Automotive protection: TVS, thermistor, or both?

Electrical Engineering Asked by orange1234 on September 7, 2020

This is a follow up question regarding my previous post where I was informed my issue was due to inrush current.

I have a DC1601B-C buck-boost converter outputting 12V and up to 20 Amps. It is hooked up in an automotive circuit that is powered on with a key switch. The key switch turns on power from a 12V lead acid battery. There is also an alternator to charge the battery. I have a SLD8S17A TVS diode as protection on the input line for automotive load dump.

The regulator is supposed to be able to output up to 20A, and under my normal conditions I am running it at about 11 amps with various sensors, single board computers, and an ethernet hub. I can’t go into specifics on the exact devices.

At the time of the board catching fire (as described in the post linked above) it was only powering the ethernet hub and a few of the sensors and should have only been outputting 2.25 amps maximum (could have been much less). The board was working fine, then the switch was keyed off, then back on, and that’s when the fire happened.

To protect against inrush current I was thinking I would add a fuse leading to an NTC thermistor. Would this be a good solution? Should it go before or after the TVS diode? Would the thermistor work if the device was only briefly unpowered? In my research I can’t seem to find a schematic of a thermistor being used with a TVS diode.

Another question is does my TVS diode need a resistor or capacitor (and what value)? I only have the TVS diode, but after some reading I have seen that some designs include a resistor before the diode to limit current.

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