Automotive DC-DC converter issues

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Automotive application as the title suggests.
The equipment is to be powered by a DC - DC converter.
Absolute max input voltage is 32V; any more and the manufacturer states warranty void.
The equipment is to be used in both 12 and 24V systems.

Unfortunately under certain conditions input can exceed the 32V limit.

A combination of MOV /Fuse has been suggested. Sadly a MOV which might be OK at 1 mA leakage current may be unsuitable at higher currents in that the terminal voltage may be way over the 32 V limit.
A lower voltage MOV on the other hand may start conducting rather heavily at the acceptable voltage .

Recommendations are sought to the possible over voltage solution on the economic side.

Transient suppression diodes , zener diodes etc to be considered.

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It depends on the input voltage and the current.

Is it for example 50V for a few seconds, or only a spike of 200V for a few ms ?
What is the current ?
Do you have to use that DC-DC converter ? Or can you replace it with one that accepts a higher voltage ?

For a spike, I would use an inductance and capacitor. For a high voltage with low current, I would use a high voltage transistor (with a 30V zener).
For a high voltage with high current, I would use a DC-DC converter that accepts high voltages.

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Load dump is the issue we're facing in automotive applications. Spikes and noise are other issues.

32V max in for the DC-DC-converter is IMO unsuitable for the 24V systems. A fully charged 24V battery delivers 28V, ... easily.
For 12V automotive systems, all equipment must be able to withstand 60V during 10ms IIRC. You'd better check that.
Now how to protect the DC-DC converter against that ? Clamping it requires a very beefy clamp. It's easier to use a zener of let's say 25V, with an emittor follower, with or without darlington configuration (depends on the current spec ... what current are we looking at ? ), with a Vceo of 100V.

Under normal circumstances, the resistor in series with the zener will feed the (darlington) NPN emittor-follower, and depending on the current you can calculate the dissipation.

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Sometimes in these situations a cheap solution is to use a current limiter, in combination with ( zener, tvs, MOV, etc).

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I had visions of active zener barier but will have to consider power dissipation within the barrier.
Crowbar with a fuse is just not a practicable solution as it may result in excessive fuse replacements.
I will keep You informed of final solution ( sounds ominous..huh?).

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Use a voltage limiting device to protect your DC-DC converter against excessive supply voltage... zener, tvs, MOV whatever

Use a current limiting device to protect that voltage limiting device against excessive current.

When used together, your DC-DC converter, and anything down stream, will continue working as if nothing happened. No blown fuses, no glitches.

It can be as simple and cheap as a power FET and a resistor.

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mikericetga wrote:
Sometimes in these situations a cheap solution is to use a current limiter, in combination with ( zener, tvs, MOV, etc).
TE's PolyZen is like that (PTC + zener); unfortunately, it doesn't meet OP's voltage range.
Refs.
TE Connectivity, Circuit Protection, Hybrid Devices
Digi-Key PTM, PolyZen (TM)

"Dare to be naïve." - Buckminster Fuller

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Problem with MOVs is broad tolaerance and soft knee.
In addition MOVS may not be OK with load dumping; they will change properties with overstressing while showing no appreciable browning ( cooking).

Quote:

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Many car ECUs use a big MOV. You rightly point out one of the issues with MOVs- soft knee and broad tolerance - but they do take more abuse than a TVS diode. I usually put a polyfuse in series to protect the mov or tvs diode. This stops the diode melting down or the mov exploding. This means my boards survive some of the customer's stupidity.

A TVS diode (unipolar) gives overvoltage and reverse voltage protection.

Another feature of MOVs I stupidly ignored is that they have significant capacitance - don't use them on comms or high speed signals.

I think the suggestion of a mosfet and voltage detection device has merit - too many volts and it disconnects.

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NCP3712ASN: Overvoltage Protected High Side Switch has a load dump transient protection circuit in its data sheet. Zener diode to adjust the turn-off voltage; up to 300mA output.

"Dare to be naïve." - Buckminster Fuller