AN4130 datasheet(23/41 Pages) STMICROELECTRONICS

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AN4130

Design guidance

Doc ID 023315 Rev 1

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4.6.2

Primary inductance

Assume the FET ON-time takes up half the cycle time.

Equation 2

A value of 950 µH was selected because additional time is required for the resonant drain

voltage fall time of the transition-mode converter.

4.6.3

Primary peak current

This is set by the required output power, which is limited by I2R loss in the internal FET on-

resistance, to about 10 W. For this design, at US mains voltage, the peak current is about

0.7 amps.

4.6.4

Reflected voltage

For PFC-flyback transition-mode power converters on US 120 V lines the best reflected

voltage choice is 110 to 130 V. This range gives the best converter efficiency. Copper losses

can be spread more or less equally between the primary and secondary windings, and the

FET turn-on losses from discharging circuit capacitance are quite low.

A turns ratio of 2:1 primary: secondary was selected for the 18-LED load, placing reflected

voltage at about 112 V at full undimmed output.

4.6.5

Leakage inductance

This should be as low as possible - energy stored here does not contribute to LED power

and must be dissipated as heat. Using part of the primary winding as the secondary greatly

reduces leakage inductance, but does not entirely eliminate it.

4.6.6

Auxiliary winding turns ratio

The operating voltage for the HVLED815PF depends on the LED voltage and the turns ratio.

The LED winding (secondary) voltage reflects to the flyback voltage on all windings. The

turns ratio from the secondary to the auxiliary winding determines the auxiliary voltage, both

for voltage regulation (open load protection) and for the Vcc power supply.

Sufficient voltage must be available to power the HVLED815PF at low dimmer settings.

Usually the lowest LED voltage where light is visible is around 2.8 V. The reflected voltage

on the auxiliary winding under these conditions must be greater than the HVLED815PF's

VccOFF max.

4.6.7

Final transformer specification

Winding ratios, primary inductance, peak currents, and other specifications are shown on

the schematic. The vendor's specification sheet appears later in this note.

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