LED Streetlight Demands Smart Power Supply By Bernie Weir, Director of Applications, and Frank Cathell, Senior Applications Engineer, ON Semiconductor, Phoenix A two-stage architecture enables designers to meet requirements for energy efficiency, power factor correction, dimming control, ac input voltage and constant-current regulation for solid-state large-area lighting.

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s 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” in the online version of this article at www.powerelectronics.com). 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 Output satisfy all the aforementioned rerectification AC Power factor Pulse-width and current quirements for intelligent LED streetinput correction modulation control LED boost (PWM) lighting. While there are single-stage Input filter string controller controller and rectifier PFC/flyback topologies such as that implemented in ON Semiconductor’s NCP1651 controller, the requireNetwork Optocoupler ments of digital dimming of the LEDs dimming control feedback favor a two-stage approach. The bandwidth of traditional PFC control loops is normally on the order Fig. 1. All of the global requirements for driving high-brightness LED lamps can be met by this two-stage design topology. This approach supports LED string lengths up to 60 devices. of 10 Hz to 20 Hz, and LED dimming 34 Power Electronics Technology February 2008

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RSTART1 RSTART2 330 kΩ 330 kΩ LBOOST

Vac

2.5 A

L1 180 μH

F1 Fuse

C1 4.7 μF

RCTUP1 750 kΩ RCTUP2 750 kΩ

Bridge L2 4.5 mH

C2 4.7 μF

CIN 0.1 μF

Auxiliary

CCOMP 0.1 μF

RC1 short CT2 0.33 nF

CT1 1.2 nF

D2

18 V

8 1 7 2 U1 3 NCP1606B 6 4 5 CCS Open

DBOOST MUR460 ROUT1A 2 MΩ RZCD ROUT1B 100 kΩ 2 MΩ

23 nF D1N4148

R1 C3 100 Ω DAUXILIARY

CCOMP2 0.39 μF RCOMP2 54.9 kΩ

NTC 4.7 Ω

400 μH, 10:1

RDRV 10 Ω CVCC2 CVCC 100 nF 47 μF

RCS 510 Ω

Q1 SPP8N50

CBULK 68 μF, 450 V

ROUT2 24 kΩ RS1 0.1 Ω 1W

Fig. 2. This critical-conduction-mode power factor correction circuit supports power requirements for driving high-brightness LEDs used in streetlighting. The NCP1606 controller’s boost-conversion stage output voltage is determined by the maximum ac line voltage.

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 micro-

frequency is above 100 Hz to avoid visible flicker. Thus, a single-stage architecture for digital dimming is not an 02onsemi_F2 option.

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