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Achieve cost-effective LED control with high efficiency.

Although the incandescent light bulb has dominated lighting applications since its introduction in the late 19th century, its efficiency has remained at less than five percent. Worldwide ecological awareness and the search for more cost-effective lighting options have made the light bulb the target of technology, resulting in the arrival of white LEDs (Light Emitting Diodes). I.ED efficiency is five to eight times higher than that of an incandescent lamp, and these semiconductor devices offer several advantages - such as longer life time and instantaneous full brightness - even when compared to other high-efficiency illuminants such as fluorescent lamps.

To achieve optimal usage of the LED's advantages, it takes some smart electronic control that considers the requirements of the LED itself, as well as the regulations regarding issues such as harmonic distortions of the current drawn from the supply.

The article describes how to reduce power dissipation of the LED driver to avoid additional thermal load for the LEDs. The first example shows a combination of two buck converters, one to drive the LEDs, the other one to provide the IC's supply, completed by a fast start-up circuit. The second example covers an LED driver with power factor correction, applicable for high numbers of LEDs, such as for uniform illumination of areas or as a replacement for fluorescent tubes. Powered from a 120 V or 230 V AC supply line, both concepts make use of the same universal switched mode power supply IC.

Basic Considerations

LEDs, like other semiconductor diodes, feature a characteristic that shows an abrupt current increase alter a certain threshold voltage has been exceeded. Furthermore this characteristic is pretty much temperature dependent. Consequently, it is recommended to drive LEDs with constant current rather than from a voltage source to achieve a well defined operating point. To avoid sacrificing the high luminous efficiency due to poor efficiency of the drive circuit, DC/DC converters are the best choice to provide electrical power to the LEDs, especially when supply voltage is significantly higher than LED forward voltage. But even when employing a DC/DC converter, total efficiency may vary significantly with the topology and choice of components.

Buck Converter Approach

The step-down or Buck converter is commonly used to drive LEDs due to its simplicity and some inherent advantages:

* Constant load current can easily be achieved by peak current switching and a constant off-time; no control loop is needed.

* The current setting is basically independent from voltage variations, as long as the supply is higher than the LED forward voltage.

* The Buck converter is inherently open load protected; in case of on open failure of an LED or the wiring, it simply stops operating. Figure 1a shows the basic Buck converter topology powered from a line voltage of 110 V or 230 V AC. The line filter and bridge rectifier with a ceramic bypass capacitor are followed by a block named "Valley fill" which will be discussed later. The converter itself, consisting of an inductance, a power transistor, free wheel diode, the control IC, and a few passive components, provides constant current to the LEDs.

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Each electrical component dissipates power. However, a closer analysis shows that the main sources of losses are the inductor, the free wheel diode, the power transistor, and the linear IC supply. So what can be done to optimize efficiency?

First, consider the topology of the LEDs (generally spoken assuming a given output power). A chain of LEDs with high forward voltage and low current yields better efficiency than a low voltage, high current solution. Main reasons are the free wheel diode and the switching transistor. The influence of the inductance is small because higher current allows higher current ripple and thus lower inductance, and which scales nicely with the DC resistance. Reducing the inductor's specific losses (i.e. increasing L/[R.sub.DC]) means: increasing its size.

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The transistor causes static and transient losses. Static power dissipation is proportional to [R.sub.DSon] but low [R.sub.DSon] causes high gate and drain charge. Gate charge times switching frequency is the average current that has to be provided by the control circuit, and the product of drain charge, DC supply voltage, and switching frequency is dissipated by the transistor itself. Therefore the design target should be to minimize the total power dissipation, especially considering that the transistor's on-time duty cycle is typically pretty small.

To view the expanded online version of this article, visit www.ecnmag.com/tags/topics-and-features/application-solutions/

Erhard Muesch, ZMDI, www.zmdi.com
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Title Annotation:Application Solutions
Author:Muesch, Erhard
Publication:ECN-Electronic Component News
Date:Nov 15, 2010
Words:759
Previous Article:Driving LEDs: a fresh look.
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