Power supply in automotive environment

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
Sadly, I'm running up against the limits of my meager mixed-signal
design knowledge.  I'm trying to put together an embedded device
intended for use in an automotive (well, motorcycle) environment, and
am trying to figure out how to power the damn thing.

A few of the components I need to use (video overlay chip, MEMS
accelerometers, op-amps) require +5V.  And since I'm going to to be
doing analog sampling and then A/D conversion, I suspect the power has
to be fairly clean.  None of my signals are going to be above 100Hz,
so I might even be wrong on that point.

Anyway, as near as I can tell, my options include powering from the
vehicle +12V system via a linear regulator, using a lithium-ion
battery along with one of the nifty USB-powered charger chips and a
switching regulator to step up to +5V, or just whipping up a 6-cell
NiMH or NiCad pack and regulating it down to +5V, then using an
external charger to recharge the thing.

The switching option seems like the best combination of operational
and design simplicity, but I'm worried about the effect of the 50mV
P-P 1MHz power supply ripple on my already poorly-understood analog
circuitry.  It seems like I should be able to LC notch filter that
out, but is this going to mess up the feedback on the power supply
chip?

The vehicular +12V supply seems the next simplest (operationally), but
I've heard all sorts of nasty things about the quality of that power,
and how it's full of noise and spikes, occasionally reverses polarity,
etc etc.  Is handling this just a matter of putting in appropriately
rated caps and a reverse-polarity protection diode, or is there more
to it?

Is implementing this going to be so much of a pain in the ass that I
should just solder together a bunch of AA NiMH cells and accept the
need to swap out battery packs every now and again?

(note: I'm not planning on actually selling this to strangers, so I'm
not concerned about UL testing or functioning 100% of the time, just
"mostly working".)

-jake

Re: Power supply in automotive environment

Quoted text here. Click to load it

The one sentence, mostly working answer is Nat Semi Simple
switcher for +5v and a 5 volt A/D with an internal 4.096v
reference. Pay close attention to data sheet recommendations
for reference bypassing and/or filtering.

Put a diode and a fuse in series with the battery, followed
by a 20-some volt zener across the regulator input and you
should be reasonably safe.  I would not guarantee it for mass
production though.









Re: Power supply in automotive environment



Quoted text here. Click to load it

First verify that the supply is 12V, most motorcyles use a 6V supply. If
this is the case and your current depands are small a zenner diode and a
few capacitors would be my choice.


Re: Power supply in automotive environment

Quoted text here. Click to load it

That was true at one time, but not for about the last 40 or more years.

Quoted text here. Click to load it


--
========================================================================
          Michael Kesti            |  "And like, one and one don't make
We've slightly trimmed the long signature. Click to see the full one.
Re: Power supply in automotive environment
It depends on your power requirements.

If it's a few hundred mA or less, I'd use a series diode, a reservoir
capacitor and a linear regulator (78M05 etc). You could include a series L
to reduce "noise".

The limits of this circuit will likely be dissipation and cooling of the
regulator ((12v - 0.7v -5v)* Current)

For more than this, use a monolithic switcher (Linear Tech make some very
nice ones and their website has afree simulator for designing with them).
You can get wound components for these from many suppliers (e.g. Coilcraft)



Quoted text here. Click to load it



Re: Power supply in automotive environment

Quoted text here. Click to load it

You can put an LC notch filter or just a lowpass filter after the regulator,
but AFTER the feedback.  Do not include the filter inside the feedback
loop, or it will likely oscillate.  You could also put a linear regulator
after
the switcher, and if you use a low dropout part, the losses aren't very
high.  Just remember to set the output voltage of the switcher high
enough to give you the headroom required for the linear reg.

Quoted text here. Click to load it

On a 12V system, usually you have to be able to handle at least 24V
on the input side.  Put in a reverse polarity protection diode, and a
fuse (or polyswitch), then follow that with a 24V power zener to
ground to limit incoming voltage spikes.  We used to use a MR2535L
part from Motorola which was meant for this specific purpose.
A datasheet can be found here:
http://www.onsemi.com/pub/Collateral/MR2535L-D.PDF

Quoted text here. Click to load it

I would think that you should be able to get standard regulators to
do what you want, without having to resort to a battery supply.

Mike Anton



Re: Power supply in automotive environment

Quoted text here. Click to load it

As someone else also stated, do not use a bridge, just a single diode
in the positive supply line.  You should have your circuit referenced
to the vehicle ground.

Quoted text here. Click to load it

There is no issue with the negative spikes, as the MR2535L is not
bidirectional.
It looks like a standard zener diode, so since its forward voltage is around
a volt, that is what it will clamp your negative spikes to.

Quoted text here. Click to load it

Sure, but only if your rectifier bridge is fast enough, and rated for the
high
instantaneous currents involved.  But the method above is better.


Mike Anton



Re: Power supply in automotive environment
Look at the LM2940. It was designed for vehicular use; it has short-circuit
and reverse voltage protection.



Site Timeline