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.
IIRC those are 3.6A but could still be a bit marginal for this design.
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.
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.
"legg" skrev i en meddelelse news: snipped-for-privacy@4ax.com...
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.
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.
That must be one hell of a FET being able to hold all this down. Your inductor must drop almost a volt at peak.
Way, way too much peak current. I don't know what you voltage levels are, no data in the image about that.
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.
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.
BTW it makes it easier for others to follow and contribute ideas if you write below the quoted text.
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".
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.
skrev i en meddelelse news: snipped-for-privacy@k30g2000hse.googlegroups.com...
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.
"Joerg" skrev i en meddelelse news:Stpkk.20092$ snipped-for-privacy@nlpi068.nbdc.sbc.com...
Yes, mea culpa. I actually know this, it was just plain sloppiness on my part.
I stacked a few of my 1uF 50V on top of each other.
A chilling history indeed, I once experienced something similar. A colleague of mine got a small part of his earlope shot away by and exploding tantalum drop-type capacitor. He bled quite a lot and was really shocked.
Aquavit is good stuff indeed! An I soon might need some of it to calm myself down ;-) It is really really hot in Denmark these days and this switcher design of mine, does not exactly cool anything down. The bugger keeps getting hot beyond my wildest fears.
I tried experimeting with the Rsense and I have found out that anything above 20mR in value makes my output not stable when input is below 24V.
I have tried various types of mosfet that I have stocked, but no considerable change in the gate waveform came from this.
I tried changing the inductors to 33uH, but still stuff gets really hot when I draw 1A.
I have made a few new screenshots, where the scaling information is present, perhaps this could be of some help in the fine assistance I have already received.
The shots are made from a running system with 20mR Rsense and 33uH inductors of the CDRH127 series.
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.
Haven't seen it this bad but one guy had a shirt full of holes after a dozen tantalums exploded. I do not use tantalums in designs, except in low current timing circuits. And even there only once in a blue moon.
Look at the bright side: Once you've mastered a few designs like this you'll become quite the expert in SMPS design. There certainly aren't many of those engineers left in the industry. Many younger engineers just take an app note or sample design from a datasheet and when it doesn't work they give up. Others are so afraid of SMPS that they always buy "modules in a can".
The high art later will be to build switchers without special PWM chips, just from logic chips and discrete parts. Can be real fun.
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.
Bottomline your 3.6A inductors are rather marginal for the power level you need. If you want to avoid CCM altogether you need much bigger ones. But you cannot use 20mohms with the higher value CDRH127 inductors, they'll saturate.
This usually happens when the FET has really high Cgd. The LM3478 is not a very strong driver so you can't use FETs with Rdson in the range of a few milliohms, the are too large.
Those inductors cannot take 8A or current peaks.
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.
Looks like you are getting a lot of noise into that node. Do you have a full ground plane? The LM3478 needs that.
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
"Joerg" skrev i en meddelelse news:z%Fkk.16959$ snipped-for-privacy@nlpi067.nbdc.sbc.com...
Well, I try and keep myself focused on the silver lining, even when burning my fingers on various inductors ;-) The app-notes and sample designs are fine for ideas, but the true art of our trade is in understading, so only one way to go. Into the quagmire. I NEVER give up. being the youngest of 4 boys taught me that. I get beaten sometimes, but I never give up, neither on this matter ;-) Aquavit to the rescue, though. As Mr. Sean Connery so brilliantly points out in "the Rock" : "Loosers always whine about [giving their] best, winners go home and f*ck the prom queen". Well, I am no Nicolas Cage, but I certainly did marry the prom queen, so no caving in.
Well that sound like a job for the Picaso's of our esteemed trade ;-)
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.
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.
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.
I will try a produce better screenshots. Hopefully my next shots will be of a better running system :-)
The most memorable comment at the beginning of a design review when this slide came up: "What the hell does that circuit do?"
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.
formatting link
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.
400kHz should be ok here. Your FET switching losses are quite high, probably too big a FET. With higher frequency that'll become worse.
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.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.