AN4130 datasheet(28/41 Pages) STMICROELECTRONICS

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Design guidance

AN4130

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Doc ID 023315 Rev 1

4.12.1

Supporting the flyback input current

First we consider the differential current drawn by the converter. The input current waveform

at line peak is sketched below:

Figure 34.

Flyback converter input current waveform

The peak current is about 0.7 A, and the ON-time is about 3.5 microseconds. The charge

that must be delivered by the capacitor directly across the converter each cycle is about:

Equation 6

The capacitance needed was determined experimentally - 0.1 µF is a good starting point at

this power level.

So, ripple voltage for the 0.1 µF capacitor is about:

Equation 7

4.12.2

Shunting the HF noise

Next we examine the input capacitor. We use 0.1 µF here as well.

Values as small as 0.047 µF can be made to work, but the inductor value must increase to

keep the noise on the AC line low enough. The resulting inductors either become physically

large, or the resistive loss in the winding affects efficiency.

4.12.3

Limiting the noise current

At 150 kHz, the 0.1 µF capacitor has a reactance of about 10

Ω. This reactance shunts away

90% of the noise current from the LISN, leaving only about 10% to the input. So 12 Vp-p

must be reduced to about 12 mVp-p (half the noise appears on each line terminal relative to

ground).

The inductor must have a reactance of at least 1000 Ohms at 150 KHz. It was found

experimentally that a 4.7 mHy part worked well. The selected inductor has about 5.5 W of

DC resistance, so I2R loss is relatively low.

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