AN4129 datasheet(28/39 Pages) STMICROELECTRONICS

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Circuit description and design guidance

AN4129

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Doc ID 023314 Rev 1

insignificant for the damping function. So the ringing network primarily consists of the filter

inductor and the converter input capacitor.

The typical damping network across a current-sink load would consist of a capacitor and a

resistor in series. The minimum capacitor value would be 3 times the filter capacitance (3X

C1, use 0.33 µF), and a resistor of:

Equation 10

However, power dissipation in the damping resistor can be a problem. If the dimmer is set to

90 degrees conduction, the input voltage comes on at the peak of the line waveform. The

damping resistor must charge the damping capacitor 120 times a second to the peak line

voltage. For each transient, the energy dissipated in the resistor is slightly less than:

Equation 11

Other damping mechanisms exist, however, so the damping network does not need to be so

large. Reducing the size of the capacitor would allow the use of a 1/2 W resistor.

The PFC-flyback converter, unlike a regulating converter, has a POSITIVE input resistance

near the filter's resonant frequency. The input current it draws increases when the line

voltage increases, short term. This contributes some damping. The effective resistance can

be calculated from line voltage and input power. Input power is about 11 W. At nominal line

the input resistance is:

Equation 12

Note that this resistance rises and falls as the square of the line voltage.

Some additional damping is provided by the winding resistance of the inductors and the

fusible resistor. It was found experimentally that the ringing could be completely damped

with a series R-C network of 0.22 µF and 390

Ω. The final filter design is shown below:

Figure 35.

Final input filter design

AN4129 Datasheet(PDF) 28 Page - STMicroelectronics