Cirrus Logic AN364 - Manual (Page 10)

AN364
10 AN364REV3
Step 9) Calculate R
FBGAIN
(R17)
Use Equation 12 to calculate the flyback gain resistor, R
FBGAIN
(R17).
where
R17 = R
FBGAIN
in 
TT
fb
= switching period TT at full brightness (full load condition)
T2
fb
= period T2 at full brightness (full load condition)
Step 10) Determine the RMS Current in the Winding
Determining RMS current I
RMS
is necessary to properly define the wire size. Current I
RMS
is dependent on the
time when the current is present in the winding, T
current
. Use Equation 13 to calculate I
RMS
:
where
T
current
= T1 when calculating the I
RMS
current in the primary winding
T
current
= T2 when calculating the I
RMS
current in the secondary winding
TT = T1+T2+T3 (see Figure 4)
The factor 3 accounts for the current triangular shape.
Step 11) Determine Output Capacitor
The flyback output capacitor, which is usually electrolytic, must provide a low impedance to the switching
frequency. When using an electrolytic capacitor, the choice is based on its ability to carry the ripple current so
that it provides a long service life. A capacitor that fulfills the ripple, voltage, temperature and life requirements
results in a capacitor with a large capacitance that is often much greater than what is necessary to smooth the
load current. In flyback applications, the voltage ripple across the output electrolytic capacitor is determined
mostly by the ESR rather than the reactance.
Step 12) Flyback Transformer Specification
This step is the first iteration of the transformer design. Due to design constraints, the following requirements
must be met:
• The core must be big enough to fit the windings.
• The number of turns required must fit in the bobbin, along with insulation material.
• The transformer must meet insulation, creepage, and safety requirements.
• The finished transformer must not overheat in the environment.
• Magnetic and geometric constraints may not yield the exact value of L
P
and N calculated.
Once the first iteration of the design is done, calculate the new I
PK(FB)
to maintain the same T1 time as earlier
using the actual value of L
P
, as shown in Equation 14.
Calculate the new T2 value using the new I
PK(FB)
and N, as shown in Equation 15:
R
FBGAIN
4M 
TT
fb
T2
fb
------------ 128 
 
  64
–
---------------------------------------------- =
[Eq. 12]
I
RMS
I
PK FB  
T
current
3 TT 
--------------------  = [Eq. 13]
I
PK FB  
V
BST
T1   
L
P
-------------------------------- = [Eq. 14]
T2
I
PK FB  
L
P

N V
O

------------------------------- = [Eq. 15]