Ground Balance Theory

[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]

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.

[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]

Exactly what I'm talking about.

[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.

[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

[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

[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 .

[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]

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 .

[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.

[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.

[green]

Plots not as clean as PiTec, but show a trend. In the past I've used gate turn off as t 0. Didn't matter for log-lin plots. The log-log plot looks better if I use coil turn off as t 0 same as PiTec. The clay from the yard doesn't give much response. I'll try to add more gain and try to get a better curve. The scope was triggered at gate turn off,t 0. A line with a -1.28 slope was added to both plots.

[PiTec]

Thanks, green. Try to extend the flyback pulse width by clamping the flyback voltage to 100 volts or less, then the difference between the two trigger points becomes more obvious. Also, the steepness of the slope should decrease.

[Aziz]

"Himmel, Arsch und Zwirn nochmal!"

Hey guys, I must insist on t0 = mosfet switch-off time. This is the standard reference time in the VRM science.

If you take t0 at a different time p, you are just implementing G(t) = (t+p)^b.
And you get different exponents b and they won't be comparable to the parameters of the standard VRM science (exponent b around -1.0).

The VRM response is dependent on the period prior to t0 (=switch-off), i.e. the TX pulse history (shape, duration, etc.) and flyback period (the shape, duration, etc. ) .

It is quite difficult to determine/measure the end of the flyback period. But it is more precise to take the mosfet switch-off time as t0.

"Himmel, Arsch und Zwirn nochmal!"
"Himmel, Arsch und Zwirn nochmal!"
"Himmel, Arsch und Zwirn nochmal!"
Aziz