What is a Power Supply?

In a very basic sense, a power supply is any device or system that can supply consistent and usable electrical power. The important word to remember here is “usable”, and a distinction must be also be made between “power supply” and “power source”. Hoover Dam is a power source, but the power it delivers needs to be converted by a power supply in order to be usable by most electrical products.

The problem, as most engineers understand, is that the power that comes to your office, home, or factory from a generating facility is delivered at between 120 to 240 volts for commercial and residential use, and 220 to 440 volts for industrial use. That voltage must be converted and/or regulated to match the needs, or load, of the device or system that requires power.

That is what a power supply, for our purposes, does. Briefly put, a power supply is a device that takes line voltage as received from a utility and converts it to match a defined electrical load, like a computer, or a stereo system, or a medical device. The three essential functions of power supplies are: voltage change, power conversion, and power regulation.

The electrical circuitry in a power supply typically receives AC voltage from an outside source, changes it to DC, and converts it into the voltage load required by the device that needs to be powered. A component called a rectifier (diode) converts the incoming power from AC to DC. A transformer then changes the incoming voltage either up or down to match what is required. A filter (capacitor) then smooths out the power signal fluctuations from when it is converted from AC to DC. Finally, a regulator reduces any remaining fluctuations in voltage left by the filter.

Not all power supplies have all of these components, however. Unregulated power supplies are available to power devices that are not overly sensitive to power fluctuations. While these may be more cost efficient, care needs to be taken at the design stage to match the power needs of the end product to the output specs of the power supply.

Many different types of power supplies are available depending on function and output, but they all fall into three broad categories: Unregulated, Linear Regulated, and Switching. Here is a quick run-down of these basic types of power supplies:

• Unregulated: An unregulated power supply uses a transformer, rectifier, and low-pass filter. Since there is no regulation of the output, the delivered voltage may vary depending on the DC input or the load. If the input voltage varies, the output voltage will vary by a proportional amount, which may not be acceptable in many applications. Unregulated power supplies, however, offer low cost, simplicity, and efficiency.
• Linear Regulated: This type of power supply takes an AC input then steps down the voltage using a transformer prior to rectifying and filtering it into a DC output with stable voltage. Linear power supplies offer simple application, relatively low cost, and low noise. They are heavier and larger than switched mode supplies and may require heat sinking for the transformer.
• Switched Mode Regulated: A switched mode power supply (or switcher) takes an AC input, rectifies and filters it into DC, converts it back to AC at a high switching frequency, reduces the voltage with a transformer, and then rectifies and filters it again into a stable DC output. Switched more power supplies make use of semiconductor (transistor) switching to provide the output voltage. They offer high efficiency and are smaller and lighter than comparable linear power supplies, but they also produce more noise.

Within these broad categories, power supplies can be classified further into four function and usage categories depending on the amount of voltage needed along with other factors. These categories are:

• AC-DC Adapters: These small, external power supplies are enclosed in a sealed unit for safety and appearance, and plug directly into an electrical wall outlet. They offer low cost and deliver relatively low voltage typically for use with commercial devices.
• Open-Frame Power Supplies: These power supplies are installed directly on a circuit board with no protective enclosure other than the external enclosure of the larger device. They offer broad customization by voltage output, wattage, and form factors.
• Industrial Power Supplies: These are higher power devices that typically offer reliability and ruggedness for use in extreme environments.
• Bench Power Supplies: These power supplies provide variable output power, channels and resolution for use in the lab or field when testing components or systems.

Finally, power supplies may also be classified based on current output. There are two basic types which are somewhat self-explanatory: DC to DC power supplies, and AC to DC power supplies. DC to AC devices are also available, but these are referred to as Power Inverters, which operate by changing a battery source into an AC signal, and are typically used in automotive or solar panel applications.

As a crucial part of almost any electronic device or system design, a power supply needs to be specified using standard parameters. First, the power requirements of the load must be established along with how clean or noise-free that power has to be. Second, what efficiency is needed from the power supply and what physical size does it need to be to fit in the system. Finally, what safety features are necessary in the power supply to protect the load.

Once these items are addressed, individual power supply specifications can be considered. While many different specifications exist for power supplies, here are some of the basics that should be known when evaluating individual products:

• Input Voltage: Typically expressed as AC Volts with lower and upper limit tolerances. Since established voltage tolerances vary from country to country, the geographic usage area of the final end product will determine what is required.
• Output Voltage: Typically expressed as volts, and fixed or variable.
• Output Current: The maximum current (expressed as amps) that can be supplied to the load.
• Accuracy: How close the voltage output of a power supply is to its stated value, defined as a percentage above or below stated value.
• Load Regulation: How well a power supply can maintain its output with a load current change (usually expressed in millivolts).
• Ripple & Noise: Ripple is the variation in voltage when AC is converted into DC. Noise is any unwanted electronic interference produced by the power supply. These are usually combined into one measurement, usually expressed as microvolts peak-to-peak.
• Efficiency: This is the ratio of input power that is effectively converted by the power supply to the DC power output, expressed as a percentage.
• Operating Temperature: The range of temperatures within which a power supply will deliver its rated values.
• Protection: This includes overvoltage protection which can shut down the power supply and protect the load if the voltage limits are exceeded; and overload protection, which prevents damage to the load due to a short circuit or overcurrent event.

Other power supply features that may be considered include operating protection for harsh or rugged environments, output stability over time, temperature stability, smart control features, and industry or national regulatory certifications.

Power supplies, as electrical devices, are rated for safety depending on how they isolate their end applications from dangerous voltages. These ratings include: Class I devices, which employ insulation along with grounding systems; Class II devices, which employ double or reinforced insulation and no grounding systems; and Class III, which operate via a safety extra low voltage (SELV) supply circuit and cannot cause harm.

Additional safety standards and regulations vary by country or region, making it important to understand at the design stage the geographic distribution targets for the device incorporating the power supply. Some of the major certifications applicable to power supplies include: IEC, UL, CE, CSA, and NOM.

Since a source of electric power is a basic need for the function of any electronic device or system, the list of application examples for power supplies can run a bit long. Some of the more common uses include:

• Computers & peripherals
• Consumer entertainment devices (TVs, radios, video game consoles, music keyboards)
• Air conditioners
• Amplifiers
• Robots & robotic systems
• Data acquisition systems
• Automated test equipment
• Battery chargers
• DC motors & motor controls
• Medical devices & equipment

The power supply as a separate product was first developed in the early 1920’s to power radios instead of batteries. IBM developed the first switch mode power supplies in 1958. Advances in personal computing in the 1980’s drove the move to ever more efficient power supply designs. Today’s power supplies employ semiconductor technology to offer effective, efficient, and safe power delivery to an ever-growing range of products and systems for consumer, industrial, medical, aerospace, and military applications.