I'm looking for a way to protect a small circuit which is to be used inside of a car or truck (12V or 24V power system). The circuit consumes about 12-15W. I use an isolated DC/DC converter module which can regulate 9-36V down to 3.3V.
I'm looking for recommended circuits or a controller IC that can take care of the usual hazards:
- Load Dump Spikes
- Reverse Voltage
- OV/UV Protection
- General noise on the power lines.
- ... Anything I might have missed.
Currently I have my eye on the LTC4365 from Linear Technologies. I've thought about using it together with a bi-directional TVS, clamping the voltage to 32V and protecting everything with a fast blowing fuse.
Would this be a proper solution or did I miss something here?
Answer
Load-dump
... is a killer - your TVS has to turn a huge amount of energy into heat without going pop.
ISO7637 for a 12V system has a spike peaking at up to ~90V with a rise time of 5-10ms lasting up to 400ms from a source resistance as low as 0.5ohms. That's several hundred Joules of energy in less than half a second!
Not all of that has to go into the suppressor - only the excess above the clamping voltage (but still ~60V in your case)
On the bright side, load-dumps are pretty rare, so if it's a one-off and you don't mind the small risk, you could ignore it.
Fast transient spikes
These can reach 200V when the wipers switch off for example - provide a (high-voltage-rated) capacitive route for those to ground right near the input.
Longish-term over-voltage
Automotive electronics is often specified to survive 24V for several minutes (for when a car is jump-started off a 24V truck) and 48V for up to a minute (IIRC) as sometimes 2 truck batteries are used to provide a quick boost charge to get a car moving in extremis! Your spike suppressor may pop under those conditions.
Dropouts
Battery dropouts can also be significant, there's a test in the industry which involves a series of pulses battery voltage falling to 0V - you need to have enough internal capacitance to keep your supply rails up when that happens.
Real-world requirements specification
If you want an example of how gory this can get, Ford's electromagnetic compatibility (EMC), which includes transient testing, is available on the web:
Component EMC Specifications EMC-CS-2009
Search through it for "transient" and "dropout" to see what series-production designs are supposed to live up to!
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