User guide

AN375
AN375REV4 7
3.3 Flyback Stage Design
Predict T3 and
Calculate Duty Cycle
Calculate I
PK
Calculate
Primary Inductance
Calculate R
Sense
,
R
CTRL1
, and R
CTRL2
Determine the Switching
Frequency Range
Transformer Core
Flyback Specification
Determine I
IN(CC)
R
Sense
,
R
CTRL1
, R
CTRL2
Fit?
Determine
V
Zener
, V
Reflected
, and N
Steps for the Flyback Design
1. Determine the constant input current to achieve full-bright
power.
2. Select a FET that aligns with the quality standards of the
designer’s company.
3. Determine the transformer turns ratio N from the peak line
voltage V
INPK
, FET breakdown voltage, and reflected
voltage V
Reflected
.
4. Use the nominal switching frequency and an initial
estimate for time T3 to determine the value of switching
time TT at full brightness. Calculate the duty cycle in
Dimmer Mode.
5. Use voltages V
INPK
, V
Reflected
, and switching time TT to
determine times T1 and T2.
6. Use times T2 and TT, turns ratio N, and nominal load
current to determine the value of peak current I
PK
.
7. Calculate the primary side inductance L
P
using times T1
and T2.
8. Use peak current I
PK
to determine resistor R
Sense
.
9. Calculate resistor R
CTRL1
used to regulate the input line
current to a constant current I
IN(CC)
.
10. Calculate resistor R
CTRL2
used to set the output load
current I
OUT
. Ensure linearity of the load versus the dim
curve.
11. Calculate primary and secondary RMS currents using
peak current I
PK
and duty cycle.
12. Select an output capacitor.
13. Determine the flyback transformer specifications.
14. Determine if the flyback transformer fits into specified form
factor after designing and constructing the flyback
transformer. Repeat steps 1 to 12 until form factor criteria
is met.
15. Refine the circuit using the final flyback transformer
design.
16. Validate that the system meets the operating criteria.