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
Coupled Inductors--how coupled is coupled?
Started by ●January 25, 2016
Reply by ●January 25, 20162016-01-25
On Mon, 25 Jan 2016 10:14:12 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> 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 HobbsWe've measured that. The short answer is "about 0.99" Most seem to be bifalar toroids, pretty good coupling. Here's some notes on a couple of Coiltronics parts: Notes on leakage inductance: Part, OCL, SCL, K ( sqrt(1 - SCL/OCL) ) DRQ74-8R2, 8.03uH, 110nH, 0.993 DRQ74-150, 16.91uH, 270nH, 0.991 Measured on AADE LC meter. RG 16-Sept-10 DRQ127-151 150 uH 4 uH DRQ127-331 330 uH 5 uH DRQ127-102 1 mH 70 uH JL Mar 2012 -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●January 25, 20162016-01-25
On 1/25/2016 10:14 AM, 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?Looks too small to be anything far from unity - even with a small gap between ferrite bits - 0.998? The series mode inductance is spec'd to be 1880 (470*4)! ;) -- Grizzly H.
Reply by ●January 25, 20162016-01-25
On 01/25/2016 10:37 AM, John Larkin wrote:> On Mon, 25 Jan 2016 10:14:12 -0500, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> 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 > > We've measured that. The short answer is "about 0.99" > > Most seem to be bifalar toroids, pretty good coupling. > > Here's some notes on a couple of Coiltronics parts: > > > > Notes on leakage inductance: > > Part, OCL, SCL, K ( sqrt(1 - SCL/OCL) ) > DRQ74-8R2, 8.03uH, 110nH, 0.993 > DRQ74-150, 16.91uH, 270nH, 0.991 > > Measured on AADE LC meter. RG 16-Sept-10 > > DRQ127-151 150 uH 4 uH > DRQ127-331 330 uH 5 uH > DRQ127-102 1 mH 70 uH > > JL Mar 2012 > > > >Thanks. The Bourns ones I was looking at are the cheesy "shielded" kind with the big gap at the top of the core + cup, so I was hoping it would be around 0.95-0.98. 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
Reply by ●January 25, 20162016-01-25
Reply by ●January 25, 20162016-01-25
On Mon, 25 Jan 2016 10:40:33 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 01/25/2016 10:37 AM, John Larkin wrote: >> On Mon, 25 Jan 2016 10:14:12 -0500, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> 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 >> >> We've measured that. The short answer is "about 0.99" >> >> Most seem to be bifalar toroids, pretty good coupling. >> >> Here's some notes on a couple of Coiltronics parts: >> >> >> >> Notes on leakage inductance: >> >> Part, OCL, SCL, K ( sqrt(1 - SCL/OCL) ) >> DRQ74-8R2, 8.03uH, 110nH, 0.993 >> DRQ74-150, 16.91uH, 270nH, 0.991 >> >> Measured on AADE LC meter. RG 16-Sept-10 >> >> DRQ127-151 150 uH 4 uH >> DRQ127-331 330 uH 5 uH >> DRQ127-102 1 mH 70 uH >> >> JL Mar 2012 >> >> >> >> >Thanks. The Bourns ones I was looking at are the cheesy "shielded" kind >with the big gap at the top of the core + cup, so I was hoping it would >be around 0.95-0.98. > >Cheers > >Phil HobbsThe DRQs are like that. I may be wrong about the construction, but coupling seems pretty good. The ISDN transformers are bifalar toroids, very good coupling. We use one 1:1:2:2 part that is very versatile. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●January 25, 20162016-01-25
On 01/25/2016 12:10 PM, John Larkin wrote:> On Mon, 25 Jan 2016 10:40:33 -0500, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 01/25/2016 10:37 AM, John Larkin wrote: >>> On Mon, 25 Jan 2016 10:14:12 -0500, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> 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 >>> >>> We've measured that. The short answer is "about 0.99" >>> >>> Most seem to be bifalar toroids, pretty good coupling. >>> >>> Here's some notes on a couple of Coiltronics parts: >>> >>> >>> >>> Notes on leakage inductance: >>> >>> Part, OCL, SCL, K ( sqrt(1 - SCL/OCL) ) >>> DRQ74-8R2, 8.03uH, 110nH, 0.993 >>> DRQ74-150, 16.91uH, 270nH, 0.991 >>> >>> Measured on AADE LC meter. RG 16-Sept-10 >>> >>> DRQ127-151 150 uH 4 uH >>> DRQ127-331 330 uH 5 uH >>> DRQ127-102 1 mH 70 uH >>> >>> JL Mar 2012 >>> >>> >>> >>> >> Thanks. The Bourns ones I was looking at are the cheesy "shielded" kind >> with the big gap at the top of the core + cup, so I was hoping it would >> be around 0.95-0.98. >> >> Cheers >> >> Phil Hobbs > > The DRQs are like that. I may be wrong about the construction, but > coupling seems pretty good. > > The ISDN transformers are bifalar toroids, very good coupling. We use > one 1:1:2:2 part that is very versatile. > >It's sort of an interesting case--very cost sensitive, but has to be tough enough to withstand a monsoon thunderstorm in a cotton mill with very iffy grounding and long runs of thin wire. We're going to bus around +24V, panel ground, and a twisted pair for 200 kb/s RS485 data. So I'm looking at some combination of MOVs, TVS zeners, and depletion MOSFETs on all four lines, plus fully isolated power and comms. The first thing I tried was a half-bridge driving two 150 uH double-wound inductors (L1A + L2A in series, L1B + L1B in parallel for the output), followed by bridges, filters, and LDOs. That worked well with no ringing or other nonsense, and managed to get to 5V okay, but it was a bit marginal at the low voltage limit. Using MOSFETs for the bridge on the output helped some, but it was just getting too complicated. I'll probably just put in one of those 27-cent A&O buck chips and then drive a couple of 1:1 transformers. I can use the unregulated input to power the high side gate, which helps some. The bad news about regulated switchers is that they have negative input resistance, which may become a problem when the wiring runs get too long. (I can see somebody running the power on a spare CAT6 pair, 20 ohms per 1000 feet.) 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
Reply by ●January 25, 20162016-01-25
On 01/25/2016 10:38 AM, mixed nuts wrote:> On 1/25/2016 10:14 AM, 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? > > Looks too small to be anything far from unity - even with a small gap > between ferrite bits - 0.998? > > The series mode inductance is spec'd to be 1880 (470*4)! ;) >Plus or minus 20%, of course. ;) 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
Reply by ●January 25, 20162016-01-25
On 01/25/2016 10:43 AM, makolber@yahoo.com wrote:> if the "leakage inductance" is specified or can be measured, then > you can calculate the coupling.Yup. It isn't, though--the inductance in series is 2 L1 + 2 M, and in parallel it's (L1+M)/2. They list the series inductance as 4x the parallel, so k is around unity. The specs are +-20%, so k could be as low as 0.6 and it would still pass. 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
Reply by ●January 25, 20162016-01-25
On Mon, 25 Jan 2016 13:13:12 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 01/25/2016 12:10 PM, John Larkin wrote: >> On Mon, 25 Jan 2016 10:40:33 -0500, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 01/25/2016 10:37 AM, John Larkin wrote: >>>> On Mon, 25 Jan 2016 10:14:12 -0500, Phil Hobbs >>>> <pcdhSpamMeSenseless@electrooptical.net> 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 >>>> >>>> We've measured that. The short answer is "about 0.99" >>>> >>>> Most seem to be bifalar toroids, pretty good coupling. >>>> >>>> Here's some notes on a couple of Coiltronics parts: >>>> >>>> >>>> >>>> Notes on leakage inductance: >>>> >>>> Part, OCL, SCL, K ( sqrt(1 - SCL/OCL) ) >>>> DRQ74-8R2, 8.03uH, 110nH, 0.993 >>>> DRQ74-150, 16.91uH, 270nH, 0.991 >>>> >>>> Measured on AADE LC meter. RG 16-Sept-10 >>>> >>>> DRQ127-151 150 uH 4 uH >>>> DRQ127-331 330 uH 5 uH >>>> DRQ127-102 1 mH 70 uH >>>> >>>> JL Mar 2012 >>>> >>>> >>>> >>>> >>> Thanks. The Bourns ones I was looking at are the cheesy "shielded" kind >>> with the big gap at the top of the core + cup, so I was hoping it would >>> be around 0.95-0.98. >>> >>> Cheers >>> >>> Phil Hobbs >> >> The DRQs are like that. I may be wrong about the construction, but >> coupling seems pretty good. >> >> The ISDN transformers are bifalar toroids, very good coupling. We use >> one 1:1:2:2 part that is very versatile. >> >> > >It's sort of an interesting case--very cost sensitive, but has to be >tough enough to withstand a monsoon thunderstorm in a cotton mill with >very iffy grounding and long runs of thin wire. We're going to bus >around +24V, panel ground, and a twisted pair for 200 kb/s RS485 data. > >So I'm looking at some combination of MOVs, TVS zeners, and depletion >MOSFETs on all four lines, plus fully isolated power and comms.We like to start with a polyfuse (radial lead, not surface mount) followed by a unipolar transzorb. Seems to protect against user blunders, short of 120 VAC on a 24 volt bus. Lightning can be a nightmare in tropical climes and old buildings. Horror stories about gear in Florida. Protect those RS485 chips, too.> >The first thing I tried was a half-bridge driving two 150 uH >double-wound inductors (L1A + L2A in series, L1B + L1B in parallel for >the output), followed by bridges, filters, and LDOs. That worked well >with no ringing or other nonsense, and managed to get to 5V okay, but it >was a bit marginal at the low voltage limit. Using MOSFETs for the >bridge on the output helped some, but it was just getting too complicated. > >I'll probably just put in one of those 27-cent A&O buck chips and then >drive a couple of 1:1 transformers. I can use the unregulated input to >power the high side gate, which helps some. > >The bad news about regulated switchers is that they have negative input >resistance, which may become a problem when the wiring runs get too >long. (I can see somebody running the power on a spare CAT6 pair, 20 >ohms per 1000 feet.)That recently confused our test folks. We have some bench supplies that come up slow, and some products with purchased switcher bricks, and the combo wouldn't start with the supplies set for apparently reasonable current limits. We try to include UVLO and/or soft-start when we design switchers. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
