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Drives LEDs efficiently with constant-voltage topology

Posted: 13 Jul 2015 ?? ?Print Version ?Bookmark and Share

Keywords:LEDs? LED driver? AC? DC? EMI?

The solutions which IC vendors offer must eliminate visible flicker, a consequence of the AC-line cycle; manage AC-cycle inrush current to avoid momentary overloads; keep audible noise very low (due to the line cycle interacting with internal magnetics; and meet regulatory standards for power factor and minimising electrical noise (EMI). Furthermore, they must do so while ensuring compatibility with the widest possible range of dimmer types if makers of LED bulbs are not to be inundated with product returns because their products don't function as expected.

Single-stage driver for dimmable LEDs
First-generation LED driver ICs did not work well at all with standard dimmers; second-generation drivers were somewhat better but still had shortcomings, and many do not meet the latest regulatory requirements. A recent introduction, the iW3688 from Dialog Semiconductor, is an AC/DC single-stage driver for dimmable LEDs rated at up to 20W. It combines digital control techniques with embedded algorithms. At the same time, it is touted to reduce the complexity, component count and cost of the driver board that has to be squeezed into the limited space defined by the restrictive, pre-defined form factor of the LED.

Its features include the ability to detect the dimmer type. This enables the device to provide dynamic impedance adjustments so it can interface with virtually any standard dimmer and control the LED brightness at the same time. It will dim right down to 1% of maximum.

The power controller uses patented PrimAccurate primary-side sensing technology to achieve tight LED-current regulation under different AC-line and LED-load voltages, without using a secondary-side feedback circuit. The controller monitors waveform changes in the auxiliary primary-side winding then uses those measurements to regulate the voltage and current outputs. This eliminates the need for an optocoupler, reducing the BOM and increasing operating life (optocouplers wear out). When there is no dimmer on the line, the iW3688 optimises the power factor and minimises the current harmonic distortion to the AC line, with resonant control.

For further BOM cost reduction, it minimises the external-component count by simplifying the EMI filter with Dialog's EZ-EMI technology, and by integrating a current sink, switching circuits, and VCC charging function. Additionally, the power controller does not require an auxiliary winding, eliminating the need for a custom transformer and requiring only a standard off-the-shelf inductor instead. In total, compared with other controllers of this type, it requires 20 fewer external components.

The power controller operates over a wide range of line frequencies (45Hz to 66Hz) for world-wide compatibility, meets IEC61000-3-2 current-harmonic requirements, and has total harmonic distortion under 20% along with a power factor (PF) above 0.92, exceeding the regulatory mandates. It also incorporates multiple protection features that are not seen by users but are nonetheless required, including LED open-circuit and short-circuit protection; current-sensing resistor open-circuit and short-circuit protection; AC-line over-voltage protection; and over-current protection. It is available in a 14-lead SOIC package and requires only a few external components to go from AC line (dimmed or not) to compatible, regulated LED drive (figure 6).

Figure 6: With just a few passive components and one external MOSFET, the digital powerÿcontroller from Dialog Semiconductor provides a complete AC-line, dimmer-compatible, single-stageÿLED driver for loads up to 20 W.

The power controller is equally suited to use with A-style, BR, candle, GU10 and PAR bulbs, T8 LED tubes and external lighting ballast drivers.

About the author
Scott Brown is the VP Marketing Power Conversion at Dialog Semiconductor.

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