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Why is my microcontroller and Mosfets failing in this circuit?
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Civilenjuneer:

--- Quote from: OM222O on May 29, 2019, 06:27:02 am ---+-12V seems a bit low. almost any modern fet can tolerate +-20V on the gate and about 30V drain to source.

zeners are horrible for fast, high energy noise sources. they are simply too slow. TVS diodes are fast, but usually rated for really high voltages. your best bet here would be schottky diodes connected to GND and VCC in reverse polarity. that will ensure non of the parts see more than about 0.3V above VCC or below ground. just choose a suitably beefy diode so it won't burn out during high current bursts.

another simple insurance would be using an NTC thermistor between the VCC and the actual battery, followed by a decently sized capacitor. this will suppress fast transients and inrush currents if they are causing any issues (I really can't say if they are or not, since you mentioned you can't test with a scope). the NTC initially has high resistance, so it heats up quickly, reducing its resistance, therefore preventing it from heating further.

I honestly think better fets (or even BJTs or IGBTs, they are usually a better choice in automotive applications) and having some schottky diodes will solve your issues.

--- End quote ---

Thanks for the reply!

I have been looking over datasheets at DigiKey.. I found SSM3K2615R,LF‎ and ‎NVR5198NLT1G‎.  They both have much higher Vds ratings and (I think) reasonable input capacitance and Rds values.   I think either would be a better choice than my current - if you have a moment to look, I'd appreciate your feedback.  On the schottky diodes for protection - I assume you mean putting the anode on ground and the cathode at the +14.5V at the top of the LED's?  You mentioned diodeS, multiple, so do you also suggest adding one in the same configuration at points on the board, or is one for the whole circuit likely to be sufficient?
Civilenjuneer:

--- Quote from: Circlotron on May 29, 2019, 07:17:30 am ---I have had good success in an automotive environment with the following setup. +12V comes in through a 10 ohm 2 watt resistor, then 1000uF cap and 27V 1W zener across the line, then on to the voltage reg. In your case a 20V zener might be better. I'm not so sure about a diode in series with the +12V line - a spike will pull up the input caps and the diode will allow the caps to stay at the high voltage level.

Done about 8000 boards over the last 20 years and no obvious failures. Protecting the LEDs and mosfets may need an additional approach - 400mA LED current will cause too much voltage drop in the 10 ohms.

--- End quote ---

Thanks!

You mean too much voltage drop meaning too much dissipation?  I sized them appropriately and the full current only flows for a short time.  On your protection circuit... what is the function of the 1000uF cap - just to absorb the energy from a transient spike?  Are you using a high-voltage cap there or just something like a 50V electrolytic?  May I ask what you have on the other side of the protection circuit.... digital electronics or something else?
floobydust:

--- Quote from: Civilenjuneer on May 29, 2019, 05:01:00 am ---I forgot to mention I do also have a schottky diode on the power input as well as the input lines to the MCU.

--- End quote ---

A partial schematic just makes it too hard to help you.
I don't know where your series input diode is. Schottky is not suitable as they are low voltage parts. Are the LED's powered ahead or after it? This is important.
Where does the hall sensor go, long cable?
OM222O:
I agree. Please upload a full schematic. I will have a look at replacement fete and post a schematic of a clean supply from the battery tonight when I'm free.
Circlotron:

--- Quote from: Civilenjuneer on May 29, 2019, 08:51:31 am ---
--- Quote from: Circlotron on May 29, 2019, 07:17:30 am ---I have had good success in an automotive environment with the following setup. +12V comes in through a 10 ohm 2 watt resistor, then 1000uF cap and 27V 1W zener across the line, then on to the voltage reg. In your case a 20V zener might be better. I'm not so sure about a diode in series with the +12V line - a spike will pull up the input caps and the diode will allow the caps to stay at the high voltage level.

Done about 8000 boards over the last 20 years and no obvious failures. Protecting the LEDs and mosfets may need an additional approach - 400mA LED current will cause too much voltage drop in the 10 ohms.

--- End quote ---

Thanks!

You mean too much voltage drop meaning too much dissipation?  I sized them appropriately and the full current only flows for a short time.  On your protection circuit... what is the function of the 1000uF cap - just to absorb the energy from a transient spike?  Are you using a high-voltage cap there or just something like a 50V electrolytic?  May I ask what you have on the other side of the protection circuit.... digital electronics or something else?

--- End quote ---
Nah, simply too much voltage drop. 400mA means you are going to lose 4V in the 10 ohms. You would have to supply the LEDs via a separate path. Yep, 1000uF just absorbs the spike. 25V caps have been fine. Downstream is a 78M05 5V reg and a couple of 68HC908 micros. Ignition system for race car engine.
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