power supply circuit design

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And here I always thought the current flowed faster if the wall of the dam was higher...

 

Ross McKenzie ValuSoft Melbourne Australia

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The problem with load dump is every carmaker has their own specification. If you use a varistor or transzorb, you have to carefully calculate the energy involved and ensure it survives. The other strategy is to choose parts that tolerate the voltage spike. Specific auto regulators like the 2954 say they can. Another strategy is to use some transistors and a zener diode that turn off if the voltage gets too high. That way you 'bend with the wind' rather than fight it.
It is silly to go to 1V8 - i've not seen anything automotive that does this. If the voltage drops during cranking, then have an undervoltage lockout. Adopt the 'three bears' philosophy - you want the voltage to be 'just right' otherwise you do nothing. In the case of undervoltage your load is useless. In the case of overvoltage, turning off the load protects it.

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Kartman wrote:
The problem with load dump is every carmaker has their own specification.

...

Specific auto regulators like the 2954 say they can.

...

If the voltage drops during cranking, then have an undervoltage lockout.

...

MCP1791 is a 70mA LDO that meets loaded Ford test pulse G for a load dump, and it has a power good signal output (active high, 90%Vout typ)

http://www.microchip.com/wwwproducts/en/MCP1791

 

"Dare to be naïve." - Buckminster Fuller

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I'm attaching the image of my new power supply design here from my phone. I'll go back to my PC to add some info...

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You have load dump diode in series with your pcb gnd...NO (I don't think you want that)...you don't have a solid gnd...your high currentload then drops (heat) through bottom diodes

 

add 0.1uf cap at micro  (or 470nf)

 

put 1k  between micro & fet gate

often good idea to put zener between gate & source to protect gate..if gate is rated for 20v use 12v zener

 

c1 & c2??? why series

When in the dark remember-the future looks brighter than ever.

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I stuck with the relay because I know it will survive and because I am going to use this device (once I've got a version that is tested and known working) in multiple vehicle. One will have a small 1.2A LED light strip as the load and one will have a large 18A LED light assembly as a load. 

relay: FTR-P6GN012WA: http://www.mouser.com/ds/2/164/f...

 

I went with more substantial reverse connection protection diodes that are fast switching and rated for automotive use up to 60A forward current.

diodes: IDB15E60ATMA1: http://www.mouser.com/ds/2/196/I...

 

I went with a protective varistor from Littlefuse specifically designed to handle load dumps up to 6J of energy in automotive applications.

MLV: V18AUMLA1812H: http://www.mouser.com/ds/2/240/L...

I chose the capacitors to accompany the varistor  based off of a design documents from Texas Instruments.

C1 & C2: CGA2B3X7R1H104M050BB: http://www.mouser.com/ds/2/400/l...

 

I also chose the the capacitors and inductor for the LC filter section of the circuit based off of the same design documents from Texas Instruments.

C3: CGA9N3X7R1H106K230KB: http://www.mouser.com/ds/2/400/l...

LC filter inductor: XAL5030-222MEB: http://www.mouser.com/ds/2/597/x...

C4: EEE-FK1H470XP: http://www.mouser.com/ds/2/315/A...

 

The voltage regulator is also rated for automotive use, and I chose the input and output caps using the datasheet for the regulator.

input cap C5: UCZ1H471MNQ1MS: http://www.mouser.com/ds/2/293/e...

input cap C6: VJ1206Y474KXATW1BC: http://www.mouser.com/ds/2/427/v...

regulator: TLS850D0TEV50ATMA1: http://www.mouser.com/ds/2/196/I...

output cap C7: C0805C225K8RACAUTO: http://www.mouser.com/ds/2/212/K...

 

I feel like the power supply circuit design is good enough now in the "it should work" department that I am comfortable prototyping, which is what I was after.

 

It may actually be over done, since the regulator is supposed to be rated to handle an automotive electrical environment, but I'd rather be overly cautious than way under build it. 

 

If anyone sees that I have made any more blundering mistakes that scream "no way that's going to survive or allow the MCU to survive" I would appreciate any feedback.

 

Thanks again!

 

 

 

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avrcandies wrote:

You have load dump diode in series with your pcb gnd...NO (I don't think you want that)...you don't have a solid gnd...your high currentload then drops (heat) through bottom diodes

 

add 0.1uf cap at micro  (or 470nf)

 

put 1k  between micro & fet gate

often good idea to put zener between gate & source to protect gate..if gate is rated for 20v use 12v zener

 

c1 & c2??? why series

 

I put the diode on the ground side to protect from the device being connected in reverse polarity by mistake. I guess I don't understand why that's a bad idea? Not arguing, but I would like to understand.

 

There will be 100nF caps at the MCU power pins, I was just working on the circuit in segments.

 

Roger that on the resistors for the FET.

 

C1 and C2 being is series was specifically recommended in the TI design document that I read about automotive power supply circuit design (it stated that it was for a "back up" in case one failed). It went further to recommend that they were mounted at 90 degrees to each other on the PCB.

Last Edited: Tue. Aug 29, 2017 - 05:25 PM
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EDIT: I think I see what you were saying about that diode. If I just don't run the external load ground through it I think I can use a much smaller diode in that position. Fixing schematic to re-post now...

 

New ground side protective diode (2.5A forward current): BAS21-E3-18http://www.mouser.com/ds/2/427/b...

 

Last Edited: Tue. Aug 29, 2017 - 05:33 PM
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New schematic...

 

EDIT: also, the zener is 18V because I could not find a 12V SMD zener rated for enough current to operate the relay coil. I'll keep looking...

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Last Edited: Tue. Aug 29, 2017 - 05:21 PM
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EDIT: also, the zener is 18V because I could not find a 12V SMD zener rated for enough current to operate the relay coil. I'll keep looking...

NO --the zener goes direct from the fet gate to fet source (GND)..it should kick in before the max tolerable gate voltage is reached (many fets gates rated for 15 or 20v, some only a few volts) 

Normally the zener sits there and does nothing, it is only for protection of the gate

When in the dark remember-the future looks brighter than ever.

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Unless I've completely lost my mind the FET gate is receiving a 5V signal from an MCU output pin? 

I would think if 12V (the gate-source voltage of that FET) got that far along through the MCU my whole device would be shot and the FET would be the least of my worries?

Or I'm just wrong again, wouldn't be shocking...

Last Edited: Tue. Aug 29, 2017 - 05:45 PM
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Yes 5V, so unless there is a transient, or static zap of some sort, the zener protects nothing.  probably then an 8V zener is great, just never allow the gate voltage to exceed the rating under any condition

 

gates can tolerate ZERO transients--they will pop like a balloon

 

SEE THIS PART FOR SOME IDEAS  !!!!!!!!!!!!!

 

http://www.onsemi.com/pub/Collateral/NUD3124-D.PDF

When in the dark remember-the future looks brighter than ever.

Last Edited: Tue. Aug 29, 2017 - 05:50 PM
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Ok, cool.

So, anything from 8-10V should suffice?

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AN EXAMPLE (with even more bells and whistles)

 

http://www.onsemi.com/pub/Collateral/NUD3124-D.PDF

When in the dark remember-the future looks brighter than ever.

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Last Edited: Tue. Aug 29, 2017 - 06:01 PM
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avrcandies wrote:

AN EXAMPLE (with even more bells and whistles)

 

http://www.onsemi.com/pub/Collateral/NUD3124-D.PDF

 

That looks great, like it would eliminate both of the resistors and the zener diode at the FET, as well as the flyback diode, But...

Where am I over looking the gate-source threshold voltage on the datasheet?

 

NVM - I found it, 1.3-2.0V.

Nice!

That'll save me some board space...

Last Edited: Tue. Aug 29, 2017 - 06:46 PM
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LsD wrote:

Something like this?

 

BZX84C8V2LT1Ghttp://www.mouser.com/ds/2/308/B...

 

I could be wrong, but I think the zener is pointing the wrong way?

Jim

 

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ki0bk wrote:

LsD wrote:

Something like this?

 

BZX84C8V2LT1Ghttp://www.mouser.com/ds/2/308/B...

 

I could be wrong, but I think the zener is pointing the wrong way?

Jim

 

 

 

DOH! (faceplam)

 

 

EDIT: I fixed it, but I didn't re-post the schematic again just for that.

Last Edited: Tue. Aug 29, 2017 - 06:38 PM
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Getting better...

I'm starting to wonder if I'm spinning my wheels with the MLV and the LC filter with all of these "automotive rated" components?

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I feel like the power supply circuit design is good enough now in the "it should work" department that I am comfortable prototyping, which is what I was after.

 

I'll add a comment or two, but I'm definitely not an expert...

 

Using a relay just because you are switching an 18 A load is unnecessary.

A power MosFet, (IgFet), and I'm sure other devices can handle this just fine.

 

The board below, a vehicle project, has 8 channels each rated at 20 A, (The Fet can handle far more than that, actually).

 

One 40 A (or whatever it was) load used two channels in parallel.

 

For higher current carrying PCBs recall that most PCB's are done in "1 Oz" copper thickness.

For heavier currents one can order the PCB with "2 Oz" copper thickness, for additional current handling capability.

 

For this PCB the high current traces, on the 2 Oz copper board, are mirrored on the back side of the PCB as well, with an occasional via between the layers.

 

I'm sure there are other / better ways to do this, but it worked fine for many years of heavy duty vehicle service.

 

JC

 

 

 

Edit: typo

 

Last Edited: Wed. Aug 30, 2017 - 02:37 AM
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DocJC wrote:
A power MosFet, (IgFet), and I'm sure other devices can handle this just fine.
Protected MOSFETs can survive a short circuit; one low-side FET is rated for 33 amps in a 24V circuit (buses, RVs, medium trucks, semi-tractors)

ON-Semiconductor

ON Semiconductor

Protected MOSFETs

http://www.onsemi.com/PowerSolutions/parametrics.do?id=819

 

"Dare to be naïve." - Buckminster Fuller

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I think this is pretty good now.

 

polarity protection diode: http://www.mouser.com/ds/2/196/I...

voltage regulator: http://www.mouser.com/ds/2/196/I...

switching FET: http://www.onsemi.com/pub/Collat...

 

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Last Edited: Wed. Aug 30, 2017 - 02:42 PM
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You will burn up you inductor & probably IDB50e6

Don't draw your 15-20 ampload there.

diode drp would be 1.5V  1.5V*20 =30Wattss....a hughe power loss & heat

Inductor prob good for 1-2 amp max (don't know what you use)

When in the dark remember-the future looks brighter than ever.

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I don't know why I did that, sloppy mouse work?

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Your diode will likely burn up if you draw 15-20 amp...will be very hot, or large copper area needed, diode rated for 15, not 20

 

why drop from 12 to 10-10.5 volts?  Leds dimmer 

When in the dark remember-the future looks brighter than ever.

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avrcandies wrote:

You will burn up you inductor & probably IDB50e6

Don't draw your 15-20 ampload there.

diode drp would be 1.5V  1.5V*20 =30Wattss....a hughe power loss & heat

Inductor prob good for 1-2 amp max (don't know what you use)

 

I just re-read your comment and saw the part I bolded above.

I assumed that I needed to protect the (S-D side of the ) FET from load dump voltage as well. Or at least that it wouldn't be a bad idea. I know it's supposed to be "self protected", but I figured adding extra protection wouldn't hurt.

Is that line of thinking total over kill? Should I power the load and ground it (through the FET) without any additional protection?

Last Edited: Wed. Aug 30, 2017 - 03:46 PM
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avrcandies wrote:

Your diode will likely burn up if you draw 15-20 amp...will be very hot, or large copper area needed, diode rated for 15, not 20

 

why drop from 12 to 10-10.5 volts?  Leds dimmer 

 

If I read the data sheet correctly, running both sides together in parrallel gives you just under 30A forward current. That's why I chose that diode.

 

But, if I shouldn't even run the load (and subsequently the S-D side of the FET) through the diode, then I can select a much smaller diode?

EDIT: like this one: http://www.mouser.com/ds/2/427/r...

 

 

EDIT 2: I can also pull my head out of my bum and pull that power after the load dump protection and before the reverse polarity diode...

(and still use a smaller diode)

Last Edited: Wed. Aug 30, 2017 - 03:55 PM
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I promise I'm not an idiot. I just get carried away and do things without thinking them through sometimes...

 

EDIT: like deleting the line from the MCU to the FET gate on that last image...

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Last Edited: Wed. Aug 30, 2017 - 04:08 PM
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If I do that I should probably put the fuse before the diode...

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Last Edited: Wed. Aug 30, 2017 - 04:29 PM
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Early on i suggested a protected high side mosfet. If the switched signal is going beyond your circuit board, then you want to switch high side. Why? Consider where the current is flowing. Just about everything in a vehicle is high side switched.

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Kartman wrote:
Early on i suggested a protected high side mosfet. If the switched signal is going beyond your circuit board, then you want to switch high side. Why? Consider where the current is flowing. Just about everything in a vehicle is high side switched.

 

Care to elaborate?

 

If there's some legitimate technical advantage to high side switching in this case I would love to hear it. Information is always good and I'm always happy to learn. :)

 

I have a habit of installing automotive electrical accessory circuits with low side switching. But, those are simple electrical circuits, not electronic devices.

 

I am obviously not an EE and my experience level is very much at the beginner level and well below most of the people on here. But, it is my understanding that the current in a DC circuit flows from the source, through the entire circuit, and back to the source. 

 

I am aware that the circuit through the device and to the switch is always "hot" in a low side switched circuit. But that only becomes an issue if you have a short or some other issue and that is why I always use a protective device (in this case a fuse) as close as possible to the power source.

 

Last Edited: Thu. Aug 31, 2017 - 10:37 AM
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Switching the the high side has at least 2 advantages.  The vehicle is negative ground so by switching the the positive you only need to run a single wire to your load and connect the negative of your load to the vehicle ground.  If there is a short in the wire to your load it will blow the fuse when you turn it on.  If you switch the low side a short in this wire to ground will just turn the load on and drain your battery.

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And you avoid 'sneak' circuits.

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TT_ZX wrote:

Switching the the high side has at least 2 advantages.  The vehicle is negative ground so by switching the the positive you only need to run a single wire to your load and connect the negative of your load to the vehicle ground.  If there is a short in the wire to your load it will blow the fuse when you turn it on.  If you switch the low side a short in this wire to ground will just turn the load on and drain your battery.

 

Yes, I am aware of all of that.

Thank you for the input though.

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Kartman wrote:
And you avoid 'sneak' circuits.

 

Ok... "sneak circuits" is a term I've never even heard of. Would you feel like sharing more? In the mean time I'll try Google...

 

EDIT: interesting read - http://www.reliability-safety-so...

 

I'm not sure this specific design is complex enough to be concerned too much with this topic, but it is very interesting!

 

(Although if I can find an appropriately rated high side gate it would be a simple enough change to make at this point.) 

Last Edited: Thu. Aug 31, 2017 - 11:29 AM
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Cars are notorious for sneak circuits when the gnd connections corrode and other wiring issues.

The choice between using a relay or high side switch is perplexing. Relays are simple and hard to damage but mosfets are cheaper(in quantity). My car has a magic box that connects to the can bus and has relays for the trailer lights. Presumably they went for the relays as trailer wiring is frequently defective.

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Kartman wrote:
Cars are notorious for sneak circuits when the ground connections corrode and other wiring issues. The choice between using a relay or high side switch is perplexing. Relays are simple and hard to damage but mosfets are cheaper(in quantity). My car has a magic box that connects to the can bus and has relays for the trailer lights. Presumably they went for the relays as trailer wiring is frequently defective.

 

I have had my share of head scratching and using choice language due to odd circuit behavior due to bad or poor grounds in automotive and other vehicle applications. Many years ago when I was younger, before I entered the engineering field, I was a technician on wheeled, tracked, and other types of vehicles in automotive, agricultural, industrial, and military applications. Perhaps "sneak circuits" was exactly what I was experiencing, but had never heard the term. I was actually quite good at electrical system trouble shooting, and perhaps I could or should have gone the electrical route rather than the mechanical route when I decided to step it up into engineering. But, electronics is very much different from simple electrical and being young I chose the path of least resistance and went where I felt I had a bigger head start.

 

Now that I am getting older climbing around and on vehicles and cutting and welding and beating myself up is more of a chore than fun. But, I am the type of person that must have projects and something going on, and if I can learn new things in the process that's even better! So, I have been taking some free online electronics courses and researching and trying to learn things that I can do in the comfort of my AC at home on the workbench and the install side won't involve lifting anything that will put me in the bed for two days. LOL

 

But, now I'm just babbling... 

 

I had originally gone with a relay due to the belief that it was a much more robust solution, and also because they are simple and that I am very familiar. Then I was convinced by various advice sources that there was no reason not to use a FET. I have been looking through the ON Semiconductor and Infineon catalogs this morning and have been unable to locate a high side gate rated for 30+A, perhaps I am just failing at the hunt. I do like the idea of not using a magnetic relay, but it does get complicated and murky, especially at my beginner's level.    

Last Edited: Thu. Aug 31, 2017 - 12:18 PM
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https://www.infineon.com/dgdl/Infineon-BTS50010-1TAD-DS-v01_01-EN.pdf?fileId=5546d462576f34750157c38810ca55cd

You need to make sure it is adequately heatsinked. Relays are simpler in that regard.

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Kartman wrote:
https://www.infineon.com/dgdl/In... You need to make sure it is adequately heatsinked. Relays are simpler in that regard.

 

 

Thank you, I will look into that now!

 

That comment bring up another thing I've noticed... Is it just me or are heat sinks insanely overpriced for what they are? I mean, complicated ICs can be had for less than a dollar or two, but what essentially boils down to a stamped out piece of aluminum is multiple times more expensive?! Or perhaps I am just bringing up the wrong parts in my searches...

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LsD wrote:
I have been looking through the ON Semiconductor ... catalogs this morning and have been unable to locate a high side gate rated for 30+A, ...
Under 20A for a 12Vdc system at elevated temperatures on a PCB heatsink is best could find at ON in NCP45560.

FETs can split current with some small resistance on each FET's source; doesn't take much as the FET's channel resistance is very low.

 

http://www.onsemi.com/pub/Collateral/NCP45560-D.PDF

ON-Semiconductor

ON Semiconductor

Home >  Products >  Product Taxonomy >  Voltage & Current Management >  Load Switches

http://www.onsemi.com/PowerSolutions/parametrics.do?id=101881

 

Edit : URL

 

"Dare to be naïve." - Buckminster Fuller

Last Edited: Thu. Aug 31, 2017 - 01:23 PM
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LsD wrote:
Is it just me or are heat sinks insanely overpriced for what they are?
If not significantly constrained in area then the heatsink is copper on the PCB; else, it's volume constrained so go vertical with an aluminum heatsink.

fyi :

http://www.mouser.com/new/ohmite/ohmiteheatsinks/ (WC Series Ceramic Heatsinks)

 

"Dare to be naïve." - Buckminster Fuller

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Every time I think I'm getting close to something I'm comfortable enough with to start testing I get deeper in the weeds. That high side gate requires quite a few additional external components that the low side gate did not. Now I've got to decide if I want to go that route, stick with the low side gate and hope I don't have issues, or go back to a small FET controlling a relay. Oy...

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