The AC-DC MR lamp integrates the LED driver into the lamp case without the need for the ballast transformer. It is possible to achieve over 80% total power efficiency in this configuration. In general, it is not easy to build an AC-DC LED driver board into the small bulb case while also meeting PF and THD requirements in the application.
It is also preferred not to use an electrolytic capacitor whose life time is less than the other semiconductor components or passive electric components like the resistors, ceramic capacitors, and inductors. The AC-DC MR-type LED bulb design is a new design challenge.
With direct AC line input voltages, it is possible to achieve a small PCB outline adaptable to the MR lamp case. This LED driver device eliminates the need for all electrolytic capacitors typically used for the input, output, and IC Vcc voltages. Eliminating the electrolytic capacitors lengthens the product life and reduces the PCB space, while resulting in a low BOM cost. Using only a few external components, it meets PF and THD requirements, while achieving efficiency over 80%.
The buck topology also has the advantage of constant output current versus a boost design, since the inductor is placed in series with the output, namely the buck topology looks like a constant current source to the LED load. The boost topology has discontinuous output current unless an output capacitor is used to filter the ripple current.