Articles: Cases/PSU
 

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Unfortunately, one of the primary tests of our methodology – measurements of the stability of the voltages – was far from visual since we used different load patterns for almost every tested unit, and we couldn’t discuss and compare the results of different PSUs without constantly referring to the peculiarities of the employed patterns. In other words, the results of each PSU used to come with a heap of conditions and reservations, so although a comparison could be made (otherwise our testing wouldn’t make any sense at all), these reservations, alas, made it more difficult to directly compare the numbers we got and the diagrams we constructed.

This article is an introduction of our new methodology of testing power supply units that is going to replace the older voltage-stability-measurement method. The new methodology is going to provide a visual and at the same time very accurate and objective result, suitable both for comparing different PSUs in numbers as well as “by the eye”, i.e. by the look of the resulting diagrams. Our new methodology involves a construction of the so-called cross-load characteristic of the PSU as developed and introduced by our colleagues from the news source ITC Online, but we’ve improved it to make it more visual and informative.

In this article I will also cover various aspects of PSUs used in computers so that those of you who are not well versed in the circuit design of switch-mode power supplies could understand where the various parameters of PSUs come from and what they mean. If you know how switch-mode power supplies are built and how they work, you can skip the first two sections of this article to get right to the description of our test equipment and test methods.

Linear and Switching Power Supplies

As you know, an electrical power supply is a device that solves the problems of adjusting, controlling or stabilizing the input electrical power; its acts as an interface between a power source and a load.

The simplest and most widespread control method is the consumption of the excess power in the control device, i.e. the power is trivially dissipated as heat. Such power supplies are referred to as linear.

The schematic above describes an example of such a device: a linear voltage regulator. The voltage in the power grid (220 volts) is stepped down by the transformer T1 to the necessary level and is then rectified by the diode bridge D1. Evidently, the rectified voltage must always be higher than the output voltage of the regulator. In other words, some surplus power is required due to the very principle of operation of the linear regulator. In our example, this surplus power is dissipated as heat on the transistor Q1 which is controlled by a certain circuit U1 in such a way as to output the required voltage Vout.

This design has two significant drawbacks. First, the low frequency of the alternating current in the power grid (50 or 60 hertz depending on the country you live in) calls for a big and heavy step-down transformer – a 200-300W transformer would weigh several kilograms. Second, the voltage on the transformer’s output should always be higher than the sum of the regulator’s output voltage and the minimal voltage drop on the control transistor. It means the transistor generally has to dissipate quite an amount of excessive power, which is going to negatively affect the efficiency factor of the whole device.

To solve these problems, the so-called switch-mode voltage regulators were invented which control power without dissipating it right in the control device. You can imagine a simplest sample of such a device as an ordinary switch (a transistor can be used as one) attached in series with the load. In this design the average current flowing through the load depends not only on the resistance of the load and the supply voltage, but also on the frequency of the switching – the higher this frequency, the higher the current is. Thus, changing the switching frequency we can control the average current through the load, and ideally the power won’t dissipate on the switch itself, since the switch has just two states: fully open or fully close. In the former case, the voltage drop on the switch equals zero; in the latter case the current equals zero and the dissipated power (which is the product of current and voltage) is zero, too. Of course, we don’t have such an idealistic picture in reality due to two reasons. First, when transistors are used as switches, a minor voltage drop occurs on them anyway even when they are closed. Second, the process of switching doesn’t happen instantly. These losses are side events, however, and they are much smaller than the excessive power dissipated on the control device of a linear regulator.

 
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