XFX 850 W Black Edition Power Supply Review

Secondary Analysis

This power supply uses eight SBR40S45CT Schottky rectifiers on its secondary and each one is capable of handling up to 40 A (20 A per internal diode at 110° C, maximum voltage drop of 0.55 V). All rectifiers are in charge of producing the +12 V output, with +5 V and +3.3 V being generated from the +12 V output using a DC-DC converter (i.e., a small switching power supply) located on a small printed circuit board. This design is the current trend among high-efficiency power supplies. This is exactly the same configuration used on the Seasonic M12D and S12D 750 W models.

Three of the rectifiers are in charge of the direct rectification, while the remaining five are in charge of the “freewheeling” part of the rectification process (i.e., discharging the coil).

The maximum theoretical current each line can deliver is given by the formula I / (1 – D), where D is the duty cycle used and I is the maximum current supported by the rectifying diode. Just as an exercise, we can assume a typical duty cycle of 30%.

For our math we need to assume the path that has the lower limit, which is the direct rectification path. This would give us a maximum theoretical current of 171 A (40 A x 3 / 0.70). This maximum theoretical current limit is for the whole secondary, since +5 V and +3.3 V are also produced from the +12 V output. The practical limit will depend on other factors, but mainly on the coils used and on the design from the small DC-DC converter used to generate the +5 V and +3.3 V outputs. If this 171 A was solely pulled from the +12 V outputs, this would give us 2,052 W.

Figure 13: Rectifiers.

The DC-DC converter uses solid aluminum caps and two APW7073 controllers, one for each output, with seven APM2556N MOSFETs, which present a maximum RDS(on) of only 7.2 mΩ.

Figure 14: DC-DC converter in charge of generating +5 V and +3.3 V outputs from the +12 V output.

Figure 15: DC-DC converter in charge of generating +5 V and +3.3 V outputs from the +12 V output.

This power supply uses a PS223 monitoring integrated circuit, which is in charge of the power supply protections, like OCP (over current protection), over voltage protection (OVP), under voltage protection (UVP) and over temperature protection (OTP, not implemented on this unit).

Figure 16: Monitoring circuit.

Electrolytic capacitors from the secondary are also Japanese, from Chemi-Con and labeled at 105° C.