Electronic – I don’t understand the text about LED’s type and power consumption


On my web-surfing, I found some information about LED's type and power consumption.

I don't understand below (Red-lines.)

Why 'common cathode' power consumption is lower than 'common anode'?

Please explain exactly.

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Best Answer

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Figure 1. LED current vs forward voltage curves. Note that red has much lower \$ V_F \$ than green and blue.

Red LEDs have a lower forward voltage than green and blue. If connected directly to the same supply as the green and blue they would draw a very high current and would be destroyed. To prevent this we normally add series resistance. On a matrix display this would require one resistor per LED and, as they point out, would increase PCB size.

The problem could be solved by sinking the current into a dedicated negative rail of, say, +0.5 to +1V, for the red LEDs. This would be an unusual topology however and a simpler solution is to feed all the reds from a lower voltage supply with common negative rail switching for all three LEDs. This solution also has the advantage of high efficiency.

The additional Vdd supply would be very easy to incorporate into a custom switched mode power supply with the addition of another transformer winding or tap, rectifier and smoothing capacitor. The common negative switching of the common cathode LED is much easier to implement than the high-side switching of the common anode LED.