Nettop Platform for 2008 System Design White Paper

System Configuration
20 White Paper
4.3.2 Common Internal Power Supply
Common internal power supply form factors include ATX12 V, TFX12 V, SFX12 V and
Flex ATX form factor. Using a common internal power supply has a comparative lower
cost than other power solutions. A system designer should consider the power supply
cost together with the system overall size. The mechanical dimension can be found in
the Power Supply Design Guide for Desktop Platform Form Factors.
4.3.2.1 Energy Star* Qualified Internal Power Supply
Tier I of the ENERGY STAR* computer specification as documented in ENERGY STAR*
Program Requirements for Computers: Version 4.0 requires that the internal power
supplies for compliant computers be at least 80% efficient. The efficiency is specified
at 20%, 50% and 100% of the rated output capacity. In addition, the power supply
needs to have a power factor of at least 0.9 measured at 100% of the rated output
capacity. In order to meet the power factor requirements, internal power supplies will
need to incorporate active power factor correction. Active power factor correction
consists of wave shaping circuitry on the AC input side of the power supply to improve
the power factor. It is unlikely that power supplies with passive power factor
correction will be able to satisfy the 0.9 power factor requirement. To meet Energy
Star* Version 4.0 internal power supply requirement, the following items need to take
consideration:
• Overall Efficiency Measurements
Desktop computer systems typically use internal power supplies with multiple
outputs. In many cases, the sum of the output power capability for the individual
outputs exceeds the total output capacity of the power supply. For these cases, a
method of proportional loading needs to be applied in order to measure or
calculate the efficiency of the power supply. This proportional loading method is
explained in detail in the Generalized Internal Power Supply Efficiency Test
Protocol which is available from
www.efficientpowersupplies.org.
• Power Supply Sizing
Generally the efficiency of the power supply drops off significantly as the load falls
below 20%. If an oversized power supply is used, it will operate in this inefficient
range when system operates in idle state. Choose a right size power supply can
minimize the power supply loss which allows additional budget for other system
components to meet the Energy Star* requirement.
• 5 V Standby (5 VSB) Efficiency
5 VSB output is the only output present for the sleep and standby states. In order
to meet the sleep and standby targets for ENERGY STAR*, the power supply will
need to have good efficiency performance for the 5 VSB output. The Power
Supply Design Guide for Desktop Platform Form Factors available at
www.formfactors.org has guidelines for 5 VSB efficiency that can be used to guide
purchase decisions to ensure good efficiency for this portion of the power supply.