Automotive device power supply design. Do i need TVS and resistors? - Page 1
Forum section: electronics / projects

author: _dx
author_rank: Newbie
timestamp: November 01, 2015, 06:03:52 pm
content: 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 paranodOther options?

author: georges80
author_rank: Frequent Contributor
timestamp: November 01, 2015, 06:29:22 pm
content: 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.

author: steve207a
author_rank: Contributor
timestamp: November 01, 2015, 06:54:05 pm
content: hiyour 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

author: eneuro
author_rank: Super Contributor
timestamp: November 01, 2015, 07:27:53 pm
content: Quote from: _dx on November 01, 2015, 06:03:52 pmNot 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

author: _dx
author_rank: Newbie
timestamp: November 01, 2015, 08:01:58 pm
content: QuoteThank you for sharing your experience.QuoteSo 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.QuoteNot 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.

author: georges80
author_rank: Frequent Contributor
timestamp: November 01, 2015, 08:17:07 pm
content: 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.

author: _dx
author_rank: Newbie
timestamp: November 01, 2015, 08:38:32 pm
content: 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.

author: georges80
author_rank: Frequent Contributor
timestamp: November 01, 2015, 09:00:10 pm
content: 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 quicklycheers,george.

author: _dx
author_rank: Newbie
timestamp: November 01, 2015, 09:08:41 pm
content: Ooh, now i seeWill use TVS with higher Vbr, no resistors.