Automotive input protection circuit redesign

[Hengy]

I am currently designing the input protection part of a 5A buck converter for use in my car. I was reasonably confident in my initial design to start the PCB prototype, but, as it  quite often happens, while in the process, I got to thinking about my design. My conclusion is that it is likely to go up in smoke.

My design employs a power inductor on the battery input (47uH), then two SMBJ33A TVS diodes back-to-back, a N-channel MOSFET driven by a LM9061 active rectifier/reverse polarity controller, and finally 220u+100u caps. The LM9061 takes care of reverse polarity and over-current.

Upon review, I know realize that if my TVS diodes clamp at a lower voltage than any other in my car, it's likely not going to last long.

Obviously, one way to fix the problem is to choose higher voltage TVS diodes - my buck converter is rated for 60V. However, the 33V TVS diode for positive transients, according to the datasheet, has a max clamp voltage of 53V (if I am understanding that correctly).

Adding a series resistor would limit the current during a transient, however, at 4A continuous input current, I'm dissipating a ton of heat. At 4A, with a series 10 Ohms, I'm looking at dissipating 160W, and that would limit the power through the SMBJ33A to about 500W with a 70V transient. The SMBJ33A is rated for 600W. At 100V transient, it would see 1000W.

I could also use some much larger TVS diodes, but I don't really want to have my one circuit always subjected to punishment, even if that TVS diodes can handle it. Space is a limitation, and this is going to use a 2-sided, 2oz copper, 0.8mm thick PCB from OSHPark.

More info:
- Buck converters are TPS54560-Q1
- Car is, and will only ever be a 2007 Chevy Impala LTZ 3.6L
- The only limitation really is space. This is a one-off, so no bulk pricing, but there is no cost limitation, besides the ridiculous

Can anyone give me suggestions on how to implement my TVS diodes? Maybe a different order of my previously mentioned components?

Thanks, Hengy

[max_torque]

How critical is the device?  If it can turn off without a safety issue, then just fit a fuse!  In the case of the TVS conducting the fuse will blow. simples.


(if it needs to stay running regardless, then things are more tricky........)