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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:

  1. Load Dump Spikes
  2. Reverse Voltage
  3. OV/UV Protection
  4. General noise on the power lines.
  5. ... 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?

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    \$\begingroup\$ I added a link to the LTC4365's datasheet. We're trying to make users aware of the importance of this, especially for less common parts, so that others don't have to go searching for it and that everybody is sure to be talking about the same thing. Just trying to cultivate good habits. \$\endgroup\$ – stevenvh Jul 20 '11 at 5:51
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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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  • \$\begingroup\$ Thanks for the answer and the link. The pdf is very informative. One thing I'm not sure about are the Fast Transients you've mentioned. I've scanned through the document what I've found is a mentioning of Test pulses A-G. C1/C2 look like the one's you've mentioned, is that correct? \$\endgroup\$ – Nico Erfurth Jul 20 '11 at 16:32
  • \$\begingroup\$ @Masta79: Pretty much all the A-F transients are what I was meaning by "fast transients", although some are indeed faster than others. They are shorter than the load-dump though... Sorry for the confusion! \$\endgroup\$ – Martin Thompson Jul 20 '11 at 21:03
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You seem to have answered your own question. The LTC4365 is probably a good solution. The datasheet says no TVS is needed, but I still would use one.
Have the LTC4365 followed by a buffer capacitor to handle dips in the battery voltage. If the battery is also used for a starter motor it's probably unavoidable that the voltage drops, especially when you're consuming 15W (that's 4.5A at 3.3V).
If the capacitor has a rather large value you may want to use a slower fuse, otherwise it may blow when switching on. (The fuse doesn't offer extra protection over the LTC4365 other than limiting the damage in case of a component failure).

Any particular reason why you want to use an isolated DC-DC converter? They're usually not needed for battery operation.

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If you've already got a isolating DC-DC converter that can handle up to 36V in, it doesn't sound like you need much more. I don't understand what you think the LTC4365 will do for you. Your converter can already handle 36V on its own, which is actually a little more than the 34V the LTC4365 is rated for.

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    \$\begingroup\$ My idea was to only protect the LTC4365 with the tranzorb. Limiting the maximum current to it with a resistor. Everything behind would be protected by the LTC4365 and the Mosfet. Those would have to be rated for the 100V+ accordingly. As the LTC also provides inrush current limitation and reverse battery protection, that sounded like a sweet deal to me. \$\endgroup\$ – Nico Erfurth Jul 20 '11 at 16:03
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    \$\begingroup\$ @Masta: But if you can protect the LTC4365 like that, which is only rated for 34V, why can't you protect the 36V DC-DC converter the same way? Then you can lose the LTC4365 altogether. I don't see what it's adding in your setup. \$\endgroup\$ – Olin Lathrop Jul 20 '11 at 16:49
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    \$\begingroup\$ As Martin already stated if i protect the whole circuit via a transorb it has to burn the whole energy of a load dump, which can be quite a lot. With my solution I can limit the current so the LTC can still work. The LTC then can detect an OV situation and shut off the mosfet so the transient can not reach the the rest of my circuitry. With this the chance of blowing a fuse SHOULD also be greatly reduced. Unless I'm massivly confused. :) \$\endgroup\$ – Nico Erfurth Jul 20 '11 at 17:43
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For a brute force protection device: ST : RBO040

MANY thousands of devices in police and other emergency vehicle applications with this part at the connector to the +12V line. Not fancy but will save your circuit from most transient events.

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