Cirrus Logic AN368 - Manual (Page 12)

AN368
12 AN368REV2
Step 3) Determine Second-stage Parameters for a Flyback Topology
Figure 3 illustrates the steps for designing the second stage.
Steps for the Flyback Design
1. Set the boost output voltage, V
BST
.
2. Select a MOSFET that aligns with the quality standards
of the designer’s company.
3. Determine the transformer turns ratio from the V
BST
, FET
voltage, and reflected voltage, V
Reflected
.
4. Use the nominal switching frequency and an initial
estimate for time T3 to determine the value of time TT at
full brightness.
5. Use V
BST
, TT, and V
Reflected
to determine time T1 and
T2.
6. Use time T2 and TT, turns ratio N, and load current to
determine the value of the peak primary current, I
PK(FB)
.
7. Use I
PK(FB)
to determine R
Sense
.
8. Calculate the primary-side inductance using time T1.
9. Calculate flyback gain resistor R
FBGAIN
using full load
conditions. Ensure linearity of the load versus the dim
curve.
10. Calculate primary and secondary RMS currents using
I
PK(FB)
and duty cycle.
11. Select an output capacitor.
12. Determine the flyback transformer specifications.
13. Determine if the flyback transformer fits into specified
form factor after designing and constructing flyback
transformer. Repeat steps 3 to 12 until form factor
criteria is met.
14. Refinements to the circuit with final flyback transformer
design.
15. Validate that the system meets the operating criteria.
Second Stage
Flyback Specifications
Determine N, F
sw
,
V
Reflected
, and V
CLAMP
Estimate T3
Calculate TT
fb
Calculate R
Sense
,
R
FBGAIN
, and Primary
Inductance
Color System
Parameters
Fit?
Yes
No
Calculate T1,
T2, and I
PK(FB )
Calculate RMS Current
and Output Capacitor
Transformer Core
Figure 3. Flyback Stage Design