Ground Balance Theory

[PiTec]

Hi Mick,

finally a new member of the flyback club (founded by Eric 12 years ago, BTW)

Your post and the flyback discussion here is definitely not off topic. It is very important to agree on the role of the flyback pulse for further VRM experiments and data processing.

[PiTec]

Tepco, I understood your statement

If I “separate” pulses as mentioned, this is either very long charging time or CC condition, then actual charging time is irrelevant. I wanted to see effects of varying same energy pulse width, …

as if you wanted to say that the CC period would change the energy content of the pulses. I would not add the CC period to the pulse width – the pulse stops when the dI/dt resp. the H-field change drops to zero.

Sorry Deemon, looks like I misunderstood you. For a conventional PI timing you are right, of course.

[deemon]

Right thing to actually “separate” pulses is to “short circuit” coil with another switch after charging time, to prevent flyback and energy release, (sort of CC again, energy will remain in coil, with some losses due to switch and resistance) and then open it in some later time, not tried this. With this “hold” interval long enough, same situation again, I wanted to see direct response.

By the way , it's exactly what I did in my experiments with recuperative PI device . In the circuit that I published here I use one "holding" interval after flyback , but I also developed another circuit with two holding intervals - one before flyback and another after flyback . In both cases the special circuit maintains the constant current during both intervals . And this circuit allows to turn the first interval on and off , so I can compare the target response .... and what I noticed - with the little targets ( short TC ) I haven't a difference of the response magnitude with or without this first holding interval . But with big targets ( long TC ) - the difference was quite noticeable . In another words - I have the same coil current before flyback , the same flyback amplitude , but bigger target response in the case when I had a longer "magnetization time" before flyback ( rising time 150 µs + additional holding time 100 µs ) . But this worked only with a big targets - just as I told here before .

[Davor]

...So if our charging pulse is already long enough ( more than target TC ) - its further increasing cannot give us more "juice" from the target...

That's because the "on" pulse counteracts the "flyback" pulse, so the only way to make this counter action vanish is to separate the two. E.g. further than target tau. Extending the distance between pulses ruins your S/N so you have to make some compromise.
Now back to the original proposition ... "charging" is merely a broken way of saying "separating in time", otherwise you don't actually need charging at all. You could as well store all the energy you need in a cored coil, discharge it through a search coil when needed, and recuperate as much energy you can ... to the same effect. Perhaps even better because there is no "charging" pulse to ruin your samples.

[deemon]

Deemon, the driving pulse is weakening the target's response, and the flyback pulse does not need to be longer than the target's TC – the shorter, the better because of the higher dI/dt.

But I didn't say that we need a long flyback pulse . I said that we need a long enough charging pulse ( ON time , charging interval ) - the interval when a power switch connects the coil to the battery , and the coil creates magnetic field . This interval must be long enough ( longer that the biggest target's TC ) , and on the other hand , the flyback pulse really must be short enough - shorter than the little possible target's TC - that's what I mean .

[Tepco]

Tepco, the coil’s energy is determined only by the peak current, no matter how long this current may flow during an additional CC period.

Sure it is, this is why I built constant peak current control

[Mechanic]

HI Guys, I know this is off topic to this thread, but since it is being discussed here, I will reply here.

When the tx pulse is turned on and the current is rising in the coil, eddy currents are generated in the target, but due to the di/dt of the ontim current ramp the eddy currents will be a low level. Then when the tx is turned off, the magnetic field that has been generated during tx on collapses back into the coil, cutting back through the taget causing current to be inducted into the target. This current is the opposite polarity of the initial low level eddy currents caused by the tx on ramp. The initial on time eddy currents get subtracted from the turn off eddy currents, but the turn off eddy currents are stronger due to the di/dt of the turn off current ramp.

In the CC version things happen a bit different. The initial tx on causes eddy currents in the target, but when the current reaches steady state, the eddy currents in the target begin to decay away(much like they do in the off time) The longer the CC period, the less eddy currents are left in the target to the point where there may be none left at all. Then at tx off, once again the magnetic field collapses back into the coil, cutting through the target causing eddy currents in the target which are subtracted from the tx on eddys. In the case of the CC there will actually be a higher eddy response due to less eddys in the target at tx off, thus allowing more eddy currents to be generated during the tx off di/dt.

Cheers Mick

[PiTec]

Tepco, the coil’s energy is determined only by the peak current, no matter how long this current may flow during an additional CC period. The CC period is only there to keep the dI/dt at zero, so the pulses can be separated.

See first simulation in the already mentioned post: http://www.geotech1.com/forums/showthread.php?20038-Triangular-Wave-Technology&p=164732#post164732

Deemon, the driving pulse is weakening the target's response, and the flyback pulse does not need to be longer than the target's TC – the shorter, the better because of the higher dI/dt. See second simulation in the linked post. In a conventional PI timing the weakening effect of the driving pulse is of course reduced if you make it longer than the target's TC.

[Tepco]

Exactly what I'm talking about.

[deemon]

The rule is very simple - for a proper target magnetization we must make charging interval ( ON time ) equal or slightly longer that a target TC . So if our charging pulse is already long enough ( more than target TC ) - its further increasing cannot give us more "juice" from the target But if the pulse is short , and the target has big TC ( the opposite situation ) , we'll increase the target response only by increasing the ON pulse duration , even with the same current magnitude before flyback and flyback peak amplitude , of course .

[Tepco]

Yes, but what is the point then? If I “separate” pulses as mentioned, this is either very long charging time or CC condition, then actual charging time is irrelevant. I wanted to see effects of varying same energy pulse width, and try to utilize it for some useful purpose. Right thing to actually “separate” pulses is to “short circuit” coil with another switch after charging time, to prevent flyback and energy release, (sort of CC again, energy will remain in coil, with some losses due to switch and resistance) and then open it in some later time, not tried this. With this “hold” interval long enough, same situation again, I wanted to see direct response.

[PiTec]

What I want to say is, if you compare response from, say, 1, 10 and 100uS “charging time” containing same amount of energy, applied to same object, and with same flyback release, you will see difference, very clearly.

Yes, very different outcome if the flyback pulse follows directly after your 1/10/100 µs charging time. No difference or even better response if you separate the pulses far enough.

[Tepco]

As Einstein said ... Problems cannot be solved by the same level of thinking that created them.

Works for problems created by humans, not nature.


What you propose is insanely long charging time, so it will make any effect irrelevant to outcome. What I want to say is, if you compare response from, say, 1, 10 and 100uS “charging time” containing same amount of energy, applied to same object, and with same flyback release, you will see difference, very clearly.

[Davor]

...Never going to happen.

As Einstein said ... Problems cannot be solved by the same level of thinking that created them.
If you can't separate a pulse from the charging in your head, you are bound to thinking that charging is somehow essential for target response. Think again.

You can produce a simple experiment that will include charging (so that you are happy) but make it completely irrelevant for the outcome (WTF!) and see that a flyback pulse devoid of the charging period works the same. It goes like this:
- place a resistor to limit current to your coil at some tolerable value,
- make a charging period abnormally long, so that it can't influence the outcome,
- fire a flyback and behold - a target response.

By forcing a constant current to your coil, it behaves like a permanent magnet, and to any target it is not unlike Earth field, but a flyback does produce a response.

[Aziz]

Thomas,

do you have a multi-channel oscilloscope with seperate trigger input?
Do the following please:
Trigger input: TX mosfet switch logic wave form, triggered at switch-off (where the data acquisition begins, has the first data sample the time code 0?)
Ch-1 input: pre-amp output (the response signal, which we interested in)
Ch-2 input: TX coil voltage (clipped to +10/-10V or less, we don't want to see the high flyback voltage, we want to see the time lag between switch-off command and the real TX coil switch-off, but don't screw your oscilloscope!!!)

I can at least see using ch-2 data, when the f*in TX coil will be switched off and correct the acquisition time code t0 by myself.

The VRM decay calculations rely on the correct time base/time code. Unless they aren't reffered to t0=switch-off time, we can't make accurate VRM modelling anymore.

Aziz