Articles: Cases/PSU

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Power-Supplying the PC

Today all power supplies employed in computers are of the switching variety because the same wattage density and efficiency at reasonable dimensions and heat generation are not achievable with linear power sources of the same wattage. For example, the wattage density of a typical ATX power supply is 2-5 watts per sq. inch (depending on its output power), and its efficiency factor is not less than 68 percent at the highest load.

The flowchart above represents the design of a typical computer power supply, while the picture below shows you a typical variant of placement of the components in a real PSU (I use a Macropower MP-300AR as an example here – the majority of other models would look much the same):

The 220 volts (or whatever there is in your country) from the wall outlet pass through a dual or triple filter which protects the other devices attached to the power grid from the noise generated by the PSU. After this filter the voltage comes to the rectifier D1 and then to the optional Power Factor Correction circuit (which appears more frequently in newer power supply models). We’ll discuss shortly what this correction is and what it does. Right now I want to linger round the filter since it involves a couple of questions often asked by the users.

This is a schematic of a classic dual filter used in the majority of power supplies. As you probably know, there can be two kinds of interference: differential-mode interference when the interference current is flowing in opposite directions in the wires, and common-mode interference when the interference current is flowing into one direction only. We can also say that differential-mode interference is interference between two hot wires, while common-mode interference occurs between a hot wire and a ground wire.

Interference of the first type is quite easily suppressed in this schematic with the chokes Ld and the capacitor Cx. The resistance of the chokes is too high for high-frequency noise to pass, while the resistance of the capacitor is, on the contrary, too low. It’s worse with common-mode interference. The choke Lc suppresses it somewhat – this throttle’s coils are wound in such a way as to produce a high resistance to this kind of interference, but that’s not enough and the two capacitors Cy are installed to effectively get rid of common-mode interference. The point where these two capacitors are attached to each other is connected to the case of the PSU – and to ground if possible.

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