Author Topic: Lost on SEPIC parameters  (Read 414 times)

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Offline LoveLaika

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Lost on SEPIC parameters
« on: March 03, 2022, 07:38:12 pm »
I'm having trouble with choosing the parameters of a SEPIC circuit using the LT3579 IC, specifically the inductor. I designed it around certain inputs and outputs, plugging the formulas into Excel, and seeing the parameters that come out. I've attached my worksheet below as a reference, so I was wondering if my choice of an inductor is okay given the calculations.

Following the formulas, I pick my input/output voltages and the frequency to start. Then, it's just plugging things in. I'm just lost at picking the right inductor. I'm given a range of values for the inductor, and this range is dependent on the switching frequency. The higher the frequency, the smaller the range. Picking the larger inductor in the range would result in a smaller ripple current. However, you're also limited by the selection of coupled inductors available. For instance, in my calculations at 1.2 MHz, I have a range of 1.45-13.5 uH (with a typical value of 3.77 uH). Selecting one at 10 uH (as seen in my link above), the ripple current reduces to 0.68 amps (which still seems pretty high to me). My inductor pick fits the criteria, and its current rating is high enough for the peak current. Given my design methodology and parameters, does this seem like a good choice?

As a related question, what can I do to further reduce the ripple current? Furthermore, while the peak current is a little high, would a soft-start capacitor help mitigate that value? By allowing the IC to start up "slowly", the inductor shouldn't reach peak current at startup?

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