ST AN2052 Application note

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®

- APPLICATION NOTE

VIPower: IMPLEMENTING FREQUENCY MODULATION

ON VIPer53 TO IMPROVE THE EMI EMISSIONS

ABSTRACT

This report describes how the frequency modul ation reduces electroma gnetic interference on an S MPS
using Viper53 as primary PWM-switch.

For your reference, comparison of EMI measurement data will be presented.

1. TEST RESULTS
Figure 1: Original, time average at 200kHz, 53.42dBuV / m

Figure 2: FM configuration "A ", time average at 200kHz, 48.07dB uV /m

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Figure 3: FM configuration "B", time average at 200kHz, 43.92dBuV/m

As shown in figures 1, 2 and 3, using the FM technique a margin improvement will be obtained. At
frequencies lower than 1MHz, the device switching is the primary noise generator. Using the
configuration "A" or "B" a 5 dBuV/m drop is obtained for frequencies lower than 200kHz.

Between 1MHz and 10MHz, both configuration "A" and "B" ma ke a dramatic drop greater than 15dBuV/
m. Above 10MHz, FM technique benefits are reduced and capacitive coupling between copper tra cks in
PCB is dominant.

Tab le 1:

<1MHz 1Mhz ~ 1 0MH z >10MHz

Original Marginally pass Pass Enough margins

“A” Pass More margins Enough margins
“B” More margins More margins Enough margins

2. CIRCUIT AND DESCRIPTION OF TEST CONFIGURATIONS

Figure 4: Original configuration (Fixed frequency at 70 kHz, and no FM).

Vdd=10Vdc

IC1=1mAmax

10k

OSC

VIPer53

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Figure 5: Configuration "A" (Switching frequency from 63kHz to 77kHz FM cycle 20ms, 50Hz).

50Hz

900k

900k

IC1=0.83mA

11k

OSC

Vdd=10V

VIPer53

Figure 6: Configuration "B" (Frequency from 63 kHz to 77 kHz, FM cycle 1.4ms, 700Hz).

V2=10Vpeak

Oscillator

IC1=0.2mA

700Hz

50k

IC1=0.83mA

11k

OSC

V1=10V

VIPer53

Changing the resistor value between Vdd and O SC pin, it is possib le to modify the f requency oscillator
(capacitor value is fixed at 220nF). In this way the FM pe rcentage can be set . The target F M pe rcentage
is +/-10%. In the original ci rcuit a 10kohm resistor is used, obtaining a switching frequency of 70 kHz.
This resistor value gives a charging current capacitor of 1mA max.

The external circuit works in order to increase the switching frequency. Therefore, it is necessary to
change the resistor value between V dd and O S C pin i n order to reduc e m inimum swit ching frequency to
66KHz. As a result the new resistor value will be 11kohm, obtaining a charging current of 0. 9mA when
external voltage is 0V. The switching frequency variation depends on the charging current capacitor,
therefore it is proportional to the external voltage. Maximum value of the switching frequency is 77 kHz: it
is obtained when the charging current is 1.1mA where 0.9mA is given by Vdd and 0.2mA is supplied from
the external circuit.

Below, the design of the resistor value, between OSC pin and the oscillator, is explained:

- in configuration "A", maximum voltage is 374V, so the resistance is R=373V/0.2mA =1.8Mohm ;

- in configuration "B", the maximum voltage is 10V, so the resistance is R=10V/0.2mA =50kohm.
It is important to verify the frequency obtained using the oscilloscope . If necessary this frequency value

will have to be tuned to the desired value.
In figures 7 and 8, the typical clock signal is shown.

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