Re: Automotive - reverse voltage protection thing

"Przemyslaw Wegrzyn" <czajnik@xxxxxxxxxxx> wrote in message
news:ejgjih$2lo$1@xxxxxxxxxxxxxxxxxxx

Hi!

I need to design a well-protected power supply for an automotive device,
well protected from all the nasty things described in ISO 7637 and similar
norms.

It's a common way to use a transil to protect from "dump load" surge, it's
perfectly ok for me. Yet another thing to protect from is a reverse
voltage.

My first idea was to use a series diode to protect from reverse voltage. I
wanted to connect it as the first component, anode to the battery, and a
transil just after that. Seems ok, but..

Well, the diode itself need to have a high level of allowable reverse
voltage (ISO 7637 says about -150V spikes, yet I've seen some papers
saying
that reality is much worse). At the same time this diode need to handle
quite high peak current, other wise it will die after first "dump load"
thing. I guess this leads to a big/expensive element.

One possibility is not to use this diode at all, assuming that the transil
itself is enough - in case of reverse polarity, it will conduct just like
a
normal diode, effectively limiting the spike to it's -Uf. It would need a
fuse to protect the transil from permanently reversed power supply.

What is actually used in practice for such a protection? Looking for some
info I've found RBO40 component from ST, which uses a series diode, TVS
across output for load dump protection, and another across the diode.
Unfrotunately, this part is quite hard to get here.

Best Regards,
Przemyslaw

For most stuff, I use a series fuse on the + side followed by an appropriate
uni-polar TVS across the + and GND. In extreme overvoltage situations (bad
regulator in the alternator) the TVS will conduct and blow the fuse. For
short duration transients, it just shorts the excess and makes heat. It's
been my experience that these transients have very little power behind them,
and a 1500 Watt SMC TVS is plenty. In a reverse polarity situation, the TVS
acts as a regular diode and blows the fuse.

As the circuitry or system I'm protecting go up in value, things can get a
little more complex.

Chris


.

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