The problem is you cannot "see" the circuit working.
I have been investigating circuits for the past 40 years and can "see" how they work.
I wrote up the operation of the circuit weeks ago, long before ericgibbs produced a simulation and an explanation.
However ericgibbs and Mr Al does not understand the intricacies of the operation of the circuit and glosses over the points such as the vital fact that starts the “turn-off” portion of the cycle.
” It's basic inductor circuit operation where we have a (relatively) constant voltage across an inductor and that causes a continual increase (ramp) in current until something else occurs to stop that increase.”
This is entirely incorrect.
How can we have a constant voltage across an inductor and an increasing current through it???
Hi again,
I'd hate to see this thread closed myself as well, and on the plus side colin is forcing us to explain this circuit in greater and greater detail, which isnt entirely bad if you look at it from the standpoint of more and more information getting out there
There may be others that have doubts like this too so we'll try to clear them up.
colin, when you posed the question, "How can we have a constant voltage across an inductor and an increasing current through it"
you seemed to have an air of doubt that this can not be possible. However, this is exactly how an inductor works in a switch mode power supply circuit.
The voltage stays relatively constant as the output increases slightly. I can say "slightly" because the collector voltage is constant at this point and the output voltage only changes by millivolts. The current is ramping because that is what happens in an inductor with a constant voltage across it.
For example, connect a 1.5v battery across an inductor. The current ramps up. If it were a perfect inductor it would ramp up in a straight line. If the battery could supply an infinite current the current would ramp up forever, with just 1.5v across that inductor. So all it really takes is a small voltage across an inductor to create a ramp current. Actually even 0.1v would do it, but usually a real life inductor has some series resistance that limits this activity. Inductors used in power supplies however have low resistance to keep efficiency high so they behave like near perfect inductors for the most part (not exactly, but close enough for basic theory).
So what i am saying is that there is no limit to the current the inductor would draw if the transistor never turned off, or unless the output rose quite a bit to near the power supply input voltage. Something eventually limits that current and that is the transistor turning off, and the reason the transistor turns off is because it's collector current starts to rise too high for it to maintain a very low voltage drop, so the drop increases which means the collector voltage falls, slowly at first.
You're talking about a detailed view of this circuit but then i think you should look at the operation near the turn off time of Q1 in increments no larger than 100ns each. What you should see is the transistor starting to turn off (well, it develops a larger voltage drop collector to emitter) and this would be a rounded looking waveform where it decreases a little bit, and if you look at the point where it starts to decrease faster than it was before (it decreases very very slightly the whole partial cycle because the inductor is conducting more and more current during the entire 'on' part of the cycle) you'll see the inductor current still rising. As i said though you need to look at 100ns intervals to see this happening, or even at shorter time slots.
For the inductor to turn off as you are saying it would the switching cycle would have to be synced with the resonate frequency of the output circuit (inductor and cap) and as i was saying before that cycle would be longer than the switching cycle observed so it can not be turning off for that reason. Following the reasoning given above, we can trace the circuit operation from one cycle to the next without any magic at all because one thing leads to the next and those things are always well explained. And then again if it really was synced in this way (which it is not) then that would explain it and we still wouldnt need any magic
There is a chance that an inductor being used in a power supply circuit like this has too high of an equivalent series resistance to allow proper operation. What would happen in that case though is the oscillations would stall because the inductor current would stop increasing on it's own just because the voltage across it isnt high enough (with the real life ESR) to force more current through it. In fact, circuits like that usually dont even start up because the transistor Q1 never turns off because the inductor current never reaches that critical point. But this is a case of using the wrong inductor for the circuit and replacing it with the proper lower ESR inductor makes the circuit work just fine.
Im sure you looked at a lot of circuits in the past 40 years no argument there, but perhaps just this one time you might take a fresh look at this one here.
ADDED LATER:
I almost forgot to mention the case where the inductor saturates. Obviously this would be the wrong inductor for the task, but if it did in fact saturate the current would go way up fast...it would never just level off unless again the series resistance could limit the current...but then again it would not oscillate either...unless of course the transistor heats up and the beta goes down low enough for it to start to pull out of sat.