Cirrus Logic AN351 - Manual (Page 2)

AN351
2 AN351REV2
2. CONSTANT POWER & INDUCTANCE FACTOR (CPLF)
To provide the necessary surge power, the inductor must be multiplied by the constant power & induc-
tance factor (CPLF), which is determined by the following equation.
For a typical 90 W, 400 VDC design, with an input line voltage range from 90 to 265 VACrms, the value
of the inductor in normal mode can be calculated by the following equation:
Applying equation 1 for V
pk(min)
of 127 V (rectified DC) and a V
link
voltage of 400 VDC, the CPLF becomes:
Therefore, the maximum inductance that the system can support for a 90 W constant power load at start-
up is:
416 x 0.648 = 270 uH.
To accommodate an inductor with a 10% tolerance, the actual inductance should be:
270

1.1 = 245 uH.
To allow for up to a 15% tolerance, a 230 uH inductor is used.
CPLF 2.04
V
pk min  
V
link
--------------------  = [Eq. 1]
L
B
 V
in min  
 
2
V
link
V
pk min  
–
2 f
max
P
o
 V
link
 
---------------------------------------------------   =
L
B
0.95 90
2
400 127 –
2 70 10
3
   90  400  
------------------------------------------------------------------   =
L
B
416 H =
[Eq. 2]
CPLF 2.04
127
400
----------  =
CPLF 0.648 =