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When the PSU is working at its full wattage, we also measure the pulsation swing of the voltages it outputs. According to the standard, the pulsation swing in the range up to 10MHz should not exceed 50mV for the +5v rail and 120mV for the +12v rail. In practice, there can be pulsations of two frequencies at the output of the PSU: about 60 kHz and 100Hz. The former frequency is the result of the operation of the PWM controller of the PSU (which usually works at 60 kHz) and is present more or less in all PSUs. A typical pulsation is shown on the next oscillogram (the +5v rail is marked with green; the +12v rail is marked with yellow):

This is the case when the pulsation on the +5v rail has deviated beyond the permissible 50 mV. You see the classic triangular shape of the pulsations in the oscillogram, but in more expensive PSUs the switching moments are smoothed out by the output chokes.

The second frequency is the double frequency of the power grid (50Hz) that usually passes through to the output because of an insufficient capacity of the high-voltage rectifier’s capacitors or various design flaws. Such pulsations (they are given with a timebase of 4 ms/del) are usually observed in low-end PSUs and are rare in middle-range products. The swing of these pulsations grows proportionally to the load on the PSU and can sometimes go out of the allowable limits at peaks.

We also attach a generator of rectangular pulses to the PSU at 150W load to measure the amplitude of the pulses on the other wire of the PSU, i.e. not on the one the generator is attached to. Thus we check out the PSU’s overall reaction to such pulse loads and, particularly, how well it suppresses noise from each of the devices attached. Due to the voltage surges provoked by switching, the measurements are not very accurate, but sometimes they yield most curious results.

As for measurements of the efficiency factor and the power factor of the PSU, this is the least interesting and important section of our tests. Our experience says these parameters are very close among different PSU models of the same type, and minor differences are not important at all for the absolute majority of users, so we perform such measurements in rare cases only: the power factor is measured for PSUs for which PFC is declared, and the efficiency factor comes along (in fact, the value of the efficiency factor is produced automatically – no additional measurements are necessary) or is measured specifically if we suspect the efficiency factor of a particular PSU goes out of the acceptable limits, but this is a rare thing.

Winding up this article, I want to say some words about what we are not measuring and are not going to measure, although we could. We don’t trust tests that measure the absolute maximum power as outputted by the PSU – when the load on the PSU is being increased till the PSU fails or is saved by its protection circuitry. Such tests produce results that vary between two samples of the same PSU model as well as depending on how exactly the tester loads the PSU, i.e. how the load is distributed across the rails of the PSU. Besides that, the ability of the PSU to maintain certain wattage is not necessary for a normal operation of the computer. What’s important is the ability of the PSU to output voltages and pulsations within the limits described by the standard, but such tests don’t pay attention to this fact. They produce pretty numbers, which have little relation to reality.

So, our new methods of testing computer power supplies allow to examine the behavior of a PSU in more detail as well as to compare visually different PSU models. The cross-load characteristics make the tests more intuitive and they show objectively and without any additional reservations what this or that PSU is like.

 
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