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Measuring power supply ripple

Posted: 23 Mar 2009     Print Version  Bookmark and Share

Keywords:power supply ripple  scope probe  transformer 

Accurate measurement of power supply ripple is an art in itself. This document offers an example where a neophyte engineer grabbed a scope and got it all wrong. His first mistake was using a scope probe with long ground leads. His second was to place the loop formed by the probe and the ground lead in the vicinity of the power transformer and switching elements. His last mistake was allowing additional inductance between the scope probe and the output capacitor.

The problem is high frequency pick-up demonstrated in the ripple waveform. There are numerous high-speed, large-signal voltage and current waveforms within the power supply that can easily couple into the probe. They can include magnetic field coupling from the power transformer, electric field coupling from switching nodes, and common mode current generated by the transformer interwinding capacitance.

View the PDF document for more information.

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