Flyback Converter Transformer design

hi

Is it possible to design fly back converter using toroid without air gap?

Yup!

Although I've found using ferrite cores at low power levels such that the cores don't saturate works OK, better performance is easily obtained using smaller cores + small gaps (a piece of paper slipped between the core halves gives a cheap repeatable gap of a few tenths mm). Iron powder cores also work OK due to their higher saturation flux densities but their higher losses and lower permeabilities mean that windings are bulkier and (core + copper) losses are higher.

Despite these predictions of doom though, if you only need miniscule power levels and you have ungapped cores to hand, then yes, it will work

Thanks for reply.

Will it work at 10 watt output?
How core saturation can be measured / detected?

With no air gap magnetics are sensitive to temperature rise and nonlinear losses. Using air gap makes them lower inductance but more linear. Success depends on worst case load B density / B rated in Guass or pref. Tesla

> Will it work at 10 watt output?

Depends entirely upon the core volume (and operating frequency). For a core the size of a peppercorn, no it won't. For a core the size of a grapefruit, absolutely! You'll have to dig up the relevant design equations - I can't recall them offhand (although there are some among the forum regulars who can

> How core saturation can be measured / detected?

The easiest way is to monitor the switch (transistor) current (usually via the voltage developed across a small source/emitter resistance). When the core saturates, it's inductance rapidly starts to drop. This shows up as a sudden departure from the nice linear current ramp observed (given by di/dt = V/L) when the switch starts to conduct. The point when the straight-line ramp bends into an exponentially increasing current is the point (=current) at which saturation commences. Small levels of saturation are OK (although efficiency suffers) but larger levels = hot inductors, dead transistors and lots of capacitor ripple current!

Sure, if you use a powdered iron core. Basically there is no physical gap, but the sparsity of particles means that there is a distributed air gap, so they work find for energy storage. You just need to choose the core mix and size appropriately.

Of course ferrite distributed magnetic particles represents an effective air gap, but the quality of such ferrite varies greatly with type of ferite slurry and quality of supplier process. In effect the higher mu concentrations have more variability and minute flaws in mu can cause localized saturation which reduces inductance and when temperature rises can lead o a thermal runaway, which is why Flybacks are often limited to 100W unless additional larger air gapped cores are used to linearize the response. For larger powers , asymmetric forward half bridge converters are far superior.

Taking up mtwiegs comment about power cores that implement a distributed air gap, you can decide about an µr value for your flyback transformer and than check if you find a torroid core made from low µr (RF) ferrite that fits the range. Most likely you won't find any. This answers your question.

Powder cores in contrast are made with different µr values (different mixture of iron powder and resin) so you can select the best core for your application. Or refer to a classical gapped core, or make a forward converter.

SunnySkyguy said:

Of course ferrite distributed magnetic particles represents an effective air gap, but the quality of such ferrite varies greatly with type of ferite slurry and quality of supplier process. In effect the higher mu concentrations have more variability and minute flaws in mu can cause localized saturation which reduces inductance and when temperature rises can lead o a thermal runaway, which is why Flybacks are often limited to 100W unless additional larger air gapped cores are used to linearize the response. For larger powers , asymmetric forward half bridge converters are far superior.

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Agreed, there are a lot of catches to powdered iron cores, in my own experience I've always found gapped ferrite to have better performance for power conversion. But if, for some reason, you absolutely need to implement energy storage with a toroid, then it's a better option than ferrite, when properly selected.