.99: Just some colour commentary. There are indeed tiny capacitive effects but I ignored them. It's fair to say that a few things happen at the same time.
The principle event that takes place when the MOSFET switches off is that the coil has to discharge it's stored energy. The stray capacitors are very small and will charge up almost instantly and may generate a tiny sipke. However, when they get charged there is still a ton of energy that still has to be discharged out of the coil. The coil is acting as a current source, not as a voltage source when it discharges. So the current literally blasts it's way through the the reversed-biased protection diode and/or the n-p and p-n junctions inside the MOSFET. Those junctions are supposed to be off, and instead get blasted with current coming from the coil (with a minor boost from the battery). They weather the electron storm and the junctions start to heat up. Beyond a certain temperature the semiconductor material fails and you have lost the MOSFET. All in all, it's a very unhealthy experience for the MOSFET.
I am no expert in circuit ringing but let me spin a story. The interconnet wires themselves are inductors, the semiconductor junctons are capacitors, and there is stray capicatance that is probably much larger than the semiconductor junction capacitance. So when current in the interconnect wires is stopped abruptly, you ring the inductor-capacitor bells in the wires, and see a nice litte 2 MHz (?) ring-downs in many wire interconnects. The resistances in the wires themselves dampen the oscillations down to zero. <Edit: 2 MHz is not a special frequency, every individual wire interconnect will ring at its own frequency.>
Back to the circuit: In very simple terms, you know that current is flowing clockwise. When the MOSFET switches off, the coil comes to life and KEEPS the current flowing clockwise for as long as it can. The reason for this is that coils resist changes in current flow, and therefore the current HAS to flow for a short while longer, there is no stopping the coil. This is simply a property of coils. The high voltage spike is a manifestaton of the fact that the current has to keep flowing in the same direction. If you changed the coil for a much much larger coil and put it in the same circuit, it would take longer for the coil to reach it's maximum current affter the MOSFET switches on. A much larger coil will of course store much more energy, and when the MOSFET switches off, it will instantly blow up! No sheit!
More fun! lol
MileHigh
The principle event that takes place when the MOSFET switches off is that the coil has to discharge it's stored energy. The stray capacitors are very small and will charge up almost instantly and may generate a tiny sipke. However, when they get charged there is still a ton of energy that still has to be discharged out of the coil. The coil is acting as a current source, not as a voltage source when it discharges. So the current literally blasts it's way through the the reversed-biased protection diode and/or the n-p and p-n junctions inside the MOSFET. Those junctions are supposed to be off, and instead get blasted with current coming from the coil (with a minor boost from the battery). They weather the electron storm and the junctions start to heat up. Beyond a certain temperature the semiconductor material fails and you have lost the MOSFET. All in all, it's a very unhealthy experience for the MOSFET.
I am no expert in circuit ringing but let me spin a story. The interconnet wires themselves are inductors, the semiconductor junctons are capacitors, and there is stray capicatance that is probably much larger than the semiconductor junction capacitance. So when current in the interconnect wires is stopped abruptly, you ring the inductor-capacitor bells in the wires, and see a nice litte 2 MHz (?) ring-downs in many wire interconnects. The resistances in the wires themselves dampen the oscillations down to zero. <Edit: 2 MHz is not a special frequency, every individual wire interconnect will ring at its own frequency.>
Back to the circuit: In very simple terms, you know that current is flowing clockwise. When the MOSFET switches off, the coil comes to life and KEEPS the current flowing clockwise for as long as it can. The reason for this is that coils resist changes in current flow, and therefore the current HAS to flow for a short while longer, there is no stopping the coil. This is simply a property of coils. The high voltage spike is a manifestaton of the fact that the current has to keep flowing in the same direction. If you changed the coil for a much much larger coil and put it in the same circuit, it would take longer for the coil to reach it's maximum current affter the MOSFET switches on. A much larger coil will of course store much more energy, and when the MOSFET switches off, it will instantly blow up! No sheit!
More fun! lol
MileHigh
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