L6986I datasheet(37/60 Pages) STMICROELECTRONICS

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6.5.2

Transformer selection

The transformer has two essential tasks:

•

Providing the isolation between primary and secondary side in accordance with the application requirements

•

Generating the necessary secondary output voltage from the regulated primary voltage with the most

suitable turn ratio

The transformer selection implies defining the following parameters:

•

Isolation voltage

•

Turn ratio

•

Primary inductance

•

Peak and RMS currents

•

Windings resistance

•

Leakage inductance

•

Parasitic capacitances

Isolation voltage

The isolation of the transformer in terms of voltage capability (1.5 kV, 4 kV, and so on) and type (functional basic,

reinforced, etc.) is mainly driven by the application. Both parameters normally affect the size of the transformer as

well as other electrical characteristics (e.g. winding resistance, leakage inductance, etc.).

Turn Ratio

Naming Npri and Nsec the number of turns of the primary and secondary windings respectively, the turn ratio is so

defined:

N = NpriNsec

(36)

Considering the equation (25), the turn ratio must be defined so that the voltage at the secondary output is the

desired one over the whole secondary output current range:

N ≥ VOUT_sec+ Rwind_sec∙IOUT_sec + VFD1

VOUT_pri+IOUTpri∙RDSonLS+Rwind_pri

(37)

If no current is drawn from the primary output, the turn ratio should be only chosen to compensate the drops due

to the secondary winding resistance and the Schottky diode.

The effect of the leakage inductance on the secondary output voltage regulation (not included in the equation

above) should be taken into account too. Figure 17 clearly shows how the secondary output voltage can drift due

to the leakage inductance.

Primary inductance

The choice of the primary inductance does not differ so much from a standard buck. The magnetizing current,

(see Figure 43) which combines the two winding currents, has the same shape of the buck inductor current and

can be defined as:

IL_mag = Ipri+N∙Isec

(38)

L6986I

Design of the external components

DS13647 - Rev 1

page 37/60

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