Seasonic X-Series 650 W Power Supply Review

Primary Analysis

On this page we will take an in-depth look at the primary stage of Seasonic X-Series 650 W. For a better understanding, please read our Anatomy
of Switching Power Supplies tutorial.

This power supply uses two GBJ1506 rectifying bridges in its primary connected in parallel with a heatsink installed between them. Each bridge supports up to 15 A at 100° C, so in theory, you would be able to pull up to 3,450 W from the power grid; assuming 80% efficiency, the bridges would allow this unit to deliver up to 2,760 W without burning them. Talk about overspecification! Of course, we are only talking about these components, and the real limit will depend on all the other components in this power supply.

Figure 9: Rectifying bridges.

Three SPA20N60C3 power MOSFETs are used on the active PFC circuit, each one capable of delivering up to 20.7 A at 25° C or 13.1 A at 100° C in continuous mode (note the difference temperature makes) or up to 62.1 A at 25° C in pulse mode. These transistors present a maximum resistance of 190 mΩ when turned on, a characteristic called RDS(on). This number indicates the amount of power that is wasted, so the lower this number the better, as less power will be wasted thus increasing efficiency.

Figure 10: Switching transistor, active PFC diode and transistors.

This power supply uses two electrolytic capacitors to filter the output from the active PFC circuit. The use of more than one capacitor here has absolute nothing to do with the “quality” of the power supply, as laypersons may assume (including people without the proper background in electronics doing power supply reviews around the web). Instead of using one big capacitor, manufacturers may choose to use two or more smaller components that will give the same total capacitance, in order to better accommodate space on the printed circuit board, as two or more capacitors with small capacitance are physically smaller than one capacitor with the same total capacitance. X-series 650 W uses two 330 µF x 400 V capacitors connected in parallel; this is the equivalent of one 660 µF x 400 V capacitor.

These capacitors are Japanese, from Chemi-Con and are labeled at 105° C. This is good for two reasons, first, Japanese capacitors do not leak; and second, usually manufacturers use 85° C capacitors here, so it is good to see a manufacturer using a capacitor with a higher temperature rating.

The active PFC circuit is controlled by an NCP1654 integrated circuit.

Figure 11: Active PFC controller.

In the switching section, two STW20NM50FD power MOSFET transistors are used. Each transistor supports up to 20 A at 25° C or 14 A at 100° C (note the difference temperature makes) or 80 A in pulse mode at 25° C, presenting an RDS(on) of 220 mΩ.

The switching transistors are connected using a design called “LLC resonant,” also known as a series parallel resonant converter, being controlled by a CM6901 integrated circuit, which operates under PWM (Pulse Width Modulation) mode when the power supply is operating under light load but under FM (Frequency Modulation) mode under other loads.

Figure 12: Controller.

Now let’s take a look at the secondary of this power supply.