Study on Periodic and non Periodic Frequency Modulation Techniques for

EMI Suppression in SMPS

Department of Engineering and Technology, University of Isfahan (U.I.), Iran

Sina Mohseni, Arash Roomizadeh

Abstract:

Switching Mode Power Supplies (SMPS) usually emit Electromagnetic Interferences (EMI) at switching frequency and its harmonics. As the convertors power goes up these interferences increase. Also in low power converters like the ones used in cellarer phones it is important to keep EMI emitting low in respect of RF circuits EMC.

In this paper different kinds of spread spectrum EMI reduction methods have been compared with the chaotic pulse width modulation. It is also shown that how changing the chaotic variables of chaotic clock can have influence on EMI reduction.

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Overview :

Introduction

Chaos Generation

Experimental Chaos Generator Setup

EMI Reduction by Modulation of Switching Frequency

FM Modulation

FM Clock Modulation

Chaotic FM Clock Modulation

Experimental Setup

Experimental Results

Conclusion

References

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Introduction:

Nowadays, one of the most important problems facing electronic hardware manufacturers is conforming to the varying electromagnetic interference (EMI) norms of the different domains.

The Main source of electromagnetic interference emissions in power converters comes from the switching of a DC voltage due to their periodic nature and to the presence of sharp edges.

Traditionally, the problem is often addressed by counteracting the radiating accesses to suppress EMI, such as adding proper tuned filters, which reduce the conducted interference in specific frequency bands and has to be based on the expected operating condition.

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Introduction:

It is desirable for DC–DC converters to eliminate EMI without using filters. Various methods have been used to reduce power supply emissions through that modulate the switching frequency, such as frequency modulation, random modulation.

In particular, the study focuses on reducing EMI noise by frequency modulation of the switching frequency with sinusoidal, triangular, random and chaotic pattern and comparing the reduction of EMI Noise.

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Chaos Generation

This circuit consists of a capacitor C that charges by source voltage V1 thorough Resistance or discharges through resistance to earth. The flip-flop controls the charge and discharge of the capacitor. A clock signal given to flip-flop causing the capacitor C to charge if the capacitor voltage is lower than VR and discharge if the voltage of capacitor exceeds VR.

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Chaos Generation

By the nth clock, comparator checks if the VC is less than VR the Switch S1 will be turned on and equation (1) will express the system.

If the VC becomes more than VR similarly, the switch S2 will be turned on and equation (2) will define the system's Characteristics.

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Experimental Chaos Generator Setup

According to theoretical verifications and the ranges concluded for T1, T2, and B in previous section, C=100pF, R1 = 500 kΩ and f = 20kHz are chosen and VR and R2 are changeable in designed circuit.

V-I wave form

V - T wave form

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EMI Reduction by Modulation of Switching Frequency

Switching in power converters causes electromagnetic interference at certain frequencies, particularly indexes of switching frequency.

It is possible to reduce the produced EMI by modulating the switching clock with other wave forms.

Spread-spectrum clocking (SSC) simply alters the clock spectrum by spreading the concentrated clock energy over a wider frequency band, which effectively reduces the peak energy. Figure.3 demonstrates a clock’s spectrum before and after spreading. After SSC, the energy is more evenly distributed with no peak above the allowed limit.

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FM Modulation

The figure showed below clarifies the effect of frequency modulation of sinusoidal wave on the frequency spectrum, where fc is the carrier frequency and fm is modulating frequency.

It can be seen that by this modulation, side-band harmonics are generated and reduced the main frequencies magnitude. The average power of such a signal still remains constant but disturbed in a wider bandwidth.

Bh = 2.fm. (1+α.h)

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FM Clock Modulation

Unlike simple sinusoidal carrier wave, unmodulated clock used in SMPS systems, also contains more harmonics. These harmonics are referring as switching frequency harmonics.

Modulation index d: the amount of frequency variation (or spread) as a relative percentage of the desired clock frequency, fc. The more d, the more that EMI will be reduced.

Modulation frequency fm: the rate the clock frequency is modulated around fc; decides how fast the clock will change from fc.

By frequency modulating of periodic profiles such as triangular or sinusoidal, they give a flat response to unmodulated sharp edge.

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Chaotic FM Clock Modulation

Since the frequency deviation laws are periodic, the modulated signals power density at peak interferences are concentrated around specific areas and remain quite high.

Therefore, another way to modulate switching frequency is to use non-periodic signals such as random signals or discrete time chaotic system. on the contrary this chaos-based probability density, features soft and more continuous power density spectrum in switching modulation. From the discussion in the previous section, the d parameter can sweep chaotically by features of chaotic wave.

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Experimental Setup

A frequency-modulated Buck converter is designed and built. A description of the converter is given below:

-Switching frequency: 1MHz

-Output voltage: 5V

Different modulation profiles and modulation index have been applied on this converter. Many tests have been performed, but only the most significant ones summarized and will be presented.

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Experimental Results

Fig. 1 shows the conducted EMI for fixed switching frequency in proposed buck converter circuit. The amplitude peaks are at the fundamental frequency (1MHz) and it's harmonics (2 MHz, 3MHz, … ).

Fig. 2 shows the conducted EMI for Triangular modulation pattern. The EMI spectrum peaks reduced to 10 dB because of the modulation.

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Experimental Results

Fig. 3 shows random modulation result. This pattern gives better EMI suppression than Triangular pattern.

Finally Fig. 4 shows chaotic clock modulations. Different chaotic patterns (from chaos generator discussed in previous section) applied on converter.

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Conclusion An effective method to reduce EMI in SMPS is using

spread spectrum frequency applied to their switching clock. The frequency variation may be random or follow various periodic profiles.

In this paper it is shown by both theoretical basis and experimental verification that non-periodic chaotic modulation obtains most attenuation from the EMI reduction point of view. Experimental results also confirm that it is worthy to design and build a reliable chaotic clock for SMPS systems. This plug-in chaos

based will be tested on other switching converter in future investigations.

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References [1] J. Balcells, D. Gonzalez, M.

Lamich, and D. Bedford, "EMI generation models for switching mode power supplies," in proc. Space Power conf. Eur. Space Agency (ESA), Sep. 1998, pp. 421-426.

[2] M..-H. Chew, "Handling EMI in Switching Mode Power Supply Design," Application Notes No: AN1695D, On Semiconductor

[3] Tse. K.K, Chung, H.S.H., Hui, S.Y.R., So, H.C. “Analysis and spectral characteristics of a spread-spectrum technique for conducted EMI suppression,” IEEE Transactions on Power Electronics, 15,pp.399-410,2000.

[4] F.L.Lin and D.YChen. “Reduction of Power Supply EMI Emission by Switching Frequency Modulation,” IEEE Transactions on Power Electronics, Vol. l9,No.1, pp.132-137,1994.

[5] R. Rovatti, G. Setti, S. Garffi "Chaotic FM and AM of Clock Signals for Improved EMI Compliance." Int. Symposium of Nonlinear Theory and its Application (NOLTA'99), Big Island, Hawaii, November 1999.

[6] R. Rovatti, G. Setti, S. Graffi "Chaos-Based FM of Clock Signals for EMI Reduction," European Conference on Circuit Theory and Design (ECCTD'99), Stresa, August 1999.

[7] W. K. Siu and H. Wong, "Improvement of Power converter spectral performance using chaotic frequency modulation" in proc. 1999 Int'l Technical Conf. Circuit/Systems, Computer and communications, Niigata, Japan, 1999, pp.426-429.

[8] J.H.B.Deane and D.C.Hamill. Improvement of power supply EMC by chaos. Electronic Letters, 32(12):1045, June 1996.

[9] Hong Li,BoZhang, Zhong Li, Wolfgang . Halang , Guanrong Chen "Controlling DC–DC converters by chaos-based pulse width modulation to reduce EMI” Chaos, Solitons and Fractals 42 2009 pp.1378–1387

[10] S. Mandal and S. Banerjee, “Analysis and CMOS implementation of a chaos-based communication system,” IEEE Transactions on Circuits and Systems-I, vol. 51, no. 9, 2004.

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