ESD protection for microcontroller/nFET circuit?

[davekra]

I've made a magnetic on/off switch for my radio controlled airplanes and would appreciate input on making it somewhat robust to handling.
I'm using a PIC10F322 and PSMN1R3-30YL n-channel FET.
https://www.digikey.com/product-detail/en/nxp-semiconductors/PSMN1R3-30YL,115/568-4908-1-ND/2122738

The microcontroller, voltage regulator and sensor will be on one board and the FET will be on a second board connected with a 3 conductor cable. Battery-in will be on the FET board with only the circuits power going over the cable.
My other option was to have the sensor and LED separate from the controller and FET but that would require 4 conductors rather than 3.  3 conductor cables are used for servo extensions so they are easy to obtain.

The circuit works well on the bench but is there anything I could do to make it less susceptible to electrical damage from handling without ESD protection?
I was looking around and TVS diodes look like they might be beneficial but I'd appreciate any opinions or suggestions.

Thanks,
davidk

[jbb]

Good thinking.

You can start with the following:

• Add a TVS to the output terminal from the PIC board, after the 220 Ohm resistor.  For this application you want a unipolar TVS with a rating of 6V, I guess.  Place it in the circuit like it's a Zener diode. This is a good placement as the TVS will clamp to > 10V, but the 200 Ohm resistor will limit the current into the PIC.
• Add another copy of the same TVS to the MOSFET gate. It won't be perfect but will help.
• Add a pull-down resistor to the MOSFET gate (say 10k). This will make it turn off if a wire falls off.

Protecting the power rail could be a bit more difficult.  Typically you'd fit a poly fuse on the incoming supply and a TVS to ground rated somewhat higher than the max battery voltage.  Then you need to make sure your regulator can handle around 50% more voltage than the TVS rating without dying (this is what data sheet Absolute Maximum ratings are for).

[davekra]

Thanks. 
So, if I understand correctly, a TVS diode would go on each board from the "pic output to gate" to ground.
And additional TVS on the power rail.  See pic.
The regulator maximum is 12 volts so in that case the TVS clamping rating should be 6volts? 
The normal input voltage is from a 4 or 5 cell MIMH or 2cell LiPo so ~8.4 is the maximum voltage.
If the TVS is 6volts and the input voltage is 8.4 would the TVS always be clamping the voltage?  Would that be a good idea?


I'd probably omit the pull-down.  The failure mode for this should be the on state.  Without radio power it becomes exponentially more difficult to control the plane.

Thanks for the help.
davidk

:edit: Doing some reading, I think the two TVS on the positive rail could be reduced to one.  Positive doesn't contact any components on the FET board.  And, actually, the one could probably be omitted all together.  The clamping ramps up  from the working voltage of say 12volts to cutoff of 20volts.  With 8.4volts max input and 12volts max on the regulator would the TVS really be doing anything?

[jbb]

Hi davekra

Sorry for the delayed response!

On the power rail TVS:

• Agree you don't need one on the battery pack end.
• If you've got a 12V input, you probably want a 15V TVS clamp to make sure it doesn't accidentally conduct.
• You typically need to allow 30% or more 'extra' voltage over the rated voltage for a TVS clamp, i.e. 15V * 130%, plus a few V. That's say 25V, so you'll need a higher voltage regulator (note: you can use the 'absolute maximum' parameter in the data sheet for this.).
• If you use a TVS rated for less than the expected DC input then continuous current will flow, things will get hot, and something will break. So a 6V TVS is not suitable here.

On the MOSFET gate:

• If you want it to fail on, use a 15V TVS (i.e. the same as the supply one) and put a pull-up resistor (say 47 kOhm, quite gentle) to battery+ on the MOSFET board.
• On the micro controller board, use a TVS a bit above to the operating voltage of the micro to protect it's IO pin.

Hope this helps
jbb