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Posted on 06 July 2019

Streetlighting Requires Large Numbers of LEDs

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System Requirements and Existing Solutions

Backlighting with LEDs differs from general purpose lighting in the type of LEDs used. Whereas many general lighting applications use less than 10 LEDs of fairly high power such as 1W each, backlighting tends to use hundreds, possibly thousands of small LEDs running at powers of 50 to 200 mW or so. This means that the type of LED drivers used have, so far, been much different. The system architecture has also been much different.

By Christopher Richardson, Systems Applications Engineer for Lighting, National Semiconductor

 

Abstract

With advent of streetlighting (and parking lot lighting, warehouse lighting, etc.) the two worlds have taken a step closer to one another. This is because High Power Wide Area lighting, HPWA, of which streetlighting is a big piece, requires much higher total output power than a light bulb retrofit or a fluorescent tube retrofit. The end result is that a large number of LEDs is needed. Backlighting LED drivers have tackled the challenge of controlling large numbers of LEDs in series-parallel arrays by providing a linear current source for each string and then improving power efficiency by using one switching power supply with a dynamically adjustable voltage output. Up until now such systems were limited in the current per channel to 100 mA or so. National Semiconductor has taken this idea but expanded the power to as much as 500 mA per channel, as well as adding the control and protection features demanded by high reliability outdoor lighting: aka streetlights.

Introduction

Governments and municipalities are better equipped than individual consumers to evaluate the long term, cost of ownership advantage that a well-designed LED-based streetlight offers, so it´s no surprise that more and more cities and towns are considering and installing LED streetlights. Streetlighting for use on public roadways is subject to many standards, in particular regulating the minimum and maximum light output and the beam pattern. The regulations vary from country to country, but they all require higher luminous flux and therefore higher power than other common LED designs such as a light bulb (300-600 lumens, total) or a T8 fluorescent tube retrofit (1000-4000 lumens, depending upon length). A streetlight may easily require 10,000 lumens or more of minimum light output. Small LEDs running at forward currents of 20 to 40 mA are better in terms of lumens per watt, but have two distinct disadvantages. First, it would take thousands of the smaller LEDs to produce the total lumens needed, creating a reliability problem from the sheer volume. Second, having large numbers of small LEDs is perfect for spreading light uniformly over a large surface, but the controlled-beam requirements of streetlighting require lenses which are most efficient when working with a point source. Currently single-die LEDs exist with powers as high as 10W, and these would be better from the optics point of view. Removing that much conducted heat from such a small area is no small challenge, however. The heat generated makes it much more difficult to meet the high expectations for lumens per watt. Current LED streetlights commonly use 50 to 200 LEDs at a drive current 350 mA because this represents the best compromise between luminous efficacy and the total number of LEDs required.

How to drive that many 1W LEDs in a way that is efficient, simple, and economical is a question that backlighting technology can answer. However, to appreciate the advantages of backlighting technology “powered up” to the level needed for HPWA applications Part I of this series will look at the system requirements and the pros and cons of existing solutions.

Using a Single Series String

 LED light output is most proportional to forward current. To match the light output from a given number of LEDs, the simplest solution is to put them all in series. A principal concern of the Single Series String Solution used to be that one LED failing open circuit would darken the entire lamp. The LEDs themselves are now more rugged, many have anti-parallel zener diodes or thyristors which convert potential open-circuit failures into near short circuits, and if the LED does not have built-in protection, several electronic component manufacturers now offer discrete silicon controlled rectifier (SCR) clamps which can be placed in anti-parallel to each LED. Now that open-circuit failures can be forced to behave like short-circuit failures, fear of a single device taking out the entire lamp is no longer the question. Instead, the concern is in determining how many LEDs can fail before the lamp needs servicing. Figure 1 shows a high-reliability single string system using the LM3409/09HV buck LED driver.

High Reliability Buck Driver with a Single String of LEDs

What is more important for a Single String Solution is the total drive voltage, VO. Improvements of indium gallium nitride technology are bringing the forward voltage of a 1W LED closer to 3.0V, but over a distribution of forward voltage bins a VF of 3.3V per device is a reasonable estimate. Put 50 such LEDs in series and the total drive voltage, VO = n * VF is 165V. The maximum acceptable VO is largely determined by safety standards such as IEC, UL, or VDE. For example, to meet IEC specifications for Safety Extra Low Voltage the system must have a transformer isolated output, and the maximum DC voltage on the secondary must be less than 120V. Any serviceable lamp would benefit greatly from SELV certification, and for this reason the LED drive designer often discards the Single String Solution due to high VO , despite the benefits of a guaranteed equal drive current through each and every LED.

Series-Parallel

When an isolated system with a limited secondary voltage is desired, the only choice is to have more than one string of LEDs. While it is tempting to take one large current source as shown in Figure 2 or add series power resistors as shown in Figure 3, these are not acceptable solutions for a streetlight because they cannot guarantee equal drive currents from one string to the next. Cross connecting as shown in Figure 4 is sometimes cited as a way to balance currents and/or prevent one entire string from going dark in the event of an open-circuit failure, but this simply clamps the voltage of each row of LEDs. Upon further inspection, cross connecting still does nothing to match current or prevent even further imbalance should any LED fail open or short circuit.

Series-Parallel Array with One Large Current Source

Series-Parallel Array with Resistor Ballasting

Unbalanced currents cause unbalanced heating which immediately reduces the luminous efficacy of those LEDs taking too much current, as well as causing them to degrade more rapidly. An LED streetlight must put out X numbers of lumens with beam pattern Y, and must be able to operate correctly for years. The only way to guarantee that the system produces the correct amount light both on Day 1 and after five years or 50,000 hours is to have a controlled current source for each string.

Series-Parallel Array with Cross-Connected LEDs

Multiple Buck Regulators

The buck regulator is celebrated among switching regulators because it adapts naturally to use as a current source. Buck regulators are also the simplest, cheapest, and most power efficient of the classic hardswitched topologies, and buck regulator LED drivers of varying power levels and configurations are available from many different manufactures, ranging from control ICs to complete modules. The classic solution to a high power LED drive system with multiple strings of LEDs is to have a buck regulator for each string, as shown in Figure 5. Because the buck can only step down the output voltage, it is easy to guarantee the maximum DC voltage on the secondary by making the input voltage to the buck regulators less than the desired threshold. Due to the historical prevalence in telecom and the common secondary limit of 60VDC, 48VDC is a popular input voltage. This also gives a comfortable margin between minimum input voltage and maximum output voltage (LED string voltage) for a string of 12 InGaN LEDs in series.

Dedicated Buck for Each String. All VO is less than VIN

The benefits of having a buck regulator for each string of LEDs start with the guarantee of matched current from string to string – critical for high quality systems with long lifetimes. Each buck can be dimmed individually, either with PWM or by linear adjustment of LED current. Some buck LED drivers also provide fault reporting, and all buck regulators can be designed for high power efficiency over a range of output voltage. In case of faults, each buck can report to a system microcontroller and one or more can be shut down while leaving the rest enabled.

Drawbacks to Multiple Buck Solutions

The first drawback to having a buck for each string of LEDs is cost, though in streetlighting and high reliability HPWA this is not as important as in consumer products. Each buck needs a power inductor, input capacitor, and in most cases a Schottky rectifier and a power resistor for current sensing, along with assorted small resistors and capacitors for various analog functions. A second, important drawback to the Multiple Buck system that is known to switching power supply designers but might otherwise be overlooked is the electromagnetic interference (EMI) that can be generated when multiple switching regulators of significant power are all fed from the same input power rail. All switching regulators generate EMI and their fundamental switching frequencies and at the harmonics of the switching frequency. When two or more switchers that run at almost the same frequency run from the same source, additional conducted EMI known as beat frequencies often shows up at the input. Beat frequencies occur at both the sum and difference of two close switching frequencies. Buck regulators have a smooth output current which is perfect for LED driving, but they draw large discontinuous pulses of input current, which makes them even more likely to cause beat frequencies than boost regulators. This interference can be eliminated if the switching frequencies are synchronized, but unfortunately most buck LED drivers are not fixed frequency, clock based systems. Non-synchronizable hysteretic and constant on-time/constant off-time (COT) control is more common because it adapts easily to the dynamic load of an LED array, and also because hysteretic and COT control provide fast slew rates for the best PWM dimming performance. The solution requires very careful PCB layout and heavy input filtering for each buck. In many cases a discrete input inductor is needed as shown in Figure 6, something that is not desirable even if space and cost are not as tightly controlled as with a consumer product.

Input Inductors and Careful PCB Layout Help Prevent Beat Frequencies

Conclusion

Streetlights and HPWA lighting are not the first applications to tackle the dilemma of powering many more LEDs than can reasonably be placed in a single series chain. Starting with small LCD screens for mobile phones, LEDs for backlighting have found use in larger and larger screens, moving to GPS and automotive infotainment, to laptops, and now to large-screen LCD televisions. While buck regulators dominate the high power LED driver space, boost regulators coupled with multiple linear regulators are the preferred choice for backlighting systems. Part II of this article series will explore how this type of LED drive system which previously found use only in drive currents in the 40-60 mA range can be expanded in scope and power to solve the main problems of streetlighting and HPWA lighting.

 

 

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