Compare Soft Saturation vs. Regular Inductor - Switching

This design shows the importance of choosing the right inductor size and type, to avoid saturation-induced current spikes during operating transients. In this case, the start-up transient in two nearly identical open-loop buck power stages (12V -> 3V) is examined. The only difference is that the top circuit is using an inductor calibrated to a Coilcraft XAL6060-223 (22uH), which has "soft saturation" behavior. The bottom circuit is using a similarly sized inductor, but with a more typical "hard saturation" characteristic.

Both inductors work well for the steady-state load condition of 3 Amp. But during the turn-on transient, that current can go significantly above that nominal level. At that time, the instantaneous inductance of the hard-saturating device collapses, resulting in even larger current spikes in each switching interval. Note in the waveforms on the left: The soft-saturation inductance drops to 14uH minimum during the transient, whereas the hard-saturation inductance crashes to near 2uH, or only 10% of its nominal value!

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Mike Donnelly

Joined November 8, 2013
Design added Monday, October 12, 2015 | 12:43 pm PDT