L6986I datasheet(39/60 Pages) STMICROELECTRONICS

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Figure 44. Secondary winding positive peak current dependency on duty cycle (IOUT_sec = 100 mA)

0.5

1.5

2.5

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Duty cycle

Figure 45. Primary negative peak current dependency on duty cycle (VOUT_pri = 5 V, IOUT_pri = 0, IOUT_sec =

100 mA, N = 6, fSW = 500 kHz, L = 19 µH)

0.5

1.5

2.5

3.5

4.5

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Duty cycle

Reverse current

limit already

triggered

typ. reverse current

limit threhsold

Windings resistance

Winding resistances should be minimized as much as possible since they affect the secondary output load

regulations. They also contribute to power losses, hence affecting the efficiency of the total solution.

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 be only reduced but not eliminated.

As shown in the Figure 46 (resulting from a simulation), 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 transformer are characterized by poorer load regulation performances.

In addition, higher leakage inductance means a reduction of the effective current slope in the primary winding,

hence increasing the duty cycle. As consequence, the Schottky diode conduction time shorten, and higher peak

and RMS current are present at the secondary winding. Higher peaks at light load negatively impact the load

regulation.

L6986I

Design of the external components

DS13647 - Rev 1

page 39/60

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