Uninterruptible power supply with low power loss

An uninterruptible power supply comprising:

• an energy storage unit having an end selectively connected to an energy storage unit connecting node and the other end connected to a common terminal for storing electric energy;
• an AC-to-DC converting circuit for converting an input AC voltage into a first DC voltage;
• a charging circuit connected to said AC-to-DC converting circuit and said energy storage unit connecting node for converting said first DC voltage into an energy storage unit connecting node voltage required for charging said energy storage unit;
• a selection circuit connected to said AC-to-DC converting circuit, said energy storage unit connecting node and a load for selectively outputting said first DC voltage or said energy storage unit connecting node voltage to said load;
• a primary detecting circuit connected to an output loop of said uninterruptible power supply for detecting an operating status of said uninterruptible power supply and issuing a corresponding operating message;
• a primary control circuit connected to said primary detecting circuit and said selection circuit for controlling operations of said uninterruptible power supply;
• and
• a secondary control circuit connected to said primary detecting circuit, said selection circuit and said primary control circuit for controlling operations of said uninterruptible power supply, wherein said primary control circuit and said secondary control circuit are enabled or disabled according to said operating message.