Key Components Inside a Desktop Power Adapter Explained

We offer a clear and concise look into Desktop Power Adapter, helping readers grasp the essentials with ease.

Key Components Inside a Desktop Power Adapter Explained

If there is any one component that is absolutely vital to the operation of a computer, it is the power supply. Without it, a computer is just an inert box full of plastic and metal. The power supply unit, also known as a PSU, converts the alternating current (AC) line from your home to the direct current (DC) needed by the personal computer. In this article, we'll learn how PC power supplies work and what the wattage ratings mean.

In a personal computer (PC), the power supply is the metal box usually found in a corner of the case. The power supply is visible from the back of many systems because it contains the power-cord receptacle and the cooling fan. A typical PSU will have integrated connectors to send power to the motherboard, microprocessors, and SATA storage. Laptops and mini-PCs usually have their power supplies separate from the computer assembly, instead of integrated into their charging cables.

Power supplies, often referred to as "switching power supplies", use switcher technology to convert the AC input to lower DC voltages. The typical voltages supplied are:

3.3 volts
5 volts
12 volts

The 3.3 and 5 volts are typically used by digital circuits, while the 12 volt is used to run motors in disk drives and fans. The main specification of a power supply is in watts. A watt is the product of the voltage in volts and the current in amperes or amps. If you have been around PCs for many years, you probably remember that the original PCs had large red toggle switches that had a good bit of heft to them. When you turned the PC on or off, you knew you were doing it. These switches actually controlled the flow of 120-volt power to the power supply.

Today you turn on the power with a little push button, and you turn off the machine with a menu option. The operating system can send a signal to the power supply to tell it to turn off. The push button sends a 5-volt signal to the power supply to tell it when to turn on. The power supply also has a circuit that supplies 5 volts, called VSB for "standby voltage" even when it is officially "off", so that the button will work.

Prior to or so, power supplies tended to be heavy and bulky. They used large, heavy transformers and huge capacitors (some as large as soda cans) to convert line voltage at 120 volts and 60 hertz into 5 volts and 12 volts DC.

The switching power supplies used today are much smaller and lighter. They convert the 60-Hertz (Hz, or cycles per second) current to a much higher frequency, meaning more cycles per second. This conversion enables a small, lightweight transformer in the power supply to do the actual voltage step-down from 110 volts (or 220 in certain countries) to the voltage needed by the particular computer component. The higher-frequency AC current provided by a switcher supply is also easier to rectify and filter compared to the original 60-Hz AC line voltage, reducing the variances in voltage for the sensitive electronic components in the computer.

A switcher power supply draws only the power it needs from the AC line. The typical voltages and current provided by a power supply are shown on the label on a power supply.

Switcher technology is also used to make AC from DC, as found in many of the automobile power inverters used to run AC appliances in an automobile and in uninterruptible power supplies. Switcher technology in automotive power inverters changes the direct current from the auto battery into alternating current. The transformer uses alternating current to make the transformer in the inverter step the voltage up to that of household appliances (120 VAC).

Over time, there have been at least six different standard power supplies for personal computers. In the late s, the industry settled on using ATX-based power supplies, with the latest version being ATX12V 2.0. ATX is an industry specification that means the power supply has the physical characteristics to fit a standard ATX case and the electrical characteristics to work with an ATX motherboard.

PC power-supply cables use standardized, keyed connectors that make it difficult to connect the wrong ones. Also, fan manufacturers often use the same connectors as the power cables for disk drives, allowing a fan to easily obtain the 12 volts it needs. Color-coded wires and industry standard connectors make it possible for the consumer to have many choices for a replacement power supply. If you don't want to deal with so much cable management, you can also buy a non-modular PSU that comes with all its wires already attached.

Windows, Mac, and Linux systems use a string of code called the Advanced Configuration and Power Interface (ACPI) to control and monitor power consumption of components inside the computer. ACPI also decides where to send full, partial, or zero power while the machine is in sleep mode.

A 400-watt switching power supply will not necessarily use more power than a 250-watt supply. A larger supply may be needed if you use every available slot on the motherboard or every available drive bay in the personal computer case. It is not a good idea to have a 250-watt supply if you have 250 watts total in devices, since the supply should not be loaded to 100 percent of its capacity. Repeatedly maxing out and exceeding a power supply's given capacity will easily lead to overheating and multiple component failures. Don't do it.

At the heavy-duty end, power supplies can be bought providing 2,000 watts of energy and beyond, but these are only useful for large servers and supercomputers. An average desktop computer consumes about 200 to 300 watts in use, while laptops and mini PCs are made to take 50 watts or less. Upgrading your machine with things like multi-core CPUs, GPUs, SSDs, larger RAM chips, and bigger fans will naturally require more energy consumption.

With that in mind, a higher-capacity power supply is a common supporting mod when installing new components. PC marketplace NewEgg has a handy calculator on their site that you can use to input various parts of your desktop build and get an estimate of its maximum power requirements.

Power supplies of the same form factor ("form factor" refers to the actual shape of the motherboard) are typically differentiated by the wattage they supply and the length of the warranty.

The PC power supply is probably the most failure-prone item in a personal computer. It heats and cools each time it is used and receives the first in-rush of AC current when the PC is switched on. Typically, a stalled cooling fan is a predictor of a power supply failure due to subsequent overheated components. All devices in a PC receive their DC power via the power supply.

A typical failure of a PC power supply is often noticed as a burning smell just before the computer shuts down. Another problem could be the failure of the vital cooling fan, which allows components in the power supply to overheat. Failure symptoms include random rebooting or failure in Windows for no apparent reason.

For any problems you suspect to be the fault of the power supply, use the documentation that came with your computer. If you have ever removed the case from your personal computer to add an adapter card or memory, you can change a power supply. Make sure you remove the power cord first, since voltages are present even though your computer is off.

Have you ever wondered what's inside your computer's power supply? The task of a PC power supply is to convert the power from the wall (120 or 240 volts AC) into stable power at the DC voltages that the computer requires. The power supply must be compact and low-cost while transforming the power efficiently and safely. To achieve these goals, power supplies use a variety of techniques and are more complex inside than you might expect. In this blog post, I tear down a PC power supply and explain how it works.1

The power supply I examined, like most modern power supplies uses a design known as a "switching power supply." Switching power supplies are now very cheap, but this wasn't always the case. In the s, switching power supplies were complex and expensive, used in aerospace and satellite applications that needed small, lightweight power supplies. By the early s, though, new high-voltage transistors and other technology improvements made switching power supplies much cheaper and they became widely used in computers. Now, you can buy a charger for a few dollars that contains a switching power supply.

The ATX power supply that I examined was packaged in a metal box the size of a brick, with a remarkable number of colorful cables emerging from it. Removing the case reveals the components below, tightly packed to keep the power supply compact. Many of the components are hidden by the heat sinks that keep the power semiconductors cool along with the fan at the right.

The power supply, removed from the case. The large bundle of wires at the left is connected to the computer. The large component in the middle that looks like a transformer is a filter inductor. Click this photo (or any other) for a larger version.

I'll start with a quick overview of how the switching power supply works, and then describe the components in detail. Starting at the right, the power supply receives AC power. The input AC is converted to high-voltage DC, with the help of some large filtering components. This DC is switched on and off thousands of times a second to produce pulses that are fed into a transformer, which converts the high-voltage pulses into low-voltage, high-current pulses. These pulses are converted to DC and filtered to provide nice, clean power, which is fed to the computer's motherboard and disk drives through the bundle of wires on the left.

While this process may seem excessively complex, most consumer electronics, from your cell to your television, use a switching power supply. The high frequencies allow the use of a small, lightweight transformer. In addition, switching power supplies are very efficient; the pulses are adjusted to supply just the power needed, rather than turning excess power into waste heat as in a "linear" power supply.

Input filtering

The first step is for the input AC to go through an input filter circuit that blocks electrical noise from exiting the power supply. The filter below consists of inductors (the toroidal coils) and capacitors. These boxy gray capacitors are special Class-X capacitors, designed to be connected safely across the AC lines.

The input filter components

Rectification: converting AC to DC

The 60-Hertz AC (alternating current) from the wall oscillates 60 times a second, but the power supply needs steady DC (direct current) that flows in one direction. The full-bridge rectifier below converts the AC to DC. The rectifier below is marked with "-" and "+" for the DC outputs, while the two center pins are the AC input. Internally the rectifier contains four diodes. A diode allows current to pass in one direction and blocks it in the other direction, so the result is that the alternating current is converted to direct current, flowing in the desired direction.

The bridge rectifier is labeled "GBU606". Filter circuitry is to its left. To the right, the large black cylinder is one of the voltage-doubler capacitors. The small yellow capacitor is a special Y capacitor, designed for safety.

The diagram below shows how the bridge rectifier works. In the first schematic, the AC input has the upper side positive. The diodes pass the voltage through to the DC output. In the second schematic, the AC input has reversed direction. However, the configuration of the diodes ensures that the DC output voltage stays the same (positive on top). The capacitors smooth out the output.

The two schematics show the flow of current as the AC input oscillates. The diodes force current to flow in the direction indicated by their arrow shape.

Modern power supplies accept a "universal" input voltage of 85 to 264 volts AC, so they are usable in different countries regardless of the country's voltage. However, the circuitry of this older power supply couldn't handle such a wide input range. Instead, you had to flip a switch (below) to select between 115 V and 230 V.

The 115/230 V switch.

The voltage selection switch used a clever circuit, a voltage doubler. The idea is that with the switch closed (for 115 volts), the AC input bypasses the bottom two diodes in the bridge rectifier and is instead connected directly to the two capacitors. When the AC input is positive on top, the top capacitor is charged with the full voltage. And when the AC input is positive on the bottom, the lower capacitor is charged with the full voltage. Since the DC output is across both capacitors, the DC output has double the voltage. The point of this is that the rest of the power supply receives the same voltage, whether the input is 115 volts or 230 volts, simplifying its design. The downsides of the voltage doubler are that the user must put the switch in the correct position (or risk destroying the power supply), and the power supply requires two large capacitors. For these reasons, the voltage doubler has gone out of style in more recent power supplies.

The voltage doubler circuit. Each capacitor is charged with the full voltage, so the DC output has double the voltage. The grayed-out diodes are not used when the doubler is active.

Primary and secondary

For safety, the high-voltage components and the low-voltage components are separated, both mechanically and electrically. The primary side below contains all the circuitry that is connected to the AC line. The secondary side contains the low-voltage circuitry. The primary and secondary are separated by an "isolation boundary" (shown in green), with no electrical connections across the boundary. The transformers pass power across this boundary through magnetic fields, without a direct electrical connection. Feedback signals are sent from the secondary to the primary by opto-isolators, which transmit signals optically. This separation is a key factor in safe power supply design: a direct electrical connection between the AC line and the output would create a high danger of electric shock.

The power supply with main features labeled. The heat sinks, capacitors, control board, and output wires have been removed to give a better view. (SB indicates the standby supply.)

Pulses to the transformer

At this point, the input AC has been converted to high-voltage DC, about 320 volts.2 The DC is chopped into pulses by the switching transistor above, a power MOSFET.3 Because this transistor gets hot during use, it was mounted on a large heat sink. These pulses are fed into the main transformer above, which in a sense is the heart of the power supply.

The transformer consists of multiple coils of wire wound around a magnetizable core. The high-voltage pulses into the transformer's primary winding produce a magnetic field. The core directs this magnetic field to the other, secondary windings, producing voltages in these windings. This is how the power supply safely produces its output voltages: there is no electrical connection between the two sides of the transformer, just a connection by the magnetic field. The other important aspect of the transformer is that the primary winding has the wire wrapped around the core a large number of times, while the secondary windings are wrapped around a much smaller number of times. The result is a step-down transformer: the output voltage is much smaller than the input, but at a much higher current.

The switching transistor3 is controlled by an integrated circuit, a "UCB current mode PWM controller". This chip can be considered the brains of the power supply. It generates pulses at the high frequency of 250 kilohertz. The width of each pulse is adjusted to provide the necessary output voltage: if the voltage starts to drop, the chip produces wider pulses to pass more power through the transformer.4

The secondary side

Now we can look at the secondary side of the power supply, which receives the low-voltage outputs from the transformer. The secondary circuitry produces the four output voltages: 5 volts, 12 volts, -12 volts, and 3.3 volts. Each output voltage has a separate transformer winding and a separate circuit to produce that voltage. Power diodes (below) convert the outputs from the transformer to DC, and then inductors and capacitors filter the output to keep it smooth. The power supply must regulate the output voltages to keep them at the proper level even as the load increases or decreases. Interestingly, the power supply uses several different regulation techniques.

Closeup of the output diodes. At the left are cylindrical diodes mounted vertically. In the middle are pairs of rectangular power Schottky diodes; each package holds two diodes. These diodes were attached to a heat sink for cooling. At right note the two staple-shaped copper wires used as current-sensing resistors.

The main outputs are the 5-volt and 12-volt outputs. These are regulated together by the controller chip on the primary side. If the voltage is too low, the controller chip increases the width of the pulses, passing more power through the transformer and causing the voltage on the secondary side to increase. And if the voltage is too high, the chip decreases the pulse width. (The same feedback circuit controls both the 5-volt and 12-volt output, so the load on one output can affect the voltage on the other. Better power supplies regulate the two outputs separately.5)

Underside of the power supply, showing the printed circuit board traces. Note that wide separation between the secondary-side traces on the left and the primary-side traces on the right. Also note the wide metal traces used for the high-current supply and the thin traces for control circuitry.

You might wonder how the controller chip on the primary side receives feedback about the voltage levels on the secondary side, since there is no electrical connection between the two sides. (In the photo above, you can see the wide gap separating the two sides.) The trick is a clever chip called the opto-isolator. Internally, one side of the chip contains an infra-red LED. The other side of the chip contains a light-sensitive photo-transistor. The feedback signal on the secondary side is sent into the LED, and the signal is detected by the photo-transistor on the primary side. Thus, the opto-isolator provides a bridge between the secondary side and the primary side, communicating by light instead of electricity.6

The power supply also provides a negative voltage output (-12 V). This voltage is mostly obsolete, but was used to power serial ports and PCI slots. Regulation of the -12 V supply is completely different from the 5-volt and 12-volt regulation. The -12V output is controlled by a Zener diode, a special type of diode that blocks reverse voltage until a particular voltage is reached, and then starts conducting. The excess voltage is dissipated as heat through a power resistor (pink), controlled by a transistor and the Zener diode. (Since this approach wastes energy, modern high-efficiency power supplies don't use this regulation technique.)

The -12 V supply is regulated by a tiny Zener diode "ZD6", about 3.6 mm long, on the underside of the circuit board. The associated power resistor and transistor "A" are on the top side of the board.

Perhaps the most interesting regulation circuit is for the 3.3-volt output, which is regulated by a magnetic amplifier. A magnetic amplifier is an inductor with special magnetic properties that make it behave like a switch. When a current is fed into the magnetic amplifier inductor, at first the inductor will almost completely block the current as the inductor magnetizes and the magnetic field increases. When the inductor reaches its full magnetization (i.e. it saturates), the behavior suddenly changes and the inductor lets the current flow unimpeded. In the power supply, the magnetic amplifier receives pulses from the transformer. The inductor blocks a variable part of the pulse; by changing the pulse width, the 3.3-volt output is regulated.7

The magnetic amplifier is a ring constructed from ferrite material with special magnetic properties. The ring has a few turns of wire wound around it.

The control board

The power supply has a small board holding the control circuitry. This board compares the voltages against a reference to generate the feedback signals. It also monitors the voltages to generate a "power good" signal.8 This circuitry is mounted on a separate, perpendicular board so it doesn't take up much room in the power supply.

The control board has through-hole components on top and the underside is covered with tiny surface-mount components. Note the "zero-ohm" resistors marked with 0, used as jumpers.

The standby power supply

The power supply contains a second circuit for standby power.9 Even when the computer is supposedly turned off, the 5V standby supply is providing 10 watts. This power is used for features that need to be powered when the computer is "off", such as the real-time clock, the power button, and powering-on via the network ("Wake on LAN"). The standby power circuit is almost a second independent power supply: it uses a separate control IC, separate transformer, and components on the secondary side, although it uses the same AC-to-DC circuitry on the primary side. The standby power circuit provides much less power than the main circuit, so it can use a smaller transformer.

The black and yellow transformers: the transformer for standby power is on the left and the main transformer is on the right. The control IC for standby power is in front of the transformer. The large cylindrical capacitor on the right is part of the voltage doubler. The white blobs are silicone to insulate components and hold them in place.

Conclusion

An ATX power supply is complex internally, with a multitude of components ranging from chunky inductors and capacitors to tiny surface-mount devices.10 This complexity, however, results in power supplies that are efficient, lightweight, and safe. In comparison, I wrote about a power supply from the s that produced just 85 Watts DC, but was suitcase-sized and weighed over 100 pounds. Now, with advanced semiconductors, you can hold a much more powerful power supply for under $50 that you can hold in your hand.

I've written about power supplies before, including a history of power supplies in IEEE Spectrum. You might also like my Macbook charger teardown and iPhone charger teardown. I announce my latest blog posts on , so follow me at kenshirriff. I also have an RSS feed.

Notes and references

Thanks for reading! As you consider your next move, keep in mind the insights shared here about AC DC Adapters. Staying informed is the first step toward making smarter, more strategic investments.