it inputs ac line power and drives the LED directly, has a simple circuit structure, and does not use any electrolytic
capacitors, so it can be implemented at a low cost while lengthening the lifetime of the circuit. The previous ac-direct
LED driver uses several strings of LEDs as a load [22], [23]. The number of strings powered by the driver varies as the input ac voltage changes over time. This drive method results in a good power factor PF without any separate PFC circuit.
However, the previous ac-direct LED driver can operate only at a fixed input voltage determined by the highest turn-on
voltage of the LED strings, and has poor power efficiency PE because any difference between the input and LED turn-on