ASUS U-75HA 750 W Power Supply Review
Secondary Analysis
Contents
ASUS U-75HA 750 W uses eight Schottky rectifiers on the secondary, making it to be completely different from the 650 W model from the same manufacturer (which has four rectifiers on the secondary).
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 three S60SC6M Schottky rectifiers (60 A, 30 A per internal diode at 118° C, 0.67 V voltage drop), one in charge of the positive part of the rectification and two in charge of the “freewheeling” part of the rectification (i.e., to discharge the coil). For our theoretical exercise we have to consider the path with the lower current limit, which is the direct rectification one, giving us a maximum theoretical current of 86 A or 1,029 W for the +12 V output.
The +5 V output is produced by two STPS30L45CW Schottky rectifiers connected in parallel (30 A, 15 A per internal diode at 135° C, maximum voltage drop of 0.50 V). This gives us a maximum theoretical current of 43 A or 214 W for the +5 V output.
The +3.3 V output is produced by another two STPS30L45CW Schottky rectifiers in parallel, giving us a maximum theoretical current of 43 A or 141 W for the +3.3 V output.
Figure 12: Rectifiers.
The eighth rectifier, an STPS2045CT (20 A, 10 A per internal diode at 155° C, maximum voltage drop of 0.57 V), is used for the standby (+5VSB) power supply.
Instead of using a monitoring integrated circuit this power supply implements a discrete solution using LM339 voltage comparators, 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.
Some electrolytic capacitors from the secondary are Japanese from Chemi-Con, but not all; some are from Ltec and CapXon. They are all labeled at 105° C, as usual.