Buck boost regulator getting hot - Page 1
Forum section: electronics / projects

author: Pedro147
author_rank: Regular Contributor
timestamp: November 24, 2016, 04:01:41 am
content: I have been doing a redesign of a quadcopter flight control board and the 5v buck boost regulator (TPS63002 - U$3 on brd file bottom right hand corner of PCB) is getting hot. It only powers the Atmega32U4 with additional connections for bootloader flashing power and an auxiliary power socket being made available if required. Initially I assumed a minor short circuit possibly beneath either the Atmega or 5v buck boost IC but after reworking them both several times there has been no improvement regarding the heat buildup. I was wondering if anyone might take a look at my schematic and board file and see if there appears to be some design or routing flaw that might possibly explain this. I did in the initial design reduce the net classes clearance for vcc and gnd classes to 8mil to facilitate easier routing. There are no DRC errors. I am also aware that the positioning and routing of the caps for this buck boost (C11 and C12) are pretty dismal and I have swapped their positions and re-routed them for the next version. Eagle board file available here -goo.gl/qlHUp1Thanks in advance.

author: amirm
author_rank: Regular Contributor
timestamp: November 24, 2016, 04:10:46 am
content: Quote from: blueskull on November 24, 2016, 04:07:09 amDarn. You beat me to it.OP, you need a current limiting resistor for your power LED.

author: Pedro147
author_rank: Regular Contributor
timestamp: November 24, 2016, 04:11:10 am
content: No not at all. The specs for that LED indicate that running on the 3.3v from the top TPS6001 necessitates no current limiting resistor is required but thanks for you positive input that is very helpful

author: Pedro147
author_rank: Regular Contributor
timestamp: November 24, 2016, 04:26:49 am
content: Sorry I was wrong a 1 ohm resistor I'll give it a misshttp://au.mouser.com/Search/ProductDetail.aspx?R=APHCM2012QBC%2fD-F01virtualkey60400000virtualkey604-APHCM2012QBC%2fD

author: Pedro147
author_rank: Regular Contributor
timestamp: November 24, 2016, 04:48:38 am
content: It's this onehttp://au.rs-online.com/web/p/products/7729654/and thanks for taking a look for me

author: Pedro147
author_rank: Regular Contributor
timestamp: November 24, 2016, 04:57:18 am
content: No I have used that inductor with both the 3.3v and 5v buck boost in very similar circuits with no overheating issues.I will build another board and see if I have the same issue. I just thought that it was worthwhile asking if anyone here could see some design or routing flaw that I had overlooked seeing as I am a rank amateur at all thisThanks again

author: georges80
author_rank: Frequent Contributor
timestamp: November 24, 2016, 05:33:17 am
content: The switcher chips really should have decent ground planes and of course better routing of input/output cap and AGND versus PGND. Thicker traces that size down to the actual pads.You need to look at the recommended layout in the TI TPS63050 datasheet. This a 2.5MHz switcher and ground loops/noise can easily cause instability issues. Have you carefully scoped the switcher to verify the the output is stable?Have you performed input/output current measurements to determine efficiency losses?cheers,george.

author: Pedro147
author_rank: Regular Contributor
timestamp: November 24, 2016, 09:07:27 am
content: Thanks George but as I said I am a rank amateur at all thisI have no scope sorry.As for performing input/output current measurements to determine efficiency losses I wouldn't know where to start unfortunately. I realise that all the power traces should have been made thicker but is this really likely to contribute to the heat issue or it more a matter of good design principles?

author: dmills
author_rank: Super Contributor
timestamp: November 24, 2016, 10:16:47 am
content: Not seeing a trace on the bottom between the junction of C11 and C4 to tie that down to ground....Is this actually a 4 layer job with inner layers not shown, because otherwise you have a lot of things just ending at vias with nothing connected on the bottom side?Assuming it is a two layer with a rendering problem, I would personally start by ripping up the bottom layer entirely and placing a ground polygon over the entire back of the board.Tie all the ground pads down to this with local vias and then try to route the rest with a few (and as short) traces on the back as you can possibly get away with.2.5MHz switchers almost always have a section in the datasheet on layout recommendations, and they do tend to mean it.Also, that LED really does want a resistor, even if only 47 ohms or so, you never want an led across a voltage source with only its incremental resistance to protect it, sooner or later that always ends badly.Regards, Dan.

author: georges80
author_rank: Frequent Contributor
timestamp: November 24, 2016, 05:20:41 pm
content: Quote from: Pedro147 on November 24, 2016, 09:07:27 amGo read the datasheet for the switcher. It does have recommended layout information, you need to understand it and implement it. You need to understand why the AGND is separated from the main power ground loop. At switching speeds (and power transfer in the DC : DC converter loops) nasty transients can occur that can couple into sensitive control inputs. Again, read/understand the TI datasheet.Your design has a lot of issues in the layout as far as the switcher layout is concerned. There is MORE to layout than just electrically connecting each point of a net. You should have a decent ground path for the entire design (and understand the TI switcher routing/layout) and then route signals.Without a scope you really have a problem since you'll have no clue what the power rails look like (ripple/stability etc).For a project like this with your experience and equipment you would have been a LOT better off sticking to some linear regulators to power your circuits (using 2 li-ion cells in series) - it's not like they should be drawing more than a few milliamps.And yes, you DO want a series resistor with the LED. Just because the datasheet says Vf of 3.3V that is only a nominal value. Vf vs I is quite steep for LEDs and you could easily be pushing a lot more mA through it than you think. I'd agree that something in the 47 ohm range would be a good and safe choice - it's only a status/power LED, doesn't need to be super bright.cheers,george.

author: Pedro147
author_rank: Regular Contributor
timestamp: November 24, 2016, 10:13:08 pm
content: The board is a two layer with GND planes top and bottom but I just posted an image prior to ratsnest so that the circuit was easier to see. Although probably not an ideal design by professional EE standards (the basis of this board was designed by a hobbyist not an EE) the basic circuit including this 5v power supply is a proven performer with thousands of hours of trouble free operation. My modifications only involved changing this circuit to run brushless motors via proprietary electronic speed controllers (ESC). This necessitated removing the original brushed motor FET's and pull down resistors and re-routing the signal line and a GND line to the sockets and changing the power supply to the Atmega from the original 3.3v to this 5v supply. This was due to the fact that the Atmega32U4 running on 3.3v at 16 mHz is overclocked and was not providing a suitable PWM signal to control the ESC's. The original 3.3v supply is retained to power the MPU6050 gyro / accelerometer IC.Thanks for all your suggestions but all I really want is some thoughts as to whether there is anything in the routing and / or component placement including trace clearances that could be causing this overheating. I realise that it is not an ideal design but as I say this basic design works with thousands of these boards operating all over the world.

author: georges80
author_rank: Frequent Contributor
timestamp: November 24, 2016, 10:34:09 pm
content: If a switcher becomes unstable then it can overheat due to efficiency losses. We've already pointed out that the layout of both the switchers is poor and does not meet the design guidelines in the TI datasheet.Regardless if 1000's have worked before, it's a poor layout and possibly there's been performance issues all along. Are you implying there's 1000's of these in the field with the same PCB layout? Why post a layout that is not the actual layout and then expect suggestions on improving it. Post the actual layout you have issues with. Not sure how you can have ground planes top/bottom if it is only a 2 layer board...If you don't have a scope then debugging will be an issue. At least measure the current being pulled from the 5V switcher (the one you say is overheating). Measure the input current going into it.cheers,george.

author: Pedro147
author_rank: Regular Contributor
timestamp: November 24, 2016, 11:20:43 pm
content: Thanks again for all your suggestions and yes, this is the board in question and it does have a GND plane top and bottom. It is only a two layer board as stated in my last reply.Quote from: Pedro147 on November 24, 2016, 10:13:08 pmI will study the datasheet and look into all your suggestions. I have explained that the original design was done by a hobbyist and that I am just modifying some aspects of it as previously detailed. Yes I get that it's technically a poor layout but is what I have to currently work with modifying. The same basic circuit albeit with different routing works at a normal temperature and this is why I am asking for help. Thanks again.

author: georges80
author_rank: Frequent Contributor
timestamp: November 25, 2016, 12:06:46 am
content: That's not a power and ground plane. That's a 2 layer board with flooded copper in the open areas.You would need to look carefully at those flooded areas to see how they actually connect and how the current paths flow.We we say ground and power planes we typically mean an entire plane allocated to ground or to power in ADDITION to the signal layers. So a board with 2 signal layers and a power and ground plane would be a 4 layer PCB.I'll say it again, there's more to PCB layout than just electrically connecting the nets. You can often get away with poor layout, but not always...Go measure your input/output currents of each switcher core, especially the 'hot' one.cheers,george.

author: Pedro147
author_rank: Regular Contributor
timestamp: November 25, 2016, 12:28:27 am
content: Point taken George and sorry for my confusion. Like I said I am a rank amateur at all this and although I know how to measure current flow in an easily accessible circuit how would I approach doing this on a PCB with these QFN packages please. To my understanding the ammeter would need to be connected in series with those power traces so how would on be able to do this please? I apologise for these basic questions.

author: georges80
author_rank: Frequent Contributor
timestamp: November 25, 2016, 12:43:43 am
content: You would either need 0 ohm resistors in the IN+ and OUT+ traces to each power supply in your layout (that you could temporarily remove to put you ammeter) or you'll need to cut traces to probe the current. In this case since you have no 'diagnostic' 0 ohm resistors in the design, you'll need to cut traces...I assume you've measured with a voltmeter the 5V and 3.3V rails to ensure they're actually regulating correctly? Though without a scope you can't see if they are stable (not oscillating etc).cheers,george.

author: oldway
author_rank: Super Contributor
timestamp: November 25, 2016, 12:50:01 am
content: Quote from: Pedro147 on November 24, 2016, 09:07:27 amThat's your problem, you can't "see" what happens in your circuits....Buy a cheap analog oscilloscope, it will help you a lot....

author: Pedro147
author_rank: Regular Contributor
timestamp: November 25, 2016, 01:09:51 am
content: Quote from: georges80 on November 25, 2016, 12:43:43 amYes I did at least check the input and output voltages and they are within spec. I am starting to see the value of at least assembling this cheap scope I bought a while agohttp://www.banggood.com/DSO138-DIY-Digital-Oscilloscope-Kit-Electronic-Learning-Kit-p-969762.html?utm_source=google&utm_medium=cpc&utm_campaign=Scategory-ds-elc-au&utm_content=nydia&gclid=Cj0KEQiAvNrBBRDe3IOwzLn6_O4BEiQAmbK-DukA8lqdoztKjj9nAaUG2OiK9o893Y7PbOC-dnKoJ9saAogh8P8HAQQuote from: oldway on November 25, 2016, 12:50:01 amThanks for your concern oldway that is very touching

author: Buriedcode
author_rank: Super Contributor
timestamp: November 25, 2016, 03:10:15 am
content: For a 2.5MHz switcher a 1MSps scope won't help at all. Analogue bandwidth is 200kHz which is an order of magnitude smaller than the switching frequency.As other have pointed out, even with ground pour, there's a lot of skinny traces there for a switcher, and if you highlight the traces carrying the switching current - and their return paths - you'll see large loops that can be remedied by changing placement and thickening up traces.First port of call with switchers is to isolate the output, cut the trace going from the output to anything else (except of course the output cap), solder to a wire so you can add a light load to the output, say a 220 ohm resistor with a red LED.Then with a single volt meter you can measure the output voltage (output node to ground) and the output current (output node to otherside of resistor, I = V/220). if those values seem 'reasonable' then with the dummy load still connected, use a ~10ohm resistor in series with the input to measure input current. With your traces cut you can now power the rest of your circuit (that the switcher was powering) with a bench supply - that might be the problem, drawing too much current. Sorry if I'm pointing out the obvious here, others have said it as well.Measuring efficiency like this is less than ideal, as of course one would want to measure input voltage, input current, output voltage and output current at the same time but the above is a cheap method thats a good first step.

author: Pedro147
author_rank: Regular Contributor
timestamp: November 25, 2016, 03:32:07 am
content: Thanks for some very specific methodology Buriedcode I appreciate that.

author: Zero999
author_rank: Super Contributor
timestamp: November 25, 2016, 08:47:51 am
content: It appears poor thermal design is the main reason for this. Unfortunately the only real solution is another board. Pay attention to section 10 - layout on page 16 of the datasheet. Note the large ground plane and thermal vias under the pad. You could try soldering a piece of copper strip to the thermal pad with the ends sticking out, then soldering the IC to the board, to act as a heat sink but it'll be a fiddle.http://www.ti.com.cn/cn/lit/ds/symlink/tps63000.pdfAs far as the LED is concerned, yes, always use a resistor. One problem LEDs have is negative temperature coefficient, at any given current, the forward voltage drops, as the temperature rises. With no resistor to limit the current, the LED will warm up, causing the forward voltage to fall, resulting in more current drawn and it getting hotter, until it emits magic smoke; this is known as thermal runaway. One problem is, this may not happen on your work bench, but it could in this field, especially if it's hotter. One thing which might be helping to avoid thermal runaway here, is the LED is connected to the output of an MCU which will already have a significant resistance, but you should never rely on it.

author: Pedro147
author_rank: Regular Contributor
timestamp: November 25, 2016, 09:04:02 am
content: Thanks for your reply. The large thermal pad under the IC has been incorporated into the board design and as repeatably stated this basic design although not technically correct has worked well on thousands of boards. This to me would logically indicate either a hardware issue as in some minor short under either the MCU or the buck boost itself or some other routing issue of which I am unaware. Same thing in regard to the no current limiting resistor thousands of boards and no high failure rate.

author: Pedro147
author_rank: Regular Contributor
timestamp: November 29, 2016, 06:56:20 pm
content: Just as a final update I decided to assemble another board and am experiencing no excess heat issues with the 5v buck boost. I can only assume that as I first suspected it was a minor short under one of the two QFN IC's causing this issue.I have done a second revision of the board and swapped the positions of the 10uF caps to tidy up that bad routing. When I do a third revision I will certainly take all your suggestions on board and I once again thank you all for your suggestions of how to do things the right way