Switching power for PC in cars

[Kalle Pihlajasaari]

Hi,

I have had the occasion to integrate a few embeded PCs into car use
in the last while and thought I would make a few points about
powersupplies.

Firstly most PCs these days have NO use for the -5V supply rail.

The -12V rail is used in the RS-232 drivers and Analog
type of cards (ADC, DAC, Sound) and the +12V supply is used
also for the RS-232 and to drive disk drives (floppy and hard)
and also for the analog functions.

There is a range of PC modules that are about 4" x 4" and use a
pin and socket stacking mechanism for the BUS instead of the
usual ISA edge connector, they are expensive but do conform to a
standard known as the PC-104 standard (now also PC-104+ that has
a PCI bus function).  A company called AMPRO in the US was instrumental
in the early formation of these standards and also sell very good quality
parts which I use for prototypes but they are rather expensive for
productions runs.

Now all this requires power and as most PC-104 modules are made to
work on +5V alone the problem is greatly simplified but is still there.
Another company makes a 4" x 4" x 0.5" power supply that takes in
9 to 35 V DC and feeds out 5V (and optionally +12V, -12V, -5V still
in the same volume), these are rather expensive though and I pay
US$ 170 for the 5V only supply and would pay US$350 for the 4 rail
supply.  Current that can be supplied is about 3A on the 5V rail
in total (the other supplies are derived from that rail for wide
input voltage range reasons) if my memory serves me (manual not handy).

Now there are a few cheaper NON PC-104 sollutions to this problem
that I will look at one of these days.  One could take standard
PC power supplies and convert them for 12V opperation if one is
very familiar with SW PSU design as you would have to replace
the input switching transistors, rewind the high frequency
transformer and adjust some of the resistor values.  These changes
will obviously be unique for every type of supply so is only
a good idea if you want to rework quite a few (50 .. 500) supplies.
At more than 500 you could get the factory to do it for you for less
on special order.

You could purchase open frame SWPSUs with 10 to 30V input for various
power supply companies, Power-1 might have some and there are others.

If one really wants to build ones own one can use some of the
SIMPLE SWITCHER (R) power converter ICs from National Semiconductors
which I think are the ones use in the expensive PC-104 supply that
I have used and they are VERY simple to use, I plan to try it out
and they look like the right answer unless they are too expensive
for production volumes.  They require a single IC in a TO-220
5 lead style, an inductor, a Schottky diode and 2 capacitors.
Just that to get 5V at 3A from 9V to 35V in.  You will need to add
primary surge, reverse voltage and noise filters but this would
always be required.

Part number that is mentioned above is the LM2596T-5.0

Cheers
-- 
Kalle Pihlajasaari   kalle at ip.co.za   http://www.ip.co.za/ip
Interface Products   P O Box 15775, DOORNFONTEIN, 2028, South Africa
+ 27 (11) 402-7750   Fax: 402-7751    http://www.ip.co.za/people/kalle

DonTronics, Silicon Studio and Wirz Electronics uP Product Dealer