Ripple Testing

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.

Ripple was tested with a 48w load on the 12 V rail as well as with all three rails loaded heavily and with the 12 V rail alone loaded (even more) heavily. Different scope time settings were used for different shots. All shots are 10 mV per divider.

All shots are also, I am forced to admit, some of the worst ripple pictures I have taken. My camera was feeling very narcistic and was far more interested on focusing on itself than on the oscilloscope.

First up, 12 V at low load(10us/divider), full unit load(5ms/divider) and heavily crossloaded(5ms/divider):

There's a whopping 6 mV of ripple here at low load.

Here at full unit load we see about 12 mV, this is very, very good.

With the 12 V rail heavily stressed by itself we see a lot more ripple, about 40 mV. This is still quite good. The large gain in ripple from the full unit load to the 12 V crossload is makes me think that this is not in fact a DC-DC unit, despite being rated like one.

Onward to the 5 V rail! Again low load first (10us), then full unit(10us), then heavy 12 V crossload (5ms).

About 7 mV here. Excellent.

Hey my camera focused on something other than itself for once! It recorded a whopping 12 mV of ripple at full load. This is quite good. It's an interesting waveform, too.

About 22 mV with the 12 V rail heavily stressed, the PWM controller could be happier at this point and the voltage is swinging in a lovely 7.5ms high-to-high waveform, but let us remember that the spec is 50 mV! Even when it could be happier the ZT 650w does a very good job.

Onward to 3.3 V, once more low load first (10us), then full unit(10us), then heavy 12 V crossload (5ms).

Camera is up to it's old tricks, but we see 6 mV of ripple here at low load.

At full load the camera cooperated again and we get a nice shot of another interesting waveform. It's moving a hair less than 10 mV.

The 3.3 V rail is much happier with a 12 V crossload than the 5 V (or 12 V...) rail was, we have about 15 mV of ripple here.

To summarize a bit, the worst ripple I found on the 12 V rail was 40 mV, the 5 V rail had 22 mV and the 3.3 V rail had 15 mV. These results are quite good, under half the spec all the way around. Whatever the "Premium 105c capacitors" are they're well chosen and doing a good job. I think we'd better crack the unit open and see what is inside.

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