High leakage inductance
I have received two sample flyback transformers from your sample service, for use with an LNK606. The sample report gives a primary inductance of 1.23mH (exactly in line with PIXI Designer prediction) but the leakage inductances are 70uH - 17% of the primary inductance. This seems very high.
Is this my fault, in designing the transformer or the winding sequence? Or a manufacturing flaw? What can be done to rectify this? What are typical values of leakage inductance?
I attach the PIXI Designer file.
Comments
Further reading suggests improvements on leakage inductance by (a) winding the secondary adjacent to the primary and (b) interleaving the primary and secondary windings. What does Power Integrations think about these ideas? (Your reference designs do not do this).
How are you measureing the leakage inductance? It's possible you might have to experiment with a few different transformer winding layouts. Magnetics design can involve a certain amount of trial and error. Doing a split primary would likely give you better coupling to the secondary and lower your leakage inductance.
-The Traveler
Thanks "The Traveller". I have not measured the inductance myself. The values were on the printed report that came with the sample transformers from Power Integration's "rapid transformer sample service". What I am keen to know is: is this value normal? Is it the fault of my design, or the transformer manufacturing process? What practical changes can I make that would reduce the leakage inductance?
The primary is currently in three layers. Should I try for a design with two layers of primary? Could I use a split primary if I had 3 layers of primary anyway? Why do the Power Integration designs show the primary and the secondary separated by the bias and feedback windings when other references say the primary and secondary should be adjacent to give the lowest leakeage?
And how do I get a reply from PI, if there is no response in this forum?
With 3 layers of primary winding and shield windings you automatically separate the secondary winding and place it further away from the primary - This results in higher leakage....
With 2 layers you may be able to get the leakage down a little but I doubt that this is the main reason. The main reason is the placement of Bias and feedback windings between the primary and secondary. The reason this is done is so that you get better shielding and reduced EMI.
You can eliminate the bias winding and reduce leaklage at the expense of no load consumption
In my opinion, 70 uH is not that bad ... try it and ensure that your clamp is not too hot.
In general, with transformer construction, when you optimize the winding construction for very high coupling you will also get higher inter/intra winding capacitances that can cause problems with EMI. Arranging your windings to reduce the pri/sec capacitance will typically result in higher leakage inductance values and vice versa.
Overall, leakage inductance is easier to deal with from a practical standpoint as you can design your external clamp and snubber components to accomodate the leakage energy. It's harder to mitigate the EMI problems associated with high pri/sec parasitic capacitances.
-The Traveler
Correction: leakage inductance is 6% of primary inductance, not 17%. But still high.