Articles: Cases/PSU

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High efficiency

Efficiency is the ratio of input power to output power. The higher the efficiency of a PSU is, the less heat it generates and the quieter its cooling can be made. Your electricity bills will be lower if the efficiency is high, too.

The current version of the ATX12V 2.2 standard limits the PSU efficiency from below: a minimum of 72% at typical load, 70% at full load and 65% at low load. Besides that, there are optional numbers (an efficiency of 80% at nominal load) and the voluntary certification program “80 Plus” which requires that the PSU has an efficiency of 80% and higher at loads from 20% to maximum. The new Energy Star certification program to come to effect in 2007 will have the same requirements as in the 80 Plus.

The efficiency of a PSU depends on the input voltage. The higher that voltage is, the better the efficiency. The difference in efficiency between the 110V and 220V power grids is about 2%. Moreover, different samples of the same PSU model may vary in efficiency by 1-2% due to the variations in the parameters of the components employed.

Dual core CPU support

This is nothing but a pretty-looking label. Dual-core processors do not require any special support from the power supply.

SLI and CrossFire support

Yet another pretty-looking label that means two power connectors for graphics cards and an ability to yield as much power as is considered sufficient for a SLI graphics subsystem. Nothing else stands behind that label.

Industrial class components

One more pretty-looking sticker! Industrial class components are components that can work in a very wide range of temperatures. But what’s the purpose of installing a chip capable of working under -45°C into a PSU if this PSU will never be used in such cold weather?

Sometimes the term industrial class components refers to capacitors meant for operation under a temperature up to 105°C, but that’s all clear here, too. The capacitors in the PSU’s output circuits heat up by themselves and also located very close to the hot chokes are always rated for a temperature of 105°C max or their service life would be too short. Of course, there is a much lower temperature inside the PSU, but the problem is that the service life of a capacitor depends on the ambient temperature. Capacitors rated for higher max temperatures are going to last longer under the same thermal conditions.

The input high-voltage capacitors work almost at the temperature of the ambient air, so the use of somewhat cheaper 85°C capacitors there doesn’t affect the PSU’s service life much.

Advanced double forward switching design

Alluring the potential customer with mysterious terms is a favorite trick of the marketing department.

Here, the term means the topology of the PSU, i.e. the general concept of its circuit design. There are quite a number of different topologies. Besides the double forward converter, PC power supplies may use a forward converter or a half-bridge converter. These terms are only interesting for a specialist and don’t mean much for an ordinary user.

The choice of the particular PSU topology is determined by a number of reasons like the availability and price of transistors with required characteristics (they differ greatly depending on the topology), transformers, controller chips, etc. For example, the single-transistor forward converter is simple and cheap but requires a high-voltage transistor and high-voltage diodes on the PSU output, so it is only used in inexpensive low-wattage models (high-voltage diodes and transistors of high power are too expensive). The half-bridge converter is somewhat more complex, but has a two times lower voltage on the transistors. So, this is generally a matter of availability and cost of the necessary components. I can predict, for example, that synchronous rectifiers will be sooner or later used in the secondary circuits of PC power supplies. There’s nothing new in that technology, but it is too expensive as yet and its advantages don’t cover its cost.

RoHS (Reduction of Hazardous Substances)

This is a new European Union directive that limits the use of certain substances in electronic equipment since July 1, 2006. It restricts the use of lead, mercury, cadmium, hexavalent chromium, and two bromides. For power supplies this mainly means a transition to non-lead solders. Yes, we are all for ecology and against heavy metals, but a too hasty transition to new materials may have unpleasant consequences. You may have heard the story about Fujitsu’s MPG hard drives which would die due to a failure of Cirrus Logic controllers that had a packaging made of some new environment-friendly compound from Sumitomo Bakelite. The elements of the compound facilitated the migration of copper and silver that formed bridges between interconnects inside the chip case. As a result, the chip would fail almost certainly after 1 or 2 years of operation. The compound was abandoned eventually, and the involved companies exchanged lawsuits, but nothing could restore the data that were lost with the hard drives.

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