Re: LM3478MM Sepic problem

Paul E. Schoen wrote:
"Henrik [7182]" <nowak.henrik@xxxxxxxxx> wrote in message news:2564a$49ca9beb$55186548$19667@xxxxxxxxxxxxxxxxxxx
"Joerg" <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> skrev i en meddelelse news:nZvyl.15321$D32.11432@xxxxxxxxxxxxxxxxxxxxxxx
C73/C74 should not be electrolytics but ceramics instead. Also, you don't really need this much capacitance. There are no ceramic caps on input and output, there need to be some substantial ones. At least several uF.

Two other things to check:

a. Does L4 saturate?

b. What current max are the sense resistors rated for?

BTW, your FQP50 is rated rather marginally for your app, only 60V breakdown. Could it be avalanching?

Just as a hint, if the inductors in a SEPIC are not on a common core then a whole lot of ripple goes out into the input rail. It needs to stomach that and all I see there is one lone 1000uF electrolytic.

I am not too familiar with European schematics but it seems your electrolytics symbol is a bit, ahem, unorthodox. Shouldn't plus be the boxy electrode?

--
Regards, Joerg

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Dear Joerg,

Thank you for your reply.

About C73/C74, I always thought that using electrolytics was more safe. I though I read that somewhere. But I could easily be wrong and cofusing two unrelated subjects. But the psysical size of ceramics should be quite large to handle the job, shouldn't they? Do they handle currents better than electrolytics?

I'll check L4, I use these http://www.chilisin.com.tw/en_new/product/images/pdf/14-tdh.pdf
the TDH2420 series, last columns in the table. Will my 1000uF suffice for this?

My sense resistors should be able to handle 12Amps. But something made them break with a one amp load tonight.

I'll check if my MOSFET does avalanche. I think I have a few 100V lying around if need be.

Time to pull out the oscilloscope and get this beast running ;-)

I have not had experience with a SEPIC, but I have worked with several single inductor boost converters. Looking at a similar circuit in the LTSpice library, I see that the capacitors are 20 times smaller for a 1 amp output, so I think you might be able to use something like 100 uF for C73-74 and 200 uF for the output filters. Also, all the electrolytics must be lew ESR high frequency types, and should be bypassed with about 200 nF ceramics. If the capacitors get hot, they have too high ESR.

Using the scope you should be able to see if saturation occurs, which will cause very high power in the sense resistors. Use a 10 amp fuse or circuit breaker in the supply, and monitor the input current. Slowly increase the load, and watch for a sharp increase in current. Check the duty cycle at this point and see if it makes sense.


Hang a scope onto the sense resistors, increase load and watch for the telltale shark fin cropping up.


I also recommend using LTSpice to simulate your circuit. They have something similar to the LM3478. Try LT1241, LT1247, or LT1317 as a start.


Yes.


Good luck.


And wear eye protection. Seriously.

--
Regards, Joerg

http://www.analogconsultants.com/

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