Hi,

For many switching power supplies, it is recommended to have at least 20% minimum load. If that is not provided, the power supply may operate in discontuous conduction mode (the loop response is not ideal (lower in frequency)). This mode actually can draw additional current but instead of being at the base switching frequency (say 400kHz), it might be between 20kHz to 100kHz.

Power supplies might be based off of a master switcher and post regulates the other voltages. Those may be switchers too or linears. Bottom line is each switching power supply should meet minimum load requirements. That said, not all power supplies need a minimum load to guarantee regulation or prevent discontinuous conduction mode. Other topologies help with both of these issues.

The minimum load is required for each switching regulator output. It is important to make sure that each supply voltage has enough for the demands of your system components. You already indicated that suspect you may not have enough and your disks would spin-up and spin-down. You might have enough for the 3.3V, but lack +5V or +12V capacity.

I'm not advocating placing load resistors on minimally loaded supplies (being energy conscious), but, I also know that a minimally loaded or overloaded supply voltage can cause difficult to locate system problems. I have used power resistors on lightly loaded supplies (often, they resulted in poor regulation and transient response) to fix flaky problems especially under temperature and voltage margins.

Match up the system requirements to the power supply capabilities (including future upgrades) and you will have a more robust system.

Just my $0.02.

Ross