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Confusion in transformer specifications/application

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ConKbot of Doom

Wed Dec 19 2012, 12:38PM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329

Ive been working on a project involving a bidirectional forward converter array off and on for a while (paralleled forward converters, secondary side isnt recitified/smoothed, just bussed together, and the drive mosefets provide synchronous recitification on the converters that are sourcing current instead of sinking). Ive been trying to find suitable transformers, preferably pre-made and cheap. IVe purchased some cores from amidon and made my own for a proof of concept, but I need very good consistency between them, and winding toridals on your own isnt that much fun either.

As a forward converter, obviously you want transformer thats more "ideal", good coupling, low leakage inductance etc... gate drive transformers seem to fit the task. and Ive got a development board using

laid out and ready to try out when I get the time. However the above transformer is a bit large, and this is a battery powered application, not an offline application, so the ~3kv of isolation isnt strictly needed, however the ~1.4mm that typical SMT GDTs provide isnt as much as I'd like. (could work if it had to, conformal coating/epoxy etc... )

caught my eye, as the size is good, as is the spacing between terminals. The isolation is adequate. However its labeled and obviously intended for usage as a flyback device.

It was my (possibly wrong, hence asking for clarificaiton here) understanding that compared to say the GDT used above, flyback transformers had looser coupling, (I.e. airgap in some applications, different ferrite material in others) increasing leakage inductance, and providing a mechanism to store energy. However, flyback transformer datasheet for the PA0367 lists the leakage at 0.75 uH, and the P0584 GDT is specced at 0.5 uH, not a lot of difference, and not very much leakage. The flyback datasheet doesnt give a volt-second product, but I can experimentally measure it, and design accordingly.

But I was further confused, searching online finding various papers speaking of minimizing leakage inductance in flyback transformers, when I thought it was their operating mechanism. IS the energy not stored in the leakage inductance? Is there some other mechanism storing the energy that I'm missing out on?

The winding ratio of the flyback would be advantageous in my application, and a 2-switch forward converter topology would deal with the single primary, and also return the energy stored in the leakage inductance to the input capacitor rather than a snubber, also advantageous. But is there some metric that I'm missing other than the manufacturer saying "yeah this is for a flyback converter" ?

Thanks!

Dr. Slack

Wed Dec 19 2012, 01:51PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

The leakage inductance question is a by-product.

The difference comes in the primary inductance, and the current for saturation.

In a transformer for forward use, the primary inductance will be as high as possible (infinite if we could). As the primary current is accompanied by a cancelling secondary current, the saturation current can be correspondingly low. Energy storage, due to the low saturation, will be low, ideally we'd like it to be zero.

In a flyback, there is no cancelling secondary current, so we need energy storage to be high, which requires a high saturation current. This tends to result in a low primary inductance.

Leakage inductance is a result of the relative primary / secondary geometry, and will be more influenced by inter-winding electrical isolation and convenience of manufacture than forward or flyback intent.

As a result, a forward transformer used in a flyback application will have very low specific power, due to its low energy storage. A flyback transformer used in a forward application will not be so handicapped, but will suffer from excessive magnetising current due to the low inductance, so will have higher losses.

ConKbot of Doom

Wed Dec 19 2012, 05:56PM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329

Thanks! that made it all make a lot more sense, and I should be able to figure out what I need to use from raw specs a bit better. I found a series of murata pulse transformers that look like they should be suitable. I'll look more indepth when I get home. I'm assuming that if I'm well below the self resonant frequency of the primary inductance and the parasitic capacitance, I shouldnt have excessive core losses as long as I'm using an actual transformer, and not a choke/emi suppresson device? The manufacturers seem a bit vague on specifying the material or suitable range of frequencies at times.

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