Silverstone Zeus SST-ST75ZF (750W) and SST-ST85ZF (850W)
I once tested a Zeus in our lab. It was the 650W ST65ZF model, but it’s clear even visually that the ST75ZF and the ST85ZF differ from it not only in wattage: the 80mm fan has moved to another panel of the case and it can be seen through the vent grid that the electronics resides on two PCBs whereas the ST65ZF was a traditional single-card model.
The 750W and 850W models are very similar, so I will discuss them both together. According to the certificate number (UL# E166947), the actual manufacturer of the PSU is Enhance Electronics. One of the PCBs bears the marking of ETASIS Electronics.
The PSU is cooled with a single 80mm fan located on its internal panel (it is internal when you install the PSU into the system case). Using an 80mm fan in such high-wattage models usually results in noisiness. Well, I’ll check this out soon.
The cover removed, you can see a very crowded design with two main PCBs densely populated with heatsinks and auxiliary cards.
The top PCB carries an active PFC device (based on an UCC3818AD chip), line filters and a couple of high-voltage capacitors.
Most of the PSU’s electronics are located on the bottom PCB: a power transformer (in the far left corner), two heatsinks with the inverter’s transistors and diode packs, and three small additional cards. One of them accommodates the main PWM controller of the PSU (a UCC3895DW chip), but the other two are quite a surprise:
Each card carries a choke, an L6730 chip (it is a PWM controller and a synchronous rectifier controller in a single case), and a few transistors in planar cases covered with a small heatsink. These are independent regulators of the +5V and +3.3V voltages.
What’s so strange about that? I have often referred to power supplies with independent voltage regulation in this review, let alone my previous articles. But the point is that other such PSUs do not have truly independent regulation. They use the so-called regulators on magnetic amplifiers, also known as saturable-core chokes. Such a regulator has very simple design and superb efficiency, but cannot work independently from a direct voltage like a full-featured PWM controller does. It is fed a pulsating, unsmoothed voltage from the secondary winding of the main transformer and its point is in narrowing the width of the impulses thus reducing the average voltage we get after the rectification. That is, applying voltage from the transformer’s 12V winding to this regulator and setting it up to reduce the impulse width in 2.5 times, we get 5V at the output. In fact, magnetic amplifiers are used in a majority of power supplies, except for the cheapest models, to get +3.3V voltage from the transformer’s 5V winding, but I only regard a PSU as having independent voltage regulation if both +5V and +3.3V voltages are provided by independent magamp regulators. It’s only in this case that none of the PSU’s output voltages is influenced strongly by the others.
Here, each card is a complete and independent PWM regulator that receives a direct voltage on its input. The regulator’s operating frequency is 400kHz, which helps greatly reduce the size of the choke and the capacitance of the smoothing capacitors on the output. A synchronous rectifier is used to increase the efficiency. I don’t have anything against this solution, yet I would much like to know the reasons why the developers decided to go for such a complex design.