LT8309 : Mosfet as diode replacement at secondary side for flyback converter ? - Page 1
Forum section: electronics / beginners

author: BravoV
author_rank: Super Contributor
timestamp: February 16, 2014, 08:11:07 am
content: I subscribe Linear Technology's Analog Journal and at the latest Volume 23 No 24, Jan 2013 release, there is an interesting new chip called LT8309, that uses mosfet as diode replacement at the secondary side of a fly back converter.There is a small note though, that specifies it can be used ONLY with Linear Technology's "NON" opto-coupled flyback controller.Don't have experience with fly-back converter, only buck or boost all this time, hence I posted this thread under Beginner section.Just wonder if this unique chip can be used at ordinary flyback converter ? I mean, just slap in this chip + mosfet to replace the existing diode, will it work ?A whopping 7-8% better efficiency is big deal right ? imho.Quoted few interesting points from the journal on this chip :QuoteTo download the journal (PDF) ->http://www.linear.com/docs/44265BTW, by the time I posted this, the datasheet for this chip LT8309 still not available yet..

author: Bored@Work
author_rank: Super Contributor
timestamp: February 16, 2014, 08:20:14 am
content: Quotecan != mustBut anyhow, an IC where the company didn't even manage to publish a datasheet? For all practical concerns that IC is vaporware.

author: BravoV
author_rank: Super Contributor
timestamp: February 16, 2014, 08:34:32 am
content: Quote from: Bored@Work on February 16, 2014, 08:20:14 amAhhh... what a sharp eyes B@W, and always critical as usualthanks !I'm fully aware of your view regarding LT, but let say its not vapor ware, and with those few brief descriptions I quoted above, and of course without the complete datasheet, can we preliminary "ASSUME" it will work with plain-jane flyback converter ?B@W, I know you are qualified to answer this kind of noob question.

author: SeanB
author_rank: Super Contributor
timestamp: February 16, 2014, 08:38:30 am
content: Probably still in the sampling stage, available to selected volume users only for evaluation. Likely the datasheet is still not complete or is still in a preliminary state and is likely to change.For volume use I would guess they will bring out one with an integrated thermal pad combining the mosfet and controller in one package shortly. Likely to be smaller board wise, as you can put a pretty good MOSFET cheaply in the package if you tailor the drive for it on the second chip. Then it will only have a few external passives only.

author: BravoV
author_rank: Super Contributor
timestamp: February 16, 2014, 08:49:59 am
content: Quote from: SeanB on February 16, 2014, 08:38:30 amThe journal stated it came with SOT-23 package.Probably a "naive" view from a hobbyist, if this chip works as advertised, say for one off modding job for existing flyback psu, I imagine to put this chip + good mosfet + few passive components + some creative soldering/mounting job to replace the diode, with the aim to squeeze out say 5% more.Is this a realistic expectation ? Again, not for high volume product, just one off modding job.

author: SeanB
author_rank: Super Contributor
timestamp: February 16, 2014, 09:10:58 am
content: You probably would get about 1W of power loss with a high current low voltage supply so this would be good to replace in a 5v supply running over about 5A, giving a saving of a good few watts and not needing a big heatsink on the board, just a small copper area to cool the switch losses. Under 1A it likely would not be any different. In anything with high current at low voltage it would be a big improvement if there is no active rectification already.

author: BravoV
author_rank: Super Contributor
timestamp: February 16, 2014, 09:28:59 am
content: Well, I already have 2 of those mains powered bricks churning out at 12 Volt at 10 Amps !

author: Bored@Work
author_rank: Super Contributor
timestamp: February 16, 2014, 09:49:40 am
content: Quote from: BravoV on February 16, 2014, 08:34:32 amSee, that is the point. With no datasheet available, with just piecemeal information from a single marketing article, all one can do is to speculate. And to speculate about an IC that isn't even available is IMHO a waste of time. I have been burned one time too often by betting the farm on some new super component, just to learn later it never will see regular production or it being buggy as hell.But if you want a speculation, I find it suspicious that they explicitly mention non-opto feedback loops. Maybe (speculation) they have observed (speculation) or know (speculation) some issues (speculation) with that IC and opto-isolated feedback loops (speculation), i.e. loop stability (speculation) going to hell (speculation). Or maybe (speculation) it is all just to market (speculation) their non-opto controller ICs (speculation).Wait until you can buy that IC via regular distributors, plug it down on a PCB and measure. Then you'll know.

author: peter.mitchell
author_rank: Super Contributor
timestamp: February 16, 2014, 01:14:32 pm
content: Quote from: AcHmed99 on February 16, 2014, 10:50:46 amYeah, i just bought a fist full of LT3791 for a project, $9 a pop!

author: BravoV
author_rank: Super Contributor
timestamp: February 17, 2014, 12:56:36 am
content: Quote from: Bored@Work on February 16, 2014, 09:49:40 amLOL ..., just love the cynical point of view of this thing, one must had been thru experiences to able to come out with that.Yeah, better wait until they released the datasheet.Quote from: AcHmed99 on February 16, 2014, 10:50:46 amThanks, didn't know other SR controllers before.I guess LT is late comer in this kind of product, isn't it ?

author: mikenltc
author_rank: Newbie
timestamp: March 13, 2014, 12:40:51 am
content: I was using google to get the link for the LT8309 datasheet and stumbled upon this forum post. I designed the LT8309. I apologize that the datasheet wasn't available on the website when the article was published. I know that we can be pricey on digikey but you can get single pieces from our website for $2.21. The datasheet is up here.http://www.linear.com/product/LT8309I know there a few other parts out there but they didn't quite meet our needs. Last time I looked its propagation delay and accuracy are better than all the others. I admit that this isn't that important for operating in boundary mode and discontinuous mode.This part can be used for opto-coupler applications, but we like to promote our family of no opto flybacks. (LT3573, LT3748, LT8302, LT3798) Why use an opto if you don't have to?This part is designed to work best with our parts so you will have to check to make sure that your application will be ok with the minimum on-time and minimum off-time. If your application operates in CCM, there will be a brief time of cross conduction that charges up the leakage inductance and causes a voltage spike. This may cause damage to the MOSFETs if not dampened. Getting these synchronous rectifiers to work in CCM is quite a bit harder than DCM and boundary mode.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 13, 2014, 03:04:40 am
content: Synchronous rectifiers (as part of controller chips, or drivers, even self powered two-pin devices) are nothing new. You can even do itdiscreteif you like; recovery is replaced by response time, no free lunch of course as usual.I believe On Semi was making the two-pin devices, which are probably worth a look. I'm sure many manufacturers are making them these days. You'll want to check response time and compare that to your operating frequency; doing it at 50kHz is practically trivial (see above example), doing it at 1MHz is a little harder.Tim