Load Testing Part Two: 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.
The scope is set thus for the results: For 12V, 5ms / 10 mV. For 5V and 3.3V it is at 5µs / 10 mV.
Zero Load
First up, 12 V, cold ambient, zero load:
8mV is fantastic.
5 V, cold ambient, zero load:
18 mV here (note the hazy transients), that’s quite good for a DC-DC setup.
3.3 V, cold ambient, zero load:
14 mV on the 3.3 V rail. that’s great.
Full Load
12 V, cold ambient, full load:
~52 mV at full load is a solid result for the 12 V rail. It’s not a record, but its well better than average.
5 V, cold ambient, full load:
The occasional spike brings this to 42mV. From the look of it I’d guess that it’s the modular output board capturing EMI from the APFC bits. That’s just a guess, though. It could just as easily be the primary switches EMI. In any even it’s still within spec, so it’s OK. It does mar what would otherwise be an excellent result though.
3.3 V, cold ambient, full load:
34 mV here, mostly courtesy of the same transient. That’s not bad at all really.
12 V, HOT ambient, full load:
46 mV! That’s a slight improvement with a 50°C ambient. Very nice.
5 V, HOT ambient, full load:
28 mV on the 5 V rail, that’s quite good. It likes the heat!
3.3 V, HOT ambient, full load:
The 3.3 V rail is largely unchanged at ~34mV. That’s still solid.
All told ripple control is good to very good on average. Well within spec at all points.
