Impotance of Power supply (SMPS)

How to: Choose the right power supply for your PC

by Danison | 

Whether you’re building a basic desktop or a power-house gaming PC, choosing the right PSU is an important decision.

Corsair's just released it's shiny new RM range of power supplies, so it's as good a time as any to look at how to choose the right PSU for you...

When it comes to PC building, there are some really exciting decisions that are going to have an immediate effect on your system. The stars of the show are usually the processor and the video card, with the motherboard a close second; people agonise over what part to choose in order to get the most of out of their new build.

Compared to these attractive pieces of kit, the humble power supply – just as essential, but a little on the boring side – can often get overlooked in terms of research and impact.

However, choosing the right PSU can have a massive impact on your next build – and many builds after, too, as this is one part that, like monitors, can often carry over from one PC to the next. So you should really make the right choice first time. You’ll end up with better performance, and a component that will last through multiple upgrades and builds. Here’s our advice on what to look for when you’re in the market for more power.

Form factor
The ATX form factor is very much the industry standard these days, and while that means its width and height, not to mention mounting-screw placement, are locked in – and compatible with all ATX case designs – the form factor can vary in depth. If you’ve got a full-size desktop tower, you’re probably not going to have to worry about room, but smaller cases will often require a shorter PSU design, so as to leave room for neat cabling. In some shorter case designs, longer PSUs will even get in the way of optical drive cables.

The ‘standard’ drive depth is around six inches deep, though supplies with a higher wattage will be an inch or two deeper. Always make sure you know the room you’ll be working with in any build, and choose an appropriately-sized PSU.

Fan placement and size is something you’ll want to pay attention to as well. Smaller fans will have to spin faster to push more hot air out of the PSU, and will therefore be noisier. Larger fans, up to 140mm in diameter, spin slower and are therefore quieter.

If you want a quiet system, this is definitely important, but larger fans usually vent into your PC’s case. You’ll need to make sure there’s room enough for sufficient exhausting, as well as make sure that the cooling within your case are up to the task of dealing with the extra hot air.

Power to the people 
Well, components, actually.

You’ll want to choose a PSU that’s up to the task of powering all your components, with enough headroom to take care of any future upgrades, though with PC parts becoming more and more efficient, this is less of a concern these days. One of the best places to start is with the excellent online calculator at Extreme Outervision. Here you  can  list the parts of your particular build, including whether or not you intend to overclock any components. You’ll then end up with an overall wattage you should be aiming for. Alternately, various PSU makers offer similar services, such as Corsair’s PSU Finder; they may be simpler, but are often quite adequate for
most needs.

If you’re running powerful parts, you’ll also need a basic understanding of the individual ‘rails’ within your PSU. Some designs offer a single rail, others multiple rails, with different amperages on each. Your video card specs, if it’s powerful enough, will often call for a recommended power supply wattage that can help you choose the right PSU. If you have a powerful CPU and video card, you’ll want a PSU with dedicated rails for these parts.

Cabling
It’s up to you whether you want a PSU with fixed or modular cabling, as modular PSUs – with the ability to use only the cables you need, like Corsair’s RM series, pictured – are generally more expensive. But if you’ve got the cash in your budget, we’d suggest going modular.

It’s not only neater and easier to install a PSU with less cabling, but ultimately this will assist in keeping your PC running cool in the long run, and provide less surfaces for dust and other cruft to accumulate on.

You’ll also want to make sure you’ve got PCI-E connectors for any juice-hungry video cards you’re using, especially if you’re running any kind of multi-GPU rig. In the latter case, choosing a PSU that is SLI Ready or CrossFire Certified is a safe bet. Even then, check that you’ll be getting the right number of 4-, 6- or 8-pin PCI-E connectors.

You do not want to get your PSU home to find out you can’t hook up your precious video cards.

Lastly, while even lower-specced PSUs tend to come with a mess of SATA and Molex connectors for components like drives, if you’re running anything more complex like front-bay fan monitors or water-cooling loops, just make sure your PSU has enough for your needs.

Efficiency
The more efficient your PSU, the less noise it will make, the less power it will draw, and the less it will cost when you open your energy bill. PSUs are rated at five levels – 80 Plus, 80 Plus Bronze, 80 Plus Silver, 80 Plus Gold and 80 Plus Platinum, with Platinum being most efficient. There’s not a huge difference, but if you’re PC is on a lot, or you’re building multiple, powerful systems, you’ll want to go with at least a Gold rating.

We’d definitely suggest not taking any cheap option that isn’t properly rated – it’s just not worth the risk.

COMPONENTS

• power supply,
• hardware

How to pick the best PC power supply

Power supplies are a frequently misunderstood—and overlooked—PC component. Many users choose a power supply based on total wattage alone, assuming that higher is always synonymous with better.  Others pay no attention to their PSU selection at all, and settle for whatever abomination arrived with their machine. But considering how important a good power supply is to a system's stability and long-term reliability, it’s a shame that PSUs get so little attention in comparison to sexier components like graphics cards and SSDs.

It doesn’t help that the power-supply market is awash with products from unscrupulous manufacturers that use substandard components and overstate the hardware’s capabilities. Indeed, the abundance of PSU-related misinformation and deception in the marketplace would be comical if it weren’t so harmful to consumers. But finding a solid, efficient power supply is possible if you arm yourself with the right knowledge. We can help.

Choosing a power supply

Devote as much thought to your power supply as you do your PC's processor.

There is no single, universal rule for selecting a high-quality power supply. Nevertheless, various indicators provide circumstantial evidence of PSU quality, and some guidelines are generally helpful.

First, always buy a power supply from a reputable manufacturer, and look for reviews of it before you buy. Avoid cheap, generic power supplies, which tend to be substandard. Look for reputable brands that offer solid warranties and support. Corsair, Seasonic, and Antec are three manufacturers with reputations for producing high-quality power supplies, though even they may offer a few duds among all the studs. Do your homework!

Larger, heavier units are preferable to puny, lightweight models. Higher-quality power supplies almost always use bigger and better capacitors, chokes, and other internal components, and they come outfitted with larger heatsinks for superior heat dissipation—all of which translates into more weight. Larger cooling fans, which typically move more air while making less noise than smaller fans, are another plus.

SILVERSTONE

A 6+2 pin connector.

Of course, you should also check the PSU's connectors to confirm the unit is compatible with your particular system. The term 20+4 pin refers to a connector that can function as either a 20-pin connector or a 24-pin connector. In the 6+2 pin connector shown at right, you can snap two of the pins in the connector on or off to suit your needs.

The vast majority of consumer PCs use standard ATX power supplies. Smaller units and units specially designed for enterprise and server applications are also available; but for common desktop systems, ATX power supplies are it.

When searching for a power supply, keep your eyes on three crucial features: power output, rails, and efficiency. Other specifications and features are important, too, but these three directly affect the PSU’s performance.

All about output

Manufacturers usually list their power supplies' output in watts. A higher-watt PSU can supply more power. Desktop power supplies have a power output rating of from 200 watts to 1800 watts (for ultra-high-end, enthusiast-class products). Wattage ratings higher than that would exceed the capabilities of a typical 15-ampere electrical outlet. The important number here is the one for sustained or continuous power, not the one for peak power. Most power supplies can operate at peak power for only brief periods.

Ideally your unit will delivers plenty of power to your components and offers some extra headroom in case you want to attach additional components later. Most power supplies hit their peak efficiency levels with loads in the range of 40 to 80 percent. Building to about 50 to 60 percent of a PSU's capacity is advisable to achieve maximum efficiency and yet leave room for future expansion.

A glimpse inside a PSU.

For example, if the maximum power or combined TDP (total design power) of your system’s present components is 300 watts, a 600-watt PSU would be a good fit. In a high-end system loaded with components that may peak collectively at 700 watts, a 1200-watt PSU would work well. You can get by with lower-capacity units if you don’t think you’ll ever need to expand your system, but if you can afford it, choosing a higher-capacity PSU is a better bet.

Outervision and Thermaltake's handy-dandy PSU wattage calculator invites you to input your build components in exacting detail—right down to CPU overclocking voltages and specific water-cooling components—and then spits out a ballpark power-supply wattage for your system.

On the subject of wattage, one common power-supply myth holds that higher-wattage power supplies necessarily consume more power. Untrue. All else being equal, a 500-watt power supply won't consume any less power than a 1000-watt unit. That's because a system's components—not its PSU—dictate its power consumption. If you have 300 watts' worth of components in a system, the system will consume 300 watts under load, regardless of whether the system is outfitted with a 500-watt power supply or a 1000-watt one. Again, a PSU’s wattage rating indicates the maximum amount of power the unit can provide to your system’s components, not how much power it consumes from the outlet.

An efficient PSU is a better PSU

A power supply’s efficiency rating is important because higher-efficiency units tend to have better components, waste less power, and generate less heat—all of which contribute to less fan noise. A power supply with an efficiency rating of 80 percent provides 80 percent of its rated wattage as power to your system, while losing the other 20 percent as heat.

Five of 80 Plus's certification levels.

Look for units with “80 Plus” certification. Though the certification process isn't especially stringent, 80 Plus-certified units are confirmed to be at least 80 percent efficient; and 80 Plus has tiers for even more-efficient units, including 80 Plus Bronze, Silver, Gold, Platinum, and Titaniumcertifications. Power supplies in the higher certification tiers tend to command very high prices, however. Average users with average needs should probably stick to the simple 80 Plus or the 80 Plus Bronze level unless they find a particularly juicy deal on a Silver or Gold PSU.

Corsair provides a thorough overview of power supply efficiency and of the 80 Plus program, if you'd like to learn more.

The great rail debate

In addition to identifying output power, manufacturers will specify the number of +12V rails their PSUs contain. A “single-rail” power supply has a single, high-power +12V rail for feeding power to hungry system components. A “multi-rail” unit divides its output between two or more +12V rails.

In a single-rail design, all of the power from the supply will be available to any component connected to the unit, regardless of the connector or cable used. In the event of a failure, however, a single-rail power supply has the potential to shoot much more current into your components.

Meanwhile, the main disadvantage of a multi-rail PSU is that it can't share power among the different rails. For example, if you connect 25 amps' worth of components to a +12V rail with a 20-amp maximum rating, the mismatch will trigger an overcurrent protection (OCP) mechanism and shut down, even though other rails my be available with plenty of power to spare. Consequently, with a multi-rail PSU you must pay attention to which components you've plugged in to which rail, a mild nuisance that you don't have to worry about with a single-rail power supply.

On the other hand, that disadvantage becomes a major advantage if you ever encounter a catastrophic failure. The OCP mechanisms in a multi-rail power supply monitor each rail and will shut the whole unit down if they detect an overload on any of the rails. The OCP on single-rail units kicks in only at much higher amperages, which could lead to a major melt-down if a serious overload occurs.

So which is better type of power supply is better—single-rail or multi-rail? Neither, usually. From a performance standpoint, both work equally well; and in general both are very safe to use. If you're building an especially powerful system, though, multi-rail OCP provides an extra layer of safety in case something short-circuits, lessening the odds of frying your costly components during a computing catastrophe.

Cabling: Piecemeal or whole hog?

CORSAIR

Corsair's HX850 is a partially modular power supply...

Another consideration is cabling. Power supplies are available with hard-wired cabling, with partially modular cabling, or with fully modular cabling. In modular power supplies, you can add or remove cabling from the PSU as needed to avoid case clutter.

Technically, a power supply with hard-wired cabling is optimal because it requires no additional connections between the unit’s internal PCB and the connector that will ultimately be plugged into one of your components. One end of the cable is soldered into the PSU’s PCB and the other end terminates in a standard connector, with no breaks in the line. Whenever you introduce an additional connection between the PSU and your components—as happens with modular power supplies—you add more resistance and another potential point of failure into the line; and any increase in resistance translates into lost efficiency.

CORSAIR

...whereas the AX860i is fully modular.

That said, the additional resistance is normally minimal and not a cause for concern for most users. Meanwhile, modular cabling greatly simplifies keeping the interior of your case nice and clean—just don't connect any superfluous cables to keep the clutter down. Most people prefer modular PSUs, though they cost a bit