs, it can quickly disconnect the LED string and protect it from damage.
Also like the LT3795, the LT3797 allows for latch-off or hiccup mode for a restart of each channel. The mode of short-circuit protection restart can be easily programmed with the addition – or omission – of R13, between the VREF and SS pins.
When it comes to how to power and protect LEDs and their driver circuitry, there is a great deal to consider in designing the complete system, including voltage, current and thermal constraints. Nevertheless, there are ICs available that can ease and facilitate your design needs, while also optimising all performance matrices. May the light be with you.
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