Is there really anything wrong with the MC34063? - Page 1
Forum section: electronics / projects

author: SingedFingers
author_rank: Frequent Contributor
timestamp: March 17, 2017, 01:22:24 pm
content: I am building a 12->5v 100mA buck converter to drive some mixed HCT/TTL logic from a SLA battery to drive a rather ancient alarm. The 78L05 that was in it before snuffed it because it was completely overdriven and just glued to a piece of metal (). Unfortunately the glue snapped off by the looks. The hot weather suddenly a couple of days ago finished it off by the looks as it blew it's arse out. This was not my design for ref - I inherited it from my father who has a dubious engineering background. I don't want to throw it away though as it works pretty well when it isn't exploding.Soooo today, on the breadboard I am using an MC34063 (ST variety) and can get it to kick out that at around 82% efficiency and not even warm to the touch which is the main thing. It's also cheap, I've filtered the output so it's not too noisy with a Pi filter and I can't seem to destroy it whatever I try as it has a suitable current limit programmed via sense resistor. This also knocks the idle consumption of the device down by 75% which is nice so the battery will last a ton longer before I have to recharge it.However I hear all sorts of horrible things about it when googling. Is there really anything fundamentally wrong with this IC at all or any edge cases that I don't know about before I go and sling this all back together to return it to its usual duty of stopping the pikeys nicking my stuff out of the garage

author: bktemp
author_rank: Super Contributor
timestamp: March 17, 2017, 01:33:40 pm
content: It is old, it is not super efficient, it is not really a PWM controller more like a gated free running fixed duty cycle oscillator with a duty cycle reduction circuit on overcurrent. So the switching waveform often looks more random, therefore the frequency content changes depending on the load and input voltage.If you don't care about the slightly higher ripple on the output voltage (a simple filter often does not help much for a 34063 for low loads), a 34063 is ok. It is cheap and it works.

author: snarkysparky
author_rank: Frequent Contributor
timestamp: March 17, 2017, 01:37:31 pm
content: Also the output switch is a darlington configuration which means there will be 0.7V drop which causes the heating issues many report.

author: SingedFingers
author_rank: Frequent Contributor
timestamp: March 17, 2017, 01:46:00 pm
content: Quote from: snarkysparky on March 17, 2017, 01:37:31 pmThanks that actually makes sense so you're losing 70mW @ 100mA on just that transistor. Not enough to make this warm on this load though which is fine.Quote from: bktemp on March 17, 2017, 01:33:40 pmI'm not seeing much ripple on it at all even when it's switching 100% of the time. I've got 2x AVX 47uF tant and a 100uH inductor in Chebyshev low pass arrangement and I can't see anything on the scope with a 50 ohm termination which corresponds to a 100mA load on 5v.

author: coppice
author_rank: Super Contributor
timestamp: March 17, 2017, 01:59:54 pm
content: The MC34063 is only cheap to use if you get inductors for free. A more modern chip will run at a much higher switching frequency, and allow the use of cheaper inductors.

author: bktemp
author_rank: Super Contributor
timestamp: March 17, 2017, 02:03:58 pm
content: Quote from: SingedFingers on March 17, 2017, 01:46:00 pmAt 100mA it will operate in a continous switching mode. Try reducing the load to <10mA, then it will probably go into some erratic burst mode. But the behaviour also depends on the ESR of the output capacitors.

author: SingedFingers
author_rank: Frequent Contributor
timestamp: March 17, 2017, 02:15:26 pm
content: It's only driving about 25mA at average. There is a peak of 85mA when the alarm is sounding. I'm replacing the output driver with a logic level MOSFET as it's currently a tip41c.Output caps are around 1 ohm (AVX TAJ).Inductor cost 3x more than the IC did!I'm actually starting to think I should just redesign the whole thing and use a PIC I've got lying around. Or get an esp8266 and get it to send me a text message when some pikey scum breaks in.

author: cyberfish
author_rank: Regular Contributor
timestamp: March 17, 2017, 02:37:03 pm
content: For your requirements, you can actually just get a To-220 drop-in replacement.Something like:http://www.digikey.com/product-detail/en/murata-power-solutions-inc/OKI-78SR-5-1.5-W36-C/811-2196-5-ND/2259781

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2017, 02:50:03 pm
content: Quote from: cyberfish on March 17, 2017, 02:37:03 pmNote that pins 1 and 3 are the opposite of the LM78L05.

author: SingedFingers
author_rank: Frequent Contributor
timestamp: March 17, 2017, 03:10:13 pm
content: I had no idea they existed. Looks like a handy product.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 17, 2017, 05:50:44 pm
content: As I'm (in)famously known to rant,Yes.Crap operating frequency, crap current limiting (try "accidentally" shorting the output!), crap output ripple, crappy huge inductors required, crappy high-ESR filter cap required.Tim

author: David Hess
author_rank: Super Contributor
timestamp: March 17, 2017, 06:00:38 pm
content: There is nothing wrong with the MC34063 which is a low cost version of the old 78S40.It is a constant off-time switching regulator which has the advantage of not requiring frequency compensation and its current sensing protects it against many failures.

author: SingedFingers
author_rank: Frequent Contributor
timestamp: March 17, 2017, 06:51:48 pm
content: Thanks for everyone's inputs. Much appreciated.

author: fcb
author_rank: Super Contributor
timestamp: March 17, 2017, 07:16:11 pm
content: I design the occasional MC34063 into products - it's a bit shit, but it's really cheap. And whilst inductors aren't free - they can be cheap.Recently I used them in a commercial product that needed a -12V, and another commercial product that needed to take +5V to +8V.The product that had the MC34063 generating -12V also had a TPS562200 in it to generate +3V3 from the main +12V for the microcontroller & GSM, as well as 7805 & LM2951-3.3 for low noise analog supplies. So it's horses-for-courses.

author: SingedFingers
author_rank: Frequent Contributor
timestamp: March 17, 2017, 07:42:27 pm
content: Yeah it's REALLY cheap. 50 pieces in SOP8 for £1.50 on ebay at the moment.

author: Zero999
author_rank: Super Contributor
timestamp: March 18, 2017, 08:24:37 pm
content: Another option is to use a discrete switching regulator, which can be made with a couple of transistors and achieve efficiencies over 90%http://www.romanblack.com/smps/smps.htm

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 18, 2017, 09:04:31 pm
content: Although with less protection and only comparable performance (still needs huge inductors, makes lots of ripple).A nicer one doesn't take many more transistors, which is sad because Black can't be bothered with it (I actually asked, years ago). I did, though:Flyback rather than buck, but a similar approach can be taken.Tim

author: SingedFingers
author_rank: Frequent Contributor
timestamp: March 18, 2017, 10:37:56 pm
content: Interesting designs. Thanks for the info. I will analyse them when I get a few minutes. I'm always interesting in switching supply designs.I've built a couple of flyback ones in the past, one crudely breadboarded here. This little bugger, powered by a 555 and LM311 and IRF510 (does the job!) managed to kick out 75v @ 300mA quite happily which is to be quite frank, scary, especially as the max Vf on that diode is 40v and the output cap is 50v rated! This could be replaced with a 34063 and a slightly wimpier MOSFET I reckon. Operation is the LM311 compares the output voltage ratio against a dumb zener reference and turns the 555 off with the reset pin if it is on the mark. Similar operation to the 34063 but larger, more expensive and more complicated.You can see the burn marks on the board at the bottom right when it went overvoltage when there was a bad solder joint and the divider fell off ()and was discharged with a screwdriver and some croc clips.I have another design in flight which was a dumb experiment to generate typical electrostatic CRT drive voltages which uses an E42 core with two TIP41C's in a multivibrator configuration with two primaries of opposite phase. That is already kicking out a selection of nice voltages but the efficiency is completely boiled shit. Sucks up 600mA idling and sounds like someone is murdering a dolphin when under load. Did some maths wrong when calculating the reactance of the primaries me thinks and the frequency is way too low. Needs to be >20KHz.For ref, I built the 34063 into it now on a little daughterboard (stripboard - kill me) that slides into the steaming holes left by the 78L05 and it's working nicely. Total efficiency is around 82% on the final device which is good! 12mW versus 180mW when idling away now. BIG improvement. Means that SLA will last 10-15x as long before I have to charge it (currently monthly).This is far more fun than the day job

author: Mr. Scram
author_rank: Super Contributor
timestamp: March 23, 2017, 12:27:52 am
content: What would be a modern day alternative for the MC34063 with similar capabilities? Or are modern chips inherently more specialized?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 23, 2017, 02:01:13 am
content: Families, perhaps; I'm partial to a number of TPS54xxx parts. Shop for types without voltage mode control, with whatever features you need (voltage, current, efficiency), and in whatever type (sync/buck/boost/etc.) you need.Tim

author: MagicSmoker
author_rank: Super Contributor
timestamp: March 23, 2017, 03:08:07 pm
content: The only real advantage of the '34063 - and I suspect the primary reason it has been in continuous production fordecadesnow - is that it is hysteretic mode controller so does not need frequency compensation. That is to say, you get the optimum transient response (basically as fast as the switching frequency) with unconditional stability. You can't touch the transient response with a conventional PWM controller (current or voltage mode) without resorting to at least 5x higher switching frequency and/or without sacrificing some stability.I don't have quite the same dislike of the '34063 that T3sl4co1l does, but these days there is only one very specific application I use it for: when I need to supply an adjustable or software-selectable output voltage on a product's IO connector. For example, accommodating encoders and resolvers for motor speed feedback that might run on 5V, 12V or a range of voltages. Attached is the schematic for such a circuit, as well as the part of the board implementing it. NB - NCP3063 is an "improved" version of the '34063.

author: free_electron
author_rank: Super Contributor
timestamp: March 23, 2017, 03:15:14 pm
content: the only thing that can be done with a 34063 is :-first widlarize it.-douse the remains in gasoline and set it on fire.-take what is left over now and put in in one cubic metre of concrete.-bury that block 6 foot deep.-put a sign there that says not to eat whatever grows on that spot...yeah they work, sometimes , under some conditions . but just as often they just freak out and explode.these chips are old. have an erratic feedback mechanism , no fault detection . so the slightest thing that drifts out of spec and the thing goes crazy.there is much better these days.

author: Mr. Scram
author_rank: Super Contributor
timestamp: March 23, 2017, 03:35:43 pm
content: Quote from: free_electron on March 23, 2017, 03:15:14 pmSuggestions and alternatives are more than welcome

author: SingedFingers
author_rank: Frequent Contributor
timestamp: March 23, 2017, 04:00:31 pm
content: Ok I've breadboarded 4 converters now and put two into production use and I'm actually quite happy. Sure it's not Jesus' sandals compared to some but I'm really not having any problems.I cannot make one explode that is set up properly. I have tried. If you kill the sense resistor, it will explode, that is it. Other than that I have overloaded, shorted, dumped large caps across the output, built them on shitty breadboards (see below) and they just go "meh" and carry on. I've pulled up to 500mA though them fine with no significant heating. These are ST ones marked "063 CHN".I can filter the noise out adequately with a couple of 22uF tants and a 470uH inductor on each line out of the supply.I knocked up a 5v -> +/-15v converter with a couple of them last night to have a play and that works nicely.Excuse the potato camera... only had work handset handy.Efficiency is between 76 and 85% depending on the load and inductor size which isn't terrible.

author: langwadt
author_rank: Super Contributor
timestamp: March 23, 2017, 04:12:37 pm
content: Quote from: SingedFingers on March 17, 2017, 02:15:26 pmif price matter, just get something like thishttp://www.ebay.co.uk/itm/162402905270

author: BravoV
author_rank: Super Contributor
timestamp: March 23, 2017, 04:13:33 pm
content: If its not for fun or learning process, and just want to get a quick "relatively" low current step-up or step-down, there are tons of these so called mini step-up or mini step-down DC boards, and they're really dirt cheap.They work quite well, just don't push it too far and suggest to derate at half of the rated current, and it is still much-much better than 34063, much-much smaller size and more efficient.Random images from Google, these boards are pretty small and ready to use.1584 based step-down wide input from 4.5 - 28 V, 3 A rated, just use at 1.5 AMT3608 based step-up with input from 2 - 24 V , 2 A rated, just use at 1 A

author: SingedFingers
author_rank: Frequent Contributor
timestamp: March 23, 2017, 06:48:06 pm
content: Quote from: langwadt on March 23, 2017, 04:12:37 pmThat's about as cheap as it gets. Will order some of them to play with. See if I can blow them up.I was actually considering registering shitfromchina.com and reviewing stuff as a sideline

author: SeanB
author_rank: Super Contributor
timestamp: March 23, 2017, 07:14:17 pm
content: I think Julian Illet already has that pretty much covered, along with Big Clive.

author: SingedFingers
author_rank: Frequent Contributor
timestamp: March 23, 2017, 09:39:30 pm
content: Had no idea Julian Ilett existed. Sorted