Logisys PS350MA Power Supply Review

Primary Analysis

On this page we will take an in-depth look at the primary stage of the Logisys PS350MA. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.

As with most very low-end power supplies, the rectification bridge of the Logisys PS350MA uses four discrete diodes, instead of using a ready-made bridge. The diodes are not identical; there are two 1N5408 (3 A at 105° C) and two RL257 (2.5 A at 70° C). At 115 V, this bridge supports only up to 345 W. Here we can clearly see that it is impossible for this unit to be a 350 W product. Assuming 75% efficiency (which is a common value for very low-end power supplies), this bridge would allow the power supply to deliver only up to 258.75 W.

Figure 9: Rectifying bridge

The electrolytic capacitors of the voltage doubler circuit are from Fhy and labeled at 85° C.

This power supply uses two 13007 NPN power transistors in its switching section, using the half-bridge topology. Each transistor supports up to 8 A at 25° C (unfortunately, the manufacturer doesn’t say the current limit at higher temperatures).

Figure 10: Switching transistors

The primary is controlled by an EST7502B PWM controller, which is physically located in the secondary of the power supply.

Figure 11: PWM controller

Let’s now take a look at the secondary of this power supply.