On Wednesday, July 17, 2013 12:20:51 AM UTC+2, Phil Hobbs wrote:> On 7/16/2013 5:08 PM, John Larkin wrote: > > > On Tue, 16 Jul 2013 15:08:24 -0400, Phil Hobbs > > > <pcdhSpamMeSenseless@electrooptical.net> wrote: > > > > > >> So, I'm goofing off playing with small switchers. (Well, not exactly > > >> goofing off, but there are somewhat more pressing tasks waiting....) > > >> > > >> As I said in George's thread, "Opamp w/ Vsupply > 36V", I built a small > > >> half-bridge supply with positive and negative voltage doublers. I'm not > > >> that keen on it, because (a) it uses a fair number of parts for what it > > >> does, and (b) it has a nasty instability. > > >> > > >> With a capacitively-coupled half bridge, if you let the transformer > > >> saturate, it instantly discharges the coupling cap, which about doubles > > >> the volt-seconds on the next half-cycle and guarantee that it will keep > > >> saturating until the FETs cook themselves. > > >> > > >> The particular little ISDN transformers I'm using have really amazingly > > >> low leakage inductance, so I'm thinking about using a current-mode > > >> flyback instead. The UCC28C45 bicmos controller chip looks pretty > > >> suitable--it's about the same price as the IRS2153D, needs one less FET, > > >> runs up to 1 MHz, and with such a low leakage inductance I wouldn't > > >> expect to need much snubbing, if any. Plus I can run the transformer > > >> right up to its maximum volt-seconds without worrying. > > >> > > >> Ideally I'd like it to give me +-45 V at about 20 mA each. > > >> > > >> Any words of wisdom? > > >> > > >> Cheers > > >> > > >> Phil Hobbs > > > > > > The ISDNs work to pretty low frequencies, so a high frequency driver > > > and a small cap might fix the saturation problem. > > > > > > I recently saw a chip with an oscillator, flipflop, and two open-drain > > > outputs. You could drive a center-tapped primary winding with one of > > > those. The ISDN trannies caome in all sorts of ratios, like 1:1:2:2 > > > for example. > > > > The flyback idea was sort of appealing, because it lends itself to > > voltage feedback, but as Joerg says, the core won't store enough energy > > without a gap, which sort of defeats the purpose. > > > > This is for another laser noise canceller design, which needs a lot of > > reverse bias and ought to work out to at least 5 mA per photodiode. > > There's a current mirror on each one, so that's 10 mA on the signal > > channel (+40V) and as much as 20 mA on the comparison channel (-40V). > > (I may need to water-cool the photodiodes.) ;) > > > > It looks like some Royer-style thing might work better, since that > > automatically fixes the saturation problem. I don't have a spare > > winding, but I could probably use a voltage divider off the secondary. > >LT3439 ? -Lasse
Flyback vs half-bridge
Started by ●July 16, 2013
Reply by ●July 16, 20132013-07-16
Reply by ●July 16, 20132013-07-16
On 7/16/2013 5:10 PM, Joerg wrote:> Phil Hobbs wrote: >> On 07/16/2013 03:52 PM, Jim Thompson wrote: >>> On Tue, 16 Jul 2013 15:08:24 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> So, I'm goofing off playing with small switchers. (Well, not exactly >>>> goofing off, but there are somewhat more pressing tasks waiting....) >>>> >>>> As I said in George's thread, "Opamp w/ Vsupply > 36V", I built a small >>>> half-bridge supply with positive and negative voltage doublers. I'm not >>>> that keen on it, because (a) it uses a fair number of parts for what it >>>> does, and (b) it has a nasty instability. >>>> >>>> With a capacitively-coupled half bridge, if you let the transformer >>>> saturate, it instantly discharges the coupling cap, which about doubles >>>> the volt-seconds on the next half-cycle and guarantee that it will keep >>>> saturating until the FETs cook themselves. >>>> >>>> The particular little ISDN transformers I'm using have really amazingly >>>> low leakage inductance, so I'm thinking about using a current-mode >>>> flyback instead. The UCC28C45 bicmos controller chip looks pretty >>>> suitable--it's about the same price as the IRS2153D, needs one less FET, >>>> runs up to 1 MHz, and with such a low leakage inductance I wouldn't >>>> expect to need much snubbing, if any. Plus I can run the transformer >>>> right up to its maximum volt-seconds without worrying. >>>> >>>> Ideally I'd like it to give me +-45 V at about 20 mA each. >>>> >>>> Any words of wisdom? >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>> >>> Do you have a decent Spice model for the ISDN transformer? >>> >>> ...Jim Thompson >>> >> >> It's 2.2 mH CT : 8.8 mH CT. >> >> On the 9 mH side, with the 2 mH side shorted, my trusty Heathkit HD1250 >> dip meter ... > > > Hey, I've got the same one. Since childhood. > > >> ... shows it resonating at 2.1 MHz with 4.9 nF in parallel, which >> is 1.2 uH. ... > > > Careful. You've got tons of winding capacitance in there which can > seriously fool one into believing a much too small number. You have to > measure the leakage inductance at frequency-of-interest, using an > impedance analyzer or do it in a more pedestrian fashion via generator > and meter.I sort of doubt that there are nanofarads of distributed capacitance in that tiny transformer, though, because otherwise its open-circuit resonance would be way too low. It works as a transformer up to well over 1 MHz, so at 9 mH, the winding capacitance has to be down in the lowish picofarads.> > >> ... That makes the coupling coefficient about >> 1-1.2/8800 = 0.99986. >> > > I thought in SPICE it is "k" as in "koupling koefficient". >Ja, kamerad. ;) 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 USA +1 845 480 2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●July 16, 20132013-07-16
On 7/16/2013 3:53 PM, Joerg wrote:> Phil Hobbs wrote: >> So, I'm goofing off playing with small switchers. (Well, not exactly >> goofing off, but there are somewhat more pressing tasks waiting....) >> >> As I said in George's thread, "Opamp w/ Vsupply > 36V", I built a small >> half-bridge supply with positive and negative voltage doublers. I'm not >> that keen on it, because (a) it uses a fair number of parts for what it >> does, and (b) it has a nasty instability. >> > > (a) ... yes. (b) ... why? What happens? Changing to another architecture > while using the same kind of loop usually doesn't do much to improve > stability. > > >> With a capacitively-coupled half bridge, if you let the transformer >> saturate, it instantly discharges the coupling cap, which about doubles >> the volt-seconds on the next half-cycle and guarantee that it will keep >> saturating until the FETs cook themselves. >> > > Not sure what you mean here, but usually current mode control is the way > to avoid asymmetrical runaway. > > >> The particular little ISDN transformers I'm using have really amazingly >> low leakage inductance, so I'm thinking about using a current-mode >> flyback instead. The UCC28C45 bicmos controller chip looks pretty >> suitable--it's about the same price as the IRS2153D, needs one less FET, >> runs up to 1 MHz, and with such a low leakage inductance I wouldn't >> expect to need much snubbing, if any. Plus I can run the transformer >> right up to its maximum volt-seconds without worrying. >> >> Ideally I'd like it to give me +-45 V at about 20 mA each. >> >> Any words of wisdom? >> > > ISDN transformers don't have much air gap, and you need air gap for a > flyback. Plus ISDN is on the way out in many areas so if this has to > remain in production until the cows come home I wouldn't. >What about ADSL line transformers? They'll probably be around for a while. http://www.mouser.com/ds/2/54/Bourns%20sm540-1-190232.pdf Their inductance is somewhat less than the ISDN types, though.
Reply by ●July 16, 20132013-07-16
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 7/16/2013 5:08 PM, John Larkin wrote: >> On Tue, 16 Jul 2013 15:08:24 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> So, I'm goofing off playing with small switchers. (Well, not exactly >>> goofing off, but there are somewhat more pressing tasks waiting....) >>> >>> As I said in George's thread, "Opamp w/ Vsupply > 36V", I built a small >>> half-bridge supply with positive and negative voltage doublers. I'm not >> The ISDNs work to pretty low frequencies, so a high frequency driver >> and a small cap might fix the saturation problem. >> >> I recently saw a chip with an oscillator, flipflop, and two open-drain >> outputs. You could drive a center-tapped primary winding with one of >> those. The ISDN trannies caome in all sorts of ratios, like 1:1:2:2 >> for example. > >The flyback idea was sort of appealing, because it lends itself to >voltage feedback, but as Joerg says, the core won't store enough energy >without a gap, which sort of defeats the purpose.You'd still need a storage inductor if you go for a forward like topology. -- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico@nctdevpuntnl (punt=.) --------------------------------------------------------------
Reply by ●July 16, 20132013-07-16
On Tue, 16 Jul 2013 18:20:51 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 7/16/2013 5:08 PM, John Larkin wrote: >> On Tue, 16 Jul 2013 15:08:24 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> So, I'm goofing off playing with small switchers. (Well, not exactly >>> goofing off, but there are somewhat more pressing tasks waiting....) >>> >>> As I said in George's thread, "Opamp w/ Vsupply > 36V", I built a small >>> half-bridge supply with positive and negative voltage doublers. I'm not >>> that keen on it, because (a) it uses a fair number of parts for what it >>> does, and (b) it has a nasty instability. >>> >>> With a capacitively-coupled half bridge, if you let the transformer >>> saturate, it instantly discharges the coupling cap, which about doubles >>> the volt-seconds on the next half-cycle and guarantee that it will keep >>> saturating until the FETs cook themselves. >>> >>> The particular little ISDN transformers I'm using have really amazingly >>> low leakage inductance, so I'm thinking about using a current-mode >>> flyback instead. The UCC28C45 bicmos controller chip looks pretty >>> suitable--it's about the same price as the IRS2153D, needs one less FET, >>> runs up to 1 MHz, and with such a low leakage inductance I wouldn't >>> expect to need much snubbing, if any. Plus I can run the transformer >>> right up to its maximum volt-seconds without worrying. >>> >>> Ideally I'd like it to give me +-45 V at about 20 mA each. >>> >>> Any words of wisdom? >>> >>> Cheers >>> >>> Phil Hobbs >> >> The ISDNs work to pretty low frequencies, so a high frequency driver >> and a small cap might fix the saturation problem. >> >> I recently saw a chip with an oscillator, flipflop, and two open-drain >> outputs. You could drive a center-tapped primary winding with one of >> those. The ISDN trannies caome in all sorts of ratios, like 1:1:2:2 >> for example. > >The flyback idea was sort of appealing, because it lends itself to >voltage feedback, but as Joerg says, the core won't store enough energy >without a gap, which sort of defeats the purpose. > >This is for another laser noise canceller design, which needs a lot of >reverse bias and ought to work out to at least 5 mA per photodiode. >There's a current mirror on each one, so that's 10 mA on the signal >channel (+40V) and as much as 20 mA on the comparison channel (-40V). >(I may need to water-cool the photodiodes.) ;) > >It looks like some Royer-style thing might work better, since that >automatically fixes the saturation problem. I don't have a spare >winding, but I could probably use a voltage divider off the secondary. > >Cheers > >Phil HobbsTake a look at SN6501 maybe. -- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
Reply by ●July 16, 20132013-07-16
bitrex <bitrex@de.lete.earthlink.net> wrote:>On 7/16/2013 3:53 PM, Joerg wrote: >> Phil Hobbs wrote: >>> So, I'm goofing off playing with small switchers. (Well, not exactly >>> goofing off, but there are somewhat more pressing tasks waiting....) >>> >>> As I said in George's thread, "Opamp w/ Vsupply > 36V", I built a small >>> half-bridge supply with positive and negative voltage doublers. I'm not >>> that keen on it, because (a) it uses a fair number of parts for what it >>> does, and (b) it has a nasty instability. >>> >> >> (a) ... yes. (b) ... why? What happens? Changing to another architecture >> while using the same kind of loop usually doesn't do much to improve >> stability. >> >> >>> >>> Ideally I'd like it to give me +-45 V at about 20 mA each. >>> >>> Any words of wisdom? >>> >> >> ISDN transformers don't have much air gap, and you need air gap for a >> flyback. Plus ISDN is on the way out in many areas so if this has to >> remain in production until the cows come home I wouldn't. >> > >What about ADSL line transformers? They'll probably be around for a >while.With fiber around the corner? Shudder... I'd rather use the flexible transformers which are sold by Coilcraft and Wurth. -- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico@nctdevpuntnl (punt=.) --------------------------------------------------------------
Reply by ●July 16, 20132013-07-16
On 7/16/2013 7:07 PM, bitrex wrote:> On 7/16/2013 3:53 PM, Joerg wrote: >> Phil Hobbs wrote: >>> So, I'm goofing off playing with small switchers. (Well, not exactly >>> goofing off, but there are somewhat more pressing tasks waiting....) >>> >>> As I said in George's thread, "Opamp w/ Vsupply > 36V", I built a small >>> half-bridge supply with positive and negative voltage doublers. I'm not >>> that keen on it, because (a) it uses a fair number of parts for what it >>> does, and (b) it has a nasty instability. >>> >> >> (a) ... yes. (b) ... why? What happens? Changing to another architecture >> while using the same kind of loop usually doesn't do much to improve >> stability. >> >> >>> With a capacitively-coupled half bridge, if you let the transformer >>> saturate, it instantly discharges the coupling cap, which about doubles >>> the volt-seconds on the next half-cycle and guarantee that it will keep >>> saturating until the FETs cook themselves. >>> >> >> Not sure what you mean here, but usually current mode control is the way >> to avoid asymmetrical runaway. >> >> >>> The particular little ISDN transformers I'm using have really amazingly >>> low leakage inductance, so I'm thinking about using a current-mode >>> flyback instead. The UCC28C45 bicmos controller chip looks pretty >>> suitable--it's about the same price as the IRS2153D, needs one less FET, >>> runs up to 1 MHz, and with such a low leakage inductance I wouldn't >>> expect to need much snubbing, if any. Plus I can run the transformer >>> right up to its maximum volt-seconds without worrying. >>> >>> Ideally I'd like it to give me +-45 V at about 20 mA each. >>> >>> Any words of wisdom? >>> >> >> ISDN transformers don't have much air gap, and you need air gap for a >> flyback. Plus ISDN is on the way out in many areas so if this has to >> remain in production until the cows come home I wouldn't. >> > > What about ADSL line transformers? They'll probably be around for a > while. http://www.mouser.com/ds/2/54/Bourns%20sm540-1-190232.pdf > > Their inductance is somewhat less than the ISDN types, though. >Interesting, thanks. Their coupling coefficient looks quite a bit lower--maybe they have a gapped core? 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 USA +1 845 480 2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●July 16, 20132013-07-16
On Tue, 16 Jul 2013 18:12:17 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 7/16/2013 3:53 PM, Joerg wrote: >> Phil Hobbs wrote: >>> So, I'm goofing off playing with small switchers. (Well, not exactly >>> goofing off, but there are somewhat more pressing tasks waiting....) >>> >>> As I said in George's thread, "Opamp w/ Vsupply > 36V", I built a small >>> half-bridge supply with positive and negative voltage doublers. I'm not >>> that keen on it, because (a) it uses a fair number of parts for what it >>> does, and (b) it has a nasty instability. >>> >> >> (a) ... yes. (b) ... why? What happens? Changing to another architecture >> while using the same kind of loop usually doesn't do much to improve >> stability. >> >> >>> With a capacitively-coupled half bridge, if you let the transformer >>> saturate, it instantly discharges the coupling cap, which about doubles >>> the volt-seconds on the next half-cycle and guarantee that it will keep >>> saturating until the FETs cook themselves. >>> >> >> Not sure what you mean here, but usually current mode control is the way >> to avoid asymmetrical runaway. > >The IRS2153D puts a square wave out of a half-bridge--no feedback, no >current limit, nada. If you put that into a transformer via a cap, all >is well until you saturate the transformer. > >Say that happens on the positive half-cycle. >At that moment, the voltage on the cap rapidly goes from V_DD/2 to V_DD. > At the next edge, the voltage across the transformer is suddenly not >V_DD/2 as expected, but V_DD. The transformer saturates in half the >time it took previously, and the voltage on the cap goes from V_DD to 0. > Then the cycle repeats. It's really obvious on a scope when this >happens, and it's far from pretty. > >On each half cycle, roughly 0.5 C_coup*V_DD**2 gets dissipated in one of >the FETs, which gradually cook themselves.Why doesn't that happen in 60 Hz line transformers? If they get shut off at the wrong time, the core is left partly magnetized. At next turnon, there can be a huge 1/2 cycle surge, enough to make the wires in the wall jump sometimes. But it quickly self-corrects. -- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
Reply by ●July 16, 20132013-07-16
On 7/16/2013 7:41 PM, John Larkin wrote:> On Tue, 16 Jul 2013 18:12:17 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 7/16/2013 3:53 PM, Joerg wrote: >>> Phil Hobbs wrote: >>>> So, I'm goofing off playing with small switchers. (Well, not exactly >>>> goofing off, but there are somewhat more pressing tasks waiting....) >>>> >>>> As I said in George's thread, "Opamp w/ Vsupply > 36V", I built a small >>>> half-bridge supply with positive and negative voltage doublers. I'm not >>>> that keen on it, because (a) it uses a fair number of parts for what it >>>> does, and (b) it has a nasty instability. >>>> >>> >>> (a) ... yes. (b) ... why? What happens? Changing to another architecture >>> while using the same kind of loop usually doesn't do much to improve >>> stability. >>> >>> >>>> With a capacitively-coupled half bridge, if you let the transformer >>>> saturate, it instantly discharges the coupling cap, which about doubles >>>> the volt-seconds on the next half-cycle and guarantee that it will keep >>>> saturating until the FETs cook themselves. >>>> >>> >>> Not sure what you mean here, but usually current mode control is the way >>> to avoid asymmetrical runaway. >> >> The IRS2153D puts a square wave out of a half-bridge--no feedback, no >> current limit, nada. If you put that into a transformer via a cap, all >> is well until you saturate the transformer. >> >> Say that happens on the positive half-cycle. >> At that moment, the voltage on the cap rapidly goes from V_DD/2 to V_DD. >> At the next edge, the voltage across the transformer is suddenly not >> V_DD/2 as expected, but V_DD. The transformer saturates in half the >> time it took previously, and the voltage on the cap goes from V_DD to 0. >> Then the cycle repeats. It's really obvious on a scope when this >> happens, and it's far from pretty. >> >> On each half cycle, roughly 0.5 C_coup*V_DD**2 gets dissipated in one of >> the FETs, which gradually cook themselves. > > Why doesn't that happen in 60 Hz line transformers? If they get shut > off at the wrong time, the core is left partly magnetized. At next > turnon, there can be a huge 1/2 cycle surge, enough to make the wires > in the wall jump sometimes. But it quickly self-corrects. >The 60-Hz line is DC coupled to ground, so there's no capacitor to remember the misbehaviour between half-cycles. It's the gift that keeps on giving. ;) 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 USA +1 845 480 2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●July 16, 20132013-07-16
On Wednesday, 17 July 2013 05:08:24 UTC+10, Phil Hobbs wrote:> So, I'm goofing off playing with small switchers. (Well, not exactly > goofing off, but there are somewhat more pressing tasks waiting....) > > As I said in George's thread, "Opamp w/ Vsupply > 36V", I built a small > half-bridge supply with positive and negative voltage doublers. I'm not > that keen on it, because (a) it uses a fair number of parts for what it > does, and (b) it has a nasty instability. > > With a capacitively-coupled half bridge, if you let the transformer > saturate, it instantly discharges the coupling cap, which about doubles > the volt-seconds on the next half-cycle and guarantee that it will keep > saturating until the FETs cook themselves. > > The particular little ISDN transformers I'm using have really amazingly > low leakage inductance, so I'm thinking about using a current-mode > flyback instead. The UCC28C45 bicmos controller chip looks pretty > suitable--it's about the same price as the IRS2153D, needs one less FET, > runs up to 1 MHz, and with such a low leakage inductance I wouldn't > expect to need much snubbing, if any. Plus I can run the transformer > right up to its maximum volt-seconds without worrying. > > Ideally I'd like it to give me +-45 V at about 20 mA each. > > Any words of wisdom?http://home.planet.nl/~sloma000/0344_001_Baxandal.pdf It gives you an extra pi/2 (1.571) volts, at the price of a second inductor. You may know the circuit from Jim Williams Linear technology application notes AN45, AN49, AN51, AN55, AN61, and AN65. If you use MOSFETs to do the switching, you are reputed to avoid the evil "squegging" referred to in a footnote in the original paper (at the bottom of page 752). -- Bill Sloman, Sydney
