SEPIC converter design - Page 1
Forum section: electronics / projects

author: midasgossye
author_rank: Contributor
timestamp: July 01, 2014, 05:42:32 pm
content: I'm want to use a SEPIC converter in a design, because the input voltage (min. 3-4 V, max. 15V) can be lower and higher then the output voltage (5V). Max. output current will be around 0,6 - 0,7 A. I have searched digikey and mouser for controller chips and application circuits, but I find it very hard to find a suitable design and chip.Does anybody know a good chip for these specs? If it is possible, a small form-factor would be great.

author: AndyC_772
author_rank: Super Contributor
timestamp: July 01, 2014, 05:54:59 pm
content: http://www.linear.com/parametric/SEPIC_Regulators#!1646_Buck-Boost|SEPIC!1032_%3C=3!1033_%3E=15!1105_%3C=5!1034_%3E=5!1035_3.1:15.5!1107_0.7:3.5!2086_%3E20!vinmin_3!vinmax_15!vout_5!iout_.7!vout2_!iout2_!vout3_!iout3_

author: midasgossye
author_rank: Contributor
timestamp: July 01, 2014, 06:17:00 pm
content: Thanks for the link, is very useful.Are there any "traps for young players" in SEPIC converters? I'm a bit of a newbie to them.

author: mariush
author_rank: Super Contributor
timestamp: July 01, 2014, 06:24:54 pm
content: LT1613 can probably do 4.8-5v @ 500mA with 3.7-4v input. If you don't mind going down a bit to about 4.75v you may stretch it to 0.6A.Download LTSpice, it has most of Linear's regulators inside. You can put the chip on the page and then right click on it and open up a test fixture and go straight to simulating the chip (by default test fixtures for chips like lt1613 are in boost mode, so you can play with input voltage and load to see how well it works).Check other regulators on Linear's site, I mentioned lt1613 only because it's very small and can use only surface mount components, taking up very little space.Layout is CRITICAL with regulators running at high frequencies, read very carefully the datasheets and check the recommended layouts and everything.edit: LT1613 can only handle up to 10v at the input so it may not work for you, since you say you have up to 15v.

author: midasgossye
author_rank: Contributor
timestamp: July 01, 2014, 06:32:25 pm
content: Thanks for the useful info. I had forgotten that I could use LTSpice for these kind of things .Are there any other good brands for these kind of converters? I know LT make very good stuf, but the parts can be a bit expensive.

author: AndyC_772
author_rank: Super Contributor
timestamp: July 01, 2014, 06:41:56 pm
content: Expensive, yes, and functionally I've had one or two that didn't work as their LTSpice models would have you believe either.That said, LTSpice is free and probably the best PSU design teaching aid I know of. Even if you end up using a controller from another manufacturer, it will help you work out things like the optimum switching frequency, inductor currents, efficiency and so on, provided the topology and values of external components remain about the same.

author: mariush
author_rank: Super Contributor
timestamp: July 01, 2014, 06:42:17 pm
content: You can get a lot of example circuits by entering your parameters in TI's webbench software.Go to ti.com and create an account first because otherwise the site won't show you example circuits ( go on top right corner and click login and then select create account and follow the steps)Once you're registered and logged in, to in menu on Products and you should see there Webbench designer.Enter min voltage 3.3v and max voltage 6v and output voltage 5v 0.8A and click on start design. This way (min voltage below, max voltage above output) you'll get sepic circuit examples. You can later refine the results by increasing the maximum input voltage.The whole system is nice because you can somewhat sort the results by chip cost, the whole parts (BOM) cost, the area all components take on pcb etc etc. In the circuits shown you can also pick from several mosfets/inductors/resistors and re-simulate

author: midasgossye
author_rank: Contributor
timestamp: July 01, 2014, 07:18:08 pm
content: Dear Mariush,Thanks for the help. I have tried the webbench designer, but I get an error.Am I doing something wrong?See picture for error details.

author: mariush
author_rank: Super Contributor
timestamp: July 01, 2014, 07:26:28 pm
content: At regulator type, select All. Then select "compare all part types" instead of hitting one of those "choose part" buttons.The difference between one with "integrated switch" and some controllers is often just a tiny mosfet and some resistors and capacitors, so stick to all for the initial selection and then you can filter more.Also play with min/max voltages and the maximum output current, you may get more parts or the parts may be arranged differently (i think they're sorted by efficiency by default).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 02, 2014, 04:31:03 am
content: SEPIC is just boost with a separate winding referenced to a different supply (but the same AC), or flyback with equal windings (= same AC) and a common ground. Don't exclude these from your search.Tim

author: Sailor
author_rank: Regular Contributor
timestamp: July 02, 2014, 11:41:48 am
content: TI have an App note on sepic design. I'll find the link tomorrow and post it.

author: amiq
author_rank: Regular Contributor
timestamp: July 02, 2014, 11:57:37 am
content: Quote from: T3sl4co1l on July 02, 2014, 04:31:03 amIt's bugger all like a flyback ....

author: Richard Head
author_rank: Frequent Contributor
timestamp: July 02, 2014, 02:11:50 pm
content: I'm want to use a SEPIC converter in a design, because the input voltage (min. 3-4 V, max. 15V) can be lower and higher then the output voltage (5V). Max. output current will be around 0,6 - 0,7 A. I have searched digikey and mouser for controller chips and application circuits, but I find it very hard to find a suitable design and chip.Don't be too hasty to opt for a SEPIC. I have no doubt that it'll work, but there may be better/cheaper/simpler options to consider that don't require a propriatary $5 IC.For such a low minimum input voltage I would be inclined to look at some sort of self oscillating bipolar converter. I don't think you will be able to get any MOSFET based converter to start up at 3V. I think a self oscillating flyback (with isolation) with appropriate turns ratio would work. I know it may sound a little crude to some but every single photoflash unit uses that ultra cheap method and has so for years.

author: sleemanj
author_rank: Super Contributor
timestamp: July 02, 2014, 02:17:37 pm
content: Is this a one-off project, or a product?If it's a one-off, you could just buy a ready to go module of ebay etc, LM2577 based, a few bucks, away you go.Here's the first one that came up:http://www.ebay.com/itm/DC-DC-adjustable-LM2577-auto-step-down-up-power-supply-module-solar-power-panel-/251150528815

author: mariush
author_rank: Super Contributor
timestamp: July 02, 2014, 02:57:25 pm
content: LM2577 needs at least 3.5v to run and as it works at low frequency (52kHz) you end up with big inductors and low efficiency. Even worse if he's going to have such voltage swings (ex from 3v to 15v).

author: Sailor
author_rank: Regular Contributor
timestamp: July 03, 2014, 08:56:42 am
content: Here's that TI documentDesigning DC/DC converters based on SEPIC topology - Jeff Falinwww.ti.com/lit/an/slyt309/slyt309.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 03, 2014, 10:05:49 am
content: Quote from: amiq on July 02, 2014, 11:57:37 amAfraid you're going to have to explain that one in some detail.Tim

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 04, 2014, 12:44:51 pm
content: Quote from: midasgossye on July 01, 2014, 05:42:32 pmFigure 34 in the LM3481 datasheet pretty much does exactly what you want, except it covers a wider input voltage range and delivers a higher maximum output current. This chip is fairly easy to use but like most current-mode controllers you need to be especially careful with noise on the current sense pin.Two other things specific to the SEPIC that can cause problems are the size/type of capacitor that couples the input and output inductors together, and whether to use separate inductors, or couple them together on a common core. The LM3481 datasheet suggests that you can use tantalum or aluminum electrolytic types for the coupling capacitor; I think that's bad advice, as this capacitor has to handle a significant AC current so only film or ceramic types should be considered. As for whether or not to use a dual winding inductor (e.g. - common mode choke or the like), the upside is that it can save quite a bit of space and reduce the cost. The downside is that you don't want the coupling between the two inductors to be too good - ie, you actually want a bit of leakage inductance here - otherwise you can get significant ringing and high reactive currents circulating in them (especially if the coupling capacitor is too high in value).Attached are the schematic and relevant portion of a board I designed a couple years ago which used the LM3481 in a SEPIC that provided a constant 24V at up to 2.5A output with a 20-28V input. It worked exactly as advertised on the first go, but, I've been designing switchmode converters for many years now so YMMV.