In ATX power supplies, the user cannot directly power up the SMPS. The SMPS is connected to the motherboard, which contro Is it. Standby power is the minimal power (at 5 volts) that is used to keep the motherboard active even when shut down by the user.

This allows the system to be powered up manually by the user by depressing the power button on the cabinet (which actually passes a signal to the motherboard, not to the power supply), or automatically by power-up events like sending a signal over the network (Wake on LAN) or by setting a wake-up time in the motherboard BIOS.