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

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

>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

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

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

>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

On Monday, January 25, 2016 at 10:14:23 AM UTC-5, Phil Hobbs wrote:
> So in the continuing saga of cotton spark detection, I need to make an 
> isolated 2-output DC-DC converter to power an RS-485 link and a small 
> SBC--about 2W altogether.   This isn't a terribly low-noise application, 
> so I was thinking about using the Bourns
> SRF0703-471M "coupled inductor" as a flyback.  The question is, what's 
> the coefficient of couping?  There's no way to find out from the 
> datasheet.  I'll get a few to try out, but in the mean time, does 
> anybody know the approximate value of k for these beasts?
> 
> Thanks
> 
> Phil Hobbs
> 
> -- 
> Dr Philip C D Hobbs
> Principal Consultant
> ElectroOptical Innovations LLC
> Optics, Electro-optics, Photonics, Analog Electronics
> 
> 160 North State Road #203
> Briarcliff Manor NY 10510
> 
> hobbs at electrooptical dot net
> http://electrooptical.net

Are you asking for a way to measure mutual 
inductance, i.e., how to measure the coupling coefficient ?

On 01/30/2016 12:07 PM, 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.

Looks like I can mostly get rid of the line regulation sag by putting a 
resistor from V_in to the feedback point, which makes it practical to 
use the flybuck topology with an LDO per output.  At that level it does 
need a lightweight snubber to protect the Schottky rectifier, but it 
doesn't cost much power.

Probably when I've done this a few times I can make it simpler, but this 
seems pretty good.

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

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

Phil Hobbs wrote...
> John Larkin 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.
>> 
>> Does closeout = EOL?
>
> No sign of that on the Bourns website,
> and DK has lots.

 I think it means Farnell (England) is
 dropping that product line, that's all.


-- 
 Thanks,
    - Win