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Electronics Drive Greater Efficiency in Retail Lighting



Aesthetic considerations dominate the choice of lighting for retail, hospitality, and many commercial installations. However, governments are increasingly concerned about energy consumption in the retail environment. Lighting is responsible for a large proportion of the energy cost, as stores need to be kept illuminated throughout open hours. Organizations such as the California Energy Commission have drawn up recommendations to retailers, hoping to encourage them to seek more energy-efficient methods of lighting.¹ To help with the process of improving efficiency, electronic power control, and new lighting technologies are coming together to provide wider selection of light sources which offer lower power demands and greater control over color and output levels.

For example, dimming support has become important. Stores are now being divided into light zones that make it possible to harvest natural lighting close to windows and reduce energy consumption to the minimum possible, based on the weather conditions outside. Furthermore, careful control over lighting color can result in more attractive displays that also save significant amounts of energy. For example, a study by researchers at the Rensselaer Polytechnic Institute found that the use of colored lighting – in this case primarily blue-hue LEDs – along with providing a 30 percent reduction in power produced a better subjective impression among shoppers than brighter, uncolored lighting.² The study suggested that color contrast is more important than overall illumination level.

The demand for high-brightness lighting in the retail environment has led to the widespread use of halogen lighting in many public areas. Traditional retail lighting uses halogen lamps rated at 150 W to illuminate items for sale in stores. Today, the same amount of lumen output can be achieved using HID lighting elements that consume a third of that power. Traditionally, HID lamps have been used mainly for high-bay installations where high area coverage is needed but the lighting technology is moving into track-lighting fixtures suitable for the office and retail environment. This trend has been helped by the adoption of HID lighting in the automotive market, where similar-sized bulbs are employed.

Metal-halide sources are available that produce 2500 lumens from a 30 W power input, making it possible to build intense spotlights that can be used for impressive window displays in the retail environment. However, these lamps demand intelligent electronics to maintain control and deal with some potential drawbacks. Without electronic control, HID lamps can take a long time to start up and cannot be restarted for as long as 20 minutes after power has been withdrawn (Figure 1).

Figure 1: A flowchart of the startup requirements for a HID lamp (Courtesy: Microchip Technology).

Pulse-start technology makes it possible to start metal-halide lamps much more quickly. Brief, high-voltage pulses are used to ignite the lamp so that it reaches peak output much more quickly and allows the lamp to be restarted within five minutes instead of twenty.

Although HID lamps have a long average active life, it is important to design an electronic ballast so that a failed lamp can be detected. If the ballast does not recognize that the lamp has failed, it will continually try to start the lamp, which can result in damage not just to the igniter but the ballast itself.

There are other unusual characteristics of HID lamps that need to be managed. For example, acoustic resonance is a phenomenon of HID lamps caused by standing pressure waves forming in the gas inside the tube. These pressure waves damage the bulb and can even crack the glass. To stop these standing waves from forming, electronic ballasts drive the lamp with a low-frequency AC voltage in the steady state.

Although a number of off-the-shelf controller ICs are available, these are designed primarily for the automotive environment. However, advanced microcontrollers and digital signal controllers can be readily programmed to provide the necessary control and detection functions needed in ballasts and are supported by lighting-oriented evaluation kits (Figure 2).

Figure 2: Using a microcontroller to control the power-conversion stages of a HID or fluorescent ballast circuit (Courtesy: Atmel).

To make it easier to develop HID ballasts, among other microcontroller-based applications, Atmel provides the STK500 evaluation and development kit, as well as the dedicated ballast kit ATAVRFBKIT. The AVR AT90PWM has internal EEPROM that can be used to store lamp wattages and other parameters for accurate control without demanding additional components. Integrated power stage controllers help manage lamp power and provide the control signals needed to drive the multiple power stages required in HID ballasts.

For HID lamp power-control, Microchip Technology offers the dsPIC33 microcontroller, which has been incorporated into a ballast reference design, detailed in the AN1372 application note.³ Although developed primarily for automotive applications, the design principles apply to HID lamps suitable for high-intensity retail lighting.

Greater control over lighting intensity and color in combination with high illumination efficiency is available from lamps based on light-emitting diodes. In contrast to the tungsten lamps they often replace, LEDs do not usually fail catastrophically. Instead, they offer thousands of hours of operation at a constant level before their performance begins to degrade to as little as 70 percent below their peak output. Electronic controllers can correct for this reduction in light output, allowing constant light levels for a given fixture if it does not need to be driven at peak output. This, in turn, reduces overall maintenance, as lamp replacement does not need to be immediate.

For retail applications, LEDs provide one distinctive advantage over most other lighting solutions: the ability to alter the color output dynamically if three-color LED sources are used. Electronic controls can tune the light output to simulate daylight and other effects or provide more obvious color-changing sequences.

However, LEDs require specialized driver circuits to work at full efficiency over a long service life. An LED requires a constant-current source to provide a controlled level of brightness. As the device’s behavior is that of a diode, up to a threshold voltage, very little current will flow through the device. Beyond that threshold, the VI curve follows that of a diode: for very small increases in voltage current rises dramatically and the resistance falls. As more current passes, the LED’s die heats up which increases the device’s voltage drop and dynamic impedance. Although LEDs do not require the complex state-oriented controls needed for HID and fluorescent lamps, the driver circuit needs to be designed carefully as control-loop stability is important to ensure flicker-free performance. The ability of an LED to adapt light output instantaneously can result in unwanted flicker if the current source is not carefully controlled.

Dimming is frequently achieved using switching power converters driven using pulse width modulation (PWM) techniques. An important choice is deciding on the PWM frequency range. Low frequency dimming provides a wide range of intensity control: a ratio of 100:1 is achievable. To avoid visible flicker, the PWM signal needs to operate at a frequency of more than 100 Hz, although care has to be taken to ensure that the choice of frequency does not result in audible noise generated by an inductor in the switching converter – this noise tends to become more noticeable as the frequency approaches 1 kHz.

High-frequency dimming, performed in the 10 kHz range, generates less radiated EMI and power supply ripple, but at the cost of a reduced dimming range: a ratio of 5:1 is common. However, this is often suitable for retail environments where a bright light is required and the dimming is used primarily for daylight harvesting.

Devices such as the Diodes/Zetex ZXLD1350 make it possible to choose from a range of dimming techniques, including DC voltage or resistor dimming in addition to PWM techniques. An adjust pin on the ZXLD1350 will accept any of these dimming control signals, providing precise control over the current supplied to the LED and, therefore, its brightness.

The Infineon Technologies BCR401 and BCR402 LED drivers are designed to provide low-cost LED power control, making them suitable for building large arrays of lamps. Able to provide current output of up to 65 mA, the devices can be biased using an external resistor which makes it possible to build LEDs with different forward voltages into the same array and still achieve a consistent light output. For 0.5 W LEDs, the drivers can be used in parallel to avoid generating hotspots on the PCB, allowing the use of low-cost FR4-based PCB materials. There is no need to use emitter ballasting as the drivers have a negative temperature coefficient. With the BCR40x devices, PWM dimming is made possible using external digital control.

The NUD4001 from ON Semiconductor similarly uses an external resistor to determine the light output of the connected LED and is designed to be easily controllable using PWM techniques.

Figure 3: A PWM dimming circuit based around the ON Semiconductor NUD4001.

NXP Semiconductors’ SSL2102 is designed to simplify lighting circuitry. It is a switched-mode, power supply-based LED driver IC that can act in combination with a phase-cut dimmer, powered directly from rectified mains to allow easy migration from an existing lighting control infrastructure. Integrated circuitry optimizes the dimming curve, providing logarithmic correction down to 1 percent of full output power. To improve efficiency, the SSL2012 and other devices in the SSL210x family use valley switching in the power converter. This reduces switch-on losses significantly.

For illuminating wide areas in the retail or office environment, fluorescent lighting still offers high efficiency and, in combination with electronic ballasts, the possibility to dim the source close to external windows to maximize the harvesting of natural light. Tube designs such as the T5 and T8 now provide a choice of colors for more natural lighting or to support colored lighting to enhance the look of window displays.

Dimmable fluorescent lighting calls for a sophisticated ballast design that can preheat filaments, generate a high-ignition voltage, provide power control for dimming, and additional filament heating when low lighting levels – typically below 20 per cent of full output – are needed.

The International Rectifier IRS2530D was developed to support the design of dimmable fluorescent ballasts. The guiding principle behind its design was to make it possible to incorporate dimming control without demanding a high pin-count package. The IRS2530D uses the same 8-pin package as a comparable non-dimming controller.

Dimming is achieved by reducing the frequency of the switching power converter that, in turn, reduces current from a resonant tank circuit to the lamp itself. Closed-loop feedback control is used to regulate the lamp current to a reference level, continuously adjusting the half-bridge frequency.

Thanks to intelligent electronic control, it is easier than ever to incorporate attractive energy-saving lighting into the retail environment. A choice of lighting technologies makes it possible to tune the lamp technology for the application at hand, whether it is to provide spot lighting for individual products or general ambient illumination for the shop floor.

Summary

This article has provided an overview of lighting technologies for the retail environment where aesthetic considerations and concerns over energy efficiency are driving the adoption of more advanced lighting and lighting-control technologies.

References
  1. California Energy Commission, Retail Lighting - Draft Measure Information Template:
    http://www.energy.ca.gov/title24/2013standards/prerulemaking/documents/
    2011-04-04_workshop/review/Retail_Lighting.pdf
  2. Freyssinier, Jean Paul et al, Reducing lighting energy use in retail display windows:
    http://www.lrc.rpi.edu/programs/solidstate/pdf/Freyssinier-SPIE6337-51.pdf
  3. Microchip AN1372:
    http://ww1.microchip.com/downloads/en/AppNotes/01372A.pdf
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