Hi Jerson,
I put a schottky diode across the coil primary terminals. I’ve attached the scope traces taken at the coil negative (switching point). It does fix the problem completely. It proves the need for suppression diode whenever inductive load is switched on and off. Though in my case it makes the ignition system inoperative. Absence of the voltage spike at the primary side means that no high voltage will be generated at the secondary side and there will be no spark. Nevertheless thanks for the advise. I have learned something about suppression from this.
I also tied to use switch mode step-down voltage regulator LM2574N-5. It does not make much difference and requires several external components to operate.
When I touch any oscillator pin the PIC stops. I tried to remove oscillator capacitors one at a time and tried different brand oscillator. It didn’t change a lot. I tied to use external clock source. I used 16f628 at 20MHz powered from and independent power supply. Using High and LOW command in a tight loop gave me 500kHz signal. So I adjusted my original code to keep the length of the driving signal the same and changed configuration fuses accordingly. It made the system less stable.
I also changed the MOSFET (had to because it stopped working)to IGBT 12N60A4. I think the reason for the MOSFET to fail was the clamping voltage. So I lowered it to 200v intending to rise it later to 300v. It improved the stability. The problem is still there though.
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