On 01/25/2016 07:07 PM, John Larkin wrote:> On Mon, 25 Jan 2016 13:14:34 -0500, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> 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 > > Do you have a decent LC meter? Just short one winding and measure the > other. TDR will work too, to estimate small inductance. > > Sometimes I use a function generator and a scope to measure small > inductances. That helps resolve resonances, which confuse some meters.I have an AADE, some Smart Tweezers, and a Measurements 59 GDO. So far I haven't needed anything else. I just don't have any of the inductors yet, and was hoping to mooch off the assembled expertise here. ;) 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
Coupled Inductors--how coupled is coupled?
Started by ●January 25, 2016
Reply by ●January 25, 20162016-01-25
Reply by ●January 25, 20162016-01-25
On 01/25/2016 07:18 PM, DecadentLinuxUserNumeroUno wrote:> On Mon, 25 Jan 2016 16:07:24 -0800, John Larkin > <jjlarkin@highlandtechnology.com> Gave us: > >> On Mon, 25 Jan 2016 13:14:34 -0500, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> 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 >> >> Do you have a decent LC meter? Just short one winding and measure the >> other. TDR will work too, to estimate small inductance. >> >> Sometimes I use a function generator and a scope to measure small >> inductances. That helps resolve resonances, which confuse some meters. > > "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. >Of course, hence the quotation marks. Distributors seem to restrict the noble term "transformer" to things that work at 50/60 Hz, or at most audio frequencies. 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 20:01:03 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> Gave us:>On 01/25/2016 07:18 PM, DecadentLinuxUserNumeroUno wrote: >> On Mon, 25 Jan 2016 16:07:24 -0800, John Larkin >> <jjlarkin@highlandtechnology.com> Gave us:snip>> "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. >> > >Of course, hence the quotation marks. Distributors seem to restrict the >noble term "transformer" to things that work at 50/60 Hz, or at most >audio frequencies. > >Cheers > >Phil HobbsSo do RF guys with antenna baluns.
Reply by ●January 26, 20162016-01-26
On 01/25/2016 02:24 PM, Phil Hobbs wrote:> On 01/25/2016 02:21 PM, Phil Hobbs wrote: >> On 01/25/2016 02:04 PM, Lasse Langwadt Christensen wrote: >>> Den mandag den 25. januar 2016 kl. 19.13.25 UTC+1 skrev Phil Hobbs: >>>> 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. >>>> >>> >>> flybuck? >>> >>> http://www.ti.com/lit/an/snva674b/snva674b.pdf >>> >>> -Lasse >> >> Thanks. Thought of that--I've done it before, but it really only works >> when the main supply is more heavily loaded than the isolated ones. I >> need two isolated outputs and zero non-isolated ones. >> >> Also I can't be sure that one supply will always be drawing more current >> than the other--a double terminated RS485 link can draw over 80 mA, >> whereas it's much less with AC termination. The SBC can draw 160 mA >> max, but there's no minimum specified. So it looks like a buck followed >> by two 1:1 transformers, bridges, and LDOs. _Not_ the elegant solution >> I was hoping for, but there you go. >> >> Cheers >> >> Phil Hobbs >> > > Belay that--with a _sync_ buck, you can balance the voltseconds even > with no load on the non-isolated output--it's a half-bridge.The sync buck half bridge looks like a winner. I should be able to use a couple of those 27-cent Bourns coupled inductors and a $1 Richtek RT7272 to make two isolated 5V outputs. As a bonus, putting capacitors in series with the primaries roughly halves the effective V_in, which lets the buck's duty cycle be longer, which improves efficiency. The cross-regulation isn't great unless the coupling coefficient is at least 0.995, though, so I'll need some scheme for feeding back from whichever output sags the most. The usual scheme for one output is to use a TL431 with the LED of an optocoupler in series with its anode, with feedback taken from the phototransistor. I could use two of those and diode-OR them into the feedback, I suppose, but depending on cost, it's probably better to use some of those toroids of John's to get better cross-regulation. Thanks, Lasse, for prodding me that direction. 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 26, 20162016-01-26
On Tue, 26 Jan 2016 12:14:51 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 01/25/2016 02:24 PM, Phil Hobbs wrote: >> On 01/25/2016 02:21 PM, Phil Hobbs wrote: >>> On 01/25/2016 02:04 PM, Lasse Langwadt Christensen wrote: >>>> Den mandag den 25. januar 2016 kl. 19.13.25 UTC+1 skrev Phil Hobbs: >>>>> 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. >>>>> >>>> >>>> flybuck? >>>> >>>> http://www.ti.com/lit/an/snva674b/snva674b.pdf >>>> >>>> -Lasse >>> >>> Thanks. Thought of that--I've done it before, but it really only works >>> when the main supply is more heavily loaded than the isolated ones. I >>> need two isolated outputs and zero non-isolated ones. >>> >>> Also I can't be sure that one supply will always be drawing more current >>> than the other--a double terminated RS485 link can draw over 80 mA, >>> whereas it's much less with AC termination. The SBC can draw 160 mA >>> max, but there's no minimum specified. So it looks like a buck followed >>> by two 1:1 transformers, bridges, and LDOs. _Not_ the elegant solution >>> I was hoping for, but there you go. >>> >>> Cheers >>> >>> Phil Hobbs >>> >> >> Belay that--with a _sync_ buck, you can balance the voltseconds even >> with no load on the non-isolated output--it's a half-bridge. > >The sync buck half bridge looks like a winner. I should be able to use >a couple of those 27-cent Bourns coupled inductors and a $1 Richtek >RT7272 to make two isolated 5V outputs. As a bonus, putting capacitors >in series with the primaries roughly halves the effective V_in, which >lets the buck's duty cycle be longer, which improves efficiency. > >The cross-regulation isn't great unless the coupling coefficient is at >least 0.995, though, so I'll need some scheme for feeding back from >whichever output sags the most. The usual scheme for one output is to >use a TL431 with the LED of an optocoupler in series with its anode, >with feedback taken from the phototransistor. I could use two of those >and diode-OR them into the feedback, I suppose, but depending on cost, >it's probably better to use some of those toroids of John's to get >better cross-regulation. > >Thanks, Lasse, for prodding me that direction. > >Cheers > >Phil HobbsThe Versa-Pac multi-winding inductors are cool, but not cheap, $3 range. We're lately paying around $3 for the 1:1:2:2 ISDN transformer; maybe ISDN is going out of fashion. Maybe one could take the feedback off an AC winding and get pretty good regulation. Coilcraft makes some really cool planar-winding transformers. I guess one might build a transformer into a PCB, with holes for the legs of a core. Cheap, but lots of engineering. You could buy a CUI dual-out surface-mount isolated converter for around $4. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●January 26, 20162016-01-26
Den tirsdag den 26. januar 2016 kl. 18.14.56 UTC+1 skrev Phil Hobbs:> On 01/25/2016 02:24 PM, Phil Hobbs wrote: > > On 01/25/2016 02:21 PM, Phil Hobbs wrote: > >> On 01/25/2016 02:04 PM, Lasse Langwadt Christensen wrote: > >>> Den mandag den 25. januar 2016 kl. 19.13.25 UTC+1 skrev Phil Hobbs: > >>>> 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. > >>>> > >>> > >>> flybuck? > >>> > >>> http://www.ti.com/lit/an/snva674b/snva674b.pdf > >>> > >>> -Lasse > >> > >> Thanks. Thought of that--I've done it before, but it really only works > >> when the main supply is more heavily loaded than the isolated ones. I > >> need two isolated outputs and zero non-isolated ones. > >> > >> Also I can't be sure that one supply will always be drawing more current > >> than the other--a double terminated RS485 link can draw over 80 mA, > >> whereas it's much less with AC termination. The SBC can draw 160 mA > >> max, but there's no minimum specified. So it looks like a buck followed > >> by two 1:1 transformers, bridges, and LDOs. _Not_ the elegant solution > >> I was hoping for, but there you go. > >> > >> Cheers > >> > >> Phil Hobbs > >> > > > > Belay that--with a _sync_ buck, you can balance the voltseconds even > > with no load on the non-isolated output--it's a half-bridge. > > The sync buck half bridge looks like a winner. I should be able to use > a couple of those 27-cent Bourns coupled inductors and a $1 Richtek > RT7272 to make two isolated 5V outputs. As a bonus, putting capacitors > in series with the primaries roughly halves the effective V_in, which > lets the buck's duty cycle be longer, which improves efficiency. > > The cross-regulation isn't great unless the coupling coefficient is at > least 0.995, though, so I'll need some scheme for feeding back from > whichever output sags the most. The usual scheme for one output is to > use a TL431 with the LED of an optocoupler in series with its anode, > with feedback taken from the phototransistor. I could use two of those > and diode-OR them into the feedback, I suppose, but depending on cost, > it's probably better to use some of those toroids of John's to get > better cross-regulation. > > Thanks, Lasse, for prodding me that direction. >you are welcome this might give a few hints, https://datasheets.maximintegrated.com/en/ds/MAX17681.pdf -Lasse
Reply by ●January 26, 20162016-01-26
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. -- Thanks, - Win
Reply by ●January 26, 20162016-01-26
On 01/26/2016 12:46 PM, John Larkin wrote:> On Tue, 26 Jan 2016 12:14:51 -0500, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 01/25/2016 02:24 PM, Phil Hobbs wrote: >>> On 01/25/2016 02:21 PM, Phil Hobbs wrote: >>>> On 01/25/2016 02:04 PM, Lasse Langwadt Christensen wrote: >>>>> Den mandag den 25. januar 2016 kl. 19.13.25 UTC+1 skrev Phil Hobbs: >>>>>> 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. >>>>>> >>>>> >>>>> flybuck? >>>>> >>>>> http://www.ti.com/lit/an/snva674b/snva674b.pdf >>>>> >>>>> -Lasse >>>> >>>> Thanks. Thought of that--I've done it before, but it really only works >>>> when the main supply is more heavily loaded than the isolated ones. I >>>> need two isolated outputs and zero non-isolated ones. >>>> >>>> Also I can't be sure that one supply will always be drawing more current >>>> than the other--a double terminated RS485 link can draw over 80 mA, >>>> whereas it's much less with AC termination. The SBC can draw 160 mA >>>> max, but there's no minimum specified. So it looks like a buck followed >>>> by two 1:1 transformers, bridges, and LDOs. _Not_ the elegant solution >>>> I was hoping for, but there you go. >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>>> >>> >>> Belay that--with a _sync_ buck, you can balance the voltseconds even >>> with no load on the non-isolated output--it's a half-bridge. >> >> The sync buck half bridge looks like a winner. I should be able to use >> a couple of those 27-cent Bourns coupled inductors and a $1 Richtek >> RT7272 to make two isolated 5V outputs. As a bonus, putting capacitors >> in series with the primaries roughly halves the effective V_in, which >> lets the buck's duty cycle be longer, which improves efficiency. >> >> The cross-regulation isn't great unless the coupling coefficient is at >> least 0.995, though, so I'll need some scheme for feeding back from >> whichever output sags the most. The usual scheme for one output is to >> use a TL431 with the LED of an optocoupler in series with its anode, >> with feedback taken from the phototransistor. I could use two of those >> and diode-OR them into the feedback, I suppose, but depending on cost, >> it's probably better to use some of those toroids of John's to get >> better cross-regulation. >> >> Thanks, Lasse, for prodding me that direction. >> >> Cheers >> >> Phil Hobbs > > The Versa-Pac multi-winding inductors are cool, but not cheap, $3 > range. We're lately paying around $3 for the 1:1:2:2 ISDN transformer; > maybe ISDN is going out of fashion. Maybe one could take the feedback > off an AC winding and get pretty good regulation. > > Coilcraft makes some really cool planar-winding transformers. > > I guess one might build a transformer into a PCB, with holes for the > legs of a core. Cheap, but lots of engineering. > > You could buy a CUI dual-out surface-mount isolated converter for > around $4.Yeah, but they're ratiometric, which won't fly in this case because of the power supply wiring resistance. Bummer that the toroids are getting so expensive. We need this gizmo to have a long production life, so something more generic is good. I'll play with the diode-OR trick. I'm not too worried about positive excursions because there'll be an LDO on each. 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 26, 20162016-01-26
On 01/26/2016 01:16 PM, Lasse Langwadt Christensen wrote:> Den tirsdag den 26. januar 2016 kl. 18.14.56 UTC+1 skrev Phil Hobbs: >> On 01/25/2016 02:24 PM, Phil Hobbs wrote: >>> On 01/25/2016 02:21 PM, Phil Hobbs wrote: >>>> On 01/25/2016 02:04 PM, Lasse Langwadt Christensen wrote: >>>>> Den mandag den 25. januar 2016 kl. 19.13.25 UTC+1 skrev Phil Hobbs: >>>>>> 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. >>>>>> >>>>> >>>>> flybuck? >>>>> >>>>> http://www.ti.com/lit/an/snva674b/snva674b.pdf >>>>> >>>>> -Lasse >>>> >>>> Thanks. Thought of that--I've done it before, but it really only works >>>> when the main supply is more heavily loaded than the isolated ones. I >>>> need two isolated outputs and zero non-isolated ones. >>>> >>>> Also I can't be sure that one supply will always be drawing more current >>>> than the other--a double terminated RS485 link can draw over 80 mA, >>>> whereas it's much less with AC termination. The SBC can draw 160 mA >>>> max, but there's no minimum specified. So it looks like a buck followed >>>> by two 1:1 transformers, bridges, and LDOs. _Not_ the elegant solution >>>> I was hoping for, but there you go. >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>>> >>> >>> Belay that--with a _sync_ buck, you can balance the voltseconds even >>> with no load on the non-isolated output--it's a half-bridge. >> >> The sync buck half bridge looks like a winner. I should be able to use >> a couple of those 27-cent Bourns coupled inductors and a $1 Richtek >> RT7272 to make two isolated 5V outputs. As a bonus, putting capacitors >> in series with the primaries roughly halves the effective V_in, which >> lets the buck's duty cycle be longer, which improves efficiency. >> >> The cross-regulation isn't great unless the coupling coefficient is at >> least 0.995, though, so I'll need some scheme for feeding back from >> whichever output sags the most. The usual scheme for one output is to >> use a TL431 with the LED of an optocoupler in series with its anode, >> with feedback taken from the phototransistor. I could use two of those >> and diode-OR them into the feedback, I suppose, but depending on cost, >> it's probably better to use some of those toroids of John's to get >> better cross-regulation. >> >> Thanks, Lasse, for prodding me that direction. >> > > you are welcome > > this might give a few hints, https://datasheets.maximintegrated.com/en/ds/MAX17681.pdfI looked at that feedback scheme, but it requires a pretty good transformer (k > 0.995 or so, < 1 ohm Rs) in order to get decent load regulation. It falls apart with k=0.985 to 0.99--the outputs sag but the voltage on the cap stays still. I ordered a bunch of inductors, so we'll see. (Newark has them on super sale at the moment.) 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 26, 20162016-01-26
On 01/26/2016 02:38 PM, 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, which in my line of work is probably a career's worth. I've recently done the same with a lot of TO92 and DIP packaged parts. It's probably a bit under $1k in extra inventory, but I can do dead bug protos for the foreseeable future. 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
