AN3286 datasheet(28/42 Pages) STMICROELECTRONICS

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Testing the board

AN3286

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Doc ID 17998 Rev 1

2.8

Voltage feed-forward function

As briefly explained in Section 2.1, in a quasi-resonant converter the switching frequency

increases with the input voltage.

According with Equation 3, for a fixed value of the maximum peak primary current, IPK_MAX,

the maximum deliverable output power is proportional to the switching frequency.

Equation 3

If a power supply is designed in order to deliver a certain maximum output power at

minimum input voltage, its power capability at maximum input voltage can be twice or more

due to the increase of the switching frequency.

The system may not be able to detect an overload at high line because the load power

demand does not exceed the converter power capability, therefore leading to overheating of

the transformer, power MOSFET, and output diode.

In order to resolve this issue, the VIPer25 is provided with a voltage feed-forward function,

which acts by lowering the maximum drain current as the input voltage increases.

This is realized by connecting a resistor between the auxiliary winding of the transformer

and the ZCD pin (RFF resistor in Figure 35). In fact, during the ON time of the MOSFET, the

auxiliary winding voltage is negative and proportional to the flyback input voltage, therefore,

through the RFF resistor a current almost proportional to the input voltage is sunk by the

ZCD pin. Thanks to the adjustable current limitation VIPer25 function, the maximum peak

drain current is reduced proportionally to the increase of the input voltage value.

Figure 33.

Operating with secondary winding

shorted - restart mode – zoom1

Figure 34.

Operating with secondary winding

shorted - restart mode – zoom2

IN TRAFO MAX

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PK MAX

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AN3286 Datasheet(PDF) 28 Page - STMicroelectronics