BFG MX-550 Power Supply Review

Secondary Analysis

BFG MX-550 has four Schottky rectifiers on its secondary, two for the +12 V output, one for the +5 V output and one for the +3.3 V output.

The +12 V output is produced by two STPS30L60CT Schottky rectifiers connected in parallel, each one capable of handling up to 30 A at 130° C (15 A per internal diode). 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 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 514 W for the +12 V output. The maximum current this line can really deliver will depend on other components, in particular the coil used.

The +5 V output is produced by one SBR30U30CT Schottky rectifier, which is capable of handling up to 30 A at 140° C (15 A per internal diode). 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 one 15 A diode). Just as an exercise, we can assume a typical duty cycle of 30%. This would give us a maximum theoretical current of 21 A or 107 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 one SPR30L40CT Schottky rectifier, which is capable of handling up to 30 A at 110° C (15 A per internal diode). 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 one 15 A diode). Just as an exercise, we can assume a typical duty cycle of 30%. This would give us a maximum theoretical current of 21 A or 71 W for the +3.3 V output. The maximum current this line can really deliver will depend on other components, in particular the coil used.

It is always good to remember that the real current/power limit for each output will depend on other factors, like the coils and the width of the printed circuit board traces.

Figure 13: +12 V rectifier and +5 V rectifier.

Figure 14: +3.3 V rectifier and +12 V rectifier.

The +5VSB output is rectified by an SR560 diode (5 A limit).

This power supply uses a PS223 monitoring integrated circuit, which features the following protections: over current (OCP), over temperature (OTP, although this power supply does not implement this protection), over voltage (OVP) and under voltage (UVP). As you see the only protection missing is over power protection (OPP).

Figure 15: Monitoring integrated circuit.

As you can see in Figure 13, this power supply has two thermal sensors, one for each fan. We don’t understand why the manufacturer didn’t implement over temperature protection (OTP), as the monitoring integrated circuit has this protection, the only thing that would be needed was an additional thermal sensor.

All the electrolytic capacitors from the secondary are from JunFu and labeled at 105° C, as usual.