SEPIC voltage regulator, one or two inductors?

[alexg]

Hi All, I am going to try to build SEPIC voltage regulator, did not chose which particular chip I will use but I noticed that on some of them inductors marked with small dot on one end (see attached illustration), and when I try to find such inductor on digikey it seems that only type that have such marking is where two coils are wrapped around the same core or something, can I simply use two separate independent inductors?

[T3sl4co1l]

Two winding coupled inductor.  Coupled inductor gives better performance (lower input ripple).  The datasheet will indicate which pins are dotted.

It will still work with uncoupled (discrete) inductors.

Tim

[alexg]

Great, thanks, I will build both and compare performance, but its good to know that two separate inductors can be used, I can keep things "lower profile" on the PCB this way.

[chris_leyson]

Strictly speaking if you couple the two inductors it's no longer a Sepic converter. In the coupled inductor version the ripple currents increase and the closed loop performance changes. See http://www.switchingpowermagazine.com/downloads/Sepic%20Analysis.pdf for example plus a lot more at http://www.ridleyengineering.com.
There are a few papers out there on the internet stating that a Sepic benefits from a fair amount of leakage inductance if you use coupled inductors. Also un-coupled and coupled versions of the Cuk converter are completely different.

[rfbroadband]

use one coupled inductor. Make sure you select one that supports the peak current of your design. Make sure you understand the hot loops of the SEPIC architecture to minimize ripple. Linear Tech has a few excellent app notes on layout considerations for different switching architectures. Use a 4 layer board to minimize spacing to next gnd layer in your PCB stack up (even though a 2 layer board would be sufficient from a routing point of view). The 1.6mm distance to the second GND layer will hurt you for ripple in a 2 layer design.

I recently designed one using the LT3758 and works great, but you have to take the layout extremely seriously. Make sure you design your board so you can easily tune the frequency response of the control loop. No matter what you simulate, you will need to tune the loop for stable operation.