Power supply electronics

A power supply unit (PSU) is an electronic circuit that converts device input voltage fed to the device being powered to the voltage or voltages needed internally by the electronics device. In typical computers a power supply unit (PSU) converts mains AC to low-voltage regulated DC power for the internal components of a computer.

Depending on the application the power supply can take in mains voltage (typically 110-120V AC or 220-240V AC) or some other voltage (like for example car 12-14VDC or 24V DC used in industrial automation and trucks). Depending on the application the power supply can provide isolation (almost always needed in mains power supplies for safety and preferred on industrial electronics) or can be non-isolated (many 12V DC powered electronics).

AC/DC power supplies can be classified into one of two primary families: internal or external. Internal power supplies are those which will be installed within some end device as a component; external power supplies accompany an end device as a stand-alone sub-assembly. Internal and external power supplies vary greatly in the degree of engineering effort required to successfully implement the power source as an element of the final system.

Designers know that there is more to a power supply than its ability to provide a steady DC (or AC) voltage despite load and line changes, system transients, noise, and other aberrations.

Here are some article links related to power supplies worth to check out:

Do You Have the Right Power-Supply Protections? article tells that power supply must be able to protect itself against temporary and permanent faults (internal or external), that could cause damage to its load.

Over-current protection in power supplies & converters article tells that AC-DC power supplies and DC-DC converters have internal current-limiting circuits to protect the power device, and to some degree its load. The majority of over-current-protections include an automatic recover feature. There are a number of ways to implement over-current-protection (OCP).

Goodbye 3AG fuse, we’ll miss you article tells that fuses are an essential part of many system designs, and we’ve come to depend on them since the earliest days of electricity. They are simple, reliable, clear cut, and unambiguous components that are used to implement system and user protection as mandated by regulatory standards. Among the most widely-used fuse body sizes is the 3AG size, measuring 6.3×32 mm, which is available in standard ratings from 100 mA to 15 A, in fast-acting, slow-blow, precision, and time-delay versions. This type of fuse fits into a fuseholder socket. The classic fusible link design represented but not limited to the 3AG style is not physically compatible with many of today’s compact product units. To meet the size needs and non-replaceable preferences while retaining the virtues of a “hard” fuse and circuit break, vendors are now offering surface-mount fuses.

Mean Well article Leakage Current discusses on leakage current in EMC filters. Leakage current in power supplies may occur due to the EMC filters, which utilizes Y capacitors between the live and neutral conductors. This causes some leakage current to flow from the neutral or the live conductor to the power supply casing which is normally connected to the earth ground. Most power supply manufacturers specify this current which should always be lower than 3.5 mA as per the IEC-60950-1 regulations. There are standards that specify maximum leakage currents that are safe for humans under different conditions. These vary with the application and type of possible contact as well as the type of ground connection.

Medical safety standards in power supplies article tells that medical power supplies can be dangerous to patients and users if not properly designed and rated by a valid safety organization.

Installing internal power supplies article tells that internal and external power supplies vary greatly in the degree of engineering effort required to successfully implement the power source as an element of the final system. When designing an internal AC/DC power supply into a system, several factors must be considered surrounding the safety, thermal, and electromagnetic compatibility (EMC) implications of the installation. This article outlines the caveats associated with utilizing an internal power conversion solution in opposition to an external one and provides guidance on achieving a proper installation.

An inrush current limiter is a component used to limit inrush current to avoid gradual damage to components and avoid blowing fuses or tripping circuit breakers. A typical application of inrush current limiters is in the input stage of non-power factor corrected switching supplies, to reduce the initial surge of current from the line input to the reservoir capacitor when the power supply is turned on. Negative temperature coefficient (NTC) thermistors and fixed resistors are often used to limit inrush current.

https://www.edn.com/inrush-limiter-also-provides-short-circuit-protection/ design idea mentions that for containing large amounts of bulk capacitance, controlling inrush currents poses problems. The simplest approach involves placing an inrush-limiting resistor in series with the capacitor bank, but a resistor wastes power and adds a voltage drop. THere are also better alternatives like this for low voltage DC applications like this Inrush limiter also provides short-circuit protection circuit.

Most modern desktop personal computer power supplies conform to the ATX specification, which includes form factor and voltage tolerances. You can read more about them at https://en.wikipedia.org/wiki/Power_supply_unit_(computer)


  1. Tomi Engdahl says:

    UKCA marking and the impact on XP Power products
    On 31st December 2020 the UK will leave the EU Customs Union and the European Single Market.

    The Impact of EN 60335 on Electrical Appliance Safety

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  3. Tomi Engdahl says:

    EEVblog 1377 – The Amazing UNPREDICTABILITY of Fuses!

    How long does it take for your 400mA multimeter fuse to blow at 600mA?
    Grab a chair and watch!
    The amazing unpredictability of fusing current ratings at low overloads.

  4. Tomi Engdahl says:

    Engineer It – How to test power supplies – Measuring Noise

    TI’s Bob Hanrahan demonstrates how to measure noise when testing power supplies.

    For more videos on testing power supplies, check out:
    Overview: http://www.ti.com/testingpoweroverview
    Measuring efficiency: http://www.ti.com/measuringefficiency
    Measuring stability: http://www.ti.com/measuringstability

  5. Tomi Engdahl says:

    Measuring Mains Voltage with Oscilloscopes

    Problems with measuring mains voltages with an oscilloscope, the correct and safe way to do it, and other dangerous methods that must never be used.

  6. Tomi Engdahl says:

    EEVblog #594​ – How To Measure Power Supply Ripple & Noise

    Dave explains what the ripple and noise specifications on a power supply is and how to measure it using different methods on both analog and digital oscilloscopes. From bad techniques through to good, showing the effect of each one. Traps for young players aplenty in this one.
    How do you detect common mode noise issues and ensure that the signal you are measuring is really coming from your device under test?
    Single ended & differential measurement, DIY coax solutions, termination, analog vs digital oscilloscopes, bandwidth limiting, and even oscilloscope probe coax construction issues. It’s all here.

  7. Tomi Engdahl says:

    Power supply design advances like FFRZVS #circuits enable smaller #USB PD #power adapters for portable devices Infineon Technologies AG #ReferenceDesign

  8. Tomi Engdahl says:

    Review And Teardown Of Economical Programmable DC Power Supply

    Review and Teardown of a Topshak LW-3010EC Programmable Power Supply

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  10. Tomi Engdahl says:

    Diagnosing a faulty PSU

    A very common PSU fault on a fairly nice power supply from a media player.

    Don’t be fooled by the cheap SRBP (Synthetic Resin Bonded Paper) style PCB. It’s been designed with common sense and safety in mind to comply with UK standards.

    The sizing of the diode array is probably mainly for the increased passive thermal dissipation.

  11. Tomi Engdahl says:

    24V 1000W power supply – autopsy (transformer, GDT, capacitors)

    Today, the autopsy of the important components of the 24V 41.7A 1000W switching power supply from eBay. I opened the primary smoothing capacitors, which are apparently fake Nippon Chemi-Con, actually, used relabelled capacitors desoldered from e-waste (scrapped electronics). I also opened and reverse engineered the main power transformer and the GDT (gate drive transformer). I measured the winding inductances, checked the insulation safety, counted the numbers of turns and measured the wire diameters. I checked the capacitances and ESR (impedance) of all electrolytic capacitors. I also checked whether the transformers, the output inductor and the EMI inductor use copper or just copper coated aluminium (CCA). I also reverse engineered the gate drive circuitry of the power MOSFETs, quite a questionable design.

  12. Tomi Engdahl says:

    12V 50A 600W power supply – (re)winding the transformer

    Today, let’s rewind the faulty transformer of my 12V 50A 600W switching power supply. I wound it partially the original way (with some original shortcomings), just using copper instead of aluminium, and splitting the secondary into more parallel wires, leaving the original design of the primary. Of course it could be redesigned much better, fitting higher cross section of copper into it to reduce the current density. It could also use many more of thinner wires to further reduce the skin effect. The returns that waste space could be eliminated by winding each section in two layers. The secondary could be a copper strip which is more space-efficient for high current low voltage windings.

  13. Tomi Engdahl says:

    “Occasionally when i touch chasis edge(amplifier) it gives me a mild shock, low leakage current i guess.”

    Low leakage current is most common. Sometimes it is possible higher leakage on device but you were well grounded.

    “Out of curiosity when i change polarity of ac power cord everything becomes normal. What’s the reason?”

    Many modern equipment that use swich mode power supplies have filter capacitors between mains side and chassis. Depending on equipment design there can be two capacitors (designed to be grounded equipment) or one capacitor from one of of the ungrounded power connector pins to case. Depending on mains plug polarity you have it from live to chassis (leakage) or neutral to chassis (almost no leakage).

    Traditional transformer can have some leakage that can vary depending on which side of primary is on live or neutral.

    “is it dangerous for human being?”

    When equipment is properly designed, im good condition and built using proper parts, it is not dangerius.

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  16. Tomi Engdahl says:


    A couple of people have asked whether it’s worth substituting the PSUs currently supplied with the Spartan 10 as they certainly feel rather ordinary. While they look like normal modern switching wall-warts, they are in fact linear transfomers with much quieter operation and far better capacitive isolation from the mains than switchers. A low leakage capacitance is important as it stops the mains coupling into the ground path which will be connected in some form to the low voltage output to derive relative power supply rails.

    The first photo shows the linear type used by Classic Audio with only 30pF measured between the mains and low voltage side (in reality it’s more like 20pF due to leakage between the test leads). Very little AC current from the mains can be coupled from the high voltage to the low voltage side.

    The isolation is excellent and there are no components connected between the high and low voltage sides. The low voltage low frequency AC output is then rectified and regulated to very quiet DC rails inside the Spartan 10, although it’s not really necessary as the amplifiers inside the S10 have superb rail rejection. The regulation is there mainly to protect from power surges and ensure the headroom doesn’t vary with mains voltage.

    The second photo shows a switching supply commonly found with a lot of similar audio gear. Here the isolation is not great due to the need to connect a ‘suppression’ capacitor between the high and low voltage sides of the switching supply to prevent the high frequency switching transients from coupling across to the low voltage side and causing radio frequency emission from the output lead. It measures at 660pF of leakage capacitance, some 30 times higher than the linear transformer. In reality it is more than double this as it sits behind the high-voltage rectifier which skews the reading on the meter. This isn’t good news as 30+ times more HF noise from the mains can get into our ground path and our equipment will now ‘float’ on the end of this leakage capacitance if not grounded. Switching transients from the high voltage rectifier will also make their way into the ground path at a considerably greater level.

    The switching supply is cheaper and doesn’t require internal regulation inside the piece of equipment it’s connected to (although such regulation is likely to be ineffective if required as the noise on the output will be high frequency and harder for a linear regulator to reject).

    The external PSU for the S10 is a plain simple low frequency transformer. The power supply rails are derived within the unit itself with the rectifiers, smoothing capacitors and regulators. The internal components determine the performance of the power supply which as already mentioned is much better than it already needs to be, the transformer simply supplies the working voltage and ensures excellent isolation from the mains.

    Contrary to marketing lies told by others regarding highly dubious claims of ‘transient headroom’, or ‘high instantaneous current for punch and slam’ (whatever that means!) a bigger transformer will do nothing except reduce the isolation due to the larger area between the high and low voltage sides. Anyone making these claims should be treated with suspicion as they either don’t know what they’re talking about or are just straight up making fraudulent claims.

    Instantaneous current demand is met by the internal reservoir capacitors, not the external transformer which only charges the reservoirs for a few milliseconds 50 times per second. The rectifier essentially disconnects it from the reservoir and DC rails for 80% of the time! As long as the reservoirs hold up the voltage ahead of the regulators then the power supply delivers the voltage and the current. This is not a linear process and the regulator doesn’t care what the exact voltage is in the reservoir so long as it’s above 18 volts or so. In the Spartan 10 it remains around 22V at full load, ensuring an excellent margin of operation. The transformer is rated to supply the average current and then some, so the small size is quite adequate.

    So there we are! Fortunately they really are the best devices for the job! You can try this one at home also if you have a capacitance meter handy. Rant over :) !

  17. Tomi Engdahl says:

    capacitor from mains side to low voltage side in ungrounded power supplies more norm than design blunder. It is generally needed for the power supply to meet all EMC regulations. The design should also meet safety regulations, so the capacitance value needs to be low enough and the capacitor needs to meet “safety capacitor” specifications for this type of application. There will be more or less leakage through this capacitor depending on which way the adapter is plugged in (does capacitor “hot: end get connected to live or neutral).

  18. Tomi Engdahl says:

    Ungrounded SMPS capacitor between mains side and output:

    If you do the analysis of the circuitry there is a capacitive coupling that pumps the isolated output at the oscillation frequency that leakage is coupled to the supply line and ground by the class y capacitor, if it’s not there the leakage would be superimposed on the output, a grounded supply could have a screen to stop this , but it’s 2 wire


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