This power supply uses a synchronous design in its secondary, meaning that the Schottky rectifiers were replaced by MOSFET transistors in order to increase efficiency. On top of that, this unit uses a DC-DC design, meaning that this unit is basically a +12 V power supply, with the +5 V and +3.3 V outputs being generated by two small power supplies attached to the +12 V output.
The +12 V output is generated by four IPD036N04L MOSFETs, each one capable of handling up to 90 A at 25° C or up to 87 A at 100° C in continuous mode or up to 400 A at 25° C in pulse mode, with an RDS(on) of only 3.6 mΩ. These transistors are soldered directly on the solder side of the printed circuit board, and the power supply housing is used as a heatsink for them. The +5 V and +3.3 V are produced from the +12 V output, but just as an exercise if all power from this unit was pulled exclusively from the +12 V rail, this unit would have a maximum theoretical current of 249 A or 2,983 W.
Usually power supplies that use DC-DC converters in the secondary to generate the +5 V and +3.3 V outputs have two separate printed circuit boards installed in the secondary, one for each output. Like other “X-Series” power supplies from Seasonic, in the X-400 Fanless these converters are installed on the modular cabling printed circuit board.
Both outputs are managed by an APW7159 PWM controller, and each output is generated by four IPD060N03L MOSFETs, each one capable of handling up to 50 A at 100° C in continuous mode, or 350 A at 25° in pulse mode, with an RDS(on) of only 6 mΩ.
In Figure 18, you can see how there are several solid capacitors in the secondary (and the capacitors that aren’t solid are made in Japan, by Chemi-Con), and how Seasonic added some interesting heatsinks connected straight to the printed circuit board.
The secondary is monitored by a PS223 integrated circuit. This chip supports OCP (over current protection), over voltage protection (OVP), under voltage protection (UVP) and over temperature protection (OTP).