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How computer power supplies work – KitGuru Guide

Even though there are more than eight types of power supplies, for household and business equipment there are only two basic power supply designs, linear and switching power supplies.

Linear power supply units are relatively simple and common for low power applications. They fundamentally work by stepping down the AC voltage of the utility power grid, rectify it to DC voltage and finally filter it by using a capacitor.

Linear Power Supply (18V - 2A max)

In order to step the voltage from the 90-240V utility grid voltage down to a low voltage usable by the equipment, linear power supplies use a transformer. The transformer steps down the supplied AC voltage to a lower value (e.g. 18 VAC) but the original waveform remains unchanged. Then a rectifier is used to transform the sine wave AC voltage into a fully rectified pulse voltage. As this rectified voltage still resembles AC, it cannot be used yet. Filtering through a capacitor is required to transform this voltage to near-DC. Then a voltage regulation stage is necessary to adjust this near-DC voltage to true DC voltage and sustain it regardless of changes in the load current.

In very small and simple applications that can be done by using a Zener diode but most of the time a full voltage regulation circuit using a power transistor operating in its linear region is necessary. This power transistor has the ability to act as a variable resistor in series with the load, regulating the output current and voltage accordingly. It is controlled from a circuit sensing the output voltage and modifies the transistor bias to maintain a set voltage output despite of any load current changes.

Linear power supplies have many desirable characteristics. They are very easy and cheap to manufacture because of the few and common components, which also makes them very reliable when correctly designed. Their performance is excellent as well, with exceptional output voltage regulation and next to non-existent ripple. Finally even the lowest quality products show very little electromagnetic interference (EMI) and exceptionally fast response times.

Common Linear PSU schematic

With so many advantages people would expect linear power supplies to dominate today’s technology market, yet that is not possible because of their two main disadvantages. Even though linear power supplies are simple and easy to manufacture, they are only used for simple and low power applications because their size and weight increases dramatically as their power output increases.

This primarily has to do with the input voltage frequency, as we will later explain. Early linear power supplies for electronic equipment would not output more than 10A, yet still they used to weight more than 10kg and had electrolytic capacitors as large as coke cans. Linear power supplies are also extremely inefficient, with even the best of products not being able to offer an electrical efficiency higher than 45-50%.

The extremely low efficiency not only is a waste of precious power but it also is a very large heat source which needs to be dealt with by implementing corresponding cooling solutions. Imagine having a small linear power supply powering an office computer; even a 300W power supply with a 50% efficiency would need to dissipate another 300W as heat and would be at least 6 times the size of a common ATX design. These disadvantages make the linear power supplies useless for a wide range of applications. Of course the technology progressed since their first appearance but linear power supplies are still usable only for simple, low power applications such as AC chargers and laboratory power supplies.

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