In a typical PSU primary and secondary stages are usually divided on a single PCB by the main transformers which are usually placed at the center of the PCB. In order to keep the ST85ZF compact for standard ATX cases Etasis chose a dual PCB design in which secondary power stages along with the main transformer are mounted on their own PCB. This includes voltage regulation and DC output to the PC.
If you look carefully at the main transformer you can just make out the part number EPAP-750 printed on the side, yet on top we have a haphazardly placed sticker indicating the part number “EPAP-850 B”. This discrepancy will be the topic of our next section on the next page.
The secondary board is rather congested, however; given the number of circuits Etasis has done an exceptional job of organization. As stated above the secondary PCB also houses the main transformer, rectification stages and DC output stages to the PC. In addition to those mentioned the ST85ZF features “true” independent voltage regulation for the ultimate in rail stability. True
independent voltage regulation is rare. Even among high end power supplies the best we may see are dual Mag Amp
output regulation circuitry in which there is a “shared” regulation among the rails. Therefore if one rail is affected the others may be as well.
On the secondary PCB independent regulation stages occupy three daughter boards (or "daughter in law" boards given the number in the entire PSU). Seen in the photo below from left to right, the main stage card features a potentiometer, (this does not adjust rail output) while the two remaining cards regulate 5V and 3.3V legs respectively. In the thumbnails below we have a close-up of the twin transformers feeding the 5V and 3.3V independent regulation and a second view with labels.
While independent voltage regulators have their advantages most PSU manufacturers do not find this method cost effective. Fortunately Etasis/Silverstone chose stability over cost. In the close-up below the first independent voltage regulator card features a potentiometer which PCModding-Malaysia
identified as; "...adjusting the protection threshold for the power supply." This initial circuit is based on a Texas Instruments UCC3895DW
(left thumbnail) advanced phase shift PWM controller, in the next thumbnail the card is identified by the part number EPAP750-CB4. The next two thumbnails exemplify printing found on the secondary mainboard for each voltage regulation stage.
There have been questions as to why Etasis/Silverstone chose true
independent voltage regulation. In my opinion Etasis/Silverstone wanted to ensure at the very least the 3.3V and 5V rails would not cause fluctuation on the 12V rails. While I don’t know of a PSU which actually places independent regulation on each 12V rail as well, the intent here is for the 12V rails remain steadfast regardless of load (ideally). Building a multi-rail PSU correctly is difficult enough when one begins to understand the contradictions in the 12V Power Supply Guidelines.
In this case there was no need for ad hoc
solutions such as "Rail Fusion" in which an overload on any 12V line result's in a combined rail voltage to compensate. This is counterintuitive to the intentions behind multi-rail circuits and while desperate times call for desperate measures, ultimately the overload is on the consumer with so much "techno-hyperbole". The ST85ZF isolates all 12V rails at their source off the PCB, which is evident in the photo below and something I haven’t seen on any other multiple 12V rail PSU beyond the 12V1 and 12V2 level.
Turning the PCB over we see the 12V rails separated into distinct solder point sections including 12V-Rails associated with baseboard (CPU) connectors and PCIe, connectors.
Circuitry markings -->