Re: LM3478MM Sepic problem

Henrik [7182] wrote:
Hello experts

Still rather new to the world of SEPIC, I have a problem with a design.

I have made a circuit based on the data*** for the LM3478. It is intended to run with Vin from 10V to 32V and deliver 24V out with a maximum of 4Amps.

My testsetup runs at 32V input. The circuit is as can be seen here: www.impc.dk/scm/lm3478sepic.pdf

A colleague of mine recently built something quite similar but with the LTC1871 controller instead. But I'd rather use the national part, since we already has this part in stock.

When I run with no load, output is 24V and everything is nice. But when I load the darn thing, even only by one amp, everything gets so hot and eventually the sense resistors crack.

I plan to change R79 so that I run at 200kHz, because according to my calculations 100kHz is a little to slow for my 22uH/13A inductors.

This heat issue has me puzzled somewhat but before i dive into measuring, I wonder if anyone would have a comment on the schematic. Especially regarding the values for R103/C16 and R145 which I have been struggling to determine.But also if I need anything or need to change anything. Not much is to be found on google regarding SEPIC with this chip, unfortunately.

I am eager to learn, but still a beginner in this field, so be gentle ;-)

Thanking you in advance for any and all help.



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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