The LED replacement needs to be controlled as a constant-current source. A 4W led MR lamp is the equivalent to a 20W halogen lamp design. Dimming is a feature found on some models, with the trend toward dimming increasing in availability.
The top challenges for the MR11/16 design are the lack of standards on the lamp fixture and the bulb shape, the power factor, total harmonic distortion requirement (Energy Star for LED luminaries ≥0.9, integral lamp ≥0.7 for >5W), and low system power efficiency. The small space for the LED driver can be appreciated by considering the Fig. 2 lamp dimensions, which must include the driver.
There are two types of printed circuit board form factors. One shape, shown in Fig. 3, is round to adapt with the LED module back side. The round diameter should be smaller than 30 mm with taller components located within 5 mm from the center connector. The other, shown in Fig. 4, is vertical; it needs to be smaller than 30 x 20 mm.