The whole point was to make the circuit as low power as possible, so switching at the inductor Ipk point gets the most charging for the least switching loss.
Efficiency matters. A solar cell is nearly a constant-current source, so it delivers the most power at high terminal voltage, 0.45 or so maybe. It delivers zero power into zero volts. So, for best efficiency the cap should be very big and the ripple across it should be small, and it shouldn't be discharged to zero.
Yes- my issue relates to 1/2 C V^2= 1/2 L I^2 and the fact that the PV goes into photocurrent mode with a much smaller current than that supplied by the capacitor. So PV trickle charges C2 up to VBE(ON) of Q2, the latch swtches , discharging C2 through the boost L, and the R4- C4 is timed to switch everything off (via the latch and overriding the Q2 input because it is High) just when I=Ipk. Turning off sooner or later means less energy transfer to the battery per cycle. This is what the description of R4-C4 time constant being 1/4 x 1/sqrt(LC) is all about- it's not perfect but at least it tries. Allowing the voltage across C2 to determine switch off time by dropping below the VBE threshold of Q2 almost certainly does not fully discharge the available energy.
Thanks for the explanation. I'm going to have to chew on it for while. At this point what I understand is: there was indeed a rationale for the desiign. Whether it works great or only so-so isn't really all that important to me at this point.
As I noted elsewhere, a silicon solar cell is most efficient when loaded steadily at around 0.45 volts or so. Discharging the cap completely makes the cell inefficient; you get more energy out of the cap *this cycle* but on average it's inefficient.
Seriously LOL- in this particular case, the ability to deliver all the available PV current at less than maximum power available speaks to its efficiency and not its inefficiency. The idea is to stuff the battery with coulombs and not joules. In this case, joules are just a measure of how much energy it takes to accomplish that.
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What does that probably erroneous supposition have to do with the
subject at hand?
Fred stated that what's important is to force the maximum charge into
the battery without specifying time.
Why do you have a problem with that?
You don't remember Larkin's nonsensical diatribe that charge is not conserved? ...Jim Thompson
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| James E.Thompson, CTO | mens |
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Remember: Once you go over the hill, you pick up speed
The photon produces and electron, and the electron goes into the battery, what's to understand? You have some way of making two or more electrons from that photon go into the battery?
A photon is going to make less than one electron, on average. And the voltage of whatever electrons it makes is going to be around 0.5. The battery voltage is way, way more than that. So there's no way to dump photocell electrons 1-for-1 into the battery. That would violate conservation of energy.
So we need a boost converter. The best we can do is let the solar cell run continuously at its optimum voltage, around 0.45 maybe, and build an efficient boost converter to charge the battery. Ibatt is going to be a lot less then Icell, by the voltage boost ratio at best.
If there is a capacitor across the solar cell, it should not be discharged to zero, or even much below the 0.45 optimum voltage. Any time it spends below Vopt is energy lost. So it should be a big cap, to provide the boost converter with the high current pulses it needs, without much ripple.
It does make sense to let the cap charge to 0.45 or so, and then fire one shot of the boost converter, then wait for it to charge up again. But the ripple on the cap should still be small.
I'm surprised I have to explain stuff this simple.
If you have to ask a question like that, you must be in the non-believer group.
You and Thompson never discuss the electronics, you just whine about people who do. His excuse is that if he reveals any of his brilliant insights, I'll steal them. What's your excuse?
What I don't understand is how you miss the fact that the battery is being charged with carriers at the same average rate as they are produced by the solar cell, and by your own admission, the circuit delivers the most charge per switch. How can you improve on that?
In the case of switching converters between solar cells and a battery, why should charge be conserved? Energy, yes. But the charge that ends up in the battery could easily be more or less than what comes out of the solar cells.
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Actually, it's a valid question because there are more than two kinds
of people in the world and, regardless of the erroneous supposition
you made, the number of different kinds of people in the world has
nothing to do with the topic.
What you were trying to do was to discredit Fred with a snide
pseudo-authoritarian brushoff - which has nothing to do with the
subject being discussed - and is one of the tricks you use to try to
evade the issue at hand and bend a discussion to your advantage.
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