Seventeam ST-550P-AG Power Supply Review

Secondary Analysis

Seventeam ST-550P-AG uses six Schottky rectifiers on its secondary, using an updated design – meaning that the rectifiers for the +12 V outputs are more powerful than the ones used for the +5 V and +3.3 V outputs. So our suspicion that this power supply would use an outdated design just because it has only two SATA power connectors was completely wrong. This shows us the importance of analyzing the circuit used by a given product instead of assuming things based on its external appearance.

For the +12 V rectification two SBL3060PT Schottky rectifiers connected in parallel are used. Each one is capable of handling up to 30 A at 95° 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 two SBL3040CT Schottky rectifiers connected in parallel, each one capable of handling up to 30 A (15 A per internal diode) at 100° C. 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 SBL3040CT Schottky rectifiers. The maximum theoretical current the +3.3 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 141 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.

On the secondary heatsink we also found the voltage regulator integrated circuit for the -12 V output (LM7912), which has a current limit of 1.5 A.

Figure 11: -12 V voltage regulator, +12 V rectifier, +5 V rectifier and +3.3 V rectifier.

Figure 12: +3.3 V rectifier, +5 rectifier and +12 V rectifier.

The secondary from this power supply is monitored by a PS223 integrated circuit, which is physically installed on a separated small printed circuit board (see Figure 13). This integrated circuit features the following protections: over current (OCP), over temperature (OTP, not implemented on ST-550P-AG), over voltage (OVP) and under voltage (UVP). As you see the only protection missing is over power protection (OPP), but for an entry-level product this power supply has a very satisfactory number of protections.

Figure 13: Monitoring integrated circuit.

The electrolytic capacitors from the secondary are from Capxon and Samson, two Taiwanese companies, and labeled at 105° C.