Understanding transformer datasheets

Question

I came across this transformer, and there are a few points of the datasheet that seem unclear to me. I can't seem to find any references for what exactly they refer to, so I'll ask here:

• Where the datasheet says Inductance @ Irated, does that mean the primary inductance, or the mutual inductance, or something else entirely?
• Likewise, I assume Irated is referring to the primary-side rated current, not the secondary side?
• When it gives a value for Leakage Inductance, should that be assumed to be the leakage as seen on the primary side?

And finally: This is a badly-written datasheet, right? I've seen other datasheets that actually define exactly how each parameter is measured, and that seems like it would answer all of my questions if this one had that.

Answer 1

This transformer is made for LT3420, which is a flyback controller so this gives you some input to how you might specify the transformer, but I agree it should state verbatim in the datasheet.

Designing with flyback means your primary inductance (magnitizing) is of primary importance since it sets your switching parameters. Second would be lekage since this sets how much energy you need to handle in your snubber and peak voltage stress too.

• Inductance @ Irated would be the primary magnitizing inductance.
• Irated is more problematic, but with 18 ohm on the secondary, I would not like to run 1.5 A though it. Assume primary.
• Leakage Inductance. When I model it for simulation, I usually place it on the primary for flyback and secondary otherwise, but that's just me. I would assume primary since 0.75/18 (leakage/magnitizing) gives a resonable ratio (4.2%) for what you end up with for a high turns ratio transformer of this size.

When ever you are in doubt, pleask ask the manufaturer. Asking them to update the datasheet would help others too.

Answer 2

As it's intended to be used as a flyback transformer, you can certainly assume that the inductance given is the primary inductance, and at the rated primary current.

We might guess that everything else is going to be primary referred as well. 0.75uH is a reasonable and believable fraction of 18uH. A primary referral makes sense, as it represents the primary spike that will not be clamped by the secondary load, and so needs a high voltage primary transistor, or other protection.