Building a dual output (+/-) flyback regulator...Can't find the transformers! - Page 1
Forum section: electronics / beginners

author: Skimask
author_rank: Super Contributor
timestamp: December 22, 2012, 04:35:07 pm
content: Trying to build up a dual output (+ and -) flyback regulator. +5v in, +12/-12v out, using the LM2577LM2587 type simple switcher regulators from National (and other similar types)...'cause that's what I've got on hand. Bought the chips years ago for another project, so they might be outdated by now. Who knows...?I'm going straight off the examples in the datasheets and they keep referencing CoilCraft, Schott, Pulse Engineering, Renco, etc. transformers that don't seem to be available anymore.For instance, Schott 67140900, Renco RL-5531, Coilcraft Q4337...the list goes on.What's the dealio?I'm trying to get this curve tracer project of mine to run off a battery pack vs. using a PC/AT power supply for the +/-12v power rails. Ya know, make it halfway portable-ish...even if it'll only run for an hour off a stack of D size NiMH batt's

author: Bored@Work
author_rank: Super Contributor
timestamp: December 22, 2012, 04:41:39 pm
content: Quote from: Skimask on December 22, 2012, 04:35:07 pmWell, e.g. the Coilcraft Q4337 is still availablehttp://www.coilcraft.com/q4337.cfm

author: Skimask
author_rank: Super Contributor
timestamp: December 22, 2012, 09:21:51 pm
content: Thanks for the tip.Might get lucky and score a couple samples and take care of business.

author: c4757p
author_rank: Super Contributor
timestamp: December 22, 2012, 09:36:30 pm
content: Usually for small, single projects like that, I find a commercially available transformer, and then instead of buying it, I wind one myself to similar specs. That way I don't need to pay for this special transformer and that special transformer (a pack of surplus toroids from eBay and a spool of magnet wire were cheap), but the homemade one still corresponds to a commercial one if I ever decided to make more.

author: Skimask
author_rank: Super Contributor
timestamp: December 23, 2012, 12:41:18 am
content: Ya, I don't know if I'd trust myself winding my own coils...unless you got any good links to a good DIY site. meh...I'll google it after a bit. Never thought too hard about winding my own. I'm imagining there's a lot of theory behind it (besides the obvious turns ratios and such), and hence probably a lot of trial and error once you get into anything that has to move any kind of power.Awhile back, I got bored(-ish), spent a weekend with my hot rework gun, and desoldered every part I could reasonably pull off a bunch of the dead PCBs I got laying around the shop. I got a decent size box full of various transformers, and not a one of them has any good identification numbers on it. You'd think there would be at least one in there that would fit the bill.

author: c4757p
author_rank: Super Contributor
timestamp: December 23, 2012, 01:13:15 am
content: It's not too hard, really. There's a lot of theory that youcouldget into but you rarely have to. I'd at least recommend a decent inductance meter, though - I use an old-fashioned but accurate Leader LCR-740 impedance bridge. It measures inductance with 10 nH resolution on the lowest range and up to 110 H at up to 40 kHz test frequency; I got it for $20 on eBay.That would do a fine job of identifying all those transformers, too.

author: Skimask
author_rank: Super Contributor
timestamp: December 23, 2012, 05:50:13 am
content: Quote from: c4757p on December 23, 2012, 01:13:15 amI've got a bunch of stuff on my "to buy" list, the MS5308 is one of them. Started a business last month, got 3 solid (e.g. ~$60K) in contracts already, buying up a bunch of quality equipment for a change.Theory-ya, from what I've been reading the past few hours, the bulk of the theory would really only come into play if I was trying squeeze out the last ounce of efficiency & heat out of them....but....I'm failing to get how an L meter would figure out those transformers.

author: c4757p
author_rank: Super Contributor
timestamp: December 23, 2012, 06:00:26 am
content: Quote from: Skimask on December 23, 2012, 05:50:13 amInductance ratio = turns ratio squared. You can also measure other parameters - leakage inductance = primary inductance with secondary shorted, coupling factor = sqrt(1 - Lleakage/Lprimary), quality factor, and so on - to help gauge their performance in a given situation.

author: Skimask
author_rank: Super Contributor
timestamp: December 23, 2012, 07:03:53 am
content: And again, more tips. And again, thanks, more stuff to read.I'm figuring out that the LM2577/2588 flyback type circuit making +/-15v from +5 might not be the best option. In my circuit, the +/-15v will be feeding opamps, so there's a chance that one side or the other will draw more juice, which I'm sure will cause an imbalance somewhere and mess everything up royally.I'm getting there...

author: SeanB
author_rank: Super Contributor
timestamp: December 23, 2012, 09:25:40 am
content: In most cases for lowest noise on an opamp supply you wind the transformer to put out a regulated ( or not) rail in the order of 20V, then use a 7815 and 7915 to provide the 15V rails with no noise on them. On a flyback you wind 2 secondaries and connect them in series, with the ends having a single diode to do flyback rectification. If the 2 windings are wound together then they will be closely coupled, and thus each will give close to the same voltage even if the load is different on each rail ( within limits) which will be within the regulation range of the 15V regulators. If you use a push pull driver then you can have wildly varying loads on each rail and still keep reasonable regulation, though the feedback is best taken from the rail with the highest load.

author: Skimask
author_rank: Super Contributor
timestamp: December 23, 2012, 04:43:32 pm
content: You're picking up what I'm laying down...The plan was to get +/-15v out of the circuit using the LM2588 with the feedback resistors set for 15v, then post-regulate that output with a 7812/7912 pair and good filtering to keep any ripple out of there.I'm still a bit flaky on winding my own transformers. Read a few pages on the subject last night. It surely isn't as simple as putting the required turns on a nail or similar.(thinking out loud-ish...)I'm pretty sure that Coilcraft Q4337 will do what I need...single primary/split secondary-center tapped transformer with a ~2.5:1 turn ratio for each secondary half (~8 in-> ~20 out). Going to be feeding the circuit with a 8 cell NiMH pack vs. a 5volt supply as shown in the LM2588 datasheet. I was going to go LiPo, but don't want to deal with the charging at the moment. Maybe in the future.Shooting for a max current of less than 1/2 amp on each half of the secondary, meaning at the very most 4 amps on the primary (or is it 2?).meh...more reading/learning required.

author: c4757p
author_rank: Super Contributor
timestamp: December 23, 2012, 06:07:49 pm
content: Quote from: Skimask on December 23, 2012, 04:43:32 pmDepending on the composition of the nail, that very well could work... I have a bag of Sanlin SL5 power inductor/transformer toroids that I use, and it really does not require a lot of computation beyond what you'd have to do to choose a commercial transformer. I get decent (85%-lower 90s) efficiency at lower powers, don't really do much with high power so I couldn't tell you anything there. I also have a couple sets of iron plates and bobbins from E-type transformers that I acquired, discovered to be useless as they were, and unwound. These work great too.

author: Skimask
author_rank: Super Contributor
timestamp: December 25, 2012, 09:42:41 am
content: Stupid me just figured out that my HY3005D-3 power supply, when series mode is selected, can give me + and - outputs...That'll suffice for now...until I figure out how to effectively wind a decent coil...or those samples come in from Coilcraft

author: Skimask
author_rank: Super Contributor
timestamp: January 06, 2013, 09:10:27 am
content: Back to the SMPS again...Regarding the SYNC pin on the LM2588's and using multiple LM2588's to power different sections of my analog signature analyzer project...I'm going to use 3 LM2588's, one to power the op-amps that are actually putting juice out to the D.U.T.'s, one to power the op-amps doing the work of amp'ing up the resultant signals, and one to power the MCU's and LCD doing the rest of the math/display/etc.Would I be better off SYNC'ing the LM2588's, so that any noise that makes it past the output filters goes fairly equally into ALL of the op-amps, as well as the ADC in the PICs?Or everything as separated as I can without going to extremes?

author: Skimask
author_rank: Super Contributor
timestamp: January 09, 2013, 09:03:35 am
content: Ughh...seems like I can't get anything to work right...Built up the boost in the LM2588 datasheet, figure 13 -- 5v -> +/- 12v dual output flyback regulator.Powered it with a 4 cell NiMH pack (tried both AA and D cells), could only get 9.-ish volts out of it at a load of 20mA on each rail, and the pack voltage sitting at 4.4v.Used the Q4337 transformer specified by the example circuit, low ESR caps for the input and output, 1N5819's for the catch diodes, but instead of using the LM2588-12, I'm using the LM2588-ADJ with the appropriate feedback resistors/pots.If I were to use an 8 cell NiMH pack for the input voltage, that should boost the output up to where I need it, and an 8 cell pack is acceptable for my project. About the only limiting factor I can foresee is the coil itself, which is rated at 5v according to the datasheet, but I'll be pulling far less than the 3 amps maximum.Let's see what parts I can let some smoke out of next ! .............