Author Topic: Circuit to convert from 12V automotive into a microcontroller Voltage?  (Read 8959 times)

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Offline Yansi

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Well of course, that a 30+V input (better 40+V) sustainig LDO is more than enough for automotive 12V application in a hobby project.

And yes of course the phone chargers and stuff are heaps of garbage. But they at least can withstand at least 30V.
 

Online malagas_on_fire

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Normaly in Automotive the capacitors are rated for 100V and even more when it electric cars comes in to place.

Chech this paper for more details if using the AEC Q200 for passive components:

http://www.knowlescapacitors.com/getattachment/69a6c2c9-aa2d-47b1-a464-2cf1518cf4c1/AEC-Q200.aspx

The new AEC Q200 for passive components only applies for newer cars, i believe june 2010 and the examples on the paper result in 250% of the widthstand dieletric voltage and it applies for internal car and maybe not on the 12V charger / cigarrette lighter. Yes 25V is OK for that plug.
If one can make knowledge flow than it will go from negative to positve , for real
 

Offline jwhitmoreTopic starter

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This ain't Drama it's an education! Thanks for the links Yansi, I'm going to have to go through them in detail, and thanks for your descriptions Ian.M. You're correct in thinking that 3Amps is probably excessive and I might pull in the reigns a tad. I am switching higher current things on and off but at present I'm using Infineon Profets for that. So on the 5V rail there's not a lot of load. I guess I could say that the load on the 5V Rail would be less then an Amp. But as soon as I say that I'll hit some problem space that bites me. But hell I'll cross that bridge later on. (The 12V/14V supply load dump hitting a PROFET is another thing I should have a look at.)

So I think the first thing to do is dump my current/present SMPS and go with a 40V rated LMR14030 (http://www.ti.com/lit/ds/symlink/lmr14030.pdf). That might go some way to a solution. Then a lot more reading might get me a better TVS solution. And as for over engineering I currently don't know enough to tell whether I am or not. I'm not sure that this is a commercial product as yet, but who knows down the road, it may be. On top of all that this ain't for a phone charger or a audio system. It's actually for agricultural tractor machine thingy. The things that make the food and cost heap big monies. So things failing for Mr. Farmer ain't like your mobile phone charger blew. I'm not saying this is safety critical but I like to thing that it's above trivial.

thanks again for all the info!
 

Offline jwhitmoreTopic starter

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Quote
500mA @5V out is 2.5W.   Assuming 90% converter efficiency (probably a bit optimistic), that's about 2.8W which @14V in is 200mA.   That's getting into the region where a pulse rated resistor isn't going to be cost-effective.  Even if the MCU board only needs another 500mA (@5V), you'll probably run into trouble with the resistor, but lets run with 5.6W in for the sake of argument.

You *MUST* limit the TPSMA6L15A input voltage at no more than 20V (to stay under its 24V Abs. Max. rating and keep within its working range).  Lets say you need 5.6W into the switching regulator. You don't want more than about 1V drop across the resistor in normal operation, to keep the dissipation reasonably low and the efficiency reasonably high.  5.6W @ (14V -1V) is 0.43A, so you can't make the resistor over 2.3 ohms.   

If you run a sim with a worst case ISO 7637-2 Pulse 5B #  :popcorn: , with the vehicle's central clamping system voltage parameter Vs* set to 35V, into a 2.2 ohm resistor feeding a 5.6W constant power load, with 1000uF of bulk input capacitance clamped at 20V, you get a peak dissipation of 68W and about 22.5 joules dissipated in the resistor during the pulse.   To be certain your generic wirewound resistor survives that, its going to need to be a 14W one (x5 overload factor).   If you get a brand name pulse rated one  (x10 overload for 1s), you could use a 7W one.   If you dig deep into the manufacturers datasheets for I2t ratings, as the total overload duration is only about 400ms, you can probably find a 5W one that will survive and *MAY* even find a 3W one.  :-+

However if you test against a worst case ISO 7637 pulse 5A,  :popcorn: (that's the one with no clamping), the peak dissipation is 755W  and about 140 joules dissipated in the resistor, so the odds are your resistor's element is now a rapidly expanding cloud of metal plasma  and its body is hot ceramic shrapnel bouncing around inside your device.    :scared:  :--

YMMV with the results above as I cant run LTspice XVII to use its ISO 7637-2 pulse models I'm stuck with LTspice IV so have to use my carefully hand crufted ISO 7637-2 Pulse 5A and 5B sim.  Its as close as I can get it to the spec. but hasn't been independently verified.  Sim attached.

Also its *really* difficult to do high energy clamping at such a low voltage and still pass the x1.5 sustained input overvoltage test.  You need a TVS that's totally off under 18V, with a worst case clamping voltage while carrying the maximum possible peak current the supply can produce (which is equal to the maximum output current the alternator can produce unless you can limit it with a series resistor) that's under 24V.   If the clamping voltage is too high, your switching regulator fries.  If its rated holdoff voltage is too low, the TVS diode can fail during a jump start or if the alternator regulator fails and overvoltages the whole electrical system.  If you are lucky it fails shorted and the supply fuse to your device will blow. If you are unlucky, the TVS diode ruptures and no more protection.

TLDR: Above a couple of watts of load, series resistor + shunt clamping is impractical for such a low clamping voltage.  You'll need to use series MOSFET + over-voltage disconnect controller protection.   Stick a small 100V TVS diode shunting the power in in front of the MOSFET to tame fast HV spikes to what the MOSFET can handle.

Thanks for correcting my calculations and forgive me for throwing more at you but just to check I'm on the right track...

If I change to a switcher like the LMR14030 (http://www.ti.com/lit/ds/symlink/lmr14030.pdf) it's good for an input voltage of 40V. Given that switcher, I can then change the TVS from the one I'm currently using, the TPSMA6L15A (http://www.littelfuse.com/~/media/electronics/datasheets/tvs_diodes/littelfuse_tvs_diode_tpsma6l_datasheet.pdf.pdf) to use another one on that datasheet, for example.

The TPSMA6L36A has a Reverse Stand off Voltage of 36V. A breakdown voltage Vbr of min 40V Max 44.2V and a Maximum clamping voltage of 58.1V,  Maximum peak pulse current of 10.4 Amps

So those two elements combined with your 2.2 Ohm resistor  and the vehicle's clamping at 35V, from your example. So in that case the resistor wouldn't be required to have excessive spec. If I have this right my Voltage input would be in parallel with TVS, 100V Cap and then a series 2.2 Ohm resistor before it reaches the switcher. I'd then be good for a voltage spike of 58.1 Volts? That's nowhere near the ISO 7637 pulse 5A, 755W, but I figure I'll still have a cloud of metal plasma. I'll have to re-read this thread again just to check what I've missed. I'll go and have a look at FET circuit.

One final point on vehicle maintenance, which has had the finger of suspicion pointed at it in this thread. There has been the suggestion that if the battery is maintained, and all the connections then there should not be a problem. Speaking from, my limited, experience the hardest thing to track and most common problem is bad earth connections to the vehicle chassis. I'm not sure that can be ignored and called something that should not happen. It just does. Be great if there was a sensor of some sort that warned you that spikes are starting to occur and you should check your connections and battery.
 

Offline Ian.M

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LMR14030 Abs Max input voltage: 44V
TPSMA6L36A Max breakdown voltage: 44.2V
TPSMA6L36A Max clamping voltage: 58.1V

Do you see the problem?   The TPSMA6L36A doesn't clamp low enough to protect the LMR14030.

You'd need to either go down to a TPSMA6L26A to get the clamping voltage under 44V:-  Vr: 26.0V, Vbr_min: 28.90V, Vbr_max: 31.90V  Vc: 42.1V @14.3A,
or add a MOSFET over-voltage disconnect circuit after the TVS and before the bulk capacitor.

The resistor needs to be in series with the feed, before the TVS as its purpose is to limit the energy into the TVS.   Without a resistor, if the TVS clamping voltage is below that of the central protection it has to be able to handle anything up to the full worst case load dump (pulse 5B) with the only limits on the pulse energy being the max alternator output current and the wiring resistance.


If you are building this as a permanently installed one-off, you can check the specs for the central clamping used in the specific vehicle and design the protection based on that clamping voltage.  However if its a portable device you need worst case any vehicle protection.

There's also the matter of the value of the device and anything attached to it.  So far we have an android phone, a uC board and unspecified peripheral boards.   Put a dollar value on that lot and estimate what its worth spending on protection circuits to insure against damage due to the rare fault of your battery ground strap or terminals working loose while driving along.  (If either are loose initially, odds are you'll have starting problems., so you wont get as far as causing a load dump)

One final point on vehicle maintenance, which has had the finger of suspicion pointed at it in this thread. There has been the suggestion that if the battery is maintained, and all the connections then there should not be a problem. Speaking from, my limited, experience the hardest thing to track and most common problem is bad earth connections to the vehicle chassis. I'm not sure that can be ignored and called something that should not happen. It just does. Be great if there was a sensor of some sort that warned you that spikes are starting to occur and you should check your connections and battery.

The nightmare one is the ground strap from the engine block or bellhousing to the chassis.   The engine itself will be on rubber mounts, so that strap flexes every time you accelerate, brake, corner or shift gear.  Eventually it will fatigue, and if it breaks while running  with a high load on the electrical system  *BAD* things happen.   Its often in a really awkward place to get to to inspect, and with the usual dirt buildup you are unlikely to spot its got broken strands before it totally fails even if you are absolutely anal about maintenance.  If you are lucky, the strands will break gradually a few at a time and when it gets too weak it will pop like a fuse one morning when you try to crank the starter.
« Last Edit: June 16, 2018, 11:55:40 pm by Ian.M »
 

Offline jwhitmoreTopic starter

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Oops! I'm confusing the breakdown and clamping voltage specification. I should be looking at the clamping voltage.

I finally got around to going through the links that Yansi sent. The first one (http://www.ti.com/lit/an/snva717/snva717.pdf) made things even worse:

Quote
In particular there are long power supply lines that feed from the power distribution box in the engine compartment to the distant corners of the vehicle. Because of the inductive characteristic of long leads there are even higher transient levels than those that occur during load dump. The governing specification for tail light electronics is that they be able to withstand transients of +100V and -300V. This is a foreboding challenge for IC based electronics such as LED tail light regulators.

Fortunately the second link (http://www.ti.com/lit/an/snva681a/snva681a.pdf) gave me the LM5088, a 75V SMPS. So if I combine that with the TPSMA6L43A and a resistor which can take a little power it should offer some protection.

After that I should investigate the MOSFET over-voltage disconnect circuit mentioned. I've seen P Channel FETs use to protect against reverse voltage but not sure about disconnect. I'll check the Internet.

On the question of the value of the device and anything attached to it it's the tractor and machinery that is of most value. If the electronic device and Android phone got toasted that would be very small potatoes compared to a fire killing a tractor or attached machine.
 
The following users thanked this post: dario200

Offline dario200

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Hi @jwhitmore!

I know it's been two years from your last post, but I would like to know if you had the chance to finish your tests and come up with a good power supply protected circuit. I'm in a similar situation where I have to put a microcontroller in a vehicle turning on and off some actuators. I was also planning on using Infineon's Profets as a replacement for common or SSR relays to drive the solenoids. Like you were, I'm concerned about the load dump damaging the step down DC regulator as well as the Profets. Could you please share your experience and perhaps the schematics and components you used?

Thanks in advance.
Dario
 

Offline John B

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I've had 2 arduino devices running in my car for several years now without any issues. It's relatively easy if the current demands are low, ie ~50mA. I protect the 3.3/5V supply with a series input resistance, large capacitance to make an RC filter, and diode clamping. All digital inputs on the controller, or opamps should be protected with a series resistance and bipolar clamping too. I usually add come capacitance to minimise HF noise. Bandwidth or risetime is not usually a concern. I have inputs connected to switches, sensors and solenoids and have never had a failure.

If the micro is switching some other circuit, it's probably a good idea to maintain some type of isolation, ie relays etc.
 

Offline Rick Law

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Unfortunately, it is a stupid idea, because automotive signal voltages are often *very* noisy and 100V noise spikes will kill the OPAMP very quickly - in fact it will probably die at the first engine start.
...

Not that I don't believe you, my question came from my inexperienced with the auto environment and seeking to understand.

Where would the noise and spikes come from?  Just the generator?

40V is a lot for a generator to spike to, it will need quite a sudden increase in RPM.  I can see that happening during gear change.  But I can't imagine it spike up as much as 100V.  Expectation adjustment time for me?

This is also related to an issue I was puzzling with -- A relative of mine had a battery maintainer/charger which puts out 15.5V @ 8A (according to it's spec).  I was thinking about getting one myself.  Since auto's electronics are designed to deal with > 15V sustained (or am I wrong?), why would that charger specifies one should/must (forgot the wording) disconnect the battery from the car while charging.  That damn thing can't spike that much.  Perhaps just CYA?
 

Offline eblc1388

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...why would that charger specifies one should/must (forgot the wording) disconnect the battery from the car while charging.  That damn thing can't spike that much.  Perhaps just CYA?

I think every sensible manufacturer will do that to avoid taking responsibility of faulty charger damaging car electronics. As the manufacturer simply cannot guarantee the charger's output under faulty condition, limiting the claim to a faulty battery is way much better.
 

Offline David Hess

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40V is a lot for a generator to spike to, it will need quite a sudden increase in RPM.  I can see that happening during gear change.  But I can't imagine it spike up as much as 100V.  Expectation adjustment time for me?

A change in RPM is not the problem.  The alternator output is inductive and like an inductor, produces an inductive flyback while trying to maintain a constant output current if the load is removed.

Quote
This is also related to an issue I was puzzling with -- A relative of mine had a battery maintainer/charger which puts out 15.5V @ 8A (according to it's spec).  I was thinking about getting one myself.  Since auto's electronics are designed to deal with > 15V sustained (or am I wrong?), why would that charger specifies one should/must (forgot the wording) disconnect the battery from the car while charging.  That damn thing can't spike that much.  Perhaps just CYA?

The charger might not be rated for continuous operation with a constant load.

I think a good charger should operate at its rated load continuously but there are a lot of bad chargers out there.
 

Offline Rick Law

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...why would that charger specifies one should/must (forgot the wording) disconnect the battery from the car while charging.  That damn thing can't spike that much.  Perhaps just CYA?

I think every sensible manufacturer will do that to avoid taking responsibility of faulty charger damaging car electronics. As the manufacturer simply cannot guarantee the charger's output under faulty condition, limiting the claim to a faulty battery is way much better.

Yeah, you are right.  It was a senior moment for me.   With such disclaimer, if you try to claim damage for anything else... well, you were suppose have have those other things disconnected.  Who in the right mind would not CYA these days.

...
...
Quote
This is also related to an issue I was puzzling with -- A relative of mine had a battery maintainer/charger which puts out 15.5V @ 8A (according to it's spec).  I was thinking about getting one myself.  Since auto's electronics are designed to deal with > 15V sustained (or am I wrong?), why would that charger specifies one should/must (forgot the wording) disconnect the battery from the car while charging.  That damn thing can't spike that much.  Perhaps just CYA?

The charger might not be rated for continuous operation with a constant load.

I think a good charger should operate at its rated load continuously but there are a lot of bad chargers out there.


re: "... but there are a lot of bad chargers out there."

Yeah, that is what is stopping me from buying a new charger.  On the one hand, I like to update my 1.5Amp charger to something faster; on the other hand, looking at some of the specs and user-guides out there, it make you wonder what exactly you are getting.

Some of them should have this sticker:
"Warning - opening this device will void your worthless warranty and may cause you severe disappointment."
 


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