Faceoff! A look at Cougar PowerX and SilverStone SFX power supplies

Power supplies are kind of the the IT department of your computer. You never see it, think about it, or hear it unless something goes wrong. It just sits there thanklessly chugging away inside your case. But man…when it goes south, it hurts. It’s funny that all of the fancy cool parts that get glory, like video cards, motherboards, and cases, can really only hurt themselves. If they go bad, you just have to replace them (motherboards can damage other components, but it’s extremely rare). But if a power supply goes bad, your entire computer could fry in an instant. Why don’t they get more attention then? Well…pretty much they’re just boring. All you need is one that will not do what I described, and keep everything sufficiently powered, and that’s almost all there is to it.

We have a couple of new releases here from Cougar and SilverStone targeted at the lower end of power wattages, suitable for most folks that can’t afford fancy SLI or Crossfire setups. Let’s take a look at the Cougar PowerX 550w and SilverStone SFX ST45SF power supplies:

The SFX form factor of the SilverStone SFX 450W is a pretty sweet addition. The plate to make it turn into the normal ATX is a snap to screw in. Given the specs, it is quite a good price. The 80 mm fan is probably not the greatest, as smaller fans tend to be whinier, and 3 years of warranty coverage is not amazing either. Sadly, I could not scrounge up the details on the Cougar model for the life of me. Given its only 80 plus bronze, not modular, and only fits the bigger ATX standard (smaller things are always more expensive with technology!), I would expect it to be cheaper than the SilverStone model to be worth it, unless it has some kind of super warranty/MTBF. Let’s take a look at how they stack up with connectivity:

Ahhh, now there’s a slight difference. Only 1 8-pin PCIe connector on the SFX unit. It’s not that you could run SLI on a wattage that low, but with the crazy low power envelopes of say, Ivy Bridge, you could theoretically fit a dual 8 pin graphics card with that kind of power. For the rest of the numbers, both units have ample connectivity for anything I could think of that would fit in that power size. It’s possible to use a couple of extra 4-pin or SATA on the smaller PSU, but all things considered that’s pretty unlikely, and not too big of a deal even if it happens. You might have to make sacrifices and trim down to an optical drive and a mere two hard drives.

Now we get to some of the meaty, nerdy bits. Power supplies have separate voltage lines, which carry power to different components and parts of the PC. Each line has its own maximum output current, measured in amps, that it can safely and reliably deliver. These are less-known and sometimes troublesome numbers that can land you in big trouble if you don’t do your research. You have to make sure that the power you’ll be sucking up on each line doesn’t exceed the maximum, even if you’re below the power supply’s total limit.

Bu that’s not all—with some handy physics (power = current * voltage) we can calculate the actual power restriction on those lines in watts, and not just the maximum current delivery. Power is much more useful, as that’s how the power draw of most components is listed. If you’re curious, an amp is equal to one coulomb per second, so it’s the amount of electrical charge delivered per second. A watt, on the other hand, is one joule per second, so it’s the amount of actual energy (or work) being transmitted every second. Multiplying the charge by the current and resistance is what gets us energy (or work again). As if hidden limits on each lines wasn’t complicated enough, power supplies also have hidden limits for the combinations of power spread across them. Obviously the limit across all lines will be the total advertised on the box, but there are a couple of other tricky ones. There is always a limit across the combined +3.3V and +5V rails, for instance.

Ah… now we see another differentiating feature. The Cougar model has two +12V rails, where as the SilverStone only has one. There are lots of theories on what PSU rails are and what they do, but the explanation is actually quite simple. There are different traces on the circuit board delivering power, and those are monitored to ensure that power on any section is not exceeded. If any section is exceeded, the whole unit shuts down to avoid short circuiting or damaging the components receiving power. It doesn’t mean that total power, or stability, or ripple, or any of that complicated nerd stuff is affected. It does mean that multiple rails adds in extra protection by adding more checks into the power delivery, but at the cost of possibly shutting down before the power supply has actually reached its power delivery limit on the entire +12V line. Modern power supplies are usually quite smart about the internal layout of these rails to make the latter issue difficult to achieve, but it’s still possible in weird circumstances. If the 264W rail on the Cougar unit is feeding both auxiliary PCIe cables, and the 336W rail is feeding the motherboard (processor, ram, PCI/PCIe power through the physical slots), it would be almost impossible. You would have to have something like two giant video cards each pulling the 150W limit just from their single 8-pin, while at the same time the rest of your motherboard is somehow sipping power at less than 216W total so as not to exceed the 516W limit on the entire +12V line, implying that a single rail would have fixed the problem.

That would be an extremely unbalanced system and probably impossible given that both cards in this scenario would almost certainly be pulling the maximum 75W from the PCIe slots if they were that power hungry. To further decrease the odds, some power supplies can balance loads across rails on the fly, like this very Cougar unit, essentially determining which traces the power will be flowing through—although this may not happen instantaneously. Without poor circuit design, this is extremely unlikely to be an issue, but the added checks aren’t likely to matter much either, so you can pick your favorite as you see fit. As for the rest of the limits, they are all quite reasonable and unlikely to be exceeded without exceeding the total power supply’s limit unless you are doing something very strange. They make these power supplies so that you don’t have enough connectors to easily exceed the corresponding line’s power limits after all.

Before we move on, let’s take a brief look at efficiency. These “80 plus” ratings are all well and good, but how do we know if they’re really up to snuff? There are even large gaps in-between the ratings, so maybe they exceed them. Let’s find out!

Power efficiency is extremely difficult to measure accurately without a very precise load generator, so let’s borrow some research others have done for this part. Below I wrote in each power supply’s efficiency at each load that has a requirement according to 80 plus specification. In parenthesis after each measurement I noted what the requirement actually is for that unit’s claim to 80 plus fame (Cougar claims bronze, SilverStone claims gold).

The handy folks over at Plug Load Solutions provided me with this detailed info, which you can see by clicking on the appropriate power supply name in the table. Well each unit adheres to its respective claim, although the SilverStone model cuts it quite close. The Cougar is a much cleaner margin of success, although still a couple percentages away from the next rating up (silver). Even so, SilverStone definitely delivers on its claims of higher efficiency  and while the differences seem slight, keep in mind that excess power that is not used is entirely converted to heat, and will then result in higher heat dissipation requirements (fan speed). With that said, the differences aren’t immense, so the value is really up to the beholder.

Now we get to the exciting part—plugging them in and powering stuff.

Operation

Both units are certainly more than capable of powering my anemic system of a single low-power CPU with two sticks of RAM and one SATA drive. Given the low amount of cables I need, I definitely enjoyed installing the SilverStone model more, as it’s fully modular. This means only the cables I want to use have to be plugged in to the back of the power supply at all, while the rest can sit neatly back in the box. The Cougar model had lots of extra dangling wires I had to tuck away and manage, although my case has reasonable space behind the motherboard tray for such things, it was still not quite trivial. It also angered my OCD, but that’s just because I’m special.

One curious feature of the smaller SilverStone unit is that it has no actual power switch. Now, normally that doesn’t matter, as turning on the power supply is essentially achieved by hitting the power button on your computer. That sends a signal to the power supply to “turn on”, which then starts feeding the components power. The reason this is actually possible, however, is that the power supply is actually always technically on. It’s sipping power (very little) to keep the circuits active to receive this signal, and also to feed the motherboard, since that also needs to be powered to send its own power after receiving the power switch signal. For most people this is not an issue about power draw, as the amount is so small, but this means that the motherboard is technically always powered. If you want to do something like swap out sticks of RAM or something in a PCI slot, it’s extremely important that the motherboard be absolutely not powered. This means you have to flip the switch on the outside of the power supply, which completely cuts off any power into the power supply itself and the motherboard (you have to wait a little as the capacitors need to discharge, usually just 10-15 seconds). In absence of that, you need to unplug the thing. It’s not really a huge deal, as it’s just a convenience, but it is a slightly confusing omission. I guess they were so stuffed for room they couldn’t fit it in.

Once up and running, both power supplies were actually admirably quiet. I was particularly impressed with the quietness of the 80 mm fan in the SilverStone unit, although the second problem with smaller fans is that they tend to lose their silence with age more quickly, as they are spinning faster to achieve comparable air flow, or even close to it. Compounded on that is the fact that while that 80 mm fan is quite, it seems to be spinning at full blast the whole time the unit is on, where as the Cougar unit has a temperature control scheme to dynamically adjust its spin rate. It’s hard to say exactly, as I did not have the components to really tax the capabilities of these units. Still, both fans were nearly silent, and extremely quiet 80 mm fans are far from a given.

Without very important details like warranty and price, it’s hard to really come to any firm conclusions about the face-off, but the trade-offs are still pretty obvious. If you have to fit into the smaller SFX form factor, you obviously only have one choice. Other than that, the SilverStone unit gives you higher efficiency and a clean modular design. The Cougar gives you that extra bump of juice and the likely quieter 120 mm fan, and also a power switch if you’re into those. Both have ample power constraints on their given lines, and the difference in rails is unlikely if not impossible to make a difference in any way. The connectivity advantage might go in favor of the Cougar in a few circumstances as well. Also…the PowerX has an X in the name, so it’s awesome, and it actually does look cooler with its shiny red hot rod look. Choose wisely!