LM3478 design gets insanely hot
in [Design]
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From: Henrik [7182] on 31 Jul 2008 16:16 Dear Joerg, Thank you very much for your replies. I will with a 50mR or so Rsense tomorrow, Also I will try to get the scales and such nice info into the shots. Also I will try exchanging the sepic capacitor to a large 1uF ceramic capacitor that I have in stock. Thank you so very much for your help. I will sleep much better now that I see a possible solution to my problems ;-) Best regards Henrik "Joerg" <notthisjoergsch(a)removethispacbell.net> skrev i en meddelelse news:EQokk.20081$N87.6430(a)nlpi068.nbdc.sbc.com... > Henrik [7182] wrote: >> "Joerg" <notthisjoergsch(a)removethispacbell.net> skrev i en meddelelse >> news:39lkk.18899$Ri.1853(a)flpi146.ffdc.sbc.com... >>> Sorry but I cannot read this kind of format. Maybe a Linux format which >>> I don't have. Can you post them in png, tiff, jpeg or some other more >>> common format? >>> >> >> Hello Joerg, >> >> Thank you for your time, and am sorry for the fileformat, I have now >> placed each file separately as JPEG for easier access: >> >> www.impc.dk/files/LM3478_fet_gate.jpg > > > The ramp up looks really sluggish. I don't know the data of this FET but > maybe it's way too big for the LM3478 to drive. Looks like it may have too > much Cgd. > > >> www.impc.dk/files/LM3478_fet_drain.jpg > > > That must be one hell of a FET being able to hold all this down. Your > inductor must drop almost a volt at peak. > > >> www.impc.dk/files/LM3478_fet_source.jpg > > > Way, way too much peak current. I don't know what you voltage levels are, > no data in the image about that. > > >> www.impc.dk/files/schematic.jpg >> > > Aha! Unless I am mistaken your current peaks exceed your inductor rating, > big time. Isense trips around 165mV and with 7mohms you'd be at 23amps by > then. Try raising Rsense to something like 50mohm. You may have the > inductor sitting in saturation and there it won't have any inductance to > write home about, just resistive losses. > > BTW I wouldn't use an electrolytic for the SEPIC cap (C5). I'd consider a > few ceramics in parallel. Electrolytics get stressed out there and might > explode some day. Unless it's a really, really good one and can take the > ripple at frequency. But if it gets warm to the touch don't use it. > > >> I hope they are viewable at your location. >> > > Yes, they are. Next time try if you can get a real screen shot from your > scope, with the setting and all. > > Just FYI, before you release BOM and schematic some day: Replace LM347B > with LM3478. > > -- > Regards, Joerg > > http://www.analogconsultants.com/ > > "gmail" domain blocked because of excessive spam. > Use another domain or send PM.
From: langwadt on 31 Jul 2008 16:17 On 29 Jul., 14:35, "Henrik [7182]" <not.va...(a)nowhere.dk> wrote: > Hello Experts, > > This is my first time designing a SEPIC type switchmode power supply with > the LM3478MM from national as my controller. > > My design is pretty much the standard application as shown in the datasheet. > > The design is made to supply 24V out from a 15-32V input. Ouput current is > 1A. > > My problem is that my two inductors and the MOSFET gets insanely hot. Much > more hot that I had anticipated. So I wonder if there is somewhere I have > missed something vital. > > I am using CDRH127 series 22uH inductors from Sumida, which are rated at > 3.3Amp and has a DCR of 43mR > > I use a 39K resistor to obtain approx. 400kHz switch frequency. > > I use 7mR as Rsense and MBRS360 as my power diode. > > My Compensation-filter consists of 100nF and 1K. > > My Mos-Fet is NP55N055 from NEC which has 12.5mR on-resistance. > > Since I am a novice in the field of SMPS, this has me baffled quite a bit, > and I hope that someone could lead me on right path. > > Thanking you all in advance. > > Best regards > Henrik The datasheet I just checked had an example with 5V/1A it uses a 50mOhm Rsense With a typical Vsense at 165mV thats 3.3A and it that must be lower than the maximum current rating for the inductor. So why did you choose 7mOhm ? -Lasse
From: Henrik [7182] on 31 Jul 2008 16:18 Dear Sir Thank you for your valued reply, I will try bumping up the sense resistor tomorrow and post a few more and better screen shots. Best regards Henrik "legg" <legg(a)nospam.magma.ca> skrev i en meddelelse news:fa4494thjaunib4n4cpb75kgq30u1hv8g7(a)4ax.com... > On Thu, 31 Jul 2008 19:10:29 +0200, "Henrik [7182]" > <noone(a)nowhere.com> wrote: > >> >>"legg" <legg(a)nospam.magma.ca> skrev i en meddelelse >>news:3pn394he6p1qs2cd781fsl5hbrp88k176a(a)4ax.com... >>> >>> 7mR seems like a pretty low value sense resistor for a 3A output and a >>> 350mV fault threshold. Shouldn't it be 15 or 30? >>> >>> RL >> >>Actually you may be right, I did not think this could have any effect on >>my >>heating issue, so I just picked somthing so low that I didn't expect to >>get >>any problems. Perhaps I am wrong? As I wrote earlier, I am still a novice >>in >>the field of SMPS and SEPIC in particular, but eager to learn ;-) >> > > Safest sense resistor values would be slightly higher (even double) > the estimated optimum, to begin with. > > If it was large enough, you could use it to scope the current > waveform, and post this as well. > > Try to scope a single cycle, or even the rising or falling edges - > with surrounding noise. It helps with visual correlation of single > trace plots, if the waveforms are not close to 50% duty cycle, as > well. > > RL
From: Joerg on 31 Jul 2008 16:43 Henrik [7182] wrote: > Dear Joerg, > > Thank you very much for your replies. I will with a 50mR or so Rsense > tomorrow, Also I will try to get the scales and such nice info into the > shots. > BTW it makes it easier for others to follow and contribute ideas if you write below the quoted text. > Also I will try exchanging the sepic capacitor to a large 1uF ceramic > capacitor that I have in stock. > Stack a few of those for the test. 1uF may be a tad too small for a 1A switcher (tho it'll work). Nowadays you can get pretty good 10uF or at least 4.7uF caps but even there you may have to parallel in order to ease and share the current load. They have become quite cheap. With switchers it is really important to carefully study the datasheets of even the most mundane parts. We take those caps for granted in bypass situations but here they'll have to slosh around a lot of energy. I leaned that the hard way. Everything was fine for an hour or so. Then a very loud bang, pieces flying around, and what was left from the ceramic material of a capacitor had turned into green bubbly glass. Quite pretty actually but it could have been a bad situation if I hadn't worn eye protection glasses. Best to wear those when you work on switchers and similar gear that can go "exotherm". > Thank you so very much for your help. I will sleep much better now that I > see a possible solution to my problems ;-) > Since you are in Denmark this would be the perfect time for an ice-cold shotglass of Aquavit. Good stuff :-) [...] -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Henrik [7182] on 1 Aug 2008 08:05
<langwadt(a)fonz.dk> skrev i en meddelelse news:cae888a6-c839-4051-9f70-6ad28b0bfaf1(a)k30g2000hse.googlegroups.com... > The datasheet I just checked had an example with 5V/1A it uses a > 50mOhm Rsense > With a typical Vsense at 165mV thats 3.3A and it that must be lower > than the > maximum current rating for the inductor. > > So why did you choose 7mOhm ? > > -Lasse Actually I been trying to experiment a little with Rsense today, and what happes does not make me happy at all :-( Everything above 20mR will not yield the 24V output when the load is a 24R resistor and the input fall below 24V. With 7mR i can obtain my 24V output, but at the cost of the insanely hot construction :-) With no lad, the output is fine, but when I load the output, the voltage drops. Something else is wrong with my setup I think, but I cannot figure it out. I have some ugly spikes at the FB pin and suspect that these might have something to do with my problems, but I don't really know how to move on from here. Best regards Henrik |