How does leakage inductance in a flyback transformer affect the efficiency of the converter? Is there any easy way to calculate it? What are the techniques that can be used to minimize it?
The leakage inductance in a Flyback transformer negatively affects the efficiency of the converter itself. Leakage inductance stores energy in the core window area but is not transferred to the secondary. This energy is primarily dissipated either in the RCD clamp network or in a turn-off snubber as heat. Typically the leakage inductance is about 3-5% of the magnetizing inductance. The leakage inductance can be reduced by interleaving the secondary winding between two halves of the primary winding. It can also be reduced by minimizing the thickness of the insulation between the primary and secondary turns and spreading the winding on a longer center leg of the core.
You can get a crude idea of how much leakage you have if you posses an LCR meter. Measure the primary inductance of the transformer, then short the secondary. If the transformer was perfect the measured inductance would fall to zero. It doesn’t, it will fall to some percentage of the original inductance. and you can call that the leakage inductance…
Leakage is due to poor coupling between the flux generated in the core by the primary winding and received by the secondary winding. The more physically intimate the two windings are the lower the leakage. This is frequently not permitted due to safety regulations.
Yo can also reduce the leakage inductance by interleaving the windings. Interleaving is when you sandwich the secondary between two halves of the Primary winding or vice versa. Also a longer center leg of the ferrite core helps in reducing the number of layers for any winding and this improves the coupling between the windings too.