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Coupled Inductors--how coupled is coupled?

Started by Phil Hobbs January 25, 2016
>Are you asking for a way to measure mutual
inductance, i.e., how to measure the coupling coefficient ? No, that's easy. I just didn't have the parts on handm. Cheers Phil Hobbs
DecadentLinuxUserNumeroUno wrote...
> John Larkin Gave us: >> Phil Hobbs wrote: >>> mixed nuts wrote: >>>> Phil Hobbs wrote: >>>>> [ snip ] so I was thinking about using the Bourns >>>>> SRF0703-471M "coupled inductor" as a flyback. >>>> Looks too small to be anything far from unity - even >>>> with a small gap between ferrite bits - 0.998? >>> Plus or minus 20%, of course. ;) >> Do you have a decent LC meter? Just short one winding >> and measure the other. > > "coupled inductor" There is no such animal. If there > is coupling it immediately becomes and is empirically > defined as being a transformer. > > Turns count ratio and voltage transformation rules apply.
The "coupled-inductor" label comes about because it is first and foremost an inductor, with a carefully designed gap, to store energy in the winding. The second transformer winding goes along for the ride. By contrast "transformer" windings don't generally store energy, but transmit it directly to a secondary. Also, the manufacturers of coupled inductors use that name, not transformer, so if you searched with your preferred term, you wouldn't find these parts. You can treat these parts as transformer if you wish, but in many of our applications most of the analysis will concentrate on the magnetizing inductance. -- Thanks, - Win
Phil Hobbs wrote...
> Winfield Hill wrote: >> Phil Hobbs wrote... >>> Winfield Hill wrote: >>>> Phil Hobbs wrote... >>>>> >>>>> I was thinking about using the Bourns >>>>> SRF0703-471M "coupled inductor" ... >>>> >>>> Newark has those on closeout sale right >>>> now; that particular value is $0.19 each. >>> >>> I saw that, and ordered 50 of them ... >> >> Made some measurements with my hp 4192 LCR meter. >> DCR 5.00 ohms both sides (spec 5.64 ohms) >> L (100kHz) 480uH [each or parallel] (spec 470uH) >> L_series (100kHz) 1.97mH (spec 1.88mH) >> Lell (any freq) 6.6uH (1.4%, very good) >> SRF 550kHz (poor, but OK for 100kHz use) >> >> These will do well, even with unmatched loads. > > Thanks, Win. So k~= 0.986 then.
How are you choosing your preferred inductance value? Based on Newark's special sales price? -- Thanks, - Win
>How are you choosing your preferred inductance
�value? �Based on Newark's special sales price? Well, for junkbox-type protos that's not totally irrational, but in this case it's mostly the tradeoff between loss in the sync buck chip and in the coil resistance, with cost as the penalty function. The Bourns SRF1260-101 has about 350 mOhms in each winding, which makes it reasonable to drive two in parallel with an LM3103 switcher. With loosey-goosey coupling like that (k=0.985), I did have to dork the primary-side setpoint as a function of supply voltage, but it looks like it should work. I don't care too much about efficiency in this application--it's only a couple of watts overall--but installed cost matters a lot. Fun. Cheers Phil Hobbs
Winfield Hill wrote...
> DecadentLinuxUserNumeroUno wrote... >> John Larkin Gave us: >>> Phil Hobbs wrote: >>>> mixed nuts wrote: >>>>> Phil Hobbs wrote: >>>>>> [ snip ] so I was thinking about using the Bourns >>>>>> SRF0703-471M "coupled inductor" as a flyback. >>>>> Looks too small to be anything far from unity - even >>>>> with a small gap between ferrite bits - 0.998? >>>> Plus or minus 20%, of course. ;) >>> Do you have a decent LC meter? Just short one winding >>> and measure the other. >> >> "coupled inductor" There is no such animal. If there >> is coupling it immediately becomes and is empirically >> defined as being a transformer. >> >> Turns count ratio and voltage transformation rules apply. > > The "coupled-inductor" label comes about because it > is first and foremost an inductor, with a carefully > designed gap, to store energy in the winding. The > second transformer winding goes along for the ride.
I should add, these are 1:1 coupled inductors; yep, larger ratio are called flyback transformers. ;-) 1:1 coupled inductors are very useful in SEPIC, Ćuk, Zeta and inverting bipolar converters, plus of course, plain old common-mode chokes. They're bifilar wound and can be very cheap. I just made a bulk purchase at 9 cents each. -- Thanks, - Win
On 2 Feb 2016 16:32:55 -0800, Winfield Hill <hill@rowland.harvard.edu>
wrote:

>Winfield Hill wrote... >> DecadentLinuxUserNumeroUno wrote... >>> John Larkin Gave us: >>>> Phil Hobbs wrote: >>>>> mixed nuts wrote: >>>>>> Phil Hobbs wrote: >>>>>>> [ snip ] so I was thinking about using the Bourns >>>>>>> SRF0703-471M "coupled inductor" as a flyback. >>>>>> Looks too small to be anything far from unity - even >>>>>> with a small gap between ferrite bits - 0.998? >>>>> Plus or minus 20%, of course. ;) >>>> Do you have a decent LC meter? Just short one winding >>>> and measure the other. >>> >>> "coupled inductor" There is no such animal. If there >>> is coupling it immediately becomes and is empirically >>> defined as being a transformer. >>> >>> Turns count ratio and voltage transformation rules apply. >> >> The "coupled-inductor" label comes about because it >> is first and foremost an inductor, with a carefully >> designed gap, to store energy in the winding. The >> second transformer winding goes along for the ride. > > I should add, these are 1:1 coupled inductors; yep, > larger ratio are called flyback transformers. ;-) > > 1:1 coupled inductors are very useful in SEPIC, > &#262;uk, Zeta and inverting bipolar converters, > plus of course, plain old common-mode chokes. > They're bifilar wound and can be very cheap. > I just made a bulk purchase at 9 cents each.
Here I'm using one as, well, an autotransformer or something. https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/28S840_A7.pdf I'm now doing the board layout, and I've got too used to fast stuff, packing things as close together as possible. Now I'm dominated by HV clearance rules, so I have to keep parts far from each other. Freaky. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com