While designing LED products with larger number of LEDs, the designer is usually faced with this choice; to place the LEDs in series first and then ties up the strings into parallel OR to first parallel the LEDs and then string them into a series.
I strongly recommend the second choice, as illustrated in my sketch on the right side (Fig 2) of the illustration.
This Parallel first design has the following advantages:
· If one LED shorts the LED currents remain the same as the original intended design value OR zero (in the shorted parallel; so all LEDs remain safe. In the other case, if one LED shorts, the current in that string will shoot (what we call current hogging) and the full series string of LEDs will get subjected to very high current.
· If one LED opens, only the few LEDs in parallel face an increased current; the rest majority remain at original intended design current. In the other case, if one LED becomes open, the current in the balance strings shoots up.
Arun’s Design Diary