I wasn't sure if this topic belonged in 'Beginners' or 'Test Equipment' - anyway.....
I received an email from an electronics test eq. store that reads:
Most LCR meters can measure components that use a current of just a few milliamps. For capacitors and resistors this is fine, but the inductance (and properties in general) of power inductors varies with the (DC) current level.
They are promoting a test device called a Power Choke Tester. Which uses pulses, I think.
I'm not really interested in the device, but in the theory behind the need. I suppose my questions are, why would the DC current affect the inductance? Is it because at higher currents the iron core can saturate (I'm guessing) or something else? I assume the discrete part is passive.
If it has anything to do with saturation, I still don't understand why a typical LCR meter couldn't be used to read the value accurately. Another assumption - that it wouldn't be normal to want to drive the part to saturation. So testing the value at a low current (with an LCR meter) should give a good reading. But apparently I'm wrong all over the place.
I realize that a reactive passive component can have a different reactance value (measured in ohms) at different frequencies. But I always thought that the value (whether it's capacitance or inductance) was constant for that specific part (no matter the frequency). Not sure if I can even count on that, but I hope so. But at any rate, now I'm reading that the inductance can change depending on the DC current. And if this isn't some breakdown current type of thing (saturation), then I'm totally lost.
I just want to understand it. Thanks.
Hello there,
SUMMARY:
Power inductors are complex devices that have a variability that make it impossible to test with a device that does not allow us to set the needed parameters in order to get complete test results for that inductor.
Inductors made for power applications have a core material that is very magnetically active. However, there is a limited range of use for a given chunk of material. When you have that core and a coil of wire wound around it, the current in the wire excites the core and the core reacts to the field that is produced. The higher the current, the higher the field, but the character of the core is such that it can only handle so much field excitation. In the theory of magnetic domains, there are a lot of domains and each domain acts like a tiny magnet. The number of domains depends on the size of the core cross section. As the current increases the field increases, and as the field increases more and more, domains flip to align themselves with the direction of the field. If the current reverses (AC current) then the process starts over again and the domains flip the other way. For a DC current, the domains flip and stay that way. As the DC current continues to rise more and more domains flip, but eventually we start to run out of domains to flip because almost all of them are already flipped. When that happens, we call that saturation, and any further increase in DC current will have no more domains to flip and that will be the end of the inductance and the coil and core start to look only like a coil of wire with no core. That means the effective resistance to the increase in DC current appears to decrease which means it starts to act like a short circuit.
Before that happens however, the inductance just decreases little by little, so that any further increase in DC current makes the construction look like an inductor with lower inductance, and that means the current will go up even more in most applications, which leads to a further loss of inductance. This is a very important character to understand in DC to DC converters because there we see at least some DC current all the time.
For your questions, this means there is no way we can test this kind of inductor with a typical inductance tester, simply because it will not be able to supply the necessary DC test current to be able to characterize the construction properly. If the inductance is 100uH with 10ma and only 50uH with 10 amps, the typical inductance checker would measure around 100uH and would never be able to tell you the inductance drops to 50uH with a DC current of 10 amps. That's why a regular inductance checker will not work with power inductors. The core characteristics with different DC currents is just too variable meaning the very thing you are trying to measure (the inductance) cannot be measured except at very low currents.
There's an even bigger catch here too. Many core materials do not show the maximum permeability with low currents either. That means that the inductance being measured at very low currents may even be less than what it would be with a slightly higher current. That's because as the DC current varies, the inductance can start out low, then creep higher, then creep lower again. That's a sort of horizontally stretched out "S" curve.
This leads us to a more descriptive test for power inductors.
The best test for a power inductor is actually in the application it is going to be used in. That means if you are going to use it in a Buck converter that runs at 20kHz, then put the inductor in a Buck circuit that runs at 20kHz, and be sure to test it at no load, partial loads, and full load. That's the best test and probably the ONLY test that will be definitive because any other test would have to be able to run the inductor DC current up to the right full load test value as well as produce the pulses at the right frequency. That means that if you did buy a tester for power inductors, it would have to be very variable as so what you can set on it: the frequency, the pulse height, the DC current, etc. Without that it's not going to be a good enough test really. You may be able to test it to some degree, but if the settings are too different than in the actual application it will be used for, it may be a complete waste of time to test it that way.
I hope this explains it well enough but feel free to ask more about this if you like.
There is one more little interesting side to this. That is, since the inductance varies with DC current by going down with increased DC current, we can use that to our advantage in some applications. This involves understanding how efficiency varies in a converter that has to convert AC to DC, then DC to a different voltage DC, combined with the effect of the inductance going lower as the DC current increases.
As the current in a rectifier circuit with an inductor input filter increases, the filter inductor inductance starts to drop, which means it offers less resistance to the DC current flow. Since at higher loads we may not need filtering as good as when there are lower values load currents, having the inductance drop means higher efficiency for the converter circuit. An inductor designed for this application is often referred to as a "swinging choke" and was popular many years ago and maybe still today in some applications.
So we have many applications where we do not want the inductance to change much, but other applications where we actually design it to change in order to improve certain aspects of the design of the product. With your question in mind again, there is no way in hell we can test this functionality with a standard, typical run of the mill inductance tester
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