Quiet switching controller for audio amplifiers

ANP013

Analog amplifiers are still used in entertainment electronics, especially in the audio range. Switching controllers are now increasingly used as their power supplies, as they attain high efficiency. These are switched circuits that include various sources of interference and may lead to various types of electromagnetic interference. Classical interference, such as line-bound interference, is not always the main problem in every circuit. In analog amplification circuits, switching controllers can cause non-linear distortions. Nonetheless, even analog circuits can be supplied with switching controllers, provided attention is paid to filtering of interference and the design of their circuit boards. However, filters are not all the same. Different filter coils are considered depending on the frequency associated with the interference. Wire coils with a relatively low self-resonant frequency are no longer considered for high frequency interference sources. SMD ferrites are selected for such cases, as offered in a wide variety by Würth Elektronik. This Application Note addresses the EMC-compliant design of DC/DC switching controllers for analog audio amplifiers using Würth Elektronik SMD ferrites.

Summary

This Application Note illustrates the importance of correct selection of filter components and the circuit board layout. Conductor loops can couple interference voltages to neighboring circuits or conductors, which leads to increased radiation of the RF interference field. Conductor loops of this kind can be reduced by observing simple layout tips. Filtering the inputs also reduces the undesirable interference voltage and thus line-bound interference. Reduction of residual ripple at the switching controller output is achieved by means of an output filter and can then prevent an audio amplifier's non-linear distortions. It has been demonstrated that SMD ferrites are most suitable for HF suppression. The Würth Elektronik Component Selector helps in the selection of SMD ferrites and the result of the filter can be simulated easily in the LTSpice software. The example investigated in this Application Note shows that switching controller can even be used in analog amplifiers provided the points described are observed.

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