AVR based ignition coil driver circuit. (need your opinion)

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Hi

Recently I designed a High voltage circuit based on AVR microcontroller. I need your opinion or any suggestion or improvements I could add to the presented project.

Circuit operation:

Microcontroller is responisible for giving appropriate PWM pulse (of known frequency) to the MOSFET transistor. Pwm signal from the uC goes to the optocoupler (18V) and then to a buffer (18V). Circuit has two main sources one is 5V for uC and second which would be 20V from transformer (This gives me 18V for the gate of MOSFET and optocoupler and buffer).
Additionally I added keypad, LCD, and Buzzer for better controlling circuit.

Below link gives overall idea what are ignition coil driver circuits:

http://www.rmcybernetics.com/projects/DIY_Devices/homemade_ignition_coil_driver.htm

Anyway I have few questions regarding circuit:

1. Do I need necessarily use optocoupler? I have two power sources and I wonder can spikes from the ignition coils can do much harm to the uC if they go through common ground. Or asking the same question in different way - does common ground in this case is dangerous to the microcontroller?

Are 3.6kOhm ok in optocouplers? It gives 5mA CE current according to my calculations.

2. Could you explain me what exactly do ER and CS pins in IR2121 device and how should I use them?
Can I leave them unused?

3. Can buzzer be supplied by 18V?

4. The most important question do all components fit together well?

5. what would you improve in the design...

6. Buffer gives 2 A on output isnt it more than Enough for the Mosfet? I mean such high current can be a trade off for the speed of the Buffer. (higher delays)

7. I set 30mA for the input of optocouplers diodes. Atmega 8 gives me max 40 mA so I think uC withstand Anyway datasheet doesn`t say anything about optimal imput current of optocouplers. Isn`t it 10mA?

Below I am attaching diagram of the circuit and links to datasheets of used devices:

1. MOSFET http://www.audiolabga.com/pdf/17N80C3.pdf
2. Optocoupler http://www.tme.eu/dok/06_optoelektronika/4n25.pdf
3. Buffer http://www.tme.eu/dok/04_uklady_scalone/ir2121.pdf

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1. Since the ground is common, no need for opto's. Layout and grounding design important.

2. The CS pin is for current sense and limit control. I would recommend using it.

4. I suggest two mosfets, one for each coil. Also consider adding some resistance in the mosfet gate circuits. 100 Ohms may be a good test value.

It all starts with a mental vision.

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I'd suggest you use an automotive coil driver. Most auto coils are used in the flyback mode. When the mosfet is turned off, the primary voltage rises due to Lenz's law. with an auto coil driver, this voltage is limited (or clamped) and this voltage is reflected on the high voltage side.

Without this your spark energy will be much lower and will probably destroy your mosfet. As previously mentioned, using optos is not adding any value in this circuit, besides, the 4N25 is a pretty low performance opto.

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I thought that till I looked at the specs for the SPP17N80C3 800V 17A. It might work.

It all starts with a mental vision.

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Quote:
Also consider adding some resistance in the mosfet gate circuits. 100 Ohms may be a good test value.

Thanks I forgot about it...

Quote:
When the mosfet is turned off, the primary voltage rises due to Lenz's law. with an auto coil driver, this voltage is limited (or clamped) and this voltage is reflected on the high voltage side.

Yes I have decided to use IGBT transistor with some TVS diodes for clamping...

What is the best way to control loudness of the Piezzo sounder? I will have 18 V for piezzo and I wonder if this won`t be too loud? Maybe add 1kohm resistor in series and then control it with PWM? But what resistance?

Adam

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Quote:
2. Could you explain me what exactly do ER and CS pins in IR2121 device and how should I use them?
Can I leave them unused?

ERR is the emergency shutdown pin and probably can be left inactive in your app. Its meant for motor drivers in a bridge and should be activated if overshoot is encountered.
CS is a current sensing input and could be interesting if you suspect that your ignition coil will ever encounter a short-circuit. If you trust the coil you should connect it to ground. ( oops, i should have read KitCarlson's response )
Quote:
Maybe add 1kohm resistor in series and then control it with PWM? But what resistance?
If you drive the buzzer near to its resonance frequency with 18 volts even a larger resistor will give a very loud signal. If unsure ,insert a 22 Kpot and adjust it for good performance. Measure it and replace with a fixed resistor. I'd think 10k is probably a good value.

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It might be more flexible to use the timer in the output compare mode. The dwell or coil charge time can then be set to a constant time that charges the coil, typically a few mS. The repetition can be controlled by off time.

A small series resistance in the coil circuit will limit the current. It may be helpful in the debug stage if you do not use the CS feature. A ballast resistor from an old MOPAR will work.

It all starts with a mental vision.