[General] MOSFET switching instead of Darlington

I made today a full study concerning the old charging system of a coil and the duty cycle of the 555 and to the 4017 output duty cycle.
Old car system used 5/6 charging to 1/6 discharge duty cycle. The 4017 gave me 50/100 and 555 gave me 92/100 ( according to the components I use)
So I will change the duty cycle of 555 by adding a signal diode between legs 6 and 7 to reduce the duty cycle, and use potentiometers to adjust the frequency. I think that way I will get between both cycles to imitate the original ignition coil charging/ discharge necessity.
Any opinion concerning that?

Give it a try. You may find it advantageous to make the duty cycle variable as well.

One of the reasons the MOSFET/Darlington gets hot is that when turned on by a high voltage at it's gate/base it conducts and wastes power by dropping a voltage across itself (ideally this would be 0V) while passing current. Ohms law tells us the power is V x I. It takes a time for the current to build the magnetic field in the coil but any longer than that is time when the current flows for no reason and is wasted.

The other reason is to do with the rise and fall of the current as previously explained. As the power lost is V x I, making one of them as small as possible makes the loss as small as possible. In a perfect switch, when open (transistor not conducting) the voltage is high but the current is zero and when the switch is closed (transistor fully conducting) the voltage is zero, both ways producing no power. In reality, the switch is not perfect so there will always be some loss but by not providing enough current from that 4017 you were holding the switch partially conducting where both V and I were high. Keeping the time spent in the transition between off and on (rise and fall time) as short as possible not only produces more voltage from the coil but minimizes the partially conducting time.

Brian.

Dear fellows, thank you so much for your help. The circuit is functionning now perfectly. I used R1=510k, R2= 910k, duty cycle 60%, a 220 resistor between 555 output and IRF740 MOSFET.
The current across the coil is now 0.88A and the voltage 22V , just the power I need. I have been running the circuit for hours with a 4x3 cm Aluminum cooler, and the temperature is still less than 25 degrees Celsius - 77F.
It is functioning perfectly until now. Thank you a lot
PhD Rodolph Farah