HP Common Slot power supply technology, 3rd edition

7
Load Balanced and High Efficiency Modes
An HP ProLiant server configured with the HP 460-watt, 750-watt, or 1200-watt CS power supply supports the following
three scenarios:
A: Operating with a single supply
B: Operating with redundant supplies in Load Balanced Mode
C: Operating with redundant supplies in High Efficiency Mode
Figure 3 compares the efficiency of the HP 750-W CS Platinum Plus power supply under these three scenarios. As
indicated by Curve A, a single 750-watt power supply supporting the entire load of the server can achieve the highest
efficiency when operating in the middle range of its capacity, although redundancy is not provided.
For redundant 750-watt supplies operating in Load-Balanced Mode (the default mode), the load is shared equally
between the two supplies. As shown by Curve B, Load-Balanced Mode results in a significant efficiency penalty over
much of the server’s operating range compared to a single power supply supporting the entire load.
When High Efficiency Mode is enabled for redundant supplies, each power supply in the server is designated as either a
primary or secondary supply and the entire server load is shifted to the primary power supply. This allows the primary
power supply to operate at higher efficiency points on the load curve while the secondary power supply operates in idle
mode, providing no output power and consuming very little energy (typically two to four watts per supply). The result,
as shown by Curve C, is that High Efficiency Mode achieves efficiency levels that come close to those of a single supply
system.
Figure 3: Efficiency comparison of single and redundant HP 750W CS Platinum Plus power supplies
As indicated in Figure 3, High Efficiency Mode with 750-watt supplies is beneficial for loads up to approximately 450
watts. In general, Load-Balanced Mode offers better efficiency for loads requiring more than 60% of the primary power
supply capacity.
High Efficiency Mode is enabled through the server’s ROM-Based Setup Utility (RBSU) under System options →
Redundancy options. When enabling High Efficiency Mode, the user can specify that the odd or even power supplies will
be designated manually or automatically as secondary supplies. This flexibility allows users to balance the load across a
rack manually or automatically.