A proper LED driver topology can result in the best cost solution. If the input voltage is 12V or 24V DC, the LED driver DC-DC topology choice is either a boost or buck topology. If the LED total string forward voltage is higher than the rectified input voltage, use a boost topology. Otherwise, use the buck topology.
The DC-DC power stage efficiency is high, it can generally reach up to 90%. However, the ballast transformer efficiency is poor. The ballast transformer is not a switched-mode power supply, just a transformer converting 110V/220Vac to 12V/24Vac. Although the DC-DC power stage efficiency is high, the total system efficiency for the AC-DC transformer + DC-DC topology is low.
The poor system efficiency and PFC and THD requirement need to be solved for the MR LED lamp driver to fit within a limited small PCB space. The current solution using an AC-DC transformer plus DC-DC topology is the current installed infrastructure: a Halogen socket and ballast transformer. It results in saving on installation investment cost, but electrical efficiency is sacrificed. This infrastructure trend is to be replaced by a more efficient configuration. Manufacturers are starting to release an AC-DC MR lamp into the market place.