TVS Diode Layout Considerations for automotive application - Page 1
Forum section: electronics / general

author: rbm
author_rank: Regular Contributor
timestamp: August 12, 2017, 05:50:25 pm
content: 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?

author: floobydust
author_rank: Super Contributor
timestamp: August 12, 2017, 07:46:32 pm
content: 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.RXEF0170.17A 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 oneBourns 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.

author: rbm
author_rank: Regular Contributor
timestamp: August 13, 2017, 12:46:00 am
content: 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.

author: floobydust
author_rank: Super Contributor
timestamp: August 13, 2017, 04:21:07 am
content: 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.

author: Rerouter
author_rank: Super Contributor
timestamp: August 13, 2017, 04:54:22 am
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 13, 2017, 03:43:48 pm
content: 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

author: rbm
author_rank: Regular Contributor
timestamp: August 13, 2017, 07:29:59 pm
content: Thank you for the additional information, Tim. I've added the additional ground vias.Quote from: T3sl4co1l on August 13, 2017, 03:43:48 pmWell 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.Quote from: T3sl4co1l on August 13, 2017, 03:43:48 pmDoh! 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.Quote from: T3sl4co1l on August 13, 2017, 03:43:48 pmYes, I have filtering and diode protection on the inputs. All outputs are driven through open collector BJT discrete transistors.

author: Niklas
author_rank: Frequent Contributor
timestamp: August 13, 2017, 09:32:40 pm
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 13, 2017, 11:40:29 pm
content: 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.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

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 13, 2017, 11:45:03 pm
content: Quote from: rbm on August 13, 2017, 07:29:59 pmOh, good -- and that explains itself, LEDs needing enough current for the switcher.QuoteExcellent.Tim

author: rbm
author_rank: Regular Contributor
timestamp: August 14, 2017, 11:12:54 pm
content: Quote from: Niklas on August 13, 2017, 09:32:40 pmOk, 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.Quote from: Niklas on August 13, 2017, 09:32:40 pmMaximum 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.

author: floobydust
author_rank: Super Contributor
timestamp: August 15, 2017, 01:36:14 am
content: 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 aSM8A27which is a 6,600W TVS and huge in size!You use two of them in series for a 24V vehicle.

author: rbm
author_rank: Regular Contributor
timestamp: August 15, 2017, 02:45:41 am
content: 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.

author: rbm
author_rank: Regular Contributor
timestamp: August 26, 2017, 10:00:41 am
content: Quote from: Rerouter on August 13, 2017, 04:54:22 amI 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.

author: Rerouter
author_rank: Super Contributor
timestamp: August 26, 2017, 10:04:50 am
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 26, 2017, 06:46:41 pm
content: 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

author: rbm
author_rank: Regular Contributor
timestamp: August 26, 2017, 06:56:38 pm
content: Cheers guys. I'm learning a lot.

author: Deridex
author_rank: Regular Contributor
timestamp: September 17, 2017, 08:46:42 am
content: Well my optinion about protection-stuff at automotive control units:Quotei hope i could help a bit

author: JDW
author_rank: Frequent Contributor
timestamp: January 31, 2018, 02:33:17 am
content: Quote from: T3sl4co1l on August 13, 2017, 11:40:29 pmExactly. Voltage spikes on the input power line would exceed the Vmax spec of most any PTC, especially of the PTC is placed first in the circuit, before the TVS can limit the spikes. I personally put a single-blow fuse externally on the power wire of the wiring harness, thereby eliminating the need for a PTC fuse on the PCB.The inline diode used in the circuit is to protect against reverse polarity, but to ensure a lower voltage drop (and less wasted power) a Schottky diode would be best. However, Schottky diodes like a B270 (70V, 2A) have lower voltage ratings than say a 1N4004 (400V, 1A). So if a Schottky is used, it would be prudent to place itafterthe TVS, ensuring the Vc of the TVS is lower than Vmax of your chosen Schottky. But when you put the Schottky AFTER the TVS, it means you need to use a BI-DIRECTIONAL TVS diode so that reversal of polarity will not adversely affect the TVS. Putting the Schottky first allows use of a UNIDIRECTIONAL TVS, but again, a Schottky's Vmax is too low for that in a 24V application, so you would either need to use a regular 1N4004 diode first with a UNIDIRECTIONAL TVS, or use a Schottky AFTER a BIDIRECTIONAL TVS.Vcof TVS diodes is another consideration. 600W TVS diodes are often used in automotive applications, but there is more than one Vc on some datasheets. Consider this:http://www.st.com/content/ccc/resource/technical/document/datasheet/1a/3b/f2/a9/96/72/49/ca/CD00000720.pdf/files/CD00000720.pdf/jcr:content/translations/en.CD00000720.pdfLook at theP6KE47Ain that datasheet, which one might consider for 24V vehicle use. Vc@10/100us =64.8V. That would seem to be an acceptable TVS choice when paired with a robust Buck Regulator like a LinearLTC3637which can handle up to76Von Vin (absolute Max is80V). But looking more closely at the TVS datasheet we also see Vc@8/20us =84V, which would exceed Vmax of the LTC3637. If one is designing for a worst case scenario, then using the 8/20us Vc would be more prudent than simply relying on the 10/1000us Vc. That would mean choosing something like theP6KE39Awhich has a Vc@8/20us=69.7V, which would work fine with the LTC3637.On the other hand though, your TVS choice depends on the maximum "continuous" (for a length period of time lasting many seconds or minutes) voltage you expect to see in your 24V vehicle. In some situations a 24V truck could see recurring voltages rise up to36V. So if you account for that with a TVS, you would want to make sure your breakdown voltageVbr-minis 36V or higher. But perhaps the biggest consideration isjump starting. What if someone tries to jump start with48V? TheP6KE47ATVS diode has a Vr=40V (which is higher than your 36V case, so you're covered there) andVbr-max= 49.4V. So if someone jump starts the 24V truck with 48V, it's not exceeding the maximum breakdown voltage of Vbr-max, but the TVS will be conducting a significant amount of current during the duration of the jump start. For that same TVS diode, Vc@10/1000us =64.8V, but Vc@8/20us =84V. So if you use a buck converter like theLTC3637which has a Vin-max of76Vand an absolute max of80V, you would only have a problem in the event of an 8/20us load dump pulse type. So you would then ask which is more likely -- the 48V jump start case or the 8/20us case? Perhaps the former, which means your TVS diode pick would be the P6KE47A when used with a Linear LTC3637 buck regulator.Thewattage ratingof the TVS is yet another consideration. In most automotive 12V and 24V designs I see 600W TVS's being used. I believe the designer is using that lower wattage value not merely to cut cost but going on the assumption that the device will be used in newer vehicles that have protection diodes in the alternator. Without the diodes in the alternator, your Load Dump in a 12V vehicle can be 100V to 125V. In a 24V vehicle the Load Dump could be 202V. But with protection diodes, a Load Dump on a 12V car is usually capped to about 60V and on a 12V vehicle it usually won't exceed 70V or 80V. So with that in mind, a 600W TVS may suffice, versus a more expensive 1500W or 2200W version. But to do the math and be sure, you would need to know the internal resistance of the alternator, which really does require some guesswork. Even so, there is a 24V example calculation given on page 11 of the following PDF:http://www.newelectronics.co.uk/article-images/82298/automotive_circuit_protection.pdfThe absolute best engineering solution is to use"active" transient suppressionrather than a TVS, but cost, complexity and board space requirements make active suppression a more difficult choice than a TVS:https://www.maximintegrated.com/en/app-notes/index.mvp/id/4240