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.
Zero Load Test
First up, zero load. 12V then 5V then 3.3V. For 12V the scope is set to 5ms/10mV. For 5V/3.3V it’s set to 5µs/10mV.
12V zero load:
Terrible picture, great ripple. ~14mV.
5V zero load:
12mV here, very good.
3.3V zero load:
7mV for 3.3V, that’s extremely good.
Load Test
Next up, full load. 12V scope settings are 2ms/10mV, 5V and 3.3V are still 5µs/10mV.
12V full load:
44mV is quite good. I’ve seen better, but not by a lot. Doesn’t look anything like the picture on the features page though, no surprise there.
5V full load:
12mV at full load for the 5V rail. That’s excellent.
3.3V full load:
10mV for the 3.3V rail, also excellent.
At 41°C the ripple pictures are essentially identical, so I see no reason to waste your time or mine in posting more pictures.
Overall ripple results: Very good to Excellent.
