The boost inductor value is determined by the output
power and the minimum switching frequency. The
minimum switching frequency must be higher than the
maximum audible frequency band of 20kHz. Minimum
frequency near 20kHz can decrease switching loss with
the cost of increased inductor size and line filter size. Toohigh
minimum frequency may increase the switching loss
and make the system respond to noise. Selecting in the
range of about 30~60kHz is a common choice; 40~50kHz
is recommended with FAN7930.
The minimum switching frequency may appear at
minimum input voltage or maximum input voltage,
depending on the output voltage level. When PFC output
voltage is less than 405V, minimum switching appears at
the maximum input voltage, according to Fairchild
application note AN-6086. The inductance is obtained
using the minimum switching frequency:
appears