"24V DC Cooling Fan" DC distributed power system is the most widely used structure. It generally comprises an AC-DC AC front end module (or a plurality of front-end modules and connected, redundancy techniques may also be used), the front-end module converting the AC voltage into a DC voltage 24,48V or 300V, and a DC distribution bus formed. By the DC bus to the system load of each board, the DC-DC conversion module load board again generate a DC voltage required by the load. "24V DC Cooling Fan" DC-DC conversion may be necessary. For example, a load board needs 5V and 2.1V two DC voltage, 5V voltage can use a DC-DC module is available from 48V bus, 2.1V voltage DC-DC module with another to get better from 5V. It should be noted that in a typical telecommunications equipment, the front-end module is not necessarily seen as perhaps 48V dc bus came from very far away, perhaps provided by a battery. "24V DC Cooling Fan" typical DC distributed power systems