AN4339 datasheet(6/27 Pages) STMICROELECTRONICS

6 / 27 page

Description

AN4339

6/27

DocID025039 Rev 1

Figure 3. PFC_OK function in PFC operation

PFC_OK function in flyback operation

The same pin can also be used in case of flyback converter to shutdown the converter either

in case of input overvoltage or output overvoltage. For input overvoltage protection, the pin

one can be biased using the resistor divider network at rectified DC voltage to activate input

overvoltage protection at the required mains OVP level. Referring to the circuit portion

below, R3, R4 composes the network to activate mains overvoltage shutdown, where small

capacitor C2 is used to average the DC value of sensing voltage at the PFC_OK pin. In the

schematic in Figure 10 on page 18 the biasing components R1, R6, R8 and R19 comprises

the input OVP detection circuit. The divider ratio is selected to provide 2.5 V at required

mains OVP level.

If output overvoltage protection is preferred instead of mains OVP, we can utilize the

auxiliary winding output, which is used to provide the operating voltage to the controller as

well as demagnetization input to ZCD. Since we get the reflection of output voltage at

auxiliary output supply so we can simply tap the signal for PFC_OK which is programmed to

2.5 V using resistor divider network at desired output OVP. This is achieved using the

resistor divider network R4 and R5 at auxiliary output supply in Figure 4. In our circuit we

have implemented input OVP protection network to bias PFC_OK. So the R14 in the

schematic is kept open.

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