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Arnolde - mosfets don't ACTUALLY act as a switch, they are more of a dimmer. WHen they are fully on (Rdson) or fully off (R = infinity), there is not much heat generated, but when they are switching the resistance swings from infinity to Rdson and in between in burns a lot of power. The "level" of the dimmer is proportional to how charged the gate is. Think of the gate like a capacitor. If you half-fill the capacitor, it has a semi-high resistance that results in major heat generation. So the idea is to fill up and dump out that capacitor as quickly as you can...
HOWEVER - if you fill it and dump it TOO fast, then you generate a very large current change in a small period of time (aka a large dI/dt). The voltage across those coils = L*dI/dt where L is the inductor rating. So if you switch a 1A charge off in 1uS, you are generating 1V for each uH of inductance. You have what, 150uH? So if you switch off 5A in 1us you generate 750 volts. There is also a "ringing" that occurs in fast switching.
So you have a trade-off, but you sure as hell aren't switching fast enough out of a 555. And those gate drivers... more like 15A of gate current for a short period of time.
Find the gate capacitance of your mosfets and figure out how much gate current is required to fill it up in 1us. Once you have this, then you can check for ringing and voltage spikes and back it off... or shoot for 2us for safety...
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