Power Supplies for Test & Measurement: A Technical Buyer’s Guide

Though the power supply provides a simple function, its effects can be far from straightforward, with power failures or Electromagnetic Interference (EMI) being among the most prominent repercussions of a poorly matched Power Supply Unit (PSU).

Naturally, your immediate considerations in choosing a PSU will be size, output power and how the unit will integrate with your product. When choosing a power supply for environments with high electromagnetic properties (EMC) or unstable power sources, it is vital that the PSU can be relied upon to deliver both stable voltages and emit the minimum radiated and conducted emissions that will not cause interference with your apparatus or the designs primary function. Other aspects you may need to address will include operating temperatures, altitude and unusual environments, depending on the application and intended use.

Linear Power Supplies are one solution to ensuring low noise and ripple performance in addition to optimal Line and Load Regulation attributes. However, linear solutions are best suited to low-power applications given the disadvantages in size, weight and poor efficiency rates.

Switch Mode PSUs offer a far more versatile solution, being smaller & lighter whilst offering greater efficiency ratings. However, they tend to emit higher EMI but have the advantage of meeting many worldwide approval standards without the need to modify the product. They also offer universal input as standard making them suitable for global, residential or commercial applications.

As many will be aware, this does not mean that switch mode PSUs are unsuitable for EMI-sensitive applications, such as tests & measurements. Factors like common noise can be dependent on many variables, including input to output isolation, design topology and built-in filters, making a PSUs Electromagnetic Compatibility (EMC) vary from one design to another.

When selecting switch mode power solutions for your design, one of the first things you will need to identify is whether your design will fall into the category of multimedia equipment, the scope of which encompasses information technology equipment and broadcast receiver equipment as well as audiovisual and lighting equipment (as the application can dictate the type of approvals required).

For example, if your design comes under the umbrella of multimedia equipment, you will need to ensure your chosen power supply meets EMC standard EN 55032/CISPR32 Class B for intended use in residential.

As well as EN 55032/CISPR32, all electronic equipment must meet basic EMC immunity standards; these are specified by IEC/EN 61000-3 (emission specifications) and IEC/EN 61000-4 (Immunity specifications), which is an essential process for CE marking.

Universal EMC Immunity Specifications

The above standards ensure that electrical products and equipment are fit for purpose in the event of phenomena such as power surges and interruptions and that the equipment is safe in an end-user environment. In addition to the above standards, the following apply in certain applications and environments.

IEC/EN 61000-4-7

This standard relates to instrumentation which is intended for measuring spectral components of up to 9 kHz, distinguishing harmonics and inharmonics.

IEC/EN 61000-4-9

This standard relates to immunity requirements for impulse magnetic disturbances usually encountered in industrial, rail and high voltage applications such as power plants and sub-stations.

IEC/EN 61000-4-10

This standard specifies the immunity requirements for equipment which is subject to damped oscillatory magnetic disturbances, which are found in medium voltage and high voltage sub-stations. IEC/EN 61000 part 4 consists of 39 parts, which detail guidance and specifications for different aspects of EMC immunity, dependent on environment and application. Before specifying a Power Supply Unit for your application, it is important to ensure the PSU meets all applicable standards for its intended use. Of course, this may mean extra costs may be incurred where specific standards must be met, so stipulating your exact requirements are vital to sourcing the right PSU.

Find out more about EMC regulations here.

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