Ultra LSX 750 Power Supply Review

Primary Analysis

On this page we will take an in-depth look at the primary stage of the Ultra LSX 750 For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.

This power supply uses one GBJ1506 rectifying bridge, which is attached to the same heatsink where the active PFC transistors are installed. This bridge supports up to 15 A at 100° C so, in theory, you would be able to pull up to 1,725 W from a 115 V power grid. Assuming 80% efficiency, the bridge would allow this unit to deliver up to 1,380 W without burning itself out. Of course, we are only talking about this component, and the real limit will depend on all the other components in this power supply.

Figure 10: Rectifying bridge

The active PFC circuit uses two SPW21N50C3 MOSFETs, which are capable of delivering up to 21 A at 25° C or up to 13.1 A at 100° C (note the difference temperature makes) in continuous mode, or up to 63 A in pulse mode at 25° C, each. These transistors present a 190 mΩ resistance when turned on, a characteristic called RDS(on). The lower this number the better, meaning that the transistors will waste less power and the power supply will achieve a higher efficiency.

Figure 11: Active PFC transistors

The electrolytic capacitor that filters the output of the active PFC circuit is from Teapo, and labeled at 85° C.

In the switching section, another two SPW21N50C3 MOSFET transistors are used, installed in the two-transistor forward configuration.

Figure 12: Switching transistors

The primary is controlled by the omnipresent CM6800 active PFC/PWM combo controller.

Figure 13: Active PFC/PWM combo controller

Now let’s take a look at the secondary of this power supply.