MPZ2012S221A datasheet(40/56 Pages) STMICROELECTRONICS

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Leakage inductance

The leakage inductance of a transformer can be defined as an undesired inductive component due to the not

perfect magnetic linking of the two windings. Leakage inductance is inherent to the transformer construction and

can only be reduced but not eliminated.

As shown in Figure 45, the leakage inductance affects the shape of the secondary winding current. In general, a

very low leakage inductance implies that the current in the secondary winding can quickly ramp up, allowing the

charge of the secondary output capacitor and supporting the load current demand.

A high leakage inductance slows down the secondary winding current rise, limiting the charge delivery to the

output. Comparing solutions with different values of transformer leakage inductance shows that higher leakage

inductance transformers are characterized by poorer load regulation performances (as already depicted in

Figure 22).

On the other hand, the leakage inductance determines the slope of the rising edge of current in the primary

winding (the lower the leakage, the steeper the slope) and the peak value it could reach. Therefore, under certain

conditions (for example short-circuit at the secondary side) this peak can be significantly high.

Figure 45. Impact of the leakage inductance on the secondary winding peak current

A higher value of the leakage can help in these cases, jeopardizing however the load regulation performances. A

similar effect can be also achieved by increasing the resistance of the secondary winding, keeping the optimized

value of the inductance. In both cases it is also possible to adjust the N value to improve the load regulation (see

Figure 46). Both higher leakage and higher secondary winding resistance have a slight negative impact on the

efficiency.

L6983I

Transformer selection

DS14040 - Rev 1

page 40/56

MPZ2012S221A 数据表(PDF) 40 Page - STMicroelectronics