Seasonic SS-300FS Active PFC PSU

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Before moving on to what the unit does when powered up, I want to tell you something about temperature controlled fans in other PSUs. Even though Silent PC Review has yet to publish an "official" review on a thermistor fan controlled PSU, I have described their behavior in the forums and in the articles on the P4 system and PSU experiments. They usually start out quiet, but the fan(s) quickly speed(s) up to become quite noisy -- even without a load, outside a case, without any external heat to raise PSU temperature. The thermistor that controls the fan speed always seems to be positioned near hot components within the PSU that cause the thermistor to heat up and speed the fan up. This is the behavior I have seen with two different models of Enermax 350W PSUs, a SH 300W PSU, and a Zalman 300W PSU. Judging from comments by a members in our forums, the new Antec True Power PSU appears to suffer the same affliction.

The Seasonic SS-300FS is different. This PSU starts out quiet upon turn on -- and stays that way indefinitely without a load, outside a case. It is a very pleasant and unique distinction among thermistor fan controlled PSUs I have seen. I later measured the startup fan voltage to be 4.34 VDC

The PSU was started outside a case without connection to a motherboard. Running a jumper wire between the green wire contact and any black wire contact in the main ATX connector to the motherboard tricks the PSU into starting up. There is no load, which is not ideal for many PSUs, but this is a convenient way to listen to its minimum noise without fussing with motherboards. Later, I tried the PSU outside a case while connected to a motherboard, with the same results.


The 7-page documentation about the S2FC fan control circuit explained its quiet behavior. It also explained more clearly for me the reason for the increasing fan speed behavior of other thermally controlled fan PSUs. Much of the following is my interpretation and rewording of the printed material Seasonic sent; the graphs come from Seasonic's web site.

Seasonic reasons that an ideal fan control in a PSU should achieve sufficient air flow for cooling with minimal fan noise. Thermal control of PSU fans is not a new idea, but the concept of tailoring the response of the fan controller to closely follow the real cooling needs of the PSU appears unique.

At low power levels, almost any power supply can be run with minimal airflow from any 80mm fan because little heat is generated. That minimal airflow is more than enough for adequate cooling as the power load is increased, until a certain critical point is reached when the heat generated demands higher airflow to cool the PSU components adequately. At this point, the fan speed must be increased for effective cooling. But until this critical point is reached, any increase in fan speed is not needed for adequate cooling. It only results in greater noise and wear on the fan.

Seasonic says its S2FC fan control circuit ensures that the fan remains at minimal speed until extra airflow is really needed for cooling, thereby ensuring the lowest noise over the widest range of operating conditions. Once the critical power load point is exceeded, the curve becomes exponential, apparently to match thermal characteristics of the PSU and to keep fan RPM (and noise) lower until needed.

The above graph (courtesy of Seasonic's web site) shows the behavior of 3 thermal fan controller circuits under load for a 250W PSU. The vertical scale shows the fan voltage, the horizontal scale shows system load in watts, which presumably has a direct correlative temperature.

The red curve represents a linear thermal fan controller. As system load and heat increases, fan speed increases from the 5V minimum, which is the usual safe consistent start voltage for 12 VDC fans, linearly to 12V at full load. This circuit does not take into account any of the cooling effects of heatsinks, and natural convection at low power loads. In this example for the 250W model, 75W is the load at which the fan must provide additional cooling; the linear fan control already has increased the fan voltage to ~8.5V, with accompanying increase in noise. It is my belief that all the other thermally controlled fan PSUs I've had my hands on follow this simple linear curve. This is why, running idle in a cool room with minimal load, they all get louder for no good reason.

The green curve represents a thermal fan controller with exponential characteristics. As system load and heat increases, fan speed increases from minimum to 12V at full load following an exponential curve. Throughout most of the operating range, an exponential thermal fan controller will keep the fan spinning slower and quieter than the linear thermal fan controller. However, because this circuit still ignores the fact that no additional cooling airflow is required well past 75W, the fan is allowed to speed up much sooner than necessary, thereby causing unnecessary wear and tear as well as unnecessary additional noise.

The blue curve represents Seasonic's S2FC thermal fan controller. It starts as a straight horizontal line turn curves up exponentially. As system load and heat increases, fan speed remains at the minimum (startup) 4.35V level. The fan is not sped up at all until the thermistor reaches a temperature where additional cooling is needed. For the SS300, this point is reached at ~100W (at 25° C). When the fan does begin to speed up, it follows an exponential curve. This means that the increase to maximum fan speed occurs faster the closer the PSU gets to maximum rated output. The area under the curve is much smaller than for the other curves; the difference in area represents the theoretical reduction in noise and wear and tear provided by the Seasonic.

The characteristics of Seasonic's S2FC thermal fan controller should make for a PSU that is quieter throughout the operating range yet remains well cooled. A particularly salient point is the 100W speedup point of the 300W unit.

NOTE: Although Seasonic refers to power levels, the truth is that there is no direct power monitoring within the fan controller. The thermistor is the only monitoring device in the fan circuit. However, the circuit's calibration is such that the fan speed up point corresponds to the temperature reached by the thermistor when the PSU is delivering about 100W in an ambient temperature of 25° C. This explains the next graph provided by Seasonic, which shows how the speedup point shifts depending on temperature. So keeping the case cooler, whether with cooling fans or a cool room, should have the effect of keeping the PSU fan low to a higher power point.

Those who have read my article on a Super Simple Power Meter may recall that the highest measurements I obtained on total power dissipation of various systems was 120W maximum peak for a fully loaded AMD XP1700+ system. The long term maximum for that same computer was 110W. This result was obtained with the CPU at 100% utilization. Two points relevant to these results:

  1. My simple power meter may not be highly accurate for complex AC loads. One apparently knowledgeable reader said it is likely to read too high, perhaps as much 20% too high. Until tested against a better power meter, the accuracy of my meter is in doubt. I have to give some credence to the idea that my measurements are inaccurately high, which means even less power is drawn.
  2. Most applications that most people work with keep the PC at close to idle, with occasional instances of 100% CPU usage, again not for prolonged periods. In other words, the load on the PSU in a desktop PC is typically dynamic, with low averages accompanied by short burst of high power activity.

All of this suggest that the fan of the SS-300FS will not often speed up past its startup voltage in a typical PC used in a typical way. It ought to be pretty quiet most of the time.

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