SilverStone Element ST75EF 750 W Power Supply Review
Secondary Analysis
Contents
This power supply uses six Schottky rectifiers on its 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 four of these rectifiers. Two SBR30A60CT (30 A, 15 A per internal diode at 110° C, typical voltage drop of 0.53 V) connected in parallel are in charge of the direct rectification part, while two STPS60L45CW (60 A, 30 A per internal diode at 135° C, maximum voltage drop of 0.50 V) connected in parallel are in charge of the “freewheeling” part (i.e., discharging the coil). For our math we need to consider the path with the lower current limit, which is the direct rectification one. This gives us a maximum theoretical current of 86 A or 1,029 W for the +12 V output.
By the way, we are now talking about the voltage drop presented by the rectifiers. This parameter shows how much voltage is wasted by the rectifier. The lower this number is, the better, as less voltage is wasted, increasing efficiency.
The +5 V output is produced by one KCQ60A04 Schottky rectifier (60 A, 30 A per internal diode at 83° C, maximum voltage drop of 0.58 V), giving us a maximum theoretical current of 43 A or 214 W for this output.
The +3.3 V output is produced by another KCQ60A04 Schottky rectifier, so the maximum theoretical power this output can deliver is of 141 W.
All these numbers are theoretical. The real amount of current/power each output can deliver is limited by other components, especially by the coils used on each output.
Figure 13: Rectifiers.
The outputs are monitored by a PS223 integrated circuit, which supports under voltage (UVP), over voltage (OVP), over current (OCP) and over temperature (OTP, not implemented on this power supply) protections. Any other protection that this unit may have is implemented outside this integrated circuit.
Figure 14: Monitoring circuit.
Electrolytic capacitors from the secondary are Chinese from Samxon and labeled at 105° C.
The secondary from SilverStone Element ST75EF is also identical to the one from Seventeam ST-750P-AF, so these two power supplies are internally identical. Seventeam ST-750P-AF uses different rectifiers on the secondary but with the same specs, the main difference being only the manufacturer.