Forums

Flyback vs half-bridge

Started by Phil Hobbs July 16, 2013
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

-- 
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
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 -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
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. John Larkin has recently used these tiny flyback transformers. Nothing wrong with hanging two in parallel on the primary and using the 2nd one with the secondary flipped around, for the -45V. Do you need this isolated? Else you could consider just inverting and boost. -- Regards, Joerg http://www.analogconsultants.com/
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 shows it resonating at 2.1 MHz with 4.9 nF in parallel, which is 1.2 uH. That makes the coupling coefficient about 1-1.2/8800 = 0.99986. 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
On Tue, 16 Jul 2013 16:24:15 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> 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 shows it resonating at 2.1 MHz with 4.9 nF in parallel, which >is 1.2 uH. That makes the coupling coefficient about >1-1.2/8800 = 0.99986. > >Cheers > >Phil Hobbs
Close enough for government work >:-} ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
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. -- 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
On Tuesday, July 16, 2013 9:08:24 PM UTC+2, 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? >
if it is CT push-pull? ir2153 would work for that too, just skip the bootstrap auir2085 is simlar to ir2153, max 100 volt but faster, programmable dead time, more drive and has over current protection http://www.irf.com/product-info/datasheets/data/auir2085s.pdf does it need to be isolated? -Lasse
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.
> ... That makes the coupling coefficient about > 1-1.2/8800 = 0.99986. >
I thought in SPICE it is "k" as in "koupling koefficient". -- Regards, Joerg http://www.analogconsultants.com/
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.
>> 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.
So I'm discovering. The actual amount of energy I can store in that toroid isn't very large, so to get any power out of it I have to run it pretty fast. With 2.2 mH of primary inductance, that takes quite a bit of voltage, which makes the whole flyback thing sort of moot.
> > John Larkin has recently used these tiny flyback transformers. Nothing > wrong with hanging two in parallel on the primary and using the 2nd one > with the secondary flipped around, for the -45V. > > Do you need this isolated? Else you could consider just inverting and boost. >
Doesn't have to be isolated, but it does need to be quiet, hence the toroids. 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 USA +1 845 480 2058 hobbs at electrooptical dot net http://electrooptical.net
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 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