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KitGuru takes the subject of power supplies very seriously, this is why we have a dedicated team behind the scenes, experts in their specific fields to produce some of the most detailed power related reviews online. Today one of our resident experts ‘IronLaw' will explain exactly how a computer power supply works. While this can be a complex subject matter, if you take your time reading it you will find it is not a ‘dark art' subject, or something impossible to understand. We hope this will be both educational and informative.
Power supply units are perhaps the most neglected piece of equipment in the history of technology. It is no overstatement to say that they are the most critical part of almost every electronic device as they are the one component which is absolutely vital to their operation. Almost no technology equipment can operate without them. That is because no equipment can actually operate by directly using the utility grid AC voltage, which also varies in level and frequency depending on your position on the planet. It is the power supply's job to convert that AC voltage to another form, suitable for the equipment. Or, to be even more accurate, the definition of a power supply is that “it is an apparatus designed to convert one form of electric energy to another“.
Several people may believe the power supplies are limited to computer applications. That is incorrect since almost every piece of technology needs a power supply to operate. Some examples include your phone charger, your TV, your alarm clock and any other electronic household equipment. Users are simply not aware of the presence of power supplies because they are often integrated into the equipment. They are not replaceable because those devices have no expandability or upgradeability and are (hopefully!) designed to exceed the product's lifetime. For example, you cannot replace the power supply of your TV because there is no way to upgrade or expand your TV and force it to require more power, meaning that ultimately there is no reason to perform a power supply upgrade. Computers are an entirely different matter; they are fully expandable and customizable, meaning that not only each and every one of them has different power needs but that the needs of every single computer can be altered several times during its operational lifetime.
A power supply transforming the utility grid AC voltage to DC voltage for the equipment to use must perform certain functions at the highest possible efficiency and at the lowest possible cost. The basic functions usually are:
- Rectification – Convert the input AC voltage to DC voltage.
- Voltage transformation – Adjust the supplied voltage to the required levels.
- Filtering – Smoothen the ripple of the supplied voltage.
- Regulation – Control the supplied voltage regardless of line, load and/or temperatures changes.
- Isolation – Electrically isolate the input voltage source from the output.
- Protection – Prevent any damaging power phenomena from reaching or take effect at the output.
Even though there are more than eight types of power supplies, for household and business equipment there are only two basic power supply designs, linear and switching power supplies.
Linear power supply units are relatively simple and common for low power applications. They fundamentally work by stepping down the AC voltage of the utility power grid, rectify it to DC voltage and finally filter it by using a capacitor.
In order to step the voltage from the 90-240V utility grid voltage down to a low voltage usable by the equipment, linear power supplies use a transformer. The transformer steps down the supplied AC voltage to a lower value (e.g. 18 VAC) but the original waveform remains unchanged. Then a rectifier is used to transform the sine wave AC voltage into a fully rectified pulse voltage. As this rectified voltage still resembles AC, it cannot be used yet. Filtering through a capacitor is required to transform this voltage to near-DC. Then a voltage regulation stage is necessary to adjust this near-DC voltage to true DC voltage and sustain it regardless of changes in the load current.
In very small and simple applications that can be done by using a Zener diode but most of the time a full voltage regulation circuit using a power transistor operating in its linear region is necessary. This power transistor has the ability to act as a variable resistor in series with the load, regulating the output current and voltage accordingly. It is controlled from a circuit sensing the output voltage and modifies the transistor bias to maintain a set voltage output despite of any load current changes.
Linear power supplies have many desirable characteristics. They are very easy and cheap to manufacture because of the few and common components, which also makes them very reliable when correctly designed. Their performance is excellent as well, with exceptional output voltage regulation and next to non-existent ripple. Finally even the lowest quality products show very little electromagnetic interference (EMI) and exceptionally fast response times.
With so many advantages people would expect linear power supplies to dominate today's technology market, yet that is not possible because of their two main disadvantages. Even though linear power supplies are simple and easy to manufacture, they are only used for simple and low power applications because their size and weight increases dramatically as their power output increases.
This primarily has to do with the input voltage frequency, as we will later explain. Early linear power supplies for electronic equipment would not output more than 10A, yet still they used to weight more than 10kg and had electrolytic capacitors as large as coke cans. Linear power supplies are also extremely inefficient, with even the best of products not being able to offer an electrical efficiency higher than 45-50%.
The extremely low efficiency not only is a waste of precious power but it also is a very large heat source which needs to be dealt with by implementing corresponding cooling solutions. Imagine having a small linear power supply powering an office computer; even a 300W power supply with a 50% efficiency would need to dissipate another 300W as heat and would be at least 6 times the size of a common ATX design. These disadvantages make the linear power supplies useless for a wide range of applications. Of course the technology progressed since their first appearance but linear power supplies are still usable only for simple, low power applications such as AC chargers and laboratory power supplies.
Switching power supplies may be more complex than linear power supplies, yet they are much more common nowadays and irreplaceable when it comes to high power equipment.
Even though their design is much more complex than that of linear power supplies, their fundamental difference is that there are two rectification stages instead of one.
The operation principle of a switching power supply is not very hard to understand but it is a more complex design than linear power supplies nonetheless. Switching power supplies will not step down the AC voltage of the utility power grid; they will rectify it to DC voltage directly and then a switching transistor will invert it to AC voltage again but at a frequency thousands of times higher than that of the AC power grid.
The frequency of the power grid is 50Hz to 60Hz, depending on where on the planet you live in, and that is what linear power supplies use; typical switching power supplies change the transformer input frequency to anything between 100.000Hz and 2.000.000Hz. Then they will again rectify that high frequency AC voltage (or portion of it) to DC voltage of a desired value.
The higher the input frequency is, the smaller the electrolytic capacitor and the main transformer can be; a switching power supply is anywhere between 4 and 25 times smaller than a linear power supply of similar output specifications. The controllable high switching frequency of the transistor can also increase the electrical efficiency of the power supply dramatically by reducing the duty cycle to match the load's power demands, meaning that it can cut off portion of the input power and supply only the amount of power the load requires.
This approach is called pulse width modulation (PWM) and can double the power supply's efficiency; typical switching power supplies have an electrical efficiency of 75% or better.
Switching power supplies are not without disadvantages, which are many and critical. Remedying these disadvantages requires additional circuits (such as PFC controls, voltage regulation, etc) which further increases their cost and complexity. The high switching frequency of the main transistor is a serious source of electromagnetic interference (EMI) which needs to be supressed by using filters and shielding.
It also causes a lot of noise at the output, therefore using line filters is necessary to ensure the smooth operation of digital circuits and the noiseless operation of audio systems. Low quality products can cause similar noise at the input, interfering with other equipment connected on the same power line. Products without power factor correction will draw a lot of reactive power which is not used by the power supply but causes strain on the power lines and other electrical equipment, as well as making the power supply the source of strong harmonic distortions; this is why non-PFC corrected power supplies are actually prohibited by law in many countries.
The output voltage quality is actually worse than that of the linear power supplies, usually requiring the presence of voltage regulation circuits in order to reduce the voltage ripple enough to meet certain specifications. Switching power supplies also require a great in-rush current when they are turned on, meaning that momentarily they will draw an immense amount of power which can be disastrous to the utility power grid and electrical equipment, therefore yet another circuit is needed to limit that effect.
Finally, switching power supplies are less reliable by design; for example, a failure could cause a dramatic increase of the output voltage, damaging any connected equipment, or stress on the capacitors could cause them to explode, requiring careful designing and testing.
To combat these problems high quality power supplies integrate even more circuits, such as OCP (over current protection) and OVP (over voltage protection), making them even more complex and expensive.
Switching power supplies have a great deal of problems and their complexity makes designing and troubleshooting extremely difficult. Nevertheless, their much smaller size, much less weight and their high electrical efficiency makes the use of them into virtually all equipment a necessity. All computer power supplies of the past several decades are switching power supply designs, much like the power supplies of almost all consumer electronic equipment ever made.
The AC receptacle is the socket where you connect the power cable at, often accompanied by a simple on/off switch. The switch does nothing more than isolating the power supply from the power grid by interrupting the live wire.
On older power supplies you could find an AC voltage selection switch which had to be set to comply with the utility grid voltage (110V or 220V) depending on where you live at. This is no longer necessary because newer units automatically sense the grid voltage through the active PFC circuit. Since the installation of an active PFC circuit became obligatory by law in many of the world's countries, it is highly unlikely you will find a passive PFC unit (or a unit without a PFC circuit at all) being sold today.
The first thing the power supply has to do is make sure that the AC power is as clean as possible. That includes both the AC power the power supply is receiving but also the AC line itself; switching power supplies generate harmonics which must be filtered before entering the AC power grid. This is done by implementing a transient filter circuit.
The transient filter usually consists of capacitors and induction coils and sometimes includes a metal-oxide varistor (commonly known as a MOV) and a simple glass fuse for extra protection. We say “usually” and “sometimes” because there are no restrictions concerning what manufacturers can and cannot use. High quality units try to meet and exceed the ATX specification guidelines. Very poor quality units will probably have no transient filter at all, so they will flood your household power grid with electrical noise and can be seriously damaged even by a slight power surge.
The most common transient filter components are:
- Y type capacitors : Used for line bypass, connected between Live or Neutral to Ground. They are used in applications where capacitor damage may lead to electric shock and they shunt current to Ground.
- X type capacitors : X type capacitors are being used across the lines, connecting Live to Neutral. They are used where damage to the capacitor cannot induce electric shock and also operate as fire retardants.
- Induction common mode (filtering) coils : Common mode choke coils are used to suppress common mode noise (EMI/RFI filtering). This type of coil is produced by winding the signal or supply wires one ferrite core. Since magnetic flux flows inside the ferrite core, common mode choke coils work as an inductor against common mode current. Accordingly, using a common mode choke coil provides larger impedance against common mode current and is more effective for common mode noise suppression than using several normal inductors.
- Metal Oxide Varistor (MOV) : Metal oxide varistors (MOVs) are semiconductors that protect electronic components and systems from transient voltages. Their design allows current to flow in only one direction. MOVs have very high resistance at low voltage and very low resistance at high voltage, which is where their name comes from (Variable Resistor). Thus applying low to moderate voltage has little to no effect, however a high voltage triggers an “avalanche effect” and causes the diode to break down. In short, these very little components are your typical, reliable surge suppressor. MOVs tend to degrade if they are repeatedly exposed to transients.
As we mentioned in the operation principles of switching power supplies, the first thing it has to do in order to transform the utility grid AC voltage to DC is rectify the cleaned AC voltage to high voltage DC. This is done through a single chip, called the rectifying bridge, which is assisted by a number of large electrolytic capacitors (usually 1). It may or may not be connected to a heatsink, depending on the class and power output of the power supply.
The power factor correction (PFC) is also taking place at this part of the circuit. Switching power supplies are highly reactive power loads and will cause a huge current-voltage phase shift while operating, drawing a lot of useless reactive power along with the necessary active power. The PFC circuit controls the input current of the power supply and is trying to nullify the phase shift between the current and voltage.
A power supply without power factor correction usually has a power factor of 0.5, meaning that it may draw as much reactive power as active power altogether. An active PFC circuit will bring the power factor very close to 1, nullifying the reactive power draw almost completely.
The power factor will not increase the actual performance of the power supply itself directly, neither will decrease the consumption of power which home users pay for. Large businesses and industrial buildings are being billed for the amount of complex power they use, which includes the reactive power, and are fined by the utility company if the power factor is too low. Home users are being billed only for the amount of active power they use. But it is a huge mistake to believe that just because the home users are not getting billed for the amount reactive power they consume, there are no advantages at all.
The power factor is an efficiency figure, indicating how efficiently an electrical load can draw and make use of utility grid power. Despite that home users do not have to pay for the reactive power their equipment is drawing, their entire household electrical grid has to be sized according to the complex power they use. This includes wiring, safety equipment and UPS systems. In short, a power supply without a power factor correction circuit not only will strain the entire electrical grid (including that of the household) for no reason, but when purchasing an UPS a much larger and far more expensive product is necessary. (For more information, please read our “Why companies rate UPS in VAs and PSUs in Watts” article).
All switching mode power supplies, including of course computer PSUs, use one or more high frequency transformers to transform the high frequency AC input to DC. As we mentioned before the high frequency allows the use of a much smaller transformer which also is considerably cheaper.
Most power supplies have a single large transformer feeding power to each and every output line, except from the +5VSB. However there have been instances of special high output models using more than one primary transformer, such as the Corsair HX1000 which had two. A small transformer, usually found right next to the primary one, is responsible for the +5VSB line. Older units also had a second very small transformer, being used as an isolator for the PWM circuit.
The primary transformer separates the primary from the secondary side of the unit. The primary side is the one under AC voltage (the side where the rectifier bridge and main capacitor are) and the secondary side is the one under DC voltage (the side where the output cables are at).
The primary heatsink holds the number of diodes and transistors which are necessary to convert the incoming high voltage DC to high frequency AC. Other components may be found attached to it, such as several PFC components or even the rectifier bridge.
The secondary heatsink holds the Schottky diodes responsible for converting the DC output of the main transformer(s) to voltages usable by your computer parts. Depending on the power supply configuration Schottky diodes are being used to provide either a) only the +12V output and the lower voltages are being generated by DC to DC circuits, b) the +5V and the 12V output and the +3.3V voltage is generated by adding a voltage regulator to the +5V line, or c) even all three positive voltage outputs found in an ATX power supply and the +3.3V voltage line can be entirely independent or sharing its output with the +5V line. The negative voltages (-5V and -12V) are generated by simple diodes and are slowly being removed from ATX power supplies.
The number and configuration of those components indicate the class (or topology or circuit methods) of the power supply. There are several different topologies currently in use, all with their advantages, disadvantages and operating limits. Manufacturers and engineers choose their product topology by taking into account the manufacturing cost and performance requirements of the product. End users should not be concerned about the topology of their unit because each and every design has its own operating range and its performance depends on the configuration and components used in the entire product. The actual performance and efficiency of a switching power supply depends on the design and quality of the entire product, not on the topology design alone.
On the secondary side of any switching power supply you will also notice induction coils and a large number of smaller capacitors. These are being used for filtering the DC output lines. There are other circuits too, depending on the class and design of the power supply. Some of them are the protection circuits (OVP, OCP, UVP etc), the PWM control, DC to DC conversion circuits and more.
Computer power supply units deliver the power to the PC hardware through a number of cables with connectors, the number of which depends on the type and output of the power supply itself. However only the number of the connectors changes; in order to ensure compatibility, the type of the connectors used per generation is identical. The generic specifications for various PSU form factors are defined in Intel's design guides, which are periodically revised.
The largest and most important connector is the ATX 24 pin Molex main connector. As the name implies, it has 24 pins which carry all of the output voltages, several common (ground) pins and even some signal pins (such as the “Power OK” signal). The exact pinout diagram of the ATX 2.X connector can be seen below. You will notice there is a blank pin (N/C), which used to be the -5V line and has been removed from the 2.X standard. There is always only one ATX main connector.
CPUs nowadays require a lot more power than some years ago, therefore power connectors dedicated to CPU power became necessary. Most low and middle range motherboards use a P4 ATX connector, seen at the schematic below, theoretically able to provide up to 192W worth of power to the CPU. On high performance motherboards this connector has already become obsolete and they are using the 8-pin EPS connector instead, which is twice as large as the P4 connector and can supply the CPU with twice as much power.
The EPS connector started as a server/workstation motherboard component but with time it became a necessity to satisfy the ever rising power consumption needs of modern CPUs. Most EPS connectors (also called 4+4 pin connectors) can be split and used with motherboards using a P4 connector. Home consumer power supplies always have only one EPS/P4 connector.
The ever rising power consumption is not a problem related with the main CPU only; during the past few years a great deal of video cards consume great amounts of power. This has led to the addition of another connector, the PCI Express (PCIe) 6 pin connector, able to feed a PCIe video card with up to 75W.
The power demands of some of the video cards would soon lead to the release of the PCIe 8 pin connector, which is able to transfer twice as much power. Even then the power consumption of some video cards could not be satisfied and it is not uncommon to see more than one connector on several of the latest high performance video cards. Latest computer PSUs should have at least one 6 pin PCIe connector but as their power output increases so does the number of the PCIe connectors they offer.
SATA was introduced to upgrade ATA devices to a more advanced and efficient design. Almost all SATA devices, usually hard disk and optical drives, require a SATA power cable. The few exceptions are the early SATA devices which had 4-pin Molex connectors for compatibility with the power supplies at the time they were released. The number of SATA connectors depends on the type and output of the power supply alone, ranging from none to tens of connectors.
The 4-pin Molex connector is by far the most popular connector in the history of computer power supplies. It has been in use ever since the first implementations of switching PSUs designed for personal computers and it still is a very common connector, despite the introduction of the SATA connectors a few years ago. 4-pin Molex connectors are being used to power almost everything inside a PC, ranging from drives and fans to providing supplementary power to special motherboards and devices. The number of 4-pin Molex connectors depends on the type and output of the power supply alone, ranging from a mere couple to tens of connectors.
The 4-pin Berg connector was introduced when the first 3.5″ floppy drives started showing up, which is why it is also called a floppy connector. Although some late AGP video cards also used Berg connectors for extra power which the AGP slot could not provide at the time, the Berg connector never became as popular as the 4-pin Molex connector. At this point of time it is slowly being phased out, following the disappearance of the 3.5″ floppy disk drives. Most recent computer PSUs have only one or two Berg connectors at most.
Please feel free to discuss this with our community over here
Bloody Nora IronLaw, that certainly is detailed. I made it to page 2, now I need a glass of coke (cola not powder). I shall return later once I absorb the basics !
I am one of the most demanding readers on the face of the planet. The standard of editorial content on this site is without a doubt leading the way on the net right now. Some of the news stories are very ermm interesting, but the reviews are fantastic.
I have bookmarked this, I shall spend a few hours Sunday afternoon chilling and trying to work this out. I need to read this at a snails pace to get to grips with it all.
Well this is taking power supplies to the next level. I got lost in parts, and im quite technical, but its well written and rather complex. Nice idea these articles, at least you guys expect the readers to know more than the usual tech site reader
Thank you, thank you , thank you.; I always wanted something like this, with images and easy to understand text. I have struggled on page 4 and 5, but I have picked up more from this than any other PSU related article on the net.
A hell of a lot of work in this Ironlaw. Thank you very much for making all this effort to educate us peasant mortals π
brain overload………… ekkk.
I am just too stupid to understand half of this, but thanks anyway ! the last page was as far as I made in understanding it all :p
Stunning work IronLaw, technically way above my level, but I feel a little better for having read it. I love the PSU reviews on KitGuru too. at the same level as Hardocps (which are also excellent).
With standard of content like this on a regular basis, you will go far ! top marks KG
Christ on a stick. lost for words. This is more for apprentice engineers !
Normally I see these guides and think, oh here we go again, some idiot reviewer ranting on about something stupid and trying to understand it himself.
Not in this case. Ironlaw is a cut above! really really enjoyed this one.
**claps** – I understood most of it, which is surprising as I only know the basics about power supplies. I hope to see more of these. I would like one explaining what exactly we need to buy when looking for a PSU, single rail 12v? multiple rails. and making sense of actual ratings. who needs 1200W psus for instance? do we ?!
Well that was surprising. very very good reading. bit complex for me in parts, but I picked up bits and pieces.
Ironlaw, thank you. very informative reading for a friday morning π now off to work to type in numbers.
lol, I just read something so badly written on bittech, then I come here to read this. its like comparing nickelodean to the discovery channel.
I feel educated, thank you π
For such a complex article it is very informative, a little complex for this geek, but im sure many people love all this about PSU’s. The last page was helpful :p
I shall email Zardon for permission to use parts of this for an edu guide I am writing for college kids.
Brilliant piece of work. 10/10
I find power supplies totally confusing. I just buy one and get a good one after reading several reviews online. I actually ordered the thermaltake one after reading the review on kitguru. This was educational but honestly a little over my head for 70 percent of it. I learned a little, which is more than I normally do every day !
Fantastic writing and very educational as other people have said. I read the power supplies reviews here in the last month and I prefer them to johnny guru. Johnny is great with PSUs but his writing style annoys me, jokes are fine, but half the review is him trying to be funny. The reviews on this site are just as detailed but more focused. I hope you get the corsair 1200w PSU to review, I want to order it, but its 260 here and im not sure if its worth it.
read it in detail – very good work indeed Ironlaw, you know your work π
My brain hurts.
Where do i find the for dummies version?
Nice work mate π
Cool and easy to understand