How to Select an Ac-Dc Power Supply


Input Voltage Specifications for Ac-Dc Power Supplies

When it comes to choosing an AC-DC power supply, the foremost aspect to consider is identifying the power requirements of the system. This involves examining both the AC input range and the DC output specifications. To ensure compatibility with their target markets and specified applications, users should verify that the AC input voltage range of the power supply is suitable.

For most low or medium power AC-DC supplies, the required input voltage operating range is standardized worldwide to range from 85 to 264 VAC. Although there are variations in specifications for different countries, they are similar enough that the single standardized range can fulfill the needs of most applications. For instance, Japan has one of the lowest nominal input voltage requirements (100 VAC), while the UK, Canada, and the US have among the highest nominal input voltage requirements (240 VAC).

The minimum standardized range value of 85 VAC is derived from allowing a 15% voltage droop of 100 VAC, while the maximum range value of 264 VAC is derived from permitting operation at 10% above 240 VAC. In some industrial applications, one might encounter a higher (and less common) voltage specification of 277 VAC. This voltage is achieved by connecting between the phase and neutral of a 480 VAC Wye configuration. A maximum voltage of 305 VAC for this configuration can be calculated by applying the previously discussed 10% tolerance above the 277 VAC rated voltage.

Relationship between 480 Vac and 277 Vac

Fig. 1 – Relationship between 480 Vac and 277 Vac


In the realm of AC-DC power supplies, the initial step involves rectifying the AC input voltage to a direct current (DC) voltage. For a majority of AC input supplies, the input can be supplied with a DC voltage. However, it is crucial to note that the rated DC voltage range must be calculated as 1.4 times the rated AC voltage range.

If a DC input configuration is being contemplated, it is imperative that users ascertain that the power supply development team has thoroughly evaluated the supply for compatibility with a DC input voltage.

Block diagram of typical switching power supply

Fig. 2 – Block diagram of typical switching power supply


Dc Output Voltage Specifications

In the realm of AC-DC power supplies, the input voltage is standardized within a particular range, whereas the output voltages are predominantly found at fixed values of 5, 12, 24, or 48 Vdc. Although there are numerous AC-DC power supplies obtainable with other output voltages, opting for one of the popular voltage levels would ensure a more extensive range of standard supplies. If the standard output voltages do not align with the system requirements, there are alternative courses of action to consider, such as procuring a standard power supply with the necessary DC output voltage, seeking a power supply vendor willing to provide a customized design catering to the required output voltage, or employing a DC-DC converter that can generate the requisite DC voltage.


Output Power or Load Current

When choosing an AC-DC power supply, it is crucial to consider the necessary output power (or load current) to meet the demands of the load. The power supply’s required rating is determined by the peak power consumption of the load. Utilizing a power supply with a higher rating than needed will function properly, but it might be bulkier or costlier than necessary. Conversely, a power supply with an insufficient power rating might shut down or deliver an improper output voltage due to the load’s peak power requirements. Some manufacturers offer AC-DC power supplies specifically designed to deliver high peak power for brief periods, effectively reducing the size and expense of the power supply.


Operating Environment Specifications

When choosing an AC-DC power supply, it is important to consider the environment in which it will operate. The operating temperature range is a crucial factor that affects the selection of a power supply. Most power supplies come with specifications that include derating factors for extreme temperatures. For instance, when it comes to cold temperature power derating, the capacitance values of the capacitors used in the power supply design decrease, leading to a decrease in the specified output load current.

On the other hand, at high ambient temperatures, the load power that can be delivered by the power supply is limited by the maximum operating temperature of the internal components. In such cases, blowing air at the power supply can assist in cooling the internal components and increase the maximum output power rating.

Apart from the operating temperature range, there are other physical characteristics of the operating environment that can influence the power supply features required. For example, if the electronics need to be physically protected from large foreign objects, a power supply enclosed in a metal case may be required. However, a metal case may not protect the power supply circuit from dust and dirt, which is where a conformal coating can be applied to the internal circuit board to provide the necessary protection.

In addition, insulating terminal covers can be added to prevent objects from accidentally connecting to exposed input and output terminals. Taking into account these factors will ensure that the chosen power supply is suitable for the specific operating environment.


Package Designs and Mounting Styles

There is a range of packaging configurations for AC-DC power supplies, including potted, open frame, U-frame, enclosed frame, and fan-cooled options. The decision on which packaging to choose may depend on the need to protect the power supply from external objects or contaminants. Moreover, users have the flexibility to choose between PCB, chassis, or DIN rail mounting options. Typically, power supply series will be available in multiple mounting styles to cater to various mechanical environments where they will be utilized.

In summary, the packaging configuration and mounting style of AC-DC power supplies are crucial factors that should be considered to ensure optimal performance and protection of the power supply. By choosing the appropriate packaging and mounting style, users can mitigate the risk of potential damage and ensure the longevity of the power supply.


Operational and Safety Regulatory Certificates

Operational and safety regulatory certificates are mandatory for various products that use AC-DC power supplies. These certificates ensure that the products meet specific safety and operational standards. The regulations governing these certificates vary based on the type of product. For instance, medical devices must adhere to the 60601 regulatory requirement, household appliances must comply with the 60335 regulation, while audio, video, and information and communication technologies must meet the 62368 requirement.

To obtain the necessary certificates, the power supply vendor can provide copies to either the product development team or the testing facility that grants the certificates to the final system. Several regulatory agencies issue these certificates, including UL, TUV, IEC, EN, FCC, and CISPR.

It is crucial to obtain the appropriate certificates to ensure that the products meet the necessary safety and operational standards. This not only protects consumers but also helps companies avoid costly legal and financial repercussions.


Additional Ac-Dc Power Supply Features

Besides the previously discussed characteristics of AC-DC power supplies, certain models offer additional features that can be advantageous or even necessary in product design. These supplementary features may include:

  • Discrete output voltage levels, which are typically found in most power supplies.
  • Flexibility for the user to adjust the output voltage by up to 10%.
  • Some power supplies have output terminals that can be paralleled to increase the maximum current that can be supplied to the load.
  • High current output supplies may have remote output voltage sense terminals that enable voltage drop compensation along output power delivery conductors.
  • Input power factor correction is often available in high power supplies.
  • A remote on/off terminal that allows users to switch the output of the power supply with an electronic signal, while a power good terminal provides a signal indicating that the output voltage is within regulation tolerance.

In conclusion, these supplementary features add more value to the already existing features of AC-DC power supplies. Incorporating these features in product designs can improve their overall efficiency and performance.



There is a wide range of standard internal AC-DC power supplies available in the market that can fulfill the requirements of most design projects. However, it is highly recommended for design engineers to carefully analyze and evaluate the available options to ensure the selection of the best-suited power supply for their products. This can be achieved by either browsing the vendor’s website or directly consulting with their representatives.

Experienced power supply companies such as ALEXANDER ELECTRIC, have highly knowledgeable teams, who are always ready to assist their clients in the power supply selection process. They can provide valuable insights and guidance to help design engineers choose the optimal power supply for their product.

In conclusion, it is crucial to make the right decision when selecting an AC-DC power supply for your product. Take advantage of the resources available, whether it be the vendor’s website or their representatives, and don’t hesitate to seek assistance from experienced power supply companies like ALEXANDER ELECTRIC.