AN2711 datasheet(13/36 Pages) STMICROELECTRONICS

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AN2711

Filtering the LED current - electrolytic capacitors revisited

Doc ID 14425 Rev 3

13/36

Filtering the LED current - electrolytic capacitors

revisited

3.1

Strobe effects

Earlier designs used only a few microFarads (µF) of filter capacitors across the LEDs. When

the dimmer was turned right down, current flowed for less than 1 millisecond out of each half

cycle. The result was that rapidly moving objects appeared as multiple stationary images,

similar to the appearance of a hand moving rapidly in front of an old TV screen. Reflections

from moving shiny objects such as jewelry appeared as dotted lines in the air.

Many users had noticed the effect. The collective opinion was that LED peak-to-peak ripple

current should be less than 20 % to 25 % of the average current. The use of electrolytic filter

capacitors had to be reconsidered.

3.2

Benefits of electrolytic filters

Once it was understood that very large output capacitors were required, other advantages of

their use became apparent.

3.2.1

Reduced crest factor in LEDs

The bonding wires in LEDs are very small - sometimes almost invisible. High RMS current

causes the wire to heat the bonding point on the LED die, leading to failure. The RMS

current of a power factor controlled converter is higher than necessary for the same light

output, by about 1.2 times. Resistive heating is about 45 % higher than for pure DC for the

same average current.

3.2.2

Better regulation of housekeeping voltages

With a narrower range of peak current, the LED load voltage range is considerably narrower.

Reflected voltage, even with dimming, is much better controlled.

3.3

Electrolytic life calculations

An LED life cycle is advertised as 50,000 to 100,000 hours of operation at 70 % light output.

The driver electronics should be designed to match or exceed this.

A capacitor life cycle is rated at maximum operating temperature and maximum ripple

current, which typically adds 10 °C to the core temperature. The life cycle doubles for every

10 °C reduction in core temperature. So, for a 105 °C capacitor rated for 10,000 hours at 1 A

of ripple current, operated continuously at 85 °C case temperature and 1 A ripple current, a

life cycle of 40,000 hours can be expected.

In the real world, continuous 85 °C temperatures are rare. A minimum of double the

calculated life cycle can be expected. So, with the proper choice of capacitors, the life cycle

can be in the same range as the LED life cycle, and is, therefore, no longer a serious

concern.

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