Author Topic: Automotive device power supply design. Do i need TVS and resistors?  (Read 3452 times)

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Offline _dx

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Hi everyone!

I am working on little ECU to control old car petrol engine(ECU for doing microprocessor ignition advancing).
It should survive load dump, jump start and other transients present in car power system.

My current version of schematic is in attachment(little jpg)
LM2931 is a very good voltage regulator, it has a lot of protections(http://www.ti.com/lit/ds/symlink/lm2931-n.pdf)
Aditionally we have TVS(D3) and two 1206 6.2Ohm resistors(R29,R10) to limit voltage and  current.

But resistors will be heavy loaded during transients and i am not sure they will survive for a long time...
It seems to be SMD resistors are not so good at high pulsed currents.

So may be i should avoid using resistors and let TVS do the job? But i am afraid SMBJ can't absorb all load dump puse energy by itself and may be damaged.
Device consuming <100mA and has relatively small PCB and cost, so i don't want to use more powerfull TVS(if it is possible).

What should i do?
Use more durable resistors(may be not SMD)
Use bigger TVS and no resistors.
All be fine with current schematic and i am paranod  :)
Other options?
 

Online georges80

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Re: Automotive device power supply design. Do i need TVS and resistors?
« Reply #1 on: November 01, 2015, 06:29:22 pm »
I've used the LM2931 in many vehicle projects. Several 1000 pieces of a couple of designs are out in the field and most on motorbikes which can have quite nasty electrical systems. Never had any failures/returns/issues. I'd say the LM2931 on its own is very rugged, no need for resistors/zeners etc. A bidir TVS at the input would be all I would consider adding (in the 30V+ range to survive dual battery jump starts) and that would take the edge off any really nasty spikes. A common mode choke may be a good idea if you have conducted emissions 'junk' from your 'ECU' and don't want to wipe out radio gear.

Finally, a fuse to protect wiring etc from a catastrophic board failure.

cheers,
george.
 

Offline steve207a

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Re: Automotive device power supply design. Do i need TVS and resistors?
« Reply #2 on: November 01, 2015, 06:54:05 pm »
hi
your main problem is spikes on the power rails and proper screening to limit glitches extra decoupling around the pcb wont do no harm build a protatype and sit it close to the ht cables  nothing like real world to test  even the best thought out  design
 

Offline eneuro

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Re: Automotive device power supply design. Do i need TVS and resistors?
« Reply #3 on: November 01, 2015, 07:27:53 pm »
...
It should survive load dump, jump start and other transients present in car power system.
...

Not sure about this from LM2931-N datasheet-it might be issue depending on application and maybe better  solution add this regulator as first stage than another LDO which will take its output and provide CONTINOUS regulation, while not sure what this shut down in this regulator means eg. in the case when we'll have at  regular basis some kind of strange voltage spikes eg. from ignition etc-LM2931-N does  not provide any regulation during this transients  shut downs?  :-//
It could be issue if we wanted our circuit work without any shut  down delays-it looks like it can protect our circuit from damage but transient, but such shut down might be disaster, MPU restart etc...
I didn't used this LM2931-N so far but I'm interested in one of my recent automotive projects which require less than 100mA, but any shutdowns are not accepted...

Quote
During line transients, such as a load dump (60V) when the input voltage to the regulator can momentarily exceed the specified maximum operating voltage, the regulator will automatically shut down to protect both internal circuits and the load.
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“Let the future tell the truth, and evaluate each one according to his work and accomplishments. The present is theirs; the future, for which I have really worked, is mine”  - Nikola Tesla
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Offline _dx

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Re: Automotive device power supply design. Do i need TVS and resistors?
« Reply #4 on: November 01, 2015, 08:01:58 pm »
Quote
I've used the LM2931 in many vehicle projects....
Thank you for sharing your experience.

Quote
A bidir TVS at the input would be all I would consider adding (in the 30V+ range to survive dual battery jump starts)
So SMBJ18 is not enough highvoltage? It has 20Vbr and a Vcl ~40V at 100A.
Seems to be bidir TVS can kill polar electrolite capacitor on input. Am i right? Actually this is the main reason why i choose unidir, but maybe i was wrong.

Quote
didn't used this LM2931-N so far but I'm interested in one of my recent automotive projects which require less than 100mA, but any shutdowns are not accepted...
Not a problem. Output capacitor should be big enough to supply your circuit during cutoff period. The longest transient is load dump condition(about 100ms). Input capacitor also should be taken into account, because it will absorb some charge before input voltage can raise to ~30V and trigger LM2931 protection mode.
 

Online georges80

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Re: Automotive device power supply design. Do i need TVS and resistors?
« Reply #5 on: November 01, 2015, 08:17:07 pm »
If you have an electro/polarised cap then you should put reverse polarity protection at the input - before the cap.

Two battery jump start is around 28V, so you need whatever front end protection is there to NOT clamp until 30V+.

I've used ceramic front end caps with the 2931 so I don't worry about polarity issues and the 2931 will protect against reverse polarity on its own. Put the polarised electro/tantalum etc on the output side of the 2931 - it'll be protected then.

cheers,
george.
 

Offline _dx

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Re: Automotive device power supply design. Do i need TVS and resistors?
« Reply #6 on: November 01, 2015, 08:38:32 pm »
Ok, thank you agaim and  hope this will be my last question in the thread: why 28V? AFAIK jump-start is a parallel connection of two batteries.
Two 12V bat will give us about 28 when connected in series, not parallel... Parallel connection should not affect voltage. I can assume only some kind of load dump when engine is started and second battery is disconnected.
 

Online georges80

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Re: Automotive device power supply design. Do i need TVS and resistors?
« Reply #7 on: November 01, 2015, 09:00:10 pm »
Parallel - tee hee... Yeah, EXCEPT when the bloke comes along in his emergency truck to jump start you on the side of the road... Won't be the first time that they jump from the SERIES connected batteries (24V nominal)... Starts things quickly :)

cheers,
george.
 

Offline _dx

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Re: Automotive device power supply design. Do i need TVS and resistors?
« Reply #8 on: November 01, 2015, 09:08:41 pm »
Ooh, now i see  :) :) :)
Will use TVS with higher Vbr, no resistors.
 


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