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.
All of the following ripple pictures are of the scope set to 10mV/10µs. Long duration ripple was not meaningful enough to matter.
First up, zero load!
Zero load, 12v rail:
19mV! That’s excellent.
Zero load, 5v rail:
17mV here, also excellent.
Zero load, 3.3v rail:
Lastly, 14mV for the 3.3v rail. Excellent all the way across.
Next up, full load with 9°C ambients.
12v, full load:
32mV at full load. That’s excellent! Really really excellent, really.
5v, full load:
31mV on the 5v rail at full load. That’s good, but not epic.
3.3v, full load:
26mV here, a mere 1mV above the “excellent” line of 25mV. I’ll give it the Excellent anyway.
Lastly for this section, full load with ~39°C ambient temperatures. This is where things can get nasty!
12v, full load hot:
36mV for the 12v rail. That’s still excellent.
5v, full load hot:
32mV for the 5v rail, 1mV higher than cold. This thing just doesn’t care! Very nice.
3.3v, full load hot:
Still 26mV here, still excellent.
All told the ripple control is very very good to excellent. Top notch, worth writing home about when you get down to it. Let’s rip this thing apart and see how Enermax did it!
