BFG MX-680 Power Supply Review

Secondary Analysis

BFG MX-680 has five Schottky rectifiers on its secondary. Here we could already see that the configuration was different from the 550 W model, which has four rectifiers.

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%.

The +12 V output is produced by two STPS41L60CT Schottky rectifiers connected in parallel, each one capable of handling up to 40 A (20 A per internal diode at 125° C, typical voltage drop of 0.67 V). So the maximum theoretical current the +12 V output from this power supply can deliver is of 57 A or 686 W. The 550 W model uses two 30 A rectifiers here.

The +5 V output is produced by two SBR30U30CT Schottky rectifiers, each one capable of handling up to 30 A at 140° C (15 A per internal diode, typical voltage drop of 0.50 V). So the maximum theoretical current the +5 V output from this power supply can deliver is of 43 A or 214 W. The 550 W model uses only one of these rectifiers here.

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, typical voltage drop of 0.41 V). So the maximum theoretical current the +3.3 V output from this power supply can deliver is 21 A or 71 W. This is exactly the same rectifier used on the 550 W model.

It is always good to remember that the real current/power limit for each output will depend on other factors, especially on the coils used.

Figure 13: Rectifiers.

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).

Figure 14: Monitoring integrated circuit.

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.