AN4214 datasheet(7/37 Pages) STMICROELECTRONICS

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AN4214

SMPS description

2.1

PFC_OK

2.1.1

PFC_OK function in PFC operation

PFC pre-regulator output voltage monitoring/disable function: This pin senses the

output voltage of the PFC pre-regulator through a resistor divider and is used for protection

purposes. If the voltage on the pin exceeds 2.5 V the IC stops switching and restarts as the

voltage on the pin falls below 2.4 V. However, if the voltage of the INV pin falls 40 mV below

that of the pin PFC_OK, a feedback failure is assumed. In this case the device is latched off

due to feedback failure protection (FFP). Normal operation can be resumed only by cycling

VCC, bringing its value lower than 6 V before moving up to the turn-on threshold. If the

voltage on this pin is brought below 0.23 V, the IC is shut down. To restart the IC the voltage

on the pin must go above 0.27 V. This can also be used as a remote on/off control input.

Referring to the circuit portion below, R3 and R4 composes the network to activate PFC

output overvoltage shutdown.

Figure 2. Output voltage setting, OVP and FFP functions: internal block diagram

2.1.2

PFC_OK function in flyback operation

The same pin can be also be used in case of flyback converter to shut down the converter in

case of either input overvoltage or output overvoltage. For input overvoltage protection, one

can bias the pin using 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 the

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

In the schematic, the biasing components R2, R6, R7 and R22 comprise 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 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, we can simply tap the signal for PFC_OK which is programmed to 2.5 V using

AN4214 Datasheet(PDF) 7 Page - STMicroelectronics