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I built a circuit for ignition system using MOSFET, 555, ignition coil (specs unknown, but current rating is more than 2 A and less than 10 A), SMPS (12 V, 10 A for powering the the whole circuit.)

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  1. First I built circuit-1 which has worked but only for a few seconds and temperature of the MOSFET is too high during operation.
  2. Second circuit-2 which is better than circuit-2 but works for 1 minute And still MOSFET is heating (slowly). I built the circuit-2 with reference to (arduino controlled spark plug with ignition coil) @MatsK

My question here is:

  1. Why it is better than the first one and how to choose the RCD components which are connected in parallel with MOSFET in circuit-2?

  2. And I powered the circuit form 12 V, 10 A SMPS and the SMPS is struggling to provide supply and I don't know why (LED in SMPS is flickering during operation)

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  • \$\begingroup\$ 220 ohm gate resistor, frequency range is 5 Hz. \$\endgroup\$
    – HEMARAJ J
    Oct 1 '18 at 9:10
  • \$\begingroup\$ Because you have a gate resistor to high. You didn't show the gate driver circuit. \$\endgroup\$ Oct 1 '18 at 9:20
  • \$\begingroup\$ I used 555 IC to generate gate pulse, powered the 555 IC with 12v Smps. And placed the IN4148 switching diode on pin 3 to prevent the IC. output voltage from pin 3 after the diode is 5v and then place 220 ohm gate resistor. \$\endgroup\$
    – HEMARAJ J
    Oct 1 '18 at 9:43
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    \$\begingroup\$ What's the primary winding resistance of the coil? You may be saturating the coil at a frequency of 5Hz which will cause very high current flow. Automotive ignition coils are made to function at much higher frequencies. You may be able to improve the situation by reducing the duty cycle of the 555. \$\endgroup\$
    – vini_i
    Oct 1 '18 at 9:49
  • \$\begingroup\$ @vini_i not able to find spec of ignition coil but which seems similar to (regulatorrectifier.com/catalog/polaris-287). And the the operating duty cycle is 80%. \$\endgroup\$
    – HEMARAJ J
    Oct 1 '18 at 10:03
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80% duty cycle is much too high for an ignition coil. As vini_i said you are probably saturating the coil. If I remember correctly, most ignition coils have a primary coil inductance of around 8 mH. This means that its reactance at 5 Hz is about 0.25 ohms. This is extremely low. At 5 Hz with 80% duty cycle this means your 12 V source will be trying to source (12V/(0.25Ω+0.5Ω))*0.8 = 12.8 amps on average to the coil alone. No wonder your MOSFET is getting hot!

Your MOSFET definitely needs a large heatsink.

The second circuit appears to be using a snubber circuit across the MOSFET to protect against voltage spikes. However, I have never seen a snubber circuit with a diode (or two paralleled diodes) in series with the rest of the snubber. I think that would defeat the purpose and not allow current to flow when it needs to. I recommend removing the diodes in the snubber branch.

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  • \$\begingroup\$ Ok, reducing the duty cycle and increasing the frequency will be helpful. thanks \$\endgroup\$
    – HEMARAJ J
    Oct 1 '18 at 11:15
  • \$\begingroup\$ You should probably get rid of the diodes in the snubber branch too. \$\endgroup\$
    – DerStrom8
    Oct 1 '18 at 11:20

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