Chester Firek, Vicor
LED technology has developed over recent years, and is now the obvious choice for many applications from energy-efficiency architectural lighting to outdoor displays. These applications tend to vary considerably, each requiring a different number, type, color and configuration of the LEDs, which means that driver circuits need to be designed specifically for the application.
Although there is considerable diversity between the applications, some requirements are fairly universal. Typically the brightness and color of LEDs in a panel must be consistent, and the overall efficiency must be high, if the full benefits of using LED illumination are to be achieved.
Other requirements may not be so obvious. Consider architectural lighting: although the structures being lit are large, and there is little physical constraint on space needed, people want to see the building and not the power supply. So small size is a key for designing lighting power chains for systems that can be installed without being seen.
This is also true for many LED driving applications ranging from signage for transportation, where the system needs to be as small as possible to allow passengers to move around easily in the train or bus to large LED screens, which must be made as light and thin as possible, making power density a key requirement for many LED power chains.
Harsh environment, hard-to-reach spots
Many LED applications are used outdoors, requiring a degree of ruggedness and a suitable operating temperature range for the power components that are used. Often the systems are installed in inconvenient locations, placing further requirements on the power system.
The installation and maintenance of an architectural lighting system or outdoor advertising display, for example, often needs workers to climb on buildings in various weather conditions. These activities are dangerous, making installation expensive in both time and liability.
By developing solutions that are smaller and lighter, and thus faster and easier to install, power engineers can make the lighting system cheaper and safer to deploy. Improving the reliability of the system will reduce the likelihood of failure, meaning it’s much less likely that repair will be required, further reducing the need for working at height.
Two categories of LED apps
For power system designers, LED applications fall into two broad categories. The first requires each LED to be controlled individually, for example in a digital signage application, where specialized LED driver chips are used. The power chain for these applications need to provide an efficient way of stepping down the input voltage to the bus voltage, and then converting bus voltage to the supply voltage of the driver, which can be 12 V, 5 V or 3.3 V.
The second category of applications requires arrays of LEDs to be driven as efficiently as possible: this includes most LED lighting systems. Typically, these systems have a requirement to match the intensity of different banks of LEDs and may also need the level of illumination to be controlled: for example, an outdoor application that needs high brightness on a sunny day, and in the middle of the night must dim the LEDs. In this situation, the LEDs can either be driven directly by the power chain or LED drivers can be used.
Increasing the hub voltage to improve efficiency
Most systems require a front end to convert the input voltage to a hub voltage that can be used to distribute power throughout the system. Historically 12 V has been the hub voltage chosen by most engineers, although today systems are moving to 24 V or 48 V, allowing the size of the conductors to be reduced as well as cutting the I2R losses, which can be significant for physically large systems such as outdoor displays.
Standard front-end power components meet the needs of most applications, converting an AC or DC input to the hub voltage. The conversion at the point of load, however, can be more challenging.
Raising the hub voltage requires a step-down at a higher ratio than if a lower hub voltage were used. This presents a challenge as the efficiency of conventional buck converters is significantly reduced at low duty cycles. Using a two-stage conversion is not a suitable solution as although it allows the converters to operate at a higher efficiency, adding a second stage is inherently inefficient.
Avoiding LED hot spots
With a pressure to reduce manufacturing costs, and the fact that many of these applications are in places where servicing the equipment can be difficult or expensive, many engineers use a single printed circuit board with the LEDs mounted on the front and the electronics that drive them on the back. This reduces manufacturing cost, system size and makes repairs easy, as all that needs to be done is to swap out the board if a failure occurs.
Putting the driving circuitry on the back of the board, however, can affect the consistency of illumination across the panel. The converters will dissipate heat, which can change both the wavelength and brightness of the LEDs as they warm up because LEDs are sensitive to temperature. Typically, the wavelength increases by 0.1nm/°C-0.2nm/°C depending on the type of LED used; the light output will decrease with rising temperature.
The impact of heat depends upon the LED technology used, and the design of the LED. The output of blue LEDs might only fall a few percent with a 50°C rise in temperature, but the output from red LEDs can fall by 25% with only a 25°C temperature increase. The output of all LEDs, however, is affected by temperature.
Whether drivers are used, or the LEDs are driven directly, any power lost in the conversion and driving circuits will generate heat, which can affect the color and intensity of the LEDs. If this is not managed correctly, it will be possible to see areas of the display or LED cluster that are less bright, something that is unacceptable in most applications. Efficient buck converters are therefore critical to maintain a consistent brightness and color across the panel.
When LEDs do not need to be controlled individually, eliminating the drivers increases efficiency. The power chain, however, needs to generate a constant current to ensure the brightness can be accurately controlled.
Buck converters are usually configured to generate a constant voltage, rather than a constant current. There are, however, some new components that offer a constant current capability.
An example power chain for LED panels
With the wide variety of requirements, and the benefits of distributing the conversion from hub voltage to the voltage or current source required by the load, the flexibility provided by power components (rather than using a central power supply) are compelling. Figure 1 shows a high-performance power chain that has been designed using the Power Component Design Methodology to drive a large panel of LEDs, which could, for example, be used in an architectural lighting application.
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Figure 1: Power Chain to Drive LED Panel
The front end conversion is performed by a PFM in VIA packaging technology, providing a 48 V hub voltage from a universal AC input in a compact, low-profile, thermally adept package. I2R losses are minimized by using the higher hub voltage, which is then converted to the constant current required by the LEDs by ZVS Buck Converters.
For example, Vicor’s PI354X uses an efficient Zero Voltage Switching (ZVS) topology to generate either a constant voltage or constant current source. Figure 2 shows this power component configured to operate in constant current mode. The EAIN pin that is normally used to control the output in constant voltage mode is held above the 1V internal reference, leaving the device to be controlled by the voltage across the shunt that is in series with the two LEDs. The voltage drop across the shunt is only 100 mV, minimizing the power wasted in the current sensing circuit.
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Figure 2: Constant Current Mode Operation of the ZVS Buck Converter
Power chains that are optimized for driving LEDs can be very simple: in this example a couple of front end components generate the hub voltage, and the LEDs are driven with a single-stage conversion to a constant current, and operates at a high overall efficiency. Whilst conceptually straightforward, however, this approach has only recently been possible as high-performance power components with the required capabilities have been introduced.