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
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
Last edited: