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Leakage Inductance and primary inductance for Flyback converter

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ivan_mateo

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Hi everyone;

I wound transformer as shown below. Np=52 Turn, Nbias=16 Turn, Nsec=10 Turn. And when measured primary inductance is 530uH ( I made it as scratch middle of ferrit N87 and bobbins ETD29) but when I short winding expect Primer, the leakage inductane 291uH. I think it seems too high ? Can anybody help to me to decrease it?
 

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The figure is too high. What are you measuring it with?
 

From the photo, your core is not properly sized. But that wouldn't cause such a high leakage inductance. You may have to tell us how you wound the coils.
 

When you say you short winding, please specify the windings you shorted. Did you short the other two windings?

Normally, you short each of the other windings with a thick wire and then you measure the inductance across the winding of interest which is open (ie without a short on it). The measurement you get is the leakage inductance of that winding.

In your case, if you are to measure the leakage inductance across the primary, you have to short the secondary with a thick wire and the auxiliary too with a thick wire and measure the inductance across the primary.

If you are to measure the leakage inductance across the secondary, you have to short the primary with a thick wire and the auxiliary with a thick wire and the measure the inductance across the secondary.

And so on...
 
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What's the core air gap? What's the expected primary inductance according to core dimensions?
 

It seems to me like one of the windings (probably the auxiliary) was not properly shorted during the measurement. You should make the measurement again.

I am also emphasizing on using thick short wire for the short.

Then you may need to tell us how you wound the transformer. The winding process has significant impact on the amount of leakage inductance.
 

I don't believe that thin or thick shorting wire makes a big difference.
There's something seriously wrong, either with the measurement or the transformer geometry.
 

I don't believe that thin or thick shorting wire makes a big difference.
There's something seriously wrong, either with the measurement or the transformer geometry.

Yes. I have tried to change thick wire.But of course is not related. I think I did not pay atttention to winding direction. If I wound primary clock wise direction and secondary other direction, it can cause something?
 

What I do is that I start at the dot end of each winding and I make all windings in the same direction.

In essence, it turns out that the directions are reversed for opposite polarity dots but it makes winding a lot easier.
 

I get the impression that the other windings are not being shorted properly - a sketch of the actual construction is needed to get to the root cause of the problems - or he is measuring 530uH and 2.91 uH ...!
 

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