ST AN2052 Application note

	AN2052
®	- APPLICATION NOTE

VIPower: IMPLEMENTING FREQUENCY MODULATION ON VIPer53 TO IMPROVE THE EMI EMISSIONS

ABSTRACT

This report describes how the frequency modulation reduces electromagnetic interference on an SMPS 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
		Product(s)
	Obsolete
	-
	Product(s)
	Figure 2: FM configuration "A", time average at 200kHz, 48.07dBuV/m
	Obsolete

October 2004

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AN2052 - APPLICATION NOTE

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 improvementProduct(s)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" make a dramatic drop greater than 15dBuV/ m. Above 10MHz, FM technique benefits are reduced and capacitive coupling between copper tracks in PCB is dominant.

Table 1:

Obsolete

<1MHz

-

1Mhz ~ 10MHz

>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

Product(s)

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

Obsolete

Vdd=10Vdc

IC1=1mAmax

10k

OSC

VIPer53

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AN2052 - APPLICATION NOTE

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

50Hz
900k	Vdd=10V
	IC1=0.83mA
900k	11k
	OSC
	VIPer53

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

V2=10Vpeak			Product(s)
700Hz		V1=10V
		IC1=0.83mA
Oscillator
IC1=0.2mA		11k
	Obsolete
		OSC
50k
		VIPer53
-
Product(s)

Changing the resistor value between Vdd and OSC pin, it is possible to modify the frequency oscillator (capacitor value is fixed at 220nF). In this way the FM percentage can be set. The target FM percentage is +/-10%. In the original circuit 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 Vdd and OSC pin in order to reduce minimum switching frequency to

Obsolete66KHz. 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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