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I have been told (and have also seen experimentally) that an inductor with a high core loss will introduce "glitches" to the ripple current in a switch mode power supply. I have tried researching this, but can't seem to find any information. Below is an example of what I mean by "glitch". An explanation of why this is occurring would be great, and I would also love to be directed to any literature on this topic that I could research further.

Glitch in buck converter ripple current due to high loss inductor core

Edit: Below are two waveforms I just took. The circuit is a synchronous buck converter, and the parts are the exact same. Out of 30 inductors, 15 were purposely damaged and the other 15 were left undamaged. All 15 damaged parts exhibit the "glitch", while all 15 undamaged parts do not. The original waveform was actually from user 'gsills' while answering an unrelated question of mine "Ripple current in synchronous buck converter". I have also heard this elsewhere, not only from gsills.

The below waveforms are from the exact same circuit, only the inductors were swapped out.

One of the undamaged parts: enter image description here

One of the damaged parts: enter image description here

For some reason there is a little added slewing on the switch node voltage waveform. I've used this same circuit many times and that generally doesn't occur.

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    \$\begingroup\$ A circuit diagram showing the measurement points is needed. \$\endgroup\$ – Andy aka Jan 6 '16 at 15:58
  • \$\begingroup\$ A saturated inductor whould show a sudden increase in inductor current, that is not what I see here (assuming the green line is the current). What you have here are switching artifacts. \$\endgroup\$ – Bimpelrekkie Jan 6 '16 at 16:00
  • \$\begingroup\$ I have edited my original question to include some waveforms I just took. This is not just one part that is showing this "glitch", but all 15 that I purposely damaged. None of the 15 good parts are showing this glitch. \$\endgroup\$ – Brett Prudhom Jan 6 '16 at 16:57
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    \$\begingroup\$ What form of damage was done to the inductor? \$\endgroup\$ – Brian Drummond Jan 6 '16 at 17:58
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Core loss is caused by current induced into the magnetic core, which is effectively a secondary winding with a resistance across it.

Since the voltage going into this 'transformer' is a square wave, the current and voltage across the core resistance is also a square wave. If you remove the slope caused by current flowing into the buck converter's load then this low amplitude square wave is what's left.

Here is an LTspice simulation showing the effect. L2 and R2 represent the core 'secondary winding' and its resistance (shown as the equivalent 1:1 transformer - in reality it is a single turn with extremely low resistance):-

enter image description here

I(L1) is current going through the inductor. V(n002) is voltage across the core resistance.

enter image description here

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  • \$\begingroup\$ This makes sense, though seems like the core material would have to be really awful to see such a pronounced effect. \$\endgroup\$ – John D Jan 6 '16 at 17:46
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    \$\begingroup\$ I deliberately simulated a very lossy core to make the effect obvious, however this may also happen if you use an inductor above its normal frequency range or use the wrong core material. \$\endgroup\$ – Bruce Abbott Jan 6 '16 at 18:11
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Losses can be due to eddy currents. These result from the changing magnetic field inducing current flows in conductive particles in the core material. Think of then as tiny secondary wingdings on your inductor, with resistive loads. When you apply a voltage across the inductor winding, magnetic flux rises and induces an emf in the winding opposing the applied voltage. Voltage is also induced in the conductive partials and a "secondary" current flows. The flux produced by the winding if proportional to the current flowing in it. These secondary currents produce their own flux, reducing the total flux in the core. In order to maintain the same core flux, a greater current is required in the winding to overcome the contribution from the eddy-current induced flux. This is exactly the situation that you would have if you wound a secondary on the inductor core and loaded it with a resistor. The voltage across these "secondaries" is proportional to the voltage across the real inductor winding so it adds a current to the "primary" proportional to the applied voltage.

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Core loss is a domain movement or alignment phenomena. At lower frequencies the loss is caused by hysteresis, and proportional to the BH loop area. As frequencies become higher, Eddy currents become more important.

It makes intuitive sense to think of core loss as parallel to the inductor since the inductor is wound around the core. It's easy to see that for a square voltage driving inductor current, the current added as loss will be a square wave (the step) in addition to the usual ramp-up-ramp-down of the ideal inductor.

A more physical way to think about it is to visualize the BH loop path as the current ramps up and then down. For most of the path, B changes a lot while H changes little, since the permeability of the core is high. But, when the current turns around (to start ramping down) the path doesn't reverse, but branches due to hysteresis. When it branches there is a period where H changes a lot, as much or more than B. Effectively the core permeability is low for this part of the path, since H is proportional to winding current-that's where the current step shows up. This is much more pronounced with very high perm, square loop materials than with ferrites.

Here are some BH curves from McLyman as example:

enter image description here

You can see why this core loss current step is hard to see with ferrite cores compared to the higher perm square materials. It'se specially hard to see with inductors since they are low perm (gapped) cores usually.

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