Ultra LSX 750 Power Supply Review

Secondary Analysis

This power supply uses a DC-DC project in its secondary, meaning that this unit is basically a +12 V power supply. The +5 V and +3.3 V outputs are produced by two smaller switching power supplies connected to the +12 V rail. The +12 V rail uses a synchronous design, meaning that instead of using diodes for its rectification it uses MOSFET transistors. Both designs (DC-DC and synchronous) are used to increase efficiency.

The +12 V output uses three IRFB3206 MOSFET transistors, two for the direct rectification and one for the “freewheeling” part of the rectification, each one capable of handling up to 210 A at 25° C or 150 A at 100° C in continuous mode, or up to 840 A at 25° C in pulse mode, with an RDS(on) of only 3 mΩ.

Figure 14: +12 V transistors

As explained, the +5 V and +3.3 V outputs are generated using two DC-DC converters, each one available as a small daughterboard attached to the main printed circuit board. Each converter is controlled by an APW7073 PWM controller and four ME75N03 MOSFET transistors, each one able to deliver up to 86 A at 25° C or up to 70 A at 70° C in continuous mode, or up to 20 A at 25° C in pulse mode, with a maximum RDS(on) of 9 mΩ.

Figure 15: One of the DC-DC converters

Figure 16: One of the DC-DC converters

The secondary is monitored by a PS113 integrated circuit, which only supports over voltage protection (OVP) and under voltage protection (UVP).

Figure 17: Monitoring circuit

All electrolytic capacitors used in this power supply are also from Teapo, and labeled at 105° C.