Abstract: Disclosed is a firmware update system for a micro-controller unit in a power supply unit. The micro-controller unit includes a central processing unit and a flash memory connected to the central processing unit. The flash memory includes a boot program sector, a main program sector, and a temporary data sector, where the boot program sector contains a boot program and the main program sector contains a main program to be executed by the central processing unit under a normal operating mode, and the temporary data sector is set to contain a downloaded firmware code to be copied to the main program sector to replace the main program, thereby updating the flash memory. The downloaded firmware code contains a sector checksum value and a firmware signature for allowing the boot program to validate if the downloaded firmware code is valid and authentic, and the boot program sector and the main program sector are accessed by a virtual address.