MPZ2012S221A datasheet(32/56 Pages) STMICROELECTRONICS

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Eq. (25) emphasizes how the selection of the transformer and, to some extent, the Schottky diode play a crucial

role in the accurate regulation of the secondary output voltage. Figure 38 shows instead the effect of the current

drawn from the primary output on the isolated voltage, as described by the Eq. (24).

Figure 38. Isolated voltage variation due to the primary output current

Another factor affecting the secondary output voltage regulation comes from the leakage inductance of the

transformer. As described later on, the leakage inductance determines the peak current that can be reached in

the secondary winding. This peak current defines the maximum current from the secondary isolated output by

limiting the amount of charge delivered to the output. When the current demand from the secondary isolated

output exceeds the maximum deliverable charge limited by the peak current, the isolated output voltage drops.

Figure 39. Secondary output maximum current

The effect of the leakage on the load regulation was already shown in Figure 22 in Section 6.2 .

To some extent also the duty cycle can affect the secondary output voltage. Since the energy transfer from

primary to secondary occurs only during the TOFF, a too high duty cycle could reduce the achievable peak

current, hence limiting the deliverable current to the secondary output. Under this condition, the secondary output

could drop.

L6983I

Output voltage adjustment

DS14040 - Rev 1

page 32/56

MPZ2012S221A Datasheet(PDF) 32 Page - STMicroelectronics