Hi all,
I am looking at the datasheet of the PFC controller (UCC28070-q1). There is an equation which i didn't understood
Vac=73*Vrms --(Page 23)
I couldn't find any relation like this on internet. Is this equation correct?
Hello Dewang
This relationship is particular to the UCC28070 so that is why you did not find it online.
The explanation is that the calculation assumes that the controller has selected QVFF = 1 (Table 1 in the DS) with a maximum VINAC Peak voltage of 1V at the pin. This corresponds to an input voltage of 133V peak or about 94V RMS.
The designer will set the CS scaling so that the peak signal at the CS pin is about 3V. There is then about 125% headroom on the CS signal before reaching the PKLMT maximum of about 4V. As the input voltage falls the signal on the CS pin increases and reaches the PKLIMIT level when the input voltage has fallen by a factor of 0.76 from 94Vrms which is 71.4V. This voltage is measured inside the bridge rectifier, so the line voltage at this point is 73V.
This is an example and the designer has some flexibility to change the CS and PKLMT scaling.
Regards
Colin
