Pretty much just like a standard flyback
driver, except running at a very high duty ratio (95%) and only 400Hz.
I didn't include potentiometers to adjust frequency or duty ratio since
ignition coils are so alike, and the current values were found to work
well. I made the circuit while using a "high power" coil, but any coil
will work. My coil's primary has an inductance of 8.8mH, and about 3.5
ohms of DC resistance. Since the primary resistance is so high it will
limit current and protect the mosfet. At 30V the max DC current will be
8.6A, and with 8.8mH of inductance the on-time will have to be t
= (I * L) / U in order to get the most current flowing through
the primary. Since running high duty cycles had little impact on the
heating of the mosfet, I chose to run it at 95% and 400Hz, which gives
about the required on-time for max power. The funky gate waveform
shaper is something I've seen on many flyback based drivers, including
some ignition coil drivers. I'm not sure how it would improve anything
in regards to power, but it greatly lowers the peak current the 555 has
to source. All it does is ensure slow turn on and fast turn off, which
are fine in this application. Slow turn-on because the current through
the primary can only rise at a certain rate, and fast turn-off because
the current must be stopped abruptly for the resulting flyback pulse to
be a energetic as possible. Another reason for fast turn-off is the
high duty cycle used.
At 35V input I could get 4 cm sparks, suggesting 30-40kV. During my
first attempt I tried running it from 50V and using a different
circuit, but it arced right through the top cone to the + connector
immediately. To correct this I submerged the whole coil in hydraulic
mineral oil which prevents arc-overs and arcing through insulation cone.
I haven't tried it from 50V again since I have plans for this coil and
don't want it ruined. The ignition coil is very loud and intense when
sparking, so hearing protection is recommended.