Exploding IC in buck converter - Page 1
Forum section: electronics / projects

author: Teun
author_rank: Regular Contributor
timestamp: March 31, 2020, 09:06:39 am
content: Hi guys,I have an interesting problem.A client of ours has sent back several buck converters we supply. Never had much trouble with them espescially for this client.It is used to charge a mobile device with an onboard receipt printer. These are used on busses for public transport.Over time most of the PCB's have blown IC's. All on the side of the input pin.(see picture) TVS and varistors are used to handle spikes on the input. The converter should be able to deliver 9V/3A (IC rated 6A output) on the output and from what i can see with the printer activated it uses 28W of power max. The current limit/OCP of the converter is set to 3.5A.Input range is +/- 12-32V DC. (Busses can be around 30VDC)Can anyone give me an idea of why the input would be blown?

author: Ian.M
author_rank: Super Contributor
timestamp: March 31, 2020, 09:27:57 am
content: Automotive DC supply transients are *NASTY* especially on older or poorly maintained vehicles. Probably you haven't got enough headroom between the TVS and varistor clamping voltage during a high energy transient spike and the abs.max. input voltage of the IC. Post a schematic for your input protection circuit, + links to datasheets for the IC, TVS and varistor.

author: Hast
author_rank: Newbie
timestamp: March 31, 2020, 09:28:39 am
content: HiI have had a similar problem.I did a buck converter for a industrial system with "industrial 24V".At first everything worked fine and then we started to get returns where the input was blown similar to yours.After some measuring we found that the "industrial 24V" was a strange sinusoidal signal from 0-48V. The buck converter I used had a max of around 32 V in or something like that.The customer had switched out the 24V supply to a cheaper one.So measure the input voltage with an oscilloscope and not a multimeter is my advice.BR

author: max_torque
author_rank: Super Contributor
timestamp: March 31, 2020, 10:39:30 am
content: Load Dump for 24v nominal systems is speced at 212 volts! What LD and RBP (Reverse Battery Protection) do you have in place in this unit!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 31, 2020, 11:02:06 am
content: Hmm, that's exactly where the bondwire ought to be, too. That thing ate a lot of amperes, very quickly.Yeah, 30V, useless. What knucklehead designed this thing? To what standards was it tested?Automotive parts start at 50V and go up from there, and that's mainly for 12V systems with modest (70A) alternators. You might want an 80V rating here for example, just so there's enough headroom to handle the transients how ever you can (probably by disconnecting the load?).Tim

author: Teun
author_rank: Regular Contributor
timestamp: March 31, 2020, 11:30:03 am
content: Can't post the controller datasheet. But here are the input filter and varistor /TSV.https://www.bourns.com/docs/Product-Datasheets/SMAJ.pdfhttps://www.sfi.com.tw/index.php/en/2012-05-14-07-49-39/ml-c

author: Ice-Tea
author_rank: Super Contributor
timestamp: March 31, 2020, 12:20:01 pm
content: Kinda difficult to comment without posting details about the controller?

author: Teun
author_rank: Regular Contributor
timestamp: March 31, 2020, 01:25:26 pm
content: Quote from: Ice-Tea on March 31, 2020, 12:20:01 pmI know and I'm sorry for that.It has a -0.3 to 40V absolute maximum input rating. So I would think it has enough headroom.Also once again... we use them in many of our products and they work fine. It seems this problem only occures with this type of load or supply.

author: Ice-Tea
author_rank: Super Contributor
timestamp: March 31, 2020, 02:11:54 pm
content: The rating of D2 seems a bit strange to me? With a 12V nominal bus (yeah, yeah, automotive blah blah) it has a max clamping voltage of 48V? By that time, your 40V DC/DC controller is long gone..

author: Ian.M
author_rank: Super Contributor
timestamp: March 31, 2020, 02:34:56 pm
content: Well its fairly obvious what's wrong (and Ice-Tea has also picked up on it).The nearest match in your linked datasheet to the TVS in your schematic (D2, 36.8V breakdown) is the SMAJ30CA. Its max. clamping voltage passing a 8.3A peak pulse >10us is 48.4V, well above the 40V abs. max input voltage you claim for your controller. There just isn't enough headroom between the max normal operating voltage from the vehicle DC bus and your controller's abs.max. input voltage rating for *ANY* TVS diode to be effective.Load dump isn't a myth although its fairly rare, but nearly all vehicles still on the road have central load dump protection, so the 24V system unclamped 212V peak load dump will only be seen if someone is running a poorly maintained vintage truck or similar with no ECU. Without knowing the exact specs of the specific vehicle's central load dump protection, you cant know if it clamps under or over your controller's 40V abs.max input voltage rating.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 31, 2020, 03:03:11 pm
content: 36.8V nominal breakdown TVS & 40V abs. max spec - not gonna work.The actual clamping voltage, when current flows through the TVS, is much higher than the nominal value.Add unit variation on the top.

author: Teun
author_rank: Regular Contributor
timestamp: April 01, 2020, 07:23:11 am
content: Ok thanks! Trickery is that I need a reverse voltage of at least 28V for the TVS. So that's just not possible?

author: Ice-Tea
author_rank: Super Contributor
timestamp: April 01, 2020, 07:37:58 am
content: The SMAJ26CA is at least closer: 28.9 breakdown (satisfies your 28V minimum) and 42.1V max clamping voltage. Still beats your 40V max, but better at least and in a lot more cases it will not surpass the 40V max (when the current isn't too high).Still, not a good way to do things. You simply need a higher rated DC/DC controller.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 01, 2020, 12:41:47 pm
content: The problem with TVS diodes is, they are not precision devices. The clamping voltage depends on current and unit variation. This is why the downstream components need to be rated higher. If your input is x volts, and you pick nominal clamping voltage of, say, 1.25*x to ensure it's never unintentionally clamping, then the worst case clamping is at least 1.5*x or maybe even closer to 2*x.The problem in automotive systems is, the voltage varies quite a lot even in normal conditions. Assuming this is a 24V automotive system, you don't want to clamp at below 30V, otherwise you'll be dissipating power in the clamp unnecessarily. (BTW, you should have a thermally coupled polyfuse to protect against long-lasting slight overvoltage.) Now, if you pick the closestminimumguaranteed clamping voltage (with acceptable leakage figure) from 30V, the nominal clamping voltage will end up somewhere between 35-40V, and the worst-case, under load clamping will be somewhere around 45-55V. There is no easy way around this; I'd strongly consider replacing the DC/DC controller IC to something that can handle at least 60V absolute maximum.Quote from: Ice-Tea on April 01, 2020, 07:37:58 amFor an even remotely robust system, you need to look the "working peak reverse voltage" spec, which is only 26V; it will likely conduct and leak excessively in a 24V automotive system when the engine is running, causing another set of problems (excessive heating, possibly only in some units, which may only become a problem after many hours of runtime...) . SMBJ30CA would be the lowest acceptable from that series, but then again, we are back to 48.4V max clamping voltage, way over the abs. max. rating of the IC. I think there is no way out by just replacing the TVS with another.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 02, 2020, 03:56:36 am
content: Put another way, this design process is backwards. It should be:1. Start with nominal input voltage.2. That selects TVS nominal voltage. TVS peak voltage is typically 33-50% above that.3. That selects converter max voltage.4. If adequate ratings are not available, consider higher order methods, like a switching or linear prereg to deliver ride-thru, or a switch to disable load during transient.If the TVS needs to handle massive energy, enough that a TVS (as such) isn't economical: use a MOV instead, which will be selected in the same manner. Note the MOV will have a peak voltage typically 100-200% above nominal, putting much more pressure on the converter rating.Tim

author: magic
author_rank: Super Contributor
timestamp: April 02, 2020, 08:22:15 am
content: I wonder if it would be feasible to cascode the converter with a depletion FET whose gate is pulled to the input by a resistor and clamped to ground by a low power TVS/zener? I mean, if youreallycan't find a converter with suitable ratings...

author: Teun
author_rank: Regular Contributor
timestamp: April 02, 2020, 10:19:19 am
content: Well it's not that I don't want to use other converters. My boss keeps coming up with cheap ones he finds online and he expects me to to just use themBut al least it gives me another reason to try and convince him otherwiseThanks for the help so far!

author: Gyro
author_rank: Super Contributor
timestamp: April 02, 2020, 10:20:54 am
content: Do you have any resistance in series with the supply. I know you'll drop the efficiency a bit but, for example, a 1 Ohm 5-10W wirewound resistor (higher resistance if possible) in series with the input (after the fuse but before any reservoir capacitance) will significantly improve any clamping measures by limiting the peak transient current. It's also the sort of thing that could be retrofitted outside the unit (or clamped to the inside of the case if metal).This, in combination with the lower suggested TVS voltage rating, might get you past the problem.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 02, 2020, 11:20:44 am
content: Quote from: Teun on April 02, 2020, 10:19:19 amWell someone needs to do the design math, this is engineering and you can't just slap random parts together. It's not rocket science. If you are the responsible designer, do the math, show it, and just say "no" if that is the truth. It should be the end of discussion.

author: Alti
author_rank: Frequent Contributor
timestamp: April 02, 2020, 08:38:12 pm
content: Quote from: Siwastaja on April 02, 2020, 11:20:44 amIf your boss's decision was to use component X then use component X.Such decision frees you from doing the math necessary for component selection process and speeds up work. The disobedience and wasting your time for formally proving your boss makes poor decisionsis not your job, you are an ee and not a babysitter.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 02, 2020, 09:02:27 pm
content: Alti, I usually detect sarcasm quite well but now I have to ask, are you serious or joking?

author: thm_w
author_rank: Super Contributor
timestamp: April 02, 2020, 09:36:15 pm
content: Quote from: Siwastaja on April 01, 2020, 12:41:47 pmI agree that it is not an ideal design. But it looks like the maximum continuous automotive voltage tested in this certification is 28.2Vdc, does that sound right? Everything else is ms bursts.http://www.emcprima.com/FileUPLoad/DownLoadFile/635792103688970689.PDFhttp://read.pudn.com/downloads226/ebook/1062022/ISO16750-1.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 02, 2020, 10:12:51 pm
content: True enough, but I would add these two caveats:1. Add your formal complaint that you disagree with the choice, for such-and-such reasons;2. Record evidence of the decision process (conversations -- insist on e-mailed discussion; emails, design documents, datasheets, and approval by the boss guy).A faulty converter for an aftermarket automotive gizmo doesn't seem to be a very serious concern, but this is well worth keeping in mind anywhere the stakes are higher.Tim

author: mzzj
author_rank: Super Contributor
timestamp: April 03, 2020, 05:11:55 am
content: Quote from: thm_w on April 02, 2020, 09:36:15 pmReal-life is definitely worse than that. Maybe you misinterpreted the standard or its not very good stardard to go with at all.On a cold day I remember measuring 15.8v from VW alternator. That would be 31.6v for 24v nominal system.This is a graph from Bosch (Bosch Automotive Electrics and Automotive Electronics book)I wouldn't design anything for below 32v.

author: magic
author_rank: Super Contributor
timestamp: April 03, 2020, 05:27:24 am
content: Quote from: Siwastaja on April 02, 2020, 09:02:27 pmWe are in a Poe's law singularity. True believers are so crazy and trolls are willing to go so far in their exploits that there no longerisany practical difference.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 03, 2020, 06:48:29 am
content: You may want, for learning if nothing else, to look at solutions where the voltage limiting element is in series with your load, not in parallel.The parallel configuration tries to clamp the full input supply capability, such as a 100A alternator. I.e., your clamp is dissipating Vdiff * Isupply_max, where Isupply_max is limited by the resistances in the system. Adding a bit of extra series resistance before the clamp helps, but if you add too much, your load starves under normal operation; this resistance is always there, want it or not.The series configuration only needs to dissipate Vdiff * Iload, where Iload is the actual consumption of your load. Basically, this would be a pass transistor with high enough voltage rating. Yes, it is more complex.It sucks when you choose a voltage regulator IC without enough voltage margin. I once chose a 26V max recommended, 30V abs max buck converter IC for a system with a regulated 24V bus. Works great. Until I needed a -10V output as well, and "simply" BOM reused the same part to form an inverting buck-boost, without realizing the part now sees 34V. Things like this happen in a hurry. The problem was made worse by the fact that the few units in existence worked just fine; one blew up when a customer wanted to use a 26V bus voltage instead, finally revealing the problem. 36V was finally enough for this 30V abs.max. part.

author: Teun
author_rank: Regular Contributor
timestamp: April 03, 2020, 08:04:04 am
content: Quote from: Siwastaja on April 02, 2020, 09:02:27 pmI have been telling him I don't always agree with things for almost 2 years now. But I guess you should know him to know the whole situation here.Anyways mzzj is right. Alternator voltage can be over 30V on 24V systems.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 03, 2020, 09:43:55 am
content: And to generalize that -- when the load resistance is already above the generator resistance, it's better to stay on the high side, i.e. reduce the load by merely maintaining load current. When below, it's better to stay on the low side, i.e. increase the load by maintaining load voltage.(We might measure the load in terms of average load resistance: instantaneous V / I. Note this is different from the AC incremental resistance of the load, and is simply the ratio of numbers measured with a meter, no fancy differencing needed. Note also a "heavy" load is a low resistance, because we always speak of load in terms of conductance, but resistance is reciprocal, go figure.)A linear regulator passes the same load current, while dropping the excess voltage; average input resistance is thus proportional to peak voltage, and so rises during the surge.A TVS passes the same load voltage, while drawing excess current; average input resistance is thus inversely proportional to peak current, and so falls during the surge.Note that a switching converter does even better than these possibilities, because as voltage goes up, current drops roughly proportionally; it has a constant-power characteristic and doesn't care what the input voltage is, as long as it's within operating range.A switcher will still have some resistive characteristic remaining, for example switching loss tends to be significantly higher at high voltages. This can be designed to be a fraction of total load power, making it still much easier to handle than the power absorbed by a brute force (series or shunt) method.Load dump generator resistance is generally tested at 2 ohms or thereabouts (depends on the alternator ratings in the target vehicle), so the decision point is at a fairly low resistance, and lighter loads like this will graciously choose the first option.I suppose very low loads, like audio power amps, might opt for a shunt method; with a customized switching controller, they might even try and turn on both switches simultaneously, shorting out the supply and clearing the fuse. A more subtle case would be partially turning on the switches, using them as a power shunt regulator instead. Though, transistors aren't good for absorbing much energy, so this probably wouldn't work well until you get into the really big (multiple kW?) sizes, which can pretty much absorb the full pulse anyway without special effort.Which, the peak available power is in the kW -- so, anything in that middle range of impedance, and not big enough to just grin and bear it, can save a huge amount of trouble just by shutting off completely. Which can still be done through a series or shunt method -- in series, the switch opens, drops all the peak voltage, and load current drops to zero; in parallel, the switch closes (crowbars), sinks all the peak current, and load voltage drops to ~zero (which eventually turns to the series case, as a fuse must open -- there will still be a 12V baseline after the surge, and we can't have that crowbar staying on forever!).Tim

author: Alti
author_rank: Frequent Contributor
timestamp: April 03, 2020, 10:12:00 am
content: Quote from: Teun on April 03, 2020, 08:04:04 amI do not want to push this subject off-topic - I am no automotive expert. Also, I have no idea if the actions taken by Mr. Head (your boss) are wasteful or maybe actually you are the problem in this relationship. Or maybe your boss has his own Ms. Finance Department who is analyzing revenues giving him no choice but cut BOM cost. As long as you do not own shares - this is irrelevant.What I point out:1. You take no responsibility for the decisions of your boss.2. Disobedience is the worst path one could follow. This will only lead to a frustration from both sides. The project timeline will slip and guess who is going to be blamed?Quote from: Mr. D.Head

author: Siwastaja
author_rank: Super Contributor
timestamp: April 03, 2020, 10:26:49 am
content: When seen unnecessary micromanagement, I have had good results by playing along, being nice, and then, just doing The Right Thing based on actual engineering principles (like the simple calculations shown here). This pattern is easy to apply in trivial cases such as maximum voltage ratings of parts. Of course, if the manager alreadyboughtthe wrong parts, then this is gonna suck.I have never seen a case where the micromanaging bossrememberswhat their exact (faulty) instructions were, and you would be in trouble because you made a working device in a "wrong" way. Not saying it couldn't happen, just that I haven't seen it. Usually everybody's happy when the device works, and not happy when you have a recall. Blame shifting starts when things go wrong, and they go wrong when people don't catch the mistakes.What is 100% sure is that exceeding the absolute maximum voltage ratings of a part severely will 100% definitely lead into troubles, recalls, and waste alotof time, and while documenting that the blame is due to a micromanaging boss is better than not documenting it, even better is to avoid such situation completely. You can do that politely, without wasting anyone's time, and everyone's happy.Such engineering calculations do not take longer than handwaving a design with random components, and the amount of time saved by getting the design right is immense, compared to reaching a working solution through iteration with random parts. That isexactlythe job of the engineers.Like aircraft have two pilots co-operating, taught to catch other's mistakes and correct things. This is the result of recognizing that following the one with the authority just doesn't work. In general, you should give everyone the benefit of doubt, and discuss such things openly; such engineering things are just normal daily science, shouldn't be a matter of feelings, authority, or opinions. If this seems to constantly fail with a certain person no matter how you try to handle the situation, I strongly suggest looking for a job elsewhere. Not everyone sucks.It's quite typical that the ones obeying the orders are the ones who become scapegoats later on. You don't need to participate in that game.

author: Alti
author_rank: Frequent Contributor
timestamp: April 03, 2020, 11:26:21 am
content: Quote from: Siwastaja on April 03, 2020, 10:26:49 amNaaah, "Exploding IC in buck converter", I am sure this is not the case.