Secondary Analysis
Contents
This power supply comes with seven Schottky rectifiers attached to its secondary heatsink.
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 uses two PFR30L60CT Schottky rectifiers (30 A, 15 A per internal diode at 120° C, 0.60 V maximum voltage drop) for the direct rectification and two PFR40L60CT Schottky rectifiers (40 A, 20 A per internal diode at 120° C, 0.60 V maximum voltage drop), giving us a maximum theoretical current of 86 A or 1,029 W for the +12 V output.
The +5 V output uses two PFR30L45CT Schottky rectifiers (30 A, 15 A per internal diode at 120° C, 0.52 V maximum voltage drop), giving us a maximum theoretical current of 43 A or 214 W for the +5 V output.
The +3.3 V is generated using a DC-DC converter installed on the +12 V line, and available on a small daughterboard. This DC-DC converter is based on a uP6124 PWM controller and two STD85N3LH5 MOSFETS, each one supporting up to 80 A at 25° C or up to 55 A at 100° C in continuous mode, or up to 320 A at 25° C in pulse mode, with an RDS(on) of only 5 mΩ.
Figure 14: +5 V and +12 V rectifiers
Figure 15: +3.3 V DC-DC converter
Figure 16: +3.3 V DC-DC converter
The seventh rectifier is used by the +5VSB output.
The secondary is monitored by a WT7525 integrated circuit. This chip over voltage protection (OVP), under voltage protection (UVP), and over current protection (OCP) with four channels (two for +12 V, one for +5 V, and one for +3.3 V). Even though this circuit has two +12 V OCP channels, this power supply has four +12 V rails, and as we could clearly see the presence of four current sensors (“shunts”), so there is a circuit to expand these two inputs to be able to support two sensors each.
Figure 17: Monitoring circuit
Electrolytic capacitors of the secondary are all from OST.
