MPZ2012S221A datasheet(39/56 Pages) STMICROELECTRONICS

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Figure 44. Primary winding current (blue), secondary winding current (magenta) and magnetizing current

(black)

Therefore, setting the current ripple ΔIL, the inductance is so defined:

Lpri= VIN−VOUT_pri∙VOUT_pri

VIN∙fSW∙∆IL

(40)

For example, assuming IOUT_prim = 500 mA, IOUT_sec = 100 mA, N = 6, the current ripple can be set around 30%

of the total current IOUT_pri + N×IOUT_sec = 1.1 A, therefore, 330 mA. If VIN = 12 V, VOUT_prmi = 5 V and fSW = 400

kHz, the inductor value should be 22 µH.

Peak and RMS current

As any inductor, peak and RMS current for each winding must be calculated in order to define saturation and

RMS currents that the transformer should fulfill.

For the primary winding, the equations below are valid:

Ipri_pos_peak=IOUT_pri+IOUT_secN+ΔIpri2

(41)

Ipri_RMS= IOUT_pri+IOUT_secN∙ 1+112∙ ΔIpri∙N

N∙IOUT_pri+IOUT_sec

(42)

For the secondary winding, the peak current can change depending on the leakage inductance. In the image

below secondary winding current waveforms with different leakage inductances are simulated. It is evident

how the peak current can significantly vary. Considering the target leakage inductance value for an iso-buck

(recommended up to 1% of the primary inductance), the waveform can be approximated with a sawtooth shape

and the peak and RMS currents can be hence estimated as:

Isec_pos_peak=2∙IOUT_sec

1−D

(43)

Isec_RMS=Isec_pos_peak∙ 1−D3

(44)

Eq. (43) gives an indication of the peak value. Nevertheless, the waveform can significantly change with the

leakage. Therefore, the measured peak value can differ from the one provided by this equation.

A duty cycle higher than 50-60 % significantly increases the peak current in the secondary winding, therefore

affecting the negative peak current at the primary side.

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, and so affect the efficiency of the total solution.

L6983I

Transformer selection

DS14040 - Rev 1

page 39/56

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