LM3478 design gets insanely hot
in [Design]
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From: mrdarrett on 31 Jul 2008 11:53 On Jul 31, 8:49 am, Joerg <notthisjoerg...(a)removethispacbell.net> wrote: > Henrik [7182] wrote: > > "Joerg" <notthisjoerg...(a)removethispacbell.net> skrev i en meddelelse > >news:eJHjk.16580$uE5.15768(a)flpi144.ffdc.sbc.com... > >> Time to take scope shots and post them. But better not on a.b.s.e. because > >> some attorney general has shot that tool for most of us. > > >> Probe the gate directly at the FET, and source and drain. Those are the > >> most important vitals for a switcher. I did a 6-9V to 0-100V adjustable > >> SEPIC with the LM3478 a few years ago. Don't have scope plots but I kind > >> of remember how it looked. Worked like a champ. > > >> -- > >> Regards, Joerg > > >>http://www.analogconsultants.com/ > > >> "gmail" domain blocked because of excessive spam. > >> Use another domain or send PM. > > > Dear Joerg, > > > Thank you for your valued reply. I have taken screenshots at the three > > mos-fet terminals. They are located here:www.impc.dk/files/lm3478_files.rar > > > I hope that you will take a look and perhaps your past experience with the > > device could shed some light on my problems. > > > All shots have been taken with 1uS for T/Div and the Gate has 2V/Div, the > > Drain is 10V/div and the Source is 500mV/Div > > > Thanking you in advance. > > 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? > > -- > Regards, Joerg > > http://www.analogconsultants.com/ > > "gmail" domain blocked because of excessive spam. > Use another domain or send PM. ..rar is a "zip-like" format Apparently proprietary, unfortunately, so you'd have to download WinRAR and install it... or ask the OP to re-upload the file to something more common Michael
From: Henrik [7182] on 31 Jul 2008 13:10 "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 ;-) Best regards Henrik
From: Henrik [7182] on 31 Jul 2008 13:55 "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 www.impc.dk/files/LM3478_fet_drain.jpg www.impc.dk/files/LM3478_fet_source.jpg www.impc.dk/files/schematic.jpg I hope they are viewable at your location. Thanking you in advance Best regards Henrik
From: legg on 31 Jul 2008 15:28 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 15:59
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. |