Power Supply Magnetic Fields

by Doug Smith, Consultant, YouTube, Oct 20 2008

Focus:
In this approximately 5-minute video, the presenter demonstrates a simple technique for the measuring magnetic field emissions produced by a switching power supply. The technique measures emissions directly as a voltage using an oscilloscope rather than measuring the magnetic field over a frequency range. The former method gives an immediate result, while the latter method requires time-consuming calculations. The test set up consists of a 500-MHz Agilent oscilloscope connected via a 50-ohm coaxial cable to a 1-inch square loop for pickup of the power supply's radiated magnetic field. The power supply under test is an 8-A, 12-V open frame supply with a 50-W 12-V work lamp that serves as its load. The presenter measures the radiated magnetic field at different locations within or just outside the power supply, and discusses the results and their implications.

What you’ll learn:

• How to measure a switching power supply's radiated emissions using a pickup loop and an oscilloscope
• How to measure a power supply's magnetic field emissions directly as a voltage rather than measuring the emissions over a frequency range and then calculating the associated noise voltage

Notes:
Doug Smith is an authority on on high-frequency measurements, circuit design, ESD, and EMC, and has lectured and written extensively on these topics. As a consultant, he provides services in general design, EMC, and transient immunity (such as ESD and EFT), and switching power supply noise. This video is also available on Doug Smith's Circuit Advisor website, http://circuitadvisor.com/main/videoList/3/0.

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