I’ve heard an inductor is used to store energy before, but I’ve never intuitively understood that or really accepted it. I’ve always thought of it as resisting changes, like inertia resisting changes in speed, an inductor resists change in current flow. Opposite to a capacitor which I think of as accelerating the current draw.
Same thing, just a change of variables. Consider the equations of both:
Capacitor: I = C dV/dt
Inductor: V = L dI/dt
The d's may not mean much to you if you haven't taken calculus, but the swap of V and I is plain to anyone.
(What happens if you put two, er, swaps, together? They oscillate of course. That's what an LC tank is doing, the energy just "sloshes" back and forth between one and other container, between voltage and current.)
I know that a moving magnetic field produces electricity in wires and I watched video on inductor basics. I suppose it's when the magnetic field from an inductor collapses, electrical energy is produced. Is this a correct way of explaining the stored 'energy' of an inductor being released?
I never liked either phrasing. "Magnetic lines of force" works fine for relative motion, but it leads to terrifically bad intuition with relative rotation -- consider this for example:
https://mysite.du.edu/~jcalvert/tech/faraday.htmAnd it completely breaks down in a transformer. When the transformer is deenergized, where do the field lines go? There are fewer of them, or they are... less intense, or something, but then how do we go from an enclosed area containing some, to none?
The reasonable explanation would have to be that there's an infinite supply of "lines of force" sitting out at infinity. When we apply flux to the transformer, some of those lines are pulled inside, into the core. In the process, they "cut" both the primary and secondary, explaining the induced voltages in both.
But the idea of objects at infinity isn't a very beginner subject; the study of infinity, poles and zeroes, real and complex analysis, is college level, to varying depths depending on major.
As for "collapse" -- consider a mechanical analogy. You hold a rock in your hand. You drop the rock. It falls through the air, and clunks on the ground.
Did its gravitational potential "collapse"?
What if you continue holding it in your hand, and just lower your hand? Did its energy "collapse"?
This phrasing leads to an incorrect intuition about the voltage and current to expect from an inductor. For example, it's a common sight to see huge power diodes on signal relays -- even though the relay coil is switched by a piddly 2N3904 or the like. If we think about "resisting change of current", well, if that relay coil is only carrying say 100mA while it's on, then it
can only ever possibly carry 100mA through that catch diode. And the voltage is perfectly constrained by the diode's voltage drop. Nothing is poorly defined here. No need to use a 1N4001, or 1.5KE24A, or something; a 1N4148 will do.
Whereas if you didn't have that diode, the turn-off voltage would merrily rise up to, well, whatever the 2N3904 can handle (about 60-100V), which may toast it in short order (again, the peak current isn't any higher, but the peak power is -- 100V * 100mA = 10W, quite a bit more than a 2N3904 can handle, at least for very long).
In the ideal case, that is with an ideal switch that instantly goes to infinite resistance and has no breakdown voltage -- the flyback voltage can be very high indeed. (Infinite, for an ideal inductance. But also for zero duration... so, ehh?) But such a case can't happen in reality. The closest we can get is perhaps, say, undesired arcing of a real switch, in which case we might ask if we can constrain the voltage to prevent that arcing, or the momentary shock hazard, or for various other reasons.
In an SMPS, the voltage is always perfectly constrained, or left to decay in which case it pretty quickly drops to zero. When the driving/input switch is on, Vin (or Vin-Vout, or some such permutation) is applied to the inductor; or the switch is off and the load switch (usually a diode) is on, and Vout (or such) is applied to the inductor; or neither is on, and the inductor's voltage freewheels down to zero, at currents much lower than when driven, and the remaining energy being dissipated into circuit resistances (losses).
Well, in the same way that we can pick up and put down rocks, we can "pick up" and "put down" the current in an inductor, and at all times the voltage (force) is well defined and controlled.
Or likewise, the voltage in a capacitor. A capacitor only "collapses" when you drop a screwdriver across it. That was clearly your fault for putting too low a resistance across it; likewise the same for the inductor, if
too high a resistance is put across it.
Since you have to put energy in to charge an inductor, I guess you get that out when it's discharged (conservation of energy and all that).
Yup. Minus some that leaks away over time, because real components aren't ideal at energy storage. That's, fairly directly, the flyback supply: an inductor is charged with one polarity of voltage (up to some peak current), then discharged with the opposite polarity of voltage (at the same initial peak current, and by "same" I mean same direction as well). It doesn't tell you anything about the in-circuit dynamics (what the voltages and currents are doing over time), but that's the thermodynamic level view, yup.
Ahh, I wanted to ask about a low pass filter with the capacitor. I thought the electrolytic caps were just for making DC after the bridge rectifier at first.
It seems fairly typical for the input choke to be placed before the bridge rectifier (you could say it's the traditional place for it?). Why not always sandwich the input choke between two electrolytic caps after the bridge rectifier? That will then mean you'd need a smaller choke, right? I assume it will save money and perhaps be easier to implement, or give you better noise suppression. What am I missing? Probably something obvious once I hear the answer.
Depends. Putting it at the AC line does give you one bonus: the rectifier itself produces some noise (basic harmonics if nothing else, but more importantly, reverse recovery can generate quite high harmonics), and it will be left unfiltered otherwise.
On closer inspection, I see the present example actually puts the CMC after the FWB, not before. Might be it didn't make enough difference for their design (...or they just didn't bother meeting actual regulatory standards...) and the layout was just better this way. Or something.
Ah, and yeah, it's between the electrolytics at that.
There are almost always relatively large caps in parallel, on either side of the CMC: on the AC line, these are X1/X2 rated film caps, maybe 47nF to 1uF, sometimes more. Or in this case, just some uF of electrolytic on the DC side. This works with the leakage inductance of the CMC, forming a (differential mode) CLC filter. The motivation is reducing lower frequency ripple (near the switching frequency).
You might also see a single (two pin) choke between electrolytics caps, which is a cheap way to reduce low frequency ripple, without also using a CMC, or in combination with one to reduce it even further.
Devices with very little common-mode coupling (small size, no output wires, etc.?), may not need a CMC at all. CFL and LED bulbs are a good example of this. They may use a single choke and two caps, nothing more.
Tim
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