The Seasonic SS-300M1U and SS-350M1U have similar specifications except for the overall wattage and a slight difference in the load capacity of their +3.3V and +5V rails. They can deliver nearly all of their output power via the +12V rail whereas the load capacity of the other rails is rather low (70-80 watts, which is more than enough for modern computers).
These PSUs comply with the 80 PLUS Gold efficiency standard.
Working together with our APC SmartUPS SC 620, the SS-300M1U was stable at its full load of 300 watts when powered by the mains and could switch to the UPS’s batteries at the same load, too.
The SS-350M1U was also stable at its full 350 watts when powered by the mains. It could only switch to the UPS’s batteries at loads up to 315 watts, though.
Cross-Load Voltage Stability
The junior model is quite stable, keeping its +12V and +5V voltages within 2% of the required levels. The +3.3V voltage is no more than 3% off.
The 350W model is even better in this test. Its +12V voltage is almost always within 1% of the required level. The other voltages are comparable to those of the 300W model.
Output Voltage Ripple
The PSUs are close to each other in terms of high-frequency output voltage ripple.
Both PSUs meet the industry standard requirements here.
It’s different at the double mains frequency:
The 300W unit has some voltage ripple on the +12V rail only, but it is not strong.
The higher-wattage unit is close to the permissible limits on the +5V and +12V rails. The voltage ripple is weak on the +3.3V rail.
Temperature and Noise
The PSUs are both cooled by a 40x40x20mm ADDA AD0412XB-C71GP fan. We couldn’t find its specs but the closest model, AD0412XB-C73GP(P), has a rated speed of 10,000 RPM at 0.24 amperes whereas the AD0412XB-C71GP consumes up to 0.3 amperes.
Unfortunately, we couldn’t measure the speed of the fan with our optical tachometer due to the configuration of the vent grid and the small size of the fan itself, so we have to limit ourselves to our subjective impressions.
Well, the fan didn't actually work at all at loads up to 140-150 watts as explained on the nearby sticker. Then, it starts up at a quiet speed, becomes audible at loads about 250 watts, and gets real noisy at 300 watts. The 350W model is definitely noisier at full load than any ATX PSU we've recently tested in our labs.
We wouldn’t consider this as a serious downside, though, because such high loads may only occur in a compact gaming computer with discrete graphics.
Efficiency and Power Factor
The SS-300M1U was 87.6%, 91.9% and 91.5% efficient at reference loads of 20%, 50% and 100%, respectively. The peak efficiency of 92.7% was observed at a load of 141 watts.
As for the SS-350M1U, it was 90.1%, 91.5% and 90.4% efficient at reference loads of 20%, 50% and 100%, respectively. The peak efficiency of 92.2% was observed at a load of 100 watts.
The PSUs meet the 80 PLUS Gold requirements but the higher-wattage model is inferior throughout the entire load range except at low loads (where our measurement accuracy is lower, too).
The power factor is up to 99% with either PSU at high load, just as promised by the manufacturer.
The standby source works blamelessly in each model, so we only show you the senior model's graph.