Power


Suppressing acoustic noise in switched-mode power supplies


By Axel Schütz, Mark Schoppel and Florian Haas from Traco Electronic AG W


hen sitting in a car, the engine noise we experience is something completely normal. After all, the engine


compartment contains a machine with moving parts in it. Some of us would even define this noise as very pleasant. In fact, manufacturers of cars and other products actually have entire research departments dedicated to tinkering about with – and creating – pleasurable sound experiences. However, it is a different situation with switched-mode power supplies (SMPSs). Noises such as humming or whining may even be interpreted as a warning signal. Although power supplies are made up of a large number of electronic components, when they are operating nothing should be moving. Therefore, there shouldn’t be any noise, should there?


The most common cause of disturbing noise from AC power supplies typically used to result in a low-frequency 100 or 120 Hz hum. And, as power supplies have moved on in terms of their complexity and structure, the range of sound waves emitted from them have changed too. However, most audible noises should not be a cause for concern.


Perception and effect


Humans can hear sound waves in the 16 Hz to roughly 20 kHz frequency range (figure 1). But whether or not a sound causes distraction or irritation depends also upon the perception of that sound in the environment where it is being generated. An industrial power


supply unit that generates audible noise probably does not constitute an actual problem for people, since most people in its vicinity will experience it in the context of other background noises as a normal part of working in factories. Other noises, thanks to their frequency and volume, may also mask


42 November 2023 Figure 1: Audible frequency range of the human ear


the frequencies generated by a power supply, an effect studied in psychoacoustics and used in the compression of audio in MP3s. Such supplies are also typically built info control panels with closed doors that also help to dampen any audible noise that may be generated.


In a different environment, such as an office, the reaction to power supply noise will be significantly different. A whining or buzzing from an electrical device will likely be perceived as being unpleasant and may even raise concerns about its safety.


Causes and background: The magnetic fields


If a current-carrying conductor is located in a magnetic field, it is generally subject to a force. The effect of this force is greatest when the current and magnetic field directions form a 90° angle. In such cases, the impacting force is vertical to the flow of current and direction of the magnetic field. Three fingers of the right hand can be used to determine the direction of this force using Fleming’s right-hand rule (figure 2).


In the context of a transformer and some inductors, an iron core can also suffer from an effect known as magnetostriction, an effect first identified by James Joule in 1842. It causes ferromagnetic materials to change shape or dimension during the process of magnetisation that results from current flowing through the component’s conductor. As well as leading


Components in Electronics www.cieonline.co.uk.uk


to frictional heating, these tiny changes in material volume often generate audible noise too.


Transformers often utilize Fe-Si steel (known as silicon steel) with varying silicon content that helps to increase the electrical resistivity of the iron. Six per cent silicon steel provides the optimum level of reduction in magnetostriction but must be traded against increased brittleness.


Figure 2: Right/left hand rule The piezo effect


A further cause of noise results from the piezo effect. The word 'piezo‘ is derived from the Greek word for pressure. In 1880, Jacques and Pierre Curie discovered that pressure in various crystals, such as quartz, generated electrical charge. They called this phenomenon the 'piezo effect‘. Later, they noticed that electrical fields can deform piezoelectric materials. This effect


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