PWM調光下的led色彩漂移

　　由于固態照明的相關色溫（CCT）隨電壓的變化而變化，發光二極管（LED）對傳統照明所采用的模擬方法要求不同的調光技術。這樣的顏色變化可以被消費者察覺，并破壞了對發光二極管質量的要求。

　　建立的方法是避免模擬調光在數字技術如脈沖寬度調制（PWM）支持。制造商聲稱使用PWM確保LED在調光過程中不受顏色變化的影響。

　　然而，最近的研究表明情況并非如此，因為不同的PWM占空比改變了LED的結溫，而結溫本身也能改變芯片發出的顏色（色度）。

　　本文回顧了PWM調光，仔細研究了它對結溫的影響，并考慮到消費者是否察覺到色度的任何變化。

　　Driving and dimming LEDs

　　The efficient operation of an LED relies on a carefully managed power supply because the devices feature a non-linear forward voltage/forward current relationship.

　　An LED is， as the name states， a form of diode. In normal operation， a constant forward voltage of sufficient magnitude （typically with the LED in series with a resistor） such that the device is operating in its （narrow） conduction region， is applied. Forward voltages for commercial high-brightness devices vary， depending on whether the chip is a low-， medium-， or high-voltage device， but a typical operational range for a low-voltage device would be 2.7 to 3 V.

　　The forward current determines the relative luminous flux （essentially the brightness） of the LED. Figures 1a and 1b illustrate the forward voltage versus forward current and forward current versus relative light output for a Philips Lumileds Luxeon TX LED. This chip produces 289 lm at 1 A and 101 lm/W （at 700 mA/2.8 V）。