Unexpected amount of heat - Page 1
Forum section: electronics / projects

author: OM222O
author_rank: Frequent Contributor
timestamp: May 12, 2019, 01:59:34 pm
content: I built an evaluation board for a current sink with an op amp driving an NPN darlington pair transistor, via a 1k\$\Omega\$ base resistor.The Collector is connected to +12V and the emitter is kept at 1V with the shunt resistor being 1\$\Omega\$. I calculated the temperature rise to be 11 degrees which I knew is extremely optimistic, but it was based on the data sheet values.Thermal resistance is given to be 1C/W and the power dissipation is (12-1)*(1/1) = 11 watt. 11 volts at 1A.here is the data sheet for the darlington pair:https://www.st.com/content/ccc/resource/technical/document/datasheet/34/7a/29/f6/d1/3e/4c/58/CD00001247.pdf/files/CD00001247.pdf/jcr:content/translations/en.CD00001247.pdfI also can see that 1A at 11V is well within the DC SOA of the D2PAK package (which is the one I'm using).but in practice after about 5 seconds the package gets very warm and after about 10 seconds I can't touch the part!I have upgraded to this part since the previous pass element I used was the MJD6039T4G which ended up failing shorted and killing an expensive op amp with it (pulled the base to +12 which was connected to the output of the op amp which had a max VCC spec of 6V and was powered from 5)I'm worried that the same thing will happen with this part too. Please let me know where I'm making mistakes in my calculations and how can I prevent the parts from going nuclear?Edit: in case a schematic helps:Vset = 1.024 (just round it to 1)Rset = 1\$\Omega\$Rload = 0Vcc = 12vRbase = 1k\$\Omega\$

author: Siwastaja
author_rank: Super Contributor
timestamp: May 12, 2019, 02:18:08 pm
content: Thermal resistance is 1C/W fromjunction to case. You need to add the thermal resistance of your heatsink in series to the calculation. If your PCB is acting as a heatsink, I recommend you google some similar D2PAK datasheets to find example PCB layouts with their evaluated thermal resistancesjunction to ambient. Some device datasheets or appnotes list full junction-to-ambient resistances for two or even three different PCB layouts.Or, you can just measure the temperature you are reaching, and depending on that, improve the PCB heatsinking layout and iteratively test if it's getting good enough:- Make the collector pad bigger. After you have stretched it about an inch in each direction, the effect starts to be fairly limited, since copper is thin and thermal resistance builds up going sideways.- Add another collector copper region on the bottom, and add thermal vias to connect the regions together,If this still isn't enough,- Mount the PCB to a metal case, add thermally conductive silpad material as the interface material. You need screws close to the transistor so that there is no board flex causing gap. Heat flows from the transistor to the top copper fill, then through the vias to the bottom copper fill, from there, through the silpad, to the case.Guesstimating from experience, dissipating 11W is possible with these means, but not trivial; a small standard D2PAK footprint definitely won't do it.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 12, 2019, 02:47:04 pm
content: I can't add thermal vias as the board is single sided (other side must be used as the front panel) and the box is a plastic one.In actual operation the device won't be on more than 2seconds on the 1A range and the maximum voltage will be 9 instead of 12, so a maximum power dissipation of 8watts. I'm still worried about the device being damaged if stressed repeatedly. I try to add more copper area next to it for sure. Top and bottom have massive ground planes which are in very close proximity of the device as well, I'm just not sure how much the thermal resistance of the FR-4 material is. the data sheet also mentions nothing about junction to ambient thermal resistance ...can you please provide some links to layouts with similar packages which provide temperature calculations / thermal resistance for a D2PAK package with different layouts? that'll be extremely helpful.Also can I use some of those cheap SMD heat sinks (basic aluminum fins) to further protect the device from thermal runaway?for example this one:https://www.mouser.co.uk/ProductDetail/Aavid/7109DG?qs=sGAEpiMZZMttgyDkZ5WiujTlBOILjl7xi4aoVUO77j4%3DI'm not sure how large is the thermal resistance of the plastic casing ... I doubt this will help at all?I really don't want the device to fail, but it's ok if it operates at a high temperature (less than 100c just to be on the safe side).

author: Siwastaja
author_rank: Super Contributor
timestamp: May 12, 2019, 03:44:49 pm
content: A solderable heatsink is a good idea. The datasheet for that one provides two contradicting unitless numbers for thermal resistance, magical "9" and "11", but thermal product manufacturers are notorious for messing up the basics of documentation, including not understanding the basic units of their trade, so we just need to assume it's 11 degC/W, and finally, test to verify this assumption.The datasheet says this is valid at 75 degC heatsink dT, in freely moving air. This is a bit demanding condition: your "ambient" would be inside the box I guess, your air probably wouldn't be completely free to move, and you probably couldn't accept dt=75, especially if you are aiming for 100degC transistor temperature - you would need to guarantee your box insides staying at 25degC max! So in practice, the Rth for that particular heatsink would behigherthan 11 degC/W in your conditions.Still assuming 11 degC/W, the total thermal resistance junction-to-ambient would be 11 + 1 = 12 (degC/W) then. 11W dissipated would cause a 12degC/W * 11W = 132 degC temperature rise, clearly beyond unacceptable. So, you'd need a bigger heatsink.I guess the thermal resistance number they give already includes what the case of the D2PAK, and their recommended footprint through FR4 already dissipates, so you can't expect any extra from it, unless you add a lot of more copper. With thermal vias out of question, you are starting to lose your options.Pulse duty with short pulses is helping - D2PAK has quite a lot of thermal mass. But then, you'd need to failsafe your design somehow to prevent duty cycles too high. Add a thermistor closely coupled to the transistor, preventing operation while too hot? This would likely be the cheapest solution.Finally, I'd look into reducing the dissipation by changing what you are actually doing, and how. 11W is a lot of dissipation in a small integrated device, and very difficult to achieve in SMD design, especially when you only have a single side available. You can look at larger SMD heatsinks, of course, but then you'd need to ventilate the case as well, if you expect the possibility of large operation duty cycles, that is.QuoteFirst result on Google with "D2PAK MOSFET":https://www.vishay.com/docs/91055/sihf740s.pdfJuction-to-case 1 degC/W (no surprise, it's the same case)Junction-to-ambient, bare device, 62 degC/WJunction-to-ambient with example PCB layout of 25.4 x 25.4 mm copper pad on FR-4 - I'dassume2 oz copper as they are too lazy to define it, 40 degC/WAnother good one, see Figure 7:https://www.microsemi.com/document-portal/doc_download/124720-an-208-pd70101-pd70201layout-guidelines-application-noteBasically, on a single layer without thermal vias, you are limited to a bit over 30 degC/W, even reaching infinite copper area, and this is on 2oz copper. You likely have 1oz, I'd guess, making things worse.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 12, 2019, 03:55:43 pm
content: thanks for the detailed answer! I can place a hard lock of 5s cool down after each measurement (which again, only lasts for 2 seconds and dissipates about 8 watts). I can set up a test and measure the temperature over a time period to confirm that is enough cool down time or increase it if necessary.however a hardware solution such as a PTC would be amazing just in case. I'm just not sure if I can find one with low enough value. as I said I have to pass about 1A through it and I only have about 8 volt headroom. can you recommend a circuit which allows for limiting the temperature rise without causing too much burden voltage?also those figures seems pretty awful! There is a display on the board and I can use it's under side for vias, but it's at a really awkward position and I have to re route the PCB for that, but I'll give it a shot.edit: on a second look, this part seems fairly decent for the task at hand:https://www.mouser.co.uk/ProductDetail/Murata-Electronics/PRG21BC0R2MM1RA?qs=makukexe9nzHJ%252Bvxp0S09w%3D%3D0.2 ohm isn't bad at all and it has a "hold current" of 750mA and a max of 10A. I'm assuming combined with the pulse method this should guarantee that the D2PAK device is protected against temperature damage?even the 0.6 or 1 ohm units don't seem too bad and the curve seem to fit a lot better for the 100c max target?

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 12, 2019, 04:10:50 pm
content: Quote from: OM222O on May 12, 2019, 01:59:34 pmthis is what we should expect when 11W is drawn from such a small package. this has nothing to do with SOA, this is thermodynamics. you need a heatsink...

author: OM222O
author_rank: Frequent Contributor
timestamp: May 12, 2019, 05:50:32 pm
content: I searched a lot but I can't seem to find a thermal calculator or a design tool that allows for simulations of it. can anyone introduce a design package for thermal calculations? the only thing I found was trace width calculations which are not really important in this case. I assume there has to be a design tool since this is a real concern in a lot of power delivery systems such as DC-DC converters and LDOsedit: TI has something which is near indecipherable:http://www.ti.com/adc/docs/midlevel.tsp?contentId=76735&DCMP=hpa_design_center&HQS=Tools+OT+pcbthermalcalcP.S: I can use a larger package if such thing exists, the only requirement is being a SMD part. AFAIK D2PAK is one of the largest beast there is in terms of package sizes except for D3PAK which is not much better with handling power ... maybe the added thermal mass would help?

author: Yansi
author_rank: Super Contributor
timestamp: May 12, 2019, 06:00:12 pm
content: It simply is quite nonsense to expect a D2PAK to dump 12W of power on a single sided board. That may be possible ONLY on an aluminium substrate board with adequate additional heatsinking, or on a multi-layer board, where therma vias are used and heatsink is attached to the other side of board.Do not expect a D2PAK on a single sided board to dissipate more than like 3W. Even that is rather on the hot side.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 12, 2019, 06:07:08 pm
content: Quote from: Yansi on May 12, 2019, 06:00:12 pmit's not a continuous load + the board isn't actually single sided, there are a ton of vias stiching ground planes under screws and on the edges of the board that are in close proximity of the device. they don't exist on the evaluation board however, so this is a very "worst case scenario". Siwastaja also mentioned a lot of helpful advice. so if you don't have anything to add to the discussion, or any useful hints / recommendations, please don't derail the topic. thanks!

author: Yansi
author_rank: Super Contributor
timestamp: May 12, 2019, 06:24:46 pm
content: I have just stated obvious facts.If it not a continuous load, that work with an average dissipated power, or model the entire thing using thermal impedance (time constants), if the period of power dissipation cycle is comparable to the thermal impedance time constants.For modeling the component, use what datasheet suggest (usually, there are graphs for just that), and you can measure your heatsink/pcb thermal properties yourself.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 12, 2019, 06:51:56 pm
content: Quote from: OM222O on May 12, 2019, 06:07:08 pmThese help a bit, tying the ground planes together thermally, but the collector pad coupling to this plane will be poor if you have (assumably) 1.6mm of FR4 inbetween. If you must have ground potential on the bottom copper (your front panel), then the best thing you could do to improve thermals is to use a 4-layer board, with the stackup that has minimized distance between the top layer and the adjacent mid layer. 100um separation is 16x improvement compared to the standard 2-layer stackup! Then, this mid layer needs to transfer heat sideways a bit, to reach all the ground vias, use a lot of them. Use 2oz copper to further improve the thermals, but again, this tends to increase PCB cost further. Much better than what you have now, but I understand that a 4-layer PCB would add cost. I guess you could get down to around 20 degC/W with this. Maybe if you combined this approach with an SMD heatsink of about 10 degC/W, the combined thermal resistance of around 7 degC/W might be acceptable for the 9W dissipation, just maybe?But I don't know whether such optimization for the peak load dissipation makes much sense - as you have identified, your dissipation is pulsed, and your duty cycle is low, with pulse length of only 2s, and period of 7s, peak power of 9W, and average power of 2.6W. Now, while the D2PAK with a standard footprint isn't capable of handling this 2.6W average dissipation, it should be achievable with enlarged copper area, and coupling through FR4 to your stitched ground planes. The excercise left for you, indeed, is to guarantee staying within this duty cycle, and pulse length, and hence, average power, without assuming anything about how the user behaves. Again, a simple thermistorlogicallypreventing the operation (preventing base drive, for example) in case of overtemp might be the simplest, as it wouldn't need to handle power. Depending on your actual circuit, of course.

author: IanB
author_rank: Super Contributor
timestamp: May 12, 2019, 07:23:00 pm
content: Quote from: OM222O on May 12, 2019, 01:59:34 pmTo give you some kind of reference to guide your thinking, consider a small incandescent lamp, e.g. 6 V, 1 A, 6 W, or 12 V, 1 A, 12 W. Consider that the filament of such a bulb glows white hot, and the glass envelope of the bulb will become too hot to touch after only a short time running (you will probably burn your fingers on the 12 V lamp).Now consider the small piece of silicon inside the transistor, of roughly the same area/volume as the filament of the lamp. Imagine this also glowing white hot under a similar level of power dissipation.This may give you some grounding in the (considerable) amount of heat sinking you need for the package if it is to have any hope of surviving.

author: Gyro
author_rank: Super Contributor
timestamp: May 12, 2019, 08:18:49 pm
content: A 6W soldering iron analogy would be more appropriate and understandable than a filament lamp.

author: IanB
author_rank: Super Contributor
timestamp: May 12, 2019, 08:27:59 pm
content: Whatever gives an appreciation of what 6 - 12 W of heat generation means should be helpful. The thread title about an "unexpected" amount of heat is slightly strange. Anything that generates that amount of heat in a small space is going to get very, very hot. Hot enough to burn your fingers.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 12, 2019, 09:16:08 pm
content: Quote from: Siwastaja on May 12, 2019, 06:51:56 pmI will order some PTCs with a few different values and carry out some tests. guaranteeing the low on time can easily be achieved in software and it's not an issue. also the plane as about 10 mil gap to the sides, it doesn't have to pass through the entire 1.6mm of FR4 (just a few mills to jump to the ground plane ... I'm not sure how to calculate that). I just wonder if the thermal mass is large enough to prevent the transistor from breaking again (in the 2second on time) like the previous one did, as it will short out the op amp too (about 5$ damage replacing both of them) unless I include some schottky diodes from op amp output to +5V rail and rely on the 1k base resistor to limit the currentgenerally speaking it won't be under that amount of load for much longer than 2 second anyways since measurements aren't repeated back to back, again I'm just assuming a worst case scenario here since it happened with the other transistor ... it was just too much for the poor thing to handle. I'm not sure if the die will reach the specified 150C within 2 seconds or not and there isn't a simulation tool which I can use to find out

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 12, 2019, 09:21:19 pm
content: Quote from: OM222O on May 12, 2019, 03:55:43 pmbut you are testing it up to 10 sec in OP and surprised that it got too hot. which engineering stress test you want to satisfy? 2 sec? or 10 sec? and from the impression, you want minimal heatsinking. there are more involved, such as how air is ventilated in your product, more ventilation, less heatsink (metal) is needed, no ventilation? you can calculate maybe from thermal conductivity of various materials in your product...http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/thrcn.htmlhttps://www.engineeringtoolbox.com/air-properties-viscosity-conductivity-heat-capacity-d_1509.htmli'll just go with try and error method until i'm happy with the system. if you want PTC route, fine, thats another try and error method. ymmv cheers.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 12, 2019, 09:33:40 pm
content: You can look up the basic physical equation forspecific heat capacity, weigh the transistor, assume it's close to the specific heat capacity of copper (since the large copper tab would dominate its mass), and simply calculate the temperature rise for your energy of 9W times 2 seconds. The result would be pessimistic, assuming no heat flow out during the pulse, and ignoring the mass of solder added (which will be more significant than the mass of copper in PCB pad). This way you get a rough idea whether you can ignore the pulse length and just calculate using average power, or if there is significant per-pulse heating.For average dissipation, you need:1) surface area,2) thermally conductive paths to said surfaceFor peak dissipation, you need:1) mass,2) thermally conductive paths to said massIt's well possible that adding extra solder by adding solder stencil openings to the collector copper fill does the trick for practically free.Oh, you have software, you can easily control the duty cycle. If you fear about user rebooting to skip the wait-time, just insert an extra wait right after the boot.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 12, 2019, 10:16:46 pm
content: Quote from: Siwastaja on May 12, 2019, 09:33:40 pmyes that seems like a very reasonable approach! unfortunately I have no way of accurately measuring the mass of copper so I just assumed a 1g estimate (not sure if it's too small or too large). specific heat of copper is 0.385 J/g.k which means with a 2 second on time, it's only about 8.5 degrees temperature rise. more than acceptable! no heat sinking required. I think this is actually a very good estimate as well, as it's inline with initial testing (same estimate results in 46 degrees after 5 seconds which feels hot and 68 degrees after 10 seconds which feels uncomfortably hot). I think 73 degrees is the max you can touch without getting burnt ... not sure if it's accurate, just read it somewhere.So knowing that with pulsed load and low duty cycle I shouldn't require any heat sinking at all since the 8/9 degrees is more than reasonable here!thanks a lot for the help. you saved my day!

author: IanB
author_rank: Super Contributor
timestamp: May 12, 2019, 10:22:27 pm
content: Quote from: OM222O on May 12, 2019, 10:16:46 pmIf you know the plating thickness of the copper and you measure the area you can estimate the mass that way.QuoteThe duty cycle matters. If it doesn't fully cool down after each pulse the next pulse will reach a higher maximum temperature.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 12, 2019, 10:30:11 pm
content: I meant the copper used in the D2PAK since that is similar to junction temperature (1C/W resistance). to measure that I need to cut off the 2 legs, measure the mass of the package with a sensitive scale (don't have anything that's more accurate than 1gram at the moment) and dissolve the package in some sort of acid which then leaves the plastic package and silicon behind. finally measure the mass again and find how much was lost which equals to the mass of the tab. IMHO the 8.5 degrees increase is more than fine which again, is a very bad estimate since it ignores the thermal mass of everything else + the amount pulled away by the PCB copper

author: mikerj
author_rank: Super Contributor
timestamp: May 13, 2019, 08:16:59 am
content: You seem to be fixating on the junction to case thermal impedance, but it's almost irrelevant in this situation. Junction to case gives you a idea of how hot the junction will run given a case temperature and a power dissipation value, but you primary issue here is case temperature since you don't have sufficient heat sinking.

author: schratterulrich
author_rank: Regular Contributor
timestamp: May 13, 2019, 08:42:21 am
content: I have written a tool to simulate the thermal behaviour of pcbs.It can at least give you a feeling of how copper planes and vias affect thermal spreading.Unfortunately it is only useful for steady state scenarios.You can find it atleiterplatte.jimdo.com/thermal-pcb-sim/

author: OM222O
author_rank: Frequent Contributor
timestamp: May 13, 2019, 02:24:40 pm
content: Thanks for the simulation tool but I can't seem to be able to do anything and constantly get this error message:it couldn't even create an error log which is interesting ... can you please provide a guide on how to use this app?

author: schratterulrich
author_rank: Regular Contributor
timestamp: May 13, 2019, 03:30:31 pm
content: Hmm...It seems the program tries to save the project in the c:\Program Files\... directory, where it doesn't have write permission.Possible solutions:a) Please save the project first in a directory where you have read/write permissionsb) If a) doesn't work: copy the file XVIIx64.exe in your project directory and set the LTspice Path in the tool accordinglyIf a) and b) don't work look for "Show Console" in the settings tab and activate it. Then post me a screenshot of the log.

author: jmelson
author_rank: Super Contributor
timestamp: May 13, 2019, 08:20:26 pm
content: Quote from: Siwastaja on May 12, 2019, 02:18:08 pm11 W into a PC board pad? No, NOT going to work (at least, not without a fan blowing directly on it.)You need a "real" heat sink, like a chunk of aluminum with fins. A TO-220, or maybe even TO-247 package might be better, once you get into 10's of Watts being dissipated.Just because the data sheet says it can dissipate this power doesn't mean it can handle it without external means of cooling.And, the data sheet SOA figures are often with an infinite heat sink at 25 C.Jon