Cirrus Logic AN364 - Manual (Page 23)

AN364
AN364REV3 23
Using Equation 20, calculate R
IPK
:
Step 16) Boost Inductor Specifications
See Figure 8 in the Boost Inductor Specifications section on page 11. Choosing a maximum switching
frequency of 110kHz, find the intersection with the 230V maximum switching curve, and get the corresponding
power. This is 50Watt
 mH, the constant P
IN
 L
BST
, for this frequency and voltage. Dividing by the input
power, obtain the inductor value.
Boost inductor RMS current I
RMS
depends on the AC line RMS current, the triangular shape and the stepped
envelope. As a first approximation consider the inductor RMS current to be equal to 1.25 times the AC line
RMS current.
Calculate the auxiliary winding turns ratio by means of Equation 44.
In summary, the inductor specifications are:
• Primary inductance = 10.4mH
• Saturation Current = 0.6A @ 3000Gauss
• RMS Current = 44mA
• Turns Ratio 18.4:1
After procuring an inductor sample meeting specifications, perform the tests described in the Boost Inductor
Specifications section on page 16.
Step 17) Determine Boost Output Capacitor
Using a flyback transformer with a 90% efficiency, boost stage output power P
BST
can be determined using
Equation 45.
For a 230V design, boost output capacitor C4 must be greater than 0.5  7.3 = 3.7F. Taking into account the
tolerance and life degradation, choosing a 4.7 F capacitor will satisfy the requirements. This capacitor must
be rated at 450V.
Step 18) Determine Boost Input Capacitor
To accommodate a variety of dimmers, capacitor C3 is proportional to input power: ~4nF/W for 230V designs
and ~12nF/W for 120V designs.
R
IPK
15.625 10
3
  V
I
PK BST  
-------------------------------------------
15.625 10
3
  V
116mA
------------------------------------------- 134k  = = = [Eq. 41]
L
BST
50 W mH   
7.3W
--------------------------------- 6.8mH = = [Eq. 42]
I
RMS
P
IN
PF
--------- 1.25
V
IN
----------- 
7.3W
0.9
--------------
1.25
230V
--------------  = 44mA = = [Eq. 43]
N
P
N
AUX
--------------
405
22
---------- 18.4 = = [Eq. 44]
P
BST
P
OUT
0.9
--------------
6.6W
0.9
-------------- 7.3 W = = = [Eq. 45]