Woah, now it's looked more confusing. ahahahaha...
Okay now i'm agree with the term of the polarity change.
Thank you for the explanation sir.

The hardest part i think, when we (students), got a question and at the 'question' already defined the polarity of the inductor, and when we want to analyze it, and we should mark the polarity, my mind would be like, "Oh sh!t, my lecturer made a joke of us. If i marked polarity of the inductor like on the question, then i will have a wrong answer in reality. But if i followed the polarity as in the question, then i got the wrong anaylze of buck converter".
No one ever got 100 in the class then. ahahahaha

Yes, this is particularly difficult for the student. And the worst thing is, sometimes the instructor or publisher DOES make a mistake. But not very often.

 

But this is not the simulation result
This picture shows the result I get when measuring the voltage across the inductor via Rigol scope
http://forum.allaboutcircuits.com/attachments/newfile34-png.103708/


But here I was talking about general case. Notice that at steady state VL = 0V but IL can be larger than 0A.

I could imagine conditions under which that could be true if you're combining phase 1 and phase 2. But I thought we were trying to discuss the difference between phase 1 and phase 2. And in phase 2, the voltage on the inductor cannot be 0.

[EDIT] Oh, I have to correct myself. If a current is flowing through an inductor from an external voltage source and the inductor is saturated, then the voltage drop across it will only be due to the resistance of the windings, and will be near zero. But in a Buck regulator we should never reach saturation.

 

I could imagine conditions under which that could be true if you're combining phase 1 and phase 2. But I thought we were trying to discuss the difference between phase 1 and phase 2. And in phase 2, the voltage on the inductor cannot be 0.

[EDIT] Oh, I have to correct myself. If a current is flowing through an inductor from an external voltage source and the inductor is saturated, then the voltage drop across it will only be due to the resistance of the windings, and will be near zero. But in a Buck regulator we should never reach saturation.

Hi there,

You made some very good points in this thread, however in the case of the inductor with zero voltage across it and non zero current, it's no mystery ... it's a DC current
A DC current through an ideal inductor produces exactly zero volts across it, no matter what had happened in the past. By in the past i mean we could have had 1000 amps or just 10ma up and down for a while, but once the CHANGE stops happening, the voltage disappears.
Consider again:
v=L*di/dt

If we have no change in current over time then we cant have any voltage:
v=L*0=0

and this doesnt just happen when the core saturates, it can happen anytime there is pure DC through the coil.

For the normal buck, it should not happen, but in the more general circuit it can happen.

Of course this is still theoretical too, where we have the ideal inductor again. In real life there is usually some small resistance in series with the coil and that drops some voltage even when there is DC according to Ohm's Law:
V=R*I

Another smaller point is that when the core saturates that doesnt necessarily mean that the inductance drops to absolute zero either, there may still be a small inductance remaining due to the winding itself apart from the help of the core material. The core material just basically helps the whole construction achieve a higher permeability which minus the core, would be just 1 instead of something like 1000. A permeability of 1 is no better than an air core coil, but still a coil

 

Oh, i already fixed it sir. ahahahah

Hi,

I am happy you are getting this now. You did make it interesting anyway though because you focused on the voltage of the inductor and that brought out several points about the circuit and the drawing of the polarities which helped to clear things up for everyone.
I've been doing circuit analysis for at least 40 years both in school, in industry, and on my own as my pet hobby so i was surprised to hear some of the stuff originally in this thread. One thing i can tell you from all this is that circuit simulators help to verify results, but doing the grunt work by hand provides more insight into how these circuits work. Many of the profound truths come out of intense study along with some deep thought and reflection, something circuit simulators alone can not achieve.

 

Hi,

I am happy you are getting this now. You did make it interesting anyway though because you focused on the voltage of the inductor and that brought out several points about the circuit and the drawing of the polarities which helped to clear things up for everyone.
I've been doing circuit analysis for at least 40 years both in school, in industry, and on my own as my pet hobby so i was surprised to hear some of the stuff originally in this thread. One thing i can tell you from all this is that circuit simulators help to verify results, but doing the grunt work by hand provides more insight into how these circuits work. Many of the profound truths come out of intense study along with some deep thought and reflection, something circuit simulators alone can not achieve.

You what ? 40 years ? Are you a Gandalf sir ? ahahahaha i'm joking sir

Shouldn't i be the one who are really happy to know the 'truth' behind it ? ahahahaha....
When i learned buck converter at first, based on the question my lecturer, it's been written on the question the polarity of the inductor. And i understand the wrong idea. My lecturer said that the 'current' will change the polarity and worked as a current source, but my thought focused on the 'polarity' at the question that has been written first. If i change the polarity, then i change the question (the question has it's own polarity, then we as a student, can't change the question), and if i didn't change the polarity, then i didn't understand what my instructor has taught me. So i conclude my self that, it have 2 polarity, polarity for 'maintain' current (changed polarity), and polarity for the voltage (both of the polarity are different each other). And i just accepted those theories in my head. ahahahaha....

Oh yeah, i think i would ask another question in the future, because buck-boost converter will be my thesis for electrical motor control (controlling current from battery to motor by using super-capacitor). And i hope your help in the future sir

Thanks a lot !

 

You what ? 40 years ? Are you a Gandalf sir ? ahahahaha i'm joking sir

Shouldn't i be the one who are really happy to know the 'truth' behind it ? ahahahaha....
When i learned buck converter at first, based on the question my lecturer, it's been written on the question the polarity of the inductor. And i understand the wrong idea. My lecturer said that the 'current' will change the polarity and worked as a current source, but my thought focused on the 'polarity' at the question that has been written first. If i change the polarity, then i change the question (the question has it's own polarity, then we as a student, can't change the question), and if i didn't change the polarity, then i didn't understand what my instructor has taught me. So i conclude my self that, it have 2 polarity, polarity for 'maintain' current (changed polarity), and polarity for the voltage (both of the polarity are different each other). And i just accepted those theories in my head. ahahahaha....

Oh yeah, i think i would ask another question in the future, because buck-boost converter will be my thesis for electrical motor control (controlling current from battery to motor by using super-capacitor). And i hope your help in the future sir

Thanks a lot !

Hi,

Ha ha, that's funny, and although circuit analysis is probably my first love, comedy comes in at a close second

Gandalf? Maybe so, but unfortunately that also shows my age, and with age you tend to forget some things so some of what i had learned in the past is now forgotten and so i have to look things up sometimes too. I do like to pass what i have learned to others also so they can go on to do whatever it is they wanted to do in electronics or a related field.

Anyway, the buck circuit is probably the simplest to understand so if you learn that well you should be ready to move on to the boost circuit which involves a bit more complex theory, then maybe the inverting boost, then the buck boost. There are also good articles on the web for this stuff, like averaged AC modeling, circuits, waveforms, etc. Many of the articles have big errors though so be prepared to rethink a lot of stuff and draw up your own equations.

Also and more to the point of this thread, when i first started years ago i used to use the plus (+) and minus (-) signs for labeling the 'assumed' polarities of a circuit element but long ago switched to using the long arrow to indicate the assumed voltage across an element. This helps to prevent interpreting the signs as the absolute voltage polarities as those being measured. The long arrow is drawn parallel to the circuit element (like a resistor) with the tip of the arrow at the assumed positive node and the feather part of the arrow at the assumed negative node. This would look something like this:

o----R----o
<----------

In the above, the tip of the arrow is the assumed positive, and the tail is the assumed negative, so if the real resistor polarity was plus on left and negative on right then the voltage would be +10v for example, but if the right side was plus and the left side negative, then the voltage would be -10v, but the arrow still points to the left.
I hope this helps clarify a little.

 

I hopeful that somebody knows more about color coding of fuse resistors than I.

Hi,

Ha ha, that's funny, and although circuit analysis is probably my first love, comedy comes in at a close second

Gandalf? Maybe so, but unfortunately that also shows my age, and with age you tend to forget some things so some of what i had learned in the past is now forgotten and so i have to look things up sometimes too. I do like to pass what i have learned to others also so they can go on to do whatever it is they wanted to do in electronics or a related field.

Anyway, the buck circuit is probably the simplest to understand so if you learn that well you should be ready to move on to the boost circuit which involves a bit more complex theory, then maybe the inverting boost, then the buck boost. There are also good articles on the web for this stuff, like averaged AC modeling, circuits, waveforms, etc. Many of the articles have big errors though so be prepared to rethink a lot of stuff and draw up your own equations.

Also and more to the point of this thread, when i first started years ago i used to use the plus (+) and minus (-) signs for labeling the 'assumed' polarities of a circuit element but long ago switched to using the long arrow to indicate the assumed voltage across an element. This helps to prevent interpreting the signs as the absolute voltage polarities as those being measured. The long arrow is drawn parallel to the circuit element (like a resistor) with the tip of the arrow at the assumed positive node and the feather part of the arrow at the assumed negative node. This would look something like this:

o----R----o
<----------

In the above, the tip of the arrow is the assumed positive, and the tail is the assumed negative, so if the real resistor polarity was plus on left and negative on right then the voltage would be +10v for example, but if the right side was plus and the left side negative, then the voltage would be -10v, but the arrow still points to the left.
I hope this helps clarify a little.

Thank you sir for your information.
ahahahaha...
Yeah, we used those 'arrow' things here in college to. The end of the edge of arrow, will be positive and the other one will be negative.
Thank you a lot for your help Mr. Gandalf