PFC waveforms

AN4314

24/31

DocID024821 Rev 2

5.7

Feedback failure protection (open loop)

A kind of protection that any power supply must have is one which prevents the failure of the

feedback circuitry. If a failure occurs, for example degradation of one BJT (Q4 or Q5), the

output voltage can increase, damaging the electrolytic capacitor and the LED load.

Figure 28 and Figure 29 show the board behavior opening the loop by removing the resistor

R17 (0

Ω).

The feedback signal on C15 falls, activating the same external protection that pulls down the

PFC_OK (CH1) pin below its disable threshold. The delay in triggering is due to the

charging time of the capacitor C7 through R32. The output voltage (CH3) is limited.

Figure 28. Feedback failure protection

Figure 29. Feedback failure protection