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Tips for designing a DC Converter Please.

Hi, i am getting crazy. I opened lots of threads to solve this problem, but even after a lot of help i got no solution. I am working on a project and i need to make my board to work either with 12V and

24V (the solution itself does not matter). So i was designing a buck- boost converter with MC34063 following the AN920 made by ON for a input of 9-27V and 0.5A and an output of 14.1V. I studied all the equations, the theory and calculated a designe. According to the number I got, I was expecting a big peak current, so I got the model for Orcad and simulated my designe to see the current. At the start i really got a real big peak current as expected (about 5A). But i was expecting to lower the current using a bigger inductor. But that did not happen. The Peak got less sharp (as expected for a inductor) but the peak itself was the same, and with a bigger inductance i was getting a worst regulation. Is there any other change i can perform to get a lower peak current? And at the start of simulation i was getting an out-of-the space current (About 12A!!!!). Does that make any sense?

Any help with this problem or tips would be apreciated.

PS: I am designing with MC34063 becouse the company I am designing the board likes to work with components that local distribuitors got on stock, and here on Brazil we have a real lack of good and new components.

Thank you!

Reply to
Luis Filipe Rossi
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Is it all going to charge the output cap? That would account for high current with or without an inductor.

Does that part let you ramp up the output voltage on power up? It sounds like you need a "soft start" circuit of some sort.

--

Tim Wescott
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Reply to
Tim Wescott

d text -

Well, that would be just half of the prroblem. But i was looking at the current and i fount that i was not able to make the ciruit work on continuous mode, even using a bigget inductor (wht i think is weird) so i raised the current limiting resistor, and it worked, i got it on continuous mode, so the peak current decreased a lot. Why to use bigger inductor did not got me on continuous mode, and why limiting the current did?

thank you!

Reply to
Luis Filipe Rossi

Can't see Luis' post, maybe he is posting via Google. Anyhow, the MC43063 has a current limit function. If the sense resistor (pin 7 on

8-pin devices) is chosen correctly that muffles the charge-up peak. Anything beyond 1.5A can damage the chip. I usually try to keep it under 1A. 
--
Regards, Joerg

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Reply to
Joerg

h
1A.

Damn, Yes i am posting on google. Well after a lot of effort i got a good result, but something is weird. On the simulation the MC34063 is switching too fast... much faster than 100khz... thats weird.. any idea why thats happenign. It was not suposed to...

Reply to
Luis Filipe Rossi

You need to incorporate a soft start in your design, if initial current is what you are referring too?

There are many ways to do this how ever, with out seeing what you now have put together, it makes it hard to offer a solution, especially when we don't know the main concern of why you are worried about this initial surge.

Jamie

Reply to
Jamie

Well I am worried becouse this input will be "delivered" by the system of my custormer's customer :) , and its characteristics can vary a lot so i must consider a "worst case scenario". That would mean a very low maximum input current (i would say 1A for a 24V power supply). Ok the inrush current can be controled, but the operational current is the worst problem. At a 24V input voltage, i am getting an average input current bigger than my average output current. Thats means that a lienear regulation would be more efficient!! Anything lower than a

60% efficiency i would go like for a linear relulator with a jumper, and just a boost converter from 12 to 14.1V (and that would be acceptable for my customer)... I just the board to work with 12 and 24V. Not all the range .... But i was looking for a better solution... using National's Workbench i could find some real good SEPIC options, but again the IC itself is a problem around here... Any way, according to your experince, do you think my results on simulation (and i am ok with the starting inrush current) are compatible with the reality, or iam screwing up with something here?

Thank you!

Reply to
Luis Filipe Rossi

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