mbt28
Posts: 29
Joined: Wed Feb 04, 2015 10:35 am

Automotive grade power supply

Mon May 28, 2018 6:02 pm

Hi,

For my project I need to design automotive grade power supply to my pi. Car is a harsh environment and in my project there are linbus drivers, audio dacs and lcd controllers so I should keep things low noise and low dropout.

But I have missing knowledge on few things:

Should I first regulate 12V to 12V with lowdroput then convert the remaining voltages?
Or should I create 12V to 3.3V, 5V, and 1.8V with parallel regulators?

What should be a good stable switching frequency for my system?

Thanks

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Z80 Refugee
Posts: 358
Joined: Sun Feb 09, 2014 1:53 pm

Re: Automotive grade power supply

Mon May 28, 2018 6:41 pm

This won't be easy. At the input side, you need to accommodate potential accidental reverse polarity (somebody connects a battery the wrong way around), voltage dips and spikes, and "load dump" (the alternator output suddenly losing regulation and reaching 60V or more). You need to ensure the unit cannot drain the battery when the ignition is off.

Internally, you will need to ensure the unit complies with automotive standards for Electromagnetic Compatibility - that a certain level of external conducted or radiated interference over a specific frequency range doesn't adversely affect operation (or if it does that it fails safe), and that it does not emit interference by conduction or radiation. Compliance to the appropriate standards will need to be independently tested and certified.

That's what "automotive grade" means (and more), so if this is a commercial project that's what you will need to tackle. If this is a home brew project, you can do what you like - but don't claim it is automotive grade.
Military and Automotive Electronics Design Engineer (retired)

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mbt28
Posts: 29
Joined: Wed Feb 04, 2015 10:35 am

Re: Automotive grade power supply

Mon May 28, 2018 8:00 pm

Z80 Refugee wrote:
Mon May 28, 2018 6:41 pm
This won't be easy. At the input side, you need to accommodate potential accidental reverse polarity (somebody connects a battery the wrong way around), voltage dips and spikes, and "load dump" (the alternator output suddenly losing regulation and reaching 60V or more). You need to ensure the unit cannot drain the battery when the ignition is off.
I have a plan for power-on/power-off according to ignition status. I am planning to watch linbus+ignition+key_in cables to decide power on and off. This will be done by a micro-controller.

However voltage dips, spikes and load dumps I dont know how to create a power supply accordingly. I couldnt find schematics for them. Also I couldnt find good resource too. My problem is the basic electronics for buck converters I just started to learn their characteristics. I ordered some buck converters from china to use in my project but I dont think they are reliable to use with car. I will use them only at home to make experiments.

Please if you know any resource about how to design proper power supply share with me.
Z80 Refugee wrote:
Mon May 28, 2018 6:41 pm
Internally, you will need to ensure the unit complies with automotive standards for Electromagnetic Compatibility - that a certain level of external conducted or radiated interference over a specific frequency range doesn't adversely affect operation (or if it does that it fails safe), and that it does not emit interference by conduction or radiation. Compliance to the appropriate standards will need to be independently tested and certified.
I dont plan it to make a final product which will sold to people seriously. I am doing it for myself, but I may sell few to people. In anyway I will share all the codes and hardware schematics. So nothing that serious.
Z80 Refugee wrote:
Mon May 28, 2018 6:41 pm
That's what "automotive grade" means (and more), so if this is a commercial project that's what you will need to tackle. If this is a home brew project, you can do what you like - but don't claim it is automotive grade.
I meant automotive grade because I want a power supply which can work with car battery, sorry if it means further than I tried to tell.

So what would be your suggestion which way I should start? Try to make seperate converters for 3.3, 1.8 and 5V or first regulate 12V as safe 12 then convert to other voltages?

Thanks

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Z80 Refugee
Posts: 358
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Re: Automotive grade power supply

Mon May 28, 2018 9:19 pm

Okay, well as long as you understand this won't cut the mustard commercially and you (and any friends you pass it on to) understand you will be operating very much at your own risk (something to be carefully considered when dealing with potentially lethal machinery, in public, and in a situation that may invalidate your insurance)...

I'm not clear where your requirements for 3,3V and 1.8V come from (you don't need them for powering an RPi), but assuming you know what you're doing:

The first thing is to defend the nominal 12V input from the electrical abuse that can be present on a car battery supply. Start with a quick-blow fuse, followed by a diode and then a very beefy 12V Zener diode to 0V. That will filter out most nonsense, and blow the fuse if it's too much. Now you can feed whatever regulators you want to use. It would probably be appropriate to put a big capacitor on the supply to the regulators, and then use a separate regulator for each voltage rail you want.

Strange you say you can't find any information. I googled "designing power supplies for automotive environments" and immediately came up with a lot of information - eg http://www.st.com/resource/en/applicati ... 181783.pdf
Military and Automotive Electronics Design Engineer (retired)

For the best service: make your thread title properly descriptive, and put all relevant details in the first post (including links - don't make us search)!

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