Cirrus Logic AN364 - Manual (Page 22)

AN364
22 AN364REV3
Using Equation 13 , calculate the transformer T1 secondary RMS current:
Step 11) Determine Output Capacitor
Output capacitor C5 ripple current I
Ripple(RMS)
is the vectorial difference between transformer T1 secondary
current and the DC load current. See Equation 39.
Output capacitor C5 is selected as a 100 F, 25V capacitor.
Step 12) Flyback Transformer Specification
Specifications for the flyback transformer T1 can now be compiled to enable suppliers to design within size
and cost constraints.
Step 13) Circuit Adjustments
Now that the transformer has been defined and built, it may need to be adjusted. For more information, see
Circuit Adjustments on page 11.
Step 14) Recalculate R
Sense
Validate that the system meets the operating criteria. This may require adjusting components like R
Sense
and
R
FBGAIN
. For more information, see Recalculate R
Sense
on page 11.
4.2 Boost Stage Design Steps
Step 15) Determine I
PK(BST)
, I
SAT, and
R
IPK
(R13)
Using Equation 19, calculate I
PK(BST)
:
Parameter Value
Output DC Power 6.6W
Converter Topology CRM flyback
Switching Frequency 85kHz
Primary Inductance 11.3 mH

10%
Peak Primary Current 0.15A
Primary RMS Current 43mA
Turns Ratio N
P
/N
S
14.3
Secondary Voltage 15.4V
Secondary RMS Current 0.85A
Primary Leakage Inductance <100 H
Primary Voltage 405V
Electrostatic Shield Between primary and secondary winding
Auxiliary Winding Turns Ratio (N
P
/N
AUX
) 20
Auxiliary Wire Any convenient gauge
Rigidity Sec  Core/Primary/Aux/Shield >2,500V
I
RMS
I
PK FB  
N 
T
current
3 TT 
--------------------  = 0.131A 14.3 
7.45 s
3 12.5 s 
----------------------------  850mA = = [Eq. 38]
I
Ripple RMS  
0.85A  
2
0.44A  
2
– 0.73A = = [Eq. 39]
I
PK BST  
3.64 P
IN

V
RMS
---------------------------
3.64 7.3W 
230V
-------------------------------- 116mA = = = [Eq. 40]