The power requirement of LED street lamps being used in China falls in the range of 100~250W. It is widely agreed that if used properly, these kinds of LED street lamps deliver many advantages. What I want to explain here is the ways to make it possible for these lamps to deliver those advantages. The key factors to be considered are high efficiency, power, reliability and cost-effectiveness.

Some low-power lighting requires PFC, while high-power lamps usually require PFC combined with DC/DC requirements. In China where the AC line voltage is 220V, Boundary-Conduction Mode (BCM) PFC controllers, such as the FAN7530 and the FAN6961, become the ideal choice to maintain a balance between the efficiency and performance-cost ratio. These solutions only need a few components.

Low Rds(on) SupreMOS(TM) MOSFETs, can further decrease switch and conduction loss. When used at the boost output, the HyperFAST 2 high voltage diode family with lower Vf can also lower the conduction loss of the diode itself.

For DC/DC topology, there are many choices such as quasi-resonant (QR), double transistor forward (DTF), active-clamp, LLC and asymmetrical half-bridge (AHB). High-power lighting applications, for example in a 100W lamp, where the output voltage is usually a little high, QR working with a synchronous rectifier can achieve up to 92.5% of total efficiency. Moreover, Fairchild has integrated QR and BCM PFC into one package (the FAN6921), reducing external components and simplifying the control.

Another popular topology is zero voltage switch (ZVS). Both an LLC and an AHB can have their two bridges working in zero voltage by implementing a simple circuit. When using Fairchild’s highly-integrated solution (for example, a LLC controller and two MOSFETs in FSFR; an AHB controller and two MOSFETs in FSFA2100), the circuit can be further simplified, with few external components. And the body diode of the MOSFET has good fast recovery characteristic, which can reduce the possibility of short-through, yet provide high reliability with high efficiency. When the output voltage is high, an LLC is the better option; when the output voltage is low, an AHB is more suitable for implementing a self-driven synchronous rectifier, and both can achieve over 93~94% efficiency.

The above solutions are highly integrated solutions and require just a few components, thus delivering high efficiency, high power density, optimized thermal performance as well as high reliability.

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