Articles: Cases/PSU

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Circuit Design

This model differs from its X series cousins in the ratings of some components (for example, there is only one high-voltage capacitor in the input circuitry whereas the higher-wattage models have two capacitors there) and in the size of the heatsinks. It looks like Seasonic didn’t have to revise the circuit design much to release this fanless model.

Thus, the X-400 Fanless implements everything I described in my X-750 review. It is a cutting-edge product with all economically justifiable technologies available today: a resonance converter, synchronous rectifiers based on field-effect transistors, a main regulator for one voltage (+12 V), additional DC-DC converters for +5 and +3.3 volts, and polymer capacitors. All of this is meant achieve as high efficiency as possible.

The developers even installed an electromagnetic relay that closes the input thermistor which limits the inrush current. When the computer is turned on, this resistor has a high resistance of a few Ohms (that’s actually how it limits the current the PSU consumes from the wall outlet in the first few milliseconds). In a few seconds the resistor warms up, lowering its resistance to 0.1-0.2 Ohms, so its heat dissipation is no higher than 1 watt. However, the developers made sure to avoid even this seemingly negligible loss of power!

Cables and Connectors

The PSU is equipped with the following connectors:

  • Two connectors for a mainboard power cable
  • One connector for a CPU power cable
  • One connector for a graphics card cable
  • Three connectors for peripheral power cables

Included with the PSU are:

  • Mainboard cable with a 20+4-pin connector (59 cm)
  • CPU cable with a 4+4-pin connector (65 cm)
  • One graphics card cable with one 6+2-pin connector (60 cm)
  • One cable with three PATA power connectors (56+15+15 cm)
  • One cable with two PATA power connectors (35+15 cm)
  • One cable with three SATA power connectors (54+15+15 cm)
  • One cable with two SATA power connectors (35+15 cm)
  • One adapter from a PATA power connector to two floppy-drive plugs (15 cm)

Well, that’s a nice selection of connectors for a 400W model, especially considering its modular design (by the way, this is most valuable for a fanless PSU because you can only attach the cables you really need and prevent unnecessary cables from obstructing the airflow). The CPU cable is long at 65 centimeters, which should be enough for any system case with bottom PSU bay. It is also good that the PSU offers HDD cables of two different lengths.


The X-400 Fanless can yield up to 400 watts of power, but the manufacturer doesn’t specify the maximum allowable air temperature. The +12V rail is solid and rated for up to 396 watts.

Temperature and Noise

Seasonic products are sometimes criticized for having hissing chokes, but my sample didn’t produce any unwanted sounds.

I cannot give you any temperature results because the PSU has no external heatsinks whereas the internal ones differ in temperature. Moreover, the exact temperature depended on what point of the heatsink I could reach with the sensor. Such measurements wouldn’t be reliable.

However, I can tell you that the PSU did not show any sign of overheat after working for 1.5 hours at a load of 400 watts. The top panel was warm but not hot then. There was no smell of overheating plastic, either.

UPS Compatibility

The PSU worked with my uninterruptible power supply (APC SmartUPS SC 620) at loads up to 392 and 330 watts when powered by the mains and batteries, respectively. They had no problems switching to the UPS’s batteries and the UPS was perfectly stable.

Output Voltage Stability

The three main voltages deflect no more than 3% from their nominal values irrespective of load. This is a very good result.

Output Voltage Ripple

There is some occasional pulsation on the +5V rail. The other two rails do not have even that.

There is some low-frequency pulsation of about 15 millivolts on the +12V rail. This is far below the allowable maximum of 120 millivolts, though.

Efficiency and Power Factor

After examining the circuit design of this PSU, we can expect only best results in this test! Indeed, the X-400 Fanless is 90% efficient at a load of 75 watts whereas its maximum efficiency is as high as 92.7%! The efficiency is never lower than 86% throughout the graph which stretches from a load of 50 watts to full load.

+5Vsb Standby Source

The standby source is rated for 2.5 amperes and copes with its job. Its voltage sags to 4.86 volts only, the allowable minimum being 4.75 volts.


The Seasonic X-400 Fanless looks more like a regular PSU the manufacturer has forgotten to install a cooling fan into rather than like a model capable of delivering up to 400 watts of power without that fan. However, it is quite capable of doing that. Moreover, the X-400 Fanless boasts excellent electric characteristics and features a modular design.

A computer enthusiast may want to install a low-speed fan on its meshed top panel to get a very quite PSU with active cooling. That’s not as odd a solution as it sounds, actually. There are too few low-wattage PSUs that can be called really quiet. As for fanless models, many users do not feel sure that their cooling is sufficient and that their service life will be long. Installing a 500RPM fan would guarantee proper cooling (an active fan is always better than natural convection) at minimum noise. As for wattage, this model offers enough of power not only for an HTPC but even for a midrange gaming machine!

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