Unlike incandescent bulbs, which are all dimmable by default, LED retrofit bulbs have a built-in driver in their base. The driver converts AC power to DC power and maintains a constant current to the LED. This is at odds with a phase control dimming system, since the driver attempts to compensate for the sliced-out portions of input voltage.
LED fixtures such as downlights usually include the LED driver, either of a ‘constant current’ or ‘constant voltage’ type, depending on the LED array design. In either case, the same issue arises: the LED driver or power supply will try to patch up the missing parts of input voltage.
However, LED compatibility problems can exist, and some dimmable LED driver designs will only work with selective dimming control systems. These problems can show up in a number of ways, including flickering, flashing, and dead travel.