View Single Post

07-07-2009, 07:32 PM
 witsend Gold Member Join Date: May 2009 Posts: 1,881
Hi Altair. I did spend the day on the answer. But I was glad of the question. As a matter of interest, how do you guys see the reversal of the voltage - that spike? Do you see it has being more discharge from the battery? Or do you see it as a result of the collapsing fields over the resistor when the battery is not able to discharge?

The part I have the most difficulty with, is the sixth one. First, when you refer to the switch (Mosfet) as closed and open, there seems to be a contradiction of terms. The established convention describes a closed switch as one wich conducts current, and conversely, an open switch doesn't conduct. Your text implies the contrary. Altair

You're definitely right. I'll have to re-read the text. I should indeed have said that an open switch interrupts the flow from the battery and vice versa. Sorry if I implied otherwise. Definitely an error. I'll re-read it and edit.

The opposite polarity point - the magnetic fields over the inductor have first been extruded to allow the path for the zipon flow. That's during the ON period of the duty cycle. Then during the OFF period and they collapse to zero. That collapse - their movement to zero represents changing magnetic fields. Changing magnetic fields induce electric fields. That, in turn, induces the spike that manifests below zero. Then some ringing as this is repeated until the rining stops and the duty cycle then repeats. That first movement through zero - to me - respresents a change in polarity and a change in the directional flow of current.

But anyway I think that a good test of the theory would be to try in your circuit, a Mosfet that doesn't have the body diode. Then, by placing (or not) a reverse diode in parallel with the mosfet, it would be possible to ascertain the usefulness of that diode. If it was proven to be necessary, that would prove that there is indeed a reverse current going from ground to the top of the circuit.

Not sure what you mean. Is there such a thing as a MOSFET without a body diode? If so, then I guess this may prove it. I really can't comment.

Thinking about it, it would also be possible to check that current by just placing a shunt in series with the Mosfet and viewing it with a scope. (Unless of course that particular zipon current is undetectable with conventional instrumentation !)

Current flow is detectable and that's all that's needed. Again, as I understand it, an ammeter simply measures current from the extruded magnetic fields on a wire - which, in turn, measures the rate of transfer in the wire. But my problem with ammeters is that they don't usually measure at fast frequencies. And they do not distinguish between the direction of current flow. The circuit is intended to induce a reverse flow of current precisely to recharge the battery.

I'm going to try and get my head around why it could possibly be that current flow 'tries to hold it's direction?' I just don't understand it. Can you explain this?
__________________