AN3095 datasheet(16/55 Pages) STMICROELECTRONICS

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DC-DC converter design

AN3095

16/55

Doc ID 16555 Rev 3

●

Transformer turns ratio:

As the converter operates in boost mode, to avoid problems of controllability for low power

levels, the value of parameter “d” must always be greater than one:

Equation 17

Moreover, considering the voltage drop across the leakage inductor it is possible to operate

the converter with n=1.2 without incurring regulation problems for high input voltage values

at low power.

●

Minimum output power device breakdown voltage:

Equation 18

●

Power device selection:

According to the calculations above, four STW55NM60ND MOSFETs were selected for the

input bridge and also two STW55NM60NDs for the active rectifier. The main characteristics

of this MOSFET are reported in Table 2 and 3:

●

Rectifier diode selection:

Two STTH60L06s are selected for the diode leg. The main characteristics are shown in

Figure 3

●

Input capacitor value:

The input capacitor, C1, is designed to smooth the high frequency ripple at the input of the

PV array. If the current generated by the module is assumed to be constant and the current

drawn by the converter is assumed to be a pulse train, the following equation gives the value

of the input capacitance:

Equation 19

Table 2.

MOSFET electrical characteristics

VDS@Tjmax

RDSon_max

ID@100°C

Coss

650 V

0.06

29 A

900 pF

190 nC

Table 3.

Diode rectifier electrical characteristics

Vf_max @150 °C IF=60 A

Vrrm

Trr_max

IRM

1.4 V

600 V

85 n

60 A

10.5 A

dVin

Vout

MAX

≤

≥ →

576

400

max

MPPT

OUTPUT

BrkMos

•

min

array

Vin

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