Over the last few years momentum has been rapidly building behind the development of a new generation of LED technology-based street lights. Promising a much longer service life and higher power efficiency than existing technologies, LEDs are tailor-made for difficult-to-access street light applications where their reduced operating and maintenance costs offer enticing dividends.

Reliability is a key concern in streetlight designs – not only in the LED fixture, but in the power supply as well. Streetlights must maintain operation under oftentimes harsh environments, including rapidly changing temperature, moisture and wind conditions, from season to season, and from day to night. Power supplies designed to power LEDs in street lighting applications must typically be able to support operation across a wide -40°C to 50°C ambient temperature range.

Since driving LEDs requires a constant current DC source, an AC-DC conversion is required when the LEDs are powered by AC mains. Early implementations of LED streetlight designs featured power supplies with an AC input connected to a standard high voltage DC-DC converter and used a current balancing circuit to drive multiple LEDs either in parallel or series. While power requirements vary depending upon brightness of the LEDs and the number used in each light fixture, generally power supplies for these applications must comply with established energy regulations, offer a power factor of greater than 0.9, and feature constant current output, surge protection and low ripple. Most designs require greater than 85 percent power efficiency.

Figure 1. Reference design for a 150 W LLC high-voltage resonant converterTaking the system perspective
Initially, designers relied heavily on flyback topologies to meet the needs of LED streetlight applications. Available from a wide variety of suppliers, proven flyback converters offered a low cost, simple-to-design and fast-to-market solution compared to more complex LLC alternatives. But as regulatory agencies worldwide have focused increasingly on reducing energy waste, new regulations have emerged that demand high efficiency not only at full load, but also in standby mode and at various percentages of the rated load. Using zero-voltage switching techniques to reduce switching losses, LLC resonant converters offer designers an opportunity to maximize efficiency across the entire load range.  

Some of the newest LLC converters take that inherent efficiency advantage one step further by combining a LLC controller, low- and high-side drivers, and two half-bridge MOSFETs in a single package. These highly integrated solutions offer a number of advantages. LLC solutions using discrete controllers and MOSFETs force designers to program dead time conservatively for worst case scenarios to prevent shoot through. The designer must overlap the off-time of each MOSFET to ensure they don’t turn on at the same time and create a short conducting loop. However, the longer the dead time, the lower the converter’s efficiency. By integrating the controller, drivers and MOSFETs in a single circuit in which all elements that affect switching delay are matched, LLC converters offer more precise control of the on-time and off-time of both MOSFETs. This permits the designer to program a shorter, tighter dead time, allowing the controller to optimize operation for maximum efficiency over a specified frequency range.

Highly integrated solutions like these also bring major advances in reliability. To ensure continuous power supply operation under all environmental conditions, most of today’s LLC converters feature a wide array of fault detection and compensation functions. These include programmable brown-in/out thresholds and hysteresis, under-voltage and over-voltage protection, programmable overcurrent protection including short-circuit protection, and over-temperature protection. Some of the integrated LLC controllers now available also allow designers to replace bulky and less reliable electrolytic capacitors with more reliable SMD ceramic capacitors in the output loop. Moreover, by integrating critical functions into a single IC, they can be trimmed during manufacture to optimize pairing between MOSFETs and drivers and ensure more reliable and efficient performance.

Figure 2. Highly integrated LLC solution dramatically simplifies design

Of course, LED street light developers need power supplies that are not only efficient and reliable, but cost competitive as well. The latest LLC solutions help designers drive down costs in multiple ways. By operating at a higher switching frequency, today’s leading-edge LLC converters allow designers to use smaller transformers, smaller output inductors and smaller output filter capacitors Moreover, in designs operating at 250KHz designers can even eliminate the use of less durable electrolytic capacitors. Those advantages, in turn, translate into significantly lower bill-of-material costs. By integrating more functions into the supply, cost is further reduced by decreasing external component count, minimizing PCB layout requirements, simplifying assembly and shortening development time. As an example, the HiperLCS family recently announced by Power Integrationscan eliminate as many as 30 external components. This high level of integration not only reduces cost, it also improves reliability by simplifying the design and eliminating potential points of failure.

As LED technology becomes increasingly commonplace in street light applications, we can expect designers to continue to add innovative new capabilities. Future street light solutions, for example, will likely introduce some form of intelligence where a sensor automatically controls light brightness to increase illumination on dark, rainy daysor decrease illumination on bright moonlit nights. At the same time, designs will likely become increasingly modular to help developers drive down cost and simplify maintenance. In each case, new advances in power supply design promise to play a key role in the continuing evolution of LED streetlight functionality.