Automotive Circuit Protection

[Jeff Webb]

This is in reference to a thread Gar posted a while back about
protecting the power supply circuit in automotive environments. I'm
finally to the point in my design where I need to think about this sort
of thing.  I've done a little search of the web for transorbs and have
found some TVS manufacturers:

http://www.worldproducts.com/home/wpi/ppg/ppg.html
http://www.crydom.com/tvs/
http://www.microsemi.com/corporate/products/tvs/overview.asp

I've got a couple of questions.  I'm pretty naive when it comes to this
sort of stuff, so please be gentle.

1) I should use a unidirectional TVS, right?  It seems like
bidirectional TVS's clamp to +/- Vc.  I would guess that unidirectional
ones clamp to +Vc/~GND like a standard zener.  Is this correct?

2) Gar used a 15KP18 in his example.  As he mentioned, the 15KP's seem
to be hard to source.  Digikey has 5KP's available.  What amount of
protection is needed in an automotive environment?  I guess it's good to
be overly cautious.  I think I'll design the board for a 15KP device,
but one can build it with a smaller one if availabilty/price is an
issue.

3) Are any other components needed?  Some of the sample TVS schematics
show a resistor in series with the power supply, and some have just the
TVS.  Some also show a zener diode before the resistor.  I was planning
on running from the power supply through a fuse.  The TVS would be
placed after the fuse and before the LM2940T-5.0 regulator.  I have a
little ascii art below.  The 22uF tantalum should have an ESR of about
0.3 Ohm for LM2940 stability.  How's this look?

BATT --[ FUSE ]--+-------+-------[ LM2940 ]--+---------+--------+---- 5V
OUT
                 |       |           |       |         |        |
                 |       |           |       |         |        |
               ____/   -----         |     -----     -----    -----
              / /\     ----- 0.47 uF |     -----     -----    -----
         TVS   /  \      |   Ceramic |       | 0.1 uF  | 22 uF  | 220 uF
               ----      |    50 V   |       | Ceramic | Tant.  |
Electrolytic
                 |       |           |       |         |        |
                 |       |           |       |         |        |
                GND     GND         GND     GND       GND      GND

4) Fuse selection.  I need to select the appropriate type of fuse that
will allow the TVS to do it's job and still provide good protection. 
The main thing I need to know at this point (for the board design) is
the type and form factor of the fuse.  I like the 297 series MINI
automotive style fuse.  I've found a horizontal PC mount holder
(Littelfuse) that looks pretty good.  The fuse holder is only rated to
+85C, which is a concern since I want to mount the box in the engine
bay.  I've found components rated to +125C for just about everything but
this.  I could also go with a cylindrical fuse, but I think the blade
style fuses would be more robust to vibration, etc.  Any suggestions in
this area?

Thanks for your input,

Jeff Webb
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