AN3095 datasheet(13/55 Pages) STMICROELECTRONICS

13 / 55 page

AN3095

DC-DC converter

Doc ID 16555 Rev 3

13/55

Equation 8

By integrating the leakage current expression over the switching period and multiplying the

result by the input voltage value the expression of power transfer may be derived as:

Equation 9

where

ωs=2πf

s is the switching frequency in rad/s,

φ=ωst is the phase-shift angle and

once the operation of the converter has been described, based on the specification in

Table 1

, the power transfer function may be plotted as shown in Figure 11.

Figure 11.

Power transfer function for different input voltages

As the converter operates in boost mode the value of parameter “d” must be kept greater

than “1” for every value of input voltage in order to maintain controllability, also at low power

levels. In fact, if d<1 the converter is characterized by a minimum power level under which

the converter cannot be controlled. For this reason, transformer turns ratio has been chosen

at equal to 1.2. The value of leakage inductance must also be chosen carefully and it is a

compromise between peak current value and the maximum energy transfer between input

and output.

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