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How much power can you derive from a brick?

Posted: 14 Feb 2014  Print Version  Bookmark and Share

Keywords:intermediate bus converters  IBC  board-mounted power supplies  BMPS  AC/DC power supply 

When the high-power brick industry surfaced some 20 years ago, 100-150 W was the practical limit. Now there are suppliers offering quarter-brick intermediate bus converters (IBC) at up to 864 W, e.g. Ericsson's PKM-NH series, and power density figures that could not even be imagined a few years back. Customers are pushing power per board towards 1 kW and projections are as high as 3 kW to satisfy the most power hungry computer chips. Even if the power brick efficiency figures are increasing asymptotically towards 100%, this will put very high demands on thermal management in the power brick as well as in the end-user system.

Power density is one of the most frequently promoted parameters of board-mounted power supplies (BMPS) and high-power bricks in particular. However, of much more practical importance than the power density rating is the maximum operating temperature and efficiency. Efficiency is exceedingly important for several reasons, it determines: the losses in the system and the amount of cooling required; how much utility power is "wasted" rather than being used for the desired purpose; the physical package sizes of both the BMPS and the final system; and the operating temperatures of the components and the resultant system reliability.

Figure: Interrelationship between BMPS selection criteria.

In actuality, all three of these criteria – efficiency, operating temperature, and reliability – are very interrelated and reinforce each other. Unlike the more typical situation in engineering where desired properties need to be traded-off against each other and cannot be simultaneously optimised, here there is a synergistic relationship between three very desirable properties that can all be obtained with a proper design. That is why BMPS manufacturers put so much design effort into maximising the efficiency of their products.

Intermediate bus converters
The intermediate bus architecture is the common power solution for high-power computer board applications. A regulated or non-regulated Intermediate Bus Converter (IBC) provides isolation and the intermediate bus voltage powering the POL regulators that provides the final supply voltage for the computer chips.

Information and Communication Technology (ICT) equipment supplied by power systems with battery back-up during mains failure have a wide input voltage range of 40.5 – 72 V dc standardised by ETSI EN 300 132 2, and normally use regulated (fully regulated or semi-regulated) IBCs. The regulation function is accomplished by changing the duty cycle of the converter, i.e. the percentage of time that the converter's power switching devices are active. Conversion efficiency is very high but limited by the power losses in the free-wheeling diodes and output filter inductance and the higher voltage rated power MOSFETs required due to the relatively high maximum input voltage.

The voltages in EN 300 132-2 is defined at the interface between the power plant and the ICT equipment. The actual BMPS operational input voltage range is affected by resistive and dynamic voltage drops in the power distribution and is therefore specified to 36 – 72 V dc.

Datacoms equipment with a narrow input voltage range, e.g. 45 – 56 V dc, may use non-regulated IBCs. They operate as DC transformers with fixed 50% duty cycle and have transformer turns ratios of 4:1 or 5:1 when stepping down the voltage from the 48VDC system voltage level to an intermediate bus voltage of 12 V dc or lower. The fixed 50% duty cycle minimises the power losses in the freewheeling diodes and output filter inductance and the conversion efficiency is typically a little bit higher than in regulated IBCs.

Safety regulations
The core safety standard that applies to ICT equipment is IEC 60950 1 Safety of information technology equipment. Now in its second edition, this standard underpins regional adoptions such as Europe's EN 60950-1 and UL/CSA 60950-1, which applies in the US and Canada.

The standard describes appropriate measures to assure safety, such as the need for isolation barriers and protective earthing. The standard's guiding principle is to provide two levels of protection from electric shock and energy hazards that may trigger other dangers, such as fire. This two-level model creates a hierarchy of protection measures that build upon five categories of insulation:
 • Functional insulation
 • Basic insulation
 • Supplementary insulation
 • Double insulation
 • Reinforced insulation

Functional insulation is necessary for the product to function properly. It may reduce the likelihood of ignition and fire hazards, but provides no reliable safety protection from electric shock.

For ICT equipment, the output side of isolated BMPS, such as high-power bricks, normally shall meet the criteria for a SELV circuit that limits voltages to a safe level of maximum 60 V dc under both normal operation and single fault conditions. The isolation requirements that the brick must then satisfy depend on the level of isolation that the AC/DC front-end power supply provides, together with the system's arrangements for connection to protective ground.

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