Owner's manual

AN368
56 AN368REV2
h. Determine the RMS Current in the Winding
The RMS current in the primary winding, I
PRI(RMS)
, is calculated using Equation 29:
The RMS current in the secondary winding, I
SEC(RMS)
, is calculated using Equation 30:
i. Calculate R
Sense
(R21)
Calculate sense resistor R
Sense
(R21) for flyback using Equation 31:
where,
f
scale
= Scaling factor
R21 = R
Sense
in 
Calculate the power P
Sense
dissipated by the sense resistor R
Sense
(R21) using Equation 32:
j. Calculate Flyback Zero-current Detection
Bit VALLEYSW in register Config2 at Address 34 is set to ‘1’, enabling quasi-resonant switching (valley
switching) on the second stage. Bit POL_ZCD in register Config4 at Address 36 is set to ‘0’ configuring the
comparator output to active-low. The transformer TX1 auxiliary winding turns ratio
is calculated using Equation
33:
I
PRI RMS
I
PK1 FB
2
D
MODE1
3
--------------------


 I
PK2 FB
2
D
MODE2
3
--------------------


+=
[Eq. 108]
299mA
2
0.37
3
-----------


 237mA
2
0.225
3
---------------


+=
123.5mA=
I
SEC RMS
N
2
I
PK1 FB
2

1D
MODE1
–
3
-----------------------------


 N
2
I
PK2 FB
2

1D
MODE2
–
3
-----------------------------


+=
[Eq. 109]
5.57
2
299mA
2

10.37–
3
---------------------


 5.57
2
237mA
2

1 0.225–
3
------------------------


+=
1.016A=
R
Sense
1.4V
f
scale
I
PK1 FB
-------------------------------------
1.4V
1.1 0.299A
--------------------------------4.26===
[Eq. 110]
P
Sense
I
PRI RMS
2
R
Sense
 123.5 mA
2
4.26 65mW===
[Eq. 111]
[Eq. 112]
N
P
N
FBAUX
-------------------
2NV
OVP
R23 
1.25V R22 R23+
------------------------------------------------------
=
25.5715V1.1k
1.25V 14k 1.1k+
---------------------------------------------------------------
=
9.7=