Calculating Minimum Magnetic Core Size For A Transformer

by Gregory Mirsky, Design Engineer, Deer Park, Ill., How2Power Today, May 13 2022

Focus:
In inductors the load current is the core magnetizing current, while in transformers the core magnetizing current is separate from the load current. This is why in inductors (chokes) the size-defining parameter is operating power, while in transformers the magnetizing current ripple, which may constitute 0.5% to 1% of the input ac current, determines core saturation and thus the minimum size of the core. This article provides an analysis that clarifies and defines this relationship between magnetizing current ripple and core volume, and in the process derives the equation needed to determine minimum core size in a power converter application. It then demonstrates the application of this equation with a transformer design example. Factors contributing to a smaller value of magnetizing current (and therefore a smaller minimum core size) and the impact of winding requirements on minimum core size are also noted.

What you’ll learn:

• How to determine the minimum core size for a transformer used in a power converter application
• How to understand why core volume is a function of magnetizing current in a power supply transformer

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