Flyback vs half-bridge

So what's wrong with the measured values? I can do it over with e.g.

3300 and 5000 pF, and compare the two estimated inductances. That should be fairly diagnostic, unless I'm missing something ultra-important.

That sort of coupling coefficient is similar to large mains transformers, for instance.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Ok, the IRS2153D is just a glorified gate driver with an oscillator in there. I'd use a real switcher controller chip, some are in the same price category.

Did you use a really big cap? I've never had that happen. Usually ferrite saturates softly enough to just "nudge" the cap.

More than once have I had that, SMT parts slowly sliding down the board because the solder melted :-)

That usually rules out flybacks. They spew a lot of noise. Then I'd use a half-bridge with CM control and series inductors on the output.

--
Regards, Joerg 

http://www.analogconsultants.com/

Bifalar 1:1 transformers are often spec'd 0.998.

We measured a 1:1:2:2 ISDN at 0.99997!

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John Larkin         Highland Technology, Inc 

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In my IRS2153D thing, I used big gate resistors and gave up some efficiency. It's a couple inches away from a 12-bit, 250 MHz ADC.

--

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

1 uF. I can probably post a scope photo tomorrow, if I have time--a bunch of lawyering has just descended on my head. (Not that I mind very much--expert witness work uses a different part of my brain and generally pays somewhat better than design work.)

Fortunately the half-bridge's worst-case condition is zero load, so it isn't too hard to control. Of course the ungapped ferrite has a pretty frightening tempco of mu (it goes down by half when you hit it with cold spray), so some sort of current control is going to be required, for sure.

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 
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You need to be careful about that. Even if you have primary current mode peak limit, then inequalities in the secondary diodes forward drops can let the xformer go into staircasing, saturating the xformer.

Cheers

Klaus

Ya know, it seems that are proving to be a really useful SMPS transformer. A lot of small to moderate volume (different use) applications may keep them in production for a long time.

??-)

On a sunny day (Tue, 16 Jul 2013 15:08:24 -0400) it happened Phil Hobbs wrote in :

I have used simple sine wave oscillator and small E core (or potcore if you can get those), you know 1 transistor, feedback turn to base or emitter, tuning cap. Takes 5 minutes to wind it (1 turn per volt works usually). couple of 1N4148 diodes, some elcos. The advantage is that it does not radiate like a switcher, if that means anything to you. At 20 mA who cares about 99 % efficiency jive.

Sorry for the diverting question, but help me out here. (I find magnet material a bit magical.) So I thought the energy in an inductor was 1/2*L*i^2.

But your and Joerg's comments seem to imply the energy is mostly in the gap. Now my very limited understanding of making a gap in an inductor was that the gap sorta sets the inductance... the actual value isn't so dependent on the magnetic material. And doesn't the gap reduce the inductance?

I seem to be missing something fundamental.

Thanks, George H.

Ok, but I'd still measure it with the secondary sorted and then from a generator, see what remains. But for a bridge it won't matter, things won't get much better than with such transformers. For a flyback it isn't useful since it isn't air-gapped.

--
Regards, Joerg 

http://www.analogconsultants.com/

That's one of the more serious problems we as a country have. Legal skirmishes usually have zero productivity from a national perspective but result in the fattest "payouts". It is the core problem why our health care system is a mess. Of course, that is never brought up by body politicus, for obvious reasons :-(

They work ok with zero load if you can tolerate the voltage increase. That would be determined by the turns ratio. If it has to be 45.000V that's a different story. Then you'll have to let it go into a skipping mode.

Yes, needs current control but that's standard even in sub-Dollar chips.

--
Regards, Joerg 

http://www.analogconsultants.com/

[...]

If the current limit works properly it'll curb in time, it normally won't let it staircase into a danger zone. Of course, all this assumes regular ferrite where the core will not saturate super-hard.

I am just wondering whether all that isn't overkill for generating two

45V rails at 20mA. That's just a couple of watts. A decent gate driver should almost be able to capacitively drive that without any FETs.

--
Regards, Joerg 

http://www.analogconsultants.com/

Yes, it is. But the gap is just called air gap, usually it's some sort of filler material. High-end plastics, phenolic or FR4, and in very cheap "solutions" ... post-consumer cardboard.

One of the issues with flybacks and gapped cores is that the gap doesn't know where it is supposed to end at its sides. It extends into the surroundings and that can result in egg in the face at the EMC lab.

It does, unfortunately. As in "no free lunch" :-)

Maybe something like this helps:

[...]

--
Regards, Joerg 

http://www.analogconsultants.com/

In this case I think the xformer RDC and MOSFET RDC would be so high that a solution with just a forward converter capacitive coupled primary would be fine.

For the flyback it needs to gapped. Non-gapped flybacks is asking for trouble

Regards

Klaus

You could say much the same thing about the army, but once the shooting starts, you're glad they're there. The case I'm working on today concerns an alleged misappropriation of trade secrets by a big semiconductor company. There's a lot of money at stake, so I'm pretty much in the noise. Designing stuff for start-ups is a bit different.

I may just make a boost with a two-winding toroid, and return the other end of the second winding to the raw -15V supply. (A lot of the Murata toroids are actually dual-winding, but they expect you to wire them in parallel to get the current handling capacity.)

There are some PoE things that look like they could usefully be run backwards, but the coupling coefficients are low enough that they must have gaps.

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

Yes, the energy is mostly in the gap, and yes, the gap reduces the inductance. If you have a gap amounting to 1% of the average magnetic path length, you have an effective mu of 100. The perpendicular component of B is continuous across the air/core interface, but perpendicular H jumps up by a factor of the mu of the core, i.e. about

2000 or so.

The magnetic energy density is proportional to B*H, so the energy stored in the gap is 1% x 2000 = 20 times the energy stored in the 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 

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

Hi George,

The magnetic energy in a given volume is E=1/2 VB^2 / (u_0 u_r), so for a given B, the energy in a given volume of ferrite is u_r times less than in the same volume of air.

For a ferrite magnetic circuit with an air gap, the total magnetic energy works out as E=1/2 B^2 (A l_f/(u_0 u_r) + A l_a/u0), where A is the cross section area of the magnetic circuit, and l_f and l_a are the path length in ferrite and air, respectively. (I neglect fringing. That's good enough, usually.)

You can work this out from just two of Maxwell's four laws.

Cheers, Jeroen Belleman

Wow, great! (lunch time reading material.) Thanks Joerg. I think I see my problem. Without the gap the B field in the material is m uch higher (for a given current) and it's that much closer to saturation. ( Maybe a lot closer!) So I guess I can believe that for a gapped inductor y ou can get more energy storage... you just have to run it at a higher curre nt. (I'll have to try putting in some numbers.)

Say are there any good books about transformers/ inductors/ magnetic materi als? The subject seems to go from the trivial freshman physics transformer. To the "full hair ball" real world situation, with nothing in between.

George H.

Thanks Phil, (I'll have to go work it out for myself now.) I just remembered the one place that I finally 'got' air gap electro-magnetics was in the second volume of the Feynman lectures.

George H.