why some from references are shown in the text as small letters instead of capital letters?

Question

I wonder why in the references some words written in Capital letters are shown in the pdf file as regular letters?

For example:

In references:

@INPROCEEDINGS{7806038, 
author={J. Lin and G. Weiss}, 
booktitle={2016 IEEE International Conference on the Science of Electrical Engineering (ICSEE)}, 
title={The virtual infinite capacitor-based active submodule for **MMC**}, 
year={2016}, 
pages={1-5}, 
keywords={Capacitors;Electrical engineering;Legged locomotion;Switches;Switching frequency;Topology;Voltage control}, 
doi={10.1109/ICSEE.2016.7806038}, 
month={Nov},}

The title that is shown in the pdf file is:

The virtual infinite capacitor-based active submodule for mmc

whereas in the pdf file it should be MMC instead of mmc

Does someone know how to solve it ?