Why transformers Secondary magnetizing inductnace and Secndary leakage inductace are Not Considered while doing filter calculation for output ? Why most transformer datasheet only mention primary leakage inductance and primary magnetizing inductance ? Kindly elaborate the Role of Secondary leakage and magnetizing inductance in actual design .enter image description here


When you magnetize a core you do it through the driven winding hence it makes sense to only mention the magnetization inductance relative to the primary because that is assumed to be the driven winding i.e. the winding that magnetizes the core.

You can easily swap this around. If you know the turns ratio then the secondary magnetization inductance (if you wanted to drive the secondary rather than the primary) is \$\dfrac{L_{mp}}{(n_1/n_2)^2}\$.

But there is only one magnetization inductance i.e. don't go believing you have to account for it on both windings i.e. it's on one not both.

When it comes to secondary leakage inductance being unspecified you can estimate it by using the same formula as above. If \$L_{l1}\$ is 1 mH and the turns ratio is 10:1 then \$L_{l2}\$ would be about one-hundredth lower i.e. approximately 10 uH.

Be careful though; what some data sheets describe is the primary referred leakage inductance and this figure combines the actual primary leakage inductance AND the referred-to-primary secondary leakage inductance i.e. it is a composite figure that takes into account both leakages. You can of course split this back to individual figures if you know the turns ratio BUT, if you are making a filter it won't matter.

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  • \$\begingroup\$ Okay i understood the concept of Magnetizing inductance. The referred to primary secondary leakage inductance and primary leakage inductance are added mathematically and presented on datasheet ? leakage inductance mentioned in datasheets are with secondary winding of transformer shorted ?@Andy aka \$\endgroup\$ – Rohan Feb 1 '18 at 11:48
  • \$\begingroup\$ What data sheet? Measurement methods for determining leakage inductance do tend to short the secondary so that effects of the primary magnetization inductance are factored out of results. \$\endgroup\$ – Andy aka Feb 1 '18 at 12:24
  • \$\begingroup\$ okay. how come primary magnetization inductance gets factored out when secondary shorted ? @Andy aka \$\endgroup\$ – Rohan Feb 1 '18 at 12:41
  • \$\begingroup\$ @Rohan shorting the secondary terminals puts the secondary leakage inductance as the load. It might be 10 uH and when projected to the primary it might be 1mH and in parallel with the massively greater mag inductance hence, the dominant impedance is the 1 mH so you can basically ignore the 10 henries (or so of mag inductance) because it is in parallel with 1 mH. \$\endgroup\$ – Andy aka Feb 1 '18 at 12:49

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