Embodiments of the invention provide a power supply unit that operates in
a standby mode and in a normal mode. The power supply unit comprises a
first power supply providing output voltages during the standby mode and
a second power supply providing output voltages during the normal mode. A
pre-converter provides a stabilized supply voltage for the first and
second power supply. The pre-converter operates as an up-converter during
the normal operation mode and is switched off during the standby mode
together with the second power supply.