LED Streetlight Demands Smart Power Supply High-Brightness LEDs

Feb 1, 2008 12:00 PM
By Bernie Weir, Director of Applications, and Frank Cathell, Senior Applications Engineer, ON Semico

Boosting PFC

The power requirement for most LED streetlights is on the order of 60 W to 150 W. As a result, a critical-conduction-mode PFC topology is used, employing the NCP1606 controller. Historically, most lighting ballasts have been optimized for one region, but for flexibility, this design has a standard universal input of 90 Vac to 265 Vac to support North American as well as international ac power sources (Fig. 2).

The output voltage of the boost stage is 400 Vdc nominal and is designed to deliver 100 W. Since the NCP1606 implements a boost-conversion stage, this output voltage is determined by the maximum ac line voltage, and the characteristics and accuracy tolerance of the controller.

In the United States, it is also common to power streetlights and commercial lighting from 277 Vac. This voltage is derived from one phase of a 3-phase 480-Vac circuit. To modify the design to support this requirement, a higher dc bulk voltage at the output of the PFC boost stage is required. This involved changing the feedback voltage to generate an output voltage of 480 V. It also may increase the maximum-rated voltage of the bulk capacitor and power MOSFET.

Following the PFC stage is a fixed-frequency isolated flyback stage and a linear current regulator that regulates a constant current to a string of LEDs (Fig. 3). The regulator maintains a constant voltage drop across the linear current-regulator MOSFET (Q2).

The flyback controller is the ON Semiconductor NCP1216, a compact 8-pin controller with a current-mode architecture. This controller is based on a proprietary high-voltage process that allows the device to start up from the high-voltage bulk and contains dynamic self-supply circuitry to power the device in normal operation.

The heart of this circuit is the secondary-side control block, which provides the constant-current regulation for the LED string. It maintains a constant drop across the current regulator MOSFET regardless of the output load voltage. It also accommodates a fast logic-level PWM dimming input signal needed to adjust the luminaire light output.

By using a series-pass linear regulator to control the LED current (U5A, Q2, R10), fast response to the PWM LED dimming control signal can be achieved at pulse frequencies up to 1 kHz, with reasonably fast rise and fall times. This could not have been done very effectively with the main flyback feedback loop without fairly low frequency constraints on the PWM input signal.

Despite the use of a dissipative current regulator, very low dissipation in Q2 is achieved by regulating the voltage drop across the series-pass MOSFET to about 1.5 V (power dissipation is 550 mW) with the flyback feedback loop. This provides maximum efficiency and versatility to deal with different output voltages due to varying LED forward drops and different LED string configurations. It also allows the flyback transformer to be designed to accommodate the maximum-desired output voltage yet handle lower voltages without dissipation or circuit change issues. That would occur if the output voltage were tailored for a specific LED configuration and directly regulated.

Regulation of the voltage drop across Q2 via the flyback converter is accomplished by a sample-and-hold circuit and error amplifier composed of D6, R24, C12 and U5B. This also allows fast PWM gating of the current amplifier without significant voltage excursions on the input voltage to Q2.

Reference amplifier U4 (TL431) provides the reference voltage for the current regulator (U5A) and the feedback error amp (U5B). Feedback to the NCP1216 current-mode controller on the primary side is with a conventional opto-coupler (U2). In the event of an open LED string or no load on the power-supply output, a simple voltage-clamp feedback circuit is provided by Z1, Z2, Z3 and additional optocoupler U3.

To further minimize the size of the transformer, a 130-kHz switching frequency was selected. The NCP1216 controller also features frequency jittering capability to reduce the EMI signature.

An 800-V power MOSFET was used in this circuit to accommodate the reflected voltage from the secondary. Note that, depending on the maximum forward voltage of the LED string used, a higher or lower voltage MOSFET could be used.

The flyback transformer T1 is designed for discontinuous-conduction-mode operation with a maximum output of about 250 V. The core is a standard E21 type ferrite core. However, other core geometries, such as ETD, PQ or EC types, could have been used depending on the required maximum output voltage.

Keeping the primary and secondary wound on single layers is important to minimize leakage inductance effects. If smaller cores are desired, multiple layer windings can have potentially detrimental effects on circuit parasitics and subsequent voltage spikes and ringing.

Fig. 4 and Fig. 5 illustrate the current switching waveform as a PWM signal is applied. The upper trace is the current through a sense resistor in series with the LED string. The lower trace is the PWM input-signal waveform. The Fig. 4 waveform shows a normal case with 50% dimming, while the Fig. 5 waveform shows an extreme case. They illustrate the symmetric rise and fall times, which facilitate linear dimming control.

Network-Controlled Dimming

The example given illustrates a circuit configuration to drive long strings of LEDs from the ac main while offering excellent power-factor performance, dimming capability and fault protection. Overall, efficiency driving a string of 60 LEDs at 350 mA was approximately 85%.

One interesting aspect of having network-controlled dimming is that a microcontroller function can be embedded in the luminaire. This could allow enhanced intelligence functions such as active temperature monitoring to compensate light output based on the temperature of the LED array, which can vary dramatically by season. In addition, it can prove safety monitoring in case of a fault.

The combination of high-brightness LED technology, efficient power-supply design and intelligent control provides interesting opportunities to reduce energy consumption and lifecycle costs. Moreover, since LED efficacy and lumen output continues to advance at a brisk pace, this design was made to accommodate the fact that the number of LEDs required for a given light output will continue to fall as LED technology improves.

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