chap10 - 10 Input Filter Design 10.1 10.1.1 INTRODUCTION...

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10 Input Filter Design 10.1 INTRODUCTION 10.1.1 Conducted EMI It is nearly always required that a filter be added at the power input of a switching converter. By attenuat- ing the switching harmonics that are present in the converter input current waveform, the input filter allows compliance with regulations that limit conducted electromagnetic interference (EMI). The input filter can also protect the converter and its load from transients that appear in the input voltage therebyimproving the system reliability. A simple buck converter example is illustrated in Fig. 10.1. The converter injects the pulsating current of Fig. 10. 1 (b) into the power source The Fourier series of contains harmonics at multiples of the switching frequency as follows:
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Input Filter Design In practice, the magnitudes of the higher-order harmonics can also be significantly affected by the cur- rent spike caused by diode reverse recovery, and also by the finite slopes of the switching transitions. The large high-frequency current harmonics of can interfere with television and radio reception, and can disrupt the operation of nearby electronic equipment. In consequence, regulations and standards exist that limit the amplitudes ofthe harmonic currents injected by a switching converter into its power source [1-8]. As an example, if the dc inductor current i of Fig. 10.2 has a magnitude of several Amperes, then thefundamental component ( n = 1) has an rms amplitude in the vicinity ofone Ampere. Regulations may require attenuation of this current to a value typically in the range To meet limits on conducted EMI, it is necessary to add an input filter to the converter. Figure 10.2 illustrates a simple single-section L-C low-pass filter, added to the input of the converter of Fig. 10.1. Thi sfilter attenuates the current harmonics produced by the switching converter, and thereby smooths the current waveform drawn from the power source. If the filter has transfer function then the input current Fourier series becomes In other words, the amplitude of each current harmonic at angularfrequency is attenuated by the filter transfer function at the harmonic frequency, Typical requirements effectively limit the current harmonics to have amplitudes less than and hence input filters are often required to attenuate the current amplitudes by 80 dB or more. To improve the reliability of the system, input filters are sometimes required to operate nor- mally when transients or periodicdisturbances are applied to the power input. Such conducted suscepti- bility specifications force the designer to damp the input filter resonances, so that input disturbances do not excite excessive currents or voltages within the filter or converter.
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chap10 - 10 Input Filter Design 10.1 10.1.1 INTRODUCTION...

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