Cirrus Logic AN349 - Manual (Page 7)

AN349
AN349REV1 7
4.1 Boost Inductor Size
Like the L6562-based designs, the size of the boost inductor when using the CS1601 is determined by the amount of energy
pushed by the inductor at the minimum input voltage during full-load operation.
At the peak of the minimum input line voltage, and at full load, the operation of the CS1601 is identical to that of L6562. Therefore
as shown in Figure 9, the CS1601-based PFC system operates in CRM mode at the peak of the line. Peak currents for both the
systems are the same. Since the inductors in both the systems are pushing maximum power at the peak of the minimum line
voltage while maintaining CRM operation, the following equation for calculating the inductor size is:
Where,
L = boost inductor value
Vin
min
= minimum line voltage for which PFC is designed (RMS)
V
link
= PFC Output/Link Voltage
Fsw = frequency of PFC at the peak of the line voltage
P
in
= input power to the PFC
Comparing Figure 8 and Figure 9, it can be seen that for the CS1601, Fsw corresponds to the maximum switching frequency
(nominally 70kHz). For the L6562, Fsw corresponds to the minimum switching frequency, which is usually around 40-45kHz in
most designs. Assuming the same margin for overcurrent conditions, the CS1601 needs an inductance which is 45% smaller
than the corresponding L6562-based design.
L
Vin
min
2
V
link
2 Vin
min
  
–
  
2 Fsw Vin
min
 
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