LM3478 design gets insanely hot
in [Design]
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From: legg on 1 Aug 2008 11:02 On Fri, 1 Aug 2008 14:05:06 +0200, "Henrik [7182]" <not.valid(a)nowhere.dk> wrote: > >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. > Start of with smaller loads (higher value load resistors). Adjust the input voltage gradually over the range. Scope the waveforms and get a feel of what things look like when no fault is evident. Then induce the limiting behaviour. if this turns out to be a regular limiting effect and no strangeness shows up on the current sensing node, it safe to reduce the Rsense value. If you see unacceptible spikes on the sense resistor (use a short scope probe tip clip) or current sensing pin, you may have to improve the design, layout or possibly just the signal filtering around the IC's sensing node. RL
From: Joerg on 1 Aug 2008 16:39 Henrik [7182] wrote: [...] >> The high art later will be to build switchers without special PWM chips, >> just from logic chips and discrete parts. Can be real fun. > > Well that sound like a job for the Picaso's of our esteemed trade ;-) > The most memorable comment at the beginning of a design review when this slide came up: "What the hell does that circuit do?" >> 20mohm still allows >8 amps. That is way too much for the inductors you >> chose. >> >> Instability for higher values: The switcher then goes into what's called >> continuous conduction mode or CCM. This can lead to right half plane zero >> instability, often called RHP-zero. If you really have to live with such >> small inductors you'll have to read up on the topic and change the >> feedback compensation. Read under slope compensation, page 10 of the >> LM3478 datasheet. >> > > I hope that I am not way off in my current understanding of the matters, as > described in the following: > > I rechecked all my calculations once again and found out that 33uH should be > sufficient with my chosen frequency. I then glued in a couple of TDH series > inductors from Chilisin I had laying around. Lo and behold, they do not get > hot at all! (of course, a little, but nowhere near "hot" like I am used to). > However, the downside is that it it still only possible for me to get 24V > out using no more than 20mR as Rsense. As soon as I crank up this to 50mR > the output falls and varies with the input voltage. According to my > calculations I have duty limits of respectively 0.44 and 0.62 for my input > voltage limits, which is close to or above the 0.5 "risklimit" mentioned in > the datasheet regarding feedback compensation. > I haven't run your numbers but you may be right in that you really do need less than 50mOhms. That was just a suggestion to see whether it will run the CDRH127 without them becoming hot. Since the LM3478 is a current mode switcher and Vsense is 165mV your 20mOhms means that the inductor must withstand 8.25A plus some margin. No matter how you toss and turn that you'll need a 10A inductor there for peace of mind. Still I think something isn't quite right here. Just key the numbers into the boost model below, for example 15V in and 24V/1A out. The inductor shouldn't go beyond 2A. http://schmidt-walter.eit.h-da.de/smps_e/aww_smps_e.html > So if I understand correctly, I need bigger current capaple inductors, and i > order to make my Rsense work, I need to determine a sufficient resistor to > put into my current feedback. > > Maybe even if I get this feedback issue working, my CDRH127 serie would be > alright afterall for the 1 amp output. I calculated my Iavg maximum to 3,1A > which is still below the limit for these inductors. > AVG doesn't matter. What matters is Ipeak. The instant the current exceeds the limit for core saturation the inductance collapses. Still puzzled why you need this much when your lowest input voltage is 15V. > I can see that bumping the frequency reduces the ripple current, but I guess > there might be some problems with e.g just bumping the frequency to 500kHz? > I use 400KHz in the current setup. > 400kHz should be ok here. Your FET switching losses are quite high, probably too big a FET. With higher frequency that'll become worse. >> This you need to crank up a lot more. It's hard to see the slope at >> Isense. Either ignore the spikes or connect to Rsense via a coax to get >> rid of them. Best to connect it where that 1000pF muffling capacitor on >> the sense line is. > > I will try a produce better screenshots. Hopefully my next shots will be of > a better running system :-) > Oh, they were ok this time, just crank up the scope gain for the Vsense node. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: legg on 1 Aug 2008 22:37 On Fri, 01 Aug 2008 13:39:32 -0700, Joerg <notthisjoergsch(a)removethispacbell.net> wrote: >Henrik [7182] wrote: > >[...] > <snip> >> >> I rechecked all my calculations once again and found out that 33uH should be >> sufficient with my chosen frequency. I then glued in a couple of TDH series >> inductors from Chilisin I had laying around. Lo and behold, they do not get >> hot at all! (of course, a little, but nowhere near "hot" like I am used to). The smallest of the TDH series (open form) bobbin inductors has double the surface area and two or three times the magnetics x-section of the low profile closed parts originally used. Neither parts give much core material loss information. Losses with accumulate from both features, with core loss having a high positive temp coefficient above 60-80degC (depending on material type and grade). >> However, the downside is that it it still only possible for me to get 24V >> out using no more than 20mR as Rsense. As soon as I crank up this to 50mR >> the output falls and varies with the input voltage. According to my >> calculations I have duty limits of respectively 0.44 and 0.62 for my input >> voltage limits, which is close to or above the 0.5 "risklimit" mentioned in >> the datasheet regarding feedback compensation. You should find that the duty cycle is more highly dependant on load variations. Scoping with single traces at close to 50% duty makes visual correlation a problem. Try to trigger on the same portion of the operating period or wave edge, regardless of the display; if neccesary by taking and uncalibrated signal from the pwm oscillator to the direct trigger input. >I haven't run your numbers but you may be right in that you really do >need less than 50mOhms. That was just a suggestion to see whether it >will run the CDRH127 without them becoming hot. Since the LM3478 is a >current mode switcher and Vsense is 165mV your 20mOhms means that the >inductor must withstand 8.25A plus some margin. No matter how you toss >and turn that you'll need a 10A inductor there for peace of mind. > >Still I think something isn't quite right here. Just key the numbers >into the boost model below, for example 15V in and 24V/1A out. The >inductor shouldn't go beyond 2A. > >http://schmidt-walter.eit.h-da.de/smps_e/aww_smps_e.html > The switch carries the current of both inductors. <snip> >>> This you need to crank up a lot more. It's hard to see the slope at >>> Isense. Either ignore the spikes or connect to Rsense via a coax to get >>> rid of them. Best to connect it where that 1000pF muffling capacitor on >>> the sense line is. >> The apparent lengthy (250nSec?) negative-going excursion in the current signal (at the source)during turn-off may be an artifact of the scope digitization. You need to look at it with a shorter time-base and a shorter probe ground clip. EL
From: JosephKK on 1 Aug 2008 23:18 On Thu, 31 Jul 2008 13:43:21 -0700, Joerg <notthisjoergsch(a)removethispacbell.net> wrote: >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 :-) > >[...] Mmmm. Aquavit. Once in a while i get some Aalborg Jubilamus variety aperitif, sugary but very tasty.
From: Joerg on 2 Aug 2008 11:19
legg wrote: > On Fri, 01 Aug 2008 13:39:32 -0700, Joerg > <notthisjoergsch(a)removethispacbell.net> wrote: [...] >> I haven't run your numbers but you may be right in that you really do >> need less than 50mOhms. That was just a suggestion to see whether it >> will run the CDRH127 without them becoming hot. Since the LM3478 is a >> current mode switcher and Vsense is 165mV your 20mOhms means that the >> inductor must withstand 8.25A plus some margin. No matter how you toss >> and turn that you'll need a 10A inductor there for peace of mind. >> >> Still I think something isn't quite right here. Just key the numbers >> into the boost model below, for example 15V in and 24V/1A out. The >> inductor shouldn't go beyond 2A. >> >> http://schmidt-walter.eit.h-da.de/smps_e/aww_smps_e.html >> > The switch carries the current of both inductors. > True, they don't have a SEPIC sim. But the 2nd inductor doesn't carry that much. I really don't see where 8A peaks should be necessary. I had to swing a few designs from boost to SEPIC because clients revised the specs in mid flight. That has not caused any serious changes in peak load. > <snip> > >>>> This you need to crank up a lot more. It's hard to see the slope at >>>> Isense. Either ignore the spikes or connect to Rsense via a coax to get >>>> rid of them. Best to connect it where that 1000pF muffling capacitor on >>>> the sense line is. > > The apparent lengthy (250nSec?) negative-going excursion in the > current signal (at the source)during turn-off may be an artifact of > the scope digitization. You need to look at it with a shorter > time-base and a shorter probe ground clip. > Or with a real scope. SCNR :-) I am not all that happy with those Tek lunch-box size scopes. Often I advise clients to go on EBay and bid on a Tek 2465. When it arrives it's like as if someone had turned the lights on. The other downside that hits in cases like SMPS designs is that many Teks only have 2.5K memory. Not enough. My scope has 25K so I can trigger, digitize at full bore and then blow up individual regions to look at a spike, zoom back out to see whether my RHP-zero situation is kosher, and so on. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |