ASUS P-50GA 500 W Power Supply Review

Secondary Analysis

ASUS P-50GA uses five Schottky rectifiers on the secondary, plus an LM7912 voltage regulator to regulate the -12 V output.

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 STPS30H100CT (30 A, 15 A per internal diode at 155° C, 0.67 V voltage drop) Schottky rectifiers, giving us a maximum theoretical current of 43 A or 514 W for the +12 V output. Interesting enough the rectifier used on the direct rectification uses a TO-220 packaging (STPS30H100CT), while the rectifier used for the “freewheeling” part uses a TO-247 packaging (STPS30H100CW).

The +5 V output is produced by two STPS20L45CT Schottky rectifiers, which one capable of handling up to 20 A (10 A per internal diode at 135° C, maximum forward voltage of 0.5 V). This gives us a maximum theoretical current of 29 A or 143 W for the +5 V output.

The +3.3 V output is produced by one STPS3045CW Schottky rectifier (30 A, 15 A per internal diode at 155° C, 0.57 V voltage drop). This gives us a maximum theoretical current of 21 A or 71 W for the +3.3 V output.

Figure 12: -12 V voltage regulator and rectifiers.

Instead of using a monitoring integrated circuit this power supply implements a discrete solution, so we couldn’t check what protections this power supply really has. The small daughterboard located on the secondary is in charge of providing the protections, controlling the fan,  generating the power good signal and turning the power supply on and off.

Figure 13: Secondary daughterboard.

Electrolytic capacitors from the secondary are all from Ltec and labeled at 105° C, as usual.