Secondary Analysis

This power supply uses a DC-DC project in its secondary, meaning that this unit is basically a +12 V power supply. The +5 V and +3.3 V outputs are produced by two smaller switching power supplies connected to the +12 V rail. This design is used to increase efficiency.

The +12 V output make use of four SBR30A50CT Schottky rectifiers, each one capable of handling up to 30 A (15 A per internal diode at 110° C, 0.55 V maximum voltage drop). From the eight available diodes (two per rectifier pack), three are used in the direct rectification and five are used in the “freewheeling” part of the rectification.

XFX PRO 650 W power supplyFigure 14: +12 V rectifiers

As explained, the +5 V and +3.3 V outputs are generated using two DC-DC converters (i.e., two switching power supplies). Usually power supplies using this design have these two converters installed on separate daughterboards, but in the XFX PRO 650 W they are available on the same board, shown in Figures 15 and 16. The two converters are managed by the same PWM chip (APW7159), and use seven APM2556N MOSFET transistors, each one being able to handle up to 160 A at 25° C or 90 A at 100° C with an RDS(on) of only 4.5 mΩ.

XFX PRO 650 W power supplyFigure 15: The +5 V and +3.3 V DC-DC converter

XFX PRO 650 W power supplyFigure 16: The +5 V and +3.3 V DC-DC converter

The secondary is monitored by a PS223 integrated circuit, which supports over voltage protection (OVP), under voltage protection (UVP), over current protection (OCP), and over temperature protection (OTP). This integrated circuit has four OCP channels (+3.3 V, +5V and two +12 V), but the manufacturer decided to use only one of the +12 V channels, making this a single-rail power supply.

XFX PRO 650 W power supplyFigure 17: Monitoring circuit

All electrolytic capacitors used in the secondary are also Japanese, from Chemi-Con, and labeled at 105° C.

Gabriel Torres is a Brazilian best-selling ICT expert, with 24 books published. He started his online career in 1996, when he launched Clube do Hardware, which is one of the oldest and largest websites about technology in Brazil. He created Hardware Secrets in 1999 to expand his knowledge outside his home country.