After a lengthy hiatus, we're back with a new PSU and case reviewer. As we kick off our revised power supply testing and reviews, we wanted to cover the fundamentals of how we test and what to expect. Some of this is still a work in progress, as we have not gathered all of the equipment we would like to have, and as we move forward we will periodically provide updates to our PSU testing procedures. And with that out of the way, let's discuss how we're going to go about testing power supplies.

Effective testing of a power supply requires far more than just connecting it to a PC and using a $10 multimeter to check the voltage rails. At the very least, it requires specialized (and very expensive) equipment. At this point, most people that actually know a few things about PSUs would say, "Yes, OK, you need an adjustable load and an oscilloscope." While it's true you need those items, you can't simply grab any old adjustable load and oscilloscope. What you really need is very precise, programmable electronic loads with transient testing built-in and an oscilloscope that should comply with exact specifications, among other meters and equipment. Then of course you need to know what you are doing, as it's not simply a matter of connecting a PSU to the equipment and pressing a few buttons; there are exact loading and testing procedures, described in technical papers and guides, that need to be followed.

Programmable DC loads are an absolute necessity if you want to test a power supply. To that end, we acquired two high precision Maynuo M9714 1200 Watt and two Maynuo M9711 150 Watt electronic loads, which will allow us to draw up to 2400 Watts from 12 Volt lines and up to 150 Watts from each of the 3.3 Volt and 5 Volt lines. As these are quick-response programmable models, they will also allow us to perform transient tests in the future.

When testing a power supply, using even the best of multimeters are entirely useless. An oscilloscope is an absolute necessity and not just any oscilloscope. Intel's ATX design guide denotes that the oscilloscope should have a bandwidth of 20MHz; however, things are not nearly as simple as that. Even if you do want to purchase a proper oscilloscope, buying a 20MHz oscilloscope is a mistake. Digital oscilloscopes need to be capable of acquiring samples at least ten times faster than the frequency they are required to resolve. So, you need a 20MHz oscilloscope with a sampling rate of at least 200 MSa/s, and low range or USB connected devices cannot get anywhere close to that number.

There are of course many other minor details but we will not bore you with those. It should suffice to say that for the time being we are using a Rigol DS5042M oscilloscope, which has a bandwidth of 40MHz and a real time sampling rate of 500 MSa/s. Although that sounds impressive, actually even this device is not good enough if you want to perform transient tests properly and it cannot resolve noise out of the ripple of a signal; these are tests we plan to add in the future.

Compared to the above items, testing the efficiency of a PSU is relatively simple, once you know exactly how much power you are drawing from it. Our electronic loads tell us exactly how much power is being drawn at a given time; therefore, we only need a good AC power analyzer to tell us how much power the unit is drawing from the AC outlet. Note the "good" part, as you need a power analyzer capable of displaying true RMS values, as PSUs can generate a great deal of harmonics.

Our Extech 380803 power analyzer does a very good job at reporting the level of power that our PSU requires at any given time. We should note that all testing is being performed with a 230V/50Hz input, delivered by a 3000VA VARIAC for the perfect adjustment of the input voltage. Unfortunately, we cannot perform tests at 110V/60Hz at the moment, as that requires a high output, programmable AC power source. As a rough estimate, conversion efficiency drops by 1% to 1.5% when the input voltage is lowered to 110V/ 60Hz.

Thermal and noise testing are another complex procedure. Thermal testing is relatively simple; we only had to acquire two high precision UNI-T UT-325 digital thermometers. With four temperature probes, we can monitor the ambient temperature, the exhaust temperature of the PSU, as well as the temperature of its primary and secondary heatsinks. Noise testing however cannot be performed while the unit is being tested, as the very equipment that is used to test it generates a lot of noise. Everyone says that it is impossible to keep the unit loaded with the equipment far apart in order to perform noise testing and yes, that truly is impossible. So, it cannot be done, right? Wrong.

One of the basics of the scientific method is that you isolate the problem from a system and resolve it on its own. In other words, instead of trying to do the impossible and measure the noise of a power supply while we are testing it, there is nothing keeping us from using a non-intrusive laser tachometer to record the speed of the fan instead. Then, we can simply test the unit on its own, with the fan hotwired to a small fanless, adjustable DC PSU that we fabricated, taking noise readings with our Extech HD600 for the RPM range of the fan and cross-referencing the two tables. Not quite that difficult, was it? There is a catch however; as the unit will not be powered at the time of sound level testing, the meter cannot record any coil whine noise. Coil whine is clearly audible during testing though and we will make sure to report it if (when) we encounter a PSU whose coils could have used a little bit more lacquer. The background noise of our testing environment is about 30.4 dB(A), which figure resembles a quiet room at night. Equipment noise usually becomes audible when our instrumentation reads above 33.5 dB(A).

In order to facilitate testing power supplies more effectively, we created a test fixture for the connection between the PSU and the testing equipment, as well as a proprietary hot box. The hot box is not much more than a closed case with an air-heating device, which is controlled via a DAQ and our software. It is imperative to heat the air inside the box, not the box itself, in order to create good testing conditions. Admittedly, this self-made contraption is not perfect as it is small and has a very slow reaction rate, but it does work well for the means of simulating the environment inside a computer case. Therefore, testing will be performed at room temperature (maintained at 25 °C) and inside the hotbox (at 45-50 °C). Remember that efficiency certifications are performed at room temperature (25 °C) and a power supply can easily fail to meet its efficiency certification standards inside the hotbox!

As for the testing procedure, there are specific, detailed guidelines on how to perform it. All testing is done in accordance with Intel's Power Supply Design Guide for Desktop Form Factors and with the Generalized Test Protocol for Calculating the Energy Efficiency of Internal AC-DC and DC-DC Power Supplies. These two documents describe in detail how the equipment should be interconnected, how loading should be performed (yes, you do not simply load the power lines randomly), and the basic methodology for the acquisition of each data set. However, not all of our testing is covered and/or endorsed by these guidelines.

(Update, Apr 20th, 2015, Click for the detailed post)

Even though these documents are just a few years old, their methods fail to account for modern "enthusiast grade" computer SMPS. The industry has been making leaps on the creation of more energy-efficient devices, continuously lowering their power requirements. Nowadays, the vast majority of computers that require very powerful PSUs simply employ multiple components, such as numerous graphics cards. As the majority of energy-consuming components require a 12 V source, PSU manufacturers have been continuously driving the 12 V output of their units upwards, while the 3.3V/5V outputs remained inert or are getting weaker. There are many design rules that modern "enthusiast-grade" PSUs do not adhere to nowadays, such as the current safely limits and the maximum size of the chassis, but this particular change creates a problem with the generalized test protocol.

According to the generalized test protocol, the derating factor D of the 3.3V/5V lines should be:

Simply put, the formula is maximum rated power output of the unit divided by the sum of the power output ratings of each individual power line.

However, this formula frequently leads to the overloading of the 3.3V/5V lines with >1 kW PSUs. The effect is particularly severe in some high efficiency units, in which the designers moved the 3.3V/5V DC-to-DC conversion circuits on the connectors PCB, reducing their maximum power output significantly. Although some PSUs would operate normally even if their 3.3V/5V lines were overloaded, the continuous degradation of the 3.3V/5V lines in comparison to the 12 V line resulted to PSUs appearing in our labs that could not operate under such conditions. Therefore, we had to alter the derating factor formula in order to compensate for real world testing. Without at least two significant energy consumers, no modern system requires > 500 Watts. Greater power demand suggests the presence of devices that load only the 12 V line (i.e. GPUs, CPUs, liquid cooling pumps, Peltier effect coolers, etc.). After certain calculations and research, for units with a rated power output over 400 Watts, we will be using the following formula:

Which effectively half the impact of the 3.3V/5V lines on the calculation of the derating factor, imposing the difference on the 12V line. Furthermore, the loading criterion of the 3.3V/5V lines for a load rating X (in % of the unit's maximum output) is now changed to:

For the 12 V line(s), the loading criterion remains unchanged.

This formula results to the more realistic representation of the requirements that actual systems have, at least up to a power output realizable today.

(End of update)

Furthermore, there are no guidelines on how transient tests should be performed and the momentary power-up cross load testing that Intel recommends is far too lenient. Intel recommends that the 12V line should be loaded to < 0.1A and the 3.3V/5V lines up to just 5A. We also perform two cross load tests of our own design. In test CL1, we load the 12V line up to 80% of its maximum capacity and the 3.3V/5V lines with 2A each. In test CL2, we load the 12V line with 2A and the 3.3V/5V lines up to 80% of their maximum combined capacity.

Furthermore, it has been suggested that efficiency testing needs to be performed at specific load intervals (20% - 50% - 100%), which is considered to be the normal operating range of a PSU. However, modern systems can easily have their energy demand drop dramatically while idling, which is why we will be testing power supplies starting at 5% of their rated capacity, not 20%. Note that the conversion efficiency of all switching PSUs literally takes a dive when the load is very low, so large drops of >10% are expected and natural.

Any questions or comments on our PSU testing procedures are welcome, and as noted earlier we plan to add and/or improve some of the testing over the coming months with some additional hardware. We will provide an updated article when/if such changes are required.

POST A COMMENT

50 Comments

View All Comments

  • DanNeely - Saturday, March 1, 2014 - link

    Before spending lots of money shipping expensive hardware back and forth to maintain calibration, I'd recommend getting quotes on having it done on site. After a several thousand dollars repair bill for a high end oscilloscope that was damaged in transit my employer discovered it was cheaper to pay a company 2 hours away to have someone drive out to cal about a half dozen bits of RF hardware and a few torque wrenches than it was to ship the same hardware to their office and have it done there. Reply
  • pvdw - Friday, February 28, 2014 - link

    Great to see some real study being put into prepping for these reviews!

    Just one thought, your noise floor seems rather high by modern standards. I know Anandtech focuses more on gaming PSUs which are generally noisier, but you're not going to be able to separate quietish PSUs from truly silent ones until you can go much lower.

    I believe these dBA levels are right:

    0          The softest sound a person can hear with normal hearing 
    10         Normal breathing 
    20         Whispering at 5 feet 
    30         Soft whisper
    Reply
  • E.Fyll - Friday, February 28, 2014 - link

    Actually no, these figures are common misinformation going around the internet. The floor noise inside sound-recording studios is 18-22 dB(A); just a hint higher than what the equipment of an ISO testing lab would read with you standing in the middle of the room, just because you are breathing and your heart is beating. The floor noise of a common household room usually is over 30 dB(A), commonly near 40 dB(A). You cannot even measure figures much lower than that without a noise and vibration isolated room, let alone specialized equipment that costs tens of thousands of dollars. Even assuming that you can create a perfect, undisturbed low-noise environment anywhere near a household, it is extremely difficult to acquire such a low power energy signal and convert it to a usable electric signal with any sense of accuracy, especially when even the vibrations of the air's molecules will affect the equipment, let alone things such as electromagnetic fields and wireless signals.

    Also, dB(A) figures are not linear and stand-alone, they are logarithmic and additive. This means that if you have a floor noise of 30 dB(A) and add a 20 dB(A) noise source into the room, the floor noise will not remain at 30 dB(A); 20 dB(A) will be added to it and not linearly, logarithmically. So, a product that tests at 20 dB(A) inside a zero-dB(A) environment of a lab will not read 20 dB(A) when testing it inside a common room but it will bring up the floor noise of the room it is being tested in. Not perfect, but it is realistic testing and all we can provide for reference. Replicating ISO lab conditions in my household and acquiring tens of thousands of dollars worth of equipment = not going to happen (unless of course if I become a billionaire, in which case I promise you that I will do my best to build my own lab in my huge new back yard).

    When a manufacturer says that a fan is rated at "17 dB(A)", it does not mean that you will be hearing 17 dB(A). That fan generates 17 dB(A) in a zero dB(A) environment (ISO test lab). You will be hearing the floor noise of you room *plus* 17 dB(A). Per 3 dB(A) the magnitude of noise pressure doubles.
    Reply
  • Death666Angel - Friday, February 28, 2014 - link

    Which is why dB / dB(A) really suck for measuring loudness for the human hearing. I've liked sone based measurements, though it's just as imperfect, because our hearing is imperfect and varied and subjective and so on. :D Reply
  • E.Fyll - Friday, February 28, 2014 - link

    Actually dB(A) readings are OK for basic reference and comparisons. Sones are somewhat difficult to be perceived as a figure by most people and ridiculously complex to measure at <40dB levels. Not that I do not think of them as a better choice when you want to indicate actual loudness, just that it is unrealistically difficult to introduce them into reviews (with any sense of accuracy, at least). Reply
  • ssj3gohan - Saturday, March 1, 2014 - link

    SPCR - at least the most reputable in noise measurements - also points out that noise bandwidth is a big contributor to perception and it is much better to have up to 6dB higher wide bandwidth noise (whoosh sound) than a concentrated tone (buzz/beep). That's something that definitely needs to be evaluated in a proper fan testing environment.

    Which brings me to something you said earlier: testing the fan in isolation of the PSU. I don't know if that's a better way of doing things, because aerodynamic obstructions (fan grating, heatsinks) as well as reflective or absorptive surfaces are going to change the noise signature quite a bit, especially when dealing with badly designed fan gratings.
    Reply
  • E.Fyll - Saturday, March 1, 2014 - link

    dB(A) are the acoustic frequency; you can hear any frequency that is part of it. Perception however truly is a vital factor; other people are annoyed by high frequency noises but others will not even perceive them. I cannot quantify that however, it is mostly psychological.

    You are absolutely correct, you should not test the fan on its own, otherwise you are not really testing the noise of the PSU. That is why I do not remove the fan from the unit; the unit is just as complete as ever, with its fan hotwired to an external, fanless PSU.
    Reply
  • Death666Angel - Saturday, March 1, 2014 - link

    The only sound measurements I trust in reviews are those done by the German magazine c't which has done sone measurements since I remember. :D Since it is a linear unit it is also much more intuitive to the way most non-science people think. Reply
  • makerofthegames - Friday, February 28, 2014 - link

    This is why I read Anandtech - they actually know what the hell they're talking about, and they test things with actual tests, not just plugging it in and seeing how fast Crysis is. Reply
  • Death666Angel - Friday, February 28, 2014 - link

    Sounds like a good start, will read the full Seasonic review when I find the time. Up to now my primary PSU site was jonnyguru and you are now approaching their level of devotion to PSU tests. :) Reply

Log in

Don't have an account? Sign up now