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AN833

APPLICATION NOTE

November 2003

Devices Description

Designing PFC, an important parameter to consider and control is the high frequency noise. This noise is pro-

duced by the switching itself and its level depends on several parameters such as high frequency current ripple,

PCB layout, active switch performance and also some particular circuital solutions.

The parameter to keep under control is the electromagnetic interference (EMI) content, versus the RFI norms

and design requirements (EN60555 and VDE0871B). To get more information about this, please refer also to

the CIRCUIT FOR POWER FACTOR CORRECTION WITH REGARDS TO MAINS FILTERING (AN510). The

PFC controller L4981B version can be useful to solve the EMI problems using the frequency modulation tech-

nique that allows to spread the noise spectrum over a wider range, reducing in this way the peak of the associ-

ated energy. This section deals with the way to use this device feature getting the best results. The L4981B

performs the frequency modulation function by using Pin16, that in this version is named FREQ-MOD.

This pin (see fig. 1) is internally connected (with a current mirror) to an input of the multiplier.

The output of the multiplier is used to dynamically change the current forced into the external oscillator capaci-

tance changing in this way the switching frequency. The second terms (input) of the multiplier is connected to

the divider (1/Vrms) that, through the VRMS (Pin7), senses the input mains making the frequency modulation

not affected by the line variations (see also Pin 16 description in AN628).

Figure 1. Frequency Modulation with L4981B.

+

-

1.28V

17

I

18

COSC

10

-

+

200

8.5V

1

7

Ifm

16

FREQ-MOD

D94IN065B

ROSC

Rfm

2 POLES

FILTER

VRMS

FREQUENCY MODULATION ON L4981B