Because of their lower power consumption, longer lifetimes and less wasted illumination, LED street lights are gaining acceptance worldwide. This article takes a look at the basics of LED street light design, examining considerations for parts selection and the Bill of Materials (BOM).
Global LED street light installations are forecasted to grow by 400 percent during the next four years, according to Strategies Unlimited. Beyond the other factors mentioned above, LED street lights often have built-in intelligence and communications capabilities, allowing them to report when they need service or replacement.
While it might seem like a simple design exercise to construct an LED street light, the addition of electronic control and communications capabilities adds some complexity compared to conventional high-pressure sodium lamps.
LED Street Light Design
An LED street light reference design from Cree defines the elements of the bill of materials:
- Drive electronics—i.e. an input voltage driver
- The LEDs themselves
- The secondary optics
- Thermal epoxy
Cree specified the HLG-120H-48a driver from Mean Well USA Inc. Designed for indoor and outdoor LED lighting and signage applications, the HLG-120H-48a provides circuit protection features and conducts active power factor correction (PFC).
As for the LEDs themselves, designers have a range of choices, including small numbers of high-brightness devices, or smaller numbers of lower-brightness LEDs. Cree offers its XSP series with lumens ranging from 4,049 up to 14,160. The wattages for these devices vary from 53 watts to 168 watts.
LED street lights require a secondary optics solutions. This consists of lenses that focus the light, enhancing output efficiency and providing an appropriate distribution pattern. Cree’s design employed a secondary optics and optics module from LedLink Optics Inc., which offers a range of lenses for different types of LED applications.
Other elements of Cree’s design include heat sink for thermal management, for example, an extruded aluminum device and a metal core printed circuit board suitable for applications that generate heat, such as LED lighting.
These represent the elements of a basic LED street light solution. However, to fully realize the advantages of solid-state lighting, such systems should employ further electronic control and communications features.
Advantages of LED Street Lights
To implement such features, the STMicroelectronics reference design for LED street lights features the company’s STLUX385A, a digital lighting and power control device. The STLUX385A includes a microcontroller, memory and a Digital Addressable Lighting Interface (DALI).
The use of such a device allows users to add intelligence to street lights, for example, enabling the construction of lamps that slowly get brighter as the sun sets, and that steadily dim as dawn breaks—rather than just turning to full brightness and switching off based on a predetermined schedule.
DALI is a global standard that can be used to implement two-way communications via wired and wireless connections. Using this connection, LED street lights can be monitored for performance and failures, allowing remote diagnostics and detection. It also can accelerate and automate the process of detecting failures and dispatching repair crews.
While LED street lights may appear to be simple replacements for conventional lamps, the design process becomes more complex as computer intelligence and communications are added to the mix. Designers should employ reference designs that allow for the integration of digital controllers into their systems.