What does voltage output from an SMPS look like when inductor is saturated


I've built a buck smps around the LM2734X. This is meant to power servos, running off a 12V battery source. Scoping the +5V line using smaller servos, I see a stable voltage, with a peak to peak of 50-100mV. When I plug in a bigger servo (it's a B1226, and I've been unable to find a legitimate datasheet for it), I see the following on startup:

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I'm using an inductor that's got a 1.2A saturation current. I've been seeing input currents of about 200mA on startup (@12.6V), but once saw it spike up to 400mA momentarily. That got me thinking that the DMM may not be fast enough to show this power on spike, and that I may actually be approaching the limit of the SMPS and the inductor.

Does this look like the inductor saturating? I'm particularly confused by how even the spikes are – they seem to be right at 20ms increments.

Best Answer

The max current limit on the controller is 2.5A, so the inductor is undersized.

If you try to draw more than the saturation limit of the inductor, the inductance will decrease and the inductor current will rise rapidly and the part will go into cycle by cycle current limit until the current demand goes below the current limit.

You could easily overheat the inductor that way, so it would be better to find an inductor that doesn't saturate before 2.5A.

It would certainly cause the output voltage to drop when it happens.