LED Streetlight Demands Smart Power Supply High-Brightness LEDs

Feb 1, 2008 12:00 PM
By Bernie Weir, Director of Applications, and Frank Cathell, Senior Applications Engineer, ON Semico

As the cost of energy continues to rise, significant attention is being paid to using more efficient and more cost-effective lighting sources. A major application is large-area lighting, using metal-halide high-intensity discharge (HID), fluorescent and high-brightness white LEDs. Intelligent high-brightness LEDs are looking to oust commonly used incandescent and mercury vapor light sources, as used in streetlighting. In addition to their maintenance requirements, the latter source has lower efficiency, contains mercury, and requires a significant warm-up and restart period (see “Energy Efficiency Is a Global Issue”).

High-brightness LEDs have different characteristics and requirements from other lighting sources, posing new challenges for their driving circuits. They feature low forward voltages of 3 V to 4 V and require constant-current drive for optimal operation. They're also typically low-power devices (1 W to 3 W) with drive currents in the range of 350 mA to 700 mA. The latest high-brightness LEDs have flux levels ranging from 80 lm to 110 lm, with efficacies of 70 lm/W to 90 lm/W.

But, they need to be used in an array to obtain the same light output possible from other sources. For example, to get the same light output as a 100-W metal-halide HID lamp, an array of 30 LEDs to 80 LEDs is needed, depending on the LED drive current and flux output.

Intelligent Power-Supply Architecture

Modern high-brightness LEDs for streetlighting require a flexible and intelligent power supply that must meet several requirements. The supply must be energy efficient and needs to meet harmonic-content or power factor correction (PFC) requirements like the European Union's IEC61000-3-2 specification. It also must support a wide range of input ac voltages, regulate a constant current to an LED string and provide dimming control.

It should be noted that digital pulse-width modulation (PWM) techniques are normally used for dimming LED streetlights. In the United States, streetlighting is not normally metered, and the power is managed by the local utility company. The utility is interested in obtaining the most efficiency from its power grid, so a power factor of 0.9 or greater is generally required.

A two-stage architecture with a power factor boost stage is followed by a flyback stage (Fig. 1). This can satisfy all the aforementioned requirements for intelligent LED streetlighting. While there are single-stage PFC/flyback topologies such as that implemented in ON Semiconductor's NCP1651 controller, the requirements of digital dimming of the LEDs favor a two-stage approach.

The bandwidth of traditional PFC control loops is normally on the order of 10 Hz to 20 Hz, and LED dimming frequency is above 100 Hz to avoid visible flicker. Thus, a single-stage architecture for digital dimming is not an option.

In addition to the blocks in Fig. 1, a small low-power auxiliary supply is used to power the secondary stage. That stage consists of the current regulation loop and the microcontroller and communication interface circuitry needed for network control.

For traditional streetlighting, different bulb/ ballast combinations are used depending on the area being illuminated. This is a function of the height of the fixtures, the spacing between the fixtures and the light output required on the ground. In the case of streetlighting, the amount of light on the pavement and the light pattern are determined by established standards depending on the expected traffic flow.

Numerous parameters factor into determining the number of LEDs required for a particular application. These include the type of power LEDs, the LED drive current, the expected operating temperature conditions, thermal management techniques and the optical design. The approach used for this power supply was to create an architecture that would accommodate various LED string lengths up to 60 LEDs and to support the use of a common ballast.

To achieve long lifetimes for LEDs, it is important to maintain the LED junction temperature at a reasonable level in the range of 80°C to 90°C. This is a vendor-specific parameter. LED lumen output is reduced as the temperature increases. So, even though high-power LEDs can be rated for currents of greater than 1 A, this design is focused on providing a constant current in the 250-mA to 400-mA range. This reduces the internal heating of the LEDs.

Want to use this article? Click here for options!
© 2007 Penton Media, Inc.