Infinity 500W Power Supply – A definite Big Bang for your Buck

Load Testing: Regulation and Ripple

A decent load test of a PSU requires a decent load. Contrary to what some may believe, that means you need a known load that can fully stress the PSU. Computer hardware does not cut it. Worse, if the PSU fails during testing it might take out the computer hardware anyway. Commercial load testers cost a lot of money. I do not have a lot of money, so I built my own with juicy power resistors and a Toyota cylinder head. It works great. I’ll be using it to load this thing down fairly severely and will check voltages and ripple (more on that later) at various points. The down side to my tester is that the loads it can put on PSUs are fairly coarse, they go in increments of 48 W for 12 V, 50 W for 5 V and 22 W for 3.3V. Those wattages assume the PSU is putting out exactly the official rail voltage, a PSU putting out 12.24 V rather than 12 V will be at 49.9 W per step rather than 48 W. I file that under the “tough beans” category as I figure if a percent or two of load makes that much of a difference, the PSU manufacturer should have hit the voltage regulation more squarely. It does make calculating efficiency difficult at best.  However, given that the input power is read via a Kill-a-Watt, the efficiency numbers are dubious to begin with. Kill-a-Watts are not known for extreme accuracy on things with automatic power factor correction. For this reason, I am not listing the efficiency.

The ATX spec says that voltage regulation must be within 5% of the rail’s official designation, regardless of load. It doesn’t actually mention that the PSU shouldn’t explode, though I expect they figured it was implied. Exploding is a failure in my book regardless.

It is also worth knowing that I will be testing this PSU at both outdoor ambient temperatures (typically between 10 °C and 20 °C here this time of year) as well as in the Enclosure of Unreasonable Warmth. TEUW is a precision engineered enclosure that I use to route the exhaust air from the PSU right back into the intake fan, it is adjustable to hold the intake air temperature at (almost) any level I want it. This way I can test the PSU’s response to hot conditions as well as cold conditions. For the hot testing I will be running the intake temp as close to the unit’s maximum rated temperature as possible. TEUW, in case you’re curious, is a cardboard box.

Ripple is fluctuation of the PSU’s output voltage caused by a variety of factors. It is pretty much impossible to have zero ripple in a SMPS computer power supply because of how a SMPS works, so the question is how much ripple is there? In the regulation testing phase we found out how the PSU does at keeping the average voltage at a set level, now we’re going to see what that voltage is doing on really short time frames. The ATX spec says that the 12 V rail cannot have more than 120 mV peak to peak ripple, the 5 V and 3.3 V rails need to stay under 50 mV.

If that isn’t complicated enough for you, there are three forms of ripple to keep track of as well. Long-term ripple from the PSU’s controller adjusting the output voltage and over/undershooting, correcting, overshooting, etc. Medium-term ripple from the voltage controller charging and discharging the inductor(s) and capacitor(s) that make up the VRM, and very short-term ripple caused by the switching itself. The first and second forms are the most important, if they are out of spec it can cause instability at best or damage in extreme situations. The very short-term (I call it transient ripple) flavor is less crucial, excessive amounts can still cause issues though it takes more of it to do so. The ATX spec does not differentiate, as far as the spec goes 121 mV of transient ripple is just as much of a failure as 121 mV of medium or long term ripple.

I test ripple in a few difference ways, first I test it during the cold load testing. It is tested at zero load and maximum load first. During the hot load testing I test the ripple at maximum load again. I have recently started testing ripple at fairly random loads with the unit still hot, it’s a bit unorthodox (a bit? maybe a lot) but has found issues in the past that did not show up with other test methods.

That’s more text than I’ve typed in this review so far! For the ripple results I’m just going to use text, it’s quicker and easier than the pictures are.

At low (10°C / 50°F) intake temperatures it can sustain nearly full load for a while, after about ten minutes the voltage on the 12 V rail started to sag. At medium load and higher there is a significant 60Hz hum from the PFC coil. With an intake temperature of 15°C all three voltages sagged significantly, the “12 V” rail was in the low 11s to high 10s, the “5 V” rail was around 4.5 V, and the 3.3 V rail was more like 3 V. During this whole time the 5VSB rail sat with a 100% load (for it) of two amps, right at 5 volts, happy as a clam at high tide.

With an intake temperature in the 25-30°C range (also known as, what it’s like inside a case) the voltages dropped further, 12 V was into the mid 10 V range.

Regulation wise, the 12 V rail held itself to 35.3%, the 5 V rail to 32% and the 3.3 V rail to 45.7%. That’s a new record.

At 37°C (also known as, what it’s like inside a cheap case) the 12 V rail went downhill rapidly, bottoming out around 9.2 volts as you can see above. Shortly after that the unit died.

Decreasing the load one level on 12 V or 5 V cleaned the ripple up nicely when hot, though it still sagged regulation wise. Up till the high temperature and/or full load point this unit was doing quite well. The fan was extremely quiet and looked great, though at no time did I hear it spin up, which is likely part of the problem. The PFC hum was rather annoying, but so it goes. Unfortunately this is simply not a 500 W PSU. This is a ~300-350W PSU with a misleading (at best) label.

Let’s crack this thing open and see what’s inside, shall we?

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