Overheating Problem with MC34063A  - Page 1
Forum section: electronics / general

author: Amir2014
author_rank: Contributor
timestamp: December 18, 2014, 08:36:19 pm
content: Hi guys,Lately I have been working on design and build a Buck DC power supply (step-down DC-DC converter) using MC34063 chip (texasInstrument) with following specifications:Vin = 24VVout = 12VIout(max) = 750 mAVripple = 50mVFmin = 25 kHzI used both formulas from the datasheet, and the following calculator tool to find the values for my components :http://www.nomad.ee/micros/mc34063a/Therefore the components that I used are the following:L = 150 uH with 6A ratingCo = 220uFCin = 100 uFCt = 1nF (but I also tried it with 660pF)Rsc = 0.26 ohmR1 = 3.9Kohm, R2= 33Kohm (~11.8 Vout)PROBLEM:The problem is even-though my design is for 750 mA output current, when ever I draw 200mA form the circuit the MC34063 chip gets hot rapidly, causing it to shut down, or gets damaged !! The datasheet indicated that it can handle up to 1.5 A current throw its internal transistors, but it can't even handle 200mA, and it get overheated in matter of seconds !!!I have tried several suggestions from other websites with similar problem, but it still doesn't work, and I am running out of clues!!I will deeply appreciate any help or suggestion towards solving this mysterious problem.I have attached the a capture image of the design schematic.Thank you.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 18, 2014, 09:10:59 pm
content: Simple solution: dump it. 34063 is an ancient beast and should be left buried to history where it deserves. Proper all-in-one regulator chips are available, and they don't cost much more (you save at least as much on the massive inductor and capacitor the 34063 requires).Do you have an oscilloscope handy?Tim

author: Amir2014
author_rank: Contributor
timestamp: December 18, 2014, 09:28:14 pm
content: Thanks for reply Tim,I have started my design with MC34063, and there is not much of turning back, not to mention there have been many applications where MC34063 where used effectively.Having said that I am not a big fan of MC34063, but at this point my problem is not choosing a different switching power supply chip, but getting my current design with MC34063 work.And yes, I do have Oscilloscope handy,... And I did monitored and measured the behavior of my power supply with 34063, but non-explains as to what am I doing wrong in my design.Thanks for your advise regardless, If you have any other solutions, all be glad to take in consideration.Amir,

author: wraper
author_rank: Unknown
timestamp: December 18, 2014, 09:31:34 pm
content: What diode type do you use?

author: free_electron
author_rank: Super Contributor
timestamp: December 18, 2014, 09:44:16 pm
content: Quote from: T3sl4co1l on December 18, 2014, 09:10:59 pmthat's what my standard answer is. grind that crap to dust. got no protection whatsoever .

author: Amir2014
author_rank: Contributor
timestamp: December 18, 2014, 09:48:31 pm
content: Thanks for your reply wraper,I used the same schdttky diode as it was suggested in datasheet (1N5819), but I also tried a more powerful schdttky diode such as VSB3200.Thanks,Amir.

author: wraper
author_rank: Unknown
timestamp: December 18, 2014, 09:55:43 pm
content: Then there a to most probable possibilities: 1. You breadborded the schematic and it compromises normal operation because of the long wires/loops. 2. Use another type inductor.

author: Amir2014
author_rank: Contributor
timestamp: December 18, 2014, 10:07:46 pm
content: Thanks for reply wraper,But I don't think the problem is a breadborded circuit with long wires, because I soldered the circuit on a prototype board making sure that components neatly placed and close as possible to each other, but I also tried it on solder less breadboard, and in both case MC34063 still overheats. Other techniques such as putting a small bypass caps. where also used before.Also, I did tried a different inductors with higher and lower values (as it was suggested on other websites), and non seem to make any difference on overheating the MC34063 chip.Thanks,

author: PChi
author_rank: Frequent Contributor
timestamp: December 18, 2014, 10:09:38 pm
content: It's difficult to know what's wrong without more information. Can you post a picture of the circuit?By the way in a buck converter the input capacitor has to survive more ripple current than the output capacitor so should generally be higher capacitance.The MC34063A is an ancient heap and will get hot with higher currents the VCS(SAT) is pretty high so the power dissipation will be high.

author: Amir2014
author_rank: Contributor
timestamp: December 18, 2014, 10:39:09 pm
content: Thanks for reply PChi,I did put a relatively a large Cap. at the input, 2 of 220 uf in parallel(440 uf) as you can see on my solder less bread board picture, while the output has only one 220 uf.Also, please note that I also tried to put an external power transistor (BD139 NPN for both boards) to make sure at least MC34063 can survive, but that did not solve the over heating problem.In this case, the only difference is that Power transistor got overheated and damaged, stead of MC34063 chip. (around 150 degree C when I measured the heat at out put current of 400 mA).This proved to me that some thing on my switching power supply is not behaving properly, since a switching power supply are not suppose to dissipate too much heat, due to their high efficiency compare to linear power supply.Thanks.Amir.

author: wraper
author_rank: Unknown
timestamp: December 18, 2014, 11:02:27 pm
content: Rsc leads must be kept as short as possible. My guess it is those green resistors with long wires in series. Moreover those green resistors look very suspicious for me. If they are wirewound, they definitely will cause issues.

author: Amir2014
author_rank: Contributor
timestamp: December 18, 2014, 11:10:10 pm
content: Thanks for reply Wraper,Those green resistors are 0.13 ohm (each) 5% 1W, initially was chosen for different output current and they are not wirewound. But before I did had them soldered with no long leads, but with same overheating results.Also, not to mention, I did tried a different resistors and values for Rsc on the solder less bread board.Thanks,Amir.

author: wraper
author_rank: Unknown
timestamp: December 18, 2014, 11:19:07 pm
content: If you are destroying your ICs anyway, just put a short instead of them. Those resistors are for current sensing for IC internal protection but as it burns anyway... Why are you sure that they are not wirewound as they look exactly as they are? Rsc do not need to be high power. 1/2 w is completely enough for test or just a jumper.

author: wraper
author_rank: Unknown
timestamp: December 18, 2014, 11:24:25 pm
content: Similar resistors but blue color, wirewoundhttp://uk.farnell.com/multicomp/mc14694/resistor-0-13-ohm-1w/dp/1306667

author: Zero999
author_rank: Super Contributor
timestamp: December 18, 2014, 11:30:00 pm
content: Where did you get those resistors from?Wire wound inductors are inductive, especially the lower values. Try replacing them with carbon/metal film.

author: Amir2014
author_rank: Contributor
timestamp: December 19, 2014, 12:04:51 am
content: Thanks for reply Wraper and AcHmed99,As I mentioned, I did replace those Green Rsc with different resistors (I did try 1% metal film), but that caused the overheat on my transistor anyway.But as you said, I did replace them with just a jumper wire, and the results where still the same, too much heat build up at the transistor.But Thanks for your suggestions, and I still looking forward for any possibility towards resolving this issue.Amir.

author: Amir2014
author_rank: Contributor
timestamp: December 19, 2014, 12:18:34 am
content: As a reply to your last comment AcHmed99,The peak current is twice of the output current, and I am saying my overheating is happening at only 200 to 300 mA (thats just only 0.3 A).Not to mention I am currently using a external transistor to over come much higher current, but overheating is still there.And I don't see how my Rsc set point is at 1.5 A when according to datasheet and calculations is not.Also, I have attached a sample application Note suggesting that this circuit should have up 90% efficiency with 12 volts out voltage, 750 mA output current, and input voltage of up to 36 volts. Not to mention its even without a external transistor.But I hope I can find the answer to this mystery.Thanks,Amir.

author: rob77
author_rank: Super Contributor
timestamp: December 19, 2014, 12:50:12 am
content: btw... why are you trying to get 750mA out of the poor 34063 ? i like that little IC and i'm using it a lot, but i never got the clever idea of pumping more than 200-300mA out of it while using it as a buck converter.for 700mA use LM2576, that's a much better choiceor eventually you can use one of the modern high switching frequency ICs (those are only in SMT and designing the board is not trivial).

author: rob77
author_rank: Super Contributor
timestamp: December 19, 2014, 01:02:37 am
content: btw.. i think i identified your problem - those inductor on the attached pictures in reply #9...the soldered version has a inductor marked "330" - that's 33uH (NOT 330uH!)- and definitely not 150 as calculated. and the bredboard version has a inductor with low inductance as well (probably salvaged from a MoBo by the looks of it ?) => again just a few micro-Henrys instead of the calculated 150

author: Amir2014
author_rank: Contributor
timestamp: December 19, 2014, 06:30:44 am
content: Thanks for your reply rob77,The MC34063 is not as poor as you think they are ! If you refer to the my reply # 18, the circuit that they suggested on their Application note is proven working, and its within the chip parameters. Not to mention I am currently using external power transistor to handle higher currents, but the problem with overheating remains the same.Also, as I have said in my previous posts, there is no turning back now for choosing a different switching chip.And yes, the inductor in the solder bread board is 33 uh (I used to operated the chip at higher frequency at first), but I soldered 2 in series making it 66 uh if you see closely.And the inductor at the solder less bread board is approximately 150 uh as I measured it with LC meter my self long ago when I purchased it. (You can't just jug value of a inductor like that by looking at in a picture).thanks,Amir.But again, inductor values, doesn't seem to be the cause of over heating the chip or the power transistor any way. I tried many inductors with different values, but results are the same.

author: SteveyG
author_rank: Unknown
timestamp: December 19, 2014, 08:58:36 am
content: Quote from: AcHmed99 on December 18, 2014, 11:31:33 pmCan you explain this please?

author: SteveyG
author_rank: Unknown
timestamp: December 19, 2014, 12:03:03 pm
content: I must have missed the converter operating in DCM. That inductor is less than ideal.

author: free_electron
author_rank: Super Contributor
timestamp: December 19, 2014, 03:53:45 pm
content: a BD139 is too slow for the switching frequency of the chip and the chip can never feed enough base current to shoot that thing open fast enough.you are trying to use analog transistors where you need switching transistors.

author: Amir2014
author_rank: Contributor
timestamp: December 19, 2014, 06:12:36 pm
content: Thanks guys for your thoughts,First, The BD139 is not a slow transistor and it does switch on and off last time I look at the Vce on oscilloscope (not to mention frequency that the chip set for is not high either). But as I have said this before, regardless if I use the transistor or just the internal transistors of the chip, it can not handle more than 200mA of out current as it get hot rapidly.Second, No I haven't take the Rsc measurements from oscilloscope yet, but I will soon. And suppose if the inductor is causing the saturation, what do you suggesting I should do?And no, there are no miss-wiring, as I have rebuild and check the circuit over and over again. Not to mention I have shown to many of my friends working with me on the project. Beside, it does regulate the 12 voltage, but it can't handle 200mA and up, when I see its within the capabilities of the chip and the external transistor.Thanks,Amir.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 19, 2014, 06:16:30 pm
content: Switching speed is masked by the inductive load. The collector voltage might transition in 50ns, but it may take 500ns or more to cease current flow.In particular, I see no evidence of a base-emitter turn-off resistor on your breadboard. The "triple Darlington" structure thus created (including the two transistors inside the 34063 itself) will drop a considerable voltage while operating, even if switching properly.BD139 is an utterly unremarkable device, and like 2N2222 and MC34063, is best left to history. Better devices are available, including MOSFETs.Tim

author: PChi
author_rank: Frequent Contributor
timestamp: December 19, 2014, 07:24:17 pm
content: Thanks for the picture.I don't know why the MC34063 is failing though it will run hot even at 200 mA due to the voltage drop across the transistor switch.Its' possible the part is misbehaving due to layout issues.The parts may be failing due to voltage transients. I would recommend soldering a circuit with some low ESR capacitors and reasonable layout.Is the inductor OK up to 6 A without the core saturating. Can you post some waveforms of the voltage across the Schottky diode?Application Notes are written to help sell the device. They aren't necessarily completely truthful about device limitations.Bipolar power transistors are pretty slow, suffering from both storage time and slow fall time.As T3sl4co1l states if it was driven fron MC34063 pin 2 it would get hot because it would be dropping an extra 0.7 V approximately. It would also need a low value base emitter resistor to be in with a chance of working at 25 kHz. At a guess a 100 ohms.