Categories LED

LED High Bay Luminaires – Ready for Prime Time

High bay lighting is the most common type of lighting used in commercial facilities that have high ceilings and require high foot-candle levels. They are ideally suited for warehouses, cold storage, airport concourses, grocery stores, gymnasiums, convention centers and other large indoor spaces with mounting heights between 15 and 40 feet, and ambient temperatures between -4°F and 131°F.  While high bay lights have traditionally used high intensity discharge (HID), metal halide (MH) or florescent lamps, many specifiers and facility managers are changing to LED luminaires. 

Properly designed and engineered LED-based high bay luminaires can offer big advantages for commercial applications. However, it’s important to consider LED luminaires that take a systems-level approach that includes driver design and thermal management, rather than just retrofitting LED “bulbs” into existing fixtures.  Thermal management is critically important to achieve the reliability expected from LED luminaires.  Extreme temperatures, both hot and cold, are common in high bay environments and can have a negative impact on the performance of electrical components.

Compelling LED Lamp Life 

  • 50,000 hours or better (5 years or more)
  • Minimizes the cumbersome maintenance of high ceiling applications

Advantages of LED Technology

  • Exceeding government mandated efficiency standards
  • Controlled distribution of light for enhanced uniformity
  • Higher luminaire efficacy

Let’s examine in more detail the many advantages of LED technology for high bay fixtures and a few application examples.

Warehouse Lighting
According to the Department of Energy, lighting uses as much as 29 percent of the electricity generated in the US and for industrial facilities, traditional lighting:

  • Uses 38 percent of the energy in a typical warehouse
  • Requires 15 percent of the energy in a refrigerated warehouse
  • Consumes 75 percent of a warehouse facility’s energy expenditures when maintenance is factored in with energy costs

Here’s where LED luminaires’ dramatic energy efficiency really makes an impact, particularly because many facilities that illuminate with high bays are in operation 18 to 24 hours a day. Typically, lighting is viewed as a fixed expense, but it shouldn’t be; energy costs can be dramatically reduced, up to 75 percent, and maintenance can be virtually eliminated through the installation of LED luminaires. Additionally, paired with occupancy sensors and/or dimmable components they provide even greater energy efficiency.

Further power savings are achieved from turning off the fixtures when not in use. Workers often leave the traditional lights on continuously because they take so long to warm up to full brightness. LED luminaires light immediately, eliminating the need to have them on all the time.

Many LED retrofit installations don’t require a one-to-one replacement so the combination of using fewer fixtures for shorter periods of time provides a lower energy bill and significantly reduced maintenance expense.

Cold Storage Lighting
With large, open spaces to cool, as well as sizable lighting requirements, cold storage facilities can consume vast amounts of energy.  As in any business, owners and managers of cold storage warehouses are often faced with minimizing their operating costs.  The energy used by the refrigeration system is often a major contributor to this cost of operation.

Conventional lighting and refrigeration systems typically work against each other.  Lighting systems generate heat, which the refrigeration system needs to remove.  In addition, lower temperatures typically reduce the efficacy of lighting systems.  Therefore, more power is required to generate the desired illumination, which in turn, increases the load on the refrigeration system.

Facilities can save tens of thousands of dollars in yearly electric costs, and cut harmful emissions by thousands of tons by implementing a handful of simple, cost-effective efficiency measures to reduce electrical consumption and have a payback period of three years or less such as installing LED luminaires. [1]

Only certain technologies, such as LED luminaires, are capable of functioning for cold storage needs at temperatures that range from zero degrees to -40°C.

Gymnasium Lighting
For years, the standard method of lighting gymnasiums has been the 400 W MH high bay.  This has led to gyms with deteriorating light levels and poor playing conditions that are expensive to operate. The MH system is essentially an “on-off” system that provides little control over light levels.  Also, these lights require 10 minutes or more before they reach their full light level.  After they are turned off, they require a similar amount of time before they can be turned back on again.  As a result, these lights are typically turned on in the morning and kept on until the building closes, regardless of whether there are any activities in the gym.  Additionally, MH use a lot of energy but produce less light as they age, giving gyms and other facilities poor illumination.

LED high bay luminaires deliver instant white light with no restrike or run-up delay.

LED Lamp Life
Correctly designed LED will not fail catastrophically, but rather slowly dim. LED luminaires are determined to have “failed” when light output reaches 70 percent of original output. In fact, well designed fixtures can last over 50,000 hours making non-scheduled equipment downtime due to lamp failure nonexistent.

How long is 50,000 hours?
Based on the length a fixture is illuminated per day, here is what a 50,000 lifetime translates into on an annual basis:

Hours of Operation –  50,000 hours is:
24 hours a day 5.7 years
18 hours per day 7.6 years
12 hours per day 11.4 years
8 hours per day           17.1 years

With LED luminaires, maintenance costs are minimized as relamping may not be required during usable lifetime of the product. Another important consideration given that high bays are ceiling-mounted and may need the use of a lift to change out the burned fixture.

LED Illumination – Ready for Prime Time
Debate continues about whether LEDs have the output in lumens. Through advancements in technology and manufacturing, bright white LED luminaires for commercial lighting applications are in the market. Recent legislation in the US has led to the phase-out of mercury vapor ballasts and lamps as well as 150 to 500 watt MH luminaires. LED technology fills these needs, while far exceeding government mandated efficiency standards.

A LED luminaire incorporates an array of point sources that direct light precisely where it’s needed, with very little scattering or loss. Light distribution is controlled by the placement of LEDs, as well as by efficient use of optics that take advantage of the focal point presented by each individual LED.

Since traditional lamps are high-intensity near-point sources, the optical design for these luminaires causes the area directly below the luminaire to have a much higher illuminance than areas farther away from the luminaire. In contrast, the smaller, multiple point-source and directional characteristics of LEDs can allow better control of the distribution, with a resulting visible improvement in uniformity.

LED luminaires use different optics than traditional lamps because each LED is, in effect, an individual point source. Effective luminaire design exploiting the directional nature of LED light emission can translate to lower optical losses, and higher luminaire efficacy.

Categories Lighting

The March of Mid-Power LEDs into General Lighting

The research firm IHS just released a 2014 forecast for the LED general lighting market, which indicated that mid-power LEDs would represent >80 percent of the LED units shipped and roughly 48 percent of the revenue.  This is quite a change from a few years ago where revenue of high power LEDs dominated the general LED lighting market. There are numerous factors that have driven this shift including improved LED efficacy, adoption of lower cost plastic packaging, and a shift from magnetic transformers to mechanical approaches to achieve safety isolation.  This is especially true in integral bulbs and as a result the LED string voltage is no longer limited to 30 to 60 V depending on the regional safety requirements.

In some cases mid-power LEDs are a combination of several low power LEDs in series in a single package.  As a result, high voltage mid-power low current LEDs are available with typical forward voltages ranging from 9 to 100 V. One impact of using these LEDs is that alternate driver topologies instead of the classical isolated flyback can be used that result in lower electronics cost and/or higher power conversion efficiency.


Figure 1. Single LED string direct AC drive operation

One of the simplest direct AC approaches involves a bridge rectifier, constant current regulator (CCR) and a string of LEDs as seen in Figure 1. If the LED string voltage is sufficiently high compared to the input AC voltage, the losses in the CCR can be kept to a reasonable level while achieving > 0.9 power factor.  This approach does have lower LED utilization since for a portion of the AC cycle, no current flows through the LEDs as seen by the red line.  In this example there is no electrolytic capacitor in the circuit for energy storage so this type of driver results in 100 percent optical flicker at 100 / 120 Hz (2x the AC line frequency) but the driver circuit is simple, small and can have a long operating life time.   As it is not always practical to match the LED string voltage to the AC line voltage a switching topology is needed.

To better understand how to determine the right switching topology based on the string voltage, we wanted to compare several mainstream topologies and provide some general guidelines to help designers understand how the selection of the LED string voltage impacted the topology choice.  To do this a figure of merit (FOM) was created that combined the maximum voltage stress and peak current through the power switch as a function of the LED forward voltage and the input line voltage.  This FOM is a good proxy for both efficiency and cost in a switching LED driver as the lower the peak current, the lower the losses in the switch, inductor and diodes.

Two general use scenarios were considered. First, applications where high power factor and low THD (<20 percent) were required and the second where high power factor is not required.  As an example for ENERGY STAR LED lamps, there is no power factor requirement for lamps < 5 W and in the EU there is a special input line harmonic exception in EN61000-3-2 for lighting products < 25 W so high power factor is not required.  An added benefit of not needing high power factor is that it is easy to achieve low optical flicker.

The buck, buck-boost, boost and isolated flyback with a turn’s ratio of 3 were all compared based on the FOM (the lower the better) as a function of the VF to Vin peak ratio.  In the case of the high power factor case, the total harmonic distortion was limited to < 20 percent for analysis and we assumed a 600 V MOSFET (80 percent derating).  For VF/Vin ratios from 20 to 40 percent, the power factor corrected buck is the best topology. The upper limitation of the buck is related to THD and power factor and not stress on the power switch.  Interestingly it turns out the best topology from the FOM analysis is the boost, which is not the first topology to come to mind for many designers.  An example of a complete high PF 10 W boost design is shown in this design note where a 220 V LED string was driven at 30 mA across an input voltage of 90 to 135 Vac.


Figure 2. Topology comparison for high power factor >0.9



Figure 3. Topology comparison for low power factor (EN61000-3-2 Class C Exception)


When looking at applications were it is not critical to achieve high power factor, the clear winner for strings of higher voltage LEDs is the buck topology as seen in Figure 3 as it has the best FOM across a wide range of forward voltages.  An added benefit of this approach is that it is one of the simplest to implement and can have low optical flicker. The reason that the curve ends below 70 percent is due to meeting the requirements of the EN61000-3-2 Class C exception where the input capacitor is undersized to control the input current shape. For applications in the US market where compliance to EN61000-3-2 Class C is not required, this range can be extended. An example of a complete 3.8W buck design is shown in this design note where a 150 V LED string was driven at 25 mA across an input voltage of 200 to 265 Vac and achieved 85 percent efficiency.

This FOM comparison is a good reference tool at the beginning of the LED selection and architecture definition.  In some cases, especially at more narrow VF/Vin, there are several options that should be considered since the differences between different topologies is relatively small but in many cases this can give clear guidance in what topology is best for lowest system cost and better efficiency.