THE SUPPLY POWER OF
Everything you need to know about your PSU, and some things you probably didn’t
YOUR RIG DRAWS every watt and volt it uses from one power supply unit (probably). All being well, it will sit quietly in a corner of your case and keep the magic happening. There are only three reasons to buy a new one: your existing one has gone bang or started to smell funny; you are about to embark on serious upgrading; or you’re building a new rig from scratch.
Its job may be vital, but the PSU is often left until last on a component list, using the remaining budget after you’ve picked out the good stuff. A power supply is a power supply—as long as it’s big enough, you just plug it in, and it works, right?
There’s a bit more to it than that. A good one lasts a long time, stays cool, and enables future expansion and stable overclocking. Get it wrong, and you’ll be back ordering another, and possibly other components, too, because when a PSU goes bang, it can take other parts along with it.
The current standard for PSUs is ATX, which surfaced in 1996, replacing the ancient AT. The latest iteration is ATX 2.31, dating from 2008, which shows how leisurely the development of PSU standards is. That’s not to say PSUs aren’t evolving; the huge appetite for power of the graphics sub-system, and the world of overclocking, has put great demands on the PSU. The latest ones are adding digital control circuits; you can even get ones with Bluetooth, so you can run them from a phone app. So, how many watts do you really need? And why do prices vary so much?
THE ELECTRICITY FROM your wall outlet is completely wrong for your PC. It delivers a 120V 60Hz alternating current (technically it is 120V +/-5 percent; the median value is actually 117V). This is great for chunky electric motors or making toast, but inimical for the delicate circuit boards inside your PC—nothing can use this unmodified, it’s way too powerful, and everything wants a direct current instead. The PSU’s one and only job is to transform this supply into tech-friendly low-voltage DC varieties. The original IBM PC’s power supply delivered 63.5W—things are a little more taxing now.
A PSU is an SMPS—a switched mode power supply. It switches from full power to none and back quickly. The voltage is regulated by adjusting the switching ratio between full-on and off. The idea has been around since the 1830s, but it took modern transistors to put them into everyday consumer electronics. An SMPS is smaller, lighter, and more efficient than plain old linear designs, which waste excess power as heat. Old-timers may remember when transformers were heavy bricks; no longer. The main drawback is that the switching introduces high-amplitude, high-frequency interference, which needs to be filtered out.
There are eight main stages in a switched power supply. These are: input filtering, primary bridge rectifier, primary capacitors, primary switching transistors, transformers, secondary rectifiers, output filtering, and finally feedback and protection circuits. The bridge rectifier is an arrangement of four diodes that only allows electricity to pass one way, thus the sine wave of an AC current is turned into a saw-toothed DC one. This is then switched on and off very quickly, and fed into the main transformers to be stepped down to the required voltage. And, yes, we have skirted around some of these stages, because, quite frankly, you don’t need to know, and it may involve diagrams.
What every component in a PC really wants is a steady voltage, but all the business of transforming and rectifying adds noise to the signal. Enter the capacitors. These are passive devices used in PSUs to smooth out the ruffles in a current. They are rapidly charged and discharged, but as they resist rapid changes in voltage, the peaks and lows of the signal are never reached. This turns a jagged signal into a much smoother one. You’ll find them both on the primary side of the PSU, dealing with the incoming AC current, and on the secondary side, smoothing the DC current from the rectifier.
The Japanese have something of a reputation here. Years of extremely high production standards, purity of materials, and quality control have given them an enviable reputation. Many spec sheets proudly boast of Japanese 105 C rated electrolytic capacitors, the ones to look for. Good capacitors are expensive, and this is one area where budget PSUs cut corners.
A PC requires its juice in three strengths: 12V, 5V, and 3.3V. The main 12V is used for the processor and PCIe cards, as well as fans, drive motors, and anything else that needs a decent amount of power. The 5V supply is used for drive controllers, parts of the motherboard chipset, including the BIOS and CMOS, and other sub-systems—USB, PCIe control circuitry, and such. The 3.3V rail typically runs your RAM, some graphics card functions, and other control circuitry. The majority of the power requirement of your PC falls on the 12V supply, and it’s this that needs the most attention.
The motherboard also has a number of VRMs—voltage regulation modules— to supply the more delicate components. The processor has its own 12V supply, for example, but you couldn’t send that through any processor made since the 1970s. VRMs use switching transistors to lower a DC current. They are multiphase synchronous buck converters. Note the term “multiphase.” Each phase means adding a set of transistors, which increases the stability of the output, reducing ripple (we’ll get to that). More phases doesn’t mean more power—that depends on the amperage rating of the transistors. More is definitely better, though.
To supply the main voltages, the PSU has a number of rails. A rail is a separate line of connections, which has a set monitored and limited output, to avoid excessive heating and dangerous loads. There are 12V, 5V, and 3.3V rails, plus a small 5V standby rail, and a now virtually unused -12V rail kept for compatibility. 2003’s ATX 2.0 spec calls for two 12V rails: one for the processor, and one for everything else. Originally, these were rated at 20A each. This is equivalent to 240W (watts = amps x volts).
Generally, a single 12V source was split into two rails, with the lower voltage rails
“A PC wants a steady voltage, but transforming and rectifying adds noise to the signal.”
running off the 12V supply, using VRMs to drop the voltage. Building fully independent rails is expensive. A few specialist PSUs boast “real” multiple rails—however, almost all actually run one and split the output. The high demands of graphics cards, all of which fell on a single rail, soon caused problems, and the power demands hit the buffers of the ATX specifications. PSUs started to split out more rails to cope. These were configured (or should have been; there were some early failures) to spread the power-hungry PCIe loads. Each rail’s power rating is its limit before circuit protectors cut in. It does not mean the PSU can supply that level of power across all the rails; there’s still a maximum 12V it can deliver, which may be lower. In 2007, ATX 2.3 finally dropped the 20A limit, one which was increasingly being breached.
Enter the one big 12V rail design. Gamers have traditionally liked these—you don’t have to worry about spreading loads. Just connect everything, and let the hardware draw what it needs from a common pool. Job done. Splitting down is inherently safer, though—if a part fails on a single rail PSU, it can potentially get a huge amount of power sent through it, a hefty shock, and possibly a fire. The single rail design is in favor these days, especially for enthusiast rigs, although the server room boys still like their safer multiple rail designs. Some PSUs offer both, with the ability to be configured as one big rail, or multiple lower amperage ones. A neat solution.
And now back to those ripples. These are tiny, rapid fluctuations in the voltage. They are a consequence of the AC to DC conversion, a harmonic distortion at the switching frequency, and are expressed as a deviation in mV (millivolts). The ATX specifications lay down allowable levels: 120mV on the 12V supply, and 50mV on the others. Excessive ripple can cause damage to hardware. Fortunately, not the burning and smoking kind, but it does stress electrolytic capacitors in particular, lowering their lifetimes.
Good ripple suppression means better overclocking potential. Faster clock speeds mean less time for the VRM to reach the required voltage; a ripple at the wrong moment of a clock cycle can mean it never reaches the required operating voltage, and it becomes “unstable.” If you are intent on some serious overclocking, look for a PSU with good ripple suppression on the 12V rail. A lot of progress has been made here recently, and high-end PSUs can manage down to 30mV; the very best boast 10mV.
Decent PSUs are reliable, but like everything, they don’t last for ever. The main reason for PSU failure is the fan. If this goes, your PSU rapidly overheats under load. A good PSU has a good fan—it should be as good as or better than your processor cooler and case fans. There’s no point in a PSU with a life of 100,000 hours being cooled by a sleeve-bearing fan with a life of 50,000. Single-fan PSUs always have an exhaust fan, sucking air from inside the case, across the PSU’s heatsinks, and out the case. Look for designs that can run without a fan at low loads, known as Eco mode. Some don’t need the fan until loads hit 40 percent—well above idle.
Capacitors have a limited lifespan, too, which is affected by how hot they get. They can leak, which gets nasty. At this point,
you may get some capacitor squeal—this is gas venting through a pinhole in the casing, making a whistle. If you can hear this, failure is imminent. Less worrying, and more likely as a source of an annoying sound, is coil whine. This is when induction coils start to vibrate under load; this varies in pitch and intensity with the current. If these coils are inside your PSU, there is not a lot you can do. They may “bed in” after a while; they may not. A good PSU will have tested and damped its coils properly.
It may be tempting to use an old ATX power supply you happen to have hanging around, doing nothing, in a new system—not recommended. It may technically fall within ATX specifications, but these are pretty loose, and a lot has changed in the last few years, particularly the distribution of power requirements, which has moved off the 5V and 3.3V rails and on to the 12V one. A perfectly good, but old, ATX supply makes a poor partner for modern rigs. If it still has a 20A 12V rail, for example, you’ll struggle if you load the PCIe system.
Power is delivered to your system through a set of cables with standardized connectors for each job. The wires are color-coded (or used to be; it appears to be going out of fashion): The +12V power line is yellow, the +5V is red, and the +3.3V is orange. Time was a PSU came with a mess of cables dangling from it, each hardwired with a connector. This is unruly, because any connectors you don’t require are left dangling about inside, and you may have to start using adapters and splitters to get everything powered.
Better PSUs now have either a partly or completely modular cable system. The PSU has blocks of sockets, and it comes with a set of separate cables, often snazzy rounded, flattened, or braided ones. You only need to install the cables you need. This looks good, improves airflow and is much neater.
PSUs only draw the power they are asked for, so over-specifying won’t cost you, other than the initial outlay. The chances of you putting too much strain on the 5V and 3.3V rails is next to none. These components simply don’t draw much power, and any decent PSU allows more than enough. It’s the 12V side you need to match with your hardware. So, where is the 12V power going? There are two devices that soak this up, and the culprits are the processor and the graphics card. Compared to these, everything else is a lightweight.
Processors have a TDP figure, expressed in watts. This is the thermal design power, and is the maximum heat a processor generates under a heavy load. It can be exceeded in certain conditions, and isn’t the same as electrical power consumption—
“If your budget has scope, go for a better quality PSU in preference to a bigger one.”
however, it is a pretty good indication of what you will need. For example, a Ryzen 1700X has a TDP of 95W. Under very heavy test loads, it actually needs feeding about 105W. An i7-6900K has a high TDP of 140W, and needs about 148W at its absolute worst. These figures are enough to run the rest of the system put together. The most extreme desktop chip is AMD’s Threadripper, which has a TDP of 180W.
The processor isn’t the big one, though— that’s the graphic card. The technology has really bloomed over the last few years, but so has power consumption. Originally, the PCIe slot was enough; it carries 75W, taken from the motherboard’s 12V feed. This was soon supplemented by a six-pin PCIe power connector, which added more 12V power lines direct from the PSU, and 75W. A second six or eight-pin connector added up to another 75 or 150W, bringing our total to a potential draw from one graphics card to 300W. This pretty much matches the rest of the system, including processor, added together. Some cards are going beyond the PCIe 3.0 specifications by using two eightpin PCIe connectors, for a maximum of 375W. PCIe 4.0 is expected to standardize this, or something similar, shortly.
The novice mistake is to spend your budget on a big wattage supply that you don’t really need. One matched to your components, with a moderate amount of breathing room, is enough. If your budget has scope, go for a better quality PSU in preference to a bigger one. Look for better ripple suppression, high-quality capacitors, fan and energy efficiency, sophisticated circuit protection, and such, rather than just getting the biggest headline number of watts. It’s about supplying the power reliably, efficiently, and smoothly for the whole life of your rig. Get it right, and you won’t be buying another for some time.
The $ 450 EVGA SuperNOVA: 1,600W of power—far too much, of course. Unless you really do want four graphics cards.
Corsair’s flagship AXi PSUs were the first to use digital control circuits, enabling you to remotely monitor and control them.
The 6+2 PCIe 12V power connectors. These feed the appetites of GPUs. If your PSU has two of these, and two six-pin versions, it was designed to run two graphics cards.
With modular cabling, you only install the cable you need. Better PSUs are fully modular, mid-range ones are often partly so; both reduce the colored spaghetti look.
Of course, there are LEDs. Thermaltake’s $120 850W Toughpower Grand boasts a 14 RGB LED fan—plus some decent specs.
CoolerMaster makes much of its fans, as you might expect. This compact V650 comes in at about $180. Poky enough for a very demanding single graphics card box.
A basic $ 30 PSU, here from EVGA. This still delivers 400W in total, with 360W through a single 30A 12V rail.
Some nice Japanese electrolytic capacitors help to smooth out the current.