Load Testing: Ripple
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.
For all the following pictures the scope is set to 5ms / 10mV. First up, cold zero load ripple:
12V rail, cold, no load:
6mV. Six millivolts, that’s it. That’s nothing. That’s the lowest I’ve seen, ever, on anything. Admittedly this is zero load, but even so! Very impressive.
5V rail, cold, no load:
4mV. That’s even more impressive.
3.3V rail, cold, no load:
Seriously? THREE MILLIVOLTS!? Apparently so. Wild.
Load time! 600W of load to be exact, with 13C intake temperatures.
12V rail, cold, full (600w) load:
31mV is a bit more like what I expected. It’s also very, very good.
5V rail, cold, full (600w) load:
16mV? Yup. Lowest DC-DC ripple I’ve seen? Yup. Well done, Enermax!
3.3V rail, cold, full (600w) load:
21mV here, this is a bit closer to what I expected. Still excellent, though.
Time for HEAT!
12V rail, hot(43°C), full (600w) load:
28mV, that’s right I cranked the temperature up by 30°C or so and the ripple got 3mV lower.
5V rail, hot(43°C), full (600w) load:
5V ripple is unchanged at 16mV. Still excellent!
3.3V rail, hot(43°C), full (600w) load:
3.3V is also unchanged, at 21mV.
All told the Enermax Platimax 600w kicked the hell out of the ripple tests, this is a fantastic pass right here.
