Author Topic: TVS Diode Layout Considerations for automotive application  (Read 1799 times)

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Online rbm

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TVS Diode Layout Considerations for automotive application
« on: August 13, 2017, 03:50:25 AM »
Hi,  I'm designing a circuit to be used in a motorcycle application, combining digital logic and independent analog circuitry. I say independent because the functional blocks of the design (there are 3 - gear decode, tachometer and speedometer conditioning) operate independently from each other.  I've arranged the PCB layout to place all digital circuitry and traces on the right side, all analog circuitry and traces on the left side for noise consideration.  The digital logic is essentially up to two 74XX logic gates; the analog circuitry is an LM358.  I'm trying to keep the overall board size as small as possible, currently 55mm X 37mm.

I am using the following power design - a reverse protection diode, TVS and 78XX-compatible switching DC-to-DC converter with 500mA capacity.



I had a couple questions about the TVS diode layout that I hope I might get advice on.

The PCB layout for the input power section is below:


Traces to D1 and TVS from the terminal block are 50mil wide, and return path back to the terminal block's Pin 2 is through a continuous top ground pour.  There is a ground pour on the bottom of this 2-layer board.

1) should I layout several vias near the anode of the TVS to sew together the top and bottom ground planes and provide a lower impedance path to ground for any transient current that the TVS may conduct?  Or is the size of the top ground plane sufficient?

2) I chose a S1A diode for reverse polarity protection but I've read online designs that suggest a PTS instead to trip incase the load dump current is excessively long duration which might destroy the TVS.  I'm not sure what is best. Should I replace D1 with a suitably sized PTC fuse?
« Last Edit: August 13, 2017, 03:52:37 AM by rbm »
- Robert
 

Offline floobydust

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Re: TVS Diode Layout Considerations for automotive application
« Reply #1 on: August 13, 2017, 05:46:32 AM »
You need some series resistance to lower spike currents into the TVS. Otherwise the TVS can fail short and then tell me about your fuse. It's probably huge and your board/wiring can burn up then.

In vehicles, I've used low value resistors or PTC's ahead of Vreg (TVS) it works well. A few ohms to lower TVS clamp currents and safety if something shorts.
RXEF017 0.17A 3-5 ohms but through-hole and OK on 24V trucks. PTC's can false-trip if very hot as trip point drops with heat.
I would design in a SMT one Bourns PTC. MF-R090 is good too.

Load dump is a one-case scenario with bad battery wire and very difficult to design for. I wouldn't bother. Automakers rate the lifetime of an ECU as 2-3 load dumps.
But inductive load switching spikes happen all the time and you need to cover that.
S1A not enough at 50PIV. ISO 7637-2 plan for -200V transients, so S1D or better. You must have the reverse-diode to protect the Vreg.

Ensure the max. clamping voltage of the TVS (32.4V at 18.5A) does not kill the Vreg or input cap.
I would say your circuit is OK just add a PTC.

As far as PCB layout, you need to withstand current spikes on the input (TVS) loop, but they are transient and traces/vias don't melt unless very small. 50 mil is plenty.
Remember the input terminal block is completely unfused (upstream of PTC), so I back off the ground pour but had a design where the thermal reliefs (GND) overloaded when customers had open GND connection and pulled current through the other terminals on the PCB.

You have a small trace right alongside Vin, it will pick up noise. I would route it another way, away from input power.
 

Online rbm

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Re: TVS Diode Layout Considerations for automotive application
« Reply #2 on: August 13, 2017, 10:46:00 AM »
Thanks for the critique Floobydust.

I added the Littlefuse Polyswitch device; inserted it between the D1 and the TVS.  I'm changing the S1A to S1D as you recommend.  I selected the TVS based on the clamp voltage (32V) being less that the maximum input voltage for the Vreg (36V) and C9 (250V)  And I'm appreciative of you pointing out the errant trace.  I have been pouring over the board looking for potential sources of interference but missed that guy.  It's relocated away from Vin now.

New schematic:


New Layout:


I understand what you are saying about the danger of unconnected ground and ground currents finding other paths, destroying circuits on its way.  However, I'm not sure how to handle this situation in Eagle. So, the -ve terminal of the power connector is still connected to the ground plane.
- Robert
 

Offline floobydust

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Re: TVS Diode Layout Considerations for automotive application
« Reply #3 on: August 13, 2017, 02:21:07 PM »
Ahh that is much better and I would say not give you troubles. This is a basic front-end for a 12V vehicle that will not cause obvious grief. Motorcycles are not as bad as cars, trucks for transient energy.
I've had so many jr. engineers mess that up and their designs fail w/upset customers. Rented a ISO-7637-2 transient generator and had some fun learning there.

If you have two or more field ground connections on your design i.e. input power and sensor GND, I have seen vehicles melt thin GND traces/thermal reliefs on PCB's. It was caused by two reasons.
1. Open input power GND so the sensor GND provides the GND for the board. OK if no high current outputs, like cooling fan etc.
2. Poor battery-engine block GND so the sensor (on the engine block) provides the GND during cranking. Poof!
I think if your thermal reliefs are fairly thick spokes at the terminal block, it will be fine, they will not be an unintentional fuse.

Vibration on vehicles is a problem, so you might need to tack the through-hole parts or try go all SMT. The Vreg seems high current at 500mA.
On a motorcycle I would consider silicone-potting the board, water everywhere too. Not sure how to keep this dry.
 

Offline Rerouter

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Re: TVS Diode Layout Considerations for automotive application
« Reply #4 on: August 13, 2017, 02:54:22 PM »
Suggestion, add a few ground plane vias right below the TVS  ground plane pad, that way you prevent an inductive loop,

Those traces under the the input diodes pad will see quite a kick, ideally you want nothing but ground plane under your "aggressor" traces.
 
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Offline T3sl4co1l

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Re: TVS Diode Layout Considerations for automotive application
« Reply #5 on: August 14, 2017, 01:43:48 AM »
Yes, vias in ground are always a good thing.  Just in general, not even in this area specifically (but it helps here, too, so don't be shy). :)

Mind that, a 28V TVS won't really protect a 30V 7805 at all.  An 18 or 24V part would be safer.  If the lower voltage rating is a concern for operating life (it'll surely be fried by load dump, but that's rare; it's the every day starter and ignition transients you need to handle reliably), you may need more series resistance (as flooby suggested), or a bigger diode, or a protection circuit (so that the input can handle much more than 30V, and an MOV can be used to clamp much higher energy pulses).

Are you really using 74xx TTL logic?  You can save an awful lot of current consumption (and locating obsolete parts!) with 74HC or the like.  As long as all logic devices are compatible, you don't need to worry about fanout or input/output ranges.  The reduced consumption would seem to be of prime importance: you're dropping about 7V in that 7805, and every watt means a bigger and bigger heatsink!

Mind that all inputs, to logic or the LM358, need to be well filtered.  Logic will happily respond to 10s of MHz, and LM358 will detect (rectify) RF on its inputs.  ESD protection is also desirable, at least a 5V zener diode, say (or series clamp diodes, like BAT54S or BAV99, for lower capacitance).

Outputs need similar consideration, if they're going any distance.  Logic outputs are more robust than inputs, but not by much -- ESD clamp diodes there can be a saver, too.

Tim
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Online rbm

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Re: TVS Diode Layout Considerations for automotive application
« Reply #6 on: August 14, 2017, 05:29:59 AM »
Thank you for the additional information, Tim.  I've added the additional ground vias.

Mind that, a 28V TVS won't really protect a 30V 7805 at all.  An 18 or 24V part would be safer.  If the lower voltage rating is a concern for operating life (it'll surely be fried by load dump, but that's rare; it's the every day starter and ignition transients you need to handle reliably), you may need more series resistance (as flooby suggested), or a bigger diode, or a protection circuit (so that the input can handle much more than 30V, and an MOV can be used to clamp much higher energy pulses).
Well the TVS is I speced had a clamp voltage of 32V.  The voltage rectifier is not a linear 7805, it is a bulk DC-DC converter with a 7805 form-factor.  The DC-DC converter, a CUI VXO78-500 non-isolated DC switching regulator, is 95% efficient and has a max input of 36V.  I had poor experience with a linear LDO in a previous version of this design.  The Voltage regulator is also driving an off-board 7-segment display which will be the greatest consumer of current from the regulator.  I'll re-spec the TVS for a clamp voltage of 27V which gives me a stand-off voltage of 17V and a breakdown voltage of 19V.  That should be enough margin I hope.

Are you really using 74xx TTL logic?  You can save an awful lot of current consumption (and locating obsolete parts!) with 74HC or the like.  As long as all logic devices are compatible, you don't need to worry about fanout or input/output ranges.  The reduced consumption would seem to be of prime importance: you're dropping about 7V in that 7805, and every watt means a bigger and bigger heatsink!
Doh!  I don't know what I was thinking when I wrote 74XX logic.  Must have had a brain fart.  I'm actually using CMOS logic.

Mind that all inputs, to logic or the LM358, need to be well filtered.  Logic will happily respond to 10s of MHz, and LM358 will detect (rectify) RF on its inputs.  ESD protection is also desirable, at least a 5V zener diode, say (or series clamp diodes, like BAT54S or BAV99, for lower capacitance). Outputs need similar consideration, if they're going any distance. 

Logic outputs are more robust than inputs, but not by much -- ESD clamp diodes there can be a saver, too.


Tim
Yes, I have filtering and diode protection on the inputs.  All outputs are driven through open collector BJT discrete transistors.
« Last Edit: August 14, 2017, 05:40:56 AM by rbm »
- Robert
 

Online Niklas

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Re: TVS Diode Layout Considerations for automotive application
« Reply #7 on: August 14, 2017, 07:32:40 AM »
C9 looks a bit small to be 10uF/250V. Have you checked that there is a part number that fits the spec? Perhaps split it up on 2 packages to get more capacitance with higher voltage rating?

The datasheet for the dc/dc recommends an emi filter at the input. Maybe the filter can be omitted depending on your application, but it also includes some buffer capacitance. Maybe you should make a quick test with just the regulator, C9, C10, a resistor load and some wire harness before you finish the PCB layout?

I would suggest that you rotate C9 to have its GND connection closer to the regulator GND pin. Also rotate C10 180 degrees to achieve the same thing.

How about swapping Vbat and GND at the input connector? Looks like it could straighten up some traces and shorten the return path from the TVS.

What is the expected max ambient temp for the application? A PTC can trip just from the ambient temp or give you a much smaller current threshold. This is more likely with an SMD PTC as it has a better thermal path to the power dissipated by other components on the board.
 
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Offline T3sl4co1l

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Re: TVS Diode Layout Considerations for automotive application
« Reply #8 on: August 14, 2017, 09:40:29 AM »
Oh, and about PTC "fuses" --

They will fuse as such, but the time required is huge: 100ms to 10s+.  That's a lot of fault current into your circuit, most likely enough to destroy the TVS.

Large surges (like load dump) also supply a lot of energy to the fuse itself.  Take the peak current and voltage ratings seriously: if exceeded, the fuse will still clear, but it will clear once and for all, not resetting itself afterwards. ;D

For something as unlikely as load dump, that's not a bad failure mode, really.  If you were designing for more frequent overvoltage, this might be a problem.

If your surge voltage has a known maximum, you may be able to select the fuse so that its minimum resistance is enough to limit current to a safe value, at the peak voltage.

Polyfuses aren't very practical beyond 30V and some number of amps, depending on situation.  The fault energy is just too much for the polyfuse to handle, or it becomes impractically large (and slow).  You could still use an active circuit at that point -- though things get increasingly bulky, as the circuit has to deal with both inrush and fault energy, and by a few hundred volts of capacity, you're looking at a handful of transistors...

Tim
« Last Edit: August 14, 2017, 09:42:22 AM by T3sl4co1l »
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Offline T3sl4co1l

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Re: TVS Diode Layout Considerations for automotive application
« Reply #9 on: August 14, 2017, 09:45:03 AM »
Well the TVS is I speced had a clamp voltage of 32V.  The voltage rectifier is not a linear 7805, it is a bulk DC-DC converter with a 7805 form-factor.  The DC-DC converter, a CUI VXO78-500 non-isolated DC switching regulator, is 95% efficient and has a max input of 36V.  I had poor experience with a linear LDO in a previous version of this design.  The Voltage regulator is also driving an off-board 7-segment display which will be the greatest consumer of current from the regulator.  I'll re-spec the TVS for a clamp voltage of 27V which gives me a stand-off voltage of 17V and a breakdown voltage of 19V.  That should be enough margin I hope.

Oh, good -- and that explains itself, LEDs needing enough current for the switcher.

Quote
Yes, I have filtering and diode protection on the inputs.  All outputs are driven through open collector BJT discrete transistors.

Excellent. :)

Tim
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Online rbm

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Re: TVS Diode Layout Considerations for automotive application
« Reply #10 on: August 15, 2017, 09:12:54 AM »
C9 looks a bit small to be 10uF/250V. Have you checked that there is a part number that fits the spec? Perhaps split it up on 2 packages to get more capacitance with higher voltage rating?

The datasheet for the dc/dc recommends an emi filter at the input. Maybe the filter can be omitted depending on your application, but it also includes some buffer capacitance. Maybe you should make a quick test with just the regulator, C9, C10, a resistor load and some wire harness before you finish the PCB layout?

I would suggest that you rotate C9 to have its GND connection closer to the regulator GND pin. Also rotate C10 180 degrees to achieve the same thing.

How about swapping Vbat and GND at the input connector? Looks like it could straighten up some traces and shorten the return path from the TVS.

Ok, I've redone the PCB around the power input, regulator and capacitors C9 and C10 according to your very good suggestions.  You were right about the capacitor size and everything.  I looked through Digikey's DC-DC converter product offerings and found a ROHM unit that looks better at least for board real estate and design simplicity.  It's their BP5293-50.  I've also adjusted the TVS and PTC devices to suit the new choice in regulator.

What is the expected max ambient temp for the application? A PTC can trip just from the ambient temp or give you a much smaller current threshold. This is more likely with an SMD PTC as it has a better thermal path to the power dissipated by other components on the board.
Maximum expected ambient temperature is 40*C; I've measured it.  The board is user-installed but most everyone chooses to install it in the spacious fuse box under the seat of the motorcycle, well protected from the elements.
- Robert
 

Offline floobydust

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Re: TVS Diode Layout Considerations for automotive application
« Reply #11 on: August 15, 2017, 11:36:14 AM »
When I did ISO 7637-2 transient testing, with short pulses (usec) the TVS saw the PTC, input diode, line series resistance limiting surge current. I.e 75V surge and 75-32.4V/~3 ohms = 14A which was fine. Longer duration pulses the PTC heats up (along with the TVS diode) and it was a thermal battle. As long as the diode was tougher than the PTC. SMC sized parts are better.

Known TVS meeting all 12V automotive transient standards is a SM8A27 which is a 6,600W TVS and huge in size!
You use two of them in series for a 24V vehicle.
 

Offline blueskull

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Re: TVS Diode Layout Considerations for automotive application
« Reply #12 on: August 15, 2017, 12:17:00 PM »
With a PTC, immediately after load dump, before PTC has cooled down, your circuit will not be powered, so be sure you have enough output capacitance.
The better way to handle cranking and load dump is to use a series passing element that can regulate output voltage in case a load dump occurs, and behaves like a pass through in normal operation.
Check out this: http://www.nxp.com/docs/en/application-note/AN5082.pdf.
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Online rbm

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Re: TVS Diode Layout Considerations for automotive application
« Reply #13 on: August 15, 2017, 12:45:41 PM »
Load dump on a BMW K-bike won't happen during cranking.  The load shed relay disconnects the battery from all but the essential circuits needed for starting during cranking until the engine fires.  My board would be one of the disconnected circuits.
- Robert
 

Online rbm

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Re: TVS Diode Layout Considerations for automotive application
« Reply #14 on: August 26, 2017, 08:00:41 PM »
Suggestion, add a few ground plane vias right below the TVS  ground plane pad, that way you prevent an inductive loop,

Those traces under the the input diodes pad will see quite a kick, ideally you want nothing but ground plane under your "aggressor" traces.
I have a question about the best pattern to use in the layout to achieve this ground plane sewing.  I have laid out two possible via hole patterns using tented vias with 0.3mm drill size, shown below (this is an Oshpark rendering of the Gerber files):

Pattern #1 based on tented vias surrounding the pad:


Pattern #2 based on via-in-pad (VIP)


Which of these is best, from the aspect of achieving the low impedance conduction path to the lower ground plane? 

Please note that I will be hand soldering the components - I don't have access to reflow or wave soldering equipment -- so I probably won't experience capillary action migrating solder away from the TVS.
- Robert
 

Offline Rerouter

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Re: TVS Diode Layout Considerations for automotive application
« Reply #15 on: August 26, 2017, 08:04:50 PM »
Both are fine in this case, as before it had to detour quite a bit on the top layer. now it has a straight hop down.

Pattern 1 is the best for "Designing for Manufacture" and is what i would recommend, the themal pad connections to the plane will allow you to solder it without much issue,

Pattern 2 i would generally only use for mechanical things, e.g. an SMD connector that i don't want to tear away without putting up a good fight.
 
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Offline T3sl4co1l

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Re: TVS Diode Layout Considerations for automotive application
« Reply #16 on: August 27, 2017, 04:46:41 AM »
I avoid via-in-pad for reflow processes unless absolutely necessary (e.g., QFN pads), or it's too beneficial to ignore (thermal vias).  It's not a big deal here, so I'd go with maybe three or four vias, around the pad in question, in the direction where they'd be most useful.

(For hand soldered builds, you can do via-in-pad all you like. ;) )

Tim
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Online rbm

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Re: TVS Diode Layout Considerations for automotive application
« Reply #17 on: August 27, 2017, 04:56:38 AM »
Cheers guys. I'm learning a lot.
- Robert
 

Offline Deridex

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Re: TVS Diode Layout Considerations for automotive application
« Reply #18 on: September 17, 2017, 06:46:42 PM »
Well my optinion about protection-stuff at automotive control units:
Quote
Shematic:
- The circuits-tests for quite some automotive stuff(normal cars!) involve operating-voltages up to 32V (maybe there are even worse operating voltages). Keep that in mind.
- The TVS-Diodes seem to have quite quite some spread among the values. Always count with the worst case.
- Also i can only recommend to simulate the circuit in spice with the ESD-pulses/transients and such stuff.
- Some C's can also help at the transients
- I did not read through the whole Datasheets ... but there seems to be no reversepolarity-protection.

Layout:
- Remember that the current at breakthrough can be quite huge. So you need to connect the TVS-Diode very well, as some other guys mentioned already.
- i recommend to place the protection as close as possible to the connector, so the transients wont spread accross the entire board.
- i suggest to keep the via's out of the pad, aslong it can be done. Otherwise it might cause trouble at soldering.

i hope i could help a bit
 


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