Protection for automotive circuit

Thread Starter

DBTech7

Joined Apr 21, 2022
30
I'm working on a remote start circuit for a 12v system and I've been working for almost a week now reading different papers to figure out the best way to protect the circuit from harsh automotive transients. In order to make the device reliable for years of operation under my dashboard I need reverse voltage and overvoltage protection, overcurrent disconnect, and transient suppression.

Here is what I've come up with:
Overcurrent- inline fuse or fuse socket on pcb, maybe both?
Reverse Voltage- series PMOS with gate connected to negative pcb input
Overvoltage/Transients- bidirectional TVS diodes connected between positive and negative of pcb (these should only be needed during load dump or other short transient periods)

ISO 16750-2 Pulse A specifies a load dump voltage of 101v max, with an ESR from .5 to 4 ohm, for a period of less than 400ms max which translates to around 200a of possible current max. These are EXTREME Max and I'm fairly confident the circuit will hardly ever experience this if at all, considering modern alternators contain suppression which can reduce the spike to as high as 79v, that still leaves me with a large amount of current to sink through the TVS diodes.

Due to availability for SMT assembly, one of my only viable choices is this diode SMBJ15CA Datasheet which has a Vrwm of 15v, a Vc of 24.4v, and a current rating for 24.6A

My main concern and question here is would it be safe to parallel several of these diodes and use one wide trace attaching all anodes and cathodes and thermally coupling them with a shared copper area?

I've included an ltspice simulation of what I'm proposing, Green V line is Vin and Blue V line is the Protected Side
diode.png
 
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Sensacell

Joined Jun 19, 2012
3,098
What is the current requirement for the load?

Your scheme effectively clamps the cars ENTIRE electrical system.
Consider adding some impedance in the circuit to limit the energy, so you don't need all those diodes.
 

Thread Starter

DBTech7

Joined Apr 21, 2022
30
What is the current requirement for the load?

Your scheme effectively clamps the cars ENTIRE electrical system.
Consider adding some impedance in the circuit to limit the energy, so you don't need all those diodes.
Rms current will typically be 15-20a but can peak as high as 30-40a for a 1s-20s durations, the boards power input will be directly connected to the battery via vehicle harness with only a fuse to protect it.

These conditions are extremely rare but I would rather implement simple protections, especially if the cost is low since the system is for an older mechanical diesel engine where transients are more likely than modern vehicles
 
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