iMicro PS-IM400WH Power Supply Review

Secondary Analysis

This power supply uses three rectifiers in 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. Since this unit is based on the half-bridge topology, the duty cycle used is of 50%.

The +12 V output is produced by one F16C20C rectifier, which supports up to 16 A (8 A per internal diode at 125° C, maximum voltage drop of 1.30 V, which is extremely high). This translates into a maximum theoretical current of 16 A or 192 W for the +12 V output. It is important to understand that this rectifier isn’t a “Schottky” model, but rather a “Fast” model, which presents a higher voltage drop (translation: lower efficiency).

The +5 V output is produced by one S20C45C Schottky rectifier, which supports up to 20 A (10 A per internal diode at 125° C, maximum voltage drop of 0.65 V). This translates into a maximum theoretical current of 20 A or 100 W for the +5 V output.

The +3.3 V output is produced by one HBR2045 Schottky rectifier, which supports up to 20 A (10 A per internal diode at 150° C, maximum voltage drop of 0.7 V). This translates into a maximum theoretical current of 20 A or 66 W for the +3.3 V output.

Of course these are theoretical numbers, and the real limits will depend on other components as well.

It is interesting to note how the +5 V and +3.3 V rectifiers are “stronger” than the +12 V rectifier. This is a typical scenario of 10 years ago, but nowadays most of the current/power pulled by the computer is concentrated on the +12 V rail (because of the CPU and the video cards).

Figure 12: +3.3 V, +12 V and +5 V rectifiers

One thing that caught our attention was the absence of filtering coils in the secondary of this power supply. Without these components, this power supply is prone to deliver high noise and ripple levels in its outputs.

Figure 13: Absence of filtering coils in the secondary

The power good signal and the available protections are implemented using an AS339 integrated circuit, which has four voltage comparators inside.

Figure 14: Voltage comparator

The electrolytic capacitors in the voltage doubler circuit are from LCZ, while the electrolytic capacitors in the secondary are from BH and Fcon.