Cirrus Logic AN368 - Manual (Page 38)

AN368
38 AN368REV2
The AC line current does not follow the inductor peak current envelope because the circuit operates in CRM
and DCM. The switching frequency and duty cycle changes across the AC line phase, resulting in a changing
average value after the EMI filter smoothing.
During circuit adjustment, connect an electronic load in CV mode for testing and clamp protection. Set the
electronic load so that the boost output voltage is 425V for a 230V system (CS1631) or 215V for a 120V
system (CS1630). Measure the switching frequency at the nominal line voltage. Adjust I
PK(BST)
to obtain the
desired waveform at the peak of the line voltage, as shown in Figure 18.
Step 9) Boost Inductor Specifications
The CS1630 controls I
PK(BST)
and keeps the boost stage operating in CRM. Boost inductance L
BST
only
controls the average switching frequency. The instantaneous frequency changes to meet the I
PK(BST)
imposed
by the controller, and the duty cycle imposed by the CRM/DCM algorithm.
The boost inductor should be designed for 600mA at 3000Gauss and the maximum ambient temperature. For
a given input voltage design, the product of (L
BST
·
P
IN
) is constant. Choose the frequency range on Figure 19
to find the corresponding (L
BST
·
P
IN
) product, and divide the product by P
IN
to obtain L
BST
.
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0 15 30 45 60 75 90 105 120 135 150 165 180
Cur
rent (A
)
Phase Angle (°)
AC Line Current
Inductor Peak Current
Figure 18. Current vs Phase Angle