I believe that the oscillations are between the inductance and the remaining capacitances in the circuit, like the MOSFET's own capacitance and the parasitic capacitance of the wiring.
That does not compute because the period of these oscillations is 240μs.
If the inductance of the coil is 110mH then the capacitance to make this oscillation in an LC circuit would have to be 13nF. ( using the formula C=1/4Lπ
2f
2 )
I don't think we can get that much from the parasitic capacitance of the wiring and the output capacitance of an open MOSFET (200pF according to datasheet).
The energy comes from the turn-off spike (which comes from the input energy in the first place) and it is the _same_ energy being sloshed back and forth between inductance and capacitance.
...but this electric energy is conducted by D1 and is used up to charge C2.
Once C2 becomes charged during the first ¼ of the cycle, it cannot give the energy back to L1 because D1 prevents it. ..so no bidirectional "sloshing" is possible between C2 and L1. ...yet we have 10 peaks visible on the scopeshot
That is, each separate spike represents the same bit of energy, not new energy, and it is dissipating at a rate that results in the decreasing amplitude of the spikes over time. A little of the energy is lost in heat and radiation with every "slosh" so the spikes decrease.
That does not match well the known resistances in the circuit (10Ω) because the rate of decay of these oscillations is 50%/2ms and to get such decay we would need to have 76Ω of resistance in the circuit. ...but these oscillations are not sinusoidal so who knows.
Note, that the green waveform appears to have a triangular shape! (@Itsu: please magnify it, before you alter the circuit, so we can be sure of that).
Also, these oscillations and reversals of current flowing through R1 are even more unexpected than if we had such current waveform flowing through L1.
This is because R1's current can be related only to Q1's Drain current, while L1's current can be independent from the Drain current (flowing through C2).
The current reversals show the direction of the "slosh": from inductance to capacitance, or the other way around.
Which capacitance did you have in mind? C2 or the MOSFET's effective output capacitance ( C
O(tr) ) + wiring's stray capacitance (C
S) ?
If "C2" then the energy can "slosh" only once in one direction because of D1.
If "C
O(tr) + C
S", then I don't think they can make up the 13nF needed to support the 4kHz oscillation.
The voltage of the sloshing is clipped on the bottom by the diode blocking so the sensed voltage at the probe doesn't go below the zero baseline.
Which diode did you have in mind? D1 or the MOSFET's body diode ?
You might compare the case with the diode shorted by a short jumper and see if the drain voltage trace ringdown gets more symmetrical around the zero baseline.
Yes, I also think that we are at a point where the circuit needs to be variously altered in order to see how these oscillations will respond.
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Topic: Magnetic CARA - Proof of Concept (Read 65178 times)
