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# Magnetizing inductance

#### abhishek.2138

##### Full Member level 2
In high frequency switching power supply transformer, does magnetizing inductance is always related to inductance of primary winding?
If yes, then what is the significance of secondary winding inductance?

The core magnetising flux is equally associated with pri and sec coils.
Think of a current sense transformer where you have to think specifically of the sec magnetising current....as if not careful it will result in your signal going down instead of up.

But in a full bridge smps say, you usually think of the magnetising current when the primary fet is on......and see if it saturates the core.......

You are seeing the main or magnetizing inductance from primary and secondary side, scaled by factor L1/L2 = (n1/n2)^2.

cupoftea

### cupoftea

Points: 2
Yes, what you ask is good question, and because everybody always says the magnetising inductance of a transformer is "L(primary) with L(secondary) open circuit".

...That is not strictly true.......the transformer does not "know" which of its coils is primary and which is secondary....it knows only the flux in its core......and it has no idea whether that flux came from current in the primary or secondary current flow.

To make it worse, the concept of flux, inductance etc are mere "parameters" invented by humans in their efforts to control physical things.......when you drill down into the true meaning, you actually find that the entire physical model that we have built up, is without any sense at all. (see things like "string theory", etc etc)...........but who cares....if you can use your "model" to design something that works as you want....then do you care that the method you used to get there was inherently incorrect. I am sure you already know this.

In a dual active bridge transformer ( 1:1 )driven the same from each side - each side contributes half the magnetising current ... or thereabouts