Feature: Power supplies
Managing audible power-supply noise By Prasad Joshi, Field Applications Engineer, Monolithic Power Systems
T
he design requirements of today’s switching power supplies are driven by efficiency requirements not only at full load, but also under sleep mode
or no-load conditions when the cable is disconnected. Power system integrators have to meet new regulations from agencies such as Energy Star, 80 Plus and the European Commission’s certificate of conformity, regardless of the power supply’s load condition. To meet these requirements, power
supplies must reduce switching frequencies below 20kHz, occasionally as low as a few kHz. Because the human ear can hear sound frequencies below 20kHz (and is most sensitive between 2kHz and 5kHz), audible noise is difficult to miss and can be extremely annoying. Tis is especially true for consumer applications such as phone or laptop chargers, or LED drivers that can be found in every living room.
Causes of power supply noise Power supply components most susceptible to audible noise are usually MLC ceramic capacitors, inductors, or transformers. Magnetic components, such
First, measure the mechanical SRF to
The mechanical force causes the coil or core to vibrate, which displaces the air around it and manifests as sound waves
as inductors and transformers, are stressed by high-voltage pulses at frequencies that result in a physical effect, such as the reverse piezoelectric effect on the coil or magnetostriction on the core. Te reverse piezoelectric effect and
magnetostriction are mechanisms that convert the applied electrical energy into mechanical force. Tis mechanical force causes the coil or core to vibrate, which displaces the air around it and manifests as sound waves. As a result, the mechanical self- resonant frequency (SRF) of these power- supply components must be addressed, since these vibrations are amplified many times at the resonant frequency.
see if it falls within the audible noise range. If it does, identify factors that contribute to this resonance. Lastly, choose electrical parameters during the design stage that limit the range of switching frequencies. By avoiding the mechanical SRF, noise can be more easily mitigated.
Mechanical self-resonance Mechanical self-resonance has been studied, and there are models to identify and define the controls that can be used. One particular model is Hooke’s Law. Figure 1 shows equations for a spring mass system. This system is similar to the helical coil of an inductor and the mass of the PCB assembly the magnetic component is soldered onto. The mass of the red ball, m, is the same
as the mass of the PCB assembly. The displacement, x, is caused by the force of either the reverse piezoelectric effect or magnetostriction. The relationship between the force applied and the weight of the board is best described as a second-order differential equation; also shown in Figure 1. Therefore, the resonant frequency of this mass spring system can be calculated
www.electronicsworld.co.uk November/December 2020 33
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