Pilot projects are underway at numerous sites to install high brightness LED street lights. These street lights are often replacing HID and HPS street lamps. While HID lamps are long-lasting and have high luminous efficacy (high luminous efficiency), well-designed LED-based systems have the advantage of directing light downwards, providing ground-level illumination without radiating too much light. With properly rated components and sufficient heat sinking, LED systems can last up to 50000 hours. LED-based systems can be dimmed in a wide range of settings, unlike HID lamps which can only dim to around 50%. To ignite HID lamps, you need a high voltage pulse. The voltage pulse is usually between 1 and 4kV for cold lamps, but can go up to 20kV for hot lamps. Most HID lamp ballasts are unable to provide hot re-strikes. It is necessary for the lamp to cool down for several minutes before it can turn on. Street lamps based on LEDs consume between 120 and 200W, while HID systems use 300 to 400W.
However, the lumen levels are comparable at ground level due to the fact that the LED light source produces a higher Lumen per Watt than HID. The LED optics allow the light to be directed exactly where it is needed. The circuit topology used to switch power supplies to provide the required power levels for LED street lighting is different from lower-power LED drivers. Although flyback converters are well-suited for power supplies below 50W, they are inefficient at 100W or more. Therefore, a multistage design is preferred. The front end of this power supply consists of a power factor correction booster regulator that provides a high voltage DC bus. This type of active power factor correction circuitry (PFC), is used extensively in power supplies and electronic ballasts, despite a slight decrease in efficiency. PFC stages typically have a 90% efficiency, which rises with higher power levels. An isolating step-down back stage follows the PFC stage to provide a regulated current output. This stage can be either a Flyback converter, forward converter, or half-bridge resonant convert. Modular LED street lights use a modular design where each panel is connected to an LED power supply or driver. Each module is powered by a constant current driver. This type of system produces an isolated fixed voltage, typically 24V or 48V. It is low enough to be within the UL safety limits. Current regulators drive the output channels of each LED module using current supplies from the central isolated low-voltage bus. This is called a multi-channel LED driver.
Modular systems are convenient and allow you to make light fixtures in different sizes and shapes without having to use multiple power supply models. However, they are less efficient than power supplies with one output. This power supply would drive one panel with an array of LEDs that operate at a fixed current. An example would operate at 1.4A, with four strings of 350mA LEDs running parallel. The forward voltage for each string will be very close to the one before. This power supply could be optimized to deliver this output voltage and current. Regulation could be provided by pulse width modulation (PWM), which is used in the back-end stage. It eliminates the need for additional output current regulators. This provides greater flexibility but also allows for higher efficiency. The PFC stage Boost regulator operates in a critical conduction mode (or transitional mode) between discontinuous or continuous modes. This means that the switching cycle starts as soon as all energy has been transferred from the inductor to the output. There are many PFC control ICs that operate on this principle. However, more sophisticated ICs capable of controlling this stage and the Flyback stage as well as the resonant bridge or Flyback stages are being developed. The PFC stage supplies a regulated high-voltage DC bus with a wide input voltage and load. This allows the LED power supply to operate at high power factor and low distortion over a wide range AC line input voltages (typically between 90 and 305VAC), which is sufficient to cover most locations. This allows for the second stage to be simplified as its transformer and power semiconductor parts can be optimized to a fixed input voltage. The second stage of an LED power supply is best suited to the resonant half-bridge. It transfers power continuously through the transformer, instead of periodically storing or transferring energy. This is unlike the Flyback converter. The transformer is significantly smaller than that of the Flyback's air gapped coupled inductor.