Cirrus-logic AN375 Manuel d'utilisateur télécharger pdf (Page 7)

AN375

AN375REV4 7

3.3 Flyback Stage Design

Predict T3 and

Calculate Duty Cycle

Calculate I

Calculate

Primary Inductance

Calculate R

Sense

CTRL1

, and R

CTRL2

Determine the Switching

Frequency Range

Transformer Core

Flyback Specification

Determine I

IN(CC)

Sense

CTRL1

, R

CTRL2

Fit?

Determine

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

7. Calculate the primary side inductance L

using times T1

and T2.

8. Use peak current I

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

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.

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Cirrus-logic AN375 Manuel d'utilisateur Page 7

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