Leakage inductance is one of the most important parameters in a high frequency transformer, but it is often misunderstood. In some circuits it is a problem to minimize. In others it is part of the intended energy transfer or resonant behavior. The practical question is not whether leakage inductance is good or bad. The question is how much the circuit can tolerate and how consistently the transformer manufacturer can produce it.
BaoHui Tech manufactures high frequency transformers and custom magnetic components for switching power supplies, inverters, chargers, UPS systems, and industrial power electronics. Leakage inductance is one of the values we review carefully during custom transformer development.
What leakage inductance means
In an ideal transformer, all magnetic flux from one winding couples to the other winding. In a real transformer, some flux does not link both windings. That uncoupled portion behaves as leakage inductance. It is influenced by winding distance, winding order, bobbin structure, insulation thickness, core shape, and the physical arrangement of copper.
Because leakage inductance comes from geometry, it is not controlled only by the turns ratio. Two transformers with the same electrical ratio can have very different leakage inductance values.
Impact on switching stress
In many switching power supplies, leakage inductance can create voltage spikes when current changes quickly. These spikes may stress MOSFETs, diodes, snubbers, clamps, and insulation. Excess leakage can reduce efficiency and create heat in protective circuits.
For flyback converters, leakage inductance is especially visible because the stored leakage energy has to go somewhere when the switch turns off. A stable clamp design depends on knowing the leakage range, not just the nominal value.
Impact on EMI
Leakage inductance can contribute to ringing with parasitic capacitances. That ringing may appear as conducted or radiated noise. Reducing leakage may help, but interwinding capacitance also matters. A winding arrangement that lowers leakage can sometimes increase capacitance and common-mode noise.
This tradeoff is why transformer design and EMI filter design should be reviewed together. A single parameter target can create a different problem elsewhere.
When leakage is intentional
In resonant converters, leakage inductance may be used as part of the resonant tank or as a controlled series inductance. In this case, the target is not zero. The target is repeatability. The transformer must be built so the leakage value stays within the range expected by the circuit.
For these designs, BaoHui Tech needs the converter topology, switching frequency range, resonant component values, and acceptable leakage tolerance before sampling.
How manufacturers control leakage inductance
Leakage inductance can be adjusted through winding order, interleaving, sectional winding, insulation spacing, bobbin selection, and core geometry. Interleaving often reduces leakage, but it may affect insulation and capacitance. Sectional winding may improve isolation but increase leakage.
Production consistency requires controlled winding process, clear work instructions, and test limits. The first sample result is useful, but repeatability across batches is what matters in production.
FAQ
Is lower leakage inductance always better?
No. Lower leakage can reduce spikes in some circuits, but certain resonant designs need controlled leakage. The target depends on the topology.
Can BaoHui Tech control leakage inductance in custom transformers?
Yes. BaoHui Tech can adjust winding structure and production testing to meet leakage inductance targets for custom high frequency transformers.
Leakage inductance should be specified as an engineering range tied to the circuit behavior. That approach gives the transformer manufacturer a practical target and gives the power supply designer a more predictable component.