BFG 800 W Power Supply Review

Secondary Analysis

This power supply uses eight Schottky rectifiers on its secondary.

The +12 V output is produced by four 48CTQ060 Schottky rectifiers connected in parallel, which can deliver up to 40 A each (20 A per internal diode, measured at 111° C). The maximum theoretical current the +12 V 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 (which in this case is made by four 20 A diodes in parallel). Just as an exercise, we can assume a typical duty cycle of 30%. This would give us a maximum theoretical current of 114 A or 1,371 W for the +12 V output. The maximum current this line can really deliver will depend on other components, in particular the coil used. As you can see this stage is highly overspec’ed.

The +5 V output is produced by two STPS30L45CT Schottky rectifiers connected in parallel, which support up to 30 A (15 A per internal diode, measured at 135° C) each. The maximum theoretical current the +5 V 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 (which in this case is made by two 15 A diodes in parallel). Just as an exercise, we can assume a typical duty cycle of 30%. This would give us a maximum theoretical current of 43 A or 214 W for the +5 V output. The maximum current this line can really deliver will depend on other components, in particular the coil used.

The +3.3 V output is produced by two other STPS30L45CT Schottky rectifiers connected in parallel, which support up to 30 A (15 A per internal diode, measured at 135° C)
each. Following the same math, this output would be able to deliver up to 43 A or 141 W, in theory.

Even though this power supply has a separated rectifier for the +3.3 V output, this rectifier is connected to the same transformer output as the +5 V line, so the maximum current +5 V and +3.3 V can pull together is limited by the transformer.

Figure 12: Four of the eight Schottky rectifiers used on the secondary. The diode on the left is used by the +5VSB output.

Figure 13: The other four rectifiers.

On this power supply the thermal sensor is installed inside the +12 V coil. The purpose of this sensor is to control the fan speed according to the power supply internal temperature.

On this power supply the active PFC capacitor is rated at 85° C, while the electrolytic capacitors from the secondary are rated at 105° C and are manufactured by CapXon, a Taiwanese company.