ASUS Atlas A-45GA 450 W Power Supply Review

Secondary Analysis

The secondary heatsink has five Schottky rectifiers attached to it, plus an LM7912 voltage regulator in charge of regulating 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 generated by two STPS20H100CT Schottky rectifiers connected in parallel, each one capable of handling up to 20 A (10 A per internal diode at 160° C, 0.88 V maximum voltage drop), giving us a maximum theoretical current of 29 A or 343 W for the +12 V output.

The +5 V output is generated by two STPS4045CW Schottky rectifiers connected in parallel, each one capable of handling up to 40 A (20 A per internal diode at 145° C, 0.94 V maximum voltage drop), giving us a maximum theoretical current of 57 A or 286 W.

The +3.3 V output is generated by another STPS4045CW Schottky rectifier, giving us a maximum theoretical current of 29 A or 94 W for this output.

Note how the +5 V rectifiers are more powerful than the +12 V rectifiers, a typical configuration used by power supplies based on old projects. Nowadays power supplies must have a greater current limit on the +12 V rail, since the components that most pull current (and power) in the computer, the CPU and the video cards, are attached to this output.

Figure 14: -12 V voltage regulator, +3.3 V, +5 V and +12 V rectifiers

This power supply uses a monitoring integrated circuit called a DWA105, which we couldn’t find any information about, so we can’t comment on what protections this unit really has. An LM339 voltage comparator is also used.

Figure 15: Monitoring circuit

Electrolytic capacitors from the secondary are from Ltec and labeled at 105° C.