Secondary Analysis
Contents
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 makes use of six 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). The 750 W model has five of these rectifiers, while the 650 W model has four of them. This gives us a maximum theoretical current of 129 A or 1,543 W for the +12 V rail.
Figure 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 850 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. The DC-DC converter board is identical to the one used in the PRO 750 W and in the PRO 650 W.
Figure 15: The +5 V and +3.3 V DC-DC converter
Figure 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.
Figure 17: Monitoring circuit
All electrolytic capacitors used in the secondary are also Japanese, from Chemi-Con, and labeled at 105° C.
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