Anatomy of a Power Supply Unit (PSU)

Monday, February 3rd, 2020 - Teknologi

Anatomy of a Power Supply Unit (PSU)

Every single desktop PC, console, or laptop has one of these. It doesn’t boost your frame rate or churn out cryptocurrency; it doesn’t have billions of transistors and it’s not made using the latest semiconductor process node. Sounds boring, right? Not at all! This thing is super important because without it, our computers would do absolutely nothing.

Power supply units don’t break headlines like the latest CPUs do, but they’re awesome pieces of technology. So let’s put on our gowns, masks, and gloves, and pull open the humble PSU — breaking down its various parts and seeing what each bit does.


What’s the name of the game?

Lots of computer parts have names that require a bit of technology knowledge to understand exactly what it does (e.g. solid state drive) but in the case of a power supply unit, it’s pretty obvious. It’s a unit. It supplies power!

Since we can’t just dust off our hands and proudly say ‘article done’ with that kind of statement, we better start having a look at one. We’re using a Cooler Master G650M — it’s a fairly generic design, with a specification found in dozens more like it, but it sports one particular feature that not every power supply unit has.

This PSU is a standard-sized one and by that we mean it complies with the ATX 12V v2.31 form factor, so it fits inside lots of computer cases.

There are other form factors, though: ones for smaller cases or unique ones for specific vendors. Not every unit follows the exact sizes set by the standard form factors, they might be the same width and height, but they could be longer or shorter.

They’re also usually labelled by how much power they can supply as a maximum; in the case of the Cooler Master, it can provide up to 650 watts of electrical power. We’ll see what that actually means in this article, but you can get PSUs that deliver just a small number of watts, as not everything computer-wise needs hundreds of watts to run. The majority of desktop PCs will run fine in the range of 400 to 0 W, though.

PSUs like this one are contained within a metal box, usually black or bare metal, so they can be heavy. Laptops nearly always have a PSU that sits externally to the computer and is almost always plastic, but the insides are very similar to what we’ll see in this one.

Most desktop PC power supplies come with a switch to isolate the mains electricity supply and a fan to keep things nice and cool, but not all do (or need to). Not all of them will have a metal Centeng full of holes, either — those found in servers rarely have them.

But as you can see in the picture above, we’ve already taken a screwdriver to our example, so let’s rip off the lid and jump inside.

I’m back in black

Before we start rummaging about with the insides of a PSU, let’s think about why we even need one in the first place. Why can’t we have the computer connect directly into the mains outlet? The answer lies in the fact that modern computer parts are expecting the electrical power to be delivered in a very different form to that provided by the outlet.

The graph below displays how mains electricity (US = blue and green lines; UK = red line) is supposed to be. The x-axis shows time in milliseconds and the y-axis shows voltage in volts. The best way to think about voltage is that it’s a measure of energy difference between two points.

If a voltage is applied across a conducting material (e.g. a length of metal wire), the difference in energy will make electrons in the material flow from the higher energy level to the lower one. These are one of the building blocks of atoms, that make up the material, and metals have lots of electrons free to move about. This flow of electrons is called a current and gets measured in amps.

One good analogy for the techno-speak is that electricity can be thought to be like water in a hose: voltage is akin to the pressure you’re using, the flow rate of the water is the current, and any restrictions in the pipe acts the same as electrical resistance.

We can see that mains electricity varies over time and this is known as an alternating current voltage supply – or just AC, for short. In the US, the mains voltage alternates 60 times per second, reaching a peak of 340 V or 170 V, depending on the location and supply. The UK hits a slightly lower peak, and varies a little slower, too. Almost all countries around the world have mains outlet voltages like this, with just a few having lower or higher peak voltages.

The need for a PSU lies in the fact that computers don’t work with AC: they need a constant voltage, one that never changes, and it also needs to be of much lower level. Using the same graph scales, it looks something like this:

It’s so much lower that it’s barely visible, but the requirements of a modern computer aren’t for one constant voltage, but four – namely +12 volts, -12 volts, +5 volts, and +3.3 volts. And because these values are constant, they’re called direct current or DC, for short. So a big part of what a PSU does, is convert AC to DC (cue the guitars…). Time to open the unit and have a look at how it does this!

… a big part of what a PSU does, is convert AC to DC (cue the guitars…). Time to open the unit and have a look at how it does this!

At this stage, we should warn you to not try this if you don’t know what you’re doing. Messing about with the insides of a PSU can be potentially very dangerous. There are components inside every unit that store electrical energy, and some store a lot.

The layout of this PSU is similar to many others, and although the make and model of the various parts used inside will be different, they fundamentally do the same thing.

The mains outlet connection to the PSU is at the top-left hand corner of the picture and the supply essentially runs clockwise around the picture, until reaching the output of the PSU (big cluster of colored wires, bottom left-hand corner).

If we flip the circuit board over, we can see that compared to connections on a motherboard, these are broad and deep — they’re designed to have lots of current flowing through them. Something else that’s immediately obvious is the big gap running down the middle, like a river cutting a path in a field.

This highlights the fact that all PSUs have two clearly defined sections to them: primary and secondary. The former is all about setting up the input voltage so that it can be efficiently changed from the mains supply level; the latter is everything about that change and the processes afterwards.

He’s a smooth operator

The very first thing the PSU does to the mains electricity isn’t about changing it from AC to DC, or dropping voltage — instead, it’s all about smoothing out the input voltage. Because we have lots of electrical devices in our homes, offices, and business that switch on and off, as well as emit electromagnetic signals, the varying AC is often lumpy and with the occasional spikes (the length of the variations isn’t constant either). Not only do these make it harder for the PSU to adjust the mains, it can also damage some of the components inside it.

This PSU has two stages of so-called transient filters, the first of which is directly applied to input socket, using 3 components called capacitors to do the job. Think of these as being like a speed bump for sudden changes in the input voltage.

The second stage of filtering in this PSU is more complex, but essentially does the same thing.

The yellow blocks are more capacitors, whereas the green rings wrapped in copper wire are inductors (although they’re usually called chokes when used this way). Inductors store electrical energy in a magnetic field but this field also ‘pushes back’ on the voltage supplying the energy — so a sudden spike in the voltage results in a sudden kick back from the magnetic field to suppress it.

The two little blue discs are yet more capacitors and just below them (hidden under a black plastic cover) is a metal oxide varistor (MOV). These are also used to help counter jumps and spikes in the input voltage; you can read more about different types of transient filter circuits here.

This section of a PSU is often the first sign of where costs have been cut to ensure the model hits a specific budget. Cheaper ones will have less filtering, and the cheapest of all will have none at all (which is not what you want!).

Now that we’re all smooth and chilled, let’s get on with the day job of a PSU: changing the voltage.

Rock down to electric avenue

Remember that the PSU needs to change an AC voltage that might be averaging 0 volts (technically, it’s root mean squaring 0 volts, but that doesn’t exactly roll off the tongue) and hack that down into DC voltages of 12, 5, and 3.3 volts.

The first thing that gets done is an AC to DC conversion, and this PSU uses a component called a bridge rectifier. In the picture below, this is the flat black object glued to the chunk of metal (which acts as a heatsink).

Once again, this is another area where a PSU manufacturer can cut costs, with cheaper components doing a worse job of the AC-to-DC conversion (e.g. emitting more heat). Now, if the input voltage peaks at 170 volts (which is the case for 120V mains), then the bridge rectifier will output 170 volts DC.

This gets passed on to the next stage of the PSU and in the one we’re looking at, it’s called an active power factor correction converter (APFC). This circuit adjusts the current flow in the unit to take into account that it is full of components that store and release energy in a complex way; this can result in the actual power output of the unit being less that what you’re supposed to be getting.

Other supply units use passive converters, that essentially do the same job. They’re less effective but fine for low power units — they’re also cheaper, so you can guess what kinds of PSUs have these, when they really shouldn’t!

The APFC can be seen in the image above – those big cylinders on the left are capacitors and they store the adjusted current, before sending them on to the next step in the PSU’s chain of processes.

This section tucked behind the APFC is called a pulse width modulation circuit (PWM, for short). Its job is to take the DC voltage and use several field effect transistors to switch the voltage on and off at a very high rate — it essentially converts the DC voltage back into an AC one. It does this because the part of the PSU that turns the mains voltage right down into 12 volts is a transformer. These devices use electromagnetic induction and a set of two coils of wire (one having more loops in the coil than the other) to step down the voltage; however, transformers only work with an alternating voltage.

The frequency of the AC voltage (the rate at which it varies, measured in hertz, Hz) significantly affects how efficient the transformer is — higher is better — which is why the 50/60 Hz mains supply gets changed into one that varies at something like 50/60 thousand Hz. The more efficient a transformer is, the smaller it can be. This super fast switching of the DC voltage is the source of the name for this type of device: a switched mode power supply (SMPS).

You can see 3 transformers in the picture below – the largest generates just the 12 volt output; in other PSUs, the big transformer might make all of the voltages. The next larger one creates a single 5 volt output that we’ll talk about in a bit, and the smallest one acts as an isolator for the PWM circuit, keeping it safe from harm and also stopping it from causing interference with other voltages in the PSU.

Various PSUs will have different ways of creating the required voltages, isolating the PWM circuit, and so on. It’ll all depend on budget constraints and how much power the unit needs to offer. All of them, though, will need to take the output off the transformer and turn it back into DC.

In the image below, the large chunk of metal is the heatsink for the bridge rectifiers that do this conversion. We can also see in this specific PSU, the circuit board in the middle of the picture corresponds to a cluster of voltage regulation modules (VRMs) that create the 5 and 3.3 volt outputs.

At this stage it’s worth talking about something called ripple.

In a perfect world, with perfect PSUs, the varying AC voltage would be converted into a constant, never-wavering DC voltage. In reality, though, it’s not 100% spot on, and the DC voltages do vary ever so slightly.

This variation is called the ripple voltage and for a PSU, you want it to be a small as possible. Cooler Master doesn’t provide the size of the ripple voltage in the specifications for this PSU model, so we’ve turned to a detailed review to find them. One such analysis was done by and they found that the +12V line in their tests had the ripple voltage peaking at 0.042 volts (42 millivolts).

The image below shows you how this compares to what is required. The red line is the targeted constant +12V DC, the varying blue line is what we’re actually getting (although the ripple itself isn’t constant).

The quality of the capacitors used throughout the PSU play a significant role. Smaller, cheaper ones would result in the ripple being bigger, which is not what we want. If it’s too big, then the complex electronic circuitry in the rest of the computer might operate in an unstable manner. Fortunately, in our example, 40-odd millivolts is okay: not great, but not bad.

No matter what is used to create the output voltages and ensure they’re DC in form, there’s still a few more bits of circuitry needed before we start waving cables out the place. It all relates to managing the PSU’s outputs, ensuring that if a high demand for power is taking place on one particular voltage, then the others aren’t going to be nobbled in the process.

The chip you can see here is called a supervisor and monitors the outputs, checking that they’re not delivering too much or too little voltage and current. It’s not very sophisticated, though, as all it does it shut off the PSU, if any of those problems occur.

More expensive supply units use digital signal processors (DSPs) to monitor what’s going on, and these can also adjust the voltages if required, as well as send details about the status of the PSU to the computer using it. Not overly useful for the typical PC user, but for computers used as servers, compute machines, and so on, it’s often a desired feature.

Plug in baby

All power supply units come with long bundles of wires, sprouting out of their back. The number of bundles, and how they’re connecting to the main unit will differ across the vast array of models available, but they’ll all provide some standard connections.

Since voltage is a measure of difference, there needs to be two wires for a given output: one for the indicated voltage (e.g. positive 12 volts, or +12V for short) and a reference wire that the difference is measured against. This wire is known as the ground or common line, and the two form a loop: running out the PSU, to the device needing the power, and then back into the unit.

The flow of current runs through these loop wires, but since some of the loops will only have a small amount of current flowing in them, several ground wires can be shared by different loops.

The first of which is the obligatory 24-pin ATX12V version 2.4 connection – it offers multiple wires for the various voltages, plus a few specific ones.

An important one is the +5V standby wire – for as long as the PSU is switched on and plugged in, this wire is always live. This is because a computer doesn’t really switch off, when you tell the operating system to shut down. The motherboard draws the power it needs to remain active off the standby connection.

There will also be another 8 pin connector for the motherboard, that provides two sets of +12V and ground wires, and most PSUs will also provide at least one PCI Express 6 or 8-pin power connector.

Graphics cards can only take a maximum of 75W from the motherboard PCI Express slot, so this connector offers additional power for today’s monstrous GPUs.

This particular PSU actually runs two PCI Express power connectors off the same wires, for cost reasons, so if you had a really powerful graphics card in the computer, it would be best to use a separate bundle of wires.

The difference between the 6 and 8-pin PCI Express connector, is an extra two ground wires. This allows a higher level of current to flow down the +12V wires, helping to feed hungrier GPUs.

Over the past few years, we’ve seen an increase number of power supply units proudly wear a tag of ‘modular’ in their description. All this means is that some of the power connectors are wired to another connector, that slots directly into the PSU. So instead of having a mass of cables and connectors clogging up the inside of the computer case, you can remove what don’t need to save some space.

This Cooler Master model, like many others, uses a fairly basic connection system for the modular cables.

Each connector provides one each of +12V, +5V, and +3.3V wires, along with two ground wires, and depending on what device the cable is going to be attached to, the connector at the other end of the cable will either use the same wiring configuration, or something simpler.

The Serial ATA (SATA) connector above is used to provide power for hard drives, solid state drives, and peripherals such as DVD burners.

This familiar shape goes by the snappy name of an AMP MATE-N-LOK 1-480424-0 power connector. Well, most people call it a Molex connector, but that’s actually the name of the company that developed it. It provides one +12V, one +5V, and two ground wires.

The output supply cabling of a PSU is another area where costs can be saved or given a higher budget, either to improve looks or flexibility of the wires. The thickness (or gauge) of the metal wire used in the cables plays a part, too, as thicker wires have less electrical resistance than thinner ones, which results in less heat being generated as current flows through them.

(Something inside) So strong

At the beginning of this article, we said that most power supply units are named after the maximum amount of power they can offer. At the simplest level, electrical power is simply voltage multiplied by current (e.g. 12 volts x 20 amps = 240 watts) and while such a statement will have many engineers chomping at the bit to correct this remark, it works well enough for our purposes.

Like most branded or generic models, our PSU comes with a label providing various snippets of information about how much power each voltage line can provide.

Here we can see that the total power available from all the +12V lines added together peaks at 624 W; add in all the others stated on the label and we get a total of 760 W, so what gives? Well, it’s down to the fact that the normal +5V and +3.3V lines are created using VRMs off the +12V output of the PSU.

And, of course, all of the output voltages come from a single source: the mains outlet. So the rating of 650 W is the maximum the PSU can provide as a total across all lines. So if you were using 0 W on the +12V output, you’ll only have 50 W left for everything else. Fortunately, the majority of hardware inside a modern PC takes the bulk of its power off the 12V lines anyway, so it’s rarely a problem assuming you’ve picked the right PSU model for your needs.

Next to the power specifications, there’s a label saying “80 Plus Bronze.” This is an efficiency rating used in the industry in an entirely voluntary way (i.e. there are legal requirements for PSU manufacturers to comply with the rating system). The efficiency also depends on what size of load the PSU is attempting to serve (i.e. how much current is being drawn down the various lines) .

If we take our Cooler Master unit, running so that it’s providing 325 W of power (50% of its max rating), then we can expect it to have an efficiency of 80 to 85%, depending on the mains supply voltage.

This would result in the unit drawing 382 to 406 W from the wall outlet. A higher 80 PLUS rating doesn’t mean the PSU gives you more power, it just wastes less during all the filtering, rectifying, switching, and transforming stages.

Also note that the peak efficiency is somewhere between 50 and 100% loading; some manufacturers provide charts showing how you can expect the unit to perform under different loads and supply voltages.

It’s worth paying attention to this information sometimes, especially if you’ve tempted to lay down a stack of bucks on a 1000 W PSU. If your computer is going to use anywhere near that power level, then its efficiency is going to take a bit of a sting.

You might see some PSUs claiming to be single rail or multi-rail (or offer a switch to flip between the two). The term rail is just another word for the specific voltage that the power unit generates. Our Cooler Master example has a single 12 volt rail and all the various power connectors that provide +12V draw current off that rail, if used. A multi-rail PSU will have two or more systems providing the 12 volts – however, there’s a big difference in how this is implemented.

PSUs for data center applications or compute servers will have multiple rails for fault tolerance, so if one fails, it won’t affect the others. A desktop computer with a multi-rail PSU might have such a setup, but they’re more likely to be just taking the main 12V output and splitting it in two or three. For example, our example provides up to 52 amps of current off the +12V line, equating to 624 watts of electrical power. A cheap multi-rail version of the same unit might have two +12V lines shown in the specification, but each will only provide 26 amps of current (or 312 W).

A well designed desktop computer PSU, using quality components, doesn’t require a multi-rail +12V system, so don’t worry about it!

Money for nothing?

Power supply units come in all kinds of price tags. A quick run through the listings on Amazon, for the same size format, has them as low as $15 for a generic 400 W unit, and all the way up to $180-240 for a fully modular 1000 W nuclear power station from EVGA or Seasonic. What are you getting for your money? What sort of things cost over $0?

The ability to deliver more power is the obvious one, but it’s how that power gets delivered. The ultra cheap model permits up to 25A of current on the +12V lines, whereas the wallet buster provides over 3 times more, at 83A. Today’s CPUs and graphics cards use the +12V lines for almost all of their power requirements, but surely 25A is enough?

Given that you can now buy a ‘desktop’ CPU with 32 cores and pair it with an equally titanic graphics card, both with an appetite for 300W at full load, the cheap PSU absolutely would npt be up to the demand; on the other hand, though, the most expensive one would have plenty of headroom to cope. And since the combined price of such a CPU and GPU could easily top $3,500 or more, shelling out a few extra hundred perhaps isn’t going to be much of a shock for some customers.

But what you’re really paying for is the quality of the components used inside the PSU. Go back to the start of this article and look at the guts of the Cooler Master unit we’ve been taking apart. There’s not a massive amount of parts there, and since virtually every bit is critical to the operation of the device, it’s not hard to see why spending more is not always money for nothing.

And with that, we bring our dissection of the PSU to a close (and leave a trail of bits all over the floor). It’s a fascinating piece of a kit and the level of engineering involved in designing and manufacturing a good one is surprisingly complex. If you’ve got any questions about power supply units or the one currently sitting in your computer, quietly doing its job, fire them our way in the comments section below as usual. Stay tuned for more anatomy series features.

Shopping Shortcuts
(Picks from TechSpot staff, from least to more expensive):
  • Thermaltake Smart 600W on Amazon
  • EVGA 0 BR on Amazon
  • Cooler Master MasterWatt 750 W on Amazon
  • Corsair RM750 750 W on Amazon
  • SilverStone Strider ST80F 800 W on Amazon
  • Seasonic Prime PX-1000 W on Amazon

Anatomy of a Power Supply Unit (PSU)

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